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

Background: Anal endosonography has become an important imaging method in the diagnosis of anorectal disorders. However, little information exists as to whether anal endosonography reliably defines pelvic floor structures. The aim of this study was to correlate endoanal sonography with cross-sectional anatomy and histology.

Methods: Endosonographic tomograms were obtained from 9 human cadavers before fixation and cross-sectioning at identical levels. Muscular layers were defined by visual inspection, histology, immunohistology, and morphometry using three-dimensional sphincter reconstructions.

Results: Endosonography visualized only two muscular layers, whereas anatomic sections always revealed three. Comparisons revealed identical findings with regard to internal sphincter volumes and asymmetries. However, due to its failure to identify the longitudinal muscle, endosonography largely overestimated external sphincter volumes. In contrast to current beliefs, anatomic 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 mechanisms 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 imaging techniques with cross-sectional anatomy may enhance our understanding of pelvic floor anatomy.