The last stage of digestion is the collection of and passing from the body of solid wastes. These wastes (what is left of what we eat after the water and nutrients have been taken by our bodies) collect in the rectum and then are expelled through the anus.
The inside of the anus is lined with glands and four to six crypts, or pockets. Sometimes one of these pockets gets filled with stool. This can cause the gland to become infected and develop an anorectal abscess.
The abscess either opens by itself and drains or is surgically drained. A fistula is a tract or opening that forms under the skin from the anus to outside of the body. In about half of the cases where an abscess has occurred and drained, a fistula will form between the inside or the pocket and the opening where the infection drained. A fistula will not heal without treatment, which involves removing the pocket where the infection started.
Having had an anorectal abscess usually precedes the development of a fistula. Signs of an anorectal abscess include:
- A vague feeling of being unwell or uncomfortable
- Swelling and discomfort around or near the anus
- Redness around the area
- Drainage of pus or fluid from the area
A person who has had these symptoms should see his or her doctor to ensure that a fistula does not develop as a result of the infection.
Causes and Risk Factors
The biggest factor in whether a person develops a fistula is whether they have had an anorectal abscess.
A physician will take the patient's medical history and do an examination to determine if a fistula exists.
Fistula tracts must be treated because they will not heal on their own. There is a risk of developing cancer in the fistula tract if left untreated for a long period of time.
Most fistulas are simple to treat. Either the tract or fistula can be opened or the tract and the pocket inside are completely removed. Because the fistula travels through the sphincter muscle, which encloses the anal canal, surgery must be done with care to avoid cutting too deeply into the muscle and allowing leaks of gas or liquid stool.
The surgeon determines how much muscle the fistula crosses before he begins surgery. If the fistula goes through most of the muscle, the surgeon will place a rubber band, or Seton, instead of surgically dividing the muscle. This will divide the muscle gradually over several weeks. Alternatively, the surgeon will leave the Seton in place as a drain only. Either method protects the muscle function and reduces the possibility of incontinence.