Topographical distribution of blood supply to the anal canal.

BACKGROUND: It has been suggested that anal fissure is an ischaemic ulcer caused by a combination of poor blood supply to the posterior midline of the anal canal and spasm of the internal anal sphincter. This study investigated the topographical distribution of blood supply to quadrants of the anal canal above and below the dentate line.
METHODS: Cadaveric anal canals were removed and 1-cm blocks were cut above and below the dentate line. Blocks were sectioned at 10 microm and every 25th section was mounted. Using the technique of systematic random sampling, fields in the subanodermal space and the internal anal sphincter in posterior, lateral and anterior quadrants of the anal canal were chosen. The numbers of small arterioles in each field were counted. Mean counts were compared for both subanodermal space and internal anal sphincter between quadrants and levels above and below the dentate line using Page's L test for trends.
RESULTS: Anal canals from eight cadavers were examined. There was a significant trend to an increasing number of arterioles from posterior to anterior in the subanodermal space at all levels and at two of three levels in the internal anal sphincter.
CONCLUSION: The arteriolar density is less in the posterior quadrant throughout the anal canal. It may be that this poor blood supply predisposes to the development of anal fissures at their most common site in the posterior midline.