Magnetic Resonance Imaging Evaluation of Pregnancy and Aging on External Anal Sphincter Volume in Squirrel Monkeys

Both humans and squirrel monkeys are susceptible to pelvic floor injury and prolapse from pregnancy, delivery, and aging. The mechanisms for external anal sphincter (EAS) injury in squirrel monkeys have not been evaluated in detail. This study evaluates a method for measuring EAS volume using magnetic resonance imaging (MRI) in squirrel monkeys and demonstrates the feasibility of serial measurements.

Using a previously described procedure, MRI was performed on 10 squirrel monkeys prior to euthanasia. After euthanasia, tissue blocks of EAS were cryo-sectioned and stained with succinic dehydrogenase to identify EAS striated muscle fibers. EAS volumes from both MRI and histological measurements were calculated using image analysis software. MRI measurements were obtained by two independent investigators. A cohort of four monkeys, each having five pregnancies and 11 MRI studies over a 5-year span, was used to evaluate EAS volume changes serially within females.

Volumes measured by MRI for each squirrel monkey were similar for the two researchers (Cronbach alpha of 0.97 with 95% lower confidence limit of 0.92), and they were statistically consistent with the volumes obtained from analysis of histology (linear regression with R-squared of 0.97 and p < 0.0001). This validated technique was used to measure EAS volumes in four breeding females and demonstrated sufficient power to detect a decrease (p < 0.00001) from 32.1 ± 3.2 mm³ (mean ± SE) prior to the first pregnancy to 10.7 ± 1.5 mm³ after five pregnancies in 5 years.

Magnetic resonance imaging is a valid technique for measuring EAS volume changes in squirrel monkeys with sufficient sensitivity to detect EAS volume changes such that effects of aging and serial pregnancies can be evaluated.