Effects of bilateral levator ani nerve injury on pelvic support in the female squirrel monkey.

OBJECTIVE: To determine whether experimental denervation of the levator ani (LA) and its subsequent atrophy contribute to the development of pelvic organ prolapse in the squirrel monkey. STUDY DESIGN: Thirty-seven female monkeys were evaluated including 7 that underwent bilateral LA neurectomy (bLAN), 17 nulliparous monkeys without prolapse, 7 parous monkeys without prolapse, and 6 parous monkeys with prolapse. Magnetic resonance imaging was used to calculate LA muscle volumes and obtain measurements of the position of bladder and cervix. Repeat observations in bLAN females occurred at different times in relation to parturition. RESULTS: LA volumes were reduced in bLAN monkeys (P = .02). Bladder (P = .03) and cervix (P = .04) positions varied between groups, with nulliparous females having the most cephalad positions and females with prolapse having the most caudal positions. Bladder descent was observed in a subset of 4 bLAN females that experienced vaginal parturition (P = .04) and correlated with external findings of vaginal prolapse. CONCLUSION: Bilateral transection of the LA nerve results in atrophy of denervated LA muscles but not a loss of pelvic support in nulliparous monkeys, suggesting that connective tissue components compensate for weakened pelvic floor muscles. LA denervation may accelerate the onset of vaginal prolapse subsequent to parturition.

Magnetic resonance imaging of the levator ani in the squirrel monkey: a comparison of muscle volume between a cohort with pelvic organ prolapse and matched normals.

OBJECTIVE: Magnetic resonance imaging was used to test whether squirrel monkeys with pelvic organ prolapse have reduced pelvic muscle volumes, compared with matched normals. STUDY DESIGN: Levator ani and obturator internus volumes obtained from T1-weighted axial scans of matched groups were measured. Muscle volumes and weights were compared for animals necropsied after magnetic resonance imaging. RESULTS: Two observers concurred on measures of levator ani and obturator internus (Kendal tau > or = 0.60 with P < .003). Levator ani volume was related to mass (R2 = 0.62, P = .0009). Animals with pelvic organ prolapse did not differ (P = .67, Wilks multivariate test) from those without pelvic organ prolapse in age, parity, and weight. Levator ani differed between groups (pelvic organ prolapse = 520 mm3 versus normals = 392 mm3, P = .015) and not sides (P = .80). The obturator internus did not differ between groups (P = .29) or sides (P = .72). CONCLUSION: Magnetic resonance imaging demonstrates that levator ani volumes in parous squirrel monkeys with pelvic organ prolapse were not reduced, suggesting that prolapse is not related to pelvic muscle size reduction in this species.