Disorder of sulfur-containing amino acid metabolism (homocystinuria): clinical guidelines

List of Abbreviations

CbS – cystathionine beta-synthase enzyme

CBS – gene encoding cystathionine beta-synthase

GNMT – glycine-N-methyltransferase

IQ – intelligence quotient

MAT – methionine adenosyltransferase

MTHFR – methylenetetrahydrofolate reductase

tHcy – total homocysteine

fHcy – free homocysteine

MSUD – maple syrup urine disease

HUS – hemolytic uremic syndrome

ABSt – acid-base status

LC-MS/MS – tandem mass spectrometry method

CNS – central nervous system

Terms and Definitions

Subluxation (luxation) of the lens – subluxation (dislocation) of the lens

1. Brief information on the disease or condition

1.1 Definition of the disease or condition

Homocystinuria (HCU) is a heterogeneous group of inherited metabolic disorders classified as aminoacidopathies, caused by impaired metabolism of sulfur-containing amino acids, primarily methionine. Their common manifestation is an elevated level of total homocysteine (tHcy), while the amino acid profile varies significantly among different forms of HCU.

There are several forms of homocystinuria, of which classical HCU is frequent; it is caused by mutations in the CBS gene and is characterized by connective tissue involvement signs (asthenic build, chest deformity, kyphoscoliosis, lens subluxation, myopia, hernias, etc.), thrombosis, intellectual disability, and psychiatric symptoms. Other forms of homocystinuria are also known, which manifest as nervous system pathology with a variable age of onset and/or involvement of other systems [1–4].

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

Several forms of the disease are distinguished depending on the primary biochemical and molecular genetic defect.

Impairment of the transsulfuration pathway (classical homocystinuria: B₆-dependent form of homocystinuria or B₆-resistant form) or disruptions in the methionine remethylation process.

Homocysteine is converted to methionine in a process known as remethylation, involving vitamin B₁₂. Subsequently, methionine can be cleaved to S-adenosylmethionine, which is converted back to homocysteine. In a reaction called transsulfuration, homocysteine is converted to cysteine, catalyzed by pyridoxal-5'-phosphate (PLP) and cystathionine beta-synthase. An elevated tHcy level suggests a defect in one of the components of this reaction.

Homocystinuria type I, or the classical form of the disease, is caused by a deficiency of the cystathionine beta-synthase (CbS) enzyme involved in the conversion of methionine to cysteine, and manifests in the presence of homozygous or compound heterozygous mutations in the CBS gene located on the long arm of chromosome 21 (21q22). The inheritance pattern is autosomal recessive [4].

In classical HCU, due to cystathionine beta-synthase deficiency, the conversion cycle of the sulfur-containing amino acid methionine is impaired.

The accumulation of tHcy leads to the activation of Hageman factor, promoting the thrombosis process. Furthermore, hyperhomocysteinemia causes endothelial dysfunction due to reduced bioavailability of the endogenous vasodilator nitric oxide and oxidative stress. These changes impair the stability of the arterial wall and cause intra-arterial thrombosis, longitudinal tears of the arterial wall with blood penetration from the arterial lumen into its wall, forming an intramural hematoma and arteriopathy, thereby increasing the predisposition to strokes [4–6].

There is also evidence indicating altered connective tissue metabolism in patients with HCU. Increased homocysteine concentration leads to impaired cross-linking of sulfhydryl groups in proteins such as elastin, which explains lens dislocation and skeletal abnormalities [7,8].

The cystathionine beta-synthase (CbS) enzyme is a pyridoxine (vitamin B₆)-dependent enzyme; therefore, classical HCU is divided into B₆-responsive and B₆-resistant forms of the disease. Impairment in the activity of the next enzyme in this metabolic chain—cystathionase—is also possible (Appendix A3.3),

Other forms of HCU are associated with genetically determined defects in methionine remethylation arising from impaired activity of methionine synthase reductase, which catalyzes the remethylation of homocysteine to methionine, certain defects in cobalamin synthesis, and impaired activity of the N(5,10)-methylenetetrahydrofolate reductase (MTHFR) enzyme. These forms are accompanied not by an increase, but by a decrease in blood methionine concentration [1].

Certain genetic disorders of intracellular cobalamin (cbl) transport and processing (cblC, cblD, cblF, and cblJ) cause impaired synthesis not only of methylcobalamin, but also of adenosylcobalamin, a cofactor of methylmalonyl-CoA mutase. These combined remethylation disorders are associated with elevated levels of tHcy and methylmalonic acid (MMA) [9].

These clinical guidelines discuss in detail classical homocystinuria and the variant associated with MTHFR deficiency, while forms of methylmalonic aciduria with homocystinuria are discussed in more detail in the clinical guidelines for methylmalonic aciduria, as their therapeutic approaches differ from those for HCU.

1.3 Epidemiology of the Disease or Condition

The average prevalence of classical HCU according to literature data is approximately 1:100,000–1:200,000, varying from 1:1,800 to 1:900,000 depending on ethnicity [10]. This disease is most prevalent in Qatar, with a carrier frequency of approximately 2% [11,12].

1.4 Features of Coding the Disease or Condition

E72.1 – Disorders of sulfur-containing amino acid metabolism.

1.5 Classification of the disease or condition

The following clinical and genetic forms of homocystinuria are distinguished:

• Homocystinuria caused by impaired transsulfuration (classical homocystinuria, subdivided into B₆-responsive and B₆-resistant);

• Homocystinuria caused by impaired remethylation.

The modern classification of various forms of homocystinuria is given in Appendix A3.4 [1,9].

1.6 Clinical presentation of the disease or condition

The clinical presentation of the disease ranges from mild to severe forms manifesting in the first year of life. Both _B6-responsive and _B6-resistant forms are distinguished [2, 4, 13–16].

In the _B6-responsive form, patients have a milder phenotype with a later age of onset [14, 17]. The disease follows the mildest course in patients who show a pronounced positive response already upon the administration of low doses of #pyridoxine**. Most of them are homozygous for the p.I278T substitution in the CBS gene [18].

Manifestation of the disease in the _B6-resistant form occurs earlier, and the clinical manifestations of the disease are more severe than in the _B6-responsive form.

In classical homocystinuria, lesions of the nervous, vascular, and skeletal systems, as well as the organs of vision, are observed.

Ocular involvement

A typical symptom of classical homocystinuria, which often appears in the second year of life, is myopia, which precedes the development of dislocation or ectopia of the lens. The disease involves the destruction of fibrillin-rich microfibrils of the ciliary zonule and the impairment of its structure and function. The degree of lens displacement varies from an unnoticeable subluxation to a complete dislocation.

The most common ocular finding in patients with homocystinuria is lens subluxation (ectopia lentis) (in more than 50% of untreated patients, usually manifesting by the age of 8 years). Iridodonesis (tremor of the iris) is a frequent sign of lens dislocation. Myopia associated with weakening of the lens leads to passive spherical deformation (spherophakia). Myopia becomes noticeable upon lens subluxation. Lens subluxation can be the first sign of the disease; a case of manifestation at the age of 4 weeks has been described in one patient. The timing of the appearance of this sign varies widely, but by approximately the 2nd year of life, the appearance of lens subluxation is noted in many patients. After this age, patients with the _B6-resistant form have a higher rate of lens dislocations than patients responding to #pyridoxine** therapy [7, 8, 19].

A frequent complication of lens ectopia is acute glaucoma caused by the displacement of the lens into the anterior chamber of the eye [20].

Thrombosis

The second most frequent manifestation in classical homocystinuria is vascular involvement. Venous thrombosis is more common than arterial thrombosis and can occur in any organ and system, including cerebral vessels, coronary arteries, and pulmonary arteries. Thromboses occur at any age. Nearly 50% of patients develop vascular complications, presented as thromboses, by the age of 29 years. In some cases, vascular lesions may be the only symptom of the disease, appearing in the 2nd–3rd decade of life (especially in patients with the most frequent homozygous missense mutation c.833T>C in the CBS gene) [21].

Skeletal system lesions

Skeletal system disorders are often represented by tall stature, dolichostenomelia, arachnodactyly, scoliosis, pectus carinatum or pectus excavatum, valgus deformity of the limbs, high-arched hard palate, pes cavus, and osteoporosis/osteopenia. Osteoporosis is observed in almost half of the patients by the second decade of life [5, 22] and is a severe complication affecting the skeletal system. In patients with the _B6-resistant form of the disease, osteoporosis develops at an earlier age than in _B6-dependent (responsive) forms.

Neurological and psychiatric manifestations

The most frequent clinical manifestation of central nervous system involvement is psychomotor retardation and mental retardation. Quite often, cognitive impairment may be the first symptom of the disease.

The intellectual abilities of patients can vary significantly depending on the age of therapy initiation. According to an international survey, IQ scores ranged from 10 to 138, with a median of approximately 64. Patients with the _B6-responsive form with early initiation of therapy have higher IQ values (median 78) compared to patients resistant to vitamin _B6 (median 56) [23].

In the second decade of life, patients frequently experience episodes of depression, anxiety, obsessive-compulsive disorders, and psychoses. Various behavioral disorders are characteristic of classical homocystinuria: aggression, alcoholism, drug addiction. Psychiatric problems are more common among patients with the _B6-resistant form and when patients refuse treatment and dietary therapy.

In individual patients, psychiatric disorders may be the sole symptom of the disease.

There are predominantly 4 categories of psychiatric disorders: personality disorders, behavioral disorders, transient depressions, and obsessive-compulsive disorders. There are even reports that schizophrenia is more common in patients with CBS deficiency [4, 25, 26].

In 21% of cases, untreated patients with classical homocystinuria experience epileptic seizures in early infancy, most frequently generalized tonic-clonic seizures. Other neurological disorders include various extrapyramidal disorders, in particular, dystonia and tremor. Focal neurological deficits correspond to the affected area during a prior acute cerebrovascular accident [4].

Other manifestations of classical homocystinuria

Cases of classical homocystinuria presenting with acute pancreatitis [26–31], chronic diarrhea, skin and hair hypopigmentation, and facial flushing have been described.

Remethylation disorders

Remethylation disorders do not present with any specific symptoms that would allow this pathology to be suspected at the clinical level. Diseases in this group are multisystemic, most commonly involving the nervous and hematopoietic systems, blood vessels, and kidneys. It is the combination of neurological and hematological symptoms, often accompanied by growth retardation, that should raise suspicion of a remethylation defect. Most patients exhibit mental, intellectual, and physical development delays, polyneuropathy, seizures, movement disorders, muscle tone abnormalities, and visual pathology. These may be accompanied by macrocytic anemia, atypical hemolytic uremic syndrome (HUS), or glomerulopathy, which are primarily associated with microangiopathy.

Homocystinuria due to severe methylenetetrahydrofolate reductase (MTHFR) deficiency

Homocystinuria due to severe methylenetetrahydrofolate reductase (MTHFR) deficiency manifests in the neonatal period with neurological symptoms similar to organic acidurias—depression of consciousness ranging up to somnolence and coma, generalized muscular hypotonia, and apnea spells. Feeding difficulties, poor weight gain, and microcephaly are also noted.

A later-onset form of the disease, manifesting in childhood and adulthood, has been described. Its clinical picture is more variable and is represented by psychomotor retardation and regression, behavioral disturbances, psychiatric symptoms, ataxia, polyneuropathy, and spasticity. Among extraneural disorders, thromboses are frequent. Megaloblastic and/or macrocytic anemia is usually absent.

Among the 24 adolescent/adult patients with MTHFR deficiency described in the literature, clinical manifestations included gait disturbance (96%, of central or peripheral motor origin), cognitive decline (74%), epileptic syndromes (50%), encephalopathy (30%), psychotic symptoms (17%), and thrombotic events (21%). 41% had a single neurological manifestation that could remain isolated for at least 3 years, which would delay diagnosis. Brain MRI revealed changes predominantly in the periventricular white matter in 71% of cases. All patients stabilized or improved after metabolic treatment [32]. Despite the fact that MTHFR deficiency in adolescence/adulthood is rare, it should be included in the differential diagnostic workup for various unexplained neuropsychiatric syndromes, epilepsy, and spastic paraparesis, since metabolic therapy has been shown to be highly effective in MTHFR deficiency.

The age of onset and the clinical picture of MTHFR deficiency correlate with the residual enzyme activity. A study involving 33 patients showed that the mean age of symptom onset was 1.25 months (median 21 years; range from 0.1 to 216 months). In 14 patients, the first symptoms were observed during the first month of life; and in another 11 patients, symptoms were observed by the 6th month of life. In the remaining five patients, symptoms manifested at the age of 2, 5, 11, 13, and 18 years [33].

Severe MTHFR deficiency is biochemically characterized by hyperhomocysteinemia, homocystinuria, elevated cystathionine, and low or low-normal blood methionine levels, in contrast to CBS deficiency, which presents with elevated methionine levels. In cultured fibroblasts, residual activity is less than 20% of the mean control value [34].

Clinical manifestations of combined remethylation disorders (cblC, cblD-MMA / HC, cblF, cblJ)

In remethylation disorders with methylmalonic aciduria (cblC, cblD-MMA / HC, cblF, cblJ), growth retardation and feeding difficulties in infancy are common. In most cases, the first symptoms manifest in the neonatal period or early childhood. In newborns, the disease may debut acutely with impaired consciousness (lethargy, coma); seizures and muscular hypotonia are also characteristic, while laboratory tests often reveal megaloblastic anemia, neutropenia, or pancytopenia. Nystagmus is noted in 50% of cases, against the background of retinopathy or optic atrophy [35,36]. Microangiopathic kidney or lung disease most commonly manifests in the early stages of the disease, but may also be present in older children and adults [37,38].

Children, adolescents, and adults exhibit acute or chronic behavioral or psychiatric manifestations, cognitive impairment, signs of peripheral neuropathy, and ataxia [37,39,40]. Optic disc pallor is frequently detected, but other ocular manifestations rarely occur in the late-onset form of the disease [36,41].

Clinical manifestations of combined remethylation disorders (cblD-HC, cblE, cblG)

The clinical manifestations of cblE and cblG deficiency are similar. The disease typically manifests in infancy, primarily with hematological disorders, muscular hypotonia, and acutely occurring neurological symptoms. One-third of patients experience impaired consciousness and seizures, and one-fourth have visual impairments. Older children and adults may exhibit signs reflecting spinal cord involvement (subacute combined degeneration of the spinal cord) or psychiatric symptoms [42]. Isolated macrocytic anemia without neurocognitive impairment has been reported in single cases [43].

Pregnancy in classical homocystinuria and homocystinuria due to severe methyleneenetetrahydrofolate reductase (MTHFR) deficiency

In HCU, both classical and due to severe methyleneenetetrahydrofolate reductase (MTHFR) deficiency, there is a high risk of thrombosis and thromboembolic disorders in mothers, both throughout pregnancy and in the postpartum period. These include multiple arterial thromboses, ileofemoral thrombophlebitis, and thrombosis of the superior sagittal sinus and cortical veins. There is evidence of preeclampsia developing in mothers with homocystinuria. However, the majority of pregnancies in mothers with classical homocystinuria proceeded without obstetric complications.

In maternal MTHFR deficiency, neurological damage in the child is possible [44].

Differential diagnosis

Differential diagnosis of classical HCU should be carried out within the group of hereditary connective tissue disorders with Marfan and Beals syndromes (congenital contractual arachnodactyly). All of these conditions are characterized by a marfanoid phenotype, dolichostenomelia, arachnodactyly, kyphoscoliosis, chest deformity, and mitral valve prolapse. A symptom frequently detected in Marfan syndrome and homocystinuria is ectopia lentis (lens subluxation).

Unlike Marfan syndrome, homocystinuria may present with normal height, skin and hair depigmentation, pronounced osteoporosis, and thrombosis. Beals syndrome is characterized by specific auricular deformities, congenital heart defects, and joint contractures, which are not found in homocystinuria.

Differential diagnosis should also be performed with a group of hereditary diseases accompanied by strokes: Fabry disease, MELAS syndrome, congenital disorders of glycosylation (Jakine disease), certain organic acidurias (methylmalonic, propionic, isovaleric, glutaric aciduria types 1 and 2), and aminoacidopathies (urea cycle disorders).

There are also other non-genetic causes of elevated tHcy, but in these conditions, tHcy levels range from 15-50 µmol/L, whereas in hereditary forms of HCU it is typically above 50 µmol/L. These include:

• Vitamin B₆ or B₁₂ deficiency;

• Folic acid deficiency;

• Hypothyroidism;

• Obesity;

• Diabetes;

• Use of certain medications (e.g., carbamazepine**, atorvastatin**, fenofibrate**, methotrexate**, phenytoin**, and nicotinic acid).

2. Diagnostics, medical indications and contraindications for diagnostic methods

The diagnostic criteria for classical HCU are: tHcy and fHcy in blood and urine, elevated or normal blood methionine levels, and the detection of mutations in the CBS gene.

For MTHFR, the diagnostic criteria are: high levels of tHcy and fHcy in blood and urine, decreased or normal methionine levels, normal blood methylmalonic acid levels, and the detection of mutations in the MTHFR gene.

The diagnostic algorithm for remethylation disorders is presented in Appendix B2.

2.1 Complaints and Anamnesis

Classical form of homocystinuria associated with mutations in the CBS gene

Thromboses occur predominantly in adolescent and young adult patients and are the main cause of myocardial infarction or stroke with the development of focal neurological symptoms.

The severity of clinical manifestations in the B₆-resistant form is more pronounced than in the B₆-dependent form. The intelligence of patients with classical HCU is frequently reduced: IQ (intelligence quotient) ranges from 32 to 85 units (normal 85-115 units). In children with the B₆-dependent form of homocystinuria, mental development may be normal.

Nervous system:

- Psychomotor development delay

- Mental retardation

- Paresis, paralysis

- Gait disturbances (ataxia, unsteadiness)

- Strokes

- Seizures

- Depression, psychosis

Cardiovascular system:

- Thrombosis, thromboembolism

- Infarctions, strokes

- Mitral valve prolapse

Skeletal system:

- Pectus carinatum or pectus excavatum

- Spinal curvature

- Osteoporosis

- Fractures

Visual system:

- Myopia

- Glaucoma

- Lens subluxation

- Cataract

- Retinal detachment

- Optic atrophy

Other disorders:

- Hernias

- Pancreatitis

Homocystinuria due to methylenetetrahydrofolate reductase (MTHFR) deficiency

Early manifestation

- Lethargy, apnea in the neonatal period

- Diffuse muscular hypotonia

- Refusal to eat, vomiting, poor weight gain

- Motor and psych Speech development delay

- Microcephaly

- Seizures

- Nystagmus

- Thrombocytopenia or pancytopenia, megaloblastic anemia

- Hemolytic uremic syndrome

- Cardiomyopathy

With late-onset presentation

- Behavioral disorder

- Cognitive deficit

- Psychological developmental regression

- Psychiatric symptoms

- Ataxia

- Spastic paraparesis/tetraparesis

- Extrapyramidal disorders

- Focal neurological symptoms as consequences of stroke

- Paresthesia, numbness, and tingling in the hands and feet

- Urinary incontinence

- Progressive limb weakness (legs > arms)

- Hemolytic uremic syndrome

- Thromboembolic events

- Recurrent venous thrombosis

- Pulmonary embolism

- Cerebrovascular events

2.2 Physical Examination

Classic form of homocystinuria associated with mutations in the CBS gene

- Phenotypic features: soft slightly wavy light brown hair, gentle flush on the cheeks, blue iris color

- Marfanoid phenotype (tall stature, asthenic body type, long thin limbs, arachnodactyly of hands and feet)

- Valgus knee deformity

- Kyphoscoliosis

- Pectus excavatum or pectus carinatum

- Lens subluxation

- Cataract

- Paresis, paralysis

- Gait disorders (ataxia, unsteadiness)

Homocystinuria due to methylenetetrahydrofolate reductase (MTHFR) deficiency

Early manifestation

- Microcephaly

- Feeding disorders

- Skin pallor

- Poor weight gain

- Altered mental status

- Apnea

- Diffuse muscular hypotonia

- Delayed motor, psychological, and speech development

- Seizures

- Nystagmus

With late-onset presentation

- Behavioral disorder

- Cognitive deficit

- Psychiatric disorders

- Ataxia

- Spastic paraparesis/tetraparesis

- Extrapyramidal disorders

- Focal neurological symptoms as consequences of stroke

- Progressive limb weakness (legs > arms)

- Paresthesia, numbness, and tingling in the hands and feet

2.3 Laboratory Diagnostics

Comments: for screening, a test card with dried blood spots is sent within the established timeframe to a specialized regional neonatal screening laboratory. Methionine is used as the primary marker in most neonatal screening programs. Data from retrospective cohorts indicate that the detection of hypermethioninemia in combination with elevated homocysteine levels allows for the timely diagnosis of classical homocystinuria. However, the sensitivity and specificity of methionine as a screening biomarker are low (PPV 5–15%), and both false-positive and false-negative results are possible. Newborns with suspected homocystinuria due to cystathionine beta-synthase deficiency (methionine level >80 µmol/L) should undergo retesting. If the retest is positive, the total homocysteine (tHcy) blood level must be determined. If the tHcy concentration exceeds the normal value (>11 µmol/L), the diagnosis of homocystinuria can be confirmed at the biochemical level. The family should be summoned for genetic counseling and therapy prescription, and blood samples should be taken for DNA diagnostics at a reference center.

Newborns with suspected homocystinuria due to methylenetetrahydrofolate reductase deficiency (methionine level less than 7 µmol/L) should undergo retesting. If the retest is positive, the total homocysteine (tHcy) blood level must be determined. If the tHcy concentration exceeds the normal value (>11 µmol/L), the diagnosis of homocystinuria can be confirmed at the biochemical level, but additional confirmation by DNA analysis methods is required.

Comments: classical homocystinuria is characterized by elevated blood serum methionine and elevated blood serum and urine homocysteine, alongside decreased blood serum and urine cystine (Appendix A3.5). Importantly, blood homocysteine concentration (study of homocysteine blood levels) must be determined in a patient who has not received #pyridoxine** (including as part of multivitamin preparations) for at least two weeks. There is evidence that some patients require a longer period of #pyridoxine** discontinuation: 1–2 months.

MTHFR deficiency is characterized by a high level of homocysteine, a decreased or normal level of methionine, and a normal level of methylmalonic acid. The tHcy level is typically >50, but in most cases >100 µmol/L in untreated patients with remethylation disorders [47–50].

Usually, tHcy is measured using chromatographic or immunoenzymatic methods. Chromatographic methods are more specific than immunoenzymatic assays and are typically coupled with tandem mass spectrometry (MS/MS), which allows simultaneous determination of other compounds of interest for the diagnosis and monitoring of these disorders, such as Met, cysteine [51], or potentially cystathionine [52,53].

Various collection tubes are suitable for drawing blood for tHcy testing (EDTA, heparinized, citrate, gel separator tubes). However, blood samples must be centrifuged within 1 hour or kept cold prior to centrifugation (<6 hours). After removing blood cells, tHcy remains stable for several weeks at +4 °C and for several years after freezing [51]. Free homocystine in plasma or urine measured by traditional ion-exchange chromatography may remain undetectable even in the presence of significantly elevated plasma tHcy [54,55]. Free homocysteine should also not be measured in place of total homocysteine.

Comments: confirmatory diagnosis can be performed using either direct automated sequencing or massively parallel sequencing (NGS). Identifying mutations in the CBS gene is important for confirming the diagnosis, providing effective genetic counseling, and predicting disease severity.

Comments: Genetic testing for MTHFR deficiency should be interpreted with caution, as there are numerous variants in this gene that are not associated with homocystinuria (HCU). Thus, the most studied thermolabile variant c.677C>T in MTHFR is not responsible for severe remethylation disorders and can only lead to a mild form of HCU in combination with severe pathogenic variants.

Comments: There are no data on various micronutrient deficiencies in classical HCU; however, they are believed to be similar to those in patients with phenylketonuria. Dietary correction is performed based on the results.

2.4 Instrumental Diagnostics

Comments: Tubular bones, knee joints, and the spine are most frequently examined (upper extremity radiography, lower extremity radiography, knee joint radiography, cervical spine radiography, cervicodorsal spine radiography, thoracic spine radiography, thoracolumbar spine radiography, lumbar spine radiography, lumbosacral spine radiography, spinal radiography, special studies and views, functional spinal radiography, vertical spinal radiography).

Comment: The study is performed on average once every 3–5 years starting from adolescence (usually no more frequently than once every 2 years).

Comment: During follow-up, the examination is performed as indicated (upon the appearance of neurological symptoms).

If necessary, anesthesia is used for the examination.

Comments: The examinations should be performed at diagnosis and, if necessary, during follow-up, including the development of strokes and myocardial infarctions. The frequency is determined individually.

Comments: During follow-up – at least once a year.

Comments: During follow-up – at least once a year. According to the literature, it is preferable to perform video-EEG monitoring including sleep.

2.5 Other Procedures

2.5.1 Diagnostic test with #pyridoxine**

Comment: The purpose of the test is to differentiate two phenotypic variants of classical homocystinuria: B₆-responsive and B₆-resistant [73].

The test is not performed in a catabolic state. Prior to testing, on a normal diet without protein restriction and in the absence of folic acid and vitamin B₁₂ deficiency, blood levels of homocystine, homocysteine, and methionine are determined in duplicate; the patient is then given 10 mg/kg/day of #pyridoxine** orally (minimum dose 100 mg/day, maximum – 500 mg/day), and 1 week later, the concentration of these homocysteine metabolites is measured again. The response is considered positive if there is a decrease in the studied homocysteine markers to <50 µmol/L. If the response is negative, therapy is continued, homocysteine levels are re-evaluated after 2 weeks, and if still negative, once more after 6 weeks. If the blood homocysteine level decreases by more than 20% but remains >50 µmol/L, additional treatment must be considered (diet and/or specialized dietary medical nutrition product with betaine). If the blood homocysteine level decreases by less than 20%, the patient is considered a non-responder to #pyridoxine**, and the disease form is classified as B₆-resistant [5].

In newborns diagnosed via neonatal screening, when performing the #pyridoxine** test, it is better to use a relatively high dose of the drug (100 mg/day) for a shorter period (2 weeks) [5], however, there are reports that a gradual dose escalation is preferable: 10 mg/kg/day for the first 4 days, 20 mg/kg/day for the subsequent 4 days, and a maximum of 40 mg/kg/day from days 9 to 12 due to the potential manifestations of rhabdomyolysis and respiratory disorders [74].

The algorithm for determining #pyridoxine** sensitivity is presented in Appendix B3.

2.5.2 Specialist consultations, management, and follow-up

Comment: initial and follow-up visits (examinations, consultations) by a geneticist, ophthalmologist, neurologist, nutritionist, hematologist, pediatric hematologist-oncologist, cardiologist, pediatric cardiologist, orthopedic traumatologist, nephrologist, pediatrician/general practitioner (family physician), psychiatrist, medical psychologist, obstetrician-gynecologist, as well as other specialists are indicated for patients with homocystinuria who have dysfunctions of the corresponding organs and systems.

Anthropometry and peripheral blood pressure measurement should be performed at every visit.

Neurological status assessment should be performed at least once a year.

Assessment of intellectual development (e.g., IQ tests) should be performed at least once every 5 years throughout childhood.

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

3.1 Conservative Treatment

The treatment strategy for homocystinuria depends on the nature of the metabolic block causing the disease.

A fundamental point in the treatment of classical homocystinuria is the determination of sensitivity to #pyridoxine** (see subsection 2.5).

The treatment algorithm for newborns with homocystinuria is presented in Appendix B4.

3.1.1 Therapy with #pyridoxine**

Comment: for patients with B₆-responsive forms of homocystinuria, the dose of the drug is equal to the minimum amount of #pyridoxine** that produces the maximum biochemical effect, i.e., contributes to a significant decrease in blood homocysteine concentrations, which for most is about 200 mg/day, and in patients with p.P49L, p.I278T – 10–40 mg. For patients with a partial response to #pyridoxine**, #pyridoxine** is prescribed in higher doses, and additional therapy (betaine and/or diet) is also prescribed. It should be borne in mind that taking high doses of #pyridoxine** may have side effects such as peripheral neuropathy, rhabdomyolysis, or apnea; therefore, #pyridoxine** should be used only under medical supervision.

It is important that infants and preschool children should not receive more than 300 mg of #pyridoxine** per day for a prolonged period to avoid side effects, such as respiratory disorders, which disappear after discontinuation of #pyridoxine**.

Use #pyridoxine** doses above 200 mg/day with extreme caution in newborns and calculate the dose by body weight if possible [74].

#Pyridoxine** may also be prescribed to patients with the B6-resistant form, usually in doses of 100–200 mg per day (there is data on patients receiving 500–1000 mg per day) [4].

3.1.2 Dietary therapy

Comment: dietary therapy can be used either as the sole treatment or as adjunctive therapy together with #pyridoxine** and/or specialized medical nutrition products with betaine [46].

Dietary treatment consists of reducing methionine intake by restricting the consumption of natural protein foods. To prevent protein-energy malnutrition, specialized nutritional products based on synthetic amino acids without methionine, but enriched with the amino acid cysteine (which becomes essential for homocystinuria patients under conditions of CbS enzyme deficiency), are prescribed.

The amount of dietary methionine is calculated by a nutritionist and provided through natural plant-based foods and special low-protein products, monitored by determining the concentration of homocysteine and methionine in plasma.

When prescribing diet therapy to patients with homocystinuria, the focus should be on maintaining normal levels of methionine and cysteine and ensuring a minimum level of homocysteine in blood plasma. In patients who do not respond to #pyridoxine**, the target total homocysteine level should be no more than 100 µmol/L, because it is at this value that the risk of life-threatening thrombosis is minimal. The corresponding reference levels for these amino acids are given in Appendix A3.6.

The source of dietary methionine for infants in the first year of life is breast milk and/or an adapted infant formula for feeding children from birth. Starting from 6 months, in addition to this, fruit-, vegetable-, and grain-based (dairy-free) complementary foods are used; for patients older than one year, regular foods (vegetables, fruits, and in limited quantities, grains) are used. The source of additional amounts of cysteine in the patients' diet is a specialized product based on a mixture of crystalline L-amino acids that does not contain methionine. This same product is an additional source of total protein nitrogen and essential amino acids. The energy deficit of the diet can be covered in patients older than one year through the use of specialized low-protein products based on starch, fats, and added sugar.

When calculating the composition of the patients' diet, the approximate requirements for cysteine and methionine in homocystinuria patients of different ages should be taken into account (Appendices A3.7, A3.8). Specialized amino acid-based products without methionine registered in the Russian Federation and their chemical composition are presented in Appendix A3.7.

3.1.3 Other conservative treatment

Comment: Betaine (N,N,N-trimethylglycine) is formed in the body from choline and is present in small amounts in a standard diet. Betaine can also act as a chemical chaperone and correct partial misfolding of the CBS protein. Betaine reduces plasma homocysteine concentration by converting it into methionine, but increases plasma methionine concentration. A specialized dietary food product for medical purposes containing betaine is typically prescribed to children at an initial dose of 50 mg/kg twice daily, increasing by 50 mg/kg per week. Increasing the dose above 150–200 mg/kg/day is usually impractical. In children over 3 years of age and in adults, a dose of 3 g twice daily is used. In patients with B₆-resistant forms of homocystinuria, a decrease in blood plasma homocysteine levels has been observed with the adjunctive administration of a specialized dietary food product for medical purposes containing betaine. One of the potential side effects of a specialized dietary food product for medical purposes containing betaine is a "fishy odor" of urine and body odor. Reducing the dose usually eliminates the odor.

Comments: prescribing specialized medical nutritional products containing betaine before the onset of neurological symptoms improves disease outcomes and prevents severe manifestations.

Comment: #Folic acid** is also capable of optimizing the conversion of homocysteine to methionine, thereby helping to reduce plasma homocysteine concentrations.

Comment: Drugs from the group "Vitamin B12 (cyanocobalamin and its analogues)" are also capable of optimizing the conversion of homocysteine to methionine, thereby helping to reduce plasma homocysteine concentrations.

Comments: in accordance with guidelines for thrombophilia. Drugs from the group B01AC Platelet aggregation inhibitors excluding heparin (salicylic acid and its derivatives, platelet aggregation inhibitors excluding heparin, direct factor Xa inhibitors, platelet aggregation inhibitors (excluding heparin) in combinations) are usually prescribed.

Vitamin K antagonists may be considered for patients with a history of venous thrombosis.

Comments: in accordance with guidelines for osteoporosis and reduced bone mineral density.

Comments: patients receiving diet therapy during prolonged intercurrent illnesses may require more frequent monitoring of metabolic parameters.

Pediatric patients are more frequently affected by complications from intercurrent infections.

Comment: This recommendation has been in place since the 1970s, when this group of drugs contained higher concentrations of estrogens. The safety of modern medications in patients with classical homocystinuria has not been studied.

3.1.4 Management of pregnancy in patients with homocystinuria

During pregnancy, labor, and particularly in the postpartum period, the likelihood of developing thrombosis increases in female patients with classical homocystinuria and MTHFR deficiency.

Data indicate an increase in blood homocysteine concentrations despite ongoing treatment in female patients with classical homocystinuria during the postpartum period during uterine involution.

There are no studies on the optimal timing of initiation and duration of anticoagulant therapy in female patients with homocystinuria; therefore, general approaches for patients with thrombophilia are applied.

Comment: Currently, there are no studies on the optimal timing for initiating anticoagulant therapy or the method of administration; general thrombophilia guidelines should be followed.

In classical homocystinuria, heparin group drugs (low molecular weight heparins) or heparin sodium** (5,000 IU twice daily [92]) or a drug from the salicylic acid and its derivatives group at low doses are typically prescribed. In the postpartum period, warfarin** may be prescribed (at a dose sufficient to achieve a full anticoagulant effect for the treatment and prevention of thrombosis) [4, 5, 92].

In MTHFR deficiency, heparin group drugs (low molecular weight heparins) or a drug from the salicylic acid and its derivatives group are also prescribed [44, 79].

Comments: Patients with MTHFR deficiency may also require this test in accordance with thrombophilia guidelines.

Comments: Doses are selected individually and average 100–250 mg/day, with reported ranges from 50 mg/day to 200 mg three times daily.

Comment: Data exist regarding the safety of using specialized medical nutritional products containing betaine during pregnancy.

Comments: postpartum, methionine intake should be reduced to a level not exceeding that before pregnancy. At the same time, it is necessary to meet the protein requirements recommended by the WHO/FAO/UNU (World Health Organization / Food and Agriculture Organization of the United Nations / United Nations University) during lactation: an additional 20 g of protein per day, and after 6 months – an additional 12.5 g per day. Some of this should be administered as methionine-free formulas, under biochemical monitoring performed for homocystinuria (see section "Laboratory Diagnostic Tests").

Comments: a number of pregnant patients with classical homocystinuria may also exhibit a deficiency of other vitamins, as well as zinc and selenium [5, 80].

3.2 Surgical Treatment

The need for surgical treatment (single- or bilateral lens extraction) is determined by an ophthalmologist.

With appropriate anesthetic precautions and modern microsurgical techniques, the risks associated with the surgical treatment of ocular complications of homocystinuria are reduced. Surgical treatment should be considered, especially in cases of recurrent lens dislocation into the anterior chamber or pupillary block glaucoma. If a conservative, non-surgical approach is chosen, these patients must be carefully monitored for recurrent episodes of lens dislocation [19].

Comments: the use of regional anesthesia techniques (epidural, spinal) is accompanied by an increased risk of thromboembolic complications caused by sympathetic blockade and peripheral vascular stasis at the microcirculatory level.

The goals of anesthetic management should be:

• maintaining high cardiac output and adequate blood circulation time;

• prevention of thromboembolism;

• reduction of peripheral vascular resistance and improvement of peripheral perfusion;

• prevention of dehydration and hypoglycemia;

• rapid patient recovery and early mobilization.

In the preoperative period, it is necessary to perform:

1. Adequate treatment and dietary therapy to maintain low methionine levels;

2. When planning surgical interventions, mandatory measurement of serum homocysteine levels. These values should be <45–50 µmol/L, since with elevated levels, anesthesia and surgical interventions are contraindicated.

3. The patient must be under the supervision of a hematologist / pediatric oncologists-hematologist.

4. Monitoring of screening hemostasis parameters: prothrombin time, thrombin time, activated partial thromboplastin time, fibrinogen, D-dimer, fibrin monomer.

5. Adequate preoperative hydration (using infusion therapy at a volume of 1.5 of physiological requirement: 1:1 solution of 5% dextrose** and 0.9% sodium chloride** solution) with volume load monitoring by standard methods to avoid fluid overload. Prevention of hypercoagulation should preferably be carried out with heparin group drugs (low molecular weight heparins). However, there are currently no definitive recommendations on the use of these drugs in patients with homocystinuria prior to surgery. The need for their use should be determined individually. Compression stockings should also be prescribed to prevent thrombosis.

In the intraoperative period, it is advisable to perform the following measures:

1. Maintenance of cardiac function with high cardiac output.

2. Use of elastic bandages or stockings to prevent peripheral blood stasis.

3. Administration of 5% dextrose solutions^** mixed 1:1 with 0.9% sodium chloride solution until the patient is able to take fluids orally to prevent hypoglycemia (monitored to avoid hyperhydration).

4. Nitrous oxide** should be avoided, as its use may increase blood homocysteine levels due to inhibition of the enzyme methionine synthase.

5. The use of opioids (narcotic drugs) and anesthetics that cause respiratory depression is contraindicated.

In the postoperative period, the following should be performed:

1. Monitoring the patient's condition for 48–72 hours for the potential development of thromboembolic complications;

2. Subcutaneous administration of a heparin group drug (low molecular weight heparin) until the patient is able to get out of bed;

3. Monitoring blood glucose levels to prevent hypoglycemia;

4. Early mobilization, if possible;

5. The decision to continue antithrombotic agents (anticoagulants and antiplatelet agents) should be made jointly by the surgeon and the anesthesiologist-intensivist based on a bleeding risk assessment.

Possible complications during surgical interventions:

• Spontaneous thromboembolism: clinical manifestations depend on the affected organ. Serious complications may include: optic atrophy, hemiparesis, arterial hypertension due to renal infarction, partial epileptic seizures, and pulmonary embolism.

• Hypoglycemia – may develop due to impaired insulin secretion in patients with high serum methionine levels. This complication can be prevented by additional glucose administration during periods when the patient receives no food.

It is important to note that there are currently no official randomized-trial-based recommendations for general anesthesia in patients with homocystinuria. The recommendations presented above are based on the international experience of individual experts in pediatric anesthesiology and intensive care within the field of orphan diseases.

4. Medical Rehabilitation and Sanatorium-Resort Treatment

Rehabilitation measures are aimed at correcting visual impairments, bone deformities, cardiovascular disorders, and motor and neuropsychological development. Given the polymorphism of clinical manifestations and complications of the disease, the profile and scope of rehabilitation measures should depend on the form and degree of functional impairment, activity limitations, and health restrictions formulated in accordance with the International Classification of Functioning, Disability and Health for Children and Youth [82, 83].

Comments: techniques are used to increase postural stability and movement symmetry, aimed at minimizing joint deformities, facilitating communication, and promoting cognitive and social adaptation.

5. Prevention and Dispensary Observation

5.1 Prevention

Preventive measures include genetic counseling and prenatal diagnosis in informative families (if the proband's genotype is known).

Comments: given the autosomal recessive inheritance pattern of all forms of homocystinuria (HCU), the risk of having a child with this condition is 25% for each pregnancy. Prenatal diagnosis is performed using molecular genetic methods by analyzing DNA extracted from chorionic villus biopsy samples and/or amniotic fluid cells and/or fetal blood. Screening of the patient's parents is necessary to identify heterozygous carrier status, with subsequent use of the test results for prenatal diagnosis. Considering the availability of disease management options, all risks associated with prenatal diagnosis should be discussed with the family, and the benefits of early treatment initiation should be explained [5,9].

Comments: there is no data indicating that patients with HCU experience unusual reactions to vaccination.

5.2 Dispensary monitoring

In classic HCU, the frequency of testing depends on the severity of the disease (e.g., presence/absence of response to #pyridoxine** therapy), the treatment being administered, patient compliance, age, and a history of complications (e.g., thrombosis).

Comments: the complete blood count (CBC) with differential must be monitored at least once a year. Laboratory parameters (homocysteine and methionine in plasma) must be monitored at least once a year; plasma vitamin B12 and folic acid levels are determined as clinically indicated. In adult patients with a full response to #pyridoxine**, blood homocysteine levels are typically monitored every 6 months; children on dietary therapy with a resistant form (no response to #pyridoxine**) require more frequent testing. In patients with the B₆-responsive form of HCU, the target plasma tHcy level should be <50 µmol/L. These values are generally not achieved in patients with partial sensitivity to #pyridoxine**. Available publications indicate that patients with the B₆-resistant form of HCU should aim to maintain a tHcy level of <100 µmol/L, as this reduces the risk of thrombosis.

Methionine and blood amino acid levels as a result of treatment should be close to age-specific reference values. Inadequate protein intake causes physical developmental delay and acrodermatitis. Diet correction is performed based on the results. Pancreatitis is a rare complication of HCU; therefore, blood pancreatic alpha-amylase activity and urinary alpha-amylase activity should be assessed as clinically indicated.

Comments: these tests are performed annually.

Comments: creatinine clearance (glomerular filtration rate) should be calculated using the Schwartz formula (this service is not listed in the Nomenclature of Medical Services).

Comments: monitoring of actual nutrition (dietary chemical composition) to prevent deficiency states should be conducted at least once every 7–10 days in infants under one year of age, and once every 1–3 months in patients older than 1 year.

Comments: the frequency of examinations is determined individually.

Comments: the frequency of examinations is determined individually, but should be at least once a year.

Comments: intervals between visits should be determined individually, taking into account age, disease severity, metabolic stability, and adherence to dietary therapy. Following diagnosis, anthropometric parameters must be monitored for all patients (once a month under 1 year of age, once every 3 months from 1 to 3 years, once every 6 months from 3 to 7 years, and subsequently once a year); dietary management is monitored and adjusted with the same frequency.

Comments: specialist consultations are scheduled as clinically indicated. Parents and adult patients must be trained in the rules of therapy management.

6. Organization of Medical Care

6.1 Measures in case of emergency conditions

Emergency situations in homocystinuria requiring urgent medical care are caused by the onset of venous or arterial thrombosis, as well as acute psychiatric disorders.

Inpatient emergency care is required in cases of acute thromboembolism, the risk of which is very high during various surgical interventions and when plasma homocysteine levels exceed 50 µmol/L. When thromboembolic complications develop (myocardial infarction, stroke, etc.), medical care is provided according to the relevant clinical protocols.

7. Additional Information

The prognosis of the condition and the level of mental development of patients depends on many factors: the form of the disease (_B6-responsive or _B6-resistant) and the associated severity of the enzyme defect; the timing of initiation and adequacy of specialized treatment; and the effectiveness of thromboembolism prophylaxis. The prognosis of the disease is relatively favorable for life with early diagnosis and careful metabolic control.

Criteria for Evaluating the Quality of Medical Care

Quality assessment criteria for primary health care

Criteria for assessing the quality of specialized care

References

Appendix A1. Working Group Composition

1. Anisimova I.V. – PhD in Medicine, Medical Geneticist, Head of the Medical Care Organization Department, Academician N.P. Bochkov Research Centre for Medical Genetics, Member of the Association of Medical Geneticists.

2. Baranov A.A. – Academician of the RAS, Professor, MD; Honorary President of the Union of Pediatricians of Russia, Advisor to the Head of the Research Institute of Pediatrics and Child Health of the Central Clinical Hospital of the RAS, Chief freelance pediatric specialist of the Russian Ministry of Health.

3. Batysheva T.T. – Professor, MD, Chief freelance pediatric specialist in medical rehabilitation of the Ministry of Health of the Russian Federation, Chief freelance pediatric specialist in neurology of the Moscow Healthcare Department, Director of the Scientific and Practical Center for Pediatric Psychoneurology of the Moscow Healthcare Department, Honored Doctor of the RF, Head of the Department of Pediatric Neurology, Physical and Rehabilitative Medicine, Higher School of Medical Education of RUDN University.

4. Borovik T.E. – Professor, MD, Head of the Laboratory of Nutrition for Healthy and Sick Children, National Medical Research Center for Children's Health of the Russian Ministry of Health, Member of the Association of Medical Geneticists (AMG).

5. Bushueva T.V. – MD, Leading Researcher at the Laboratory of Nutrition for Healthy and Sick Children, National Medical Research Center for Children's Health of the Russian Ministry of Health, Member of the Association of Medical Geneticists (AMG).

6. Bykova O.V. – MD, Head of the Research Institute Department at the Scientific and Practical Center for Pediatric Psychoneurology of the Moscow Healthcare Department, Professor at the Department of Pediatric Neurology, Physical and Rehabilitative Medicine, Higher School of Medical Education of RUDN University.

7. Vashakmadze N.D. – MD, Department of Faculty Pediatrics, Pediatric Faculty, Pirogov Russian National Research Medical University, Research Institute of Pediatrics and Child Health of the Central Clinical Hospital of the Russian Ministry of Science and Higher Education.

8. Vishnyova E.A. – MD, Deputy Head for Scientific Work at the Research Institute of Pediatrics and Child Health of the Central Clinical Hospital of the Russian Ministry of Science and Higher Education, Professor at the Department of Faculty Pediatrics, Pediatric Faculty, Pirogov Russian National Research Medical University of the Russian Ministry of Health, Member of the Union of Pediatricians of Russia.

9. Voronin S.V. – Chief Medical Officer of the Research Centre for Medical Genetics, Chief freelance specialist in medical genetics of the Russian Ministry of Health in the Far Eastern Federal District, PhD in Medicine.

10. Globa O.V. – PhD in Medicine, Physician at the Department of Psychoneurology and Psychosomatic Pathology, National Medical Research Center for Children's Health of the Russian Ministry of Health, Member of the Association of Medical Geneticists.

11. Grebenkin D.I. – Medical Geneticist at the Department of Medical Genetics, National Medical Research Center for Children's Health of the Russian Ministry of Health, Member of the Association of Medical Geneticists (AMG).

12. Davydova Yu.I. – Medical Geneticist at the Department of Medical Genetics, National Medical Research Center for Children's Health of the Russian Ministry of Health, Member of the Association of Medical Geneticists (AMG).

13. Zhurkova N.V. – PhD in Medicine, Leading Researcher at the Department of Orphan Diseases and Prevention of Disabling Conditions, Medical Geneticist at the Research Institute of Pediatrics and Child Health of the Central Clinical Hospital of the RAS, Member of the Association of Medical Geneticists (AMG), Member of the Union of Pediatricians of Russia.

14. Zakharova E.Yu. – MD, Head of the Department of Molecular Mechanisms of Hereditary Metabolic Disorders, Academician N.P. Bochkov Research Centre for Medical Genetics, Member of the Russian Society of Medical Genetics, Member of the Society for the Study of Inborn Errors of Metabolism (SSIEM).

15. Zvonkova N.G. – PhD in Medicine, Senior Researcher at the Laboratory of Nutrition for Healthy and Sick Children, National Medical Research Center for Children's Health of the Russian Ministry of Health, Member of the Association of Medical Geneticists (AMG).

16. Karaseva M.S. – Pediatrician, Researcher at the Department of Standardization and Study of Evidence-Based Medicine Fundamentals, Research Institute of Pediatrics and Child Health of the Central Clinical Hospital of the RAS, Member of the Union of Pediatricians of Russia.

17. Kondakova O.B. – PhD in Medicine, Medical Geneticist, Head of the Department of Medical Genetics, National Medical Research Center for Children's Health of the Russian Ministry of Health, Member of the Association of Medical Geneticists (AMG).

18. Kuzenkova L.M. – Professor, MD, Head of the Center for Pediatric Psychoneurology, Head of the Department of Psychoneurology and Psychosomatic Pathology, National Medical Research Center for Children's Health of the Russian Ministry of Health, Member of the Association of Medical Geneticists (AMG).

19. Kutsev S.I. – Academician of the RAS, MD, Director of the Academician N.P. Bochkov Research Centre for Medical Genetics, Chief freelance specialist in medical genetics of the Russian Ministry of Health, Chairman of the Ethics Committee of the Russian Ministry of Health, President of the Association of Medical Geneticists of Russia.

20. Lobanova V.S. – Neurologist at the Department of Hereditary Metabolic Disorders, Morozov Children's City Clinical Hospital of the Moscow Healthcare Department, Member of the Society for the Study of Inborn Errors of Metabolism (SSIEM).

21. Mikhailova S.V. – MD, Head of Department, Russian Children's Clinical Hospital of the Ministry of Health of the Russian Federation, Member of the Russian Society of Medical Genetics.

22. Nazarenko L.P. – Professor, MD, Deputy Director for Scientific and Clinical Work, Head of the Laboratory of Hereditary Pathology, Research Institute of Medical Genetics, Tomsk National Research Medical Center of the RAS, Member of the Association of Medical Geneticists (AMG).

23. Namazova-Baranova L.S. – Academician of the RAS, Professor, MD, President of the Union of Pediatricians of Russia; Past-President of EPA/UNEPSA; Head of the Research Institute of Pediatrics and Child Health of the Central Clinical Hospital of the RAS, Head of the Department of Faculty Pediatrics, Pediatric Faculty, Pirogov Russian National Research Medical University of the Russian Ministry of Health, Chief freelance pediatric specialist in preventive medicine of the Russian Ministry of Health.

24. Nikolaeva E.A. – MD, Chief Researcher, Head of the Department of Clinical Genetics, Academician Yu.E. Veltischev Research Clinical Institute of Pediatrics, Pirogov Russian National Research Medical University of the Russian Ministry of Health, Member of the Association of Medical Geneticists (AMG).

25. Pak L.A. – MD, Neurologist, Head of the Center for Rare Diseases, National Medical Research Center for Children's Health of the Russian Ministry of Health.

26. Podkletnova T.V. – PhD in Medicine, Neurologist, Senior Researcher at the Laboratory of Nervous System Diseases in Children, National Medical Research Center for Children's Health of the Russian Ministry of Health.

27. Pushkov A.A. – PhD in Biology, Leading Researcher at the Laboratory of Molecular Genetics and Medical Genomics, National Medical Research Center for Children's Health of the Russian Ministry of Health, Member of the Russian Society of Medical Genetics.

28. Repina S.A. – Candidate of Medical Sciences, Medical Geneticist at the Research Centre for Medical Genetics (Bochkov Research Centre for Medical Genetics), Member of the Russian Society of Medical Geneticists, Member of the Association of Medical Geneticists (AMG).

29. Savostyanov K.V. – Doctor of Biological Sciences, Head of the Centre for Fundamental Research in Pediatrics, Head of the Laboratory of Molecular Genetics and Medical Genomics at the National Medical Research Center for Children's Health of the Russian Ministry of Health, Member of the Russian Society of Medical Geneticists.

30. Selimzyanova L.R. – Candidate of Medical Sciences, Leading Researcher at the Research Institute of Pediatrics and Children's Health at the Central Clinical Hospital of the Russian Academy of Sciences of the Ministry of Science and Higher Education of the Russian Federation, Associate Professor at the Department of Faculty Pediatrics, Pediatric Faculty, Pirogov Russian National Research Medical University of the Ministry of Health of Russia, Associate Professor at the Department of Pediatrics and Pediatric Rheumatology, Sechenov First Moscow State Medical University of the Ministry of Health of Russia (Sechenov University).

31. Sviridova V.V. – Medical Geneticist at the Department of Medical Care Organization, Junior Researcher at the Laboratory of Mutagenesis, Research Centre for Medical Genetics (Bochkov Research Centre for Medical Genetics).

32. Semyachkina A.N. – Doctor of Medical Sciences, Chief Researcher at the Veltischev Research and Clinical Institute of Pediatrics of the Pirogov Russian National Research Medical University of the Ministry of Health of Russia, Member of the Russian Society of Medical Geneticists.

33. Smirnova O.Ya. – Medical Geneticist, Senior Researcher at the Department of Standardization and Study of Evidence-Based Medicine Foundations, Research Institute of Pediatrics and Children's Health at the Central Clinical Hospital of the Russian Academy of Sciences.

34. Subbotin D.M. – Researcher at the Laboratory of Neurogenetics, Medical Geneticist at the Research Centre for Medical Genetics (Bochkov Research Centre for Medical Genetics), Member of the Association of Medical Geneticists.

35. Tepaev R.F. – Doctor of Medical Sciences, Head of the Intensive Care Unit at the National Medical Research Center for Children's Health of the Russian Ministry of Health, Associate Professor at the Department of Pediatrics and Pediatric Rheumatology, Sechenov First Moscow State Medical University of the Ministry of Health of Russia (Sechenov University).

36. Fisenko A.P. – Doctor of Medical Sciences, Professor, Honored Doctor of the Russian Federation, Director of the National Medical Research Center for Children's Health of the Russian Ministry of Health, Honored Doctor of the RF, Director of the National Medical Research Center for Children's Health.

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:

• Pediatricians

• general practitioners (family physicians)

• general practitioners (internal medicine)

• medical geneticists

• dietitians

• ophthalmologists

• cardiologists / pediatric cardiologists

• endocrinologists

• neurologists

• psychiatrists

• medical psychologists

• medical university students, residents, postgraduate students

Description of methods used for evidence collection/selection:

publications included in the Cochrane Library, EMBASE, and MEDLINE databases served as the evidence base for the guidelines. The search depth was 5 years.

Methods used to assess the quality and strength of evidence:

• Expert consensus;

• Assessment of significance in accordance with the rating scale.

Table 1. Scale for assessing levels of evidence (LE) for diagnostic methods (diagnostic interventions)

Methods used for evidence analysis:

• Reviews of published meta-analyses;

• Systematic reviews with evidence tables.

Description of methods used for evidence analysis:

When selecting publications as potential sources of evidence, the methodology used in each study is examined to ensure its validity. The result of this examination affects the level of evidence assigned to the publication, which in turn affects the strength of the recommendations derived from it.

The evaluation process is undoubtedly subject to potential subjectivity. To minimize potential errors, each study was assessed independently, i.e., by at least two independent members of the working group. Any discrepancies in evaluations were discussed by the full working group. If consensus could not be reached, an independent expert was involved.

Evidence tables:

Evidence tables were completed by the members of the working group.

Methods used to formulate the recommendations:

Expert consensus.

Table 2. Scale for assessing the levels of evidence (LE) for prevention, treatment, and medical rehabilitation methods, including those based on natural therapeutic factors (preventive, therapeutic, and rehabilitation interventions)

Table 3. Scale for assessing the grades of recommendation (GR) for prevention, diagnostic, treatment, and medical rehabilitation methods, including those based on natural therapeutic factors (preventive, diagnostic, therapeutic, and rehabilitation 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

Appendix A3.1 Main regulatory legal acts

1. Federal Law No. 323-FZ of November 21, 2011 (as amended on December 28, 2024) "On the Fundamentals of Protecting the Health of Citizens in the Russian Federation".

2. Decree of the Government of the Russian Federation No. 890 of July 30, 1994 "On State Support for the Development of the Medical Industry and Improvement of the Provision of Medications and Medical Devices to the Population and Healthcare Institutions" (with amendments and addenda).

3. Order of the Ministry of Health and Social Development of the Russian Federation No. 185 of March 22, 2006 "On Mass Screening of Newborns for Hereditary Diseases (along with the 'Regulations on the Organization of Mass Screening of Newborns for Hereditary Diseases', 'Recommendations for Blood Sampling during Mass Screening of Newborns for Hereditary Diseases').

4. Procedure for providing medical care to patients with congenital and (or) hereditary diseases (Order of the Russian Ministry of Health No. 274n of April 21, 2022).

5. Decree of the Government of the Russian Federation No. 403 of April 26, 2012 (as amended on June 5, 2020) "On the procedure for maintaining the Federal Register of Persons Suffering from Life-Threatening and Chronically Progressive Rare (Orphan) Diseases Leading to a Reduction in Citizens' Life Expectancy or Disability, and its Regional Segment."

6. Criteria for Assessing the Quality of Medical Care (Order of the Ministry of Health of the Russian Federation No. 203n of May 10, 2017, "On Approval of Criteria for Assessing the Quality of Medical Care").

7. Order of the Ministry of Health of the Russian Federation No. 834n of December 15, 2014 (as amended on April 18, 2024) "On the approval of unified forms of medical documentation used in medical organizations providing outpatient medical care, and procedures for filling them out."

8. Order of the Ministry of Health of the Russian Federation No. 1051n of November 12, 2021 "On the approval of the Procedure for giving informed voluntary consent to medical intervention and refusal of medical intervention, the form of informed voluntary consent to medical intervention and the form of refusal of medical intervention."

9. Order of the Ministry of Health of the Russian Federation No. 302n of May 16, 2019 (as amended on November 19, 2020) "On the approval of the Procedure for minors undergoing dispensary observation, including during the period of study and upbringing in educational organizations."

10. Order of the Government of the Russian Federation No. 3053-r of December 31, 2018 (as amended on February 20, 2024) "On the approval of lists of medical devices implanted into the human body when providing medical care within the framework of the state guarantee program for free provision of medical care to citizens, as well as the list of medical devices dispensed by prescription for medical devices when providing a set of social services."

Information on medicinal products: https://grls.rosminzdrav.ru/

Main regulatory legal acts governing the provision of palliative medical care

1. Federal Law No. 18-FZ of March 6, 2019 "On Amendments to the Federal Law 'On the Fundamentals of Protecting Citizens' Health in the Russian Federation' Regarding the Provision of Palliative Medical Care."

2. Order of the Russian Ministry of Health No. 345n, Russian Ministry of Labor No. 372n of May 31, 2019 "On the Approval of the Regulation on the Organization of Palliative Medical Care, Including the Procedure for Interaction between Medical Organizations, Social Service Organizations and Public Associations, and Other Non-Profit Organizations Operating in the Field of Health Protection."

3. Order of the Russian Ministry of Health No. 348n of May 31, 2019 (as amended on November 2, 2020) "On the Approval of the List of Medical Devices Intended to Maintain Human Body Organs and Systems, Provided for Home Use."

4. Order of the Russian Ministry of Health No. 505n of July 10, 2019 "On the Approval of the Procedure for Transferring Medical Devices Intended to Maintain Human Body Organs and Systems from a Medical Organization to a Patient (Their Legal Representative) for Home Use in the Provision of Palliative Medical Care." 

Other regulatory and legal documents used in the development of the clinical guidelines:

5. International Classification of Diseases, Injuries and Causes of Death (ICD-10);

6. Order of the Ministry of Health of the Russian Federation No. 205n of May 2, 2023 (as amended on December 4, 2023) "On the Approval of the Nomenclature of Positions of Medical Workers and Pharmaceutical Workers."

7. Order of the Ministry of Health of the Russian Federation No. 541n of July 23, 2010 (as amended on April 9, 2018) “On the Approval of the Unified Qualification Directory of Positions of Managers, Specialists, and Employees, Section 'Qualification Characteristics of Employee Positions in Healthcare'.”

8. Federal Law No. 489-FZ of December 25, 2018 (as amended on June 11, 2021) "On Amendments to Article 40 of the Federal Law 'On Compulsory Medical Insurance in the Russian Federation' and the Federal Law 'On the Fundamentals of Protecting Citizens' Health in the Russian Federation' Regarding Clinical Guidelines."

9. Order of the Russian Ministry of Health No. 804n of October 13, 2017 (as amended on September 24, 2020, with rev. of October 26, 2022) "On the Approval of the Nomenclature of Medical Services."

10. Order of the Ministry of Health and Social Development of the Russian Federation No. 494 of August 9, 2005 "On the Procedure for the Use of Medicines in Patients for Vital Indications."

11. Information Letter of the Russian Ministry of Health on the possibility of procuring a medicinal product by trade name (https://www.rosminzdrav.ru/news/2019/12/18/13043-minzdrav-podgotovil-informatsionnoe-pismo-po-vozmozhnosti-zakupki-lekarstvennogo-preparata-po-torgovomu-naimenovaniyu).

12. Order of the Government of the Russian Federation No. 3551-r of December 11, 2023 (as amended on January 23, 2025) "On the Approval of the List of Specialized Therapeutic Nutrition Products for Disabled Children."

Appendix A3.2. Explanation of notes

…^** – the drug is included in the list of vital and essential drugs (Order of the Government of the Russian Federation No. 2406-r of October 12, 2019 "On the Approval of the List of Vital and Essential Drugs for 2020, as well as Lists of Medicinal Products for Medical Use and the Minimum Assortment of Medicinal Products Necessary for the Provision of Medical Care" (with amendments introduced by Order of the Government of the Russian Federation No. 3073-r of November 23, 2020 "On Amendments to Order of the Government of the Russian Federation No. 2406-r of October 12, 2019).

…^**** – a specialized therapeutic nutrition product included in the list of specialized therapeutic nutrition products for disabled children (List of specialized therapeutic nutrition products for disabled children for 2021, approved by Order of the Government of the Russian Federation No. 3242-r of December 17, 2020).

#... – off-label use (outside the indications registered in the drug instructions) is carried out by decision of the medical commission, with the permission of the Local Ethics Committee of the medical organization (if any), subject to the signed informed consent of the parents (legal representative) and the patient over the age of 15;

Appendix A3.3. Homocysteine metabolism

SAM: S-adenosylmethionine; SAH: S-adenosylhomocysteine; SAHH: S-adenosylhomocysteine hydrolase; CβS: cystathionine beta-synthase; CγL: cystathionine gamma-lyase; MTR: methionine synthase; THF: tetrahydrofolate; MTHFR: 5,10-methylene-THF reductase; SHMT: serine hydroxymethyltransferase; BHMT: betaine-homocysteine methyltransferase; MMACHC: methylmalonic aciduria and homocystinuria type C protein.

Appendix A3.4. Genetics and biochemical defect in various forms of homocystinuria

Genetics and biochemical defect in various forms of homocystinuria (according to [1])

Appendix A3.5 Main biochemical parameters for the diagnosis of homocystinuria

Appendix A3.6 Biochemical monitoring in HCU

Reference levels of free methionine, cysteine, and homocysteine in blood plasma (for monitoring classical HCU therapy).

Biochemical monitoring in HCU – target values.

Appendix A3.7 Dietary therapy in classical HCU

Table 7 - Estimated requirement for methionine and cysteine in patients with homocystinuria by age.

Table 8 - Specialized amino acid-based products without methionine/with betaine

Appendix A3.8 Examples of diet calculation in classical homocystinuria

Dietary calculation for a 2-week-old patient with homocystinuria, body weight 4 kg¹.

1. Total daily protein intake based on 2.5 g/kg/day × 4 kg = 10 g.

2. Permissible daily amount of methionine: 30 mg/kg × 4 kg = 120 mg/day.

3. Amount of natural protein from breast milk (BM) / infant formula (IF), considering that 1 g of BM/IF protein contains 30 mg of methionine: 120:30 = 4 g of natural protein from BM/IF.

4. Amount of protein from the specialized product: 10 - 4 = 6 g of protein.

5. Amount (in g) of specialized dry product for dietary (therapeutic) nutrition of infants under one year of age with homocystinuria, "Nutrigen 14-met"**** (100 g of dry product contains 14 g of protein): the patient requires 43 g of dry Nutrigen 14-met per day****.

   If using the specialized infant formula for dietary medical nutrition for infants from 0 to 12 months as a primary food and as a supplementary food for children up to 3 years old, "HCU Anamix Infant" (for B6-unresponsive homocystinuria)**** (100 g of dry product contains 13.1 g of protein): the patient requires 46 g of dry HCU Anamix Infant per day****.

   The requirement for natural protein is met by breast milk or infant formula. The protein content in 100 ml of breast milk/infant formula is approximately 1.2 g; accordingly, to meet the natural protein requirement (4 g), 330 ml of breast milk/infant formula will be required.

6. The specialized amino acid mixture and breast milk/infant formula are divided in equal amounts into 6 feedings: 339:6 = 55 ml of BM/IF.

   Dissolve 43:6 = 7.2 g of the specialized dry product for dietary (therapeutic) nutrition of infants under one year of age with homocystinuria, "Nutrigen 14-met"**** in 50 ml of water.

   For one feeding, the infant will receive 105 ml of nutrition (55 ml + 50 ml).

   If using the specialized infant formula for dietary medical nutrition for infants from 0 to 12 months as a primary food and as a supplementary food for children up to 3 years old, "HCU Anamix Infant" (for B6-unresponsive homocystinuria)****, take 7.6 g of dry product per feeding and dissolve it in 53 ml of water.

   For one feeding, the infant will receive 108-110 ml of nutrition (55 ml + 53 ml).

Dietary calculation for a 10-year-old patient with homocystinuria, body weight 27 kg¹.

1. Total daily protein intake based on 2 g/kg/day = 54 g.

2. Average permissible daily amount of methionine: 10 mg/kg = 270 mg.

3. Amount of natural protein: 270 : 30 = 9 g of natural protein.

4. Amount of protein from the specialized product: 54 - 9 = 45 g of protein.

5. Amount (in g) of specialized dry product for dietary (therapeutic) nutrition of children over one year of age with homocystinuria, "Nutrigen 70-met"**** (100 g of dry product contains 70 g of protein): the patient requires 64 g of dry Nutrigen 70-met per day.

   If using the specialized medical dietary nutrition product for children with the B6-unresponsive form of homocystinuria or hypermethioninemia, "XMET Homidon"**** (100 g of dry product contains 77 g of protein): the patient requires 58 g of dry "XMET Homidon" per day.

   Using tables of the amino acid composition of foods, a daily set of natural foods is compiled that will contain a total of 9 g of natural protein.

¹The given examples are estimates. In each case, monitoring of the prescribed diet therapy and its adjustment based on laboratory results are required [77].

Appendix B. Physician Action Algorithms

Appendix B1. Physician Action Algorithms

Appendix B2. Algorithm for diagnosing remethylation disorders

Appendix B3. Algorithm for determining sensitivity to #pyridoxine**

Appendix B4. Algorithm for the treatment of newborns with HCU

Source: [5]. Published unchanged, translation performed by the authors.

Appendix B5. Algorithm for expanded newborn screening for homocystinuria

Appendix C. Patient Information

Classic homocystinuria, cystathionine beta-synthase deficiency

What is homocystinuria?

Homocystinuria is a group of inherited disorders characterized by elevated levels of homocysteine in the blood. Homocysteine affects blood coagulation processes, the cardiovascular system, the nervous system, and connective tissue.

Homocystinuria can be caused by mutations in several different genes. All of these genes are responsible for the synthesis of enzymes that are involved in how our body utilizes and processes amino acids. The most common form of homocystinuria is cystathionine beta-synthase enzyme deficiency, which results from mutations in the CBS gene. Rarer causes of homocystinuria include mutations in the MTHFR, MTR, MTRR, and MMADHC genes. Other forms of homocystinuria are associated with impaired vitamin B12 formation and, in addition to homocystinuria, are accompanied by elevated methylmalonic acid.

The most common genetic form of homocystinuria is classic homocystinuria, which is caused by a deficiency of the enzyme cystathionine beta-synthase.

How does classic homocystinuria manifest?

Children with this condition appear normal at birth. The first non-specific symptoms may include psychomotor retardation, mental retardation, skeletal deformities (valgus knee deformity, kyphoscoliosis, pectus excavatum or carinatum), cardiovascular pathology, and frequent fractures (in older children).

Ocular pathology is characteristic: subluxation (luxation) of the lenses, frequently complicated by secondary glaucoma which often has a malignant course, myopia, optic nerve atrophy, cataracts, and retinal detachment.

Adolescent and young adult patients may experience thrombosis, which is the main cause of myocardial infarction or stroke with the development of focal neurological symptoms, occurring predominantly in adolescent patients.

Other possible symptoms include bone disease (scoliosis), mental health problems, and behavioral issues (e.g., attention deficit hyperactivity disorder).

How is classic homocystinuria inherited?

Homocystinuria is an inherited disease transmitted in an autosomal recessive manner. This means that the disease only manifests if both parents are carriers and, although unaffected themselves, transmit two mutated genes to the child. Most families with a child affected by this condition have not encountered such a problem before. The risk of having another affected child in a family that already has affected children is 25% for each pregnancy.

All families with homocystinuria must undergo medical genetic counseling and receive complete information from a geneticist regarding the risk of recurrence of this disease in the family, and discuss all disease inheritance issues with a specialist. In Russia, medical genetic clinics operate in almost every region.

How is classic homocystinuria diagnosed?

Based on clinical symptoms, a physician may suspect the disease and subsequently refer the patient for specialized laboratory testing. In homocystinuria, blood levels of free and total homocysteine, as well as the amino acid methionine, are elevated. Urine organic acid testing helps exclude forms of homocystinuria with methylmalonic aciduria. Subsequently, molecular genetic testing is used to identify mutations in the CBS gene. It is very important to perform this test, as in some cases the genotype can determine whether there will be sensitivity to vitamin B6 therapy or not.

How is classic homocystinuria treated?

Methionine is an essential amino acid (not synthesized in the body, but obtained from external sources); it is necessary for normal growth and protein synthesis in the body and therefore cannot be completely excluded from the diet. Reduction of methionine is achieved by restricting dietary protein to maintain blood methionine and homocysteine levels within treatment target ranges (these differ slightly from normal values). This requires avoiding high-protein foods, including meat, fish, eggs, cheese, and legumes, as they contain large amounts of methionine and are thus permitted in homocystinuria only in very small quantities. At the same time, to ensure the body receives a sufficient amount of other amino acids, special medical formulas are prescribed in addition to the low-protein diet, in which only the methionine content is restricted while other amino acids are present in adequate amounts. The main skills that parents and the patient must acquire are planning the diet in such a way that all amino acids and other nutrients are supplied in sufficient quantities while methionine levels are restricted.

It should be remembered that methionine tolerance (safe level) varies among patients. As a rule, it may range from 80–160 mg/day to keep total homocysteine (tHcy) <100 µmol/L. It is important to determine an individual's safe level of methionine together with physicians and based on laboratory analyses measuring methionine and homocysteine. To manage a person's methionine intake, either tables showing the methionine content of each food product are used. If methionine content is not available, a rough estimate is made based on the fact that 20 mg of methionine is contained in approximately 1 g of protein.

In addition to diet therapy, vitamins are prescribed, among which the most important is B6 (#pyridoxine). This vitamin is necessary for the normal functioning of the enzyme cystathionine beta-synthase (CBS). In approximately half of all cases of classic homocystinuria, sensitivity to vitamin B6 treatment is observed. For this, a special vitamin test is performed, and biochemical blood parameters are determined. If methionine and homocysteine levels decrease, a diagnosis of B6-responsive homocystinuria is established, and many of these patients do not need diet therapy, requiring only vitamin B6 intake. It is important to strictly follow the dosage prescribed by the physician to achieve the maximum treatment effect and prevent side effects. If there is no response to vitamin B6, doctors recommend following a strict diet and, as a rule, prescribe a specialized dietary therapeutic product with betaine. This compound is able to help "neutralize" homocysteine by triggering a special enzymatic reaction.

If surgery is necessary, it is mandatory to consult the attending physician, perform a blood homocysteine test, and take thrombosis prevention measures specified in these guidelines.

What monitoring should be performed for patients?

A multidisciplinary approach to the monitoring and treatment of patients with homocystinuria by specialists from various fields is necessary to ensure comprehensive therapy and its timely adjustment when needed. Patients with homocystinuria require regular examinations prescribed by a physician.

How do patients receive treatment in the Russian Federation?

Homocystinuria is among rare inherited metabolic disorders and is included in the list of rare (orphan) diseases, the treatment of which is funded by the regional budget. After diagnosis, the patient must be included in the regional segment of the registry for life-threatening rare (orphan) diseases to ensure future provision of necessary medical nutrition and medications. In different regions, different specialists are responsible for maintaining the registry, but most often they are medical geneticists, so after diagnosis, it is mandatory to contact the regional medical geneticist.

How can family and loved ones help?

Do not forget — the success of treatment also depends on the family. Recommendations must be followed, "lapses" in diet therapy must not be allowed under any circumstances, and attention should be paid to rehabilitation and scheduled examinations. All family members must know that the child needs a special diet, and close relatives must master the skills of diet calculation.

The parents of a patient with homocystinuria, and subsequently the patient themselves, must be trained in the rules of therapy management.

Homocystinuria associated with MTHFR deficiency

What is homocystinuria?

Homocystinuria is a group of inherited disorders characterized by elevated levels of homocysteine in the blood. Homocysteine affects blood coagulation processes, the cardiovascular system, the nervous system, and connective tissue.

Homocystinuria can be caused by mutations in several different genes. All of these genes are responsible for the synthesis of enzymes that are involved in how our body utilizes and processes amino acids. The most common form of homocystinuria is cystathionine beta-synthase enzyme deficiency, which results from mutations in the CBS gene. Rarer causes of homocystinuria include mutations in the MTHFR, MTR, MTRR, and MMADHC genes. Other forms of homocystinuria are associated with impaired vitamin B12 formation and, in addition to homocystinuria, are accompanied by elevated methylmalonic acid.

Homocystinuria associated with MTHFR deficiency is a genetic disorder caused by impaired function of the enzyme methylenetetrahydrofolate reductase. This enzyme is encoded by a gene also named MTHFR. MTHFR is important for a chemical reaction in the body involving vitamin B6 and folic acid.

How does homocystinuria manifest?

Homocystinuria is usually asymptomatic in newborns. In the absence of treatment, infants develop signs and symptoms of severe homocystinuria. Homocystinuria caused by MTHFR variants may be mild and manifest in later childhood or adulthood. Symptoms may include blood clotting disorders (increased thrombosis), developmental delay, seizures, ataxia, polyneuropathy, intellectual disability, and microcephaly.

How is MTHFR deficiency-related homocystinuria inherited?

Homocystinuria is an inherited disease transmitted in an autosomal recessive manner. This means that the disease only manifests if both parents are carriers and, although unaffected themselves, transmit two mutated genes to the child. Most families with a child affected by this condition have not encountered such a problem before. The risk of having another affected child in a family that already has affected children is 25% for each pregnancy.

All families with homocystinuria must undergo genetic counseling and receive comprehensive information from a geneticist regarding the risk of recurrence of this disorder in the family, and discuss all questions related to disease inheritance with a specialist. In Russia, medical genetic clinics operate in almost every region.

Homocystinuria due to methyleneenetetrahydrofolate reductase deficiency is associated with mutations in the MTHFR gene. Mutations in the MTHFR gene cause the enzyme to function abnormally, leading to the accumulation of homocysteine in the body.

How is MTHFR deficiency-related homocystinuria diagnosed?

Based on clinical symptoms, the physician may suspect the disease and subsequently refer the patient for specialized laboratory testing. In this form of homocystinuria, the blood levels of free and total homocysteine are elevated, while the level of the amino acid methionine is decreased. Sometimes the concentration of methionine may be within the normal range. Urine organic acid testing helps rule out forms of homocystinuria associated with methylmalonic aciduria. Subsequently, molecular genetic analysis identifies mutations in the MTHFR gene. It is very important to perform this analysis because in some cases the genotype can determine whether there will be sensitivity to vitamin B6 therapy or not.

How is MTHFR deficiency-related homocystinuria treated?

Patients with this form of homocystinuria do not require diet therapy. Treatment for MTHFR deficiency includes the intake of a specialized medical nutrition product containing betaine, as well as folic acid**, vitamins B6 and B12, and methionine. If the diagnosis is established early and treatment with betaine is initiated immediately, the therapeutic efficacy is very high. Treatment started later does not eliminate symptoms, but may improve the condition. Treatment is prescribed individually by a physician in each specific case. Strict adherence to the physician's prescriptions allows achieving the maximum therapeutic effect.

If surgical intervention is necessary, be sure to consult your attending physician, perform a blood homocysteine test, and take the thrombosis prevention measures specified in these guidelines.

In homocystinuria caused by severe MTHFR deficiency, there is a high risk of thrombosis and thromboembolic events in women both throughout pregnancy and in the postpartum period. These include multiple arterial thromboses, ileofemoral thrombophlebitis, and thrombosis of the superior sagittal sinus and cortical veins. There are reports of preeclampsia developing in mothers with homocystinuria. However, the majority of pregnancies in mothers with classical homocystinuria proceed without obstetric complications.

In maternal MTHFR deficiency, nervous system damage in the child is possible. Therefore, female patients with MTHFR deficiency receiving a specialized medical nutrition product with betaine must continue this therapy during pregnancy to maintain target blood homocysteine levels and, accordingly, prevent complications.

What monitoring should be performed for patients?

A multidisciplinary approach to the monitoring and treatment of patients with homocystinuria by specialists from various fields is necessary to ensure comprehensive therapy and its timely adjustment when needed. Patients with homocystinuria require regular examinations prescribed by a physician.

How do patients receive treatment in the Russian Federation?

Homocystinuria is among rare inherited metabolic disorders and is included in the list of rare (orphan) diseases, the treatment of which is funded by the regional budget. After diagnosis, the patient must be included in the regional segment of the registry for life-threatening rare (orphan) diseases to ensure future provision of necessary medical nutrition and medications. In different regions, different specialists are responsible for maintaining the registry, but most often they are medical geneticists, so after diagnosis, it is mandatory to contact the regional medical geneticist.

How can family and loved ones help?

Do not forget — the success of treatment also depends on the family. Recommendations must be followed, "lapses" in diet therapy must not be allowed under any circumstances, and attention should be paid to rehabilitation and scheduled examinations. All family members must know that the child needs a special diet, and close relatives must master the skills of diet calculation.

The parents of a patient with homocystinuria, and subsequently the patient themselves, must be trained in the rules of organizing therapy.

Appendix B1. Example of an "emergency card"

Guidelines for acute glaucoma attack.

An acute glaucoma attack (AGA) is a critical condition characterized by a sharp increase in intraocular pressure (IOP), decreased visual acuity, and severe pain due to sudden and complete closure of the anterior chamber angle (ACA). Without treatment, the condition threatens rapid and irreversible loss of visual function due to the development of glaucomatous optic atrophy.

The child complains of a significant deterioration in vision, the appearance of rainbow halos around light sources, severe pain in the affected eye, which may radiate along the branches of the trigeminal nerve (upper orbital margin, forehead, temple, zygomatic region), as well as nausea, vomiting, and bradycardia. Congestive injection (redness) of the eyeball, blepharospasm, and possible reactive eyelid edema are observed; the cornea loses its characteristic luster, smoothness, and mirror-like appearance, appearing semi-transparent and rough (compared to the healthy eye). – Uneven pupil dilation is visible with a sharp slowdown or absence of light reaction (compared to the healthy eye).

Help for an acute glaucoma attack must be provided immediately!

First of all, you need to call an ambulance!

The main goal is to lower intraocular pressure and normalize blood circulation in the eye to restore the nutrition of the retina and optic nerve.

Administer analgesics to relieve pain.

To reduce blood flow to the eyes, hot foot baths for an hour and mustard plasters to the calf muscles are prescribed. The patient must be hospitalized in the ophthalmology department of a hospital.

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

Not applicable.