anal fistula

Definition: Anal fistula, or fistula-in-ano, is an abnormal connection between the epithelialised surface of the anal canal and (usually) the perianal skin.

Anal fistulae are common and usually develop secondary to chronic infection of the anal glands or ducts. Many are idiopathic but they are also frequent in patients with Crohn disease and in occasional patients with ulcerative colitis.

Anal fistulae originate from the anal glands, which are located between the two layers of the anal sphincters and which drain into the anal canal.

If the outlet of these glands becomes blocked, an abscess can form which can eventually point to the skin surface. The tract formed by this process is the fistula.

Abscesses can recur if the fistula seals over, allowing the accumulation of pus. It then points to the surface again, and the process repeats.

Anal fistulas per se do not generally harm, but can be very painful, and can be irritating because of the pus-drain (it is also possible for formed stools to be passed through the fistula); additionally, recurrent abscesses may lead to significant short term morbidity from pain, and create a nidus for systemic spread of infection.

Microscopy

On microscopic examination, specimens from fistulae contain a central core of active and chronic inflammation with granulation tissue, possibly with foreign body giant cells, surrounded by scar.

Occasionally, granulomas are present.

Squamous or ATZ epithelium may extend into the tract and partially epithelialize it.

Treatment

Surgery is considered essential in the decompression of acute abscesses; repair of the fistula itself is considered an elective procedure which many patients elect to undertake due to the discomfort and inconvenience associated with a draining tract.

See also

 anal lesions
 anal anomalies