Electrophysiologic Testing for Acquired and Congenital Myasthenia Gravis

Repetitive nerve stimulation, needle electromyography, and single fiber EMG are useful in sorting out myasthenia gravis from clinical look-alikes. Routine nerve conduction studies are almost always normal in myasthenia gravis but are abnormal in the Lambert-Eaton Syndrome, botulism, some intoxications, and many neuropathies. In health, depolarizing the motor axon causes complete depolarization and contraction of all the muscle fibers in its motor unit. The action potential causes enough calcium to enter the nerve terminal to release enough vesicles filled with enough acetylcholine to bind with enough acetylcholine receptors to cause sufficiently high end plate potentials to make the muscle fibers depolarize and contract. This excess of acetylcholine+receptor interactions beyond what is necessary is called the 'safety factor.'