Reversible F-wave hyperexcitability associated with antibodies to potassium channels in Isaacs' syndrome

We report a case of Isaacs' syndrome showing F-wave hyperexcitability which was reversible after either epidural block or plasmapheresis. A 14-year-ol d girl with progressive muscle cramp and myokymia in the legs showed high a mplitude, long duration, polyphasic F-waves after either tibial nerve or pe roneal nerve stimulation. Potassium current of PC-12 cell membrane was supp ressed hy her serum, IgM of which strongly reacted with proteins of approxi mately 50 kDa of the lysates of a PC-12 cell line. After an epidural nerve block,vith I per cent lidocaine 10 mi, the myokymia temporally improved, du ring which time the hyperexcitable F-waves became normalized. We treated he r with double filtration plasmapheresis, which resulted in remarkable impro vement of myokymia; simultaneously, the abnormal. F-waves showed a decrease in amplitude, duration, and in the number of phases. We concluded that the re was hyperexcitability of the proximal site of motor fibers in our patien t, and possible immune mechanisms gave rise to the myokymia and unusual F-w ave features. Eur J Neurol 6:95-98 (C) 1999 Lippincott Williams & Wilkins.