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6.1 ANAL ATRESIA

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EPIDEMIOLOGY/GENETICS

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Definition Anal atresia is the congenital absence of an anal opening.

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Epidemiology Occurrence is 1 5000 births (M3:F2).

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Embryology Anal atresia results from an arrest in the division of the cloaca into the urogenital sinus and rectum that occurs during the ninth week of fetal development. There are two types: (1) high anal atresia, which is above the levator sling and is associated with urinary tract fistula in males or complex cloacal anomalies in the female; and (2) low anal atresia, which is below levator sling and is not associated with urinary fistula in males or the vagina in females. More than 80 genetic, chromosomal, and sporadic syndromes have been reported with anal atresia. Associated abnormalities occur in 85% of cases and include spinal/skeletal abnormalities, genitourinary abnormalities (50%), tracheoesophageal abnormalities, and cardiac malformations. The VACTERL (vertebral defects, anal atresia, cardiac defects, tracheoesophageal fistula [TEF], renal anomalies, limb abnormalities) association should be considered in these cases. Caudal regression and the OEIS (omphalocele, exstrophy of the bladder, imperforate anus, and spinal defects) sequence also may have anal atresia as a feature. Trisomies 18 and 21 both have been reported in cases of anal atresia.

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Inheritance Patterns Inheritance is sporadic, with a 3% to 4% recurrence risk in first-degree relatives. However, the genetic syndromes with anal atresia as a feature include ones that are autosomal recessive, autosomal dominant, and multifactorial.

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Teratogens Alcohol, thalidomide, and maternal diabetes are teratogenic.

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Prognosis Prognosis is dependent on associated malformations and the nature of the anal atresia. Of isolated cases, 80% to 90% will have successful functional repair.

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SONOGRAPHY

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FREQUENT FINDINGS
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  • Absence of an “anal dimple”

  • Dilated colon/rectum (15% to 40%)

  • Associated anomalies (85%)

    • Urogenital anomalies (54%)

      • Bilateral renal agenesis

      • Urethral atresia

      • Cloacal anomaly

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LESS FREQUENT
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  • Calcified meconium due to vesicocolic fistula

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KEY FINDINGS/PITFALLS
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  • There is a low prenatal detection rate: sensitivity (8%); positive predictive value (15%).

  • First-trimester prenatal diagnosis has been reported.

    • Early bowel dilatation may resolve.

  • Primary diagnostic clue: enterolithiasis: Coprolites form in the intestine in the presence of urine.

  • A V- or U-shape segment of dilated bowel in the lower pelvis suggests anorectal atresia.

  • The absence of the fetal anal sphincter can be used to diagnose an imperforate anus.

    • A transverse image of the anus has a “bull’s-eye” appearance.

  • Associated anomalies allow prenatal diagnosis.

    • Most frequently, it is diagnosed as part of VACTERL.

  • Anal atresia is associated with complete or partial urorectal septal malformation (URSM).

    • In complete URSM, there is an absence of the perineal and anal opening.

      • Usually, this is lethal due to pulmonary hypoplasia from oligohydramnios.

    • In partial URSM, there is a single outlet for urine and feces.

  • Dilated bowel may not be apparent until the third trimester.

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