Anal Fistula

How an anal fistula forms

Small glands sit just inside the anus. If one becomes blocked and infected, it forms a perianal abscess, a collection of pus. The abscess either bursts through the skin or is drained by a doctor. In roughly a third to half of people, the tract between the gland and the skin does not close, leaving a fistula with an internal opening in the anal canal and an external opening on the skin.

Fistulas are described by how they relate to the anal sphincter muscles that control continence:

A simple fistula involves little sphincter muscle. A complex fistula involves more muscle, has branches, is linked to Crohn's disease or radiation, or lies at the front in women.

Causes and risk factors

• previous anal abscess (most cases)
• Crohn's disease
• hidradenitis suppurativa
• diabetes and weakened immune systems
• tuberculosis, HIV and some sexually transmitted infections
• previous anal surgery, radiation treatment or trauma
• rarely, cancer

Anal fistulas are more common in men, typically aged 20 to 50.

Symptoms

• a small opening near the anus that discharges pus, blood or occasionally stool
• recurring swelling and pain that builds up and eases when the fistula drains
• skin irritation and itching from discharge
• pain on sitting or opening the bowels
• fever if an abscess re-forms

Diagnosis

• Examination: a doctor looks for the external opening and may feel a cord-like tract. A gentle rectal exam and proctoscopy may show the internal opening.
• MRI of the pelvis: the best imaging to map complex or recurrent fistulas and hidden abscesses.
• Endoanal ultrasound: an alternative to MRI in some centers.
• Examination under anesthesia (EUA): often done before or at the start of surgery to trace the tract fully.
• Colonoscopy or other tests if Crohn's disease is suspected.

Treatment

Fistulas rarely heal with antibiotics alone. Surgery aims to remove or close the tract while protecting continence.

Fistulotomy

The surgeon opens the full length of the tract so it heals from the inside out. It cures most simple, low fistulas, with success rates above 90 percent, but is avoided when much sphincter muscle would be cut.

Seton

A thin silicone or suture loop is threaded through the tract and tied loosely. A draining seton keeps the tract open to prevent abscesses, often for weeks to months, or long term in Crohn's disease. A cutting seton is tightened gradually and is used less often.

Sphincter-sparing procedures

For complex fistulas:

• LIFT (ligation of the intersphincteric fistula tract): the tract is tied off and divided between the sphincter muscles.
• Advancement flap: a flap of healthy tissue covers the internal opening.
• Fistula plugs, fibrin glue, laser closure and video-assisted techniques: less invasive, with lower and more variable success rates.

Crohn's-related fistulas

These are managed jointly by surgeons and gastroenterologists. Draining setons plus medicines such as anti-TNF biologics are the mainstay.

Recovery

Most procedures are outpatient. Recovery involves pain relief, sitz baths, stool softeners and dressing changes for a few weeks. Fistulas can recur, and some need more than one operation.

Complications

• recurrence or new abscesses
• fecal incontinence, mainly after operations that divide sphincter muscle
• in long-standing complex fistulas, very rarely, cancer in the tract

Frequently asked questions

Can an anal fistula heal on its own?

Rarely. Most fistulas persist and continue to discharge or cause repeated abscesses. Antibiotics can calm infection but do not close the tract. Surgery is usually needed for a cure.

Is anal fistula surgery painful?

There is some pain for the first days to weeks, managed with pain relief, warm baths and stool softeners. Most people return to normal activities within 1 to 2 weeks for simple procedures, though complete healing of the wound takes longer.

What is the difference between an anal fistula and an anal fissure?

An anal fissure is a small tear in the lining of the anus, causing sharp pain and bleeding with bowel movements. A fistula is a tunnel from inside the anus to the skin, usually after an abscess, causing discharge and swelling. They are treated differently.

Will surgery for an anal fistula affect bowel control?

Surgeons choose procedures to protect the sphincter muscles. For simple, low fistulas, the risk is low. For complex fistulas, sphincter-sparing techniques or setons are used to minimize risk. Discuss the specific risks of your procedure with your surgeon.