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Anal atresia, or imperforate anus, refers to a spectrum of anorectal abnormalities ranging from a membranous separation to complete absence of the anus.
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The estimated incidence is 1 in 5000 live births.
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
Clinically there is no anal opening and failure to pass meconium.
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.
- 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 of 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-positives may occur if the image is taken in the first 24 hours of life or if there is impacted meconium within the distal rectum 6.
For radiographic technique, see invertogram view and prone cross-table lateral view articles.
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 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/post-operatively 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