The clinical and laboratory evaluation of the patient with suspected metabolic myopathy is reviewed from the Department of Neurology, Children’s Hospital, Boston, MA. Myopathies are classified as static characterized by proximal weakness, generalized weakness, and developmental delay; and dynamic with recurrent episodes of reversible muscle dysfunction, sometimes myoglobinuria, related to exercise intolerance, fasting, exposure to cold, anesthesia, intercurrent infection, or a low-carbohydrate, high-fat diet. Both forms are common in mitochondrial myopathies. The type or duration of exercise inducing weakness may be specific: Prolonged, low-intensity activity (eg walking) - induced weakness occurs with fatty acid oxidation (FAO) defects; high-intensity exercise (eg weight lifting or sprinting) - glycogen or glucose metabolism defects. Myoglobinuria may be induced by inborn errors of glycogen/glucose metabolism, FA metabolism, and some mitochondrial cytopathies. Laboratory tests include CK, elevated in glycogen defects and lactate dehydrogenase deficiency; blood lactate and pyruvate elevated in mitochondrial myopathies; liver transaminases elevated in FAO defects; and abnormal carnitine, acylcarnitine, free fatty acids, and hypoketotic hypoglycemia in lipid metabolic disorders. EMG, Forearm Ischemic Exercise Test, and muscle biopsy with molecular studies may be required in diagnosis. [1]
COMMENT. A helpful algorithm for the step-by-step diagnosis of metabolic myopathies is provided by the authors.