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- 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 described as low and 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 by type of associated fistula: rectal, bladder, urethra, or vaginal.
- Prevalence is estimated to be in the range of 1:3,000–1:5,000.
- High lesions are more common in males (2:1).
- Low lesions occur with equal frequency in both sexes.
- Can be an isolated defect or part of a syndrome or association
- Syndromic disorders that contain IA are associated with defects on chromosomes 6, 7, 10, and 16.
- Can be associated with trisomy 21 (typically IA without fistula)
- Can be part of omphalocele-exstrophy of the bladder-IA-spinal defects (OEIS) complex or cloacal exstrophy (EC)
- 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.
- Females: Most common defect is a recto-vestibular fistula where the rectum opens into the vestibule.
- Males: Most common defect is a rectourethral fistula from the rectum to lower posterior urethra (bulbar) or upper posterior urethra (prostatic).
Commonly Associated Conditions
- Other anomalies are present up to 50% of patients with an IA.
- IA can be associated with vertebral, cardiac, tracheoesophageal fistula, renal, and limb anomalies (VATER or VACTERL).
- Other associated anomalies include urologic, spine/sacrum (hypoplastic sacrum, sacral agenesis, presacral mass, myelomeningocele, tethered cord), gastrointestinal (esophageal/intestinal atresia, malrotation, omphalocele, annular pancreas), gynecologic (duplicate uterus, septate vagina, vaginal atresia), and cardiovascular defects (septal defects).