A proposed mechanism for intermediate atresia ani (AA), based on a porcine case of AA and hypospadias

BACKGROUND: Atresia Ani (AA) is one of the most common malformations of the lower gastrointestinal tract. It occurs in 1 in 1500 to 1 in 5000 human births. These patients suffer physically and psychologically from this disorder, and thus there is a research interest in the condition. There are 3 subcategories of AA: high, intermediate, and low. Each of these forms is likely related to separate etiological processes. METHODS: An anatomical study was performed on a porcine case of AA with a urorectal fistula and malformed urethra. RESULTS: This animal was found to have the intermediate form of AA. CONCLUSIONS: A new mechanism is hypothesized, distinct from that described for the high and low forms of AA. This proposed mechanism involves the persistence of the cloacal membrane. Evidence to support this hypothesis includes: location of the urorectal fistula in the region of the embryonic cloacal duct, the lack of anomalies usually seen in conjunction with AA associated with mutations of the Sonic Hedgehog gene, and the presence of a malformed urethra, which is significantly correlated to intermediate AA. This form of AA could be related to a failure of the cloacal membrane to break down at the appropriate time during urorectal separation.