Cerebral dysgeneses and their influence on fetal muscle development.

Abnormal suprasegmental influences of the brainstem and cerebellum on the developing motor unit during the histochemical stage of muscle development (20-28 weeks gestation) may alter the rate of maturation of striated muscle or may cause abnormal proportions and relative sizes of histochemical fibre types. Such aberrations without primary myopathic or denervative changes are commonly found in children with cerebral malformations, particularly associated with cerebellar hypoplasia. Upper motor neuron disease during embryonic life may explain histochemical alterations in some nonprogressive 'congenital myopathies' such as congenital muscle fibre-type disproportion, nemaline rod disease, and central core disease. Suprasegmental factors also may contribute to some aspects of the muscle pathology in the muscular dystrophies, especially the Fukuyama type of congenital muscular dystrophy regularly associated with cerebral dysgenesis. Fibre-type predominance or delayed histochemical differentiation thus may serve as a useful marker, in the muscle biopsy, of upper motor neuron disease. During the critical period of muscle development implicated, the corticospinal tract probably is of much less importance in muscle maturation than the multiple small bulbospinal pathways which also subserve motor control in nonmammalian vertebrates.