Evaluation of the complexity of motor unit potentials in anal sphincter electromyography

Objective Motor unit potential (MUP) morphology can be quantified using parameters describing electrophysiological size and shape (complexity). Traditionally, MUP complexity has been estimated using parameters number of phases and turns. Recently, ‘irregularity coefficient’ (IR), measuring the length of the MUP curve normalized with its amplitude, has been introduced. The aim of this study was to evaluate IR in the external anal sphincter muscle. Methods Sensitivity was examined in 61 patients with chronic cauda equina lesions, and specificity in 75 controls using a standard concentric EMG needle and EMG system with multi-MUP analysis. Results When evaluated separately the sensitivity of IR was 16% lower, and specificity 5% higher compared to number of turns, with both differences decreasing to only 2 and 1%, respectively, when each of these parameters was added to MUP area and duration. Conclusions Our present results suggest that IR provides a similar diagnostic usefulness, but a more appropriate description of MUP complexity compared to traditional MUP parameters. Significance IR seems to be suitable to complement one of MUP parameters measuring electrophysiological MUP size in the future quantitative EMG.