Imperforate Anus

Imperforate Anus is a topic covered in the 5-Minute Pediatric Consult.

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Basics

Description

  • Imperforate anus (IA) is a congenital abnormality in which the bowel fails to perforate or only partially perforates the pelvic muscular floor.
  • IA may also perforate the epidermal covering of the pelvic muscular floor (anal membrane).
  • Spectrum of anorectal malformations that range in severity from imperforate anal membrane to complete caudal regression
  • IA has been classically subcategorized into low or high anomalies.
    • In a low lesion, the colon remains close to the skin and there may be a stenosis of the anus, or the anus may be missing altogether, with the rectum ending in a blind pouch.
    • In a high lesion, the colon is higher up in the pelvis, with a fistula connecting the rectum and the bladder, urethra, or the vagina.
  • More recently, classification of IA is based on the presence or absence of fistula and the type of fistulous connection (perineal, bladder, urethra, vagina).

Epidemiology

  • Prevalence is estimated to be in the range of 1:4,000 to 1:5,000.
  • High lesions are more common in males (2:1).
  • Low lesions occur with equal frequency in both sexes.

Risk Factors

Genetics

  • An isolated defect (~1/3) or part of a syndrome or association (~2/3)
  • Chromosomal abnormalities in 4.5–11%. Most common anomaly is trisomy 21 where patients typically have IA without fistula.
  • Increased risk in 1st-degree relatives; reports of familial inheritance
  • Can be part of omphalocele-exstrophy of the bladder-IA-spinal defects (OEIS) complex or cloacal exstrophy (EC)
  • Genes implicated in animal studies: SHH, Wnt5a, and Skt

Pathophysiology

  • During the 6th week of fetal development, the hindgut comes into contact with the cloacal membrane. The hindgut is divided into a ventral urogenital and dorsal rectal component. By the 8th week, the dorsal 1/2 perforates to the exterior. In IA, the process is arrested during this critical period.
  • There is a wide spectrum of anatomic variants of IA; commonly associated with urologic and spinal defects
  • Classification of anatomic variants is based on the relationship between the rectum and the puborectalis muscle: supralevator (high) and translevator (low) malformations
  • Cloaca is a complex defect, where the rectum, urethra, and the vagina drain into a common channel that communicates with the perineum.
  • A fistula communicating from the rectum to the external opening (perineal fistula) or to the urogenital system is present in 90% of cases.
  • In females, the most common defect is a rectovestibular fistula where the rectum opens into the vestibule.
  • In males, the most common defect is a rectourethral fistula from the rectum to lower posterior urethra (bulbar) or upper posterior urethra (prostatic).

Commonly Associated Conditions

  • Anomalies present in 50–67% of patients with IA
  • VACTERL is the most common associated anomaly and includes vertebral, cardiac, tracheoesophageal fistula, and renal and limb anomalies.
  • Associated organ-specific anomalies:
    • Genitourinary (33–50%): vesicoureteral reflux, renal agenesis, horseshoe kidney, multicystic kidney, ectopic kidney, obstruction, hypoplasia/dysplasia, and chronic renal failure (2–6%).
    • Spine/sacrum (33–50%): tethered cord (most common 20–30%), hypoplastic sacrum, sacral agenesis, presacral mass, vertebral defects formation/fusion, myelomeningocele
    • Gastrointestinal (5–10%): tracheoesophageal atresia and esophageal atresia; others include malrotation, omphalocele, annular pancreas.
    • Gynecologic (~7–17%): vaginal atresia, absent vagina/cervix/uterus, septate vagina, and duplicate uterus
    • Cardiovascular defects (10–30%): atrial and ventricular septal defects
  • Other associated syndromes:
    • Currarino (sacral agenesis, presacral mass, and IA)
    • Trisomy 21
    • Townes-Brock syndrome
    • Fragile X
    • McKusick-Kaufman
    • Johanson-Blizzard
    • Opitz-Kaveggia

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