Anorectal Atresia

Anorectal atresia refers to a spectrum of congenital anorectal malformations with an unclear etiology. These anomalies range from a simple imperforate anus in an otherwise normal anorectal region to complex anomalies involving the urogenital system. Anorectal atresia is sometimes seen as an isolated finding, but it may also occur as part of a multi-organ syndrome. Diagnosis is often made during initial newborn examination or after a delay in passage of meconium for over 24 hours after birth. Prognosis varies based on complexity. Treatment is primarily surgical.

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Overview

Epidemiology

  • 1 in 5,000 births in the United States
  • Boys have a higher incidence than girls.
  • Most cases are sporadic (no family history).
  • Parental risk factors:
    • Paternal: smoking
    • Maternal: obesity and diabetes
  • Associated anomalies occur frequently and are easily remembered using the acronym VACTERL:
    • Vertebral anomalies
    • Anal artesia
    • Cardiac defects
    • TracheoEsophageal fistula/atresia
    • Renal anomalies and Radial aplasia
    • Limb anomalies
  • There is an association between anorectal atresia and trisomy 21 and 18.

Etiology

Anorectal atresia arises out of deviations from the normal embryological development of abdominal organs to create the adult rectum and anal canal.

  • At 78 weeks’ gestation, the hindgut and allantois open into a common cloaca.
  • The cloaca is endoderm lined and separated from the surface ectoderm by the cloacal membrane.
  • By 7 weeks’ gestation, the cloaca is divided by a urorectal septum into the urogenital sinus and rectum/anal tubercle.
  • Following a ventral urogenital opening, the cloacal membrane dorsal to the urorectal septum (i.e., anal membrane) gradually resorbs, connecting the ectoderm to endoderm and creating the anal opening.
  • Developmental arrest anywhere during these stages results in anorectal malformation.
Cloaca of human embryo

Cloaca of the human embryo at 25–27 days

Image by Lecturio.

Classification

Anorectal malformations may be divided into high and low lesions based on the level of the pelvic floor (levator ani muscle):

  • High/supralevator lesions
    • More difficult to correct and prone to fecal incontinence
    • Boys:
      • Rectobulbar fistula (most common type)
      • Rectoprostatic fistula
      • Rectovesical fistula
    • Girls: 
      • Rectovestibular fistula
      • Rectovaginal fistula
      • Cloacal malformation has the worst prognosis.
  • Low/infralevator lesions
    • Relatively easy to correct, but prone to constipation
      • Rectoperineal fistula in both sexes
      • The rectum communicates with the skin but the canal is not protected by a sphincter. 
    • The anus may also end in a blind pouch.

Diagnosis

Defects are often identified during the newborn physical examination. Missed cases are usually identified within 24 hours following failure to pass meconium and abdominal distension.

Physical examination

  • Clues about the neonate’s type of malformation:
    • Perineal opening: indicates a low defect 
    • Urine sampling for meconium:
      • In boys without a perineal opening and girls without a perineal or vestibular opening
      • Positive sample indicates urinary tract fistula.
    • Palpation for abdominal masses: may indicate hydronephrosis or hydrocolpos
  • Discovery of anorectal atresia should prompt a close examination for associated congenital anomalies:
    • Umbilical stump exam
      • Absence of an umbilical artery often associated with anal atresia
      • Potentially associated absent kidney
      • Boys: verify presence of descended testes
      • Girls: check vaginal patency
    • Cardiac exam: Auscultation of the heart may reveal the murmurs of Tetralogy of Fallot or a ventricular septum defect.

Imaging

  • Prenatal ultrasound may identify suggestive features such as
    • Polyhydramnios
    • Intra-abdominal cysts
    • Hydrocolpos
    • Hydronephrosis  
  • By 24 hours of age, if the infant has not passed stool:
    • Lateral prone radiograph should be obtained.
    • Ultrasound may also identify associated genitourinary and gastrointestinal anomalies.
    • Other radiographic studies such as urethrogram, cologram, and fistulagraphy may be used.
    • Spinal magnetic resonance imaging (MRI) may be indicated in patients with abnormal pelvic radiography.

Management

Medical

  • Once defects are identified, the infant should not be fed orally.
  • Individualize intravenous fluids and electrolytes to meet nutritional needs for the infant.
  • If urinary fistula is suspected: broad-spectrum antibiotics
  • Treatment of life-threatening anomalies should take precedence.

Surgical

  • Primary repair: often performed on rectal pouches
    • < 1 cm identified on imaging
    • With or without perineal/vestibular fistulas
    • Without associated urogenital anomalies
  • Colostomy + staged repair: performed in children who do not qualify for primary repair
    • Within or after 24 hours: early colostomy 
    • Several weeks later: posterior sagittal anorectoplasty (most common procedure)
    • When wounds have healed: colostomy closure
Imperforate anus with rectopenile fistula surgery

A urethrography (a) showing the urethra and bladder, with a small amount of contrast appearing in the rectum. The location of the fistula entering the urethra is displayed (arrow). Limited posterior sagittal anorectoplasty (b) with division of the rectum (upper arrow) and a catheter in the fistula (lower arrow).

Image: “Urethrography showed the urethra and the bladder” by Department of Pediatric Surgery, West China Hospital, Sichuan University, Chengdu, 610041, People’s Republic of China. License: CC BY 4.0

References

  1. Rosen NG. (2019). Pediatric Imperforate Anus. Medscape. NewYork NY. WebMD. Retrieved Oct 6, 2020, from https://emedicine.medscape.com/article/929904-overview#a2
  2. Cuschieri A; (2002). EUROCAT Working Group. Anorectal anomalies associated with or as part of other anomalies. Am J Med Genet.
  3. Anorectal malformations in neonates. Afr J Paediatr Surg.  2011; 8(2):151-4 (ISSN: 0974-5998)

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