Myotonia permanens is associated with a G1306E mutation in the SCN4A gene.Two sporadic patients have been reported,but the clinical phenotype has not been fully characterized.The authors report a family in which the disease is autosomal dominantly inherited.The patients have severe myotonia,but the clinical picture is not qualitatively different from that seen in other nondystrophic myotonias. Myotonia permanens is associated with a G1306E mutation in the SCN4A gene. Two sporadic patients have been reported, but the clinical phenotype has not been fully characterized. These authors report that a family in which the disease is autosomal dominantly inherited. These patients have severe myotonia, but the clinical picture is not qualitatively different from from that seen in other nondystrophic myotonias.