How Do We Evacuate Stool? Insights From Combined High-Resolution Anorectal Manometry With Defecography

Abstract

Introduction: The sense of incomplete evacuation is a feature of a defecatory disorder- however the mechanism of evacuation is not well understood. The aim of this study was to use simultaneous barium defecography and high resolution anorectal manometry (HRAM) to elucidate the mechanism of defecation in patients and controls Methods: Simultaneous seated HRAM and defecography (using180ml barium paste) were performed in 26 constipated women and 22 asymptomatic controls. Anal diameter and rectal pressures during evacuation and percentage of barium emptied, and the rectoanal pressure gradient during evacuation were calculated. Values less than 25th percentile for controls was considered abnormal. Results: Healthy women evacuated 80% of barium paste (median, IQ range-18-90%) while patients evacuated 21% (0-70%). Five controls and 12 patients evacuated < 18% of barium (25th percentile for controls). They were classified as “poor-evacuators” or PE); of these 9 had an abnormal balloon expulsion test (>60 seconds). The rest were classified as “adequate evacuators” (AE). Rectal and anal pressures increased initially during evacuation in all women. Thereafter, anal pressures declined, but the rectoanal pressure gradient exceeded 0 mm Hg only in AEs, enabling evacuation in this group. Rectal pressure at the onset of evacuation (median 87 vs 47mmHg, p=0.1) and anal diameter during evacuation (13 vs 0mm, p=0.002) was greater in AEs than PEs. Among AEs, a moving average of recto-anal pressure gradients (median = 4.2 mmHg) was positive (>0 mm Hg) for a median duration of 5s, which is 60% of the median duration of evacuation (8.5s). Among constipated PEs, disturbances were inadequate rectal pressure (4 patients) or impaired anal relaxation (8 patients). In a multivariate linear regression model, the rectoanal gradient, average anal canal diameter, and duration of anal relaxation during evacuation explained 40% of the variation in percent of barium emptied (p=0.0001). Conclusion: Adequate evacuation of barium (≥18%) is characterized by an initial simultaneous increase in rectal and anal pressure followed by anal relaxation. Conversely, either an inadequate increase in rectal pressure or in anal diameter during evacuation leads to incomplete evacuation. Not all women who have poor evacuation are symptomatic. The simultaneous increase in rectal and anal pressures during evacuation may be mediated by a reflex induced by increased abdominal pressure. Simultaneous defecography with HRAM may be useful in the clinical evaluation of chronic constipation.