Standardization of anal sphincter electromyography: normative data

Objectives: Electromyography (EMG) of the external anal sphincter (EAS) is important in the evaluation of conus/cauda lesions, the differential diagnosis of parkinsonism and anal incontinence. The aim of our study was to establish normative data in a sufficiently large group of healthy subjects, using a rigorously standardized examination technique. Methods: Sixty-four subjects (aged 19–83 years) without pelvic or neurological disorders were included. Motor unit potentials (MUPs)/interference pattern (IP) samples were obtained from the EAS using multi-MUP and turn/amplitude analyses, respectively. The effect of age, gender, parity, and constipation on MUP/IP parameters was studied. For MUP parameters the lower/upper limits for mean values, and ‘outlier’ limits, and for IP parameters normal ‘clouds’ were calculated. Results: From 112 muscles 15–30 MUPs were sampled. As no effect of evaluated factors on mean values could be demonstrated, common reference values were calculated. Lower/higher limits for mean values were: amplitude 148/661 μV, duration 3.2/7.8 ms, area 87/625 μVms, and number of phases 2.3/3.7. ‘Outlier’ limits for individual MUPs were: amplitude 84/1315 μV, duration 1.6/13.8 ms, area 46/1222 μVms, number of phases 2/6. From 95 muscles 2706 IP samples were obtained. Conclusions: The presented normative data should allow valid quantitative EMG of the EAS muscle in patients.