Isaacs' syndrome: clinical and electrophysiological response to gabapentin.

A patient with Isaacs' syndrome had generalized muscle spasms, twitching, and progressive muscle stiffness for 21 years. Electrodiagnostic study showed continuous spontaneous motor-unit activity and the presence of M-wave afterdischarges. He responded dramatically to treatment with carbamazepine but developed a drug rash; his treatment was changed to gabapentin with continued improvement. Subsequent nerve conduction studies showed decreased amplitude and later an absence of M-wave afterdischarges. Gabapentin thus appears to be an effective treatment for Isaacs' syndrome.