Anal atresia, or imperforate anus, refers to a spectrum of anorectal abnormalities ranging from a membranous separation to complete absence of the anus.
The estimated incidence is 1 in 5000 live births.
Clinically there is no anal opening. Subtypes can be classified into two broad categories: high (supralevator) or low (infralevator), depending on the location of the atretic portion.
Most cases are sporadic, with occasional familial forms
There are frequent associations with other congenital abnormalities.
- other atresias
- VACTERL association
- caudal regression syndrome: associated sacral agenesis and lower limb hypoplasia
- Currarino's triad; anorectal malformations with sacral anomalies and presacral mass lesion 6
- fistulous tracts to the urethra or vagina may be present or may have a single cloacal opening
- can be variable depending on the site of atresia (i.e. high or low), level of meconium impaction and physiological effects such as straining
- may show multiple dilated bowel loops with an absence of rectal gas
air within urinary bladder suggests high type 6
- calcified meconium in the bowel loops would suggest high type (meconium calcifies due to urine exposure) 6
A coin/metal piece is placed over the expected anus and the baby is turned upside down (for a minimum 3 minutes).
The distance of the gas bubble in the rectum from the metal piece is noted:
- >2 cm denotes high type
- <2 cm denotes low type
False positive: if image is taken in the 1st 24 hours of life or impacted meconium in distal rectum 6.
Fluoroscopy (contrast study)
- to detect a rectourinary, rectovaginal, or rectoperineal fistula
- the fistula is considered low (below the levator ani plane) if it is below the pubococcygeal line (PCL) and high if above it
- the anus may be seen as an echogenic spot at the level of the perineum and in anal atresia this echogenic spot may be absent 4
- may show bowel dilatation
- an infracoccygeal or transperineal approach may allow differentiation between high and low subtypes 4
- kidneys should be assessed in such patients 6
- spinal US can reveal spinal cord lesions like tethering of cord 6
Can be used pre/postoperatively to study pelvic floor, renal and spinal abnormalities 6.
Treatment and prognosis
- low subtypes are treated with anoplasty
- high subtypes are treated with colostomy with subsequent potential repair
- 1. Harris RD, Nyberg DA, Mack LA et-al. Anorectal atresia: prenatal sonographic diagnosis. AJR Am J Roentgenol. 1987;149 (2): 395-400. AJR Am J Roentgenol (abstract) - Pubmed citation
- 2. Berrocal T, Lamas M, Gutieérrez J et-al. Congenital anomalies of the small intestine, colon, and rectum. Radiographics. 19 (5): 1219-36. Radiographics (full text) - Pubmed citation
- 3. Han TI, Kim IO, Kim WS. Imperforate anus: US determination of the type with infracoccygeal approach. Radiology. 2003;228 (1): 226-9. doi:10.1148/radiol.2281011900 - Pubmed citation
- 4. Entezami M, Albig M, Knoll U et-al. Ultrasound Diagnosis of Fetal Anomalies. Thieme. (2003) ISBN:1588902129. Read it at Google Books - Find it at Amazon
- 5. Donnelly L, Jones B, O'hara S et-al. Diagnostic imaging. AMIRSYS. ISBN:141602333X. Read it at Google Books - Find it at Amazon
- 6. Lee Alexander Grant, Grant, Nyree Griffin. Grainger and Allison's Diagnostic Radiology Essentials. (2019) ISBN: 9780702034480