Chapter 90. Congenital Malformations of Digestive System: Hindgut Malformations - Review of Medical Embryology Book

Malformations of the hindgut: some form of imperforate anus is usually seen, occurs in 1/5000 births, and is more common in males. Most anorectal malformations result from an abnormal development of the urorectal septum, resulting in an incomplete separation of the cloaca into the urogenital and anorectal parts

Imperforate anus or absent anus: classical anal imperfections include many varieties, and all require surgical intervention
Insufficient anus leads to problems of meconium evacuation and should be treated without delay
Ectopic sinus: less serious than (a) or (b) since it does allow some intestinal transport, but it, too, is usually functionally insufficient

Superficial malformations; due to anomalies in formation and fixation at superficial perineal levels. Here we see

Anal agenesis or insufficient anus (with or without fistula): the canal may end blindly, and there may be an ectopic opening (ectopic anus) or fistula opening in the perineum or vulva or male urethr Due to incomplete separation of the cloaca by the urorectal septum
Membranous atresia or covered anus (with or without fistula): very rare; anus is in normal place, but a thin layer of tissue separates the anal canal from the exterior. Due to a failure of the anal membrane to perforate at the end of week 8
Anorectal agenesis with or without fistula: rectum ends blindly above the anal canal, but there is usually a fistula to the urethra in the male or vagina in the femal The defect is similar to (i) above

Deep malformations: where the anomaly affects septation of the cloaca by aberrant migration of the urorectal septum

Pure rectal atresia: complete failure of the formation of the inferior part of the rectum and anal canal
Rectal atresia with fistula: always insufficient. The length and degree of anastomosis differentiate the various types of anomaly

May involve abnormal anastomoses of the anus to the perineum and reflect both perineal and cloacal abnormalities