Anal atresia, or imperforate anus, refers to a spectrum of anorectal abnormalities ranging from a membranous separation to complete absence of the anus.

Epidemiology

The estimated incidence is 1 in 5000 live births.

Associations

There are frequent associations with other congenital abnormalities.

• other atresias

  • oesophageal atresia ⁴

• VACTERL association

• caudal regression syndrome: associated sacral agenesis and lower limb hypoplasia

• Currarino triad: anorectal malformations with sacral anomalies and presacral mass lesion ⁶

• trisomy 21

Clinical presentation

Clinically there is no anal opening and failure to pass meconium.

Pathology

Subtypes can be classified into two broad categories: high (supralevator) or low (infralevator), depending on the location of the atretic portion.

Genetics

Most cases are sporadic, with occasional familial forms.

Radiographic features

Plain radiograph

Abdominal radiograph

• 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

Invertogram

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)

The fistula is considered low (below the levator ani plane) if it is below the pubococcygeal line and high if above it

Ultrasound

• 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. This sign may be appreciated on antenatal sonography as well

• 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

MRI

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  

Complications

• meconium peritonitis

See also

• anorectal malformation

• duodenal atresia

• small bowel atresia