Maturational arrest of fetal muscle in neonatal myotonic dystrophy. A pathologic study of four cases.

Skeletal muscles from four infants with a severe neonatal form of myotonic muscular dystrophy showed histopathologic features of immaturity. Three of the infants died in the neonatal period and were studied at autopsy; one of these and the still-living infant had a gastrocnemius muscle biopsy. The most severely involved muscles were those associated with arthrogrypotic joints regardless of function as flexors or extensors. Pharyngeal muscles and the diaphragm were also severely involved. Immature features included irregularly distributed small, round muscles fibers with large vesicular internal nuclei and sparse myofibrils. Histochemical differentiation was incomplete and fiber types often could not be distinguished. Muscle fiber degeneration and other features of myotonic dystrophy in adult muscle were lacking. Electron microscopy showed fine granular chromatin and convoluted nuclear membranes of centronuclear fibers, dialated transvers tubules that were aligned longitudinally as in fetal myotubes, poorly formed Z-bands, simple mitochondria, and many satellite cells. We suggest that these features represent an arrest in fetal muscle maturation due to unresponsiveness of an abnormal sarcolemma to trophic influences of normal innervation.