Anal atresia

Last revised by Joshua Yap on 7 May 2024

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.

There are frequent associations with other congenital abnormalities.

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. 

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

  • low subtypes are treated with anoplasty

  • high subtypes are treated with colostomy with subsequent potential repair  

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