Neonatal myotubular myopathy: neuropathy and failure of postnatal maturation of fetal muscle.

The natural course of the pathologic features in striated muscle was studied in a full-term infant with myotubular myopathy. At 5 days of age a muscle biopsy revealed that more than 90 percent of muscle fibers fulfilled histologic, histochemical and electron microscopic criteria of fetal myotubes. The infant died unexpectedly at 9 months of age from spontaneous rupture of a multifocal cavernous hemangioma of the liver. Postmortem examination revealed that progressive maturation of the fetal muscle had not occurred postnatally, and more than 90 percent of myofibers were still apparent myotubes. This maturational arrest was generalized to all striated muscles. The only changes detected since the neonatal period were hypertrophy of the small population of large fibers, but with minor cytoarchitectural alterations, and loss of the incomplete histochemical differentiation with ATPase stains or dedifferentiation not attributed to postmortem diffusion. Involvement of the gubernaculum testis accounted for the undescended testicles. The brain and spinal cord appeared normal. Evidence of degenerating and regenerating axons in the sciatic nerve suggested that the etiology of this maturational arrest of fetal muscle may be neurogenic.