Anal Atresia

Anal atresia is also known as imperforate anus.

(Also see Overview of Congenital Gastrointestinal Anomalies.)

In anal atresia, the tissue closing the anus may be several centimeters thick or just a thin membrane of skin. In simple cases, the anal canal ends blindly at the anal membrane, which forms a diaphragm between the endodermal and ectodermal portions of the anal canal. In more severe cases, a thick layer of connective tissue may be found between the terminal end of the rectum and the surface because of either a failure of the anal pit to develop or atresia of the ampullar part of the rectum (rectal atresia) (1).

In supralevator lesions (in which the atresia ends above the levator sling), a fistula often extends from the anal pouch to the perineum or the urethra in males and to the vagina, the fourchette, or, rarely, the bladder in females. Cutaneous fistulas are usually present in infralevator lesions.

The incidence of anal atresia is 1 in 5000 live births. This disorder is frequently associated with other congenital anomalies (50% of cases) such as VACTERL (vertebral anomalies, anal atresia, cardiac malformations, tracheoesophageal fistula, esophageal atresia, renal anomalies and radial aplasia, and limb anomalies). Before surgery, neonates with anal atresia should be evaluated for other congenital anomalies.

Anal atresia is obvious on routine physical examination of the neonate because the anus is not patent. If the diagnosis of anal atresia is missed and the neonate is fed, signs of distal bowel obstruction soon develop.

The urine should be examined for meconium, indicating the presence of a fistula to the urinary tract. Plain x-rays and fistulograms with the neonate in a lateral prone position can define the level of the lesion. Ultrasound is also helpful for classifying the type of lesion and for assessing any other associated anomalies. A cutaneous fistula generally indicates low atresia. If no perineal fistula exists, a high lesion is likely.

Neonates with a cutaneous fistula and a low lesion can undergo primary perineal repair. Neonates with a high lesion should have a temporary colostomy; definitive repair is deferred until the infant is older and the structures to be repaired are larger. Several malformations may be repaired as a single-step procedure (2).