Issues & opinions: Single muscle fiber discharges (insertional activity, end-plate potentials, positive sharp waves, and fibrillation potentials): A unifying proposal

Abstract

The exact origin and precise morphologic explanation of positive sharp waves (PSWs) are presently lacking. Observing normal needle electromyographic insertional activity reveals two types of waveforms: (1) biphasic negative/positive spikes, and (2) positive spikes followed by a small negative phase. In the end-plate region, it is possible to occasionally observe a biphasic end-plate spike transform into a monophasic positive end-plate waveform. It is postulated that this waveform is simply a form of intracellular recording for the biphasic end-plate spike or a form of extracellularly recorded but blocked single muscle fiber discharge. Similarly, the observed monophasic positive insertional activity may be an intracellularly recorded single muscle fiber discharge or a blocked extracellular discharge originating about the needle electrode. Applying this reasoning to PSWs suggests that they may also be an intracellular recording of a fibrillation potential, or needle-induced extracellular blocked local single muscle fiber discharge. This unifying concept is applied to various clinical situations purported to demonstrate "different" types of PSWs.