An Artifact in Concentric Needle EMG Recordings.

Introduction/aims: In healthy subjects, we observed high amplitude motor unit potential (MUP) waveforms that resembled the cannula potential (CP) with a positive sharp wave (PSW)-like waveform. We analyzed the source of this signal, its prevalence, and its effects on the analysis of electromyographic waveforms.

Methods: Three channel recordings were performed to explore the contribution of the needle core and cannula to the MUP. In 7 control subjects with no neuromuscular disease, the EMG interference pattern was quantified for the amplitude asymmetry (AASM) of its positive and negative going peaks.

Results: The large amplitude MUP resembling a CP with a PSW-type waveform is recorded by the needle core, not the cannula. With a slight change in needle position, its amplitude decreased and the waveform had a normal appearance. Thus, it is an artifact. Such potentials (AASM > 150%) are commonly encountered (frequency 7%) and occur in all muscles and subjects in this study.

Discussion: The artifact is an enigma and appears related to the interaction between the needle and muscle fiber(s) similar to that in PSW signals. Failure to recognize these waveforms may result in a false impression of abnormality. AASM calculations can help confirm this artifact in automated analysis algorithms to preclude false positive neurogenic patterns.