A dominantly inherited muscle disease that begins in early childhood and is characterized by severe myotonia (delayed relaxation of a muscle) after forceful voluntary contractions. Muscular hypertrophy is common and myotonia may impair ambulation and other movements. Myotonia typically becomes less severe with repetitive voluntary contractions of the affected muscles. Generalized myotonia (of Becker) is an autosomal recessive variant of myotonia congenita that may feature more severe myotonia and muscle wasting. (From Adams et al., Principles of Neurology, 6th ed, pp1476-7; Joynt, Clinical Neurology, 1997, Ch53, p18).