Topographic and manometric characterization of the recto-anal inhibitory reflex

Abstract

Background: Recto-anal inhibitory reflex (RAIR) is an integral part of normal defecation. The physiologic characteristics of RAIR along anal length and anterior-posterior axis are unknown. The aim of this study was to perform topographic and vector-graphic evaluation of RAIR along anal canal using high definition manometry (HDM), and examine the role of various muscle components.

Methods: Anorectal topography was assessed in 10 healthy volunteers using HDM probe with 256 sensors. Recto-anal inhibitory reflex data were analyzed every mm along the length of anal canal for topographic, baseline, residual, and plateau pressures during five mean volumes of balloon inflation (15 cc, 40 cc, 71 cc, 101 cc, 177 cc), and in 3D by dividing anal canal into 4 × 2.1 mm grids.

Key results: Relaxation pressure progressively increases along anal canal with increasing balloon volume up to 71 cc and thereafter plateaus. In 3D, RAIR is maximally seen at the middle and upper portions of anal canal (levels 1.2-3.2 cm) and posteriorly. Peak residual pressure was seen at proximal anal canal.

Conclusions & inferences: Recto-anal inhibitory reflex is characterized by differential anal relaxation along anterior-posterior axis, longitudinally along the length of anal canal, and it depends on the rectal distention volume. It is maximally seen at internal anal sphincter pressure zone. Multidimensional analyses indicate that external anal sphincter provides bulk of anal residual pressure. Our findings emphasize importance of sensor location and orientation; as anterior and more distal location may miss RAIR.

© 2012 Blackwell Publishing Ltd.

Figures

Figure 1

The top panel shows a continuous recording of the rectal and anal manometric pressure sequences after a 70 cc rectal distention induces RAIR in one subject. The lower panel shows the topographic pressure map of the anal canal during the baseline, residual, and plateau pressures. The top panel exhibits the rise in rectal pressure with distention, the reflexive relaxation of the anal canal, and its partial recovery of pressure. The puborectalis, IAS, and EAS high pressure zones are marked in the baseline pressure map in the lower panel. The IAS provides pressure in the anterior and posterior superior anal canal, while the puborectalis in just the superior posterior anal canal. The pressure map at the residual pressure indicates a decrease in pressure at the puborectalis and IAS zones, with the EAS zone still providing anal canal pressure. The plateau topographic pressure map indicates a partial return to the baseline pressures in the IAS and puborectalis regions.

Figure 2

Each panel represents a single continuous recording of the rectal and anal manometric pressure sequences during RAIR at 10cc, 40cc, 70cc, 100cc, and 160cc in a single subject. The panels show the progressive nature of the RAIR with increasing rectal distention volumes. The nadir of the RAIR is progressively lower till 70cc rectal distention and the plateau pressure also gradually decreases with larger rectal distention volumes. The pressure scale on the left corresponds to the anal manometric pressure and the right to the rectal manometric pressure.

Figure 3

Mean relaxation pressure from all subjects at every millimeter along the length of the anal canal. The nadir of the pressure for each rectal distention group is progressively lower till an average 71 cc rectal distention, at which the anal canal cannot relax any further. The peak relaxation pressure is at 1.7 cm from the anal verge for 40 cc, 71 cc, 101 cc, and 177 cc mean rectal distention and at 2.3 cm from the anal verge for the 15 cc mean rectal distention group.

Figure 4

Mean relaxation pressure from all subjects every 4 millimeters along the anterior and posterior anal canal at a mean 15 cc and 101 cc rectal distention volume. The mean 15 cc rectal distention volume shows a constant 20 mmHg relaxation pressure ranging from 1.2 cm to 3.6 cm in the anterior and posterior anal canal, while the 101 cc rectal distention volume shows a peak at 1.7 cm from the anal verge and a larger pressure change in the superior-posterior anal canal region compared to the superior-anterior region, indicating both the IAS and puborectalis are relaxing.

Figure 5

Mean residual pressure from all subjects at every millimeter along the length of the anal canal. The peak residual pressure for the mean 15 cc rectal distention volume is at 1.6 cm from the anal verge. The peak shifts distally with larger rectal distention volumes, moving to 0.9 cm from the verge at 71 cc distention volume. This indicates the increasing relaxation of the IAS.

Figure 6

Mean residual pressure from all subjects at a mean 101 cc rectal distention volume along the anterior and posterior half of the anal canal every 4 millimeters. The residual pressure in the anterior and posterior vectors is at 0.9 cm from the anal verge. This corresponds to the EAS and indicates this sphincter provides anal canal pressure during the residual pressure phase of the RAIR.