Normal values for assessment of anal sphincter morphology, anorectal motion, and pelvic organ prolapse with MRI in healthy women

Background: Endoanal MRI and MR defecography are used to identify anal sphincter injury and disordered defecation. However, few studies have evaluated findings in asymptomatic healthy people. The effects of BMI and parity on rectoanal motion and evacuation are unknown. Methods: In 113 asymptomatic females (age 50 ± 17 years, Mean ± SD) without risk factors for anorectal trauma, anal sphincter appearance, anorectal motion, and pelvic organ prolapse were evaluated with MRI. The relationship between age, BMI, and parity and structural findings were evaluated with parametric and non-parametric tests. Results: The anal sphincters and puborectalis appeared normal in over 90% of women. During dynamic MRI, the anorectal angle was 100 ± 1º (Mean ± SEM) at rest, 70 ± 2° at squeeze, and 120 ± 2° during defecation. The change in anorectal angle during squeeze (r = −.25, P <.005), but not during evacuation (r =.13, P =.25) was associated with age. In the multivariable models, BMI (P <.01) and parity (P <.01) were, respectively, independently associated with the intersubject variation in the anorectal angle at rest and the angle change during squeeze. Ten percent or fewer women had had descent of the bladder base or uterus 4 cm or more below the pubococcygeal line or a rectocele measuring 4 cm or larger. Only 5% had a patulous anal canal. Conclusions: In addition to age, BMI and parity also affect anorectal motion in asymptomatic women. These findings provide age-adjusted normal values for rectoanal anatomy and pelvic floor motion.