The levator ani muscles are critical for providing upward support to the pelvic organs and minimizing the load on the connective tissue that attaches these organs to the pelvis. When the muscles fail, pelvic organ prolapse may ensue, making surgery necessary. Vaginal birth substantially increases the risk of prolapse in occurring in parous women, but it is not clear whether levator ani defects lead to prolapse later in life. The possibility that this may be the case has lent support to cesarean delivery on request.
This case-control study compared the structure and function of the levator ani muscle in 151 women with prolapse and 135 control subjects matched for age, race, and hysterectomy status. Case patients had prolapse of a vaginal wall, a hysterectomy scar, or the cervix extending at least 1 cm above the hymen during a Valsalva maneuver. MR imaging served to identify major defects with more than half the levator ani missing, and minor defects with less than half the muscle missing. An instrumented vaginal speculum was used to quantify vaginal closure force at rest and during maximum pelvic muscle contraction.
The incidence of major levator ani defects was 55% in cases and 16% in controls, for an adjusted odds ratio (OR) of 7.3 (95% confidence interval [CI], 3.9-13.6). Women in the two groups were, however, about equally likely to have minor defects. Incidence rates of major defects were 53% for women reporting having had a forceps delivery and 28% for the others (adjusted OR, 3.4; 95% CI, 1.95-5.78). Women with prolapse had lower estimates of vaginal closure force during pelvic muscle contraction than did control subjects (2.0 versus 3.2 Newtons). Women with levator ani defects generated less force than those lacking defects (2.0 versus 3.1 Newtons). The genital hiatus was 50% longer in case women than in controls (4.7 versus 3.1 cm). In both the case and control groups, women without levator ani defects had higher maximal contraction force estimates than those with defects.
This case-control study showed that women having pelvic organ prolapse more often have defective levator ani muscles than control women, and generate less vaginal closure force during maximal muscle contraction.