Levator ani muscle morphology and function in women with obstetric anal sphincter injury

OBJECTIVES

To estimate the prevalence of, and explore the risk factors for, levator ani muscle (LAM) injury in women with clinically diagnosed obstetric anal sphincter injury (OASI). The secondary aim was to assess the association between LAM injury and pelvic floor muscle contraction, anal incontinence (AI) and urinary incontinence (UI) in women with OASI.

METHODS

This was a cross-sectional study of 250 women with OASI, recruited between 2013 and 2015 from a tertiary referral center at Croydon University Hospital, UK. AI symptoms were assessed using the modified St Mark's incontinence score and UI was assessed using the International Consultation on Incontinence modular Questionnaire for Urinary Incontinence - Short Form. All participants underwent three/four-dimensional transperineal ultrasound at rest and on maximum pelvic floor muscle contraction. Major LAM injury was defined as a unilateral or bilateral defect in all three central slices on tomographic ultrasound imaging. Muscle contraction was assessed using the modified Oxford scale (MOS) and measured on ultrasound as the proportional change in the anteroposterior (AP) levator hiatal diameter between rest and contraction. Multivariable logistic regression analysis was used to study risk factors for LAM injury. Differences in contraction and AI and UI symptoms between women with intact and those with injured LAM were studied using multivariable ANCOVA and the Mann-Whitney U-test.

RESULTS

Of the 248 women with OASI for whom ultrasound volumes of adequate quality were available, 29.4% were found to have major LAM injury. The prevalence of LAM injury was 23.6% after normal vaginal delivery and 40.2% after operative vaginal delivery (adjusted odds ratio, 4.1 (95% CI, 1.4-11.9); P = 0.01). LAM injury was associated with weaker pelvic floor muscle contraction, with an adjusted mean difference for proportional change in AP diameter of 5.0 (95% CI, 3.0-6.9) and MOS of 0.6 (95% CI, 0.3-0.9) (P < 0.001 for both). AI and UI symptom scores were similar between women with intact and those with injured LAM.

CONCLUSIONS

Operative vaginal delivery was a risk factor for LAM injury in women with OASI. LAM injury was associated with weaker pelvic floor muscle contraction. Special attention is recommended for women with OASI and LAM injury, as they are at high risk for future pelvic floor disorders. The benefits of implementation of an intensive, focused and structured pelvic floor rehabilitation program need to be evaluated in these women. Copyright © 2018 ISUOG. Published by John Wiley & Sons Ltd.