14 - Basic Electromyography: Analysis of Spontaneous Activity

Abstract

The analysis of spontaneous activity is the first part of the needle electromyography (EMG) examination of every muscle. The presence of abnormal spontaneous activity on EMG is of key importance and yields several important pieces of information. First, the distribution of abnormal spontaneous activity often indicates the neuroanatomic localization of the lesion. Second, the type of abnormal spontaneous activity often provides specific diagnostic information. Certain types of spontaneous activity are associated only with specific disorders. Third, the degree or amount of spontaneous activity often helps to determine the severity of the lesion. Lastly, the presence of abnormal spontaneous activity may yield information regarding the time course of the lesion.The identification of any spontaneous activity can be achieved by either pattern recognition or analysis of the waveform. With experience, the characteristic appearance and sound of each waveform become easily recognizable. However, when first learning needle EMG or when encountering an unusual waveform, one must be able to systematically analyze the waveform according to the following attributes: (1) morphology, (2) stability, and (3) firing characteristics. Using this information, nearly every spontaneous waveform can be identified correctly. The electromyographer must be able to identify waveforms generated in the following locations: at or near the neuromuscular junction: endplate noise and spikes; the muscle fiber: fibrillations potentials and positive sharp waves, myotonic discharges and complex repetitive discharges; and the main axon / neuron: fasciculation potentials, doublets, triplets, multiplets, myokymic discharges, cramp potentials and neuromyotonic discharges.