A new method for assessing anal distensibility with a barostat and magnetic resonance imaging in healthy and constipated women.

Abstract

Defecation requires relaxation of the internal and external anal sphincters. High anal resting pressure is associated with painful constipation, defecatory disorders, and increased healthcare utilization in constipated patients; the mechanisms are unclear. Perhaps patients with a high anal resting pressure have a less distensible canal, which impedes defecation. In 50 of 64 participants (33 healthy and 17 constipated women), anal pressures and distensibility were measured, respectively, with manometry and balloon distention combined with magnetic resonance imaging; rectal balloon expulsion time (BET) was also studied. The BET (P=.006) was longer, and the mean (SD) rectoanal pressure gradient (-58[40] vs -34[26] mmHg, P=.03) was more negative in constipated than healthy women; anal resting pressure was not different. During anal distention, the balloon expanded rapidly at an opening pressure of 49 (18) mmHg, which was lower (P<.0001) than resting pressure (90 [25] mmHg). The resting pressure was correlated with the opening pressure (r=0.57, P<.0001) and inversely (r=-0.38, P=.007) with maximum volume but not with anal distensibility (volume-pressure slope). In healthy women, the difference (opening-resting pressure) was correlated with anal relaxation during evacuation (r=0.35, P=.04). Anal distensibility and sensory thresholds were not different between constipated and healthy women. Among healthy and constipated women, a greater anal resting pressure is correlated with greater opening pressure and lower maximum volume during distention, and, hence, provides a surrogate marker of anal distensibility. The difference (opening-resting pressure), which reflects anal relaxation during distention, is correlated with anal relaxation during evacuation. Anal resting pressure and distensibility were comparable in healthy and constipated women.