Design & Development of Rectal EMG Probe for the Assessment of Anal Sphincter Muscles

Abstract

Faecal incontinence (FI) is a debilitating condition that severely impairs a patient’s quality of life ranging from mild distress to progressive social isolation. It is a common condition that results in the inability to control the passage of faecal contents. A prevalence of 2-15% was reported based on Western population studies, with a prevalence of 11-15% in New Zealand. In general, FI is multifactorial with anorectal neuropathy being a key contributor to FI in females, particularly after obstetric injury and with aging. The treatment for FI still remains challenging and the current clinical tools used in the diagnosis of FI are unable to assess the caudal and pelvic (pudendal) nerves supplying the pelvic floor and anal sphincters as they are deeply located. This makes it difficult to be targeted by the traditional clinical methods like cutaneous or needle stimulation and perineal electromyography (EMG). These methods also induce greater discomfort to patients and are inefficient in providing comprehensive anatomical and electrophysiological information, which are essential for studying the sphincter muscle innervation and identification of neuromuscular injuries. In order to overcome these limitations, this research aims to establish a framework to provide comprehensive analysis of human anorectal EMG data with the use of noninvasive multichannel high-resolution (HR) surface electromyography (sEMG) recordings. In this regard, an anorectal probe was designed to hold the multi-electrode array circumferentially and longitudinally to follow the main muscle fibre orientation. The HR-EMG recordings were obtained from patients affected with FI (n=6) and healthy individuals (n=4) during normal and tight contraction periods. The EMG amplitude analysis was performed using the root mean square (RMS) computation and peak-to-peak analysis, since EMG amplitude is an indicator of muscle’s functional behavior. Results revealed 8 out of 10 subjects have a greater EMG amplitude during tight contraction compared to normal contraction, with a mean RMS value of 0.05 mV and 0.03 mV respectively. A similar trend was observed in the peakto- peak EMG amplitude analysis whereby healthy subjects exhibited mean amplitude of 0.151 mV for normal contraction and 0.178 mV for tight contraction, while FI patients showed 0.166 mV and 0.271 mV for normal and tight contractions respectively. Overall, the healthy individuals exhibited an average increment in peak-to-peak EMG amplitude of 0.092 mV while transitioning from normal to tight contraction, while the FI patients showed only a minimal increase of 0.02 mV. This observation was also supported by the statistical analysis test, which revealed a significant rise in EMG profile for control subjects over FI patients with a p value < 0.05. There is a direct correlation between the frequency of motor potentials firing and force exerted by the underlying muscle. Therefore, the FFT (Fast Fourier Transform) frequency analysis was performed and it showed a two-fold increase in dominant frequency while shifting from normal contraction (34-40 Hz) to tight contraction (64-73 Hz). This observation was more prominent in control subjects where an increase by 29 Hz was observed while only 4 Hz increment was observed in FI patients. Furthermore, the skewness and kurtosis analysis computed to characterise the dispersity of FFT envelope, exhibited a shift towards higher frequency while transitioning to tight contraction. This trend was generally observed across the study population. The skewness of FFT envelope for normal to tight contraction revealed an increment from 2.5 to 2.9 and kurtosis of 9.6 to 13.3 respectively. The integration of anatomical structures of External Anal Sphincter (EAS) muscles with EMG amplitude profile was established with 2D spatial amplitude mapping. Through the geometrical mapping, it was found that the EMG amplitude was escalating in a circular direction indicating the dominant influence of EAS circular muscles during the voluntary contractions. Furthermore, the amplitude profile for 8 out of 10 participants increased as it approached the anal verge and most of the innervation occurred in the posterior regions of anal canal. Lastly, 2D time maps were constructed for investigating the peak propagation activity of motor unit (MU) potentials. It was found that the MU peak potentials in control subjects propagated on average 1.8 times faster than FI patients. In summary, the EMG analysis aligns with the hypothesis that EMG amplitude shares a direct relationship with muscle denervation. Henceforth, the EMG analysis performed in this study can be applied to improve the diagnostic value in understanding the muscle function in FI patients and provide new opportunities for patient specific treatments.