Autosomal dominant monosymptomatic myotonia permanens.

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.