Sonographic Investigation of the Rectoanal Inhibitory Reflex: A Qualitative Pilot Study in Healthy Females.

PURPOSE: The rectoanal inhibitory reflex has been studied using various methods, e. g., anometry and electromyography. The aim of this study was to apply ultrasound for direct visualization of the rectoanal inhibitory reflex. METHOD: The rectoanal inhibitory reflex was induced in ten healthy females (age range, 21 - 55 years) by injection of small amounts of water (7, 12, and 20 ml), into the rectum. The intra- anal pressure was measured with a microtransducer and the rectoanal inhibitory reflex was visualized with real- time transvaginal or transperineal sonography. RESULTS: The rectoanal inhibitory reflex consisted of a reduction in the intra- anal pressure and relaxation of the internal anal sphincter, manifested as an increase in the inner diameter of the internal anal sphincter from the mean of 11 to 16 mm (P < 0.001). Simultaneously, a wave of rectal contents entered the anal canal. The distance from the most distal border of the rectal contents to the anal verge decreased from a mean of 33 to 20 mm (P < 0.001). The rectoanal inhibitory reflex ended with a retrograde transport returning anal contents into the rectum. During the retrograde transport a contraction in the internal anal sphincter was observed. CONCLUSIONS: The rectoanal inhibitory reflex can readily be visualized with ultrasound as a wave of rectal contents entering the anal canal. The transport into the anal canal was not of voluntary origin and could be either noticed or not noticed by the subjects. The observed retrograde transportation in the anal canal was not noted by the subjects; it is related to a contraction in the internal anal sphincter and visualized for the first time using ultrasound.