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

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