Opening and closing function of the anal canal assessed by acoustic reflectometry

Anal acoustic reflectometry (AAR) is a technique that is currently under investigation for the assessment of faecal incontinence. It uses reflected sounds waves to measure cross sectional area at different pressures leading to a profile of the anal canal, and in particular the high pressure zone of the anal sphincters. The cross sectional area from the high pressure zone is then plotted on a graph to give seven characteristic parameters. AAR has been shown to be reproducible and reliable, able to distinguish between continence and incontinence, correlate with the severity of incontinence and able to discriminate between the three patterns of incontinence (urge, passive and mixed). Opening pressure has been shown to be an independent predictor of success with peripheral nerve evaluation, the trial period before sacral nerve stimulation. This thesis aimed to validate AAR against manometry and explore its physiological and clinical potential. A retrospective analysis of 265 patients who had undergone AAR was undertaken in order to develop a surrogate marker for anal canal length. The surrogate marker did find the expected difference between men and women but this was not clinically significant. Furthermore, the surrogate marker was unable to differentiate between incontinence and continence. A technical limitation (Gibbs phenomenon) of AAR was subsequently shown to explain this unexpected result.Prior manometry could possibly interfere with the interpretation of AAR, and therefore a prospective randomised cohort study of 30 patients was conducted to assess two orders of data collection. Reassuringly it does not matter which one of these investigations is undertaken first. In order to test the hypothesis that the greater the challenge to the anal sphincter, the greater the response, the effect of two rates of anal canal stretch was investigated in a prospective randomised cohort study of 50 patients with faecal incontinence. No difference was found between normal or fast rates of AAR. This study has validated a faster method of AAR that can be used alongside manometry in any order.A pudendal nerve block was used to investigate whether AAR assesses primarily internal or external sphincter function in a prospective cohort study of 15 patients using both AAR and manometry. Bilateral pudendal nerve block reduced the function of the external anal sphincter but had no effect on the internal sphincter using both techniques. This study suggests that AAR at rest is predominately an investigation of the internal anal sphincter. A prospective study of 30 patients with faecal incontinence was carried out to establish if AAR can predict the outcome from posterior tibial nerve stimulation. Posterior tibial nerve stimulation improved rectal sensation, manometry squeeze pressures, quality of life, severity of incontinence and was more effective for patients with urge incontinence. A variety of demographic, clinical and physiological measures were unable to predict the success of posterior tibial nerve stimulation.The results presented in this thesis suggest that the full clinical potential of AAR has yet to be realised and it will be necessary to compare it with high resolution anal manometry in the future. Progress in this field would be greatly facilitated by establishing the normal values for this technique and the development of a robust AAR assessment of the external anal sphincter.