[Isaacs' syndrome with abnormal F response and effects of double filtration plasmapheresis: a case report].

We reported a case of Isaacs' syndrome with abnormal F response detected electrophysiologically. A 14-year-old female was admitted to Hirosaki University Hospital with complaints of progressive myokymia and muscle cramp. PHT and CBZ were partially effective, but discontinued for drowsiness. A neurological examination revealed prominent myokymia and muscle cramp in the legs. The myokymia were worsened by exercise, bathing and diet. An electrophysiological examination showed characteristic F-response; high amplitude, long duration and increased number of phases. The epidural nerve block brought about a disappearance of the myokymia and an improvement of the abnormal features of F response. After repeated double filtration plasmapheresis, the myokymia and abnormal features of F response were remarkably reduced. Although Isaacs' syndrome is thought to have a hyperexcitability at the site of distal peripheral nerve, we suggested that the hyperexcitability might exist at the site of proximal region, and that immunological mechanisms underlie the cause of myokymia and unusual F-response in this case.