autosomal dominant neuromuscular disorder which usually presents in early adulthood, characterized by progressive muscular atrophy (most frequently involving the hands, forearms, and face), myotonia, frontal baldness, lenticular opacities, and testicular atrophy; cardiac conduction abnormalities, diaphragmatic weakness, and mild mental retardation may also occur; congenital myotonic dystrophy is a severe form of this disorder.