A new look at the anal transitional zone with reference to restorative proctocolectomy and the columnar cuff

The anal transitional zone (ATZ) is of interest in relation to anorectal function and its fate during the operation of restorative proctocolectomy. There are only a few studies describing the anatomy of this structure. This study applied new techniques to quantify the ATZ in normal and inflammatory bowel disease states.

The epithelial linings of 28 anal canals were analysed by two techniques: whole-mount Alcian blue staining and a new method of creating a computer map of the histological findings based on longitudinal sections taken every 3 mm. The Alcian blue pictures and the histology maps were measured and compared using image analysis software.

The ATZ usually commences just above the dentate line, which is situated a median of 1.05 cm above the lower border of the internal sphincter. The median length of the ATZ measured from computer maps of the histology was 0.45 cm. This was significantly smaller than the 0.73 cm obtained by the Alcian blue technique, which overestimates the length of the ATZ. The amount of columnar epithelium in the ATZ is very small. The median lowest point of columnar epithelium in the anal canal was 1.1 cm above the lower border of the internal sphincter.

The ATZ is only half the length described previously. After double-stapled restorative proctocolectomy there is a 1.5-2.0-cm cuff of diseased columnar epithelium remaining in the upper anal canal above the ATZ. This is suitably termed the 'columnar cuff' and is an important consideration in long-term follow-up.