Functional asymmetry of external anal sphincter (EAS) innervation has been studied for its potential role in the prediction and diagnosis of fecal incontinence (FI). Intramuscular electromyography (EMG) has been utilized to study the functional asymmetry [1]. However, the low spatial coverage and inv/’asive nature of needle electrodes has prohibited the comprehensive and long-term assessment of the sphincter innervation. In this study, a novel intra-rectal high-density surface EMG (HD-sEMG) probe was employed to compare sphincter innervation asymmetry between healthy and incontinent subjects.

Subjects were pre-screened with endoanal ultrasound, and subjects with EAS defects or perineal scarring were excluded from participation. Surface EMG was recorded during voluntary sphincter contraction using a 64-channel (8 x 8) intra-rectal probe, shown in figure 1, from both healthy (n=5) and incontinent (n=2, Wexner score = 15±2) groups. Surface interferential HD-EMG recordings were then decomposed to constitutive motor unit action potential trains, using K-means clustering convolution kernel compensation (KmCKC) algorithm[2]. The innervation zone for each motor unit was identified based on signal polarity inversion and MUAP propagation along the circular direction of the resultant bipolar EMG mapping. The asymmetry index (AI) was determined by taking the distance of the innervation zone distribution barycenter to the center of the probe, with a resultant value ranging from 0 (symmetric) to 1 (completely asymmetric).

MUAPs were successfully decomposed from all seven subjects. The healthy and incontinent groups have matched average age, 64.6±3.5 years and 65.0±1.4 years respectively, thus eliminating the effect of aging on innervation asymmetry. The AI values for healthy group were calculated as 0.46±0.17 and 0.74±0.26 for incontinence group.

Prior research suggests that asymmetrical innervation may result in unbalanced pressure zones in the rectum, impairing closure efficiency and patients’ continence[3]. Our results agree with these observations, and help to further establish a relationship between EAS innervation symmetry and fecal incontinence. The average AI was remarkably larger in incontinent subjects than in the healthy control group, which indicates that asymmetric EAS innervation may influence the closure efficiency of the EAS.

Although the subject pool of the current study is small and limited to elderly subjects, our preliminary results have shown a marked difference between severe incontinence patients and healthy controls. Increased asymmetry in incontinence patients may signify a correlation between AI and incontinence in the elderly population. Larger patient recruitment is necessary to verify this hypothesis.

Enck P, Franz H, Azpiroz F, et al. Innervation zones of the external anal sphincter in healthy male and female subjects. Digestion 2004;69(2):123-30
Ning Y, Zhu X, Zhu S, et al. Surface EMG decomposition based on K-means clustering and convolution kernel compensation. IEEE journal of biomedical and health informatics 2015;19(2):471-77
Perry RE, Blatchford GJ, Christensen MA, et al. Manometric diagnosis of anal sphincter injuries. The American Journal of Surgery 1990;159(1):112-17