MRI anatomy of anteriorly displaced anus: what obstructs defecation?

Abstract

Anteriorly displaced anus is an anomaly that is debated with regard to its nomenclature, diagnosis and management. To describe MRI anatomy of the anal canal in children with anteriorly displaced anus and its impact on the process of defecation. We prospectively examined ten children (7 girls, 3 boys; age range 7months to 8years, mean 3years) with anteriorly displaced anus between August 2009 and April 2012. Noncontrast MRI examinations were performed on a 1.5-T magnet. T1- and T2-weighted turbo spin-echo images were acquired in axial, sagittal and coronal planes of the pelvis. The anorectal angle and the relative hiatal distance were measured in mid-sagittal images, and compared with those of a control group using the Mann-Whitney test. In children with anteriorly displaced anus, no anatomical abnormality was depicted at the level of the proximal anal canal. However, the distal anal canal was displaced anteriorly, running out its external muscle cuff, which remained un-displaced at the usual site of the anus. This changes the orientation of the central axis of the anal canal by passing across instead of along the fibers of the longitudinal muscle coat. Children with anteriorly displaced anus had a more obtuse anorectal angle (mean 112.1°), which was significantly greater than that of the control group (mean 86.2°). MRI is a valuable tool in studying the anatomy of the anal canal in children with anteriorly displaced anus. The abnormal orientation of the longitudinal muscle across the anal canal can explain the obstructed defecation in these children. Based on this study, it might be of interest to use MRI in studying equivocal cases and children with unexplained constipation.