What Is an Anal Fistula?

Medically Reviewed By Megan Soliman, MD
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An anal fistula is the abnormal formation of a tunnel that connects the internal anus to the outside skin around it. Also called fistula-in-ano, it is often a complication of an anal abscess. Anal fistulas are one of the most common anorectal conditions. Treatment of anal fistulas depends on the location and complexity of the fistula. Surgical removal of the fistula is often necessary.

Keep reading to learn more about anal fistulas, including the causes, diagnosis process, and treatment. 

What is an anal fistula?

An anal fistula is a small tract that starts internally at the anal canal and ends externally at the skin around the anus.

In most cases, this tunnel develops due to drainage from an anal abscess. An anal abscess is an infected collection of pus that can form in the rectum or anus.

Learn more about abscesses, including symptoms and treatments.

Types of anal fistulas

Medical illustration by Mekhi Baldwin of types of anal fistulas
Mekhi Baldwin

The anus has two sphincters inside the body to control movement. One is internal and the other is external. Doctors classify anal fistulas depending on where the formed passageway develops and ends.

TypeDescriptionPercentage of cases
intersphinctericgoes through the internal sphincter to outside skin without passing through the external sphincter45%
transphinctericgoes through both the internal and external sphincters to the outside skin30%
suprasphinctericgoes up through the internal sphincter and around the external sphincter to the outer skin20%
extrasphinctericbypasses both sphincters and connects directly from the rectum to the perineum5%

The type of anal fistula will also help determine your doctor’s recommended treatment options.

What are the symptoms of an anal fistula?

Symptoms of an anal fistula are similar to the symptoms of an anal abscess. When symptoms follow the drainage of an anal abscess, they likely indicate an anal fistula.

Anal fistula symptoms include:

  • redness and swelling on the skin surrounding the anal region
  • throbbing pain in the anus region
  • foul smelling drainage from an opening near the anus
  • blood in stool
  • painful bowel movements
  • trouble controlling bowel movements

When to contact a doctor

Contact a doctor immediately if you have symptoms of an anal fistula or anal abscess. A prompt diagnosis can improve treatment outcomes and reduce the risk of complications.

Learn other possible causes of rectal discharge.

What causes an anal fistula?

An anal fistula can develop for many reasons. Anal abscesses are the most common cause of anal fistulas.

Anal abscesses

In most cases, a person with an anal fistula has a history of an anal abscess. Up to 70% of people who receive a diagnosis for an anal abscess have already developed an anal fistula as well, according to a 2022 overview.

Other causes

Certain conditions that can cause inflammation in the anorectal area can also lead to the development of anal abscesses that produce fistulas. These include:

  • Crohn’s disease
  • diverticulitis
  • tuberculosis
  • trauma to the area
  • cancer and certain cancer treatments, such as radiation
  • sexually transmitted infection (STI)
  • structural differences present at birth

Some anal fistulas can also develop spontaneously.

Learn more about potential complications of Crohn’s disease.

What are anal fistula risk factors?

According to a 2022 overview, researchers have identified trends and risk factors associated with the development of anal fistulas. These include:

  • male sex assigned at birth, as males develop anal fistulas more than twice as often as females assigned at birth
  • age between 20 and 40, with 38 being the average age at diagnosis
  • hyperlipidemia, which is a high concentration of fats or lipids in the blood
  • obesity
  • smoking
  • diabetes
  • sedentary lifestyle

Talk with your doctor about your individual risk for anal fistulas and steps you can take to lower your risk.

How do doctors diagnose an anal fistula?

To diagnose an anal fistula, your doctor will first discuss your medical history and your symptoms.

Your doctor will perform a physical examination of the outside and inside of your anus. Because this exam can be uncomfortable, doctors typically perform it in an operating room and you will be under general anesthesia.

Tests your doctor may order to diagnose and evaluate an anal fistula include:

  • digital rectal exam, in which a doctor inserts a finger to feel for changes in the structure of anal tissue
  • anoscopy or sigmoidoscopy, in which a doctor inserts a thin, flexible tube with a camera to view the lining of the anus and rectum
  • endoanal ultrasound, which can help doctors locate and evaluate fistulas
  • pelvic MRI, which can help map the path of the fistula
  • CT scan, which can help evaluate more complex fistulas
  • colonoscopy, if the fistula may be associated with Crohn’s disease

Your doctor will discuss your diagnosis with you and how it will determine your treatment options.

What are treatments for an anal fistula?

Male patient wearing hospital gown sitting on hospital bed
Ivan Gener/Stocksy United (person appearing is a model and used for illustrative purposes only)

Surgery has traditionally been the most common treatment for an anal fistula. However, new treatment options may also be effective with fewer associated risks.

Anal fistula surgery 

One longstanding treatment for an anal fistula is a fistulotomy. This procedure involves creating an incision and removing the infected area. The surgeon then stitches the area back up so it can heal.

Fistulotomy has a very high success rate, up to 97%, according to the American Society of Colon & Rectal Surgeons (ASCRS). However, the procedure carries the risk of damaging the anal muscles that control when you pass your stool. This can result in fecal incontinence.

Other surgical treatment options do not require cutting and dividing the anal muscles. These options can reduce the risk of incontinence:

  • Endoanal advancement flap: This procedure involves covering the fistula’s opening with healthy tissue. There is still a risk of incontinence, and certain conditions can affect overall effectiveness.
  • Ligation of intersphincteric fistula tract (LIFT): This is a less invasive procedure that fuses the internal opening shut. In some cases, doctors perform a LIFT procedure following another procedure called seton placement.
  • Seton placement: A seton is a thin tube that doctors place through the fistula. They then tie the ends together outside the body. This creates a loop that collects drainage while also encouraging the fistula to heal.
  • Fibrin glue: This procedure uses a synthetic mixture of collagen to form a plug at the internal end of the fistula. Success rates are mixed, and more research is necessary.

The ASCRS notes that for people who develop anal fistulas with Crohn’s disease, medical therapy is the first-line treatment instead of surgery.


An anal fistula is the formation of an abnormal tract that connects the inner anal canal to the outer skin. It is most commonly a complication of an anal abscess.

Symptoms of an anal fistula include severe pain, redness, swelling, discharge, and blood in your stool. Contact your doctor for any symptoms of an anal fistula. They can discuss prompt treatment options, including surgical removal.

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Medical Reviewer: Megan Soliman, MD
Last Review Date: 2022 Oct 25
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