Mitochondrial myopathy and cardiomyopathy in siblings.

Two siblings with infantile lactic acidosis and mitochondrial myopathy are described. The first child, a girl, died at 5 months of age from severe lactic acidosis after about 3 weeks of progressive muscular hypotonia. The younger brother had congenital lactic acidosis but no other symptoms until 6 months of age when progressive muscle weakness appeared. Treatment with dichloroacetate lowered the serum lactic acid level but did not affect his clinical condition. At 13 months of age, cardiomyopathy was diagnosed and he died at the age of 29 months of circulatory failure. Both children had mitochondrial myopathy. Postmortem examination of the boy revealed marked morphologic changes of the mitochondria in both skeletal muscle and the myocardium; biochemical investigation of skeletal muscle mitochondria demonstrated deficiencies in both complex I (NADH ferricyanide reductase) and complex IV (cytochrome c oxidase). The disease in these siblings differs in several respects from previously reported patients with mitochondrial myopathy and cytochrome c oxidase deficiency.