Metabolische Myopathien

Abstract

Metabolic myopathies are rare congenital metabolic disorders which are caused by enzyme defects of muscle energy metabolism. Amongst these, the glycogen storage diseases (GSD) or glycogenoses and the lipid storage myopathies are the most important groups, with Pompe disease (GSD II) and McArdle’s disease (GSD V) being the most common glycogenosis. Late-onset Pompe disease (LOPD) usually manifests as a limb girdle myopathy and is characterized by respiratory insufficiency, which frequently requires non-invasive ventilation. The underlying enzyme defect can be confirmed in a dried blood spot and by molecular genetic analysis in blood or other tissues. Enzyme replacement therapy is currently the only established causative treatment available. McArdle’s disease is characterized by exercise-induced symptoms such as myalgia, cramps and muscle weakness. In some cases, rhabdomyolysis may develop and give rise to myoglobinuria and even acute renal failure. Treatment consists of symptomatic measures such as providing creatine, vitamin B6 and the intake of carbohydrates (cane sugar) prior to exercise; most important, however, is moderate aerobic training. Lipid storage myopathies are a biochemically heterogeneous group, which includes the defects of mitochondrial fatty acid uptake and β-oxidation, of coenzyme Q10 metabolism and neutral lipid storage disease. Patients with fatty acid oxidation defects are usually asymptomatic in everyday life; prolonged exercise, fasting or intercurrent infections may, however, induce episodic rhabdomyolysis with myoglobinuria and in infants also hypoketotic hypoglycemia. The analysis of plasma acylcarnitine profiles in dried blood spots is the mainstay for establishing the diagnosis of a fatty acid oxidation defect. Molecular genetic analysis is, however, mandatory for final confirmation of all metabolic myopathies. Treatment of lipid storage myopathies should focus on avoiding precipitating factors. Generally, a normocaloric, low-fat, high-carbohydrate diet is recommended. Depending on the enzyme defect, dietary supplements with medium-chain triglycerides, L-carnitine or coenzyme Q10 can be considered.