In adults diagnostic features of myotonic muscular dystrophy include reflex, percussion, and grip myotonia. This is a characteristic inability to release after exerting a grip on an object. There is also weakness and atrophy of the skeletal muscles, premature baldness, cataracts, and mental retardation. Cardiologic involvement includes arrhythmia, intraventricular conduction defects, and com¬plete heart block that may require electronic pacing. There is fibrosis and fatty infiltration with degeneration of the specialized electrical conduction tissue, particularly the sinus node, AV node, and Purkinje system.