The anus is the opening at the far end of the digestive tract through which stool leaves the body. The rectum is the pouch of large intestine that holds stool prior to defecation. In anorectal malformations, skin may be covering the area where the anus should be, and the skin may be several centimeters thick or just a thin membrane. The opening to the anus may be narrow or may be missing completely.
Anal atresia occurs when the intestines do not develop properly while the fetus is growing. It is not known why the intestines do not develop properly.
Most infants with anal atresia develop some type of abnormal connection (fistula) between the anus and either the urethra, the area between the urethra and anus (the perineum), the vagina, or rarely the bladder. Anal atresia commonly occurs along with other birth defects, such as defects of the spine, heart, kidneys, and limbs. Affected infants may also have tracheoesophageal fistula and esophageal atresia.
Infants with anal atresia do not defecate normally after birth. Eventually, if the defect is not treated, intestinal obstruction develops. Intestinal obstruction is a blockage that completely stops or seriously impairs the movement of material in the intestines. Symptoms of intestinal obstruction in infants include pain, irritability, vomiting, and a swollen abdomen.
(See also Overview of Digestive Tract Birth Defects.)
Doctors often detect the malformation by looking at the anus when they first examine the baby after birth, before symptoms develop.
Using x-rays, a radiologist can see the location of and determine other details about a fistula. Doctors may also do an ultrasound to determine the type of malformation and to look for other defects.
Anal atresia usually requires immediate surgery to create a passage for stool and to close the fistula. However, sometimes infants first have a surgical procedure called temporary colostomy (see figure Understanding Colostomy). In this procedure, the surgeon makes a hole in the abdominal wall and connects it to the colon to allow stool to flow into a plastic bag on the abdominal wall. The colostomy stays in place until the infant is older and the structures that need to be repaired have grown larger. The colostomy is closed when surgery to fully repair the defect is done.