Paper 12: 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

Abstract

OBJECTIVE: Squirrel monkeys develop naturally occurring pelvic organ prolapse (POP) that is associated with age and parity as in women. In this study, we applied magnetic resonance imaging (MRI) and 3-dimensional modeling to test the hypothesis that animals with POP have reduced volume of pelvic floor muscles compared to matched animals without POP. METHODS: A cohort of 14 adult parous female squirrel monkeys was evaluated while sedated using a 1.5 T Siemens Sonata (n=7 females with visible support defects and n = 7 females without POP matched in age, parity, and weight). Animals were placed on their back with legs and body supported in a natural position inside of the 4-channel wrist coil. Initial localizing MRI sequences were obtained to locate anatomy and set region of interest to produce serial images from L7 through C4 vertebrae. Contrast medium (0.5mmol/kg ProHance) was given intravenously 5–10 minutes prior to initiating a T1 weighted (gradient echo) axial scan that was acquired over 7.5 minutes. Axial T1 weighted images with 0.4 mm cubic voxels were visualized and 3-dimensional models constructed using 3D-Doctor software to obtain volumes of the levator ani (LA) and obturator internus (OI) muscles by 2 independent observers (LAK and TJK). Observers were compared for concordance using Kendall tau correlations for each muscle and side. Volume measures were also compared by regression analysis with muscle weights for a subset of animals undergoing necropsy following MRI procedures. Analysis of variance was used to compare treatment groups for each side of each muscle. RESULTS: Observers concurred on measures of left LA, right LA, left OI, and right OI (Kendal tau of 0.72, 0.99, 0.67, and 0.82, respectively, with P<0.02). Volume of LA muscles measured by MRI without correction for density was significantly related to mass (R2=0.51, p=0.009). Animals with POP did not differ (P=0.67, Wilks multivariate test) from those without POP in age (12.7 yr versus 15.2 yr, respectively), parity (4.7 versus 5.4, respectively), and weight (731 grams versus 699 grams, respectively). The LA differed between groups (535 mm3 for the POP group versus 398 mm3 for normals, P=0.026), but not between left and right sides (P=0.94). The OI did not differ between groups (446 mm3 versus 390 mm3, P=0.14) or sides (P=0.11). CONCLUSIONS: In the squirrel monkey model of pelvic relaxation, 3-dimensional models created with MRI demonstrate that LA muscle volumes differ between cohorts of parous females with and withoutPOP matched in age, parity, and weight. These findings suggest that prolapse is NOT a result of pelvic floor muscle wasting and subsequent weakness.