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An 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, characterized by neonatal MUSCLE HYPOTONIA, feeding difficulties, respiratory muscle weakness, and an increased incidence of MENTAL RETARDATION. (From Adams et al., Principles of Neurology, 6th ed, pp1423-5; Joynt, Clinical Neurology, 1997, Ch16, pp16-7)