Electrodiagnostics Of Myasthenia Gravis

Muscle fibers of patients with MG are easily fatigued, and thus do not respond as well as muscles in healthy individuals to repeated stimulation. By stimulating a nerve-muscle motor unit with short sequences of rapid, regular electrical impulses, before and after exercising the motor unit, the fatiguability of the muscle can be measured. This is called the repetitive nerve stimulation test. In single fiber electromyography (SFEMG), which is considered to be the most sensitive (although not the most specific) test for MG, a thin needle electrode is inserted into different areas of a particular muscle to record the action potentials from several samplings of different individual muscle fibers. Two muscle fibers belonging to the same motor unit are identified and the temporal variability in their firing patterns are measured. Frequency and proportion of particular abnormal action potential patterns, "jitter" and "blocking," are diagnostic. Jitter refers to the abnormal variation in the time interval between action potentials of adjacent muscle fibers in the same motor unit. Blocking refers to the failure of nerve impulses to elicit action potentials in adjacent muscle fibers of the same motor unit.