The internal sphincter in anorectal malformations: morphologic investigations in neonatal pigs.

The knowledge of sphincter anatomy in anorectal malformations is still inadequate and contradictory. Therefore, morphologic investigations were carried out in 33 neonatal piglets with congenital anal atresias. Of the 24 male animals 12 had high anomalies with a rectourethral fistula. The remaining 12 piglets had low anomalies; in nine cases we were able to demonstrate an anocutaneous fistula. Of the nine female animals, six had high anomalies with a rectovaginal fistula. The three female piglets with low anomalies had an anovestibular or an anocutaneous fistula. In all animals we could demonstrate a normal internal sphincter, which surrounded the proximal part of the fistulae. The position of the internal sphincter therefore depended on the localization of the fistula orifice into the rectal pouch. This differed greatly. The form of the internal sphincter also differed greatly. Sometimes the muscle had the form of a tube or an acute-angled funnel as in healthy piglets. However, mostly the internal sphincter was spread out wide and had the form of a disc or a flat dish. The proximal region of the fistulae in anal atresias has most features of a normal anal opening: (1) it is surrounded by an internal sphincter, (2) the rectal pouch in the region of the internal sphincter as well as the fistulae are hypoganglionotic, (3) the proximal fistula region is lined by transitional epithelium, and (4) it contains anal glands. We, therefore, consider that the fistula should be designated as an ectopic anal canal. The most important result was the demonstration of a normal internal sphincter even in high and intermediate anomalies of anal atresias.(ABSTRACT TRUNCATED AT 250 WORDS)