Anal fistula - Causes of an anal fistula

What causes an anal fistula?

Anal fistulas (also known as perianal fistulas) can be caused by a number of different things. 

The causes of an anal fistula include:

• An anal abscess causing an anal fistula
• Inflammation of the intestines from conditions such as Crohn’s disease or diverticulitis
• Other reasons such as:
• Hidradenitis suppurativa
• Tuberculosis (TB)
• HIV
• Complication of surgery in the anal area

Anal abscess causing an anal fistula

One of the most common causes of an anal fistula is having an anal abscess. An anal abscess happens when an anal gland, a small gland around the anus (where poo comes from) that produces fluid, becomes blocked and infected which causes pus to build up, becoming an abscess.

When an anal abscess bursts a tunnel can form from the anal gland where the abscess was exiting at the skin.

Up to 50% of people with an anal abscess will develop a fistula¹.

Inflammation of the intestines

Certain conditions can cause your intestines to become inflamed and anal fistulas can be a complication of this.

Conditions which cause this type of inflammation include:

• Crohn’s disease - those with disease in their colon and/or rectum are more likely to get perianal fistulas²
• Diverticulitis
• Ulcerative colitis (it is more rare to have anal fistulas if you have ulcerative colitis)

They are common in people who have Crohn’s disease, with up to 50% of people with Crohn’s disease developing some form of fistula at some point in the 20 years after diagnosis³.

Sometimes developing a fistula will be the first sign that someone has inflammatory bowel disease (IBD).

The exact reasons why they are so common in people with Crohn’s disease isn’t known, but the fact that the inflammation in Crohn’s disease affects every layer of the intestine is thought to play a part. In ulcerative colitis it is only the top layer of the intestine that is affected.

Other causes of anal fistula

Other causes include:

• Hidradenitis suppurativa (HS). This is a chronic inflammatory skin disease which can cause recurring abscesses. Research has shown that HS was found in 6.6% of patients with a perianal fistula⁴
• Tuberculosis (TB). Anal fistulas were found in 80-91% of cases of anorectal TB⁵
• HIV - up to 30% of people with HIV will develop an anal fistula⁶ 
• Cancer of the anorectum
• Chlamydia. Symptoms of chlamydia can include diarrhoea and perianal fistulas, causing similarities to Crohn’s disease⁷
• Complication of surgery. Fistulas can reoccur after surgery for anal fistula⁸