Congenital myopathy with type II muscle fiber hypoplasia.

An unusual congenital neuromuscular disease characterized by atypically small type II muscle fibers is reported. The patients were a 9-year-old boy and a 3-year-old boy, both of whom showed motor retardation with proximal muscle wasting and hypotonia. Muscle biopsy revealed hypoplastic type II fibers and normal type I fibers. The observed hypoplasia seems to be a primary alteration.