Congenital myopathy and cardiomyopathy with identical ultrastructural changes.

A 7-day-old girl with congenital hypotonia and unexplained episodes of bradycardia had a broad spectrum of similar skeletal muscle and myocardial degenerative ultrastructural abnormalities. Ultrastructural studies showed obliteration of cross striations, myofilament disorganization, streaming, smearing, clumping, and zigzag Z-band deformities. A decrease in glycogen, mitochondria, and T-tubular system occurred in the regions showing Z-band abnormalities of both skeletal muscle and myocardium. Concurrent structural cardiomyopathy should be considered in patients with congenital myopathies, particularly with unexplained cardiac conduction abnormalities or contractile insufficiency. Ultrastructural evaluation of skeletal and cardiac muscle may be necessary to define such disorders.