The So-Called "Anterior Anus"
The so-called "anterior anus," meaning an anus located towards the front rather than the back side of the body, is a rather controversial issue. The term "so-called" is used because whether or not the "anterior anus" actually exists depends on what anatomical features define the term.
If an "anterior anus" is defined by these features—a normal anus, surrounded by normal sphincter, with a normal anal canal located closer to the genitalia—we must say that we have never seen an "anterior anus" in our entire collective medical experience.
Many babies, however, are born with a perineal fistula meaning an abnormal anal opening, anteriorly mislocated. This opening is located somewhere between the normal center of the sphincter, the ring-like band of muscles that opens and closes the anus, and the genitalia. Sphincter activity can be elicited behind (posterior to) and to the side of the opening but there is usually no real sphincter anterior to that opening. In the overwhelming majority of these cases, the anal opening or orifice is stenotic (narrowed or constricted). The anal canal is not normal. Most patients that are labeled as having an "anterior anus" in fact have a perineal fistula.
Most Suffer from Constipation
In the spectrum of anorectal malformations, perineal fistula is the simplest. Most of these patients suffer from severe constipation. The problem of constipation is more severe in patients with low, simple anorectal defects, compared to those with high, very complex anorectal malformations. "High" and "low" refer to the relationship of the defects to the levator muscle complex, which elevates the organs and structures in the area. In "low" anorectal defects, the rectum descends through the levator muscle complex, while in high anorectal defects, the rectum fails to descend through the levator muscles.
All patients with perineal fistulas have fecal continence, meaning they can voluntarily control their bowels, except under two circumstances:
- When they are subjected to an unnecessarily large operation that damages the sphincter and nerves
- When the constipation is not treated properly and the patient suffers from chronic fecal impaction
The Posterior Shelf
Many patients are labeled as having an "anterior anus" because a contrast enema, a procedure that injects barium sulfate and air into the colon to show abnormalities, shows what is called a "posterior shelf." From our point of view, this "posterior shelf" is present in all normal human beings, although it is more prominent patients that suffer from constipation because this shelf reflects the dilatation of the rectosigmoid, the end of the sigmoid section of the colon and the beginning of the rectum.
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