Investigation of anal motor characteristics of the sensorimotor response (SMR) using 3-D anorectal pressure topography

Abstract

Desire to defecate is associated with a unique anal contractile response, the sensorimotor response (SMR). However, the precise muscle(s) involved is not known. We aimed to examine the role of external and internal anal sphincter and the puborectalis muscle in the genesis of SMR. Anorectal 3-D pressure topography was performed in 10 healthy subjects during graded rectal balloon distention using a novel high-definition manometry system consisting of a probe with 256 pressure sensors arranged circumferentially. The anal pressure changes before, during, and after the onset of SMR were measured at every millimeter along the length of anal canal and in 3-D by dividing the anal canal into 4 × 2.1-mm grids. Pressures were assessed in the longitudinal and anterior-posterior axis. Anal ultrasound was performed to assess puborectalis morphology. 3-D topography demonstrated that rectal distention produced an SMR coinciding with desire to defecate and predominantly induced by contraction of puborectalis. Anal ultrasound showed that the puborectalis was located at mean distance of 3.5 cm from anal verge, which corresponded with peak pressure difference between the anterior and posterior vectors observed at 3.4 cm with 3-D topography (r = 0.77). The highest absolute and percentage increases in pressure during SMR were seen in the superior-posterior portion of anal canal, reaffirming the role of puborectalis. The SMR anal pressure profile showed a peak pressure at 1.6 cm from anal verge in the anterior and posterior vectors and distinct increase in pressure only posteriorly at 3.2 cm corresponding to puborectalis. We concluded that SMR is primarily induced by the activation and contraction of the puborectalis muscle in response to a sensation of a desire to defecate.

Fig. 1.

Top: continuous recording of the rectal and anal manometric pressure sequences. Bottom: 4 separate snapshots or zones of the high-resolution topographic pressure profiles taken before, during, and after a sensorimotor response (SMR) in a healthy subject. A: anal and rectal resting pressure profile before rectal balloon distention. Following this, the rectal balloon distention can be seen as a rise in the pressure in the rectal pressure channel. The rectal balloon distention induces a recto-anal inhibitory reflex (RAIR) that is seen as a relaxation of the anal resting pressure throughout all anal channels. B: pressure changes just before the onset of the SMR and predominately consists of anal relaxation. C: subject signals a sensation of desire to defecate, and simultaneously there is a unique anal contractile response that is seen throughout the anal high-pressure zone, which represents the SMR. The topographic image further shows a transient contraction of the puborectalis muscle (Arrow). Following the SMR, the RAIR continues to inhibit anal pressures throughout the anal canal, and this residual pressure is represented D. All measurements were taken at a specific time point within the A, B, C, and D ranges.

Fig. 2.

The circumferential pressure changes along the length of the anal canal before and during the SMR, zones B and C, respectively. The arrows outline the location where there was a significant (P ≤ 0.05) rise in pressure during the SMR. This location (2.4–3.2 cm from the anal verge) is the same location at which the puborectalis muscle was identified to generate anal canal pressure in Fig. 3.

Fig. 3.

Mean percent increase in anal pressure during the SMR from all subjects at every centimeter along the anal canal. The superior portion shows the largest percentage increase in pressure, with the superior 2 cm of the anal canal showing a more significant (*P < 0.05) percentage pressure increase compared with the more distal sections of the anal canal. The means ± SE are shown per segment. Data are the means from all subjects.

Fig. 4.

Mean pressure changes along the length of the anal canal from all subjects in the anterior and posterior vectors during baseline, i.e., resting. The locations of the 3 anal canal muscular structures that contribute to the anal high-pressure zone as described by Mittal and colleagues (11a, 14). IAS, internal anal sphincter; EAS, external anal sphincter.