The most common cause of anal fistulas is an anal abscess, but there are several other possible causes. Most anal fistulas form in the posterior midline.
If the fistula is found elsewhere, it is often accompanied by secondary symptoms. These include:
- Inflammatory diseases of the intestines
- sexually transmitted diseases
- trauma
- Tumors including leukemia
- tuberculosis
- anorectal cancer
- HIV with anal abscess or anal fistula in almost one third of patients
Each of these potential causes of anal fistulas is discussed in more detail below.
anal abscess
An abscess is a collection of pus in a part of the body. Anal abscesses occur when small anal glands become infected with bacteria. It can be very painful and can develop into other medical conditions such as anal fistulas.
Anal abscesses are the most common cause of anal fistulas, causing approximately 80% of cases. Up to half of patients with anal abscess develop anal fistulas.
Anal fistulas are more likely to form if:
- Abscess ruptures before treatment
- Fluid is not completely drained
- Abscess does not heal completely
To prevent the progression of anal abscess to anal fistula, it must be treated in a timely manner with appropriate antibiotics. In addition, most patients require the abscess to be incised and the pus internally drained completely to prevent the development of a fistula.
intestinal inflammation
Anal fistulas can also occur as a complication of conditions that involve inflammation of the intestines, such as Crohn’s disease or diverticulitis.
Crohn’s disease is a type of inflammatory bowel disease (IBD). This chronic condition causes inflammation of the mucous membranes that line the digestive system. Diverticulitis is another condition that causes inflammation of small openings or diverticula that form numerous small pockets from the sides of the colon.
Both of these conditions cause chronic inflammation of the colon and rectum, which can lead to anal fistulas in some patients.
Hidradenitis suppurativa
This is a chronic skin condition in which the sweat glands frequently become infected, resulting in scarring and healing. If this occurs in the skin of the anal area, an anal fistula may form secondary to a superficial or deep infection.
sexually transmitted diseases
Some sexually transmitted infections can also cause anal fistulas.
HIV and AIDS involve viral infections that attack and weaken the body’s immune system. This increases susceptibility to infections and can lead to the formation of anal abscesses and anal fistulas.
Chlamydia and syphilis are also sexually transmitted diseases and can increase the risk of anal fistulas.
Trauma/abnormal structures or growths
The formation of anal fistulas is also associated with trauma to the rectal area. In some cases, abnormal growth may predispose to the development of anal fistulas. Tumors, ulcers, complications after surgery near the anus, and birth defects can all cause anal fistulas.
tuberculosis
Tuberculosis is a bacterial infection that often affects the lungs, but can occur anywhere in the body. In some cases, it can affect the rectum and cause anal abscesses to form, which can track to the surface and cause anal fistulas.
cancer
Cancer of the anorectal area rarely causes anal fistulas. Here, the rectum, the distal part of the colon where feces are stored before excretion, is involved in abnormal cell proliferation and tumor formation.