Abnormalities of the rectum and anus are thought to be due to arrest of the caudal descent of the urorectal septum to the cloacal membrane. The resulting malformations range from isolated imperforate anus to persistent cloaca. These malformations are classified as "high" supralevator lesions, which end above the levator sling and are typically associated with fistulas, and "low" infralevator lesions, which end below the levator sling and are not associated with fistulas.
Isolated transient bowel dilation has been observed as early as 12 weeks of gestation in a fetus with anorectal atresia; the distal colon may be dilated prenatally. The anal canal may appear small (anal canal diameter <95% CI of the normal range), or the anal canal and rectum may be absent.
Associated anomalies occur in up to 50 percent of cases. Anal atresia is most commonly associated with the VACTERL association (vertebral, anal atresia, cardiac, tracheoesophageal fistula, renal, limb) and occurs with increased frequency in trisomy 21.
If a vesicorectal fistula is present, urine may mix with meconium, causing it to calcify and form enteroliths. Therefore, if intraluminal calcifications are identified, anal atresia with vesicorectal fistula should be suspected.
Assessment of the fetal cul-de-sac by magnetic resonance imaging (MRI) can be helpful in distinguishing between a high anorectal malformation and cloaca. MRI may reveal dilated meconium-filled rectum in cases of anal atresia , and detection of abnormal fluid in the rectum suggests vesicorectal fistula.
From Up To Date: Prenatal diagnosis of esophageal, gastrointestinal, and anorectal atresia