Combining the strength-duration curve of the external anal sphincter with manometry for the assessment of faecal incontinence

Background The strength-duration curve of a muscle is thought to be a measure of its innervation. This study was designed to evaluate the ability of the strength-duration curve of the external anal sphincter to discriminate between controls and patients with faecal incontinence. Methods Forty-three women with faecal incontinence due to sphincter weakness were studied together with 45 age-matched women with no disorders of defaecation. Strength-duration curves of the external anal sphincter and anorectal manometry were recorded for all subjects with the additional measurement of pudendal nerve terminal motor latency in the incontinent group. Results Logistic regression was performed using resting and voluntary squeeze pressures and current strengths at 28 different pulse durations to develop a predictive equation for incontinence. Only currents at 1 and 6 ms were significant predictors. When anorectal manometry data were included, only the current required to elicit contraction at 1 ms was necessary. The following regression equation can be applied to predict continence in this population with a sensitivity of 95 per cent and a specificity of 100 per cent: logit(P) = 4.1605 - (0.0559 x squeeze pressure)- (0.1755 x resting pressure) + 0.8622I(1ms). A negative value indicates continence. Conclusion The strength-duration curve, when used in conjunction with anorectal manometry, may have a role in the investigation of faecal incontinence.