Exploration électrophysiologique des pathologies de la jonction neuromusculaire

Abstract

Electrophysiological investigations play a key role in the diagnosis of neuromuscular junction (NMJ) disorders. Low-frequency repetitive nerve stimulations (RNS) (2 or 3 Hz) are the most commonly used electrophysiological test for detecting NMJ abnormalities. They should be performed on several nerve/muscle pairs and focused on clinically deficient territories if possible. Exploration of ocular, facial, bulbar, axial and proximal territories is usually more sensitive than in distal territories. A decrement of the amplitude and surface of the compound muscle action potential (CMAP) superior to 10% is considered pathological. RNS require a rigorous technique to avoid artifacts, often related to patient movements. Their sensitivity and specificity are both imperfect; indeed, decrements may be absent in genuine myasthenia gravis, but can sometimes be found in motor neuron or muscular disorders. Classical nerve conduction studies and myography are therefore essential to avoid false positives and search for these differential diagnoses. In addition, the discovery of a low-amplitude CMAP should systematically suggest a presynaptic blocking of the NMJ, and lead to search a CMAP increment (or "potentiation") after brief exertion. Finally, in case of suspicion of pure ocular or generalized myasthenia gravis without decrement, the electrophysiological investigations may be continued with single-fiber electromyography, which enables the study of neuromuscular transmission at the muscle fiber level, and is more sensitive in these difficult cases.