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‐old girl with progressive muscle cramp and myokymia in the legs showed high amplitude, long duration, polyphasic F‐waves after either tibial nerve or peroneal nerve stimulation. Potassium current of PC‐12 cell membrane was suppressed by her serum, IgM of which strongly reacted with proteins of approximately 50 kDa of the lysates of a PC‐12 cell line. After an epidural nerve block with 1 per cent lidocaine 10 ml, the myokymia temporally improved, during which time the hyperexcitable F‐waves became normalized. We treated her with double filtration plasmapheresis, which resulted in remarkable improvement of myokymia; simultaneously, the abnormal F‐waves showed a decrease in amplitude, duration, and in the number of phases. We concluded that there was hyperexcitability of the proximal site of motor fibers in our patient, and possible immune mechanisms gave rise to the myokymia and unusual F‐wave features.