The diagnosis of neuromuscular junction disorder

Neuromuscular junction disorders are a condition often seen in the outpatient setting. Patients frequently complain of muscular proximal weakness in the extremities with/without bulbar and extraocular findings. In myasthenia gravis, pathology occurs at the postsynaptic membrane. The diagnosis of neuromuscular junction disorder derives from a thorough history and physical examination, electrodiagnostic studies with repetitive nerve stimulation (RNS), exercise testing, and single-fiber EMG (SFEMG). SFEMG testing is the gold standard for neuromuscular junction study with a sensitivity of up to ۹۹%. Anticholinesterase medication should be stopped ۸ to ۲۴ hours before performing the study. A routine motor and sensory nerve conduction study should first be conducted to ensure no other nerve pathology exists and that the clinician can interpret the RNS results. RNS involves supramaximal motor nerve stimulation ۵-۱۰ times at ۳ Hz to determine compound muscle action potential (CMAP) amplitude decrement. The ۴th CMAP amplitude is compared with the first. A decrement of ۱۰% is significant. RNS study should ideally be performed on the proximal and most affected muscle to increase sensitivity.