Obstetric risk factors for anal sphincter trauma in a urogynecological population

Introduction and hypothesis Obstetric anal sphincter tears are the single major modifiable risk factor for anal incontinence (AI) in women. We undertook a retrospective observational study in a tertiary urogynecology unit to describe the prevalence of sonographic anal sphincter defects in a urogynecological population and investigate obstetric risk factors. Methods Files of women seen for investigation of pelvic floor disorders between January 2014 and May 2021 were reviewed. Sonographic defects of the external anal sphincter were analyzed using stored 4D ultrasound imaging data. Explanatory parameters were number of vaginal births, delivery mode, age at first vaginal birth, and birthweight of the first vaginally born baby. Results Of 3,037 women seen during the inclusion period, data were missing in 219, leaving 2,818. AI was reported by 508 (18%), with a mean St Marks score of 11 (1-23) and a mean bother score of 5.9 (0-10). External sphincter defects were detected in 945 women (34%), with a "residual defect" in 343 (12%). The strongest risk factors for a residual defect were first vaginal birth and forceps, with higher-order multiparity adding risk. On multivariate analysis, forceps and vaginal parity >5 remained significant predictors, as opposed to age at first birth and birthweight of the firstborn. Conclusions In this observational study in urogynecological patients, 12% showed a residual EAS defect on imaging. The most obvious risk factors for the diagnosis of a residual defect on exo-anal imaging were the first vaginal birth and forceps, with higher-order vaginal parity conveying additional risk.