The most common cause of an anal fistula is an anal abscess, although there are several other possible causes of the condition. Most anal fistulas form in the posterior midline.
When fistulas are found in other locations, they are often associated with secondary conditions. These include:
- intestinal inflammatory disease
- sexually transmitted infections
- trauma
- tumors including leukemia
- tuberculosis
- anorectal cancer
- HIV which is associated with anal abscesses and/or fistulas in almost a third of patients
Each of these potential causes of anal fistula is discussed in more detail below.
Anal Abscess
An abscess is a collection of pus in an area of the body. An anal abscess develops when a small anal gland becomes infected with bacteria. It can be very painful and may develop into other medical conditions such as an anal fistula.
Anal abscess is by far the most common cause of anal fistula, and is responsible for causing approximately 80% of cases. Up to half of all patients with an anal abscess will develop an anal fistula.
The formation of an anal fistula is more likely to occur if:
- the abscess bursts before treatment
- the fluid is not drained entirely
- the abscess fails to heal completely
To prevent the progression of an anal abscess to an anal fistula, it should be treated in a timely manner with appropriate antimicrobial medication. Additionally, most patients will require incision of the abscess with complete drainage of the pus inside in order to prevent the occurrence of a fistula.
Intestinal Inflammation
An anal fistula can also present as a complication of medical conditions that involve inflammation of the intestines, such as Crohn’s disease and diverticulitis.
Crohn’s disease is a type of inflammatory bowel disease (IBD). In this chronic condition, there is inflammation of the mucosa lining the digestive system. Diverticulitis is another condition which leads to the inflammation of the small outpouchings or diverticula that form numerous tiny pockets from the sides of the colon.
Both of these conditions cause chronic inflammation of the colon and the rectum, and have the potential to cause anal fistulas in some patients.
Hidradenitis suppurativa
This is a chronic condition of the skin in which the sweat glands develop frequent infections and heal with scarring. When this occurs in the skin of the anal region it can lead to the formation of an anal fistula secondary to superficial or deep infection.
Sexually Transmitted Infections
Some sexually transmitted infections also have the potential to cause anal fistula.
HIV and AIDS involve a viral infection that attacks and weakens the immune system of the body. This can lead to an increased susceptibility to infection, with the formation of an anal abscess and anal fistula.
Chlamydia and syphilis are also sexually transmitted infections that can increase the risk of an individual for anal fistulas.
Trauma/Abnormal Structure or Growth
The formation of an anal fistula is also associated with trauma to the rectal area. In some cases, an abnormal growth may predispose an individual to the development of anal fistulas. Tumors, ulcers, surgical complications following operative procedures near the anus, and birth abnormalities, all have the potential to cause anal fistula.
Tuberculosis
Tuberculosis is a bacterial infection that often affects the lungs but can also occur anywhere in the body. It may sometimes affect the rectum, causing an anal abscess to form, which tracks to the surface, causing an anal fistula.
Cancer
Cancer of the anorectal region is a rare cause of anal fistula. Here the rectum, or the distal portion of the colon where the feces are stored before excretion, is involved in abnormal cell growth and tumor formation.
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