Repetitive nerve stimulation and muscle membrane excitability: case report and review.

Few muscle disorders can be diagnosed by repetitive nerve stimulation (RNS). Decreasing compound muscle action potentials (CMAP) on high frequency RNS is recorded in muscle channelopathies, and particularly in sporadic and recessive congenital myotonia. In this myopathy, decreasing CMAP after exercise test and RNS are the most sensitive electrophysiological in detecting muscle membrane dysfunction and are considered highly informative even in mildly symptomatic patients. We report on a patient with excercise-induced diffuse muscle cramps and myalgia; muscle biopsy and laboratory investigations were normal. Decreasing CMAP on high frequency RNS suggested muscle membrane conduction anomalies and, though clinical and electrical myotonia was not detected, the neurophysiological finding raised the suspicion of congenital myotonia and addressed to molecular investigation.