From the Guidelines
Metabolic myopathies in children are genetic disorders that affect muscle energy metabolism, causing muscle weakness, exercise intolerance, and sometimes rhabdomyolysis, with the most common causes including glycogen storage diseases, fatty acid oxidation disorders, and mitochondrial myopathies. The primary approach to management involves identifying the specific type of metabolic myopathy through genetic testing, muscle biopsy, and biochemical analysis 1. Treatment typically includes dietary modifications, with specific approaches depending on the underlying disorder. For glycogen storage diseases, patients should avoid prolonged fasting and maintain regular carbohydrate intake, while those with fatty acid oxidation disorders benefit from frequent meals, a low-fat/high-carbohydrate diet, and sometimes medium-chain triglyceride supplementation. Mitochondrial myopathies may be treated with coenzyme Q10 (starting at 5-15 mg/kg/day), riboflavin (100-400 mg/day), and L-carnitine (50-100 mg/kg/day) 1.
Some of the key symptoms of metabolic myopathies in children include:
- Muscle weakness
- Exercise intolerance
- Rhabdomyolysis
- Hypotonia
- Hepatomegaly
- Elevated creatine kinase (CK) levels
- Cardiac hypertrophy or dysfunction in some cases, particularly in infantile Pompe disease 1.
Differential diagnosis is crucial, and conditions to consider include:
- Limb-girdle muscular dystrophy
- Becker muscular dystrophy
- Scapuloperoneal syndromes
- Rigid spine syndrome
- Myasthenia gravis
- Spinal muscular atrophy
- Polymyositis
- Glycogen storage diseases (types IIIa, IV, V, and VII)
- Danon disease
- Rheumatoid arthritis
- Mitochondrial myopathies 1.
Exercise programs should be tailored to the specific condition, with aerobic exercise often beneficial for mitochondrial disorders while anaerobic activity may trigger symptoms in glycogen storage diseases. Acute episodes of rhabdomyolysis require immediate medical attention with IV fluids to prevent kidney damage. Genetic counseling is essential for families, as these conditions are inherited. Regular monitoring of cardiac, respiratory, and neurological function is important as these systems may be affected in certain metabolic myopathies. Early diagnosis and intervention are crucial to prevent complications and improve quality of life for affected children.
From the FDA Drug Label
The FDA drug label does not answer the question.
From the Research
Causes of Metabolic Myopathy in Children
- Metabolic myopathies are genetic disorders that impair intermediary metabolism in skeletal muscle, affecting glycogenolysis/glycolysis, fatty acid transport/oxidation, and mitochondrial metabolism 2, 3, 4
- The three main groups that cause metabolic myopathy are glycogen storage disorders (GSD), fatty acid oxidation defects (FAOD), and mitochondrial myopathies 2, 3, 4
- Mitochondrial disorders are the most common cause of metabolic myopathies, with a frequency of 1/8000 population 3
- Primary mitochondrial myopathies are genetic metabolic disorders of mitochondrial dysfunction affecting mainly skeletal muscle, and are the most common inherited metabolic disorders in childhood 5
Symptoms of Metabolic Myopathy in Children
- Metabolic myopathies can present in the neonatal period with hypotonia, hypoglycemia, and encephalopathy, or in childhood with exercise intolerance, rhabdomyolysis, and weakness 2, 3, 4
- Glycogen storage diseases manifest during brief bouts of high-intensity exercise, while fatty acid oxidation defects and mitochondrial myopathies manifest during longer-duration endurance-type activities, often with fasting or other metabolic stressors 2, 3, 4
- Infants with mitochondrial myopathies present with hypotonia, muscle weakness, and difficulty feeding, while toddlers and older children present with delayed motor development, exercise intolerance, and premature fatigue 5
- Pompe disease, a metabolic myopathy caused by a deficiency of acid-alpha glucosidase, is characterized by severe hypotonia and cardiomyopathy in its classic infantile-onset form 6