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 ≥ 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.