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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 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).
- 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.
- 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
- 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