[Neuromuscular choristoma]

Neuromuscular choristomas are malformative pseudotumoral masses composed of striated muscle and peripheral nerves. This rare condition almost exclusively affects large nerve trunks of infants and young children, and may cause neurologic deficits of variable severity. We report a case of neuromuscular choristoma identified in the lumbosacral lipoma of a 4-month-old boy. The lesion was characterized by an organoid association of myogenic and neurogenic elements reminiscent of neuromuscular units. Myosin immunophenotyping revealed disordered groups of type-I and type-II extrafusal myofibers. There was no immunoreactivity for smooth muscle specific alpha actin. Most participating axons were of the large myelinated type. Pathogenetic theories of neuromuscular choristoma involve aberrant migration of developing peripheral nerves, dysgenesis of muscle spindles, as well as mesenchymal differentiation of the primitive neuroectoderm. Neuromuscular choristomas arising in connection with congenital lipomas may derive from a dysembryogenic process of stem cells of the caudal neural tube.