DISORDERS OF MUSCLE ENERGY METABOLISM
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The two principal sources of energy for skeletal muscle are fatty acids and glucose. Abnormalities in either glucose or lipid utilization can be associated with distinct clinical presentations that can range from an acute, painful syndrome with rhabdomyolysis and myoglobinuria to a chronic, progressive muscle weakness simulating muscular dystrophy. Diagnosis usually requires biochemical-enzymatic studies of biopsied muscle.
Progressive muscle weakness usually beginning in the third or fourth decade can be due to the adult form of acid maltase deficiency (Pompe’s disease). Respiratory failure is often the initial manifestation; treatment with enzyme replacement may be of benefit. Progressive weakness beginning after puberty occurs with debranching enzyme deficiency. Glycolytic defects, including myophosphorylase deficiency (McArdle’s disease) or phosphofructokinase deficiency, present as exercise intolerance with myalgias. Disorders of fatty acid metabolism present with a similar picture. In adults, the most common cause is carnitine palmitoyltransferase deficiency. Exercise-induced cramps and myoglobinuria are common; strength is normal between attacks. Dietary approaches (frequent meals and a low-fat high-carbohydrate diet, or a diet rich in medium-chain triglycerides) are of uncertain value.