DESCRIPTION

• Anorectal malformation (ARM), formerly called imperforate anus, is a spectrum of congenital abnormalities in which the anus and the rectum fail to develop properly during the 5th to 7th week of fetal development.
• The spectrum of ARM ranges in severity, from imperforate anal membrane to complete caudal regression.
• ARM 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.
• Currently, a therapeutic- and prognostic-related classification of ARM is based on the presence or absence of fistula, and the type of fistulous connection (perineal, bladder, urethra, vestibular, vagina) is being used:
  • Males: rectoperineal, rectourethral bulbar or prostatic (most common), rectobladder neck, imperforate anus without fistula (5%), rectal atresia/rectal stenosis (<1%)
  • Females: rectoperineal, rectovestibular (most common), cloaca, complex malformations, imperforate anus without fistula, rectal atresia/rectal stenosis

EPIDEMIOLOGY

• Occurs in approximately 1 in 4,000 to 5,000 live births
• Slightly more common in males

RISK FACTORS

PATHOPHYSIOLOGY

• During the 6th week of fetal development, the hindgut comes in 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 ARM, the process is arrested during this critical period.
• There is a wide spectrum of anatomic variants of ARM, commonly associated with genitourinary and spinal defects.
  • Cloaca is a complex defect, where the rectum, urethra, and the vagina drain into a common channel that communicates with the perineum. They are further classified into short (<3 cm) or long (>3 cm) common channel.
  • A fistula communicating from the rectum to the external opening (perineal fistula) or to the urogenital system is present in 95% 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 ARM
• VACTERL is the most common associated anomaly and includes vertebral, anorectal, cardiac, tracheoesophageal fistula, renal, and limb anomalies.
• Müllerian duct aplasia, renal aplasia, and cervicothoracic somite dysplasia (MURCS)
• Omphalocele, exstrophy, imperforate anus, and spinal defects (OEIS) are also common associations.
• 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, and 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 ARM)
  • Trisomy 21, 13, and 18
  • Townes-Brock
  • Fragile X
  • McKusick-Kaufman
  • Johanson-Blizzard
  • Opitz-Kaveggia