Pelvic organ prolapse as a function of levator ani avulsion, hiatus size, and strength

Background: Obstetrical levator ani muscle avulsion is detected after 10%–30% of vaginal deliveries and is associated with pelvic organ prolapse later in life. However, the mechanism by which levator avulsion may contribute to prolapse is unknown. Objectives: This study investigated the extent by which size of the levator hiatus and pelvic muscle weakness may explain the association between levator avulsion and pelvic organ prolapse. Study Design: This was a supplementary study of a longitudinal cohort of parous women enrolled 5–10 years after first delivery and assessed annually for prolapse (defined as descent beyond the hymen) for up to 9 annual visits. For this substudy, vaginally parous participants were assessed for levator avulsion using 3-dimensional transperineal ultrasound. Ultrasound was performed at a median interval of 11 years from delivery. Ultrasound volumes also were used to measure levator hiatus area with Valsalva. Pelvic muscle strength was measured with perineometry. Women with and without pelvic organ prolapse were compared for levator avulsion, levator hiatus area, and pelvic muscle strength, using multivariable logistic regression yielding a measure of mediation. Bootstrap methods were used to calculate the confidence interval corresponding to the measure of mediation by hiatus area and pelvic muscle strength. Results: Prolapse was identified in 109 of 429 (25%) and was significantly associated with levator avulsion (odds ratio, 4.17; 95% confidence interval, 2.28–7.31). Prolapse also was associated with levator hiatus area (odds ratio, 1.52 per 5 cm²; 95% confidence interval, 1.34–1.73) and inversely with muscle strength (odds ratio, 0.87 per 5 cm H₂O; 95% confidence interval, 0.81–0.94). In a multivariable logistic model including levator avulsion, levator hiatus area, and strength, the association between levator avulsion and prolapse was substantially attenuated and indeed was no longer statistically significant (odds ratio, 1.75; 95% confidence interval, 0.91–3.39). Hiatus area and strength mediated 61% (95% confidence interval, 34%–106%) of the association between avulsion and prolapse. Furthermore, since the 95% confidence interval for this estimate contained 100%, it cannot be ruled out that the 2 markers fully mediate the effect of avulsion on prolapse. Conclusions: The strong association between pelvic organ prolapse and levator avulsion can be explained to a large extent by a larger levator hiatus and weaker pelvic muscles after levator avulsion.