Anal fistula

What is anal fistula?

An anal fistula is a small tunnel that connects the inside of the anal canal to the skin near the anus. It usually forms after an anal abscess drains and leaves a channel behind. It causes repeated discharge, irritation and swelling, and rarely heals on its own.

A fistula develops when an infected gland inside the anus forms an abscess, which then leaves a channel as it drains. Some people have a clear history of an abscess, while others only notice ongoing discharge. The path of the tunnel, and how much sphincter muscle it crosses, decides the treatment.

What causes anal fistula?

Most anal fistulas develop after an anal abscess that does not heal properly once the pus drains. The abscess starts when an anal gland becomes blocked and infected. Less common causes are Crohn's disease, diverticulitis, tuberculosis, hidradenitis suppurativa and complications of surgery near the anus.

• A previous perianal abscess
• Blocked anal glands
• Crohn's disease
• Infection or trauma near the anus
• Less commonly, conditions such as tuberculosis

What are the symptoms?

Anal fistula symptoms are repeated pus or bloody discharge near the anus, skin irritation and a throbbing pain that is worse when sitting or passing stool. A small opening is sometimes visible on the skin. Swelling, redness and fever mean an abscess has formed again and needs prompt assessment.

• Repeated pus or bloody discharge
• Irritation and itching around the anus
• Pain and swelling that worsens
• A small opening on the skin
• Fever if an abscess returns

When should I see a colorectal surgeon?

An anal fistula needs a colorectal surgeon's review, because it rarely heals without treatment. See one for discharge, irritation or swelling near the anus that persists or returns. Seek prompt care for fever with a painful swelling, large amounts of pus, or loss of control over wind or stool.

The warning signs below need prompt attention.

• Fever with a painful swelling
• Large amounts of pus
• Symptoms in someone with Crohn's disease
• Inability to control wind or stool

How is it diagnosed?

An anal fistula is diagnosed by examining the area and finding the opening, with a probe or an examination under anaesthesia if it is painful. An MRI or ultrasound scan maps the track for deep or complex fistulas. Tests for Crohn's disease are added when it is suspected.

• Examination of the area
• Assessment under anaesthesia if painful
• Probe or imaging to map the track
• MRI for complex or deep fistulas
• Tests for Crohn's when suspected

How is anal fistula treated?

Anal fistulas usually need surgery because the track rarely closes alone. A simple fistula is opened along its length (fistulotomy). A fistula crossing more muscle is drained with a seton thread or closed with a sphincter-sparing method such as an advancement flap, which protects bowel control. Crohn's disease is treated alongside.

• Fistulotomy for simple tracks
• Seton placement to drain gradually
• Advancement flap or other sphincter-sparing methods
• Treatment of underlying Crohn's disease
• Long-term review for complex cases

What are the risks and complications?

Anal fistula surgery carries a risk of leaking wind or stool if sphincter muscle is divided, so the operation is planned around how much muscle the track crosses. Other risks are the fistula coming back, infection, delayed healing and the need for more than one procedure.

• Incontinence to wind or stool if muscle is divided
• Fistula coming back
• Infection and delayed healing
• Need for more than one procedure