Muscle weakness in a Japanese family of Arg1239His mutation hypokalemic periodic paralysis

Familial hypokalemic periodic paralysis is an autosomal dominant disorder. Recently, three predominant mutations were found in the muscle dihydropyridine‐sensitive calcium channel alpha 1‐subunit gene. We present a Japanese family that displays one of these, the Arg1239His mutation. All the affected individuals of this family displayed this mutation. Two cases had a history of recurrent episodes of muscle weakness and difficulty in running before their first paralytic attack. It is suggested that there are no racial differences with this mutation, and that mild muscle weakness occurs not only after but also preceding the first attack.