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

Objectives: 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. Methods: 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. Results: The BET (P =.006) was longer, and the mean (SD) rectoanal pressure gradient (−58[40] vs −34[26] mm Hg, 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) mm Hg, which was lower (P <.0001) than resting pressure (90 [25] mm Hg). 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. Conclusions: 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.