Are Muscle Fibers Denervated in Myotonic Dystrophy?

An underlying neurogenic abnormality has recently been postulated in the muscular dystrophies. To test this hypothesis, we applied a widely accepted criterion of denervation—ie, an increase in extrajunctional acetylcholine (ACh) receptor sites—to muscle biopsy specimens from nine patients with myotonic dystrophy and three with amyotrophic lateral sclerosis (ALS). The ACh receptor sites were determined by means of iodine 125-labeled α-bungarotoxin binding, measured by scintillation counting and autoradiography. None of the myotonic dystrophy muscles showed increased extrajunctional ACh receptor sites, even in the smallest fibers. By contrast, muscle biopsy specimens from patients with ALS showed notably increased extrajunctional ACh receptor sites, especially in the small fibers. Our findings do not support the hypothesis of a neurogenic defect in myotonic dystrophy.