Congenital neuromuscular disorder with predominant mitochondrial changes in type II muscle fibers.

The muscle biopsy of a young boy presenting a marked hypotrophy of stature and weight, proximal muscle weakness, uncertain gait, and partial hearing loss, showed an abnormal distribution of the mitochondrial activities in type II muscle fibers by histochemical methods. Electron microscopy confirmed the presence of giant mitochondria in these muscle fibers, and at a lesser degree in some type I fibers. These findings contrast with the usual type I predominance of the mitochondrial changes in the different "mitochondrial" myopathies.