[A Japanese family with paramyotonia congenita which has a mutation in the muscle sodium channel gene].

We report here a Japanese family with paramyotonia congenita. The proband was a 42-year-old woman (case 1), who noticed muscle stiffness and weakness in the cold since the age of 7 years. These symptoms were alleviated by warming. Her eldest son (case 2) also experienced similar symptoms, while her younger son and daughter were healthy. Neurological examination in case 1 revealed mild weakness in facial and neck muscles. Cold-induced muscle stiffness and weakness were present. Electromyography showed myotonic discharges, intensified by cooling or repetitive exercise. The amplitude of the compound muscle action potentials was also reduced by the repetitive exercise and cooling. Serum chemistry including potassium and CK was normal. Molecular analysis of SCN4A (exon22-24) by SSCP and nucleotide sequencing revealed a C-to-T transition at nucleotide 3,938, causing a substitution of 1313methionine of threonine in case 1. This mutation was confirmed by PCR-RFLP with a mismatched primer; the proband (case 1) and her eldest son (case 2) had a heterozygous mutation, while the other family members did not. This is the first report that a mutation in SCN4A was identified in a Japanese family with paramyotonia congenita.