Interspike interval analysis in a patient with peripheral nerve hyperexcitability and potassium channel antibodies.

Neuromyotonia or Isaacs' syndrome is a rare peripheral nerve hyperexcitability disorder caused by antibodies against potassium channels of myelinated axons. We present the high-density surface electromyographic (EMG) recordings of a patient with fasciculations and cramps due to neuromyotonia. To characterize the time course of hyperexcitability, we analyzed the interspike intervals (ISIs) between fasciculation potentials, doublet, and multiplet discharges. ISI duration increased within each burst. The ISI histograms found can be explained by the recovery cycle of the myelinated axon and its dependency on the slow potassium conductance. We conclude that ISI analysis is a useful tool to understand the membrane dynamics underlying abnormal motor unit activity.