A novel method for measurement of the recto-anal inhibitory reflex using anal acoustic reflectometry.

Aim: The recto-anal inhibitory reflex (RAIR) is currently measured using manometry catheters, which potentially distort the anal canal. Anal acoustic reflectometry (AAR) is considered to be a catheter-free technique and primarily measures the function of the internal anal sphincter. We sought to determine if RAIR could be measured using AAR.

Method: Patients aged 18 and over attending the hospital for investigation of pelvic floor dysfunction were included. AAR parameters were obtained before (prerectal distension) and after (postrectal distension) inflation of a rectal balloon catheter with 100 ml of air. Minimum opening pressure (Op, cmH O), opening elastance (Oe, cmH O/mm ), closing pressure (Cp, cmH O), closing elastance (Ce, cmH O/mm ) and hysteresis (Hys, %) were recorded. The presence of RAIR was defined by a reduction of prerectal distension Op by 20% or more.

Results: Thirty-two patients were included, 26 of whom were women. The mean age of patients was 57.5 years (SD 11.3 years). Nine patients had faecal incontinence, six patients had obstructive defaecation and 17 patients had both. RAIR was seen in 30/32 patients. The mean reduction in Op was 58.8% (range -59.18% to 100%). Three patients had 100% reduction. Comparison of pre- and post-RAIR parameters showed a significant difference in Op (34.44 vs 15.41, P < 0.0001), Oe (1.39 vs 1.1, P = 0.004), Cp (20.06 vs 8.00, P < 0.0001) and Ce (1.31 vs 1.13, P < 0.0001) but not Hys (39.71 vs 39.90, P = 0.88).

Conclusion: We describe a novel method for the measurement of RAIR. Rectal distension appears to alter resting pressure and the resistance of the anal canal to opening and closing, with complete inhibition of the sphincter complex in three patients.