[Electrophysiological testing in muscle channelopathies]

Electrodiagnostic testing should always be tailored to the clinical setting; in the muscle channelopathies, provocative tests are essential to demonstrate the modifications of the muscle excitability. When a permanent muscle weakness is present, a post-exercise (or post-tetanic) potentiation should be search to demonstrate a presynaptic neuromuscular channelopathy; when the weakness is fluctuating it is useful to perform repetitive nerve stimulation to see if a decrement is present or if the jitter is prolonged, indicative of a postsynaptic neuromuscular channelopathy; when the weakness is episodic, the prolonged exercise test is the only test to demonstrate the appearance of a late post-exercise decrement (sodium and calcium channelopathies), and when a myotonic reaction is seen to search the myotonic discharges by needle emg. The concomitant presence of myotonic discharges and abnormal decrement of the motor responses during repetitive nerve stimulation train depend on the type of mutations in the chloride gene and the amount of CTG repeats in myotonic dystrophy type 1. In paramyotonia congenita there a prolonged decrement when the tests are carried out with cooling the recorded muscle. Electrophysiological testings appear therefore useful to diagnose muscle channelopathies but also give information about the prognosis of the disorder; they could also be viewed as useful tests to predict response to treatments.