Evaluation of neuromuscular junction disorders in the electromyography laboratory.

Neuromuscular junction (NMJ) disorders may be demonstrated using repetitive nerve stimulation (RNS) testing and single-fiber electromyography (SFEMG). RNS testing with low frequency stimulation reduces the safety factor of neuromuscular transmission (NMT) and may elicit decrementing compound muscle action potential (CMAP) responses. Exercise or tetanic nerve stimulation may potentiate acetylcholine release in presynaptic NMT disorders with CMAP facilitation. SFEMG is a selective recording technique assessing MFAPs within the same motor unit. Jitter is increased in NMJ disorders, and is the temporal variability between these MFAPs. Impulse blocking reflects failure of NMT. RNS and SFEMG findings in NMJ disorders are reviewed.