These malformations account for approximately 66% of the anorectal anomalies. Two varieties of the high malformation are common: (1) anorectal agenesis, and (ii) rectal atresia.
i) Anorectal Agenesis with or without a fistula. In this anomaly the anus, anal canal and inferior part of the rectum are absent. The rectum ends as a blind pouch superior to the puborectalis muscle. In the male often there is a fis-tulous connection between the rectal pouch and prostatic urethra (rectoure-
thral fistula). Less oftenly, the fistula indicated by a shallow dimple or a pig-may open into the urinary bladder (rec- mented spot. The rectum usually ends tovesical fistula). In the female the rec- blindly, but there may be a rectoure-tal pouch may be connected by a fistu- thraf fistula in the males, and rec-lous communication to the vagina (rec- tovaginal fistula in the females, tovaginal fistula) or the vestibule (rec-
tovestibular fistula). It has been pro- ii) Ectopic Anus. Due to faulty deposed that the anorectal agenesis is velopment of the urorectal septum, the caused by failure of the development of anal opening may be situated in an ec-the proctodeum .and defective forma- topic location much anterior to its nor-tion of the urorectal septum. mal place.
ii) Rectal Atresia. In this malformation jjj) Covered Anus. Sometimes the the anus and anal canal are present at urogenital folds grow in a posterior di-their normal place but the rectum ends rection excessively and fuse in the mid-blindly superior to the puborectalis |jne to cover the anus completely or muscle and no fistula is present. This partially. This condition almost always anomaly is most probably due to a lo- occurs in males (because the urogeni-cal interference in the blood supply of tal folds do not normally fuse together the developing rectum. in females). If the coverage by the uro genital folds is complete, the anus is
B) Low Malformations absent; this condition is called anocu-
These anomalies result from taneous occlusion of the anus. How-
maldevelopment of the anal pit, anal ever, in cases of partial coverage the
membrane, urorectal septum, or uro- ana' canal communicates with the sur-
genital folds (swellings which develop face through are anocutaneous fistula,
around the urogenital membrane and which opens onto the skin in the per-
take part in the development of exter- ineum. nal genitalia; for details see chapter 8).
The main varieties of the low anorectal iv) Imperforate Anal Membrane. anomalies are: (i) anal agenesis with or In this condition the anal pit develops without fistula, (ii) ectopic anus, (iii) normally, but the anal membrane be-covered anus, (iv) imperforate anal comes thickened by tissue proliferating membrane, and (v) anal.stenosis. from the urogenital folds. This thick ened membrane may fail to rupture
i) Anal Agenesis with or without a during the 8th week. This condition is
Fistula. Failure of the anal pit (procto- also called membranous atresia of
deum) to form results in anal agenesis. anus- Examination of the infant reveals
The location of the deficient anal pit is that the anus is in its normal position
but a thin layer of tissue separates the
anal canal from the exterior. The mem- developmental basis of this anomaly is
brane bulges when the child strains a subject of controversy. Some authori-
and appears blue due to the presence ties believe that this malformation re-
of meconium superior to it. The condi- suits from a posterior deviation of the
tion is easily treated by surgical exci- urorectal septum. Other workers claim
sion of the membrane. that the stenosed anus results from the
failure of the anal membrane to rupture
v) Anal Stenosis. In this anomaly the completely. Still other embryologists
anus is in the normal position but the maintain that anal stenosis results from
opening is microscopic in size. Exact poor development of the proctodeum.