SEMI-AUTOMATED VECTORIAL ANALYSIS OF ANORECTAL MOTION BY MAGNETIC RESONANCE DEFECOGRAPHY IN HEALTHY SUBJECTS AND FECAL INCONTINENCE

The anal sphincters and pelvic floor muscles preserve fecal continence and participate in defecation (1). By visualizing structural (e.g., rectocele) and functional (e.g., impaired anal relaxation) disturbances, barium defecography and magnetic resonance imaging (MRI) facilitate the diagnosis of defecatory disorders, particularly when anal manometry and a rectal balloon expulsion test are inconclusive (2-6). MRI also reveals puborectalis atrophy and impaired pelvic floor contraction in some women with fecal incontinence (FI) (7-9). However, the clinical utility of barium defecography and MRI is limited by variable reproducibility of these assessments. For example, in one study, inter-observer agreement was good for identifying enteroceles and rectoceles but suboptimal for identifying intussusception, anismus, or the puborectalis impression by barium defecography (10). Moreover, for some measurements, agreement was much better for experienced observers (10). The American Gastroenterological Association technical review concluded that “there is poor agreement between independent observers in the measurement of the anorectal angle, a parameter thought to be critical to the interpretation of defecography results” (11). This is partly attributable to lack of a standardized definition for the rectal axis, which forms one aspect of the anorectal angle and can be drawn through the anterior, middle or posterior rectal wall. Moreover, the rectum is curvilinear, and the puborectalis indentation on the posterior rectal wall is variable. Hence, even minor differences in the orientation of the rectal axis can profoundly affect the anorectal angle. Lastly, the bony landmarks (e.g., pubic symphysis, coccyx) which are used to measure perineal descent may not be visible during barium defecography but are distinctly visualized on MR images (12). Nonetheless, and despite centralized training, a NIH-sponsored multi-institutional study concluded that “measurement variability adversely affects the utility of many MRI measurements for multicenter pelvic floor disorder research,” although agreement was better for bony than pelvic soft-tissue measurements during dynamic pelvic MRI (13). By contrast, we documented substantial inter-observer agreement for anorectal angles and the anorectal location at rest, during squeeze and evacuation measured by MRI in asymptomatic women and women with defecatory disorders; however, differences between observers were statistically significant (12). While most attention has focused on anorectal motion and the puborectalis, the coccyx also moves during pelvic floor contraction and rectal evacuation (14). The coccygei, which have a variable proportion of muscle and fibrous tissue, may pull forward and support the coccyx, after it has been pressed backwards, during defecation or parturition (14-16). With the levatores ani and piriforms, they close the posterior part of the pelvic outlet. Previous studies have evaluated the relationship of coccygeal mobility to age, sex, parity, and minor trauma but not to fecal incontinence. It is conceivable that coccygeal weakness may contribute to pelvic weakness in FI. Hence, the aims of this study were to (i) to develop a user-friendly semi-automated program to measure anorectal motion by defecography; (ii) evaluate the within-subject reproducibility of anorectal motion measured by this semi-automated program; (iii) to compare measurements of anorectal motion by a radiologist and a semi-automated program; and (iv) compare anorectal motion disturbances in women with and without FI. A more robust and refined approach to measure pelvic floor motion by MR defecography may enhance the utility of this technique in clinical practice and research studies.