[Electrophysiological signs of muscle channelopathies by causal mutation]

Nondystrophic myotonias and familial periodic paralyses are disorders of muscle excitability caused by mutations in genes coding for voltage-gated ion channels. Electromyographic (EMG) diagnosis of these diseases is based on needle searching of myotonic discharges and on several exercise tests, performed by surface recording of compound muscle action potential (CMAP) before and after voluntary contractions of various durations. After a short exercise test (10 seconds contraction), three abnormal changes of CMAP have been defined in muscle channelopathies: 1) appearance of repetitive discharges (post-exercise myotonic potentials, PEMP), 2) transient or long-lasting amplitude reduction, 3) long-lasting amplitude increase. These abnormalities become more marked or decline with exercise repetition (repeated brief exercise test), depending on the causal mutation. After long exercise test (5 minutes contraction), three other abnormalities of CMAP amplitude have been described: 1) immediate increase, 2) immediate decrease, 3) progressive or delayed decrease. Combining the responses to the different tests has disclosed five main EMG types of muscle channelopathies (I to V), that correlated with different pathophysiological mechanisms and with subgroups of mutations. This classification may guide molecular diagnosis in clinical practice.