Electrodiagnostic Approach to Defects of Neuromuscular Transmission

Neuromuscular junction (NMJ) disorders such as myasthenia gravis, Lambert-Eaton myasthenic syndrome, and botulism are characterized by impaired synaptic transmission leading to weakness. This review examines the electrodiagnostic evaluation of these conditions, emphasizing the importance of techniques such as repetitive nerve stimulation (RNS) and single-fiber electromyography (SFEMG) for confirming the diagnosis and distinguishing presynaptic from postsynaptic defects. The reduced safety factor of neuromuscular transmission (NMT) in postsynaptic disorders produces a decrement in compound muscle action potential (CMAP) amplitude and area with low-frequency stimulation, whereas presynaptic disorders show small baseline CMAPs that increase markedly in amplitude and area (postactivation facilitation) after brief exercise or during high-frequency stimulation. SFEMG, the most sensitive test of abnormal NMT, measures neuromuscular jitter-temporal variability in action potential generation-and also reflects a compromised safety factor. Fiber density remains normal in primary NMJ disorders, distinguishing them from conditions with neuropathic reinnervation, such as motor neuron disease. Proper performance and interpretation of these electrodiagnostic studies are essential for accurate diagnosis, assessment of disease severity, and guiding management of NMJ disorders.