Correlation of endoanal sonography with cross-sectional anatomy of the anal sphincter

Background: Anal endosonography has become an important imaging method in t he diagnosis of anorectal disorders. However, little information exists as to whether anal endosonography reliably defines pelvic floor structures. Th e aim of this study was to correlate endoanal sonography with cross-section al anatomy and histology. Methods: Endosonographic tomograms were obtained from 9 human cadavers befo re fixation and cross-sectioning at identical levels. Muscular layers were defined by visual inspection, histology, immunohistology, and morphometry u sing three-dimensional sphincter reconstructions. Results Endosonography visualized only two muscular layers, whereas anatomi c sections always revealed three. Comparisons revealed identical findings w ith regard to internal sphincter volumes and asymmetries. However, due to i ts failure to identify the longitudinal muscle, endosonography largely over estimated external sphincter volumes. In contrast to current beliefs, anato mic studies failed to detect striated muscle fibers within the longitudinal muscle and did not show an intersphincteric space. However, anatomic cross sections demonstrated "anterior bands" as newly described anchoring mechan isms for the anal sphincters. Conclusions: Anal endosonography supplies accurate Information with regard to internal anal sphincter dimensions, but does not reliably outline deeper muscular layers. However, despite these drawbacks, comparisons of modern i maging techniques with cross-sectional anatomy may enhance our understandin g of pelvic floor anatomy.