Hemolytic-uremic syndrome: clinical guidelines

List of Abbreviations

ADAMTS13 – specific metalloproteinase (a disintegrin and metalloproteinase with thrombospondin motifs 13) responsible for the degradation of von Willebrand factor;

C3 – complement component 3;

cblC-HUS – hemolytic uremic syndrome associated with defective cobalamin C metabolism;

CFB – complement factor B;

CFH – complement factor H;

CFI – complement factor I;

CH50 – total hemolytic complement activity;

DGKE – diacylglycerol kinase ε;

EHEC – enterohemorrhagic Escherichia coli;

Gb3 – globotriaosylceramide;

IgG – immunoglobulin G;

IgM – immunoglobulin M;

INF2 – inverted formin-2;

HELLP syndrome is an acronym for the symptom complex of hemolysis (Hemolysis), elevated liver enzymes (Elevated Liver enzymes), and thrombocytopenia (LowPplatelet count), associated with preeclampsia;

HLA – human leukocyte antigen;

KDIGO – Kidney Disease: Improving Global Outcomes;

MCP/CD46 – membrane cofactor protein;

MLPA – multiplex ligation-dependent probe amplification;

NGS – next generation sequencing;

PLG – plasminogen;

pnHUS – hemolytic uremic syndrome associated with Streptococcus pneumoniae;

STEC – Shiga toxin-producing Escherichia coli;

SD1 – Shigella dysenteriae type1;

Stx – Shiga toxin;

STEC-HUS – hemolytic uremic syndrome associated with Shiga toxin-producing Escherichia coli;

TF antigen – Thomsen-Friedenreich antigen;

THBD – thrombomodulin;

AH – arterial hypertension;

aHUS – atypical hemolytic uremic syndrome;

ALT – alanine aminotransferase;

Anti-CFH-ab – anti-CFH antibodies;

AST – aspartate aminotransferase;

APS – antiphospholipid syndrome;

ARB – angiotensin II receptor blockers;

HUS – hemolytic uremic syndrome;

DIC – disseminated intravascular coagulation;

GIT – gastrointestinal tract;

TPE – therapeutic plasma exchange;

RRT – renal replacement therapy;

ACEi – ACE (angiotensin-converting enzyme) inhibitors;

IMV – invasive mechanical ventilation;

CT – computed tomography;

CK – creatine kinase;

CK-MB – MB isoenzyme of creatine kinase, most specific for the myocardium;

LDH – lactate dehydrogenase;

MAC – membrane attack complex;

MRI – magnetic resonance imaging;

AKI – acute kidney injury;

ARI – acute respiratory infection;

CVVHD – continuous venovenous hemodialysis;

CVVHDF – continuous venovenous hemodiafiltration;

PD – peritoneal dialysis;

PT – plasma therapy;

PCR – polymerase chain reaction;

eGFR – estimated glomerular filtration rate;

FFP – fresh frozen plasma;

SLE – systemic lupus erythematosus;

GFR – glomerular filtration rate;

CRP – C-reactive protein;

TMA – thrombotic microangiopathy;

TTP – thrombotic thrombocytopenic purpura;

ESRD – end-stage renal disease;

LoE – level of evidence;

LeRM – level of recommendation credibility;

CKD – chronic kidney disease;

CNS – central nervous system;

ALP – alkaline phosphatase;

ECG – electrocardiography recording, interpretation, and analysis of electrocardiographic data;

EchoCG – echocardiography;

** – a medicinal product included in the List of Vital and Essential Drugs for medical use for 2020 (Decree of the Government of the Russian Federation No. 2406-r dated October 12, 2019, as amended);

# – off-label use – in accordance with drug characteristics not specified in the instructions, carried out in accordance with Federal Law No. 323-FZ dated November 21, 2011, "On the Basics of Protecting the Health of Citizens in the Russian Federation" by decision of the medical commission, subject to the signed informed consent of the parents (legal representative) and the patient over 15 years of age.

Terms and Definitions

ADAMTS13 – a metalloprotease that cleaves ultra-large von Willebrand factor multimers, a diagnostic marker for thrombotic thrombocytopenic purpura.

cblC-HUS – hemolytic uremic syndrome associated with defective cobalamin C metabolism (including in patients with methylmalonic acidemia (aciduria)).

pnHUS – infection-mediated hemolytic uremic syndrome developing due to Streptococcus pneumoniae.

STEC-HUS – infection-mediated hemolytic uremic syndrome developing due to microcirculatory vascular endothelial injury by Shiga toxin produced by enterohemorrhagic or enteroaggregative Escherichia coli or Shigella dysenteriae type 1. Synonym: typical HUS.

Atypical hemolytic uremic syndrome (aHUS) – a systemic disease from the group of thrombotic microangiopathies with a progressive course and unfavorable prognosis, based on uncontrolled activation of the alternative complement pathway of hereditary (mutations in genes encoding complement proteins) or acquired nature (antibodies to factor H), leading to generalized thrombus formation in microcirculatory vessels (complement-mediated thrombotic microangiopathy).

Methylmalonic acidemia (aciduria) – a genetically heterogeneous hereditary disease from the group of organic acidemias, caused by the blockade of propionate metabolism at the level of the conversion of methylmalonyl-CoA to succinyl-CoA and impaired metabolism of a number of amino acids (isoleucine, valine, threonine, methionine), odd-chain fatty acids, and cholesterol.

Acute kidney injury (AKI) – an acute, potentially reversible change in the renal parenchyma accompanied by a decrease in renal function of varying severity. A term currently accepted in place of the obsolete term – acute renal failure (ARF). The latter statement is essentially incorrect, as the terms AKI and ARF are not synonymous. International consensus criteria for AKI are designed to describe three levels of renal impairment (risk, injury, failure) and two clinical outcomes (loss of kidney function and end-stage).

Multiple organ dysfunction syndrome (MODS) – severe damage to two or more functional systems of the body.

Thrombotic microangiopathy (TMA) – a clinical and morphological syndrome characterizing a specific type of microcirculatory vascular lesion based on endothelial injury followed by thrombus formation.

Thrombotic thrombocytopenic purpura (TTP) – a systemic form of thrombotic microangiopathy caused by ultra-large von Willebrand factor multimers due to acquired (autoantibodies) or hereditary (gene mutations) ADAMTS13 deficiency.

Chronic kidney disease (CKD) – persistent renal dysfunction lasting for 3 months or more, defined as structural and/or functional changes with a varying degree of clearance function reduction.

1. Brief information on the disease or condition

1.1 Definition of the disease or condition

Hemolytic uremic syndrome (HUS) is one of the leading causes of AKI in children [1]. HUS is characterized by a triad of signs with the development of Coombs-negative microangiopathic hemolytic anemia (MAHA), thrombocytopenia, and acute kidney injury. These signs are components of a clinical and morphological syndrome – thrombotic microangiopathy (TMA), characterized by occlusive microcirculatory vascular lesions resulting from endothelial injury. Endothelial dysfunction leads to platelet activation with thrombus formation, which causes mechanical red blood cell damage and vascular lumen occlusion with subsequent organ ischemia, primarily affecting the kidneys [1, 2, 3]. TMA is based on endothelial injury and dysfunction caused by various factors, which determines the diversity of its forms [1, 4].

The classification of HUS is complex, as the clinical presentation of the disease can manifest across a wide range of clinical scenarios.

The majority of HUS cases (up to 90%) result from infection with Shiga toxin (Stx)-producing bacteria: enterohemorrhagic Escherichia coli (STEC) or Shigella dysenteriae type 1 [5]. This form of TMA is termed typical HUS/STEC-HUS. STEC-HUS is one of the most common conditions requiring emergency renal replacement therapy (RRT) in children [2, 6]. Global mortality in typical HUS during the acute phase reaches 2-5% [7].

The remaining 10% of HUS cases have traditionally been grouped as atypical HUS (aHUS). However, as our understanding of HUS pathogenesis has improved over the past 20 years, it has become clear that aHUS is not a single disease, but many different disorders with similar clinical features. In most cases, aHUS is an ultra-rare (orphan) progressive disease resulting from systemic TMA driven by chronic uncontrolled activation of the alternative complement pathway [1, 5, 8]. aHUS is characterized by a lifelong relapsing course and represents a potentially life-threatening condition. It has been established that when aHUS debuts in childhood, end-stage chronic kidney disease (CKD) (CKD stage G5) develops less frequently, but mortality is higher in children than in adults [1, 5].

1.2 Etiology and pathogenesis of the disease or condition (group of diseases or conditions)

HUS presents a major therapeutic challenge in pediatrics and pediatric nephrology, serving as a leading cause of AKI with potential progression to CKD G5 at various intervals from disease onset. Although the STEC-associated form of HUS with a typical diarrheal prodrome is the most common, careful confirmation of infectious etiology is required to promptly rule out aHUS and pneumococcal infection-associated HUS.

1.2.1 Typical HUS

The majority of children (85-90%) are diagnosed with typical HUS (STEC-HUS), etiologically linked to intestinal infection with Shiga toxin (Stx)-producing strains of enterohemorrhagic E. coli or S. dysenteriae type 1. Acute intestinal infection caused by these bacterial strains is complicated by HUS in 10-15% of cases [2, 8].

Enterocolitis in the prodromal period of STEC-HUS is caused by various EHEC strains: most frequently serotype O157:H7 (45-80%), which differs from other strains by its inability to ferment sorbitol, and less frequently by O26:H11/H−, O145:H28/H−, O103:H2/H−, O111:H8/H−, O118:H2/H−, O113:H21/H−, O45:H2/H−, O80:H2/H− [7, 9, 205].

Each year, STEC infection is diagnosed in 2.8 million cases, which are complicated by the development of HUS in 3890 patients [10]. The incidence of HUS resulting from STEC infection depends on bacterial pathogenicity and for the E. coli O157:H7 serotype ranges from 3-7% in sporadic cases to approximately 20% in epidemic forms. During the 2011 intestinal infection outbreak in Europe (n=3816) caused by a novel hypervirulent enterohemorrhagic E. coli (EHEC) O104:H4 strain with unique enteroaggregative properties, the risk of HUS was 22% (n=845) [11, 12]. Cattle are the primary reservoir of STEC infection, and sheep, goats, horses, deer, cats, dogs, birds, and flies can also act as vectors. Infection is caused by bacterial contamination of water, meat, and dairy products. Transmission is possible through contact with animals, as well as from person to person and from mother to child. STEC-HUS is more common in summer and autumn [6, 9, 13].

After consuming contaminated food, bacteria enter the intestine, where Shiga toxin (Stx) is secreted E.coli. There are two families Stx – Stx1 i Stx2. It has been shown that E. coli, isolated from patients with HUS produces either both toxins (Stx1 i Stx2) or only Stx2. The genes Stx replicate spontaneously at a very low level. Exposure to stimulating factors – hydrogen peroxide produced by neutrophils, antibiotics – enhances the production Stx. Surface plasmon resonance analysis revealed that Stx1 easily binds to and dissociates from the glycosphingolipid receptor globotriaosylceramide (Gb3), unlike Stx2, which binds and dissociates very slowly, thus remaining on the cell surface long enough to be internalized. This may explain why for human endothelial cells in vitro Stx2 is 1000 times more toxic than Stx1. It has been shown that during the course of the disease, strains E. coli may lose the ability to secrete Stx, leading to false-negative results in late diagnosis of infection [14].

The pathogenesis of HUS is based on endothelial damage due to the action of Shiga toxin. Following infection, pathogenic E. coli bind to the villi of the terminal ileum and the epithelium of Peyer's patches, after which colonization of the large intestine occurs. Bacteria adhere to epithelial cells via the outer membrane protein intimin. Stx plays a role in enhancing the adhesion of E. coli to the epithelium by increasing the expression of nucleolin, another surface receptor for intimin. As the intestinal villi are progressively damaged, a watery stool diarrhea develops first, followed later by hemocolitis [15].

Free Stx is not detected in the bloodstream. Its transporters include polymorphonuclear neutrophils, monocytes, erythrocytes, platelets, as well as platelet-monocyte and platelet-neutrophil complexes. The amount of bound Stx correlates with the severity of renal injury. Normal enterocytes do not express Gb3. It is believed that Stx crosses the tight junctions of the intestinal epithelium by binding to Gb3 expressed on Paneth cells located in the deep crypts of the small intestine. At the cellular level, Stx binds to Gb3 localized on glomerular endothelial cells, mesangial cells, podocytes, and tubular epithelial cells; it is also present in other organs, particularly in large amounts in neurons and glial cells of the brain. Subsequently, the active part of Stx penetrates the cell and blocks protein synthesis, inducing cell apoptosis [1, 16].

In cell culture, the microcirculatory endothelium is more susceptible to Stx compared to larger vessels. Consequently, Stx-mediated changes in the endothelial phenotype create a prothrombotic environment, as evidenced by higher median plasma concentrations of prothrombin fragments, tissue plasminogen activator, and D-dimer in children with STEC-HUS. In glomerular vessels, furthermore, Gb3 expression and Stx exposure increase under the influence of tumor necrosis factor alpha (TNFα) and other proinflammatory cytokines capable of increasing receptor density on the cell surface. The toxin also activates P-selectin, an adhesion molecule involved in the regulation of the complement system. Complement activation in STEC-HUS is considered a "second hit" mechanism that amplifies microcirculatory endothelial injury [4, 18, 19].

Another pathogenic factor of STEC is the bacterial cell wall lipopolysaccharide (O157LPS), which circulates bound to platelets, monocytes, and neutrophils. The binding of O157LPS to platelets leads to their activation and aggregation, while Stx binds to activated platelets and monocytes [20].

Infectious enterocolitis preceding typical HUS may also be caused by S. dysenteriae type 1 (SD1), since the Stx of E. coli is identical to the Stx of SD1. S. dysenteriae type 1 produces a toxin almost identical to Stx1. However, unlike STEC, the SD1 bacterium itself possesses enteroinvasive properties, which can lead to bacteremia and septic shock [7, 21]. HUS develops in 13% of shigellosis cases caused by SD1, occurs mainly in African and Asian countries in children under 5 years of age, is characterized by a severe clinical course, and results in a fatal outcome in 36% of cases [8, 21]. In addition, Stx phages can sometimes be detected in other gram-negative bacteria, so isolated reports of other etiological factors of HUS (Citrobacter, Salmonella, Clostridium difficile) can be found in the literature [7].

The E. coli O104:H4 serotype producing Stx, which caused the outbreak of typical HUS in Europe in 2011, in addition to Stx production, E. coli O104:H4 exhibits enteroaggregative properties that may lead to more prolonged intestinal colonization and toxin release into the bloodstream. The particular virulence of this strain is explained by the combination of a chromosomally integrated bacteriophage encoding Stx and an E. coli plasmid that ensure tight adhesion of enteroaggregative fimbriae to epithelial cells [22]. The exceptional virulence of the O104:H4 strain is confirmed by unfavorable outcomes of the outbreak compared to all previously described outbreaks of E. coli O157:H7 infection and other enterohemorrhagic strains [11, 12, 23].

1.2.2 Atypical HUS

Atypical HUS is an ultra-rare, chronic, genetic systemic disease with a progressive course and unfavorable prognosis, driven by chronic uncontrolled activation of the alternative complement pathway, leading to complement-mediated TMA [1, 5, 8, 18].

The complement system is a complex network of over 40 proteins that forms a major part of the innate immune system and helps regulate the adaptive immune response. Complement is activated via three distinct pathways (Appendix A3.3.). The common convergence point for all three pathways is the cleavage of the C3 complement component. The abnormalities underlying the development of aHUS involve the alternative activation pathway.

Unlike the first two pathways, whose activation begins after binding to immune complexes or microorganisms, the alternative pathway is in a state of constant activation with a low baseline level (the so-called " tick-over" mechanism), which is maintained by spontaneous hydrolysis of the C3 complement component. The resulting C3b fragment can bind to both pathogens and the host's own cells. On a foreign (e.g., bacterial) surface, C3b binds to factor B (CFB), resulting in the formation of the C3 convertase (C3bBb complex). The latter repeatedly amplifies C3 cleavage through the formation of the so-called "amplification loop." When additional C3b fragments attach to the C3 convertase, the C5 convertase (C3bBb(C3b)) is formed—an enzymatic complex that cleaves the C5 complement component. Cleavage of C5 yields the C5b fragment, which triggers the assembly of the C5b-9 membrane attack complex (MAC), causing lysis of bacterial cells.

The surface of host cells is normally protected against local amplification and C3b deposition. This protection is provided by strict control from a number of complement regulatory factors, represented by both plasma and membrane-bound proteins anchored on the surface of endothelial cells. The main plasma proteins regulating the alternative complement activation pathway are factor H (CFH) and factor I (CFI), while membrane-bound regulators include membrane cofactor protein (MCP/CD46) and thrombomodulin (THBD).

CFH is the primary regulatory factor of the alternative complement activation pathway. It blocks the formation of the C3 convertase and directly accelerates its decay. In addition, factor H acts as a cofactor for CFI in the inactivation of C3b, leading to the formation of the inactive fragment iC3b, which is unable to bind to factor B to form the C3 convertase. The CFH molecule has two C3b-binding domains. The first is localized in the N-terminal region, where C3b binding regulates alternative pathway amplification in plasma. The second binding domain is located in the C-terminal region of the molecule, within exons 19 and 20; binding to these disrupts the ability of C3b to anchor on the endothelial surface, leading to local inactivation of the alternative pathway. Thus, CFH plays a pivotal role in protecting endothelial cells from complement activation. Furthermore, the important role of CFH in regulating platelet function by blocking complement activation on their surface has been established, which in turn leads to a decrease in functional activity and helps reduce the risk of thrombosis.

CFI is a serine protease that cleaves C3b, leading to the formation of inactive iC3b in the presence of soluble and/or membrane-bound cofactors.

MCP/CD46 is an integral transmembrane protein expressed on the cell surface, where it binds C3b and acts as an additional cofactor for CFI.

THBD is an endothelial glycoprotein with anticoagulant, anti-inflammatory, and cytoprotective properties. It also serves as a complement regulatory protein, functioning as a membrane-bound cofactor for CFI. It binds C3b, accelerating its inactivation by CFI in the presence of CFH.

Thus, the alternative complement pathway is regulated by four proteins—CFH, CFI, MCP/CD46, and THBD—whose interaction converts C3b into the inactive iC3b molecule, thereby blocking the key activation mechanism: the generation of increasing amounts of C3 convertase followed by uncontrolled MAC production.

It is hypothesized that mutations in genes encoding regulatory proteins in patients with aHUS lead to impaired protection of endothelial cells against complement activation due to a deficiency or, more frequently, functional impairment of these proteins. As a result, MAC formation is enhanced on the surface of endothelial cells, causing their damage with exposure of the subendothelial matrix, transformation of the antithrombotic phenotype into a prothrombotic one, and subsequent thrombus formation. An additional contribution to thrombosis in patients with CFH mutations may also come from complement activation on the platelet surface, leading to enhanced functional activity [1, 5, 8, 24, 25, 27].

Along with the most common mutations in regulatory proteins that impair their function in controlling alternative complement pathway activity (loss-of-function), mutations in factor B (CFB) and complement component C3 have also been described, causing a significant increase in activity (gain-of-function) due to stabilization of the C3 convertase and its resistance to inactivation, respectively, which results in excessive complement activation. The predominant renal involvement in aHUS is likely due to the particular susceptibility of the fenestrated glomerular endothelium to damage caused by dysregulated complement control [1, 5, 18, 24, 25].

Molecular diagnosis of aHUS is complex and currently involves at least 9 different genes (CFH, MCP/CD46, CFI, C3, CFB, THBD, CFHR1-5, and DGKE), high-risk haplotype screening (CFH-CFHR3 and MCPggaac), as well as copy number variation analysis, hybrid genes, and other complex genomic rearrangements in the CFH/CFHRs genomic region [24, 28].

aHUS is associated with CFH mutations in 20–30% of patients, MCP/CD46 mutations in ≈8–10%, and CFI mutations in ≈6%. Factor B (CFB) mutations occur in 1–4% of cases, whereas mutations in the C3 complement fraction are identified in 6–10% of patients. Thrombomodulin gene (THBD) mutations are rare (3–5%). Approximately 12% of patients with aHUS harbor mutations in two or more complement genes. The CFH gene exists within a gene cluster (regulators of complement activation cluster) alongside five highly homologous genes encoding factor H-related proteins (CFHR1–5) [27]. It is now well established that mutations, deletions, and genomic rearrangements are common in this region. The frequency of genes encoding CFHR1–5 proteins is 3–10%. In more than 10% of aHUS patients, predominantly children, autoantibodies against CFH—the main regulatory protein of the alternative complement pathway—are detected, leading to the same consequences as its mutations. "Antibody-mediated" aHUS tends to frequently recur. Currently, 30–48% of aHUS cases cannot be adequately explained by molecular genetics [29, 30, 31, 32]. However, this group of patients exhibits a disease severity and relapsing clinical course comparable to that of genetically confirmed aHUS.

Mutations in the CFH, CFI, THBD, and C3 genes predominate in patients under 1 year of age; in patients aged 1 year and older, MCP/CD46 mutations are identified in addition to the aforementioned mutations. In children with disease onset between 7 and 11 years of age, antibodies against CFH are more commonly detected [31].

Familial or sporadic aHUS is familial (siblings, parents, grandparents affected) in ≈25% of patients. Sporadic cases of aHUS have a better prognosis than familial forms. A negative family history does not rule out the possibility of genetic transmission of the disease. Only half of mutation carriers within a family develop clinical manifestations during their lifetime [29, 33].

In some cases, molecular genetic testing reveals mutations in non-complement genes (DGKE — diacylglycerol kinase ε, INF2 — inverted formin-2, PLG — plasminogen) and mutations in genes leading to defective cobalamin C metabolism (MUT, MMAA, MMAB, MMACHC, MMADHC, MCEE) [28].

It is now established that complement genetic abnormalities, previously considered the primary cause of aHUS, are merely a predisposing factor for TMA. The realization of this predisposition requires additional factors that can influence the development or progression of aHUS. Such factors, which trigger further complement activation in predisposed individuals and are therefore termed complement-activating conditions, include respiratory tract infections (18% of cases), gastrointestinal infections (24%), pregnancy (7%), organ transplantation (5%), etc. [2, 17, 31, 32]. However, in nearly 1/3 of aHUS patients, trigger factors cannot be identified [34].

1.3 Epidemiology of the Disease or Condition

1.3.1 Typical HUS

STEC accounts for 2.8 million acute illnesses and 3,890 cases of HUS annually [7, 23, 28]. STEC-HUS accounts for up to 90% of HUS cases in children and is the most common cause of acute kidney injury (AKI) in pediatric practice. STEC-HUS is most common in children aged 6 months to 5 years, although this form of TMA can occur at any age. The global incidence is 0.2–8 per 100,000 population (approximately 6.1 per 100,000 among children under 6 years of age). Among adults, the incidence is 0.5 per 100,000/year [7, 9].

The incidence of typical HUS has remained relatively stable since its discovery in the 1980s, with only a slight decline after 2000 [10]. The incidence of STEC-HUS varies significantly across regions (depending on the rural population proportion, water supply characteristics, etc.), reaching 10–17 cases per 100,000/year in children under 5 years old in endemic areas (Argentina and Uruguay). In Europe and North America, the annual incidence is 0.6–1.1 cases per 100,000 children <15–18 years old and 1.9–2.9 cases per 100,000 children <3–5 years old [7, 10, 206, 207, 208].

Approximately 2,000 cases of STEC-HUS caused by the E. coli O157:H7 serotype are registered worldwide each year [207, 209]. Non-O157 organisms represent a heterogeneous group with a risk of developing HUS of up to 1% [209, 210]. In India, Africa, and parts of Asia, HUS develops in 13% of acute intestinal infection cases caused by S. dysenteriae type 1, is characterized by a severe clinical course, and results in mortality in 36% of cases [211].

STEC-HUS is characterized by peaks in incidence between June and September, alongside sporadic cases. In Russia, outbreaks of STEC-HUS caused by the E. coli O157:H7 serotype have been reported in the Moscow and Volga regions, North Ossetia-Alania, Krasnodar Krai, and the Omsk, Ivanovo, Orenburg, and Rostov oblasts.

1.3.2 Atypical HUS

Atypical HUS accounts for 5–10% of all HUS cases in children and is primarily the result of complement dysregulation. Currently, over 1,000 patients with aHUS are known to have identified mutations in complement protein genes [36].

According to various studies, the incidence of aHUS in Europe is 0.23–1.9 cases per 1,000,000/year, and in Australia, 0.44 cases per 1,000,000/year. Among individuals <20 years of age, it is 0.26–0.75 per 1,000,000/year, and in adults, 0.42–1.9 cases per 1,000,000/year [1, 9, 35, 36].

The prevalence of aHUS ranges from 2 to 10 per 1,000,000 individuals. In individuals aged <20 years: from 2.21 to 9.4 per 1,000,000 individuals. Among patients under 4 years of age, the prevalence was 3 per 1,000,000 children, and in children aged 5 to 15 years, 0.3 per 1,000,000 children. Among adults in Europe, it was 5.75 per 1,000,000 population, and in Australia/New Zealand, 2.4 per 1,000,000 population. The largest reference population of aHUS patients is registered in Europe at 16,427,365, with a disease prevalence of 4.96 per 1,000,000 of the total population [1, 9, 36, 37].

There are no exact statistical data in the Russian Federation. The incidence is about 0.8 per 1,000,000 children per year. Approximately 27.5 new cases of aHUS are diagnosed annually. The prevalence of the disease is 1 case per 143,000 children.

Among children, boys and girls are affected equally. In 60% of cases, aHUS onset develops in childhood. Furthermore, in children under 6 months and from 6 months to 2 years, the rates are equal at 28% and 28%, respectively. At the age of 2–15 years, aHUS is diagnosed in 44% of children. Infection is the most frequent complement-activating condition in the realization of the disease. Vaccination ranks 2nd among the causes provoking the development of aHUS (only in children with a genetic defect) [35, 36, 37, 259, 260, 261, 262, 263].

Among affected adults, females predominate, as pregnancy is a frequent triggering event. Among patients aged 20–40 years, aHUS is diagnosed in 48%, in those aged 50–60 years in ≈ 8% of patients, and at >60 years in ≈ 1% [1, 27, 30].

1.4 Features of Coding the Disease or Condition

D59.3 Haemolytic-uraemic syndrome

N08.2* Glomerular disorders in diseases classified elsewhere

Note: code N08.2 is used only in relation to haemolytic-uraemic syndrome

1.5 Classification of the disease or condition

To date, the classification of HUS remains a subject of discussion. Various proposed classifications are either not focused on the main pathophysiology of the disease or are too complex.

It should be noted that a number of authors recommend naming all forms of TMA, except TTP, as HUS, while distinguishing: STEC-HUS, pneumococcal-associated HUS, infection-induced HUS (influenza A, human immunodeficiency virus, cytomegalovirus, Epstein-Barr virus, parvovirus B19, varicella-zoster virus, Coxsackie virus, hepatitis viruses A, B, and C, Salmonella typhi, Bartonella, leptospirosis, rickettsiosis, malaria, Dengue fever), atypical HUS, secondary HUS, and HUS due to defective cobalamin C metabolism [38]. Other researchers propose distinguishing hereditary and acquired TMA, which may overlap. Thus, triggers may be required for the development of hereditary TMA, and acquired TMA may have a genetic basis. It is also recommended to classify TMA depending on the response to therapy [39].

Taking into account that the pathomorphological basis of any form of HUS is TMA, the following classification determining the treatment strategy was proposed in 2023 [212].

1. Thrombotic thrombocytopenic purpura:

• congenital: ADAMTS13 mutation;

• immune: antibodies to ADAMTS13.

2. Complement-mediated TMA:

• mutations in complement-associated genes: C3, CFB, CFH, CFI, MCP/CD46, CFHR1-5;

• antibody-mediated complement dysregulation: anti-CFH, anti-CFI, anti-CFB;

• mutations in non-complement genes: DGKE, PLG, THBD, VTN;

• complement role under investigation: INF2, WT1, COL4A5, POLR3B, EXOSC3.

3. Metabolic form of TMA due to defective cobalamin C metabolism

4. Infection-mediated TMA:

• Bacterial: Shiga toxin-producing Escherichia coli, Streptococcus pneumoniae, Campylobacter jejuni, Klebsiella pneumoniae.

• Viral: influenza, HIV, EBV, CMV, BK virus, parvovirus B19, SARS-CoV-2.

• Fungal: histoplasmosis.

5. Secondary TMA:

• systemic diseases: APS, SLE, scleroderma, vasculitis, rheumatoid arthritis, dermatomyositis, Sjögren's syndrome;

• malignant hypertension;

• associated with solid organ or haematopoietic stem cell transplantation;

• others: haemophagocytic lymphohistiocytosis, sickle cell anaemia, Castleman disease and its variants;

• drug-induced TMA (cyclosporine**, tacrolimus**, sirolimus, everolimus**, gemcitabine**, mitomycin**, bevacizumab**, sunitinib**, bortezomib**, carfilzomib**, adalimumab**, clopidogrel**, simvastatin**, estrogens, progestogens, combined progestogens and estrogens, etc.).

Traditionally, aHUS is also subdivided into familial and sporadic. In the structure of aHUS, familial cases (diagnosed in at least 2 family members) account for only 10–20% according to various sources, whereas sporadic aHUS, with no family history, occurs in 80–90% of patients with this pathology. Mostly, aHUS is inherited in an autosomal dominant manner with 50% penetrance, and significantly less often in an autosomal recessive / polygenic manner [28]. It should be remembered that the absence of the disease in relatives does not exclude its hereditary nature, as healthy family members of an aHUS patient may turn out to be carriers of pathogenic mutations.

1.6 Clinical presentation of the disease or condition

Typical HUS develops mainly in children aged 6 months to 5 years. In STEC-HUS, the history indicates a current or preceding episode of an infectious process in the form of fever, liquid stools mixed with blood, followed by the development of weakness, decreased appetite/refusal to eat, repeated vomiting, sharp pallor, sometimes with elements of cutaneous haemorrhages, decreased urine output/anuria, symptoms of intoxication associated with AKI, oedema (peripheral, cavitary), and sometimes macroscopic haematuria. aHUS can manifest at any age, starting from the neonatal period. In 31% of cases, aHUS starts suddenly. Onset with non-specific symptoms such as weakness, fatigue, general malaise, or flu-like syndrome is possible. In adult patients, an insidious onset with slow progression is noted in 20% of cases. However, in children, aHUS onset is more often associated with a complement-activating condition: diarrhoeal prodrome/gastroenteritis (23–28%), acute respiratory infection (18%), vaccination, trauma, surgical intervention, etc. The onset of TMA without a diarrhoeal prodrome or pneumococcal infection is highly suspicious for the development of aHUS. Also in aHUS, a relatively gradual development of TMA against the background of gastroenterocolitis (haemocolitis is possible) with a decrease in renal function over several days, a positive family history, and recurrent episodes of neurological disorders (focal symptoms, confusion, transient ischaemic attacks) are possible. In 30–50% of cases with AKI development in aHUS, a decrease in diuresis is not observed, and macroscopic haematuria may sometimes be present. In 17% of cases, the pathological process in the kidneys is characterised by proteinuria and haematuria without the development of AKI signs. Most patients with aHUS associated with anti-CFH antibodies have signs of gastrointestinal tract (GIT) involvement in the prodrome of the disease. Consequently, the presence of GIT symptoms cannot exclude aHUS.

1.6.1 Typical HUS

Risk factors for the development of HUS in STEC infection include age from 6 months to 5 years, female sex, dehydration, fever, vomiting, leukocytosis, haemocolitis, the use of anti-acid medications (proton pump inhibitors, H2-receptor antagonists), and gastrointestinal motility-lowering drugs (with antiperistaltic effect) during the prodromal period. The use of antibiotics in STEC-HUS is currently controversial. A number of researchers believe that the use of antibiotics can lead to increased release of Stx from dead bacteria or to changes in the intestinal flora that promote further attachment of STEC to the intestinal wall, induction of phage production, and expression of Stx genes, which can lead to disease progression and clinical deterioration [213, 214]. The results showed that antibiotic treatment had no statistically significant effect on symptom progression, faecal pathogen shedding, or the incidence of HUS [215, 216].

Adequate parenteral rehydration does not prevent the development of STEC-HUS, but contributes to a more favourable course of the disease [2, 7, 8, 10, 13, 65].

The course of STEC-HUS distinguishes a prodromal phase and a period of full-blown clinical manifestations. The prodrome is characterised by diarrhoea in 90–95% of patients, vomiting in 30–60%, and abdominal syndrome. After 1–2 days, haemocolitis develops in 70% of cases. It has been established that patients with haemocolitis in the prodrome of STEC-HUS have a longer duration of anuria, more frequent development of neurological symptoms, and a higher mortality rate [5, 23, 27].

STEC-HUS manifests 2–14 days (on average on the 6th day) from the onset of diarrhoea. Deterioration of the child's general condition, increasing lethargy, the appearance of pallor and icterus of the skin, and pastosity of the eyelids and legs are characteristic. A haemorrhagic syndrome may occur, manifested by petechial rash, ecchymoses, nasal and gastrointestinal bleeding [6, 8, 10]. In 5–10% of patients with STEC-HUS, the onset of the disease is not associated with a history of preceding diarrhoea, which emphasises the importance of microbiological testing in all patients with TMA regardless of medical history [27].

STEC-HUS is characterized by the rapid development of oliguria or anuria; changes in urine color due to hematuria and/or hemoglobinuria are possible. Oligoanuric AKI requiring RRT develops in approximately 50–70% of patients. Typically, against the background of enteral losses, anuria is diagnosed late. In the absence of adequate fluid balance control, hyperhydration occurs: peripheral edema, signs of polyserositis, pulmonary edema, and volume-dependent arterial hypertension (AH). Progressive worsening of respiratory failure is possible, up to the need for respiratory support [8, 217].

Cardiovascular system involvement occurs in 5–35% of STEC-HUS cases [217, 218]. In the acute period of STEC-HUS, AH associated with hyperhydration occurs in 72% of cases, characterized by a persistent course and poor response to therapy. With the restoration of diuresis, a second rise in blood pressure is observed, associated with increased renin production. Volume overload, electrolyte disturbances, toxic myocarditis in AKI, and cardiac TMA cause heart failure in some patients during the acute period of STEC-HUS. Clinically, hemodynamic disturbances manifest as tachycardia, muffled heart sounds, enlarged heart borders, and in some cases, the development of heart failure [40]. Acute myocardial infarction, although less common, is another potentially life-threatening complication of STEC-HUS [41, 217, 218].

CNS involvement develops in 11–34% of cases and manifests as psychomotor agitation or depression, focal or generalized seizures, impaired consciousness of varying degrees of severity (delirium, sopor, coma; cortical blindness, decerebration involving the brainstem, and focal symptoms such as paresis and paralysis are possible) [42, 217, 218]. Consumption coagulopathy and AH can lead to hemorrhagic stroke [8, 10, 43].

The kidneys and brain are the organs most vulnerable to STEC-HUS, but involvement of other organs has been described and must be considered when evaluating patients with this pathology. Gastrointestinal tract manifestations typically include enterocolitis of varying severity (ranging from slightly frequent, loose stools to profuse bloody diarrhea with severe abdominal pain). In some cases, ulcerative enterocolitis with necrosis and perforation of the intestinal wall develops, necessitating surgical intervention. Hepatosplenomegaly may be detected. Acute pancreatitis and cytolytic syndrome (elevated liver transaminases) occur in 20% of patients with STEC-HUS, but usually do not lead to organ failure. Nevertheless, about 3% of patients suffer from hyperglycemia in the acute phase [44], and survivors of STEC-HUS have a significantly higher incidence of diabetes, possibly as a consequence of thrombosis of vessels supplying the islets of Langerhans, as confirmed by a series of autopsies. Diabetes mellitus may be transient, but partial loss of the number of islets of Langerhans can lead to the recurrence of diabetes after a distant period [8, 45, 217, 218]. Rare extrarenal manifestations include visual impairment in the form of Purtscher-like retinopathy and skeletal muscle involvement leading to rhabdomyolysis [217, 218].

1.6.2 Atypical HUS

The development of aHUS requires the interaction of genetic abnormalities in the complement system with environmental factors that act as triggers, provoking additional complement activation in predisposed individuals.

The clinical presentation of aHUS is characterized by significant symptom polymorphism. However, the main manifestations of the disease are thrombocytopenia, MAHA, and AKI, which constitute the classical triad of TMA. An insidious onset of aHUS with slow progression cannot be excluded (in 20% of cases). In this case, mild anemia, transient thrombocytopenia, and preserved renal function are noted for several weeks or even months. The development of "incomplete" TMA is also possible, with the absence of thrombocytopenia (13–40%) or, less commonly, MAHA (6%) [38]. In some cases, the development of only renal signs of TMA in the absence of hematological abnormalities has been described.

In most cases, renal injury manifests as AKI with or without oligo/anuria. When diuresis is preserved, proteinuria is observed, sometimes massive, up to the development of nephrotic syndrome, especially with the gradual development of the disease. Hematuria may appear. It should be noted that in aHUS, AKI may not be the first manifestation of the disease. Approximately 17% of patients demonstrate only a moderate urinary syndrome without impaired renal function. The disease may also debut with isolated proteinuria and signs of early CKD. Half of children and the majority of adult patients with aHUS require RRT at the time of hospitalization. CKD stage G5 develops in many patients regardless of the nature of the disease manifestation [33, 34, 35, 37, 39].

AH develops in the majority of patients due to volume overload in the presence of oligo/anuria and/or hyperreninemia resulting from renal tissue ischemia caused by TMA.

The generalized nature of TMA in aHUS causes the development of extrarenal signs of the disease associated with damage to the microvasculature of various organs and systems, including the brain, heart, lungs, gastrointestinal tract, and visual organ. Extrarenal manifestations of the disease are observed in 20% of patients, of whom almost two-thirds have more than one extrarenal sign [1, 8, 33, 34, 35]. In some cases, it is difficult to distinguish primary organ damage due to TMA from secondary complications (hypertension / fluid overload / electrolyte disturbances).

The majority of patients have pronounced edematous syndrome, the main manifestations of which are massive peripheral edema up to anasarca and effusion in body cavities (hydrothorax, hydropericardium, ascites). The cause of edema is sharply increased vascular permeability induced by C3a and C5a complement components through the release of large amounts of histamine.

Almost half of the patients are diagnosed with CNS damage of varying severity (drowsiness, irritability, seizures, visual disturbances, hemiparesis or hemiplegia, stupor, coma). In some cases, the development of cerebral edema due to increased vascular permeability is possible [1, 8, 24, 247].

In 40% of patients, myocardial TMA develops, the main manifestation of which may be dilated cardiomyopathy with signs of gradually increasing or acute heart failure. A small number (about 3%) of patients with intramyocardial TMA demonstrate the development of acute myocardial infarction, which can cause sudden death [46, 47].

Pulmonary TMA can cause hemorrhagic alveolitis or adult acute respiratory distress syndrome. Respiratory failure increasing due to these causes in some cases requires mechanical ventilation. Quite often, patients with aHUS develop bilateral pulmonary infiltrates, which complicates diagnosis verification and requires differential diagnosis with vasculitis and infectious pathology [8, 35, 46, 47].

Gastrointestinal tract involvement occurs in approximately 30% of patients with aHUS. Bowel damage is most commonly observed, manifesting as diarrhea, nausea, and vomiting, although abdominal pain syndrome may develop. The development of acute pancreatitis with characteristic clinical signs up to pancreonecrosis is often noted. Acute development of diabetes mellitus has been described. Ischemic liver necrosis is less common [8, 35, 47].

Also, in a number of cases, visual organ damage is diagnosed in the form of Purtscher-like retinopathy, manifesting predominantly as bilateral visual impairment with optic disc edema, intraretinal hemorrhages, soft cotton-wool exudates, and macular edema. Currently, the most accepted theory is microembolization of the retinal vascular network, leading to precapillary occlusion of arterioles and microvascular infarction of the nerve fiber layer [49].

A rare manifestation of aHUS is skin lesion with the development of extensive necrotic foci [50]. Digital ischemic gangrene leading to amputation of fingers and toes also occurs [51].

Approximately 6% of patients experience multiple organ failure associated with diffuse TMA involving the central nervous system, myocardial ischemia, pulmonary hemorrhage and respiratory failure, pancreatitis, hepatic cytolytic syndrome, and gastrointestinal bleeding [8, 47]. Extrarenal manifestations of TMA occur both in the acute phase and after it (years later) as a result of chronic complement dysfunction and in combination with endothelial cell injury factors (drugs, hypertension, irradiation, etc.). At the same time, it must be remembered that severe extrarenal manifestations can occur in the absence of an obvious disease relapse and hematological abnormalities, since aHUS is a chronic disease whose course continues even if the patient's condition appears stable.

2. Diagnostics, medical indications and contraindications for diagnostic methods

Due to the fact that all TMAs, regardless of their pathogenesis, share similar clinical and laboratory manifestations and a common histological picture, differential diagnosis between the main forms of primary TMA – STEC-HUS, aHUS, and TTP – is extremely important.

In children, the differential diagnostic workup should include, in addition to STEC-HUS, aHUS, and TTP, methylmalonic acidemia (defective cobalamin metabolism) and neuraminidase-producing pneumococcus-associated HUS. Thus, diagnosing HUS initially requires establishing the presence of TMA, followed by differential diagnosis between primary and secondary TMAs.

Diagnostic criteria for typical hemolytic uremic syndrome (STEC-HUS):

If TMA symptoms develop against the background of gastrointestinal symptoms (diarrhea and/or nausea/vomiting and/or abdominal pain and/or gastroenteritis), STEC-HUS must be ruled out. It is diagnosed based on a characteristic clinical presentation with a diarrheal prodrome and must be confirmed by laboratory data that exclude other TMAs. TMA diagnosis is based on the presence of MAHA and thrombocytopenia combined with signs of renal damage and/or extrarenal manifestations [1, 4, 18, 25, 27, 28].

The presence of MAHA is established by detecting Coombs-negative anemia and schistocytes (fragmented erythrocytes in a peripheral blood smear with a diagnostically significant TMA content of >1%), elevated lactate dehydrogenase (LDH), and decreased haptoglobin. Hyperbilirubinemia (due to an elevated indirect fraction), free hemoglobin, and reticulocytosis are also non-specific indicators of erythrocyte hemolysis.

Thrombocytopenia is diagnosed when the platelet count is <150,000/mm³. If the platelet count exceeds this value, platelet consumption can be inferred from a decrease of >25% from the baseline level (if known). Giant platelets can be found in peripheral blood smears, and a normal number of megakaryocytes can be seen on bone marrow trephine biopsy.

In patients with MAHA and thrombocytopenia, the presence of AKI, either isolated or combined with symptoms of GI, CNS, pulmonary, cardiac, or other organ involvement, serves as the basis for diagnosing TMA.

The course of STEC-HUS with involvement of other organs and systems in addition to the kidneys largely determines the prognosis and outcome. Extrarenal manifestations of the disease may be associated with electrolyte imbalances, metabolic disorders, fluid overload, intoxication, systemic inflammatory response, and in some cases, the development of TMA.

Once TMA is established, the diagnosis of STEC-HUS can only be verified by ruling out aHUS and TTP. Screening for STEC-HUS is indicated in the presence of GI symptoms. It is known that 5–10% of patients may not have a history of preceding diarrhea, which emphasizes the importance of microbiological testing in all patients with TMA regardless of disease history. Laboratory tests should be performed during the first 24 hours of hospital admission prior to initiating antibacterial therapy. The diagnosis of STEC-HUS can be rejected by ruling out the presence of Stx in blood and stool. To exclude STEC-HUS, the following are indicated:

√ stool culture to detect STEC (on MacConkey agar for E. coli O157:H7);

√ identification and isolation of enterohemorrhagic E. coli strains from clinical material (stool/rectal swab) by PCR;

√ determination of Stx in blood serum;

√ determination of serum antibodies to the lipopolysaccharide of the most prevalent E. coli serotype in the region.

PCR analysis of primary fecal cultures is the most sensitive and specific test for screening toxin-producing organisms. A negative screening result when STEC-HUS is suspected is reliable if the sample is collected before the initiation of antibacterial therapy. When antibacterial therapy is administered, the detection rate of EHEC DNA in stool by PCR is significantly higher only during the first 6 days of diarrhea. Therefore, detecting Shiga toxin in stool/rectal swabs by PCR at later stages appears impractical.

A promising approach for routine practice is the detection and identification of EHEC virulence genes (rfb_O157 – O157 lipopolysaccharide, eae – intimin adhesion factor, Stx1,2 – Shiga toxin type 1,2) using multiplex PCR.

The presence of serum IgM antibodies against specific E. coli lipopolysaccharide serves as important evidence of a recently acquired STEC infection (ELISA, passive hemagglutination assay) [38].

STEC-HUS always develops acutely. A fluctuating, relapsing course is not characteristic of this form of TMA. Following the initial drop in hemoglobin and platelets, their levels gradually rise, although signs of renal injury may persist for a long time.

Criteria for suspecting/establishing the diagnosis of STEC-HUS:

1. Absence of a family history.

2. Age at HUS onset: 6 months–5 years.

3. Development of HUS against the background of an intestinal infection (signs of GI involvement, diarrhea, hemocolitides).

4. Development of HUS during an STEC infection outbreak.

5. MAHA: decreased hemoglobin, schistocytosis >1%, reticulocytosis, negative Coombs test, elevated LDH, decreased haptoglobin concentration.

6. Platelet consumption: platelets <150,000/mm² or platelet count reduced by >25% without falling below the normal range.

7. Organ dysfunction: involvement of the kidneys, cardiovascular system, lungs, GI tract, visual system, etc.

8. Positive stool culture result or presence of Escherichia coli/Shigella dysenteriae type 1 in fecal culture by PCR.

9. ADAMTS13 activity >10%.

10. No changes in acylcarnitine levels according to tandem mass spectrometry.

11. Relapse-free course!!!

Diagnostic criteria for atypical hemolytic uremic syndrome:

To date, there are no specific diagnostic markers for aHUS. The diagnosis of "aHUS" is a diagnosis of exclusion. It is established based on a combination of data: medical history, presence/absence of a triggering event, the characteristic TMA triad, and must be confirmed by laboratory data excluding other forms of TMA. The course of aHUS is often associated with a combination of AKI and extrarenal manifestations. Clinical suspicion for aHUS is necessary in the presence of a gastroenterocolitis symptom complex in children aged <6 months, a history of previously unexplained anemia, an insidious or relatively gradual onset with declining renal function over several days, a fluctuating course with recurrent drops in hemoglobin and platelet levels, suspected prior HUS, or familial cases of HUS occurring at different times.

In the event of TMA development, the diagnosis of aHUS can only be verified by excluding other forms of primary and secondary TMAs: STEC-HUS, TTP, pneumococcus-associated HUS, and HUS caused by defective cobalamin metabolism, including in the setting of methylmalonic acidemia.

The diagnosis of STEC-HUS can be rejected by ruling out the presence of Stx in blood and stool, and diagnosing a viral etiology of diarrhea.

To exclude TTP, all patients with TMA require the determination of ADAMTS13 metalloprotease activity. ADAMTS13 activity testing should be performed prior to initiating plasma therapy. Normal ADAMTS13 activity is 80–110%. A decrease to <10% favors TTP, which subsequently requires testing for anti-ADAMTS13 antibodies in the blood to verify the genetic or autoimmune form of the disease for treatment selection. In patients with aHUS, ADAMTS13 activity may be reduced, but its value always exceeds 10%. Reduced ADAMTS13 activity, besides aHUS, can be observed in systemic diseases (catastrophic APS, SLE), sepsis, and DIC. If immediate testing of ADAMTS13 activity is impossible in a patient with TMA and extreme clinical severity posing an immediate life threat or high risk of severe renal and/or extrarenal complications, a rule should be used whereby serum creatinine values >150–200 µmol/L (1.7–2.3 mg/dL) combined with a platelet count >30,000/µL practically rule out the diagnosis of TTP. Since the severity of thrombocytopenia directly correlates with ADAMTS13 activity, recent data indicate that in stable moderate thrombocytopenia not lower than 70,000/µL, metalloprotease activity may not be determined [28, 38].

In children with clinical and laboratory signs of TMA, differential diagnosis should also be performed between aHUS, Streptococcus pneumoniae-associated HUS, and HUS associated with defective cobalamin metabolism in children, including methylmalonic aciduria.

Streptococcus pneumoniae-associated HUS (pnHUS) is most commonly observed in children under 2 years of age, accounting for approximately 5% of all HUS cases in pediatric practice. HUS develops in 0.4–0.6% of invasive pneumococcal infection cases. Despite vaccination against pneumococcal infection altering the "microbial landscape", pnHUS has not become any rarer. In 72% of cases, pnHUS develops against the background of pneumonia and/or pleural empyema, in 23%—meningitis, and in 5%—pneumococcal sepsis [53]. Previously, it was believed that the Thomsen–Friedenreich antigen (TF antigen), located on the surface of erythrocytes, platelets, endothelial cells, and glomerular cells and masked by sialic acids, played the primary role in the pathogenesis of pnHUS. It was hypothesized that pneumococcal neuraminidase A cleaves cell-surface N-acetylneuraminic acid, exposing the TF antigen, which is then attacked by circulating IgMs, causing hemolysis, endothelial cell damage, and platelet aggregation [8]. It has now been proven that pnHUS may be associated with complement consumption in the early stages of the disease via its alternative activation pathway. Upon desialylation, factor CFH cannot bind to C3 convertase on the cell surface, leading to active MAC formation [54]. Pneumococci efficiently bind human PLG via the bacterial surface proteins Tuf and PspC. Upon plasmin activation on the bacterial surface, the active protease damages endothelial cells, causes endothelial retraction and matrix exposure, disrupting local homeostasis and promoting a thrombogenic state.

Thus, the link between membrane desialylation and unregulated complement activation can occur without the interaction of the TF antigen and anti-TF antibodies [53, 54]. The absence of erythrocyte agglutination at 37°C also argues against the role of anti-TF antibodies. Furthermore, it has been established that TF antibody binding is significantly higher at temperatures below 4°C and negligible at 37°C, which once again confirms the invalidity of the previously presented pathogenetic model of the disease under in vivo conditions [54]. pnHUS develops on days 3–13 of the course of invasive pneumococcal infection. Diagnostic features of pnHUS include: the development of AKI, thrombocytopenia, and MAHA with a positive direct Coombs test (60–90%) in the absence of laboratory evidence of DIC [1, 8, 28, 38, 55]. Patients with pnHUS are characterized by: prolonged oliguria, the development of AKI in all patients with a requirement for RRT in 40–80% of cases, as well as the presence of extrarenal manifestations (purpura fulminans, pancreatitis, cholecystitis, thrombosis, cardiopathy, deafness) [1, 35].

Cobalamin C defect-associated HUS (cblC-HUS) is a monogenic autosomal recessive disorder. There are 37 known cases of cobalamin metabolism defect-induced HUS (cblC-HUS) with manifestation under 1 year of age in 17 observations. This form is a rare case of TMA in childhood, caused by a genetic orphan disease—methylmalonic acidemia. The disease debuts in 5% of cases in children ≤1 year old, and in 25%—older than 1 year. There are 2 forms of the disease: early-onset (<1 year) and late-onset (≥1 year) [38, 57]. The features of cblC-HUS presentation in early-onset children include: feeding difficulties (vomiting, regurgitation, food refusal), development of failure to thrive, central nervous system involvement (muscle hypotonia, lethargy, developmental delay, seizures, microcephaly/hydrocephaly), visual impairment (pigmentary retinopathy/nystagmus), and severe arterial hypertension. A fatal outcome is practically programmed in 100% of children without treatment. Late manifestation of cblC-HUS is characterized by the development of pulmonary hypertension (40%), central nervous system involvement (cognitive impairment, ataxia, seizures, myelopathy), and severe AH [1, 58].

Diagnostics of cblC-HUS is based on the detection of MAHA, macrocytosis, reticulocytes <3‰, thrombocytopenia with/without leukopenia and neutropenia, proteinuria with/without nephrotic syndrome, hematuria, AKI, progressive CKD, severe metabolic acidosis with an elevated anion gap, hyperhomocysteinemia, elevated blood/urine methylmalonic acid levels (gas chromatography), elevated blood methylmalonylcarnitine and propionylcarnitine levels (tandem mass spectrometry), and pathogenic mutations in the MUT, MMAA, MMAB, MMADHC, MCEE, and MMACHC genes [1, 59].

The exclusion of STEC-HUS, TTP, pnHUS, and cblC-HUS in a patient with unquestionable TMA allows for the diagnosis of aHUS. In addition, it must be kept in mind that in 20% of cases, incomplete forms of aHUS are diagnosed in the form of a symptom dyad instead of a triad: a combination of MAHA with AKI, or thrombocytopenia with AKI. Furthermore, extrarenal manifestations of aHUS may be noted without laboratory signs of TMA in patients with an already established diagnosis [60].

Criteria suggesting aHUS:

1. Age <6 months / >5 years.

2. In an infant <6 months, a negative urine test for methylmalonic acid, normal blood levels of methylmalonylcarnitine and propionylcarnitine according to tandem mass spectrometry.

3. Debut of TMA without a diarrheal prodrome or pneumococcal infection.

4. Relatively gradual onset against the background of gastroenterocolitis (hemocolitis possible) with a decline in renal function over several days.

5. Possible positive family history.

6. Symptoms of severe fatigue, developmental delay.

7. Episodes of neurological disorders (focal symptoms, altered mental status, etc.) with ADAMTS13 activity >10%.

8. Signs/symptoms of other organ involvement (myocardial infarction, cardiomyopathy, chest pain, dyspnea, respiratory failure, pancreatitis, hepatitis, colitis, etc.).

9. Previous episodes of TMA/HUS.

10. MAHA: decreased hemoglobin, schistocytosis >1%, reticulocytosis, negative Coombs test, elevated LDH, decreased haptoglobin concentration.

11. Platelet consumption: platelets <150,000/mm² or platelet count decreased by >25% from baseline, without dropping below the normal range.

12. Organ dysfunction: involvement of the kidneys, cardiovascular system, lungs, gastrointestinal tract, organ of vision, etc.

13. Development of nephrotic syndrome in the absence of AKI against the background of TMA.

14. AKI without a decrease in diuresis.

15. In case of a diarrheal prodrome, a negative Escherichia coli / Shigella dysenteriae type 1 stool culture by PCR.

16. ADAMTS13 activity >10%.

17. TMA with the development of AKI and extrarenal manifestations, provided that the latter are not related to volume overload.

18. Plasma resistance / plasma dependence of a patient with TMA.

19. Absence of signs of SLE, APS, drug-induced endotheliotoxicity.

Criteria confirming the diagnosis of aHUS:

1. burdened family history: cases of aHUS in the family.

2. Development of TMA following vaccination, ARI, trauma, or without a triggering event.

3. Development of TMA in patients with a diarrheal prodrome and a negative Escherichia coli / Shigella dysenteriae type 1 stool culture by PCR (the test is performed prior to antibiotic administration).

4. Repeated episodes of hemolysis and platelet consumption during the disease manifestation period.

5. Appearance of extrarenal symptoms during resolving TMA.

6. Relapses separated by periods of remission and progression to CKD stage G5.

7. Pathogenic/likely pathogenic mutations in genes encoding complement proteins described in aHUS.

8. Positive clinical and laboratory effect of complement-blocking therapy (eculizumab**).

The absence of pathogenic mutations in genes encoding complement proteins does not preclude the diagnosis of aHUS due to the possibility of variants in genes other than classical aHUS genes and variants that may be missed by high-throughput sequencing (e.g., large genomic rearrangements or intronic variants not covered by traditional panel testing).

2.1 Complaints and Anamnesis

When examining patients with suspected typical/atypical HUS, attention should be paid to the following complaints:

− weakness, lethargy, fatigability, lack of appetite, dizziness;

− nausea, vomiting, abdominal pain, frequent loose stools;

− peripheral edema, decreased urine output, changes in urine color;

− dyspnea;

− hemorrhagic syndrome: hemorrhagic skin rash, bleeding;

− headaches, psychomotor agitation, altered mental status, seizures;

− visual impairment of varying severity.

When taking the medical history in patients with suspected typical/atypical HUS, pay attention to the following information:

− cases of TMA in the family;

− previous episodes of TMA;

− development of TMA following acute intestinal infection (E. coli, S. dysenteriae type 1), pneumococcal infection, vaccination, acute respiratory infection, ARVI with intestinal syndrome, trauma, or without a triggering event [38, 66].

2.2 Physical Examination

(Level of Evidence C, Grade of Recommendation 5)

Comment: the main goal of the examination is to detect signs of kidney damage and extrarenal manifestations.

Performed as part of initial/follow-up consultations and physical examinations by a nephrologist.

An initial examination prior to referral to a nephrologist at a specialized clinic may be performed by a pediatrician (initial/follow-up pediatric consultations and physical examinations).

2.3 Laboratory Diagnostics

2.3.1 Evaluation of patients with signs of TMA:

(CGR C; LOR 5)

Comment: The frequency of follow-up tests is determined by the clinical situation. Decompensated metabolic acidosis is an indication to initiate RRT, regardless of the azotemia level. Detecting severe metabolic acidosis with a high anion gap may suggest cblC-HUS.

(CGR C; LOR 5)

Comment: The frequency of follow-up tests is determined by the clinical situation. Patients with HUS exhibit decreased hemoglobin, reticulocytosis, and schistocytes ≥1%. STEC-HUS is characterized by leukocytosis with a left shift and an elevated ESR.

The International Council for Standardization in Hematology (ICSH) has developed guidelines for identifying schistocytes. Fragmented erythrocytes on a blood smear appear as crescent-shaped, tri-cornered (Napoleon's hat), helmet-shaped, triangular, with two to three sharp protrusions, or as small irregular fragments with a fracture line. It was also proposed to consider micro-spherocytes as schistocytes when other forms of schistocytes are present on the blood smear [52].

Platelet optical control (Fonio method) is mandatory, because automated hematological analyzers may identify disintegrated erythrocytes as platelets, thereby overestimating the platelet count in the final analysis result. If the degree of thrombocytopenia does not correspond to the severity of TMA, drug-induced thrombocytopenia should be ruled out (valproic acid** [and its formulations], cefoperazone + sulbactam**, hydrochlorothiazide**, etc.).

(CGR C; LOR 4)

Comment: STEC-HUS, aHUS, TTP, cblC-HUS, and secondary forms of TMA are typically characterized by Coombs-negative anemia, whereas in pnHUS, the Coombs test is positive in 60-90% of patients. The indirect Coombs test assesses the presence of anti-erythrocyte antibodies circulating freely in blood plasma. To rule out an autoimmune nature of hemolytic anemia, a direct Coombs test is necessary to detect anti-erythrocyte antibodies/complement components fixed on the surface of blood erythrocytes. If the direct Coombs test is positive, autoimmune hemolytic anemia and Evans-Fisher syndrome must be excluded. It should also be borne in mind that possible causes of a positive Coombs test may include blood transfusion without prior cross-matching (post-transfusion hemolytic reaction), systemic lupus erythematosus, lymphoma, infectious mononucleosis, mycoplasma infection, or drug-induced hemolytic reactions (cephalosporins, purine analogs, methyldopa**).

(CGR C; LOR 5)

Comment: Patients may require, for instance, correction of anemia with packed red blood cells, surgical intervention, etc., sometimes on an emergency basis.

(CGR C; LOR 5)

Comment: Performed in all patients for diagnostic purposes and dynamic monitoring, including treatment, during the acute period of the disease and during aHUS exacerbation. The frequency of follow-up tests is determined by the clinical situation.

The development of AKI is accompanied by elevated urea and creatinine levels, and potential electrolyte disturbances (hyponatremia, hyperkalemia, hyperphosphatemia, hypocalcemia). An indication to initiate RRT is hyperkalemia >7.5 mmol/L, regardless of the severity of azotemia. Total protein and albumin levels may be moderately decreased. Uric acid may be elevated in patients with AKI due to reduced excretion. Hyperuricemia can also accompany neoplastic diseases of the hematopoietic and lymphatic systems, which in some patients with anemia and thrombocytopenia necessitates a bone marrow trephine biopsy. Hyperbilirubinemia as a marker of hemolysis is not always diagnosed in patients with TMA. Transaminases may be elevated.

Elevated serum iron and ferritin can also serve as markers of hemolysis. However, when hyperferritinemia (≥500 mcg/L) is combined with hepatosplenomegaly, cytopenia involving ≥2 of 3 hematopoietic lineages, hypertriglyceridemia, and/or hypofibrinogenemia, a bone marrow trephine biopsy is required to rule out hemophagocytic lymphohistiocytosis. Patients with TMA invariably exhibit increased LDH activity. Tracking LDH activity dynamics in the blood allows for a rough estimation of the rate of hemolysis. Normally, all fractions of the enzyme in blood serum show low activity as part of the total LDH value.

Elevated serum LDH is an indicator of virtually any damage to cells where the enzyme is localized (in the cells of the liver, heart, kidneys, skeletal muscles, and erythrocytes). Total LDH activity is determined, comprising the activities of all 5 existing isoforms (LDH-1, LDH-2, LDH-3, LDH-4, LDH-5). For the myocardium and brain tissue, LDH-1 is the primary isoform; for erythrocytes, platelets, and renal tissue, it is - LDH-1 and LDH-2. In the lungs, spleen, thyroid and pancreas, adrenal glands, and lymphocytes, LDH-3 predominates. LDH-4 is found in all tissues containing LDH-3, as well as in granulocytes, the placenta, and male germ cells, which also contain LDH-5. Accordingly, in diseases accompanied by tissue damage and cell destruction, LDH activity in the blood increases, which is observed during the development of TMA.

Elevated blood creatine phosphokinase (CPK) activity in TMA indicates damage or destruction of cells rich in this enzyme (myocardium, skeletal muscle, brain, thyroid gland, lungs). Determining the activity of the CPK-MB isoenzyme (the cardiac isoenzyme that changes with myocardial cell injury) is important in patients with TMA for diagnosing cardiac involvement and monitoring their condition.

All patients with signs of gastrointestinal dysfunction, regardless of the TMA form, may develop pancreatic involvement, which necessitates dynamic monitoring of pancreatic enzymes (lipase, amylase) and glycemic levels.

An elevated blood CRP concentration in TMA may indicate an ongoing bacterial infection and myocardial injury.

¹Glomerular filtration rate should be calculated using the formula (estimated by the Schwartz formula) – (the service is not included in the Nomenclature of Medical Services). Evaluation of nephron function via creatinine clearance (Rehberg's test) is not performed in children. For details, see the clinical guidelines «Chronic Kidney Disease» (pediatric age group).

(CGR C; LOR 4)

Comment: in 60–80% of patients with aHUS, serum C3 is within the reference range. In 87% of observations, the CFH concentration remains within normal limits. A normal C3 level does not exclude the diagnosis of aHUS, and a detected decrease in this complement component may serve as an additional argument in favor of this diagnosis [33, 34, 38, 61]. In STEC-HUS, a decrease in C4 levels may be observed at the onset of the disease, with normalization after 10 days, indicating the activation of the classical and/or lectin pathways involving C4 consumption.

Performed for diagnostic purposes in the acute period and, if necessary, for dynamic monitoring, including during treatment. The frequency of follow-up tests is determined by the clinical situation.

(CGR C; LOR 5)

Comment: haptoglobin binds free hemoglobin released from destroyed erythrocytes. A decreased haptoglobin level in TMA indicates its excessive consumption and is a sign of enhanced hemolysis, specifically intravascular hemolysis, as extravascular hemolysis is not accompanied by a decrease in haptoglobin. Haptoglobin decreases earlier in hemolysis than LDH rises, thus serving as an early marker of hemolysis. In nephrotic syndrome, patients without signs of hemolysis may also exhibit decreased haptoglobin due to urinary losses. In the setting of a severe bacterial process, the haptoglobin level may be normal during active TMA because this protein is an acute-phase reactant.

The test is performed dynamically, if necessary. The frequency is determined by the clinical situation.

(Level of Evidence C; Grade of Recommendation 4)

Comment: the frequency of follow-up tests is determined by the clinical situation.

(CGR C; LOR 5)

Comment: the frequency of tests and the choice of parameters for dynamic monitoring are determined by the clinical situation. Differential diagnosis with disseminated intravascular coagulation (DIC) syndrome is required. Decreased fibrin degradation products and antithrombin III with normal haptoglobin, prolonged prothrombin time or activated partial thromboplastin time, decreased fibrinogen level, elevated markers of fibrin formation and fibrinolysis activation (soluble fibrin monomer complexes, D-dimer), inflammation (CRP, procalcitonin, presepsin), and platelet and clotting factor consumption support DIC syndrome. However, differential diagnosis is not always possible, as HUS can also be complicated by DIC syndrome. Acute lung injury and/or shock are more frequently accompanied by DIC syndrome, whereas central nervous system dysfunction is more characteristic of TMA. Furthermore, microangiopathic hemolytic anemia (MAHA) is rare in DIC syndrome, whereas in TMA it occurs in almost 100% of cases. Hypertension frequently develops in any form of HUS, whereas sepsis with DIC syndrome presents with vasoplegia and hypotension. It should be noted that if thrombocytopenia or hemolysis persists after the resolution of sepsis, the diagnostic workup for their causes must be continued [93].

(CGR C; LOR 5)

Comment: in some cases, the frequency of follow-up tests is determined by the clinical situation.

(CGR C; LOR 4)

Comment: in most cases, acute myocardial infarction in patients with aHUS develops in the absence of obstructive coronary artery disease [41, 46]. Diagnosis of acute myocardial infarction requires confirmation of acute myocardial injury (characteristic troponin I kinetics with an increase and/or decrease, with results exceeding the upper limit of normal in at least one measurement) combined with evidence of myocardial ischemia (ECG, echocardiography). However, an elevated troponin I level is not specific solely to acute myocardial ischemia resulting in myocardial infarction. Troponin I may be elevated in patients with congestive heart failure, renal failure, and cardiomyopathy, which emphasizes the importance of serial troponin testing and evaluation of test results within the appropriate clinical context.

(CGR C; LOR 5)

Comment: metalloprotease activity is assessed prior to plasma therapy. ADAMTS13 activity values >10% exclude TTP [38, 60, 63]. In STEC-HUS/aHUS, metalloprotease activity may be reduced (yet above 10%) or fall within the reference range. The severity of thrombocytopenia correlates directly with ADAMTS13 activity; therefore, if stable thrombocytopenia is not lower than 70,000/µL, metalloprotease activity testing may be omitted [28, 38]. A blood platelet count >30×10⁹/L and a creatinine concentration >200 µmol/L in a patient with TMA practically exclude the diagnosis of TTP [61].

(CGR C; LOR 5)

Comment: all patients with suspected aHUS must be screened for anti-CFH antibodies in blood samples collected prior to plasma exchange or plasma infusion. The formation of anti-CFH antibodies is associated with CFHR3/CFHR1 deletion in 90% of cases, which is more characteristic of children than adults (25–50% vs 5–10%). Timely detection of anti-CFH antibodies is critical for choosing the treatment strategy, as their high titer requires the addition of immunosuppressants to complement-blocking therapy. Once the antibody-mediated form of aHUS is confirmed, subsequent monitoring of anti-CFH antibodies every 3–6 months during treatment is required until their levels normalize.

(CGR C; LOR 5)

(CGR C; LOR 5)

(CGR C; LOR 5)

Comment: molecular genetic testing (PCR) is required to confirm the diagnosis when pneumococci are isolated from sterile body fluids (blood, pleural effusion, cerebrospinal fluid). This test is necessary to confirm invasive pneumococcal infection as the cause of pnHUS.

The service "Molecular biological testing of pleural fluid for pneumococcus (Streptococcus pneumoniae) by PCR" is currently not included in the Nomenclature.

(CGR C; LOR 5)

Comment: the latex agglutination test is a rapid diagnostic method for detecting pneumococcal antigens in blood and CSF in suspected invasive forms of pneumococcal infection to rule out pnHUS.

(CGR C; LOR 5)

Comment: SLE and APS must be ruled out first. The combination of clinical and laboratory manifestations of TMA with the presence of antiphospholipid antibodies supports the diagnosis of catastrophic APS, regardless of whether the patient has clinical and immunological signs of SLE or not. The diagnosis of systemic pathology in patients with TMA is essential, as the identified marker profile determines the therapeutic strategy [60].

(CGR C; LOR 4)

Comment: in patients with genetic mutations leading to defective cobalamin C metabolism (MUT, MMAA, MMAB, MMADHC, MMADHC, MCEE), cblC-TMA can develop against the background of genetically determined cobalamin C deficiency. Detection of elevated homocysteine levels with normal vitamin B₁₂ and folic acid levels is a probable indicator of the cblC-HUS diagnosis [38, 57].

Hyperhomocysteinemia can also be diagnosed in thrombophilia caused by folate cycle gene mutations, vitamin B12 and folic acid deficiency.

Blood vitamin B12 and folic acid levels may be monitored dynamically due to the likelihood of micronutrient deficiency during the development of CKD or AKI, including in patients receiving hemodialysis, for timely correction [219, 220, 221].

(CGR C; LOR 5)

Comment: see clinical guidelines for methylmalonic acidemia/aciduria.

(CGR C; LOR 4)

Comment: tandem mass spectrometry is a substance testing method based on determining the mass-to-charge ratio of ions. It allows both qualitative and quantitative analysis of organic mixtures. Blood (capillary/venous) is collected on a standard filter paper card. The sample is dried for 2-3 hours at room temperature (heating and direct sunlight exposure are strictly prohibited). A diagnosis of methylmalonic acidemia should be suspected in the presence of megaloblastosis, elevated homocysteine and decreased plasma methionine concentrations, as well as increased urinary excretion of homocysteine and methylmalonic acid. Molecular genetic testing is required to confirm the diagnosis (mutations in MUT, MMAA, MMAB, MMADHC, MMADHC, MCEE genes) and provide medical genetic counseling [104].

It should be noted that aHUS in patients with combined remethylation disorders may also develop in children older than 6 months and even in adults, therefore these tests may be performed based on clinical necessity in other age groups.

The service "Determination of propionylcarnitine level in blood by tandem mass spectrometry" is currently not included in the Nomenclature.

(CGR C; LOR 5)

Comment: if CH50 > 30% in patients receiving eculizumab**, changing the dosing interval or increasing the drug dose is recommended due to incomplete complement blockade [64]. To date, there are no reliable biomarkers confirming disease activity for the diagnosis of aHUS, including relapse.

CH50 testing is not performed during ravulizumab therapy because this parameter is not informative for assessing correct drug dosing [222, 223].

(CGR C; LOR 4)

Comment: the following may be performed: Platelet aggregation test, Platelet aggregation test using aggregate hemagglutination assay, Determination of factor XI activity in blood serum, Determination of factor X activity in blood serum, Determination of factor IX activity in blood serum, Determination of factor VIII activity in blood serum, Determination of factor VII activity in blood serum, Determination of factor V activity in blood serum, Determination of antithrombin III activity in blood, Protein C level test in blood, Determination of protein S activity in blood, Determination of anticardiolipin antibodies in blood, Determination of antiphospholipid antibodies in blood, Plasminogen level test in blood, Plasmin (fibrinolytic) system test, Determination of 675 4G/5G polymorphism (guanine insertion at position 675) in the type 1 plasminogen activator inhibitor (PAI-1) gene, etc., depending on the clinical situation.

(CGR C; LOR 5)

Comment: the frequency of follow-up tests is determined by the clinical situation. Proteinuria and hematuria may be detected in the urinalysis in any form of TMA.

(CGR C; LOR 4)

Comment: the test is performed in a morning urine sample. The frequency of follow-up tests is determined by the clinical situation.

(CGR C; LOR 5)

Comment: a diarrhea prodrome can occur in aHUS, therefore a comprehensive set of tests recommended for STEC-HUS is indicated for all patients.

(CGR C; LOR 5)

Comment: a diarrhea prodrome can occur in aHUS, therefore a comprehensive set of tests recommended for STEC-HUS is indicated for all patients.

(CGR C; LOR 5)

Comment: diarrhea may also be a manifestation of a viral infection; therefore, all patients with suspected aHUS should undergo the panel of tests recommended for STEC-HUS.

(CGR C; LOR 5)

Comment: determination of influenza A virus RNA in nasopharyngeal swabs by PCR and/or determination of influenza B virus RNA in nasopharyngeal swabs by PCR and/or determination of influenza C virus RNA in nasopharyngeal swabs by PCR and/or determination of human adenovirus DNA in nasopharyngeal swabs by PCR, or determination of human herpesvirus 6 (HHV-6) DNA in oropharyngeal swabs by PCR, qualitative assay and/or determination of human herpesvirus 6 (HHV-6) DNA in oropharyngeal swabs by PCR, quantitative assay and/or molecular biological testing of a nasopharyngeal scrape for herpes simplex virus and/or determination of Epstein-Barr virus DNA in oropharyngeal swabs by PCR, qualitative assay and/or determination of Epstein-Barr virus DNA in oropharyngeal swabs by PCR, quantitative assay and/or determination of cytomegalovirus DNA in oropharyngeal swabs by PCR, qualitative assay and/or determination of cytomegalovirus DNA in oropharyngeal swabs by PCR, quantitative assay and/or testing for the novel coronavirus infection (COVID-19): determination of SARS-associated coronavirus (SARS-CoV-2) RNA in nasopharyngeal swabs by polymerase chain reaction / determination of SARS-CoV-2 coronavirus RNA in nasopharyngeal swabs by real-time polymerase chain reaction / determination of SARS-CoV-2 coronavirus RNA in nasopharyngeal swabs by reverse transcription loop-mediated isothermal amplification / determination of SARS-CoV-2 coronavirus RNA in oropharyngeal swabs by real-time polymerase chain reaction / determination of SARS-CoV-2 coronavirus RNA in oropharyngeal swabs by reverse transcription loop-mediated isothermal amplification / determination of SARS-CoV-2 coronavirus antigen in nasopharyngeal swabs by immunochromatographic assay) [224, 225].

The choice of study is determined by the epidemiological situation and the clinical capabilities.

2.4 Instrumental Diagnostics

(CGR C; LOR 5)

Comment: the frequency of follow-up imaging is determined by the clinical situation. Complaints of shortness of breath, pain in the sternum, precordial region, noisy breathing, cough, abnormal sputum characteristics (purulent, bloody sputum), and persistent high body temperature are indications for this study.

Chest CT may be performed for the same purposes if necessary (insufficient data from chest X-ray, questionable situations).

(CGR C; LOR 5)

Comment: the recommended imaging method serves as a screening tool in patients both with and without signs of gastrointestinal tract involvement. HUS is characterized by involvement of hollow organs (primarily the colon) and parenchymal organs (liver, pancreas, spleen). In hyperhydration, echographic signs of free fluid in the abdominal cavity (hydroperitoneum) are detected. The frequency of follow-up studies is determined by the clinical situation.

(CGR C; LOR 5)

Comment: the frequency of follow-up studies is determined by the clinical situation. Renal ultrasound makes it possible to determine kidney size, position, structural features, and other parameters. Doppler ultrasound evaluates the state of renal blood flow.

(CGR C; LOR 5)

Comment: the frequency of follow-up studies is determined by the clinical situation.

(CGR C; LOR 5)

Comment: signs of volume overload (hydropericardium), hypertension, and specific lesions caused by thrombosis of small and intramyocardial vessels in TMA can be detected in any form of HUS. Patients with aHUS carrying CFH, CFB, C3 mutations, or anti-CFH antibodies are at risk of developing cardiovascular complications [41, 46]. The frequency of follow-up studies is determined by the clinical situation.

(CGR C; LOR 5)

Comment: TMA can lead to unilateral or bilateral visual impairment with optic disc edema, intraretinal hemorrhages, soft cotton-wool exudates, and macular edema [49]. The frequency of follow-up examinations is determined by the clinical situation. It is also performed in case of aHUS recurrence.

(CGR C; LOR 5)

Comment: Magnetic resonance imaging is a more sensitive method for visualizing ischemic lesions in TMA. Brain MRI with angiography allows not only assessing the condition of blood vessels but also characterizing blood flow dynamics in patients with endothelial dysfunction associated with TMA. The frequency of follow-up examinations is determined by the clinical situation.

The use of iodine-containing contrast agents should be avoided in renal impairment.

General anesthesia is performed if necessary.

(CGR C; LOR 4)

(CGR C; LOR 5)

2.5 Other Procedures

(CGR C; LOR 5)

• If atypical HUS is suspected and in preparation for transplantation in any form of TMA leading to stage 5 CKD, it is recommended to perform molecular genetic testing using NGS and Sanger sequencing to identify genetic mutations associated with aHUS (CFH, MCP/CD46, CFI, C3, CFB, THBD, CFHR1, CFHR3, CFHR5). Multiplex ligation-dependent probe amplification (MLPA) is performed to detect deletions or duplications [60].

(CGR C; LOR 5)

Comment: if an NGS-detected genotype supports the diagnosis, confirmation of the genotype using the most accurate method is recommended. Such methods recognized worldwide include Sanger sequencing, i.e., letter-by-letter reading of a given DNA segment 500–1000 nucleotides long. In patients with aHUS, identification of complement factor mutations is not required for diagnosis and treatment decision-making. Genetic testing is necessary to determine prognosis and the duration of complement-blocking therapy (eculizumab**/ravulizumab). It should be remembered that mutations in genes encoding regulatory proteins of the alternative complement activation pathway are not detected in all patients with aHUS. However, a negative genetic screening result in a patient with TMA does not rule out the presence of aHUS. Currently, genetic testing is recommended even in the presence of anti-CFH antibodies. If a mutation is identified in a patient with anti-CFH antibodies, the decision on further treatment should be made on an individual basis, depending on the antibody titer and the functional consequences of the mutation. In addition, this test must be performed in all patients in need of kidney transplantation, regardless of the form of TMA; prior to discontinuing eculizumab** therapy; and for the diagnosis of rare forms of TMA (ADAMTS13, DGKE, INF 2, PLG, MUT, MMAA, MMAB, MMADHC, MCEE). In cases where the cause of TMA is not established, whole-exome sequencing is recommended for personalized treatment [35]. A database of gene variants is available at: http://www.complement-db.org/home.php .

(CGR C; LOR 5)

Comment: renal biopsy is not mandatory for diagnosing a specific form of TMA. When performing a nephrobiopsy in patients with TMA, there is a high risk of bleeding due to hypertension, thrombocytopenia, and uremia. However, renal biopsy can confirm the diagnosis of TMA in questionable and unclear situations: doubt regarding the HUS diagnosis, massive proteinuria in patients with anemia and thrombocytopenia, absence of the full laboratory symptom complex of TMA (most commonly thrombocytopenia), persistence of reduced renal function despite the resolution of hematological manifestations of TMA, suspicion of secondary forms of TMA, or ineffectiveness of eculizumab**.

(CGR C; LOR 5)

Comment: the primary specialist is a nephrologist (initial/follow-up nephrologist consultation/examination). Consultations with other specialists may also be required if necessary: initial/follow-up gastroenterologist consultation/examination, initial/follow-up neurologist consultation/examination, initial/follow-up pediatric cardiologist consultation/examination, initial/follow-up ophthalmologist consultation/examination, pathologist consultation/examination, initial/follow-up dermatovenerologist consultation/examination, initial/follow-up pulmonologist consultation/examination, initial/follow-up geneticist consultation/examination, initial/follow-up hematologist / pediatric oncohematologist consultation/examination, initial/follow-up pediatrician consultation/examination / initial/follow-up general practitioner (family physician) consultation/examination, etc.

(CGR C; LOR 5)

Comment: the primary specialist is a nephrologist (initial/follow-up nephrologist consultation/examination). Consultations with other specialists may also be required if necessary: initial/follow-up gastroenterologist consultation/examination, initial/follow-up neurologist consultation/examination, initial/follow-up hematologist / pediatric oncohematologist consultation/examination, initial/follow-up ophthalmologist consultation/examination, pathologist consultation/examination, initial/follow-up pediatrician consultation/examination / initial/follow-up general practitioner (family physician) consultation/examination, etc.

Differential diagnosis of typical HUS

(CGR C; LOR 5)

Comment:

• Atypical HUS;

• TTP;

• Hemolytic anemias;

• Multisystem inflammatory syndrome in children associated with SARS-CoV-2;

• HUS associated with defective cobalamin C metabolism (cblC-HUS);

• Sepsis.

Differential diagnosis of atypical HUS

(CGR C; LOR 5)

Comment:

• STEC-HUS;

• TTP;

• Pneumococcus-associated HUS;

• Multisystem inflammatory syndrome in children associated with SARS-CoV-2;

• Hemolytic anemias;

• Paroxysmal nocturnal hemoglobinuria;

• Megaloblastic anemia;

• Evans syndrome;

• Hemoblastosis;

• Hemophagocytic lymphohistiocytosis;

• Acute tubulointerstitial nephritis;

• Secondary forms of TMA (SLE, APS, systemic sclerosis, malignant hypertension, drug-associated TMA, etc.);

• Rare forms of HUS (DGKE, cblC-HUS);

• Sepsis.

If TMA develops in the presence of gastrointestinal symptoms, screening for STEC-HUS is indicated first; subsequently, verification of STEC-HUS is recommended after ruling out TTP and with "no suspicion of aHUS". The diagnosis of STEC-HUS cannot be excluded based solely on negative results of infection screening. A negative STEC-HUS screening result is considered reliable only if the sample was collected prior to the initiation of antibacterial therapy. In cases where antibacterial therapy is administered, the detection rate of EHEC DNA in stool by PCR is significantly higher during the first 6 days of diarrhea. Involvement of organs and systems other than the kidneys largely determines the outcome and prognosis [30, 40, 42, 47]. A "relapsing" (repeated episodes of hemolysis and platelet consumption) and recurrent course of TMA is not characteristic of STEC-HUS [2, 4, 5, 8, 10, 18, 27].

Based on the definition of aHUS, other causes of TMA—primarily STEC-HUS—must be ruled out to establish the diagnosis in a child. In some cases, diarrhea or respiratory tract involvement is also observed at the onset of aHUS; therefore, timely exclusion of STEC infection as well as Streptococcus pneumoniae infection is necessary [28, 34, 35, 38, 60]. In addition, systemic pathology, SARS-CoV-2-associated multisystem inflammatory syndrome, infections (HIV, H1N1), pre-existing malignant arterial hypertension, HELLP syndrome in parturients, medication use (cyclosporine**, tacrolimus**, etc.), methylmalonic acidemia, etc., should be excluded as potential causes of TMA (Appendix 1).

To rule out TTP, it is necessary to test the activity of ADAMTS13—the natural regulator of microthrombosis responsible for the inactivation of von Willebrand factor—in all patients presenting with TMA. A marked deficiency of ADAMTS13 (ADAMTS13 <10% of normal) leads to TTP. In STEC-HUS, aHUS, and other TMAs, ADAMTS13 activity may be reduced, but its level always exceeds 10%. Clinically, TTP and HUS share many similarities. TTP should be suspected in patients with severe persistent thrombocytopenia (<30,000/µL) with mild AKI or without it [35, 38]. However, severe AKI occurs in 10–12% of patients with idiopathic TTP [65]. In children, TTP can be either congenital due to ADAMTS13 deficiency or acquired as a result of anti-ADAMTS13 antibody production. Congenital TTP (Upshaw-Schulman syndrome) has a variable phenotype and may manifest in newborns with MAHA and jaundice or in children with unexplained thrombocytopenia. This form of TTP is characterized by ADAMTS13 activity <5%, absence of anti-ADAMTS13 antibodies, and homozygous/heterozygous mutations in ADAMTS13 [120]. TTP is characterized by the prevalence of neurological symptoms with moderate renal impairment [4, 24, 25, 35, 37].

It is necessary to test the level of anti-CFH antibodies, which are elevated in the antibody-mediated form of aHUS [35, 38]. The "autoimmune" form of aHUS is mainly seen in children aged 5 to 15 years. The action of anti-CFH antibodies, like CFH mutations, leads to excessive activation of the alternative complement pathway. The production of anti-CFH antibodies is associated with a deficiency of CFH-related proteins 1 and 3 (CFHR1, CFHR3). It has been established that 90% of patients with the antibody-mediated form of HUS have a homozygous deletion in the CFHR1 and CFHR3 genes. If the patient's age at disease onset is <3 years or if plasma exchange was performed prior to screening, only the detection of the deletion helps confirm antibody-mediated disease in patients with low anti-CFH antibody titers. Given its therapeutic significance, screening for anti-CFH antibodies should be performed in all patients with aHUS.

Since many mutations tend to impair the function of complement proteins rather than alter their plasma concentration, the levels of CFH, CFI, C3, and CFB may remain normal even in the presence of mutations. Molecular genetic testing itself requires significant time, and obtaining its results in the acute phase of the disease is practically unrealistic. Therefore, genetic testing has no diagnostic value for aHUS or for initiating treatment. At the same time, genetic screening is necessary during patient follow-up to determine their long-term therapy strategy [35, 59, 60, 62].

Multisystem inflammatory syndrome in children associated with SARS-CoV-2 is a critical form of COVID-19. This syndrome features virus-induced secondary hemophagocytic lymphohistiocytosis (cytokine storm), manifested by uncontrolled activation of the immune system with the release of pro-inflammatory cytokines (TNF-alpha, IL-1, IL-2, IL-6, IL-8, IL-10, granulocyte colony-stimulating factor, monocyte chemoattractant protein 1) and inflammatory markers (CRP, serum ferritin), which can lead to acute respiratory distress syndrome, multiorgan failure, and may cause patient death. Multisystem inflammatory syndrome often develops in the late stages of the disease. A positive SARS-CoV-2 PCR is not detected in all patients with multisystem inflammatory syndrome, but antibodies to the COVID-19 causative agent can be identified in almost all of them. The main clinical manifestations of multisystem inflammatory syndrome in children are persistent fever and systemic inflammation involving various organs and systems—the blood system, cardiovascular and nervous systems, gastrointestinal tract, kidneys, and skin. Evidence suggests that in COVID-19, complement dysregulation-associated TMA is an important pathophysiological mechanism in addition to or as part of thromboinflammation [121, 122, 123]. Collectively, COVID-19-associated coagulopathy appears to represent a complex pathological process driven by a cascade of thromboinflammation, endothelial injury, and complement activation, leading to TMA.

3. Treatment, including drug and non-drug therapies, diet therapy, and pain management

The goal of therapy for any form of HUS is to resolve the clinical and laboratory manifestations of TMA, preserve and improve the function of affected organs, prevent the development of CKD G5, eliminate the need for dialysis modalities, prevent involvement of internal organs other than the kidneys, improve patients' quality of life, and ensure better patient survival.

3.1 Conservative Treatment

3.1.1 Typical HUS

To date, there is no universally accepted treatment regimen for STEC-HUS with clinically proven efficacy. The complex of therapeutic measures in typical HUS includes etiotropic, syndromic, and RRT.

(CGR C; LOR 5)

Comment: hospitalization in such medical facilities is necessitated by the need for dialysis treatment modalities (hemodialysis (HD), sustained low-efficiency daily dialysis / venovenous hemodiafiltration (SLEDD/CVVHDF), peritoneal dialysis (PD)), mechanical ventilation (MV), and plasma therapy (PT).

(CGR C; LOR 5)

Comment: preference should be given to dialysis, especially in the presence of severe hyperkalemia and metabolic acidosis, the correction of which by administration of solutions containing sodium bicarbonate (e.g., sodium bicarbonate**) and dextrose** may exacerbate fluid overload [124].

(CGR C; LOR 5)

(CGR C; LOR 5)

Comment: a small prospective cohort study in 2005 demonstrated that the amount of sodium administered prevents the development of oligoanuric AKI in HUS [125]. Since then, accumulating evidence and a meta-analysis showed that with a hematocrit of ≤23%, the risk of oligoanuric renal failure increases by 2.38-fold [126]. Infusion therapy may be the first effective individualized measure to prevent STEC-HUS and improve prognosis, representing a paradigm shift since fluid restriction was the mainstay of treatment until recently [127]. Infusion therapy helps improve renal perfusion, prevents ischemic organ damage, and maintains tubular blood flow. Infusion therapy protocols have not been established. Most studies used isotonic sodium chloride solution. The intravenous fluid volume should be calculated based on a clinical assessment of intravascular volume to avoid fluid overload. This requires invasive hemodynamic monitoring, as well as assessment of fluid intake, urine output, and losses from stool and vomiting [7]. Signs of fluid overload may include weight gain, arterial hypertension, edema, and hyponatremia.

• Nutrition, as well as water and electrolytes, is recommended to be provided enterally, via nasogastric tube if necessary (nasogastric tube insertion, nasogastric tube care, feeding a critically ill patient orally and/or via nasogastric tube) in order to maintain adequate gastrointestinal function [2, 128].

(CGR C; LOR 5)

Comment: calorie and protein intake should be 100% of the recommended daily requirement. Parenteral nutrition is necessary in case of persistent vomiting, diarrhea, and symptoms of colitis.

(LetC, Level of Evidence 4)

Comment: the use of gastrointestinal motility stimulants (prokinetics) and systemic antimicrobial agents (with bactericidal effects) may contribute to increased toxemia, thereby increasing the risk of typical HUS. Data have been published on the feasibility of using bacteriostatic antibiotics in STEC infection. It is known that during the outbreak of typical HUS in Germany (2011) caused by E. coli O104:H4 infection, in centers using a combined antibiotic therapy strategy (meropenem** and ciprofloxacin**), E. coli was excreted in feces for a shorter period, convulsive syndrome was less pronounced, mortality was lower, and serum creatinine levels decreased rapidly compared to patients who did not receive antibiotic treatment; these patients did not require surgery [133, 134]. Thus, the obtained treatment results justify the prescription of systemic antibacterial drugs in already developed HUS.

(Level of Evidence C, Grade of Recommendation 5)

Comment: in the development of STEC-HUS, parenteral antibiotic use is performed during central venous catheterization, peritoneal catheter implantation, and other surgical interventions.

Choice of drug according to local and individual pathogen susceptibility.

(Level of Evidence C, Grade of Recommendation 5)

Comment: in the acute period of STEC-HUS, red blood cell transfusions should be used at a rate of 10-15 ml/kg. Packed red blood cell transfusion is indicated for the development of hemic hypoxia (dyspnea, tachycardia, weakness, and decreased exercise tolerance) regardless of the hemoglobin level. To prevent immune reactions and complications after packed red blood cell transfusion (leukocyte antigen alloimmunization, febrile non-hemolytic transfusion reaction, graft-versus-host disease, transfusion-related acute lung injury), leukoreduced red blood cell units (leukocyte-depleted red blood cells) are preferred. In cases of transfusion of blood of a different group, hyperkalemia, or allergic reactions, washed red blood cells are preferred. In the resolution period of STEC-HUS with persistent anemia not associated with hemolysis, short-acting erythropoiesis-stimulating agents (ATX group B03XA Other antianemic preparations) should be used, the frequency of which is determined by the hemoglobin level.

(Level of Evidence C, Grade of Recommendation 5)

Comment: platelet concentrate transfusions may increase the manifestations of TMA in children with STEC-HUS because they provoke new episodes of microthrombosis with further platelet consumption. Platelet concentrate transfusion is indicated only in the event of bleeding or when using invasive diagnostic procedures / surgical interventions (central or peritoneal catheter placement, abdominal surgeries) with a high risk of bleeding [7].

(CGR C; LOR 5)

Comment: severe STEC-HUS is an absolute indication for initiating RRT. Early initiation of dialysis plays a leading role in life prognosis and renal survival, as a direct correlation exists between the timing of dialysis initiation and outcomes. When determining indications for RRT, general clinical indicators and signs of organic AKI development should be taken into account, as evidenced by the lack of response to loop diuretics under conditions of normo- or hypervolemia. Repeated diuresis stimulation is contraindicated, as its continuation only worsens renal injury. An absolute indication for starting RRT in STEC-HUS is AKI in the failure stage. If there is no choice regarding the need for RRT in anuria, general clinical indications come to the fore in oliguria: fluid overload with pulmonary edema and/or respiratory failure; metabolic disorders: hyperkalemia (above 7 mmol/L), acidosis (BE < -12 mmol/L), azotemia (creatinine increase of more than 50-70 µmol/L/24h); uremic encephalopathy; uncontrolled hypertension; the need to ensure adequate nutrition for the child during prolonged oliguria.

(CQR C; LE 5)

Comment: when choosing the dialysis modality, the severity of the clinical condition, degree of metabolic disorders, presence of contraindications to a specific dialysis method, as well as the age, height, weight of the child, and availability of RRT modalities in a specific medical institution are taken into account. Indications for peritoneal dialysis include: "isolated AKI", pronounced neurological disorders, presence or high risk of hemorrhagic complications, pronounced circulatory failure, and inability to perform vascular access. In young children, the peritoneal surface area relative to body surface area is larger than in adults, which ensures high procedural efficiency and makes PD the primary method of choice for RRT in early childhood. As a starting RRT method, CVVHDF is used in cases of severe fluid overload with pulmonary edema, cerebral edema, hyperazotemia, decompensated metabolic acidosis, extremely severe patient condition, hemodynamic instability, and contraindications to PD. HD is the method of choice when treating a patient with preserved hemodynamics, pronounced azotemia and fluid overload, and contraindications to other RRT methods.

Procedures: Replacement of peritoneal dialysis catheter extension set, Reintegration of peritoneal dialysis catheter using video-endoscopic technologies, Intraabdominal fixation of peritoneal dialysis catheter, Implantation of peritoneal dialysis catheter using video-endoscopic technologies, Implantation of peritoneal dialysis catheter, Implantation of peritoneal dialysis catheter, Balloon angioplasty of vascular access for extracorporeal dialysis, Stenting of vascular access for extracorporeal dialysis, Percutaneous angioplasty of vascular access for dialysis, Vessel bougienage for extracorporeal dialysis access, Implantation of vascular graft (vascular access) for dialysis, Closure of vascular graft (vascular access) for dialysis, Removal of vascular graft (vascular access) for dialysis.

(LetC, Level of Evidence 4)

Comment: arterial hypertension in children is defined as a persistent elevation of blood pressure above the 95th percentile for sex, age, and height. Arterial hypertension can cause posterior reversible encephalopathy syndrome (PRES) [142]. Calcium channel blockers and/or diuretics in case of fluid overload should be preferred for the management of hypertension in STEC-HUS.

(CGR C; LOR 5)

(CGR C; LOR 4)

Comment: proteinuria detected in the acute period is transient in nature. There is no proven benefit of steroid therapy, and the risk of secondary infection is high.

(CGR C; LOR 4)

Comment: usually prescribed 3–6 months after recovery from STEC-HUS. ACE inhibitors/ARBs may preferably be used after the acute phase of STEC-HUS [151, 152]. The use of ACE inhibitors is contraindicated when eGFR ≤ 30 mL/min/1.73 m²].

The use of off-label drugs – outside the indications approved in the drug's instructions – is carried out by decision of the medical commission, with the permission of the Local Ethics Committee of the medical organization (if available), subject to signed informed consent from the parents (legal representative) and the patient over 15 years of age.

(LoE C; GoR 5)

3.1.2 Atypical HUS

(CGR C; LOR 5)

Comment: hospitalization in such hospitals is necessitated by the need for dialysis treatment modalities (HD, CVVHD/CVVHDF, PD), and mechanical ventilation. Patients with aHUS/TMA require management of fluid balance, anemia, nutritional status (up to parenteral nutrition), hypertension, electrolyte balance, acid-base status, hypoxia, encephalopathy, and seizure disorders. Hemodialysis should be preferred, especially in the presence of severe hyperkalemia and metabolic acidosis, the correction of which with sodium bicarbonate-containing preparations (e.g., sodium bicarbonate**) and dextrose** may exacerbate fluid overload [124].

• In hospitalized patients with signs of TMA, in most cases, blood transfusion (red blood cell suspension transfusion, preferably leukoreduced red blood cell suspension (leukocyte-depleted red blood cells)) is recommended to correct anemia [226, 227].

(LetC, Level of Evidence 4)

(Level of Evidence C, Grade of Recommendation 5)

Further treatment is recommended with eculizumab** or ravulizumab depending on indications.

The dosing regimen of eculizumab**/ravulizumab for children and the frequency of administration are determined by the child's body weight (Tables 1 and 2, respectively). The goals of therapy are: prevention of death with timely initiation of therapy, achievement and maintenance of hematological remission, absence of need for plasma therapy, improvement/maintenance/continuation of improvement in renal function, avoiding the need for transplantation in a number of cases, and improving quality of life. Long-term use prevents relapses of acute complement-mediated TMA [60, 63, 154, 155, 156, 157, 158, 160, 228, 229, 230, 231, 232, 233].

(LoE A; GoR 1)

Comment: the therapeutic approach to aHUS has been radically improved with the discovery and introduction into clinical practice of eculizumab**, a humanized monoclonal antibody against the C5 terminal fraction of the complement cascade. Eculizumab** prevents the cleavage of C5, leading to the formation of pro-inflammatory C5a and pro-thrombotic C5b-9 components, thereby eliminating their pathogenic effect. Published results of controlled trials have demonstrated high efficacy of eculizumab** in terms of rapid elimination of hemolysis activity (on average within 7–14 days) and signs of active TMA in 88% of patients, with the duration of hematological remission maintained throughout the entire treatment course in the majority of patients. An increase in GFR by an average of 32 mL/min/1.73 m² from baseline was noted, with the greatest magnitude when treatment was initiated early. In children, the increase in GFR was more pronounced (64 mL/min/1.73 m²). In some cases, a slow improvement in renal function was observed over 2 years. Most patients became free of the need for dialysis [15, 16, 27]. Notably, this effect was observed regardless of the presence or absence of mutations in complement genes. When comparing groups of patients who received and did not receive eculizumab**, a significant difference was noted in the percentage of patients reaching end-stage chronic kidney disease (CKD) at one year of follow-up (25% and 63%, respectively) [155, 156]. According to >5 years of follow-up data in patients with aHUS, the use of eculizumab** reduces the incidence of TMA relapses by 13.7-fold compared with the untreated period [159]. Whenever possible, treatment with eculizumab** should be initiated within 24–48 hours of aHUS manifestation or admission to the hospital [157]. The drug is reconstituted in 0.9% sodium chloride solution**, 0.45% sodium chloride solution** (the drug is not commercially available in finished form in the Russian Federation and is prepared in hospital pharmacies), or 5% dextrose solution**. The required volume of diluted eculizumab** to a final concentration of 5 mg/mL is calculated based on 60 mL of solution for every 300 mg of the drug.

Ravulizumab is a humanized monoclonal antibody targeting the same epitope on C5 as eculizumab**, but has a four-fold longer half-life due to enhanced endosomal C5 dissociation and antibody recycling, providing an extended duration of terminal complement inhibition [234]. The tolerability of ravulizumab is generally assessed as favorable and is also similar to eculizumab** [229, 230, 231, 232, 233]. In the Russian Federation, ravulizumab is not included in the VJNLV list (vital and essential drugs).

Differences between ravulizumab and eculizumab** include the dosing frequency, which is once every 2 weeks for eculizumab compared to once every 4–8 weeks for ravulizumab in children depending on body weight, as well as immediate, complete, and sustained complement inhibition (serum free C5 ≤0.5 mcg/mL) immediately after the first infusion and throughout 8 weeks. According to studies, eculizumab** and ravulizumab demonstrated equal efficacy in the treatment of aHUS, close to 100%. The efficacy and safety of ravulizumab were evaluated in adults and children with aHUS, both treatment-naive and previously treated with eculizumab**, in two single-arm, multicenter, 26-week Phase III studies. Ravulizumab provided immediate, complete, and sustained C5 inhibition, leading to rapid improvement in renal and hematological parameters over 26 weeks and a further safe increase in response rates over a longer period in patients (adults and children) with aHUS who had not previously received complement inhibitors. In children with aHUS, switching from eculizumab** to ravulizumab led to the sustained maintenance of stable renal and hematological parameters for 1 year [231, 232].

Principles of drug selection:

In the Russian Federation, eculizumab** is included in the list of vital and essential drugs (VED). At the meeting of the Expert Council of the «Circle of Kindness» Foundation on March 6, 2024, a decision was made to include aHUS in the list of severe life-threatening and chronic diseases, including rare (orphan) diseases. At this meeting, ravulizumab was included in the list of medicinal products, medical devices, and rehabilitation equipment procured by the Foundation for the treatment of aHUS. The experts approved the categories of children for whom ravulizumab is indicated: patients with an unfavorable prognosis and a high risk of disease recurrence, in whom mutations in at least one of the genes (CFH, CFI, C3, CFB) are confirmed, with the prospect of lifelong therapy; patients with individual intolerance to eculizumab**.

• Eculizumab** is the drug of choice in the treatment of the acute stage of aHUS, ensuring careful disease control, efficacy, and safety of therapy (Table 1).

• Ravulizumab is indicated for patients with an unfavorable prognosis and a high risk of disease recurrence, in whom mutations in at least one of the genes (CFH, CFI, C3, CFB) are confirmed, with the prospect of lifelong therapy; and for patients with individual intolerance to eculizumab** and a body weight not <10 kg (Table 2).

Table 1. Eculizumab** dosing regimen in children with aHUS

Table 2 – Dosing regimen and administration frequency of ravulizumab in children with aHUS (Adapted from Syed YY [34])

Note: ^a – body weight at the time of treatment, ^b – for patients switching from eculizumab**, administration should be performed 2 weeks after the last eculizumab** infusion, ^c – starting on day 15 and every 8 weeks thereafter in patients with a body weight of ≥20 kg, or every 4 weeks thereafter in patients with a body weight of <20 kg.

The decision to switch a patient from eculizumab** therapy to ravulizumab is made by a medical commission involving nephrologists specializing in the treatment of aHUS. Administration of eculizumab**/ravulizumab is carried out via a peripheral venous catheter (peripheral catheter) or a port system (implantable infusion/injection port***), which are installed in accordance with the guidelines "Venous Access, 2019" (https://msestra.ru/download/file.php?id=4763) using the necessary medications. Port systems are installed in accordance with Order of the Government of the Russian Federation No. 3053-r dated December 31, 2018, "On approval of the list of medical devices implanted into the human body when providing medical care within the framework of the state guarantee program for the free provision of medical care to citizens, as well as the list of medical devices dispensed on prescriptions for medical devices upon the provision of a set of social services".

(CGR C; LOR 4)

Comment: positive results regarding hematological changes and renal function parameters have been described with the combined use of plasmatherapy (plasma exchange with 1.5 plasma volumes (60–75 ml/kg) per session for 5 days or until hematological remission is achieved, then every other day at 40 ml/kg for 2 weeks, followed by a regimen of twice a week for 2–3 weeks) combined with pulse intravenous administration of #cyclophosphamide** – usually 0.5 g/1.73 m² every 4 weeks for 2 doses, up to 1 g/1.73 m² for 5 doses [162] or 0.5 g/m² every 3–4 weeks for 5 doses [165], glucocorticoids (prednisolone** 1 mg/kg/day for 4 weeks, then every other day for the next 4 weeks, followed by a dose reduction every 2 weeks, on average, to 0.2 mg/kg/48h for 10–12 months) (in severe cases, pulse therapy with methylprednisolone** up to 1 g/day for 3 days) and #rituximab** (375 mg/m² intravenously every 7 days for 2 doses; plasma exchange is not performed for 48–72 hours after infusion. Positive clinical dynamics were accompanied by a decrease in anti-CFH antibody titers. The combination of plasmatherapy with immunosuppressants can reduce the risk of death or progression to CKD stage 4 during the first year from 59% to 24%. Maintenance immunosuppressive therapy with glucocorticoids (0.2–0.3 mg/kg/48h for 10–12 months in terms of prednisolone**), #azathioprine** (1–2 mg/kg/24h for 12–24 months), or #mycophenolate mofetil** (0.5–0.75 g/m²/24h for 12–24 months) [164, 165] further reduced the 1-year relapse risk of this form of aHUS from 21% to 8% [38, 165]. In the treatment of anti-CFH antibody-mediated aHUS, FFP transfusions are not recommended as a substitute for plasma exchanges. Anti-CFH antibody titers are closely monitored between days 7 and 28 of treatment, and then every 3–6 months. Relapses of anti-CFH antibody-mediated aHUS occur following intercurrent illnesses within the first 2 years [38, 162].

(}`)...(})

Comment: the duration of therapy with eculizumab** is 6–12 months, followed by withdrawal and switching to immunosuppressive therapy (#mycophenolate mofetil** (0.5–0.75 g/m²/24 h) with quarterly monitoring of anti-CFH antibody levels for 12–24 months [38, 164]. Treatment with eculizumab** does not affect the production of anti-CFH antibodies. Their elevated titer during the first 12–24 months is associated with an increased risk of relapse [164], therefore immunosuppression may be required. Discontinuation of immunosuppressive therapy is possible upon normalization of anti-CFH antibody levels or their absence.

If plasmapheresis is performed during treatment with eculizumab**, an additional dose of the drug should be administered [168].

Additional administration of the drug is necessary due to changes in the metabolism of eculizumab** during plasmapheresis (Table 3) [158].

Table 3 Administration of an additional dose of eculizumab** in case of plasmapheresis

Complement-blocking therapy should not be interrupted during an intercurrent illness not caused by encapsulated bacteria, due to the high risk of aHUS relapse [169].

(CGR C; LOR 5)

(CGR C; LOR 5)

Comment: most data on infectious complications and the need for vaccination were obtained from studies of eculizumab**; however, given the similar mechanism of action, experts recommend extrapolating these to ravulizumab [229, 233, 235, 236].

In view of isolated reports on the risk of meningococcal meningitis during eculizumab**/ravulizumab therapy, preliminary vaccination is performed, since blockade of the C5 complement component by eculizumab** and ravulizumab impairs the immune response to encapsulated bacteria (Neisseria meningitidis, Haemophilus influenzae type b, Streptococcus pneumoniae). The mechanism of action of eculizumab** and ravulizumab entails an increased risk of meningococcal infection, which necessitates vaccination of aHUS patients, preferably before initiating eculizumab**/ravulizumab therapy or after its initiation once the patient's condition stabilizes. The primary vaccination series with meningococcal polysaccharide conjugated vaccine (serogroups A, C, W, Y)** or meningococcal vaccine** (oligosaccharide conjugated, serogroups ACW135Y) involves the administration of 2 doses with a 3-month interval, while the second vaccine dose does not affect the initiation of eculizumab** therapy. For critically ill patients, eculizumab** therapy is initiated on vital indications under cover of prophylactic antibiotic therapy. The lack of vaccination should not delay the initiation of therapy [172].

Meningococcal vaccination schedule 2 weeks prior to initiating eculizumab** therapy:

√ patients aged 9 – 23 months are administered 2 doses of meningococcal polysaccharide conjugated vaccine (serogroups A, C, W, Y)** or meningococcal vaccine** (oligosaccharide conjugated, serogroups ACW135Y) with a 3-month interval;

√ patients older than 23 months who have not received the primary meningococcal vaccination series are administered 2 doses of meningococcal polysaccharide conjugated vaccine (serogroups A, C, W, Y)** or meningococcal vaccine** (oligosaccharide conjugated, serogroups ACW₁₃₅Y) with a 3-month interval, or a single dose of meningococcal polysaccharide vaccine (serogroups A and C)**;

√ patients older than 23 months who were vaccinated more than 2 years ago with a single dose of meningococcal polysaccharide vaccine (serogroups A and C)** or more than 5 years ago with a single dose of meningococcal polysaccharide conjugated vaccine (serogroups A, C, W, Y)** or meningococcal vaccine** (oligosaccharide conjugated, serogroups ACW₁₃₅Y), are prescribed 1 dose of meningococcal polysaccharide conjugated vaccine (serogroups A, C, W, Y)** or meningococcal vaccine** (oligosaccharide conjugated, serogroups ACW₁₃₅Y).

Meningococcal vaccination schedule when less than 2 weeks remain before initiating eculizumab** therapy:

√ patients aged 9–23 months are administered the first dose of meningococcal polysaccharide conjugated vaccine (serogroups A, C, W, Y)** or meningococcal vaccine** (oligosaccharide conjugated, serogroups ACW₁₃₅Y) under cover of prophylactic antibiotic therapy for two weeks starting from the initiation of eculizumab**. Meanwhile, the second vaccine dose must be administered 3 months after the first vaccination and does not affect the initiation of eculizumab** therapy.

√ patients older than 23 months who have not been vaccinated according to the complete meningococcal schedule are administered the first dose of meningococcal polysaccharide conjugated vaccine (serogroups A, C, W, Y)** or meningococcal vaccine** (oligosaccharide conjugated, serogroups ACW₁₃₅Y) or a single dose of meningococcal polysaccharide vaccine (serogroups A and C)** with the addition of prophylactic antibiotic therapy for two weeks starting from the initiation of eculizumab**. Meanwhile, the second vaccine dose must be administered 3 months after the first vaccination and does not affect the initiation of eculizumab** therapy.

√ patients older than 23 months who received a single dose more than 2 years ago of meningococcal polysaccharide vaccine (serogroups A and C)** or meningococcal polysaccharide conjugated vaccine (serogroups A, C, W, Y)**, or who were vaccinated more than 5 years ago with a single dose of meningococcal vaccine (oligosaccharide conjugated, serogroups ACW₁₃₅Y) or meningococcal vaccine (serogroups A, C, W, Y)**, are administered a single dose of meningococcal polysaccharide conjugated vaccine (serogroups A, C, W, Y)** or meningococcal vaccine** (oligosaccharide conjugated, serogroups ACW135Y) with the prescription of prophylactic antibiotic therapy for 2 weeks after the initiation of eculizumab** therapy.

Meningococcal vaccination schedules in various clinical scenarios in aHUS patients:

√ If eculizumab** therapy is vitally necessary, immunization with meningococcal polysaccharide conjugated vaccine (serogroups A, C, W, Y)** or meningococcal vaccine** (oligosaccharide conjugated, serogroups ACW₁₃₅Y) is permitted in patients under 9 months of age.

√ Patients who have completed the primary meningococcal vaccination series should be administered an additional dose of conjugate meningococcal vaccine every 5 years to ensure the most effective protection.

√ Patients who interrupted eculizumab**/ravulizumab therapy and resumed treatment after a certain period are recommended revaccination against meningococcal infection if more than 2 years have passed since the first dose of meningococcal polysaccharide vaccine (serogroups A and C)** and more than 5 years (after the administration of meningococcal polysaccharide conjugated vaccine (serogroups A, C, W, Y)** or meningococcal vaccine** (oligosaccharide conjugated, serogroups ACW₁₃₅Y)).

√ If an aHUS patient has not been vaccinated previously, children require vaccination against meningococcal, pneumococcal, and Haemophilus influenzae type b infections as part of preparation for transplantation and prophylactic use of eculizumab**. Vaccines can be administered simultaneously in different syringes at different anatomical sites.

(CQR C; LE 5)

Comment: immunization against pneumococcal infection is not a reason to postpone the initiation of Eculizumab** therapy and can be performed in patients receiving this drug. Patients aged 2 months and older are administered 2 doses of the pneumococcal conjugate vaccine (Pneumococcal infection prevention vaccine** [conjugate, 13-valent]) according to the schedule at 2 and 4.5 months, followed by revaccination at 15 months. Patients aged 12 to 24 months who have not been previously vaccinated are administered 2 doses of the pneumococcal conjugate vaccine (Pneumococcal infection prevention vaccine** [conjugate, 13-valent]) with a 2-month interval (a 3-month interval is allowed if it can be combined with the 2nd dose of the meningococcal conjugate vaccine).

In accordance with [173], for patients under 2 years of age with chronic inflammatory diseases receiving immunosuppressive therapy, vaccination with the 13-valent pneumococcal conjugate vaccine is carried out according to the 3+1 schedule, i.e., three times with an interval of 1 month, followed by revaccination in the second year of life at 15 months of age, but no earlier than 4 months after the last dose of the primary vaccination series.

Patients aged 2 to 18 years who have not previously received a single dose of the pneumococcal conjugate vaccine (Pneumococcal infection prevention vaccine** [conjugate, 13-valent]) are vaccinated once. For children aged 2–5 years, the 10-valent pneumococcal conjugate vaccine (Pneumococcal infection prevention vaccine** [conjugate, 10-valent]) can be used according to a 2-dose schedule, where 2 doses of the vaccine are administered with an interval of at least 8 weeks between them. If the patient has previously been vaccinated with the pneumococcal polysaccharide vaccine (Pneumococcal infection prevention vaccine** [polysaccharide, 23-valent]), then vaccination with the pneumococcal conjugate vaccine (Pneumococcal infection prevention vaccine** [conjugate]) is carried out no earlier than 1 year later.

Considering the high risk of severe pneumococcal infection, patients with aHUS require an additional administration of the 23-valent pneumococcal polysaccharide vaccine (PPV23) with an interval of at least 8 weeks, optimally 12 months after completing the pneumococcal conjugate vaccine immunization schedule, to broaden the coverage of pathogen serotypes [173].

(CQR C; LE 5)

Comment: patients under 6 months of age are prescribed 3 doses: at 3, 4.5, and 6 months, followed by revaccination at 18 months of age. Patients aged 6 to 12 months are administered 2 doses with a 1-month interval, followed by revaccination at 18 months. Patients older than 12 months receive a single vaccination. Revaccination is not required.

(CGR C; LOR 5)

Comment: usually, drugs from the following groups are prescribed: broad-spectrum penicillins, beta-lactamase-sensitive penicillins, and other beta-lactam antibacterial drugs. Prophylactic administration of a systemic antimicrobial agent should be started simultaneously with vaccination and continued for at least 2 weeks if vaccination is performed on the day of the first eculizumab** infusion. If vaccination is not possible by the time treatment starts, antibiotic prophylaxis can be extended for up to several months depending on the timing of vaccination. In some cases, antibiotics may be prescribed even to timely vaccinated patients. Indications for their use are determined by the attending physician. The need for prophylactic administration of antibiotics in patients with aHUS is due to the absence of a vaccine against the N. meningitides serotype B, meaning vaccination does not completely rule out the development of meningitis [34].

(CGR C; LOR 5)

Comment:the question of the optimal duration of treatment with eculizumab** has not been definitively resolved. The risk of aHUS relapse is determined by the mutation type. It has been demonstrated that when the drug is discontinued after 2 years, aHUS relapses occur more frequently in patients with mutations in the CFH, C3, and THBD genes, whereas patients with a mutation in the MCP gene and anti-CFH antibodies do not relapse. Therefore, discontinuing the drug in certain cases is possible after assessing the risk level of aHUS relapse. Cessation of eculizumab** treatment leads to relapses in 60–72% of patients with CFH gene mutations, in 43%–with C3 mutations, 37–50% with MCP/CD46 gene mutations, and ~10% without identified defects [166]. Currently, there is no optimal strategy for monitoring complement activity and the development of aHUS, especially its subclinical form in the absence of pronounced symptoms. Cessation of Eculizumab** therapy potentially exposes the patient to the risk of disease progression and the development of a new TMA episode, unpredictable in severity and time of onset, as well as the risk of losing the described clinical benefits of prolonged therapy (e.g., improvement in renal function, which in some patients occurs over 2 or more years). Children are at high risk, frequently experiencing complement-activating conditions (e.g., infections or vaccination). Discontinuation of eculizumab** therapy should only be considered in the absence of disease activity and at a low risk of relapse, on an individual basis. The risk of aHUS relapse depends on the mutation type. The low-risk category includes patients with an isolated mutation in the MCP/CD46 gene; patients with anti-CFH antibodies who have had negative test results for these antibodies over a long period during follow-up. The medium-high risk category includes patients with an isolated mutation in the CFI gene; patients with persistently low levels of anti-CFH antibodies; patients without identified mutations in complement system genes. The high-risk category includes patients with a history of aHUS relapses; patients with relatives having a relapsing course of aHUS; patients with mutations in the CFH, CFB, and C3 genes. To address the issue of discontinuing therapy, it is necessary to take into account: disease severity, duration of the organ function normalization/stabilization period, family history, genetic analysis results, presence of complement-activating conditions, patient compliance, and the possibility of rapidly restarting eculizumab** therapy [160, 182].

(CGR C; LOR 5)

Comment: the duration of eculizumab** therapy is determined by the age at aHUS onset, clinical course features, history of relapses, and the patient's genetic profile. In the absence of mutations in genes encoding complement proteins, discontinuation of eculizumab** is indicated after achieving a 2-year aHUS remission (Table 4). In the presence of mutations in genes encoding complement proteins, the duration of complement-blocking therapy is determined by stratified relapse risks. When aHUS develops at an early age or life-threatening TMA manifestations occur, regardless of the genetic profile, the duration of pathogenetic treatment should be determined individually.

Table 4 - Duration of eculizumab** use in patients with aHUS

Current studies have demonstrated that drug discontinuation is possible, although approximately 16.4–29.6% of patients develop TMA relapse after cessation of therapy, more frequently in those with mutations associated with aHUS [169, 237, 238, 239]. It should be noted that current guidelines regarding the lifelong duration of eculizumab** therapy cannot be considered definitive, and this issue continues to be studied.

(Level of Evidence C; Grade of Recommendation 4)

Comment: it has been established that the aHUS relapse rate ranges from 23 to 30%, most frequently observed within the first 3 months after eculizumab** discontinuation [169; 238]. According to Emirova Kh.M. et al. (2024) the aHUS relapse rate was 16.4%. All relapses occurred within the first 9 months after eculizumab** discontinuation, with half of the cases occurring in the first 3 months [239]. Following the discontinuation of complement-blocking therapy, patients require careful monitoring and control of key TMA laboratory criteria for the purpose of diagnosing relapse and early restart of complement-blocking therapy. Laboratory tests are performed 2 weeks after the last dose of eculizumab**, then weekly for 1 month, every 2 weeks for 2 months, monthly for 3 months, and subsequently every 3 months. Outpatient monitoring of blood pressure, proteinuria, and hemoglobinuria using test strips is also recommended for patients [157, 241]. An alternative monitoring schedule after eculizumab** discontinuation may be used: laboratory tests are performed 2 weeks after the last dose of eculizumab**, then every 2 weeks for 1 month, monthly for 6 months, and thereafter every 3–4 months long-term [242]. Additional laboratory tests may be performed in the event of an acute illness or surgical intervention [186, 187].

Currently, there are no exact data on monitoring patients after ravulizumab discontinuation; given the similar mechanism of action, it is suggested to use the same approaches as for eculizumab** [240].

In the event of an aHUS relapse, eculizumab** therapy must be resumed.

(CGR C; LOR 5)

Comment: timely diagnosis of infections, which most frequently precede aHUS, and subsequent adequate antibacterial therapy are of particular importance.

(Level of Evidence C, Grade of Recommendation 5)

Comment: in an acute episode of aHUS, packed red blood cell transfusions should be administered at a dose of 10–15 mL/kg. Packed red blood cell transfusion is indicated when hemic hypoxia develops, regardless of the hemoglobin level. To prevent immune reactions and complications following packed red blood cell transfusion (leukocyte alloimmunization, febrile non-hemolytic reaction, graft-versus-host disease, transfusion-related acute lung injury), leukocyte-reduced red blood cells (leukocyte-depleted red blood cells) are preferred. In cases of cross-group blood transfusion, hyperkalemia, or allergic reactions, washed red blood cells are preferred. For persistent anemia not associated with hemolysis in aHUS patients, short-acting erythropoiesis-stimulating agents (ATX group B03XA Other antianemic preparations) should be used, with the frequency of administration determined by the hemoglobin level.

(Level of Evidence C, Grade of Recommendation 5)

Comment: platelet concentrate transfusions may exacerbate TMA manifestations in children with aHUS, as they provoke new episodes of microthrombosis with further platelet consumption. Platelet concentrate transfusion is indicated only upon the development of bleeding or when using invasive diagnostic procedures / surgical interventions (insertion of a central or peritoneal catheter, abdominal surgeries) with a high risk of bleeding.

(CQR C; LE 5)

Comment: when determining indications for RRT, general clinical parameters and signs of organic AKI development should be taken into account, as evidenced by the lack of response to loop diuretics under conditions of normo- or hypervolemia. Repeated diuresis stimulation is contraindicated, as its continuation only worsens renal injury. An absolute indication for initiating RRT in aHUS is AKI in the failure stage. While in anuria there is no choice when deciding on the need for RRT, in oliguria general clinical indications come to the foreground: hyperhydration with pulmonary edema and/or respiratory failure; metabolic disorders: hyperkalemia (above 7 mmol/L), acidosis (BE < -12 mmol/L), azotemia (creatinine increase of more than 50–70 µmol/L/24h); uremic encephalopathy; uncontrolled hypertension; the need to ensure adequate nutrition in a child with prolonged oliguria.

(CGR C; LOR 5)

Comment: when choosing a dialysis modality, the severity of the clinical condition, the degree of metabolic disorders, the presence of contraindications to a specific dialysis method, as well as the age, height, weight of the child, and the availability of RRT modalities in a specific medical institution are taken into account. Indications for peritoneal dialysis include: "isolated AKI", pronounced neurological disorders, presence or high risk of hemorrhagic complications, pronounced circulatory failure, and the impossibility of vascular access. In young children, the peritoneal surface area relative to body surface area is larger than in adults, which ensures high procedural efficiency and makes PD the primary method of choice for RRT in infants and toddlers. CVVHDF is used as a starting RRT method in cases of pronounced hyperhydration with pulmonary edema, cerebral edema, hyperazotemia, decompensated metabolic acidosis, extremely severe patient condition, hemodynamic instability, and contraindications to PD. HD is the method of choice when treating a patient with preserved hemodynamics, pronounced azotemia and hyperhydration, and contraindications to other RRT methods.

(LetC, Level of Evidence 4)

Comment: arterial hypertension in children is a persistent elevation of blood pressure above the 95th percentile for sex, age, and height. Arterial hypertension can be the cause of posterior reversible encephalopathy syndrome (PRES syndrome) [142]. The prescription of ACE inhibitors is contraindicated at an eGFR ≤30 mL/min/1.73 m².

(CGR C; LOR 5)

Comment: treatment with ACE inhibitors / ARBs helps reduce the progression of renal failure to end-stage renal disease while simultaneously controlling blood pressure levels.

(CGR C; LOR 5)

3.2 Surgical Treatment

3.2.1 Typical HUS

• Kidney transplantation is recommended as a modality of RRT in patients with CKD G5 resulting from STEC-HUS [1, 6].

(Level of Reliability of Evidence 3; Level of Consistency of Evidence C)

Comment: there is no risk of recurrent STEC-HUS after kidney transplantation. The possibility of living-related donor transplantation should be discussed. Cyclosporine** is not contraindicated. Based on the analysis of the disease course, atypical HUS must be ruled out, if necessary by molecular genetic testing [195].

3.2.2 Atypical HUS

• Kidney transplantation is recommended as a renal replacement therapy modality in patients with stage 5 CKD resulting from aHUS [73]. Kidney transplantation may be performed in patients with aHUS who have reached stage 5 CKD, have no clinical manifestations of extrarenal TMA, and no earlier than 12 months after the initiation of dialysis.

(Level of Evidence C; Grade of Recommendation 4)

Comment: the risk of aHUS recurrence immediately after transplantation is extremely high in patients with CFH mutations (~ 80%), CFI, and C3 (>50%). Transplantation has been performed in only 3 patients with CFB mutation, all of whom lost their grafts due to recurrent aHUS. Since the transplanted kidney does not contain mutated MCP/CD46 protein, the likelihood of aHUS recurrence with these mutations is very low. Graft loss from recurrent aHUS occurs in the majority of patients in less than 1 year.

Only in isolated patients has it been possible to prevent recurrent aHUS by performing sessions of renal replacement therapy before surgery and in the post-transplantation period. More optimistic prospects are associated with the use of eculizumab in the pre- and post-transplantation periods. There are reports of 13 patients with previous graft loss in whom the administration of eculizumab** a few hours before transplantation and within 24 hours after it, followed by transition to a standard treatment regimen, helped prevent rejection and TMA recurrence [73, 193, 194, 195].

(CGR C; LOR 5)

Comment: the donor and recipient must be fully informed about the development of possible transplantation complications. At the same time, if mutations responsible for the development of aHUS are found in neither the donor nor the potential recipient, living-related transplantation should be abandoned.

(CQR C; LE 5)

(CGR C; LOR 5)

Comment: a low risk of aHUS recurrence after kidney transplantation is present in the following cases: in patients with an isolated MCP/CD46 mutation; in patients with anti-CFH antibody-mediated aHUS with persistently zero titers of these antibodies and no additional complement regulatory gene mutations. Moderate recurrence risk is due to: isolated CFI mutations; mutations with unstudied effects; persistent low anti-CFH antibody titers and no additional complement regulatory gene mutations. Patients in whom mutations could not be identified also belong to the moderate recurrence risk category. High recurrence risk is determined in patients: with a history of early aHUS recurrence in the patient themselves or their relatives; with CFH mutations or the presence of a hybrid CFH/CFHR1-5 gene; with gain-of-function C3 and CFB mutations. In general, the risk is determined predominantly by the type of mutations, but partially depends on other peri-transplantation factors. Severe ischemia-reperfusion injury of the graft, high blood concentrations of calcineurin inhibitors (cyclosporine A and tacrolimus) in the recipient, acute graft rejection, especially antibody-mediated, viral infections (cytomegalovirus infection and other herpesvirus infections, influenza, BKV infection, parvovirus B19 infection, infectious gastroenteritis, etc.) may contribute to the development of recurrence [60].

(Letal Level of Evidence A; Level of Reliability of Evidence 2)

Comment: on the day of transplantation, at least one hour before reperfusion of the donor organ, an infusion of eculizumab** must be performed. The dose of the drug for children per administration depends on body weight. The second administration of the drug is carried out 24 hours after kidney transplantation. Subsequent administrations of eculizumab** are carried out once a week, switching to a once every 2 weeks regimen according to an individual protocol.

(Letal Level of Evidence B; Level of Reliability of Evidence 3)

(CGR C; LOR 4)

Comment: microscopic examination of the urine sediment should also be monitored. The frequency is determined individually.

(CGR C; LOR 5)

4. Medical Rehabilitation and Sanatorium-Resort Treatment

Specific rehabilitation methods do not exist. Rehabilitation measures are aimed at residual symptomatic manifestations (arterial hypertension, neurological disorders, etc.). Also, a number of patients with STEC-HUS and aHUS may develop CKD as an outcome of AKI, which requires rehabilitation methods provided for in the guidelines for CKD in children.

5. Prevention and Dispensary Observation

5.1 Prevention

5.1.1 Typical HUS

(CGR C; LOR 5)

Comment: numerous studies have shown that antibiotic therapy during the diarrheal period increases the risk of developing HUS due to the release of Stx resulting from bacterial lysis. However, this risk has not yet been proven. There is no consensus on whether it is necessary to prescribe antibiotics that do not cause bacterial lysis, such as macrolides (azithromycin**), to patients with STEC-positive HUS.

5.1.2 Atypical HUS

(CGR C; LOR 5)

Comment: patients receiving eculizumab**/ravulizumab should be vaccinated using inactivated polio, influenza, and type b Haemophilus influenzae vaccines, conjugated vaccines (pneumococcal, meningococcal), and vaccines containing an acellular pertussis component (DTaP). Combined vaccines are preferred. The use of live vaccines is not contraindicated [253]. For emergency indications (including in case of contact with a patient with measles or mumps), the use of immune sera and immunoglobulins (immune sera, specific immunoglobulins, normal human immunoglobulin) in accordance with Sanitary and Epidemiological Rules 3.3686-21 "Sanitary and epidemiological requirements for the prevention of infectious diseases" is possible. Vaccination is performed on a routine basis and is not a ground for delaying eculizumab**/ravulizumab therapy. In individual patients with aHUS, relapses have been noted after vaccinations. Nevertheless, the benefits of the latter significantly outweigh the risk. Vaccination should be carried out with caution, under the control of clinical and laboratory data.

(CGR C; LOR 5)

Comment: patients receiving eculizumab**/ravulizumab should be immunized against influenza once annually using inactivated vaccines that do not contain adjuvants. In the first year of eculizumab**/ravulizumab therapy, a two-dose vaccination schedule is recommended, with an interval of at least 1 month between doses. The same schedule applies to infants in their first year of life who have not been previously immunized against influenza.

Also, in clinical practice, eradication of chronic foci of adenoid, tonsillar, and dental infection is usually performed, since most aHUS episodes are triggered by infections; however, evidence-based studies on this intervention have not been conducted.

(CGR C; LOR 5)

Comment: immunostimulatory/immunomodulatory therapy (including herbal and synthetic immunomodulators, bacterial lysates) is contraindicated in patients with a confirmed diagnosis of aHUS due to the risk of relapse associated with immune system activation and complement system dysfunction.

5.2. Condition monitoring (patient management)

5.2.1 Typical HUS

(CGR C; LOR 5)

Comment: children who have suffered from HUS require long-term follow-up by a nephrologist (initial/follow-up nephrologist consultation (examination), dispensary nephrologist consultation (examination)), as primary kidney injury can lead to persistent proteinuria, reduced GFR, and hypertension.

Follow-up examination in the 1st year after HUS: at 1, 3, 6, and 12 months:

• initial/follow-up nephrologist consultation (examination), dispensary nephrologist consultation (examination),

• blood pressure measurement on peripheral arteries (age-appropriate normal value),

• complete blood count (CBC) with differential (including platelet count),

• routine urinalysis,

• determination of urine protein and urinary creatinine (in morning urine with calculation of the protein (g)/creatinine (g) ratio < 0.2 g/g) and/or determination of urine albumin and urinary creatinine (in a morning urine sample with calculation of the albumin (mg)/creatinine (g) ratio < 30 mg/g),

• serum creatinine test with GFR calculation (within reference ranges).

If blood pressure (age-appropriate normal), microalbuminuria, and eGFR remain stable – further examinations should be performed annually until the age of 18.

If arterial hypertension (AH) is detected – initial and follow-up pediatric cardiologist consultation (examination), with 24-hour ambulatory blood pressure monitoring (ABPM) or maintenance of a home BP diary by parents in infants/young children, initiation of antihypertensive therapy, and echocardiography control (according to the clinical guidelines "Arterial Hypertension in Children").

If chronic kidney disease (CKD) develops continue monitoring, nephroprotective therapy, and preparation for renal replacement therapy (RRT) (see CG "CKD" (pediatric age group)).

If persistent proteinuria/microalbuminuria persists 6 months after the acute phase of HUS consider nephroprotective therapy (see CG "CKD").

Repeat hospitalizations are indicated primarily for the progression of renal failure in accordance with the CG "CKD" (pediatric age group).

If CKD stage G5 develops and there are indications for RRT (kidney transplantation) perform molecular genetic testing to rule out involvement of complement system genes (aHUS panel) and prevent aHUS recurrence in the peritransplant period.

5.2.2 Atypical HUS

(CGR C; LOR 5)

Comment: considering the severity and prognosis of the disease, aHUS requires continuous monitoring by a nephrologist (initial/follow-up nephrologist consultation (examination), dispensary nephrologist consultation (examination)) with the involvement of multidisciplinary specialists as needed (initial/follow-up pediatrician consultation (examination) / initial/follow-up general practitioner (family physician) consultation (examination), initial/follow-up gastroenterologist consultation (examination), initial/follow-up neurologist consultation (examination), initial/follow-up pediatric cardiologist consultation (examination), initial/follow-up geneticist consultation (examination), initial/follow-up hematologist / pediatric oncologists-hematologists consultation (examination), initial/follow-up infectious disease specialist consultation (examination), initial/follow-up anesthesiologist-resuscitologist consultation (examination), initial/follow-up pediatric surgeon consultation (examination), pathologist consultation (examination), etc.).

Patients receiving regular infusions of eculizumab** every two weeks / ravulizumab every 4–8 weeks are generally followed up at the hospital where these infusions are administered.

Prior to the infusion of complement inhibitors (eculizumab**/ravulizumab), laboratory tests are performed:

• complete blood count (CBC) with differential (including blood platelet count),

• routine urinalysis,

• general general-clinical blood biochemistry (total blood protein test, total blood protein test, blood urea test, serum creatinine test, blood aspartate aminotransferase activity assay, blood alanine aminotransferase activity assay, blood glucose test, blood sodium test, blood potassium test, blood total calcium test, blood inorganic phosphorus test),

• determination of urine albumin (microalbumin in a single urine sample), eGFR.

If the patient's condition is stable, scheduled administration of eculizumab**/ravulizumab is possible in an inpatient/day-care setting. In the absence of eculizumab**/ravulizumab therapy, visits to a specialist (nephrologist (initial/follow-up nephrologist consultation (examination)/ dispensary nephrologist consultation (examination))) are indicated every 2–3 months, along with diagnostic tests for the early detection of a potential aHUS relapse and resumption of complement-blocking therapy:

• routine urinalysis,

• complete blood count (CBC) with differential (including blood platelet count),

• serum creatinine test,

• urine protein-to-creatinine ratio (urinary creatinine test, urine protein determination),

• blood LDH activity assay.

at intervals:

√ 1st schedule: 2 weeks after the last administration of eculizumab**, then weekly for 1 month, every 2 weeks for 2 months, monthly for 3 months, and thereafter every 3 months. 2nd schedule: after discontinuation of eculizumab**: laboratory tests are performed 2 weeks after the last administration of eculizumab**, then every 2 weeks for 1 month, monthly for 6 months, and thereafter every 3–4 months long-term.

Outpatients should also monitor their blood pressure (peripheral blood pressure measurement), proteinuria, and hemoglobinuria using test strips.

Additional laboratory tests may be performed in the event of an acute illness or surgical intervention.

Currently, there are no exact data on monitoring patients after ravulizumab discontinuation; given the similar mechanism of action, it is suggested to use the same approaches as for eculizumab** [240].

In case of recurrent TMA signs (anemia, thrombocytopenia, deterioration of renal function, and appearance of extrarenal symptoms), immediate hospitalization is indicated.

6. Organization of Medical Care

6.1 Indications for hospitalization

Indications for planned hospitalization in a medical organization for patients with STEC-HUS and aHUS:

1. provision of diagnostics and treatment requiring prolonged (day-care or round-the-clock stay) medical observation and monitoring of clinical and laboratory parameters;

2. need for various types of assessments or examinations in a medical organization when outpatient execution is impossible (including issuing a federal concilium/medical commission opinion, performing a renal biopsy if necessary);

3. initiation / scheduled re-administration of complement-blocking therapy in non-life-threatening variants of aHUS; playroom or

4. need to clarify the reasons for the ineffectiveness of complement-blocking therapy in a non-life-threatening course of aHUS.

Indications for emergency hospital admission of patients with STEC-HUS and aHUS:

1. development of severe organ- and life-threatening manifestations and complications of the disease requiring urgent treatment, including intensive care (RRT, mechanical ventilation, plasmapheresis, etc.), round-the-clock medical monitoring, and special diagnostic and treatment procedures;

2. necessity for the immediate initiation of complement-blocking therapy in patients with a first acute episode / relapse of aHUS.

Indications for patient discharge from a medical organization:

1. absence of life-threatening risks and risks of complications requiring urgent treatment upon completion of diagnostic procedures;

2. stabilization of the patient's condition and main clinical and laboratory parameters of the pathological process, no need for prolonged medical supervision (day or inpatient care);

3. the diagnostic and treatment plan has been completed, and recommendations for further follow-up and management have been provided;

4. necessity of transferring the patient to another medical facility or social care institution;

5. upon the written request of the patient's legal representative, provided that discharge does not threaten the patient's life and is not dangerous to others. In this case, discharge may be carried out only with the permission of the chief medical officer or the deputy chief medical officer for clinical work.

The organization of therapeutic nutrition for patients during inpatient treatment in medical organizations is carried out in accordance with Appendix 3 of the Order of the Russian Ministry of Health No. 1008n dated September 23, 2020, "On approval of the procedure for providing patients with therapeutic nutrition".

6.2 Principles of organizing medical care for patients with HUS/aHUS

The diagnosis of any form of HUS raises many questions for the patient's family and for specialists working with such patients. Numerous problems arise upon the detection and progression of the disease, the resolution of which requires competent organization of care involving a multidisciplinary team of specialists and adherence to the basic principles and approaches of its provision.

These principles must be observed at any stage of medical care, both at the time of diagnosis and at any stage of patient follow-up.

1) The patient and their representatives must receive complete information about the disease, its course, pathogenesis, prognosis, complications, and potential therapy methods.

2) The diagnosis of HUS/aHUS implies the possibility of providing all types, conditions, and forms of medical care stipulated by the legislation of the Russian Federation, depending on necessity.

3) Upon diagnosing HUS/aHUS, the family should be referred to a specialist experienced in working with such patients, possessing up-to-date information on the disease course and opportunities for participation in clinical trials (of new medicinal products and/or medical devices). It is also important to inform families about existing patient advocacy organizations operating for this patient group.

4) A multidisciplinary team of specialists must be involved in the follow-up of a patient with HUS/aHUS, consisting of: a pediatrician, nephrologist, infectious disease specialist, anesthesiologist-intensivist, neurologist, pediatric cardiologist, gastroenterologist, geneticist, nutritionist, ophthalmologist, physical therapist, and pathologist. Other specialists should be involved as specific problems arise.

Comment: the nephrologist provides primary follow-up care for patients with HUS. The duties of the nephrologist include examination and diagnosis, informing the family about the disease, drawing up a multidisciplinary follow-up plan, monitoring the patient, assessing the dynamics and degree of disease progression during regular examinations, and prescribing supportive therapy. When drawing up the multidisciplinary follow-up plan, the nephrologist determines the frequency of consultations with other specialists of the multidisciplinary team according to the patient's current condition. At each examination, the nephrologist reassesses the patient's current condition and makes adjustments to the multidisciplinary follow-up plan accordingly. In their recommendations, the nephrologist indicates the medicinal products and other measures needed by the patient in accordance with the identified patient needs.

The organization of therapeutic nutrition for patients during inpatient treatment in medical organizations is carried out in accordance with Appendix 3 of the Order of the Russian Ministry of Health No. 1008n dated September 23, 2020, «On approval of the procedure for providing patients with therapeutic nutrition».

7. Additional Information

7.1 Complications

• severe hemorrhagic colitis;

• toxic megacolon;

• intussusception;

• colon perforation or marked necrosis, secondary colonic stenosis;

• mesenteric thrombosis;

• necrotizing pancreatitis;

• transient or permanent insulin-dependent diabetes mellitus;

• heart failure resulting from hypervolemia and hypertension;

• myocardial ischemia with heart failure;

• arrhythmias;

• myocarditis;

• cardiac tamponade;

• stroke;

• partial optic atrophy;

• CKD stages G3–G5.

7.2 Outcomes and Prognosis

7.2.1 Typical HUS

Mortality, mainly resulting from CNS involvement, was 1–5% in the 2000s. In most cases, hemolytic anemia and thrombocytopenia resolve, and diuresis normalizes within less than 1–2 weeks. Despite this, on average within 4 months of follow-up, approximately 10% of children develop stage 5 CKD. In addition, 25% of children show residual renal abnormalities: a decrease in GFR to 70–80 mL/min/1.73m², hypertension, or proteinuria.

According to various studies on the long-term follow-up of children who have had typical HUS, up to 70% of patients recover [245, 255, 256]. Renal function recovery (eGFR) is spread over time: in 50% of cases by 1 year, in 75% by 2 years, and in the remaining 25%, maximum renal function recovery is noted after 5 years. Following the acute period of the disease, up to 7.3% of children require chronic dialysis, but renal function improves over several months, allowing RRT to be discontinued [255].

In addition to decreased eGFR, up to 23% of children following STEC-HUS exhibit significant proteinuria and up to 17% develop hypertension during a 10-year follow-up [108, 257]. In some patients, hypertension develops for the first time after 10 years of follow-up, which is particularly characteristic of children who experienced STEC-HUS under the age of 1 year [108]. Young age during the acute phase of typical HUS is an adverse prognostic factor, and an age of less than 1 year is characterized by a high risk of hypertension. In 36.2% of cases, patients without baseline renal pathology subsequently develop hypertension, proteinuria, and CKD 1 year later [257, 258]. The main prognostic factors influencing the long-term prognosis of the disease are the duration of anuria and the need for RRT during the acute period of STEC-HUS [245, 257].

Continuous clinical follow-up by a nephrologist helps improve the prognosis of patients who have undergone STEC-HUS. These patients require long-term monitoring, as CKD of varying severity develops in 20–25% of cases. The more time that passes since the onset of HUS, the higher the likelihood of CKD. Thus, after 12 years of follow-up, CKD is diagnosed in 51% of cases [255, 257]. The average follow-up time from the onset of STEC-HUS to the development of stage G5 CKD is 9.8 years [257]. Stage G5 CKD requiring RRT develops in 1.4–6% of cases after typical HUS [108, 245, 255, 258].

1. Risk factors for an unfavorable prognosis in typical HUS are:

• anuria >8 days,

• hospitalization >4 weeks,

• CNS involvement,

• marked leukocytosis (>20 thousand/µL),

• prolonged anemia (>30 days),

• ischemic colitis,

• arterial hypertension,

• proteinuria ≥1 year after resolution of the acute stage,

• cortical necrosis, involvement of >50% of glomeruli, arteriolar type of injury,

• renal tissue dysplasia,

• duration of RRT >4 weeks.

Comment: the most significant risk factor for long-term renal complications is the severity of renal injury during the acute episode. Severe AKI may be associated with long-term hypertension, proteinuria, decreased GFR, and neurological complications. Patients rarely recover renal function if the duration of dialysis exceeds 4 weeks. The development of proteinuria within 1 year may be associated with adverse future outcomes. Some patients who recover serum creatinine and eGFR levels but have persistent proteinuria are at risk of developing CKD, including stage G5 CKD, more than 5 years and sometimes 20 years after the acute illness [1, 20, 243, 244, 245].

Following an episode of typical HUS, all patients should be monitored for at least 5 years, and the duration of follow-up for patients who have had severe typical HUS cannot be time-limited in the presence of proteinuria, hypertension, or decreased eGFR.

Comment: if renal function remains preserved for 1 year, it will stay stable for another 1 to 5 years after HUS in the majority of children, and patients with persistent proteinuria, hypertension, abnormal ultrasound findings, and/or decreased eGFR within 1 year require further follow-up. Nevertheless, some authors suggest that 2 to 5 years may pass before any clinical manifestations appear, and therefore all patients with a history of tHUS should be monitored long-term for the timely detection of late-onset complications. Delayed proteinuria and/or renal function decline may be the result of hyperfiltration and subsequent sclerosis of the remaining glomeruli. This is indirectly supported by studies of the glomerular renal functional reserve, which showed that a protein load did not lead to the expected increase in eGFR in some patients who had fully recovered from HUS. However, the clinical relevance of these findings remains uncertain.

The presence of STEC infection in the prodrome of HUS does not always allow for a diagnosis of typical HUS [253]. In some patients, STEC infection can act as a trigger for aHUS; therefore, any child with a history of typical HUS requires clinical vigilance regarding the possibility of TMA recurrence, especially during intercurrent illnesses (acute respiratory infections, acute intestinal infections) as well as during vaccination. In the event of CKD stage 5 development and the need for renal replacement therapy (kidney transplantation) in a patient with a history of STEC-HUS, molecular genetic testing of the aHUS panel is indicated to rule out a complement-mediated disorder and to address the issue of peri-transplantation complement-blocking therapy.

7.2.2 Atypical HUS

Overall, the prognosis of aHUS is unfavorable. Mortality in the acute phase is 5–10%. Approximately 50% of patients develop end-stage renal disease (ESRD), most commonly within 1 year of disease onset. Extrenal manifestations, most frequently central nervous system involvement (seizures, coma, ischemic lesions on MRI), are observed in at least 20% of patients.

Relapses of aHUS occur in all variants, most frequently in patients with MCP/CD46 mutations. Triggering infections in this mutation are accompanied by acute hemolysis, thrombocytopenia, and AKI resulting from hemoglobinuria. In most of these cases, renal function completely recovers. The time interval between relapses can range from several weeks to many years.

The most favorable prognosis is observed with the MCP/CD46 mutation, and the most unfavorable – with CFH and combined mutations. In a French study, death or ESRD within less than 1 year from disease onset was observed in 60% of patients with the CFH mutation, 37% with the CFI mutation, 33% with the C3 mutation, 60% with combined mutations, 32% in the group with unknown etiology, and 0% with the MCP/CD46 mutation [73]. In patients with anti-CFH antibodies, the disease has a favorable course if plasma exchange is initiated early.

√ CKD stage 5 or death develops in 36% of children and 64% of adults within 3–5 years of disease onset.

√ The use of eculizumab** has changed the prognosis of overall and renal survival in aHUS, and has prevented the exclusion of kidney transplantation in many cases.

√ The prognosis of renal survival is determined by the timing of eculizumab** administration and the results of molecular genetic testing.

√ Early initiation of appropriate therapy is associated with better treatment outcomes and a greater increase in eGFR.

√ Eculizumab** treatment is equally effective in all genetic variants and in patients without identified variants [26, 73, 186, 202, 228, 230].

Given the relatively limited global experience with ravulizumab, such detailed and long-term data for this drug are lacking, but available studies indicate a favorable long-term efficacy and safety profile provided that indications, contraindications, and infection prophylaxis are observed [246].

Criteria for Evaluating the Quality of Medical Care

References

Appendix A1. Working Group Composition

Emirova Kh. M. – MD, Professor, Department of Pediatrics, Russian University of Medicine of the Russian Ministry of Health; Nephrologist at the Center for Blood Gravitational Surgery and Hemodialysis, Children's City Clinical Hospital St. Vladimir of the Moscow City Health Department, member of the Pediatric Nephrology Scientific Society (PNSS), Russian Dialysis Society (RDS).

Abaseeva T. Yu. – PhD, Associate Professor, Department of Pediatrics, Russian University of Medicine of the Russian Ministry of Health; Nephrologist at the Center for Blood Gravitational Surgery and Hemodialysis, Children's City Clinical Hospital St. Vladimir of the Moscow City Health Department, member of PNSS, RDS.

Albot V. V. – PhD, Assistant Professor, Department of Pediatrics, Irkutsk State Medical Academy of Postgraduate Education – branch of the Russian Medical Academy of Continuing Professional Education of the Russian Ministry of Health, Deputy Chief Physician for Medical Work, Irkutsk Regional Children's Clinical Hospital, Head of the Dialysis Department, Chief Pediatric External Expert in Nephrology and Dialysis of the Ministry of Health of the Irkutsk Region, member of PNSS, RDS.

Baranov A. A. – Academician of the Russian Academy of Sciences, Professor, MD; Honorary President of the Union of Pediatrists of Russia, Advisor to the Head of the Research Institute of Pediatrics and Child Health of the Central Clinical Hospital of the Russian Academy of Sciences, Professor, Department of Pediatrics and Pediatric Rheumatology, Sechenov First Moscow State Medical University of the Russian Ministry of Health (Sechenov University), Chief Freelance Pediatrician of the Russian Ministry of Health.

Vashakmadze N. D. – MD, Head of the Department of Orphan Diseases and Prevention of Disabling Conditions, Research Institute of Pediatrics and Child Health of the Central Clinical Hospital of the Russian Academy of Sciences, Professor, Department of Faculty Pediatrics, Pediatric Faculty, Pirogov Russian National Research Medical University of the Russian Ministry of Health.

Vishneva E. A. – MD, Deputy Head for Research, Research Institute of Pediatrics and Child Health of the Central Clinical Hospital of the Russian Academy of Sciences of the Ministry of Science and Higher Education, Associate Professor, Department of Faculty Pediatrics, Pediatric Faculty, Pirogov Russian National Research Medical University of the Russian Ministry of Health, member of the Union of Pediatrists of Russia.

Generalova G. A. – PhD, Associate Professor, Department of Pediatrics named after G.N. Speransky, Russian Medical Academy of Continuing Professional Education of the Russian Ministry of Health, Nephrologist at the Center for Blood Gravitational Surgery and Hemodialysis, Children's City Clinical Hospital St. Vladimir of the Moscow City Health Department, member of PNSS, RDS.

Zakharova E. Yu. – MD, Head of the Laboratory of Hereditary Metabolic Diseases, Research Centre for Medical Genetics named after Academician N.P. Bochkov, member of the Russian Society of Medical Geneticists, member of the Society for the Study of Inborn Errors of Metabolism (SSIEM).

Kalyuzhnaya T. A. – PhD, Leading Researcher, Department for the Development of Scientific Approaches to the Immunization of Patients with Health Status Abnormalities and Chronic Diseases, Pediatrician at the Research Institute of Pediatrics and Child Health of the Central Clinical Hospital of the Russian Academy of Sciences of the Ministry of Science and Higher Education of the Russian Federation, Associate Professor, Department of Faculty Pediatrics, Pediatric Faculty, Pirogov Russian National Research Medical University of the Russian Ministry of Health, member of the Union of Pediatrists of Russia.

Kvaratskhelia M. V. – PhD, Associate Professor, Russian University of Medicine of the Russian Ministry of Health, Anesthesiologist-Intensivist at the Center for Blood Gravitational Surgery and Hemodialysis, Children's City Clinical Hospital St. Vladimir of the Moscow City Health Department.

Kutsev S. I. – Corresponding Member of the Russian Academy of Sciences, Professor, MD, Director of the Research Centre for Medical Genetics named after Academician N.P. Bochkov, President of the Association of Medical Geneticists (AMG).

Lupan I. N. – Candidate of Medical Sciences, Associate Professor at the Department of Pediatrics and Neonatology, Institute of Additional Professional Education, South Ural State Medical University; Chief Freelance Pediatric Nephrologist of the Ministry of Health of the Chelyabinsk Region; Member of the TODN; Member of the Union of Pediatricians of Russia.

Namazova-Baranova L. S. – Academician of the RAS, Professor, Doctor of Medical Sciences, President of the Union of Pediatricians of Russia; Past President of EPA/UNEPSA; Head of the Research Institute of Pediatrics and Children's Health of the Central Clinical Hospital of the RAS; Head of the Department of Faculty Pediatrics, Faculty of Pediatrics, Pirogov Russian National Research Medical University of the Ministry of Health of Russia; Chief Freelance Pediatric Specialist in Preventive Medicine of the Ministry of Health of Russia.

Makarova T. P. – Doctor of Medical Sciences, Professor at the Department of Hospital Pediatrics, Kazan State Medical University; Member of the TODN; Member of the Board of the Tatarstan Regional Branch of the Union of Pediatricians of Russia.

Margieva T. V. – Candidate of Medical Sciences, Chief Specialist at the Methodological Center for Accreditation of Specialists, Sechenov First Moscow State Medical University of the Ministry of Health of Russia (Sechenov University); Nephrologist; Member of the Union of Pediatricians of Russia.

Mstislavskaya S. A. – Candidate of Medical Sciences, Associate Professor at the Department of Pediatrics, Russian University of Medicine of the Ministry of Health of Russia; Nephrologist at the Center for Gravitational Blood Surgery and Hemodialysis, St. Vladimir Children's City Clinical Hospital of the Moscow Healthcare Department.

Muzurov A. L. – Candidate of Medical Sciences, Associate Professor at the Department of Pediatric Anesthesiology, Intensive Care and Toxicology, Russian Medical Academy of Continuous Professional Education of the Ministry of Health of the Russian Federation; Anesthesiologist-Intensivist, Head of the Center for Gravitational Blood Surgery and Hemodialysis, St. Vladimir Children's City Clinical Hospital of the Moscow Healthcare Department.

Orlova O. M. – Candidate of Medical Sciences, Assistant at the Department of Pediatrics, Russian University of Medicine of the Ministry of Health of Russia; Nephrologist at the Center for Gravitational Blood Surgery and Hemodialysis, St. Vladimir Children's City Clinical Hospital of the Moscow Healthcare Department.

Pankratenko T. E. – Candidate of Medical Sciences, Associate Professor at the Department of Transplantology, Nephrology and Artificial Organs, Moscow Regional Research Clinical Institute named after M. F. Vladimirsky; Nephrologist at the Center for Gravitational Blood Surgery and Hemodialysis, St. Vladimir Children's City Clinical Hospital of the Moscow Healthcare Department; Member of the Russian Dialysis Society (RDS).

Petrosyan E. K. – Doctor of Medical Sciences, Professor at the Department of Hospital Pediatrics named after Academician V. A. Tabolin, Faculty of Pediatrics, Pirogov Russian National Research Medical University of the Ministry of Health of Russia; Head of the Nephrology Department, Russian Children's Clinical Hospital.

Prikhodina L. S. – Doctor of Medical Sciences, Head of the Department of Hereditary and Acquired Kidney Diseases named after Professor M. S. Ignatova, Research Institute of Pediatrics and Child Surgery named after Academician Yu. E. Veltiщеv, Pirogov Russian National Research Medical University of the Ministry of Health of Russia (Pirogov University); Professor at the Department of Pediatrics named after Academician G. N. Speransky, Russian Medical Academy of Continuous Professional Education of the Ministry of Health of Russia; President of the Creative Association of Nephrologists.

Popa A. V. – Candidate of Medical Sciences, Associate Professor atthe Department of Propaedeutics of Childhood Diseases, Institute of Maternity and Childhood, Pirogov Russian National Research Medical University of the Ministry of Health of Russia; Anesthesiologist-Intensivist at the Center for Gravitational Blood Surgery and Hemodialysis, St. Vladimir Children's City Clinical Hospital of the Moscow Healthcare Department.

Shilova M. M. – Candidate of Medical Sciences, Associate Professor at the Department of Faculty Pediatrics, Faculty of Pediatrics, Pirogov Russian National Research Medical University of the Ministry of Health of Russia.

Fedoseenko M. V. – Candidate of Medical Sciences, Head of the Department for the Development of Scientific Approaches to Immunization of Patients with Health Deviations and Chronic Diseases, Leading Researcher, Pediatrician at the Research Institute of Pediatrics and Children's Health of the Central Clinical Hospital of the RAS of the Ministry of Science and Higher Education of the Russian Federation; Associate Professor at the Department of Faculty Pediatrics, Faculty of Pediatrics, Pirogov Russian National Research Medical University of the Ministry of Health of Russia; Member of the Union of Pediatricians of Russia.

Tsygin A. N. – Doctor of Medical Sciences, Professor, Head of the Nephrology Department, National Medical Research Center for Children's Health; Professor at the Department of Nephrology, Scientific and Educational Institute of Clinical Medicine named after N. A. Semashko, Russian University of Medicine of the Ministry of Health of Russia.

Shavkin A. L. – Head of Dialysis Department No. 20, Anesthesiologist-Intensivist at the St. Petersburg City Children's Multidisciplinary Clinical Specialized Center for High Medical Technologies; Member of the Coordination Council of the RDS; Member of the European Renal Association.

The authors declare no financial support or conflicts of interest that need to be disclosed.

Appendix A2. Methodology for Developing Clinical Guidelines

Target audience for these clinical guidelines:

1. Nephrologists

2. Pediatricians

3. General practitioners (family physicians)

4. Dietitians

5. Geneticists

6. Anesthesiologists-intensivists

7. Pediatric oncologists-hematologists.

8. Gastroenterologists

9. Pediatric surgeons

10. Infectious disease specialists

11. Neurologists

12. Cardiologists;

13. Pathologists

14. Clinical pharmacologists

15. Medical students

16. Residents and postgraduate students

Methods used for evidence collection/selection:

- searching electronic databases.

Methods used for evidence analysis:

- reviews of published meta-analyses;

- systematic reviews with evidence tables.

Methods used to formulate the recommendations:

- Expert consensus;

- Assessment of significance according to the rating scheme.

The strength and quality of recommendations according to the conducted evidence analysis (Tables 5, 6, 7) are provided in the text.

Table 5. Scale for evaluating the levels of evidence (LE) for diagnostic methods (diagnostic interventions)

Table 6. Scale for evaluating the levels of evidence (LE) for prevention, treatment, and rehabilitation methods (preventive, therapeutic, and rehabilitative interventions)

Table 7. Scale for evaluating the grades of recommendation (GR) for prevention, diagnostic, treatment, and rehabilitation methods (preventive, diagnostic, therapeutic, and rehabilitative interventions)

Procedure for updating clinical guidelines.

The mechanism for updating clinical guidelines provides for their systematic revision at least once every three years, as well as upon the emergence of new evidence-based medicine data on the diagnosis, treatment, prevention, and rehabilitation of specific diseases, or in the presence of justified additions/comments to previously approved clinical guidelines, but no more frequently than once every 6 months.

Appendix A3. Reference Materials

1. Order of the Ministry of Health and Social Development of the Russian Federation No. 366n dated April 16, 2012, "On Approval of the Procedure for Providing Pediatric Care".

2. Information on medicinal products: State Register of Medicines: https://grls.rosminzdrav.ru

3. Federal Law No. 323-FZ of November 21, 2011, "On the Fundamentals of Protecting the Health of Citizens in the Russian Federation" (Collected Legislation of the Russian Federation, 2011, No. 48, Art. 6724).

4. International Statistical Classification of Diseases and Related Health Problems (ICD-10).

5. Order of the Ministry of Health of the Russian Federation No. 205n of May 2, 2023, "On Approval of the Nomenclature of Positions of Medical and Pharmaceutical Workers."

6. Order of the Ministry of Health of the Russian Federation No. 541n of July 23, 2010, Unified Qualification Handbook of Positions of Managers, Specialists, and Employees, Section: Qualification Characteristics of Positions of Healthcare Workers.

7. Federal Law No. 489-FZ dated December 25, 2018, "On Amendments to Article 40 of the Federal Law 'On Compulsory Medical Insurance in the Russian Federation' and the Federal Law 'The Fundamentals of Healthcare Protection in the Russian Federation' Regarding Clinical Guidelines."

8. Order of the Russian Ministry of Health No. 103n dated February 28, 2019, "On approval of the procedure and timelines for the development of clinical guidelines, their revision, the standard template of clinical guidelines, and requirements for their structure, composition, and scientific validity of the information included in the clinical guidelines."

9. Order of the Ministry of Health of the Russian Federation No. 804n dated October 13, 2017, "On Approval of the Nomenclature of Medical Services."

10. Order of the Russian Ministry of Health dated November 24, 2021, No. 1094n "On approval of the Procedure for prescribing medicinal products, prescription form templates for medicinal products, the Procedure for issuing these forms, their accounting and storage, prescription form templates containing prescriptions for narcotic drugs or psychotropic substances, the Procedure for their manufacture, distribution, registration, accounting and storage, as well as the Rules for issuing prescription forms, including in the form of electronic documents."

11. Medical contraindications to preventive vaccinations with preparations of the National Immunization Schedule: Guidelines. – M.: Federal Center for State Sanitary and Epidemiological Surveillance of the Ministry of Health of Russia, 2002. – 16 p.

12. "Methodological recommendations for the detection, investigation, and prevention of adverse events following immunization" (approved by the Ministry of Health of Russia on April 12, 2019)

13. Order of the Ministry of Health of the Russian Federation No. 1008n dated September 23, 2020, "On Approval of the Procedure for Providing Patients with Therapeutic Nutrition."

Examples of diagnoses:

Diarrhea-associated hemolytic uremic syndrome (STEC-HUS). Acute kidney injury. Anuric stage. Renal replacement therapy (peritoneal dialysis/hemodialysis).

Hemolytic uremic syndrome. Atypical variant (CFH gene mutation p.His272Tyr), relapsing course. Chronic kidney disease stage III. Complication of the main diagnosis: Secondary encephalopathy with convulsive syndrome. Secondary arterial hypertension. Cardiomyopathy.

Appendix A3.1 Differential diagnosis and treatment of TMA variants

Appendix A3.2 aHUS: genetic basis, clinical characteristics, and recurrence risks upon eculizumab** discontinuation and post-transplant recurrence [203]

Notes: ¹ESRD – end-stage renal disease; ²KTx – kidney transplantation.

Appendix A3.3. Complement cascade diagram and its role in the pathogenesis of aHUS

Appendix B. Physician Action Algorithms

Appendix B1 Management algorithm for a patient with typical hemolytic-uremic syndrome (STEC-HUS)/atypical hemolytic-uremic syndrome (aHUS)

Appendix B2 Diagnostic algorithms for typical HUS (STEC-HUS) in children

Appendix B3 Diagnostic algorithms for atypical HUS (aHUS) in children

Appendix C. Patient Information

HUS is one of the leading causes of AKI in children. It is characterized by a triad of signs: Coombs-negative hemolytic anemia with fragmented erythrocytes (schistocytes), thrombocytopenia, and AKI. These signs are components of TMA – widespread microvascular occlusion by thrombi resulting from endothelial injury. As a result of endothelial cell damage, mechanical injury to erythrocytes occurs, along with activation of platelet aggregation leading to microvascular thrombosis, particularly in the kidneys [23, 24].

In the majority (90–95%) of children, typical HUS develops, which is secondary to Shiga toxin-producing Escherichia coli (Shigatoxine; Stx) (Stx-producing E. coli, STEC) infection. Less common infectious triggers include Shigella and Streptococcus pneumoniae. This form of HUS does not relapse.

Another form of HUS, termed atypical (aHUS), is much rarer (5–10% of all cases) and results from abnormalities (usually genetic) in proteins regulating complement activation [6, 13, 20]. aHUS is a disease with a progressive course and an unfavorable prognosis under natural history. With modern therapy, the prognosis of the disease improves.

Appendix G. Assessment Scales, Questionnaires, and Other Evaluation Tools

Appendix G1 Diagnostic criteria for DIC syndrome [204]

Russian title: Diagnostic criteria for DIC syndrome

Source: Gando S., Levi M., Toh C.H. Disseminated intravascular coagulation. Nat. Rev. Dis. Primers. 2016; 2: 16037

Type: rating scale

Purpose: diagnosis of DIC syndrome

Content (template):

Key (interpretation): Summation of scores:

• ≥5 points – overt DIC syndrome;

• <5 points – DIC syndrome not detected, daily scoring should be continued.