Impaired muscle fiber type differentiation in a child with nemaline myopathy

SINCE NEMALINE MYOPATHY was first described by Shy et al., 1 a n u m b e r of cases have been reported. The disorder is charac ter ized clinically by generalized muscle hypotonia f rom early infancy and by muscle weakness predominant ly involving the neck and face. Al though the presence of rod (nemal ine) bodies in the small-cal ibered fibers is a character is t ic and diagnostic finding in nemal ine myopathy, the n u m b e r of rod bodies per se is occasionally out of proport ion to the clinical severity. In additon, most of t h e pat ients have shown abnormal fiber type distribution, including hypoplast ic type 1 fibers and type 1 fiber p redominance . The abnormal muscle histochemistry, probably induced by an impaired neural influence on the developing muscles, and muscle fiber immatur i ty have been assumed to generate muscle weakness. 2-6 We describe an incidence of nemaline myopathy t ha t followed a typical course but produced extremely severe neck muscle weakness. In addit ion to the presence of numerous rod bodies in the muscle fibers, most (96%) of the fibers behaved as type 2C on ATPase staining, indicat ing impaired muscle fiber differentiat ion from immature type 2C fibers into ei ther ma tu re type 1, 2B, or 2A fibers.