Gender differences in the longitudinal pressure profile of the anal canal related to anatomical structure as demonstrated on three-dimensional anal endosonography

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Abstract
Background: Anal canal squeeze pressure is assumed to be due to external sphincter contraction, but the contribution of other muscles has not been explored. Methods: Ten male and ten nulliparous female asymptomatic subjects had three-dimensional anal endosonography and manometry. Incremental squeeze pressures at 0·5-cm intervals, expressed as a percentage of the maximum pressure recorded anywhere in the canal, were related to the following anatomical levels: puborectalis, overlap between external anal sphincter (EAS) and puborectalis, external and internal anal sphincters, and external anal sphincter only. Levels were determined by coronal and sagittal endosonographic reconstructions. Results: Puborectalis was the same length in men and women (median 23·9 versus 27·1 mm) but represented a greater proportion of the anal canal in women (45 versus 61 per cent; P = 0·02). At the level of puborectalis alone, the pressure generated as a proportion of maximum anal canal pressure was 71 (range 32–100) per cent in men and 62 (range 32–100) per cent in women. At the level of the EAS alone, the pressure was 60 (4–98) per cent in men and 82 (41–100) per cent in women; where the external sphincter was overlapped by puborectalis, the pressure was 98 (60–100) per cent in men and 75 (47–100) per cent in women. Conclusion: Maximal anal canal squeeze pressure is found where the puborectalis overlaps the EAS. This segment represents a significant proportion of anal canal length in women.