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Anal Fistula: Causes, Symptoms & Treatment

Medically reviewed by Dr. Sim Hsien Lin, Consultant General Surgeon (Colorectal Surgery)

What Is an Anal Fistula?

An anal fistula is a small passage that forms between the anal canal or lower part of the rectum and skin surrounding the anus, due to a bacterial infection in the rectum. Such infection leads to a collection of abscesses (pus) in the surrounding rectal tissue. As the pus drains out, it leaves behind a tunnel (fistula).

The anus is made up of small glands that excrete mucus. When these glands get clogged, an infection may set in, leading to the formation of abscesses. About half of these abscesses (bacteria) may increase in size, forming a track that leads to the skin around the anus.


Anal abscesses and clogged glands are the leading causes of anal fistula. Other possible causes of an anal fistula include:

  • Inflammatory bowel disease
  • Diverticulitis (colon infection, especially in older people)
  • Crohn’s disease (intestinal inflammation)
  • Cancer and its treatment procedures like radiation therapy
  • Sexually transmitted diseases
  • Tuberculosis

Signs and Symptoms

Anal fistula causes unpleasant and uncomfortable symptoms. These include skin irritation that keeps getting worse with time. Other common symptoms include:

  • Feeling itchy around the anus as a result of skin irritation
  • Smelly discharge (abscess) from an opening near the anus
  • Persistent throbbing pain that gets worse when walking, sitting or coughing
  • Rectal bleeding and pain when passing faeces
  • Chills, fever and general body weakness

Diagnosis and Treatment


Your doctor may perform several tests to diagnose an anal fistula, but the first step is the physical examination. Your colorectal surgeon will examine the anal area for any physical opening or skin inflammation.

In the case of a physical opening, your doctor will perform further analysis to determine the depth and direction of the tract (fistula).

However, some fistulas may need additional tests. These include:

  • Anoscopy – A special tube with a video camera and source of light at the tip is inserted into the rectum through the anus. This enables the doctor to check for any abnormalities in the rectum and anal canal.
  • MRI or Ultrasound – This is a non-invasive procedure that helps the doctor get a better view of the fistula.

Diagnostic methods used to rule out other disorders such as cancer or inflammatory bowel disease include:

  • Flexible sigmoidoscopy. A thin, flexible tube with a lighted camera inside the tip allows doctors to view the lining of the rectum and sigmoid colon as a magnified image on a television screen.
  • Colonoscopy. Similar to sigmoidoscopy, but with the ability to examine the entire colon or large intestine.


Surgery is the most effective ways to treat fistula. Your colorectal surgeon will perform the surgery, balancing between clearing the fistula and keeping the patient’s sphincter muscles intact. Any damage to the sphincter muscles may lead to bowel incontinence. Common treatments for fistula include:

Fistulotomy – This treatment works best for fistulas that have few sphincter muscles involved. The surgeon will cut open the skin and muscle around the fistula and convert the tunnel into a groove. This ensures the tunnel heals from the bottom up.

Seton Procedure – Your doctor places a surgical thread through the fistula and ties it to create a continuous ring between the outside and the inner openings of the tunnel. This thread remains for a couple of weeks as it helps with the drainage of abscess, thus preventing further infection. Once the fistula heals, the surgeon will perform a further procedure to close the fistula’s opening.

Fibrin Glue – The doctor injects a combination of fibrinogen, calcium and thrombin to form a clot within the fistula, allowing healthy tissue development to fill up the tunnel.

Endorectal Advancement flap – Your surgeon takes a healthy flap of your bowel lining and covers the fistula’s internal opening. As the healthy flap heals up, the tunnel drains the abscess and healing begins.

Fistula Plug – Your surgeon uses animal tissue to create a plug that blocks the fistula’s internal opening. This infection-resistant plug spurs development of normal tissue that eventually fills up the fistula tract.

The LIFT procedure – Your surgeon will open up the skin around the anus, dissect between the anal sphincters, locate the tract and tie off the fistula.

Your colorectal surgeon will discuss these options with you before the surgery. This will be based on their judgment regarding the severity of the condition and patient’s preferences.

Colorectal surgery is generally straightforward, and you may not need to stay in the hospital for more than a day. An experienced colorectal surgeon will ensure the process is seamless to avoid recurring cases of abscesses and tampering of the sphincter muscles.


Anal fistula is an invasive condition with uncomfortable signs and symptoms. Fortunately, the condition is highly treatable. However, you can reduce the chances of suffering from anal fistula by keeping your stools soft, avoiding constipation and emptying your bowels as soon as you feel the urge. Drinking lots of fluids also helps clear your digestive system and prevent the build-up of bacteria that may end up forming abscesses in the rectum.

Dr. Sim Hsien Lin’s main sub-specialty is in colorectal surgery. In addition, her sub interests include laparoscopic colorectal surgery, transanal endoscopic microsurgery (TEMS), surgical management of piles, inflammatory bowel diseases, complex anal fistulas as well as diagnostic and therapeutic endoscopic procedures.

Dr. Sim Hsien Lin

Consultant General Surgeon (Colorectal Surgery)
  • MBBS (Singapore)
  • MRCS (Edinburgh)
  • MMed (General Surgery)
  • FRCS (Edinburgh)

Dr. Sim Hsien Lin is a specialist colorectal surgeon in Singapore with over 10 years of experience in colorectal surgery. Her sub interests include laparoscopic colorectal surgery, transanal endoscopic microsurgery (TEMS), surgical management of piles, inflammatory bowel diseases and complex anal fistulas.

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