Histological and manometric studies of the external anal sphincter before and after excision of the internal anal sphincter in dogs

ABSTRACT

The histologic changes in the external anal sphincter after internal sphincter excision were studied in 20 dogs. An external sphincter biopsy was taken before internal sphincterotomy, 2 weeks and monthly thereafter for 10 months. The excised material was studied microscopically after being stained with hematoxylin-eosin and Verhoeff-Van Gieson's stains. A manometric record of the anal pressure before and after excision of the internal sphincter was also done. 70% of external sphincter specimens before internal sphincter excision showed smooth muscle fibers scattered between the striated fibers. These smooth fibers could be responsible for the resting tone of the external sphincter. After internal sphincter excision, characteristic histologic changes could be identified in the external sphincter. From the 2nd week to the 5th month post-excision, the external sphincter showed degenerative and hypertrophic changes. From the 6th to the 10 month, there was a regeneration of the striated muscle fibers and an increase in the number of smooth muscle fibers, so that the 10th month a "compound muscle" of mainly smooth and scanty striated muscle fibers was identified. Theories were put forward to explain the smooth muscle preponderance in the external sphincter after internal sphincter excision. The increased non-striated element in the external sphincter seems to be a structural-functional adaptation, so that the external sphincter takes on the involuntary function of the excised muscle. Results of the manometric study showed that the mean anal pressure before excision was 78 cm H20. Two weeks after excision, there was a rapid drop of the anal pressure. This was followed by a rapid rise of the pressure till the 5th month, then slow rise up to 69 cm H20 in the 10th month