Rectal adaptation to distention: implications for the determination of perception thresholds.

Chronic changes in rectal compliance and perception are often associated with constipation, but the mechanisms responsible for these changes are not known. These studies evaluated the dynamic response of the rectal wall to distention and, in a separate investigation, the influence of adaptive relaxation on perception thresholds. In Study 1, seven healthy volunteers were evaluated using a computer-controlled barostat to maintain continuous isobaric distention of the rectum at urge threshold over a 25-min period. Changes in intrabag volume were evaluated at minutes 1, 5, and 25. Study 2 investigated changes in perception thresholds with different interstimulus intervals (30 s vs. 60 s) in 16 healthy subjects. Pressure was incremented in steps of 2 mmHg up to discomfort threshold. The mean intrabag volume, pressure, and the compliance index for the first and second 5-s intervals were compared to the last 5 s interval. Statistical analyses were performed using the Wilcoxon Sign-Rank test with Bonferonni corrections. Study 1 showed a significant relaxation of the rectal wall in response to balloon distention with volumes consistently increasing from minute 1 to minute 25. Study 2 showed a significant change in the compliance index at the threshold for moderate urge and intense urge during the 60-s distention that resulted from progressive relaxation of the rectal wall. Study 1 showed an adaptive response of the rectum to distention. Study 2 confirmed these findings and implied a role for this adaptive response in the determination of rectal sensory thresholds.