Evaluation of the Rectal Reflex Using Anal Acoustic Reflectometry

Anal Acoustic Reflectometry (AAR) is a reliable and reproducible technique that has been studied in our department over the last 6 years. Sound waves pass into a balloon placed in the anal canal and are used to measure the cross-sectional area. By gradually increasing and decreasing the pressure in the balloon the investigators can measure the pressure at which the cross-sectional area starts to increase and decrease, and the anal canal starts to open and close. This assessment mimics the natural opening and closing of the anal canal and the effect of squeezing the muscles.

The gold standard investigation of the anal sphincter muscles has been manometry which measures anal canal pressure at rest and during squeeze. However, it has limitations. In previous studies AAR has shown promise in the assessment of faecal incontinence and, that unlike manometry, it has been able to distinguish between different types of incontinence.

The Recto-anal Inhibitory Reflex (RAIR) is a normal response when the rectum fills with faeces, fluid or air, whereby there is a change in the pressures within the anal canal to determine the type of contents. This can be absent or altered in patients who have difficulty in opening their bowels. The RAIR is currently measured by anal manometry using a 4.9mm catheter, resulting in an anal canal which is already partially opened prior to the measurement, and potentially distorted.

AAR is considered a catheter free technique as the balloon has a cross-sectional area of only 0.4mm2 when collapsed. The investigators propose to measure the RAIR using a 1.7mm diameter catheter alongside the AAR balloon to determine the effect that its placement has on the recorded parameters of AAR. This aims to improve our understanding of the opening and closing of the anal canal in response to distension of the rectum.