ANORECTAL ATRESIA

- IMPERFORATE ANUS

Anorectal malformations occur in 1:5000 live births and range in severity from a persistent cloaca to anal atresia (1,2). The anal canal is either absent or ends ectopically in a fistulous canal.

ETIOLOGY

Link to Etiology

CLASSIFICATION

• "High" lesions = atresia above the levator ani muscle (supralevator).
• "Low" lesion = atresia below the levator ani muscle.

ULTRASOUND

• Dilatation of the colon (usually only apparent in the third trimester). The exact level of obstruction may be difficult to discern antenatally.
• The most common sonographic appearance of the dilated colon is a septated "V" or "U" configuration of anechoic bowel in the abdomen or pelvis (2,3).
  This is probably caused by two loops of bowel that lie adjacent to one another (4,5).
• The degree of colonic dilatation is dependent on the menstrual age of the fetus.
• Enterolithiasis.
• Vesicoenteric fistula.

ASSOCIATED ANOMALIES (ABOUT 50%)

• VATER / VACTERL association.
• Sacral abnormality (myelomeningocele), sacral agenesis or sirenomelia occur in up to 50% of fetuses with high lesions (2,3,6).
• Genitourinary tract abnormalities, especially crossed renal ectopia, renal agenesis, duplex ureters, bladder diverticuli and urachal remnants occur in 40-50% of cases (6).
• VSD / Patent ductus arteriosis (10%).
• Tracheo-esophageal fistula (10%) (3,7).
• Trisomies 18 and 21 (1,3).

REFERENCES

1. Boles ET Jr. Imperforate anus. Clin Perinatol 1978;5:149-161.
2. Harris RD, Nyberg DA, Mack LA et.al. Anorectal atresia: Prenatal sonographic diagnosis. AJR 1987;145:395-400.
3. Petrikovsky BM, Walzack MP, D'Addario PF. Fetal cloacal abnormalities; Prenatal sonographic findings and differential diagnosis. Obstet Gynecol 1988;72:464.
4. Lande IM, Hamilton EF. The antenatal sonographic visualization of cloacal dysgenesis. J Ultrasound Med 1986;5:275.
5. Bean WJ, Calonje MA, Aprill CN et.al. Anal atresia: A prenatal ultrasound diagnosis. J Clin Ultrasound 1978;6:73.
6. Karrer FM, Flannery AM, Nelson MD et.al. Anorectal malformations: Evaluation of associated spinal dysraphic syndromes. J Pediatr Surg 1988;23:47.
7. deVries PA, Cox KL. Surgery of anorectal anomalies. Surg Clin N Am 1985;65:1139.