Mitochondrial DNA depletion syndrome, TK2-related: clinical guidelines

List of Abbreviations

VC — vital capacity;

FVC — forced vital capacity;

CK — creatine kinase;

mtDNA — mitochondrial DNA;

NIV — non-invasive ventilation;

PEF — peak expiratory flow;

TK2 — thymidine kinase 2;

MDAs — medical devices for rehabilitation;

EMG — electromyography;

PaCO₂ — partial pressure of carbon dioxide in arterial blood;

PaO₂ — arterial oxygen partial pressure;

PtcCO₂ — transcutaneous carbon dioxide partial pressure.

Terms and Definitions

Mitochondrial myopathies — mitochondrial disorders primarily affecting the muscular system.

1. Brief information on the disease or condition

1.1 Definition of the disease or condition

Mitochondrial DNA depletion syndrome, TK2-related (TK2-related myopathy, OMIM #609560) is a clinically heterogeneous group of autosomal recessive disorders characterized by a myopathic phenotype with a variable age of onset [1, 2, 10].

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

The disease is inherited in an autosomal recessive manner. More than 40 pathogenic variants in the TK2 gene (16q21) have been identified [2, 10], the majority of which are missense variants (70%) leading to impaired function of the TK2 protein product [3].

The range of described phenotypes (from infantile to adult-onset forms) can likely be explained by varying levels of residual thymidine kinase activity [4].

Given the extremely rare occurrence of the disease, a clear genotype-phenotype correlation cannot be definitively established, although the following observations have been reported: the pathogenic variant p.Arg130Trp is likely associated with a more severe phenotype involving the central nervous system in the first months of life [5]; the p.Lys202del variant has been associated with the adult form of TK2 myopathy [2].

The enzyme thymidine kinase 2 (encoded by the nuclear TK2 gene) is the first enzyme in the mitochondrial deoxypyrimidine salvage pathway. The enzyme catalyzes the conversion of the nucleosides deoxycytidine and thymidine into nucleoside monophosphates, which are subsequently phosphorylated to form deoxynucleoside triphosphate. The latter are essential for maintaining the nucleotide pool used for mitochondrial DNA (mtDNA) replication. Depletion and multiple deletions of mitochondrial DNA lead to impaired function of mitochondrial respiratory chain complexes involved in oxidative phosphorylation [6, 7].

1.3 Epidemiology of the Disease or Condition

According to literature data, the prevalence is less than 1 in 10,000,000 individuals. Fewer than 200 patients have been described [3, 11, 12]. However, it is worth noting that the clinical presentation is relatively nonspecific, and patients may be followed under other diagnoses.

1.4 Features of Coding the Disease or Condition

According to ICD-10, the disease belongs to Class VI — Diseases of the nervous system, ICD-10: G71.3 Mitochondrial myopathy, not elsewhere classified

1.5 Classification of the disease or condition

Three main forms of TK2-related myopathy have been described [1]:

- infantile form;

- childhood (juvenile) form;

- late-onset (adult) form.

1.6 Clinical presentation of the disease or condition

1. Infantile form is characterized by an early acute/subacute onset (under 2 years of age) with rapid progression of clinical features and an early fatal outcome.

Common symptoms:

- diffuse muscular hypotonia, rapidly progressive muscle weakness (96%);

- elevated blood CK levels up to 5–30 times the upper limit of normal (95%);

- respiratory failure (91%), up to 89% of patients require respiratory support;

- regression of previously acquired motor skills (88%);

- hyporeflexia (79%);

- lactic acidosis (67%);

- facial diplegia, ptosis (10%).

Central nervous system involvement is noted in 25% of patients: 32% manifest clinical epileptic seizures (epilepsy), cognitive dysfunction (7%), lissencephaly (3%), with isolated cases of sensorineural hearing loss.

Overall, in addition to musculoskeletal symptoms, 33% of patients demonstrate manifestations involving other organ systems, including isolated cases of nephropathy, biventricular hypertrophy, cardiomyopathy, anemia, bilateral chylothorax, and multiple fractures.

2. Juvenile/childhood form (ages 2–18):

- proximal muscle weakness, Gowers' sign in up to 100%;

- regression of previously acquired skills (60%);

- respiratory failure requiring respiratory support (NIV / IMV) in 60%;

- facial diplegia, ptosis (30%);

- elevated blood CK levels up to 5–10 times the upper limit of normal.

A smaller number of patients (about 18%) demonstrate extra-musculoskeletal manifestations such as hearing loss, long QT syndrome, arrhythmias, multiple fractures, renal tubulopathy, and gynecomastia. Central nervous system involvement is present in only 10% of cases.

3. Adult form (late-onset, mean age of onset — 30 years) [8]:

- the first symptom is often slowly progressive proximal muscle weakness in the extremities and neck muscles;

- facial muscle involvement (facial diplegia) in more than 40% of cases;

- loss of the ability to walk independently up to 22%;

- ptosis, ophthalmoparesis;

- swallowing difficulties (dysphagia), weight loss;

- sensory axonal polyneuropathy;

- sensorineural hearing loss;

- respiratory failure requiring respiratory support.

2. Diagnostics, medical indications and contraindications for diagnostic methods

Please note that in accordance with the requirements for the development of clinical guidelines, each recommendation statement must indicate the strength of recommendations and the evidence base in accordance with the grading scales for the levels of evidentiary certainty (LEC) and the levels of recommendation persuasiveness (LRP). For many statements, the LRP and LEC will be low due to the lack of high-design clinical trials dedicated to them. Despite this, they are necessary elements of patient examination to establish a diagnosis and choose treatment tactics.

Criteria for establishing the diagnosis/condition.

The diagnosis of TK2-associated myopathy is established based on a combination of the following data:

1) anamnesis data;

2) physical examination;

3) instrumental studies;

4) laboratory tests;

5) molecular genetic testing.

Definite confirmation of the diagnosis is possible only through molecular genetic testing. It requires the detection of biallelic (in trans) pathogenic/likely pathogenic variants in the TK2 gene, the presence of which is confirmed by a reference method. For example, for single-nucleotide variants identified by next-generation sequencing methods, such a method is direct automated Sanger sequencing.

TK2 myopathy should be differentiated from the following nosologies [3]:

- childhood-onset myopathy-associated mitochondrial DNA depletion syndromes associated with DGUOK, AGK, DNA2, MGME1, POLG2, SLC25A4 genes;

- Pompe disease;

- Prader-Willi syndrome;

- 5q proximal spinal muscular atrophy and other forms of spinal muscular atrophy;

- congenital myopathies.

2.1 Complaints and Anamnesis

When collecting complaints and anamnesis, it is important to pay attention to the following symptoms (presentations vary depending on the type and age of disease onset):

- acute/subacute progressive muscle weakness;

- fatigue;

- regression of gross motor skills;

- pathological gait abnormalities;

- lumbar hyperlordosis;

- decreased or absent tendon reflexes;

- "impoverished" facial expression, ptosis;

- difficulty swallowing liquids and/or solid food, sensation of food getting stuck in the throat during swallowing, coughing and/or choking during or after meals;

- elevated creatine kinase (CK) levels greater than 5 times the upper limit of normal;

- elevated blood lactate levels;

- hypoventilation symptoms: elevated pCO₂ levels according to ABG > 45 mm Hg (decreased saturation < 94 % during sleep, elevated CO₂ according to transcutaneous capnography);

- recurrent pneumonia.

2.2 Physical Examination

Physical examination of patients with suspected TK2-related myopathy includes:

1. Assessment of anthropometric parameters: measurement of height, weight, head and chest circumferences, calculation of SD scores, weight-to-height ratio, and dynamic BMI to evaluate nutritional status.

   Patients with TK2-related myopathy often show alterations in nutritional status. Bedridden patients and those with dysphagia have an increased risk of malnutrition. After diagnosis, all patients should have their height and body weight monitored at least every 3–6 months for young children and annually for older children;

2. Evaluation of chewing and swallowing function should be performed to rule out dysphagia.;

3. Gait assessment should be performed (myopathic/waddling or steppage gait);

4. Assessment of muscle weakness: special attention should be paid to the neck extensor muscles;

5. Assessment of tendon reflexes: diminished or complete loss;

6. Positive Gowers' signs: use of myopathic maneuvers when rising from the floor; use of auxiliary myopathic maneuvers and neck muscle weakness when lifting the head from a supine position;

7. Examination of the musculoskeletal system: lumbar hyperlordosis, joint hypermobility (recurvatum of the knee/elbow joints), scoliosis, "winged" scapulae, chest deformity;

8. Respiratory system examination: increased respiratory rate (tachypnea possible), decreased breath sounds on auscultation, diminished cough reflex / history of previous pneumonia.

2.3 Laboratory Diagnostics

Early diagnosis of patients with TK2-related myopathy is the key to proper patient management, timely pathogenetic therapy, and rehabilitation to prevent complications and preserve the quality and duration of life.

Comments: elevated CK levels are not an obligatory, yet frequently encountered symptom (up to 95% of cases) in patients with infantile and juvenile forms of the disease. An elevation of blood CK levels up to 5–10 times the upper limit of normal is characteristic.

Comments: respiratory function must be monitored, as some patients develop respiratory failure. Evaluation of blood hydrogen ion concentration (pH), transcutaneous monitoring of partial pressure of oxygen, blood carbon dioxide levels, and blood buffer system levels during wakefulness and sleep is indicated.

Comments: lactic acidosis occurs in 67% of patients with the infantile form of the disease [1].

Comments: initial and follow-up consultations (examinations) with a medical geneticist are mandatory to determine the DNA diagnostics strategy and correctly interpret the obtained data. The diagnosis is confirmed only when biallelic pathogenic (likely pathogenic) variants in the TK2 gene are detected. Molecular genetic diagnostics can be performed using massively parallel sequencing methods (gene panel sequencing, clinical or whole exome sequencing, whole genome sequencing). Identification of biallelic variants of uncertain clinical significance, or only a single variant in the gene, does not confirm the diagnosis. All variants detected by massively parallel sequencing methods must be confirmed by reference methods (e.g., for single nucleotide variants, gene variants in a biological sample, other or unspecified, unclassified in other categories, must be determined using Sanger sequencing). Given the autosomal recessive inheritance pattern, the trans position of the variants must be confirmed.

2.4 Instrumental Diagnostics

The listed instrumental research methods are necessary for initial and differential diagnosis of the disease.

Comments: EMG is an additional examination method when TK2-associated myopathy is suspected. This method helps differentiate TK2-related myopathy from other neuromuscular disorders and determines the level of lesion (primary muscle, neurogenic, neuromuscular junction). It is important to remember that this study is an auxiliary functional test, and the result heavily depends on the correct choice of the muscle for diagnosis, as well as the experience of the specialist performing the procedure.

Comments: axial slices of the upper and lower extremity muscles in T1 and T2 STIR modes are required. In certain cases, the study helps in the differential diagnosis of the level of lesion, as well as with other primary myopathies that have a specific MRI lesion pattern. It is noted that the gluteus maximus, sartorius, semitendinosus, semimembranosus, and gastrocnemius muscles are most severely involved in the pathological process.

Comments: pulmonary function assessment is performed in all patients capable of understanding and correctly executing the physician's instructions (usually older than 5–6 years); evaluation of forced respiratory volumes may also be indicated).

Comments: for bedridden patients, at least once every 3 months.

Comments: for bedridden patients, at least once every 3 months.

Comments: obviously, after recording the electrocardiogram, interpretation, description, and analysis of the electrocardiographic data are required. These examinations are performed at least once a year, as well as prior to surgical interventions.

Considering the possible development of spinal deformities, patients are indicated for spinal computed tomography (one section) to assess spinal deformities (in certain cases, surgical intervention may be considered). Patients older than 5 years are indicated for X-ray densitometry once a year, as well as prior to orthopedic interventions to rule out osteoporosis.

2.5 Other Procedures

(Scales — Appendices G1-G5).

Comments: not applicable at the non-ambulatory stage of the disease.

Comments: to assess motor skills over time, evaluation using the following scales is necessary: CHOP INTEND scale (for patients under 2 years of age or unable to sit independently), Hammersmith Functional Motor Scale Expanded (HFMSE), and North Star Ambulatory Assessment.

Comments: during the initial consultation, the medical geneticist performs differential diagnosis of the condition and determines the DNA diagnostics strategy. A follow-up medical geneticist consultation is required upon receiving DNA diagnostic data to interpret the results. The diagnosis is confirmed by the detection of biallelic pathogenic (or likely pathogenic) variants in the TK2 gene. It is important to remember that DNA diagnostics cannot rule out the hereditary nature of the disease.

Comments: upon initial presentation and subsequently during follow-up, at least once a year. Some patients may have an increased risk of developing epilepsy.

Comments: at least once a year. Prevention of lower respiratory tract infections and timely detection and treatment of respiratory failure are essential. It is necessary to assess respiratory system functions, monitor pulmonary function testing parameters (mandatory in both supine and sitting positions), analyze acid-base balance and blood gases, perform spirometry (testing of forced respiratory volumes), and carry out cardiorespiratory monitoring (cardiopulmonary exercise testing) to assess for nocturnal hypoventilation, address the need for symptomatic therapy; in some cases, BiPAP therapy (non-invasive mechanical ventilation) is indicated.

Comments: nutritional disorders and the development of dysphagia are possible in patients. In some cases, symptomatic therapy may be indicated; if there is a high risk of aspiration, the placement of a nasogastric tube or gastrostomy should be considered.

Initial and follow-up consultation (examination) with a nutritionist every 3–6 months for infants and young children, and annually for other patients, for diet selection and adjustment, aiming to prevent both malnutrition and overweight.

Comments: Sensorineural hearing loss may be detected in some patients. Consultations are required annually or more frequently if hearing loss symptoms appear. The following investigations may be indicated: monitoring of auditory sensitivity thresholds, pure-tone audiometry in a free field, speech audiometry with bilateral free-field stimulation, speech audiometry with bimodal free-field stimulation, and electroaudiometry (promontory test).

Comments: Initial and follow-up consultations (examinations) are indicated with a geneticist, neurologist, pulmonologist, gastroenterologist, nutritionist, traumatologist-orthopedist, pediatrician, general practitioner (family physician), otorhinolaryngologist-audiologist, physiotherapist, and palliative care physician, as well as initial and follow-up consultations (testing) with a medical psychologist.

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

Treatment of TK2 myopathy includes both disease-modifying (pathogenetic) treatment and symptomatic therapy.

3.1 Conservative treatment. Pathogenetic therapy.

Currently, an ongoing phase 3b prospective, multicenter, open-label study is evaluating the efficacy and safety of MT1621 (an oral combination of pyrimidine nucleosides: thymidine and deoxycytidine [dC and dT] at a dosage of 400 mg/kg/day each) in children and adolescents with TK2-related myopathy (ClinicalTrials.gov ID NCT04581733).

Comments: Targeted pathogenetic oral nucleoside therapy (thymidine/deoxycytidine) restores mitochondrial function via the cytosolic nucleoside salvage pathway. With supplemental substrate administration (thymidine/deoxycytidine), phosphorylation occurs via cytoplasmic enzymes (thymidine kinase 1 and deoxycytidine kinase), ensuring a sufficient copy number of mtDNA [9]. Initiation of thymidine/deoxycytidine therapy with a gradual dose escalation regimen [7]:

50 mg/kg/day once daily per os for 3 days,

100 mg/kg/day twice daily per os for 3 days,

150 mg/kg/day in 2 divided doses per os for 3 days,

200 mg/kg/day in 2 divided doses per os for 5 days.

Further escalation of each nucleoside dose by 50 mg/kg every 5 days until reaching the target working dose of 400 mg/kg/day.

Side effects of pathogenetic nucleoside therapy [7]:

- dose-dependent diarrhea in up to 50% of patients;

- transient abdominal pain.

3.2 Conservative treatment. Symptomatic therapy.

Management of patients with TK2-related myopathy involves a multidisciplinary approach with the mandatory participation of neurologists (especially neuromuscular specialists), geneticists, cardiologists / pediatric cardiologists, pulmonologists, gastroenterologists, physical therapists, and other specialists with experience in treating this rare disease. If symptoms of target organ involvement are present, a consultation with the relevant specialist is minimally required.

3.3 Other Treatment

Dietary therapy.

Dietary therapy is necessary to correct nutritional deficiencies resulting from dysphagia.

Postural management during feeding.

Patients with TK2-related myopathy need appropriate positioning of the body, head, and arms to enable independent feeding, including the use of special devices, to improve the safety and efficiency of swallowing.

Food consistency.

It is advisable to modify food consistency to semi-solid for patients with TK2-associated myopathy suffering from dysphagia, in order to compensate for masticatory muscle weakness and reduce meal duration. Thicker liquids are safer regarding aspiration than more fluid ones. It is necessary to use denser nutritional formulas, jelly-like food forms, and liquid purees. Industrially produced maltodextrin-based liquid thickeners can be used.

Enteral nutrition formulas can be used in patients receiving tube and gastrostomy feeding to ensure a balanced intake of essential nutrients. Modern enteral nutrition formulas are balanced in their composition of essential nutrients—proteins, fats, carbohydrates—and contain vitamins, macro-, and micronutrients. They vary in caloric density (hypo-, normo-, and hypercaloric) and protein content. Enteral nutrition formulas are selected individually depending on the patient's needs.

Comments: calculation of dietary caloric content is required (correction of food caloric content, fluid intake, macro- and micronutrients). When formulating a diet for patients with TK2 myopathy, the need for calories (calculated via resting energy expenditure with activity level correction), macro- and micronutrients, and fluids is assessed. General standardized formulas are used for nutritional calculations.

Comments: the choice of intervention is determined by the attending physician; care for the nasogastric tube / gastrostomy, training in gastrostomy care, replacement of the gastrostomy tube, and the procedure of feeding a severely ill patient via a nasogastric tube / gastrostomy performed by medical staff may also be required.

Comments: pathogenetic therapy may also be prescribed by a geneticist.

Comments: pathogenetic therapy may also be prescribed by a neurologist.

Comments: nutritional disorders and the development of dysphagia are possible in patients. In some cases, symptomatic therapy may be indicated; if there is a high risk of aspiration, the placement of a nasogastric tube or gastrostomy should be considered. Initial and follow-up appointments (examinations, consultations) with a dietitian every 3–6 months for infants and young children, and annually for other patients, are necessary for dietary correction and selection, avoiding both deficiency and overweight.

Comments: patients with TK2-myopathy need physical activity appropriate to their condition and disease stage, supervised by parents / legal representatives and relevant medical personnel to ensure proper physical activity and prevent the occurrence of contractures.

Comments: at least once a year. Prevention of lower respiratory tract infections and timely detection and treatment of respiratory failure are important. It is necessary to assess respiratory function, monitor pulmonary function tests (mandatory in both supine and sitting positions), test acid-base balance and blood gases, perform spirometry (evaluation of forced respiratory volumes) and cardiorespiratory monitoring (cardiopulmonary exercise testing) to assess for nocturnal hypoventilation, decide on the need for symptomatic therapy; in some cases, BiPAP therapy (non-invasive mechanical ventilation) is indicated.

Respiratory support.

Due to the presence of myopathic syndrome in the clinical picture, TK2-associated myopathy can be classified among muscular dystrophies. Therefore, it is rational to use respiratory management approaches similar to those used in neuromuscular diseases.

Assessment and support of respiratory function should be a high priority in the management of a patient with TK2-associated myopathy, ensuring that airway clearance and ventilation measures are initiated only when reliable indications are present. In the earlier stages of the disease, a preventive approach with stepwise implementation of these therapies is necessary, starting with pulmonary secretion management.

The treatment plan abroad is determined by a respiratory therapist, whereas in domestic practice this involves a fairly wide range of specialists—anesthesiologist-resuscitators, pulmonologists, neurologists, and pediatricians experienced in muscle diseases.

Key respiratory problems include the following: impaired coughing leading to inadequate lower respiratory tract clearance of secretions, nocturnal hypoventilation, chest and lung hypoplasia, and recurrent infections that exacerbate muscle weakness. The main ways to address these issues are airway clearance and non-invasive positive pressure ventilation (NIV).

Comments: NIV is initiated in the presence of hypoventilation symptoms and at least one of the following findings:

- 24-hour PaCO₂ level ≥ 45 mm Hg;

- nocturnal PaCO₂ level ≥ 45 mm Hg;

- nocturnal PtcCO₂ level ≥ 50 mm Hg for more than 30 min.;

- daytime normocapnia with a nocturnal increase in PtcCO₂ ≥ 10 mm Hg;

- rapid decline in FVC > 10% from baseline over 3 months.

Nocturnal hypoventilation also indicates nocturnal desaturation. Consequently, prolonged nocturnal desaturation (mean SpO₂ < 90% or SpO₂ < 90% for at least 10% of the recorded monitoring period) may also indicate clinically significant hypoventilation. However, nocturnal desaturation alone cannot serve as an indication for initiating non-invasive ventilation (NIV), as other causes of desaturation must also be considered (obstructive sleep apnea, pulmonary infection, bronchial secretions accumulation).

If FVC is < 70% and declines by more than 10% over 3 months, nocturnal NIV can be initiated regardless of further test results due to the rapid disease progression [18, 19].

Comments: an individualized approach to each patient is essential when deciding on the need for this procedure. Tracheostomy is indicated in the following situations, strictly with the consent of the patient's parents or legal guardian:

- NIV intolerance;

- NIV failure;

- severe bulbar symptoms with recurrent aspiration;

- inefficacy of non-invasive airway clearance techniques;

- inability to transition back to NIV following intubation and invasive ventilation;

- isolated cases of 24-hour NIV.

The patient and their family must be fully informed about the disease course prior to making a decision; consent from the patient's parents or legal guardian is mandatory. The decision must be made by a multidisciplinary medical team (critical care specialists, pulmonologists, neurologists, and pediatricians specializing in neuromuscular disorders).

Oxygen therapy in patients with TK2-associated myopathy should be used with caution. Secondary hypoxemia caused by hypoventilation may be confused with hypoxemia caused by other factors, such as mucous plugging or atelectasis. Capnometry, pulmonary function testing, or blood gas analysis are necessary to determine the appropriate oxygen therapy regimen.

Comments: electric suctioning (upper airway aspiration) and mechanical insufflator-exsufflators should be used in any patient with proven ineffective cough, frequent recurrent lower respiratory tract infections, and chronic aspiration syndrome. Indicated when FVC ≤ 50%, PEF < 270 L/min, or maximum expiratory pressure < 60 cm H₂O. The use of mechanical insufflator-exsufflators is critically important, as the inability to cough effectively due to respiratory muscle weakness leads to an increased risk of atelectasis, pneumonia, ventilation-perfusion mismatch, and progression of respiratory failure. The main methods of sputum clearance for bedridden and seated patients involve combining several techniques: a mechanical insufflator-exsufflator with manual chest compressions or postural drainage combined with an electric suction device. In young and/or non-communicative children, it is advisable to use mechanical insufflator-exsufflators with an inspiratory trigger function to prevent lung injury if such children hold their breath and resist coughing. A relative contraindication to mechanical insufflator-exsufflator use is chronic lung disease (e.g., emphysema). The use of a mechanical insufflator-exsufflator in such cases must be weighed against the potential risks of pneumothorax.

Comments: patients' parents must be provided with medical equipment — a reusable breathing circuit bag (Ambu bag), and if necessary, a battery-powered electric suction pump (universal medical suction pump, electric nasal aspirator), a home BPAP ventilator, and home respiratory monitoring equipment (pulse oximeter, and where possible, a transcutaneous capnography device [carbon dioxide monitor/capnograph]). Parents must be trained in their proper use.

Comments: parents must be trained in the correct use of medical devices.

4. Medical Rehabilitation and Sanatorium-Resort Treatment

4.1. Rehabilitation Goals:

- preserving the ability for passive verticalization, stable and symmetrical posture, and upper limb function;

- maximum independence and socialization;

- postural correction and prevention of contractures;

- minimization of surgical intervention;

- speech and intellectual development;

- pain relief and easing of caregiving.

4.2. Rehabilitation Tactics:

- regularity of rehabilitation;

- regular and continuous follow-up of the child by specialists: exercise therapy physician, physical therapist, neurologist, orthopedic trauma surgeon, speech and language therapist;

- appropriate orthotic management to prevent contractures;

- optimal selection of assistive technology and rehabilitation devices (ATRD);

- psychological and pedagogical rehabilitation;

- psychological counseling for all family members.

4.3. Physical Rehabilitation Methods:

- exercise therapy (kinesiotherapy);

- medical massage;

- hydrokinesiotherapy (individual and group sessions);

- physiotherapy (combined physical therapy modalities not classified elsewhere);

- robotic mechanotherapy;

- use of ATRD (functional bracing, verticalization, prone and seated positioning).

The rehabilitation program must be comprehensive, involving specialists such as: neurologist, exercise therapy physician, physical and rehabilitation medicine physician, physical therapist, pulmonologist, pediatrician, orthopedic trauma surgeon, speech and language therapist, and medical psychologist.

4.4. Postural Management

Proper positioning reduces the need for expensive and complex surgical interventions. To reduce the negative impact of poor posture on the body, it is necessary to select various correction options and train specialists and parents in positioning and safe transfer techniques. Evaluation of the child's posture in the lying, sitting, and standing positions is required using a postural chart. The postural chart addresses the positioning regimen, helps everyone interacting with the child to position them correctly, and facilitates positioning in the home environment. The main sections of the postural chart include information on devices used, ATRD and orthopedic products, general daily positioning recommendations, individual recommendations, and guidelines for walks, hygiene routines, and all possible child positioning options.

Stages of the postural management program in family life: consultation, assessment of the child's posture in lying, sitting, and standing positions, training, practical sessions, assistance in selecting and adjusting ATRD, adaptation of ATRD, and ongoing family support. Comprehensive support must be integrated into family life. Training in safe, ergonomic transfers for mobility-impaired patients must be an integral part of medical care for special-needs children and their families.

Proper positioning ensures necessary comfort, reduces the risk of secondary complications, the development of deformities and contractures, and ensures patient safety. The use of functional systems and sets of molding rolls, pillows, and special U-shaped mattresses—mattresses filled with millet, which conform to the child's body shape, evenly distribute weight, and help maintain proper body alignment. Regular position changes and massage help reduce or prevent the appearance of pressure ulcers (on the back, side, and abdomen, whenever possible).

Moving a young child involves providing an anti-gravity position to reduce the risk of aspiration. To move an adolescent or adult, special equipment (a hoist) must be used. 

4.5. Exercises

Exercises that develop muscle endurance are important. To train strength, low resistance, high repetition, and optimal biomechanics should be used. Avoidance of excessive loads and high-resistance eccentric exercises. Preference should be given to active exercises, encouraging the child themselves to perform the available range of motion.

4.6. Use of Assistive Devices

To facilitate care, it is advisable to use assistive devices for rehabilitation. The selection of assistive devices is carried out taking into account motor impairments. Muscle and ligament traction is effective if the duration of exposure during the day is at least 6 hours. Deformity correction is achieved through adequate orthotization and the use of assistive devices.

Assistive devices must be properly sized, taking into account the activity, functional and anatomical features of the specific child. An indoor functional wheelchair is advisable for use starting from 6 months of age, and a standing frame from 12–14 months. Orthoses are custom-made, and replacement occurs as the child grows or their condition changes. Standing can offer a child a great deal: prolonged, passive muscle stretching, increased joint range of motion, proprioceptive correction for postural control development. Axial load is very important for bone strengthening and growth, passive standing promotes hip joint formation and reduces the risk of subluxation and dislocation, reduces the risk of scoliosis and lower extremity contractures, urinary tract infections, improves digestive function (reduces constipation), improves respiratory function, increases head control, promotes upper limb function development; stimulates involvement and the ability to explore the environment from a different perspective than from a lying position; social interaction, and increased self-confidence. Standing must be safe, regular, and properly organized. To achieve maximum effect from organized standing, the patient should use a standing frame for at least 1 hour per day (can be divided into sessions of any duration, but no more than 60 minutes continuously and in accordance with the child's capabilities). The duration of standing is determined by the child's activity in the standing position.

4.7. Hydrokinesitherapy

Hydrokinesitherapy is physical therapy in water, including therapeutic swimming, therapeutic swimming with counter-current, therapeutic swimming with objects and devices, underwater therapeutic shower-massage, vibrational underwater shower-massage, etc. The recommended pool water temperature is 30 degrees. Swimming helps the child reduce the effect of gravity and allows them to perform movements that they lack the physical strength to perform on land. Hydrostatic pressure acts on the surface of the body immersed in water. Water, stimulating the entire complex of body receptors, affects nerve centers and tonifies the nervous system, which provides a sense of vigor and increases working capacity.

One of the consequences of hydrostatic pressure is an increased load on the chest during inspiration and on the respiratory muscles during forced expiration into water. This stimulates the development of respiratory muscles, chest mobility, an increase in its dimensions and vital capacity (VC), establishes a correct breathing rhythm, increases respiratory rate, increases heart rate, and alters the tone of peripheral blood vessels. During swimming, the child is in a state of hydrostatic weightlessness, which unloads the musculoskeletal system by reducing body weight pressure on it; creates conditions for correcting posture disorders and restoring motor functions. The lack of a firm support increases motor capabilities and promotes their development. One of the features determining the effect of swimming on the body is the horizontal body position when performing movements with the arms and legs. Swimming engages almost all body muscles, which contributes to the harmonious development of musculature and mobility in major joints. Such a body position, cyclic movements associated with muscle work, water pressure on the subdermal venous bed, deep diaphragmatic respiration, and the state of hydrostatic weightlessness promote blood return to the heart and significantly facilitate its work.

Specific features of swimming are associated with motor activity in the aquatic environment, and are also determined by the physical properties of water: density, viscosity, pressure, temperature, and heat capacity. Exercises for stretching, balance, and coordination are used in water. Water sessions allow relieving stress from the spine, joints, and heart, train the vestibular apparatus and breathing, and increase muscle tone. All this helps the child feel their body better. The aquatic environment, providing a powerful stimulating effect through tactile, temperature, muscular, and other receptors, has a positive impact not only on the process of physical rehabilitation, but also on the mental sphere of children. When performing physical exercises in the aquatic environment, the body, as a single integral system, responds with a set of complex reactions of the skin, cardiovascular, nervous, endocrine, muscular systems, heat exchange, and oxidation-reduction processes. The therapeutic action of physical exercises in water is carried out on the basis of a tonic effect, trophic action, and the formation of compensation.

4.8. Speech Therapy Care

Conducting speech therapy massage and articulation gymnastics for the purpose of training chewing and swallowing muscles (medical facial massage, general massage and gymnastics in early childhood, medical-speech therapy procedure for dysphagia, medical-speech therapy procedure for dysarthria). Preservation and improvement of chewing and swallowing functions, prevention of contractures in the temporomandibular joint. Classical and special fascial techniques for restoring speech functions, kinesiological taping, differentiated speech therapy massage (manual, probe, hardware massage, unclassified in other categories), acupressure are used; correctional work is also carried out to develop the mobility of articulation organs, form sound pronunciation, phonemic perception, and prosody.

5. Prevention and Dispensary Observation

Vaccination of patients with TK2 myopathy is carried out for the prevention of infectious diseases on a general basis. Contraindications to vaccination in patients with TK2 myopathy do not differ from those in other categories of citizens and are carried out in accordance with the National Schedule of Preventive Vaccinations, with the exception of the oral polio vaccine.

Comments: prevention and discussion of preconceptional screening options are particularly relevant for parents of a patient with TK2 myopathy, for couples where each partner is a confirmed heterozygous carrier of a pathogenic / likely pathogenic variant in the TK2 gene, as well as for patients with confirmed TK2 myopathy, in order to prevent the recurrence of a child with TK2 myopathy in the family. Given the autosomal recessive inheritance pattern, the risk of recurrence of a child with TK2 myopathy in the proband's parents within the same marriage is 25% (high genetic risk category) for each pregnancy, regardless of the sex of the unborn child. In families with an affected child, prenatal and preimplantation diagnosis can be performed. For this, parents need to contact specialized diagnostic laboratories and medical centers. Molecular genetic testing of the TK2 gene can be performed (searching for pathogenic / likely pathogenic variants previously detected in the family in the fetus), as well as preimplantation genetic testing of the embryo for known pathogenic / likely pathogenic variants in the TK2 gene. Testing is possible at various stages of pregnancy: 8–12 weeks (chorionic villi), 15–18 weeks (amniotic fluid), 20–24 weeks (cord blood). Chorionic villus sampling or placenta biopsy is optimal: in case of an unfavorable prognosis, the pregnancy can be terminated (artificial termination of pregnancy (abortion)) within the timeframe of a standard medical abortion.

Comments: spirometry testing of forced respiratory volumes is also indicated. If vital capacity (VC) decreases below 50%, pulse oximetry is indicated to screen for obstructive sleep apnea syndrome. Cardiorespiratory monitoring (cardiopulmonary exercise testing) is important to detect hypoventilation and hypopnea in all patients with saturation below 94% (at least once every 3 months for bedridden patients).

Comments: patients with TK2 myopathy require physical activity appropriate to their condition and disease stage, supervised by parents / legal guardians and relevant medical staff to ensure adequate physical activity and prevent the development of contractures.

Comments: neurological status, motor functions, and range of motion must be regularly assessed, at least once every 6 months. Validated methods and scales should be used for objective assessment.

Comments: initial and follow-up follow-up consultations (examinations) are indicated by a geneticist, neurologist, pulmonologist, gastroenterologist, nutritionist, trauma surgeon / orthopedist, pediatrician, general practitioner (family physician), otorhinolaryngologist-audiologist, and palliative care physician, as well as an examination (consultation) by a physical therapist, and initial and follow-up testing and consultation by a medical psychologist (if necessary).

Patients with clinical signs of chest deformity and/or hypoventilation are indicated for targeted chest X-ray and/or lung X-ray (as clinically indicated), and computed tomography of the chest cavity to detect potential pulmonary atelectasis and determine the degree of chest deformity. Given the potential development of spinal deformities, patients are indicated for computed tomography of the spine (single region) to assess spinal deformities (in some cases, surgical intervention may be considered). Patients older than 5 years are indicated for X-ray densitometry annually and prior to orthopedic interventions to rule out osteoporosis.

6. Organization of Medical Care

Indications for elective hospitalization (for infantile and childhood forms):

1) initial diagnosis establishment;

2) all patients with TK2-related myopathy every 6 months for children under 3 years of age, and once a year for children over 3 years of age for functional motor assessment, screening for disease complications—respiratory, cardiac, musculoskeletal, and gastroenterological—and for rehabilitation;

2) all patients with TK2-related myopathy requiring evaluation prior to elective orthopedic surgery.

For the adult form:

1) initial diagnosis establishment;

2) dynamically — depending on the severity of the disease course (every 2 years or more frequently, if necessary).

Indications for discharging a patient after elective hospitalization:

1) execution of the patient's evaluation plan, including assessment of respiratory disorders;

2) delivery of rehabilitation measures;

3) recommendations for further follow-up and treatment are provided.

Indications for elective hospitalization in a surgical department:

1) patients with TK2-related myopathy requiring gastrostomy placement;

2) patients with TK2-related myopathy requiring tracheostomy placement; sometimes a tracheostomy is placed due to emergency indications, requiring emergency hospitalization.

Indications for elective hospitalization in a palliative care unit or consultation with a palliative care physician:

1) need for home respiratory support;

2) severe pain syndrome refractory to outpatient treatment, including home-based care;

3) severe manifestations requiring symptomatic treatment under inpatient medical supervision;

4) adjustment of the treatment regimen to continue care at home;

5) performance of medical interventions that cannot be carried out in an outpatient setting, including at home.

If a patient is diagnosed with a comorbid condition requiring surgical intervention, it is advisable to perform it electively with preliminary preparation and evaluation. The risks of elective surgery are much lower than the risks associated with emergency interventions.

Indications for emergency hospitalization:

1) Development of intercurrent infections in a patient with TK2-related myopathy (acute respiratory viral infection, moderate to severe bronchitis [accompanied by desaturation and/or feeding difficulties]; acute intestinal infections accompanied by dehydration), poisoning, trauma, or other conditions or diseases (at the discretion of the attending physician) requiring active treatment and round-the-clock medical monitoring.

The goal of patient care in an acute situation is to normalize gas exchange by preventing atelectasis and clearing the airways. In most cases, these tasks should be addressed, whenever possible, using non-invasive mechanical ventilation.

2) Development of severe life-threatening respiratory disorders with hypoventilation and apnea, without comorbid conditions, where respiratory disorders require resuscitation measures; hospitalization is indicated in medical organizations structured with an anesthesiology and resuscitation department or an intensive care unit/step-down unit providing round-the-clock medical monitoring and treatment for children.

When implementing emergency protocols, it is necessary to contact a multidisciplinary team (neurology and pulmonology / intensive care), typically including a neurologist who is familiar with the clinical course and potential problems of the specific patient. Oxygen in patients with TK2-associated myopathy should be used with caution. Secondary hypoxemia caused by hypoventilation can be confused with hypoxemia caused by other reasons, such as mucus plugging or atelectasis. Capnography or blood gas and acid-base analysis will help choose the correct oxygen administration regimen.

3) Development of severe life-threatening cardiac disorders without concomitant diseases (heart failure, cardiac arrhythmia).

Hospitalization is indicated in medical organizations that have an anesthesiology and intensive care unit or an intensive care unit (ward) and provide round-the-clock medical monitoring and treatment for children. When implementing emergency protocols, it is necessary to contact a multidisciplinary team (neurology and cardiology), typically including a neurologist who is familiar with the clinical course and potential problems of the specific patient.

Indications for discharging a patient after emergency hospitalization:

1) absence of a threat to the patient's life;

2) absence of a threat of complications requiring urgent treatment;

3) stabilization of the patient's condition and main clinical and laboratory parameters of the pathological process regarding the underlying disease;

4) absence of the need for continuous medical and round-the-clock observation for the underlying disease;

5) the need to transfer the patient to another medical organization.

7. Additional Information

Pathogenetic therapy has a significant positive impact on the survival of patients with early-onset TK2 myopathy.

Early initiation of nucleoside therapy can reverse symptoms of muscle weakness.

Against the background of therapy, significant functional improvements are shown in patients with childhood onset: positive effects of therapy were confirmed by functional tests (6-minute walk test, The North Star ambulatory assessment system, Hammersmith scale (HFMSE)).

Therapy makes it possible to reduce dependence on respiratory support, up to its complete withdrawal. In patients with late-onset, the effect of therapy is less pronounced, but in all cases, it was possible to achieve stabilization of the condition, including respiratory function, as well as a decrease in the frequency and severity of respiratory infections.

The conclusions are based on the results of published international [7] and Russian studies.

Criteria for Evaluating the Quality of Medical Care

References

Appendix A1. Working Group Composition

1. Inga V. Anisimova — Cand. Sc. (Med.), Medical Geneticist, Head of the Department of Medical Care Organization, Research Centre for Medical Genetics, Member of the Association of Medical Geneticists (AMG).

2. Irina N. Artamonova — Junior Researcher at the Research Laboratory of Physiology and Pathology of Newborns, Pediatrician at the Almazov National Medical Research Centre of the Russian Ministry of Health.

3. Aleksandr V. Borodin — Junior Researcher at the Research Laboratory for Unknown, Rare, and Genetically Determined Diseases, Center for Personalized Medicine, Almazov National Medical Research Centre, Pediatric Cardiologist.

4. Aleksandr V. Bryzzhin — Head of the Anesthesiology and Intensive Care Department for Children at the Almazov National Medical Research Centre of the Russian Ministry of Health, Member of the Association of Pediatric Anesthesiologists and Intensivists of the Russian Ministry of Health.

5. Elena S. Vasichkina — D.Sc. in Medicine, Head of the Research Center for Unknown, Rare, and Genetically Determined Diseases, Center for Personalized Medicine, Almazov National Medical Research Centre; Scientific Supervisor of the Department of Pediatric Cardiology and Medical Rehabilitation, Almazov National Medical Research Centre; Professor at the Department of Pediatric Diseases, Faculty of General Medicine, Institute of Medical Education, Almazov Centre; Chief freelance specialist pediatric cardiologist of the Northwestern Federal District of the Russian Ministry of Health, Member of the specialized commission under the chief freelance pediatric cardiologist of the Russian Ministry of Health, Member of the Association of Pediatric Cardiologists of Russia, Russian Cardiological Society, All-Russian Scientific Society of Arrhythmologists, Pediatric Cardiologist, Arrhythmologist.

6. Elena Yu. Gurkina — Ph.D. in Medicine, Deputy Chief Physician for Medical Work at the Almazov National Medical Research Centre, Pediatric Endocrinologist, Pediatrician.

7. Ekaterina Yu. Zakharova — D.Sc. in Medicine, Head of the Laboratory of Hereditary Metabolic Diseases, Academician N.P. Bochkov Research Centre for Medical Genetics, Member of the Russian Society of Medical Geneticists, Member of the Society for the Study of Inborn Errors of Metabolism (SSIEM).

8. Natalia Yu. Kolbina — Head of the Department of Pediatrics and Medical Rehabilitation No. 1, Almazov National Medical Research Centre, Pediatrician, Neonatologist, Ultrasound Diagnostics Physician, Member of the Regional Public Organization "Saint Petersburg Radiological Society".

9. Anna A. Kostareva — D.Sc. in Medicine, Director of the Institute of Molecular Biology and Genetics, Almazov National Medical Research Centre; Professor at the Department of Faculty Therapy with Clinic, Institute of Medical Education, Almazov Centre; Specialist at the Department for Supporting the Development of VIMIS CVD; Member of the European Society of Cardiology.

10. Sergey A. Kurbatov — Ph.D. in Medicine, Senior Researcher at the Research Institute of Experimental Biology and Medicine, N.N. Burdenko Voronezh State Medical University; Associate Professor at the Department of Neurology named after K.N. Tretyakov, Saratov State Medical University named after V.I. Razumovsky of the Ministry of Health of the Russian Federation; Geneticist, Neurologist, Functional Diagnostics Physician at Healthy Child LLC; Member of The World Muscle Society.

11. Sergey I. Kutsev — Academician of the RAS, Professor, D.Sc. in Medicine, Director of the Research Centre for Medical Genetics named after Academician N.P. Bochkov, President of the Association of Medical Geneticists (AMG).

12. Tatyana A. Levko — Pediatrician at the Department of Pediatrics and Medical Rehabilitation No. 1 of the Almazov National Medical Research Centre of the Russian Ministry of Health.

13. Ekaterina A. Mamaeva — Neurologist, Epileptologist at the Almazov National Medical Research Centre, Member of the European Pediatric Neurology Society, British Paediatric Neurology Association.

14. Svetlana V. Mikhailova — D.Sc. in Medicine, Head of the Department of Medical Genetics, Russian Children's Clinical Hospital, Pirogov Russian National Research Medical University of the Russian Ministry of Health.

15. Natalia A. Petrova — Ph.D. in Medicine, Associate Professor, Head of the Research Laboratory of Physiology and Pathology of Newborns, Institute of Perinatology and Pediatrics; Head of the Research Laboratory for Unknown, Rare, and Genetically Determined Diseases, Center for Personalized Medicine; Associate Professor at the Department of Pediatric Diseases with Clinic, Institute of Medical Education, Almazov National Medical Research Centre; Member of the Russian Society of Neonatologists, Russian Association of Specialists in Perinatal Medicine, International Pediatric Sleep Association.

16. Tatyana M. Pervunina — D.Sc. in Medicine, Director of the Institute of Perinatology and Pediatrics, Leading Researcher at the Research Laboratory for Unknown, Rare, and Genetically Determined Diseases, Center for Personalized Medicine, Almazov National Medical Research Centre; Professor at the Department of Pediatrics, Faculty of Medicine, St. Petersburg State University; Chief Freelance Pediatrician of St. Petersburg, Member of the Union of Pediatricians of Russia, Russian Association of Specialists in Perinatal Medicine, Member of the Presidium of the Association of Pediatric Cardiologists of Russia.

17. Olga N. Pichkova — Head of the Department of Restorative Treatment and Medical Rehabilitation No. 3, Almazov National Medical Research Centre.

18. Valeria V. Sviridova — Geneticist at the Department of Medical Care Organization, Research Centre for Medical Genetics.

19. Dmitry M. Subbotin — Medical Geneticist at the Research Centre for Medical Genetics named after Academician N.P. Bochkov, Member of the Association of Medical Geneticists (AMG).

20. Tsygankova Polina Georgievna — PhD in Medicine, Leading Research Fellow, Laboratory of Hereditary Metabolic Diseases, Academician N.P. Bochkov Research Centre for Medical Genetics, Member of the Russian Society of Medical Geneticists.

Public organization representatives

Chernova Polina Anatolyevna — Coordinator for Mitochondrial Myopathy, TK2 direction (mitochondrial DNA depletion syndrome), All-Russian Orphan Disease Association.

Conflict of interest:

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

Appendix A2. Methodology for Developing Clinical Guidelines

Target audience for these clinical guidelines:

1. Medical geneticists, neurologists, orthopedic traumatologists, neurosurgeons, anesthesiologist-resuscitators, gastroenterologists, dietitians, pulmonologists, cardiologists, exercise therapy doctors, physical and rehabilitation medicine doctors, endocrinologists, general practitioners, pediatricians, general family practitioners (family physicians).

2. Medical students, residents, postgraduate students.

Methods used for evidence collection/selection: electronic database searches.

Description of methods used to assess the quality and strength of evidence: the evidence base for the recommendations includes publications from the Cochrane Library, MEDLINE, and PubMed databases.

Search depth — 15 years.

Methods used to assess the quality and strength of evidence:

- expert consensus;

- assessment of significance according to the rating scheme (scheme attached).

Methods used for evidence analysis:

- reviews of published meta-analyses;

- systematic reviews with evidence tables.

Description of methods used to analyze evidence: when selecting publications as potential sources of evidence, the methodology used in each study is examined to ensure its validity. The results of this examination affect the level of evidence assigned to the publication, which in turn influences the strength of the recommendations. To minimize potential biases, each study was evaluated independently. Any discrepancies in evaluations were discussed by the full group of authors. If consensus could not be reached, an independent expert was involved.

In these clinical guidelines, all information is ranked by the level of reliability (evidence) or LRE, depending on the quantity and quality of studies on the issue, in accordance with the tables:

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

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 grades of recommendation (GR) for prevention, diagnostic, treatment, and medical rehabilitation methods, including those based on natural therapeutic factors (preventive, diagnostic, therapeutic, and rehabilitative interventions)

Procedure for updating clinical guidelines.

The update mechanism for clinical guidelines provides for their systematic revision—at least once every three years, as well as upon the emergence of new evidence-based data regarding the diagnosis, treatment, prevention, and rehabilitation of specific conditions, 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

Information on specialized medical nutrition products

https://nsi.eaeunion.org/portal/1995 — Unified Register of State Registration Certificates, Unified Regulatory Reference Information of the Eurasian Economic Union.

Specialized dietary food for medical purposes containing deoxycytidine.

NUTRITIONAL AND ENERGY VALUE (average values):

INDICATIONS FOR USE: the product is intended for dietary prophylactic nutrition in children starting from 1 month of age as part of specialized diets for patients with mitochondrial myopathy and syndromes accompanied by deoxycytidine deficiency.

Take in accordance with recommendations and under the supervision of a physician or nutrition specialist.

Children should take the product under the supervision of a pediatrician.

INSTRUCTIONS FOR USE: the product is recommended for consumption according to the instructions below.

Instructions for product use:

1) Deoxycytidine tablet containing 400 mg of deoxycytidine, or half a deoxycytidine tablet containing 200 mg of deoxycytidine, or powder containing 400 mg of deoxycytidine, is dissolved in 50–200 ml of water.

2) The resulting solution is administered orally in accordance with the recommendations in the table.

3) For adults and children over 1 month of age with mitochondrial myopathy and syndromes accompanied by deoxycytidine deficiency, the administration regimen may differ from the recommended one, as prescribed by the attending physician depending on the stage and severity of the disease.

CONTRAINDICATIONS: individual intolerance to the product ingredients.

Not a medicinal product. Do not use parenterally.

STORAGE CONDITIONS: store in the original closed packaging in a dry place protected from direct sunlight and out of the reach of children at a temperature from + 5 ^°C to + 25 ^°C and a relative air humidity of no more than 60%, do not freeze.

SHELF LIFE: 24 months from the date of manufacture

2. Specialized dietary food for medical purposes containing thymidine.

NUTRITIONAL AND ENERGY VALUE (average values):

INDICATIONS FOR USE: the product is intended for dietary prophylactic nutrition in children starting from 1 month of age as part of specialized diets for patients with mitochondrial myopathy and syndromes accompanied by thymidine deficiency.

Take in accordance with recommendations and under the supervision of a physician or nutrition specialist.

Children should take the product under the supervision of a pediatrician.

Appendix A3.1 Main diseases for differential diagnosis

Appendix B. Physician Action Algorithms

Appendix C. Patient Information

Mitochondrial DNA depletion syndrome (MDDS) associated with pathogenic variants in the TK2 gene.

Mitochondrial DNA depletion syndrome is a group of autosomal recessive disorders that cause a reduction in mitochondrial DNA content, leading to impaired cellular energy production and, consequently, damage to tissues and organs. All forms of MDDS are extremely rare. In mitochondrial DNA depletion syndrome associated with variants in the TK2 gene, there is a decrease in the levels of nucleotides (deoxythymidine monophosphate and deoxycytidine monophosphate) required for mitochondrial DNA synthesis. The myopathic form associated with TK2 belongs to rapidly progressive neuromuscular disorders and leads to muscle weakness and respiratory failure. Over time, symptoms progress, involving new muscle groups; patients lose the ability to walk independently and become dependent on mechanical ventilation.

Inheritance.

TK2-related myopathy is inherited in an autosomal recessive manner, meaning a child inherits one altered gene from each parent. In families where both parents are carriers of the altered genes, the risk of having an affected child is 25% for each pregnancy. Therefore, if a child with mtDNA depletion syndrome is born in a family, it does not mean that all subsequent children will be affected. There is a 25% chance of having a healthy child, and a 50% chance of having children who are carriers of the altered genes.

There are three forms of TK2-associated myopathy: infantile (onset under 1 year of age), juvenile (onset at 1–12 years), and adult form (onset after 12 years and in adults). The rate of progression and mortality depend on the age of onset: ranging from rapid progression and early death in the infantile form, to a less aggressive course with loss of motor functions and respiratory failure in the juvenile form, and preservation of the ability to walk independently in the adult form.

Clinical manifestations.

Each patient with TK2-related myopathy has individual impairments, and symptoms may vary depending on the age of onset and disease severity. Universal manifestations include:

- progressive muscle weakness and wasting, primarily affecting proximal muscles (shoulders, thighs, and pelvis), leading to gait abnormalities, lordosis, increasing generalized weakness with gradual loss of motor skills, ability to sit, and ability to walk independently;

- neck muscle weakness (dropped head), leading to progressive difficulty in lifting the head, up to complete inability to lift the head independently;

- involvement of respiratory muscles, which can lead to respiratory failure, impaired cough reflex and sputum clearance from the airways, and hypoxia;

- severe regression of respiratory function during respiratory infections; frequent infectious episodes increase muscle weakness;

- involvement of masticatory and swallowing muscles, leading to difficulties with feeding (chewing and swallowing difficulties), as well as an increased risk of food and saliva aspiration, and consequently, a high risk of airway obstruction, hypoxia, or asphyxia;

- development of contractures and chest deformities amid increasing muscle weakness.

Diagnosis.

The key diagnostic method is DNA diagnostics and the detection of biallelic pathogenic variants in the TK2 gene.

Often, due to overlapping symptoms, patients may be misdiagnosed with congenital myopathies and various types of dystrophies, Pompe disease, or spinal muscular atrophy.

Therapy.

Patients with confirmed TK2-related myopathy are indicated for lifelong intake of specialized medical nutrition products containing thymidine / deoxycytidine, which are precursors of deoxythymidine monophosphate and deoxycytidine monophosphate nucleotides. The therapy is designed on the premise that cells, utilizing these deoxynucleotides, will produce additional mitochondrial DNA, which in turn reduces clinical symptoms.

Currently, two specialized medical nutrition products are registered in the Russian Federation for the dietary management of patients diagnosed with mitochondrial myopathy and syndromes accompanied by thymidine and deoxycytidine deficiency.

The effectiveness of therapy depends on the timing of initiation of specialized medical nutrition products containing thymidine / deoxycytidine: the earlier a child begins therapy, the greater the chance of halting the aggressive course and restoring previously lost functions. In any case, therapy is considered successful if there is a significant deceleration of disease progression and stabilization of the condition. A positive effect is manifested by an increase in muscle strength, a decrease in dependence on respiratory support, and in some cases, the restoration of the ability to walk; meanwhile, no serious adverse events associated with the therapy have been reported.

Supportive measures and rehabilitation.

All patients with TK2-related myopathy, regardless of whether they receive therapy or not, require a multidisciplinary approach to medical care involving a wide range of specialists (geneticist, neurologist, pulmonologist, anesthesiologist-intensivist, physical therapist, cardiologist, ophthalmologist, gastroenterologist, endocrinologist). Given that each patient has different symptoms and manifestations, the members of the therapeutic team and the degree of their involvement may vary depending on needs and circumstances.

The following measures play a particularly important role in maintaining a stable condition and functionality at various stages of the disease.

Breathing.

Oxygen: Supplemental oxygen must never be administered without concurrent non-invasive ventilation!

We all breathe by inhaling oxygen (O₂) and exhaling carbon dioxide (CO₂). The administration of oxygen under conditions of compromised pulmonary function can undermine the body's drive to breathe independently and, consequently, lead to elevated carbon dioxide levels (respiratory acidosis). This can be dangerous and potentially life-threatening. Oxygen administration must be carried out with great caution and close monitoring of blood carbon dioxide levels. If the blood oxygen saturation level drops below 95% (according to a pulse oximeter), the blood carbon dioxide level must be checked. If carbon dioxide levels in the blood are elevated, manual, mechanical, and non-invasive respiratory support devices should be used. Oxygen deficiency can be compensated for using non-invasive positive pressure ventilation (NIV) while closely monitoring CO₂ levels.

Mucolytic drugs: Patients with reduced respiratory function and a weak cough reflex must not take mucolytic drugs! They increase the volume of secretions that the child is unable to cough up, which can lead to pneumonia. Be sure to inform your attending physician (primary care pediatrician) of this fact!

Respiratory support is central to the management of patients with TK2-related myopathy. Due to chest muscle weakness, the ability to breathe and cough effectively may be reduced, thereby increasing the risk of infections.

In TK2-related myopathy, it is important to receive regular consultations on respiratory support. It is necessary to visit a pulmonologist immediately after diagnosis and perform functional tests as soon as possible. Testing should include the measurement of FVC (the maximum volume of air a person can exhale after a maximum inhalation), maximal inspiratory pressure (how forcefully you can inhale air), maximal expiratory pressure (how forcefully you can exhale), and peak cough expiratory flow. Furthermore, the test must be performed as early as possible so that the child gets used to the equipment and the physician can establish a baseline of respiratory capacity to monitor dynamics in the future. To diagnose respiratory disorders, it is important to regularly assess respiratory function, as well as perform other nocturnal (extended) sleep studies (the physician will provide more details).

As the respiratory muscles weaken, the risk of pulmonary infections increases due to an «ineffective» cough; therefore, acute respiratory viral infections (ARVI) must be treated under medical supervision, and if saturation drops and cough-clearing techniques are ineffective, the physician decides on prescribing antibiotics or hospital admission.

Breathing problems during sleep may develop over time. The attending physician (pulmonologist / anesthesiologist-intensivist) should be informed if morning headaches or fatigue occur – these may be symptoms of excessively shallow breathing during sleep, which requires further investigation. A sleep study will determine the quality of breathing at night. If blood oxygen levels drop significantly during sleep, non-invasive mechanical ventilation (BI-PAP) is prescribed to improve the quality of breathing. The goals of therapy are to increase lung volume and maintain flexibility of the chest wall muscles (lung volume recruitment).

When the cough reflex weakens, it is crucial to use assistive devices to make coughing more effective, especially during illness (colds). To clear the upper airways in bedridden and seated patients, an electric aspirator (insufflator-aspirator) can be used; in ambulant patients, the use of this device may be required during periods of infectious diseases and other periods of weakness. To clear the lower airways and assist with coughing in the same situations, a cough assist device (mechanical insufflator-exsufflator) is used. These devices can be used at home.

At home, it is important to have a pulse oximeter to monitor blood oxygen saturation, as well as an Ambu bag to provide assisted ventilation in situations where it is necessary (the child cannot clear secretions independently) and a cough assist device is not available. It is also advisable to bring respiratory support aids when traveling, visiting the swimming pool, etc.

At the same time, a preventive approach is essential to address emerging problems—that is, starting interventions at the earliest, mildest symptoms. In case of respiratory disorders, early initiation of ventilation and respiratory muscle training are important. This can be accomplished through the following methods: breathing exercises, swimming, blowing up balloons, playing the harmonica, etc.

Movement.

Depending on the patient's individual functional status, there is a need for physical therapy and rehabilitation methods. The main areas will include:

- maintaining and improving motor skills,

- maintaining the quality of daily life and independence,

- postural management (correct body positioning), for example, posture control, management of emerging asymmetry due to scoliosis and other deformities, verticalization;

- increasing range of motion,

- using orthopedic devices and technical rehabilitation aids.

Scoliosis (curvature of the spine) is a common problem in patients with TK2-related myopathy, caused by muscle weakness. An orthopedic surgeon may recommend postural support devices (orthotics) or surgical correction of scoliosis. Lifelong continuous monitoring of the spine's condition is necessary.

In addition, there is a risk of developing contractures (reduced joint mobility), which may subsequently become permanent and limit movement; dislocations and subluxations are also possible. An orthopedic surgeon may recommend various types of orthoses and technical rehabilitation aids to address these issues.

The physical therapy program should primarily focus on stretching and preserving the range of motion. Wearing full-leg / ankle / wrist orthoses (often called braces) may be indicated for stretching and preventing loss of mobility.

Regular aerobic exercise (swimming, cycling) with an experienced instructor. It is important to avoid strenuous exercise and prevent overload, as cardiovascular and respiratory reserves may be reduced, and the risk of muscle injury increased. It is important to structure the exercise load to allow sufficient time for rest and muscle recovery while conserving energy.

At the same time, it must be understood that regular physical therapy and rehabilitation must be ongoing for all patients with TK2-related myopathy. The best approach is to incorporate physical activity and specialized exercises into your daily life.

Nutrition.

Due to muscle weakness, patients may experience difficulties with eating, requiring modification of food consistency and feeding methods.

Due to mobility limitations, excess or insufficient weight may develop, requiring dietary adjustments.

In severe progressive cases, the ability to chew and swallow may be impaired. There is a risk of aspirating food and liquid into the lungs during meals and developing respiratory infections. This may require temporary or permanent use of a nasogastric tube or gastrostomy and a partial or complete transition to tube feeding.

It is important to undergo regular examinations and consult a clinical nutritionist/gastroenterologist to manage emerging nutritional and gastrointestinal disorders.

Medical Genetic Counseling

It is crucial for families to consult a geneticist. The geneticist performs initial diagnostics, determines the diagnostic strategy, and analyzes the genetic test results. It is important to remember that only a geneticist can correctly interpret the genetic test result (i.e., determine whether the identified gene variant is associated with the disease). After confirming the diagnosis with a geneticist, families can find out the recurrence risk of having another child with TK2-related myopathy, have relatives screened if necessary, and discuss prevention strategies for having another affected child.

Remember: the success of treatment depends just as much on the family as it does on the physician. It is essential to follow recommendations and never lose hope under any circumstances. The disease may follow a fluctuating course; even during therapy, periods of worsening and a decline in certain functional parameters may be observed (especially during and after intercurrent illnesses). Periods of decline may be followed by periods of recovery and improvement in the child's condition. And even if you do not fully conquer the disease, it is within your power to change life for the better and make your loved one happy.

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

Template for including clinical rating scales, questionnaires, and other patient assessment tools

Russian title:

Original title (if applicable):

Source (official developer website, validation publication):

Type (underline):

- assessment scale

- score

- questionnaire

- other (specify):

Purpose:

Content (template):

Key (interpretation):

Explanations

Appendix G1. Alberta Infant Motor Scale

Russian title: Alberta Infant Motor Scale.

Original title: Alberta Infant Motor Scale (AIMS).

Source (official developer website, validation publication): Piper MC, Pinnell LE, Darrah J, Maguire T, Byrne PJ. Construction and validation of the Alberta Infant Motor Scale (AIMS). Can J Public Health. 1992;83:S46-50.

Type (underline):

• rating scale

• index

• questionnaire

• other

Purpose: The Alberta Infant Motor Scale (AIMS), an observational rating scale, was designed to measure gross motor maturation in infants from birth through independent walking.

Content (template): Based on literature, 58 skills were generated and categorized into four positions: prone, supine, sitting, and standing. Each skill is assigned a specific score.

Key (Interpretation): scores are summed, providing an overall assessment of motor development and deviation from/conformance to normal development in percentiles.

Alberta Infant Motor Scale

Total score: ______ percentile: ______

Notes/Recommendations: -------------------------------------------------------

INFANT MOTOR DEVELOPMENT RATING SCALE

Percentile values

Appendix G2. Medical Research Council muscle scale

Russian title: MRC scale.

Original title (if available): MEDICAL RESEARCH COUNCIL SCALE.

Source (official developer website, validation publication): van der Ploeg, R.J.O., Oosterhuis, H.J.G.H. & Reuvekamp, J. Measuring muscle strength J Neurol (1984) 231: 200. https://doi.org/10.1007/BF00313939

Type (underline):

• rating scale

• index

• questionnaire

• other

Purpose: this scale is used to assess:

• muscle strength of the tested muscles at the time of examination in points;

• dynamics of involvement of specific muscles and muscle groups;

• rate of progression of the myopathic process.

Content (template): the scale represents muscle strength in points.

Key (Interpretation): the higher the score, the greater the muscle strength.

Muscle strength according to the Medical Research Council scale

Appendix G3. 6MWT – 6-minute walk test

Russian title: 6-minute walk test.

Original title (if available): The six minute walking test (6MWT).

Source (official developer website, validation publication): Laboratories, A. T. S. C. o. P. S. f. C. P. F. (2002). "ATS statement: guidelines for the six-minute walk test." Am J Respir Crit Care Med 166(1): 111-117.

Type (underline):

• rating scale

• index

• questionnaire

• other

Purpose: determination of patient endurance to assess treatment efficacy.

Content (template): the test evaluates the distance in meters walked by the patient in 6 minutes without forced stops.

Key (interpretation): comparison is made with the results of a test performed previously (before treatment, during treatment, etc.).

6MWT – 6-minute walk test (6 minutes walking test, 6MWT)

It should be borne in mind that this test has the following absolute contraindications: unstable exertional angina and myocardial infarction within the past 1 month. Relative contraindications include: resting heart rate above 120 bpm, systolic blood pressure > 180 mmHg, and diastolic blood pressure > 100 mmHg. Stable angina is not an absolute contraindication to the test, but it should be performed with caution, while taking antianginal medications as indicated.

If the patient is on continuous oxygen support, the oxygen flow rate during the test should be maintained in accordance with the prescription of the physician who prescribed and monitors the therapy.

The test must be stopped immediately if the following occur:

1. Chest pain;

2. Intolerable dyspnea;

3. Leg cramps;

4. Severe instability and unsteadiness while walking;

5. Excessive sweating;

6. Sharp change in skin color (pallor).

The 6MWT is performed in a room familiar to the patient, featuring a sufficiently long corridor with a hard surface. The path length should be at least 30 meters with markings every 3 meters, as well as turnaround points.

The patient is instructed to walk as fast as possible (but not run) for 6 minutes.

The test evaluates the distance in meters covered by the patient in 6 minutes without forced stops.

Appendix G4. SF-36 Quality of Life Questionnaire

Russian title: SF-36 Quality of Life Questionnaire.

Original title (if available): "SF-36 Health Status Survey".

Source (official developer website, validation publication): Ware J.E., Snow K.K., Kosinski M., Gandek B. SF-36 Health Survey. Manual and interpretation guide //The Health Institute, New England Medical Center. Boston, Mass.-1993.

Type (underline):

• rating scale

• index

• questionnaire

• other (specify)

Purpose: SF-36 is a generic questionnaire for assessing quality of life. The Russian translation and validation of the methodology were carried out by the Institute of Clinical Pharmacology Research.

Content (template): 36 questionnaire items grouped into eight scales: physical functioning, role-physical functioning, bodily pain, general health, vitality, social functioning, role-emotional functioning, and mental health. Each scale score ranges between 0 and 100, where 100 represents full health; all scales form two component summary measures: mental and physical well-being.

Key (Interpretation): Results are presented as scores on 8 scales, formulated such that a higher score indicates a higher level of QoL.

SF-36 Quality of Life Questionnaire

This questionnaire asks for your views about your health at the present time and over the past 4 weeks. We would like to gain a more complete understanding of how you feel, how well you are able to carry out your daily activities, and what you think about your own health.

In this questionnaire, the word "health" refers to your overall health status. Thus, this term concerns not only aspects directly related to TK2-associated myopathy, but also your general well-being.

Please answer all questions by selecting only one answer for each question. Please choose the answers that best correspond to your own feelings. If you are unsure which answer to choose, please select the most appropriate of the possible answers. For example: to answer question 3, you can choose between "yes, limited a lot", "yes, limited a little", and "no, not limited at all". However, you may completely (or no longer) be able to perform certain activities. In such cases, you should select the number in the "yes, limited a lot" column.

Would you evaluate your general health status as:

☐ 1 = excellent

☐ 2 = very good

☐ 3 = good

☐ 4 = fair

☐ 5 = poor

Compared to your general health status a year ago, how would you rate it at present?

☐ 1 = much better now than a year ago

☐ 2 = somewhat better (slightly better) now than a year ago

☐ 3 = about the same as a year ago

☐ 4 = somewhat worse now than a year ago

☐ 5 = much worse now than a year ago

The questions below are about your ability to perform routine daily activities. Does your health currently limit you in performing these activities? If so, how much?

During the past 4 weeks, have you experienced any of the following problems with your work or daily activities as a result of your physical health?

During the past 4 weeks, have you experienced any of the following problems with your work or daily activities as a result of your emotional state (such as depression or anxiety)?

During the past 4 weeks, to what extent did your physical health or emotional state interfere with your social activities with family, friends, neighbors, or other groups?

☐ 1 = not at all

☐ 2 = slightly

☐ 3 = moderately

☐ 4 = considerably

☐ 5 = extremely

How much bodily pain have you experienced during the past 4 weeks? (the location of the pain does not matter)

☐ 1 = none

☐ 2 = very mild

☐ 3 = mild

☐ 4 = moderate

☐ 5 = severe

☐ 6 = very severe

During the past 4 weeks, to what extent did pain interfere with your normal work (including both work outside the home and housework)? (the location of the pain does not matter)

☐ 1 = not at all

☐ 2 = slightly

☐ 3 = moderately

☐ 4 = considerably

☐ 5 = extremely

The questions below are about how you felt and how things have been with you during the past 4 weeks. For each question, please give one answer that best describes how you felt. How much of the time during the past 4 weeks:

During the past 4 weeks, how much of the time did your physical health or emotional state interfere with your social activities (such as visiting friends, relatives, etc.)?

☐ 1 = always

☐ 2 = often

☐ 3 = sometimes

☐ 4 = rarely

☐ 5 = never

How TRUE or UNTRUE is each of the following statements for you?

This questionnaire is approved for use by the IQOLA (International Quality of Life Assessment) Project.

Appendix G5. The North Star, Hammersmith Functional Motor Scale (HFMSE)

1. Title in Russian: Key Motor Milestone Activities (Section 2)*

* The scale is not validated in the Russian Federation; special training is required for its use.

Original title (if applicable): Hammersmith Infant Neurological Exam - Source (official developer website, validation publication): Bishop KM, Montes J, Finkel RS. Motor milestone assessment of infants with spinal muscular atrophy using the hammersmith infant neurological Exam-Part 2: Experience from a nusinersen clinical study. Muscle Nerve. 2018 Jan;57(1):142-146. doi: 10.1002/mus.25705. Epub 2017 Jun 14.

Type (underline): rating scale.

Purpose: general assessment of motor activity in children with neuromuscular disorders; can be used to evaluate treatment efficacy.

Hammersmith Functional Motor Scale Expanded (HFMSE)

NORTH STAR ASSESSMENT FOR DYSTROPHIC MUSCULAR DYSTROPHY

Russian name: North Star Ambulatory Assessment.

Original name: North Star Ambulatory Assessment, NSAA.

Key (interpretation): NSAA is a 17-item scale used to assess the patient's functional abilities: 0 (unable), 1 (performs independently, but with modifications), 2 (performs without compensation).

Notes:

- perform tests in a standing position if there are lower limb injuries. However, the patient may be asked to perform head lifts and transition to a sitting position if able to do so. "Comments" should be noted in the worksheet;

- do NOT use mats;

- please note that time in seconds is rounded to the nearest tenth of a second for all timed tasks (e.g., rise from floor or 10 m run);

- exercise performance is scored as follows:

2 = "Normal" - no deviations;

1 = With minor modifications, but the patient completes the task without additional assistance;

0 = Unable to complete the task independently;

- if, in your opinion, the patient could score higher, but received a lower score due to failure to meet certain conditions or lack of understanding of the task, ask them to repeat the exercise and re-evaluate its performance.

- If you are unsure whether a task performance can be scored higher, give it the lower score.

Preparation.

- measure 10 meters in a long, quiet hallway or room. Mark the finish line at the 11-meter mark to ensure the patient actually runs past the 10-meter mark rather than slowing down before the finish line;

- ensure a clean floor space of 3 × 3.5 m, sufficient for performing other tasks (e.g., rise from floor).