Geometry assessment of anal sphincter muscle based on monopolar multichannel surface EMG signals

Anatomical studies on the external anal sphincter (EAS) indicate that superficial muscle fibres are circular at low depth within the anal canal. A more complex geometry of the fibres is documented for increasing depth within the muscle and along the anal canal. Monopolar intra-anal EMG signals recorded using an array of electrodes placed in circular direction have no common mode components if the muscle fibres are circular, with constant depth within the muscle and parallel to the detection array. Thus, the presence of common mode signals may provide indications about the geometry of muscle fibres of EAS. Intra-anal EMG signals were recorded from EAS of 12 subjects using an anal probe carrying three circumferential arrays of 16 electrodes at three depths within the anal canal. Contribution of common mode components in single MUAPs was lower for MUs located superficially in the muscle (Pearson correlation coefficient: R=-0.75, p≪0.001) and at a lower depth within the anal canal (non-parametric one way Kruskal-Wallis ANOVA, Χ=17.3, p<0.001), in line with EAS anatomy. A large contribution of common mode components was found in the interference signal, suggesting that the signal receives contributions from far, large muscles (e.g. puborectalis, glutei).