Delay in the maturation of muscle fibers in infants with congenital hypotonia.

Muscle biopsies of hypotonic children have shown delayed maturation of a fetal type of muscle fibers: subsarcolemmal halo devoid of activity for mitochondrial dehydrogenases, type II predominance and in some cases abnormal dispersion of fiber diameter. Fiber subtypes within group II were also abnormal. One case has definite embryonic characteristics with presence of myoblasts. Not a single clinical pattern was present in these patients and a variety of associated disorders were recognized. Some patients had a clinical picture corresponding to congenital benign hypotonia as described by Walton.