Anal fistula

An anal fistula can either be a single tunnel or, occasionally, a network of tunnels. It’s usually caused by an infection near the anus that causes pus to collect in the surrounding tissue. When the pus drains out, a tunnel or network of tunnels is left behind.

An anal fistula is a painful and often unpleasant long-term (chronic) condition. Symptoms include discharge, skin irritation, anal discharge and, in some cases, bowel incontinence.

You’re more likely to develop an anal fistula if you’ve had an anal abscess, including a perianal abscess. About one in every two people with an anal abscess also develops an anal fistula.

An anal fistula requires fistula treatment, usually anal fistula surgery.

You may notice a small hole in the skin near your anus. You may also experience symptoms such as:

• A high temperature (fever) if you also have an abscess
• Problems controlling your bowel movements (bowel incontinence)
• Pus or anal bleeding when you pass a stool
• Red, itchy, irritated skin around your anus
• Throbbing, constant anal pain and swelling around your anus
• Unpleasant discharge from the area around your anus

See your GP if you’re regularly experiencing any of the symptoms of an anal fistula. Your GP will ask about your symptoms and your general health, including any history of bowel conditions. With your permission, your GP may carry out a rectal examination. This involves examining your anus by gently inserting a gloved finger inside it.

If your GP diagnoses — or suspects — an anal fistula, they’ll refer you to a consultant called a colorectal surgeon. At your first appointment, your consultant may refer you for further investigations, including:

• A proctoscopy — to check the inside of your rectum using a proctoscope, a special telescope with a light on the end
• A sigmoidoscopy — a flexible tube with a camera on the end that takes images of your rectum and bowel
• A CT scan, MRI scan and/or ultrasound scan — to provide a detailed picture of inside your rectum

An anal fistula is often the result of an anal abscess (perianal abscess). A perianal abscess can occur when glands in your rectum that make fluids become blocked and then infected with bacteria — this causes the formation of a pus-filled abscess. 

If the abscess becomes large enough, it can form tunnels that pass through the tissue around your rectum and out to the skin around your anus. As the abscess drains, it leaves a tunnel behind.

An anal fistula can also be caused by chronic bowel conditions, including: 

• Crohn’s disease — a chronic condition where the digestive system becomes inflamed
• Diverticulitis — an infection of the large intestine, specifically of small pouches that stick out of the side of the large intestine and commonly develop over the age of 40
• Hidradenitis suppurativa — a chronic skin condition that causes abscesses and scarring

In rare cases, complications after surgery near the anus, HIV and tuberculosis (TB) can cause an anal fistula.

Anal fistula treatment almost always involves anal fistula surgery to close the tunnel. This allows an anal fistula to heal without affecting your bowel continence.

The type of surgery recommended by your doctor will depend on the position of your fistula and whether it is one tunnel or a network of tunnels. Further examination may be needed to determine the most appropriate surgery — these investigations will be carried out under general anaesthetic.

Your consultant will discuss your options with you, which may include surgical procedures or a non-surgical procedure. 

The most common type of fistula surgery is a fistulotomy. It involves opening the anal fistula by cutting along the length of it, so it closes and heals as a flat scar. A fistulotomy is effective for treating many anal fistulas but is only suitable for fistulas that do not pass through sphincter muscles as this reduces the risk of incontinence.

If your fistula passes through a considerable portion of your sphincter muscle and a fistulotomy will put you at a high risk of incontinence, your doctor may recommend an alternative surgery, such as:

• Advancement flap procedure — cutting out your fistula and covering the entrance hole to the fistula with tissue from your rectum; this procedure has a lower success rate than a fistulotomy
• LIFT procedure — this procedure has been in use since 2007, producing good results, and involves making a cut in the skin above your fistula to enable your sphincter muscles to be moved apart, then the fistula is closed at both ends and cut open along its length so it lies flat
• Seton techniques — a seton, a thin silicone thread, is inserted into your anal fistula and left there for several weeks so your fistula can drain and heal; this procedure is usually repeated so that:
• a loose seton can first be inserted to drain your fistula
• a series of tighter setons can be inserted to gradually cut through your fistula (or your doctor may recommend a series of fistulotomy surgeries to gradually open up your fistula)

Other surgical options include:

• Bioprosthetic plug — a cone-shaped plug made of animal tissue is placed inside your fistula and stitched in place to safely block the fistula opening 
• Endoscopic ablation — a thin, telescope-like tube (an endoscope) is passed into your fistula, then an electrode is passed through the endoscope and is used to effectively seal the fistula
• Laser surgery — a small laser beam is used to seal your fistula; while there are no major safety concerns, it is unclear how effective this procedure is

Fibrin glue offers the only non-surgical treatment and is not as effective as a fistulotomy. Under a general anaesthetic, a special glue is injected into your fistula to help it seal and heal. The results may not be long-lasting but this procedure may be recommended if your fistula passes through a considerable portion of your sphincter muscle. 

The major risks of anal fistula surgery are: 

• Bowel incontinence — this risk is present with most types of anal fistula treatment but severe incontinence is rare
• Infection — this can be treated with antibiotics; in severe cases, antibiotics will need to be administered at a hospital directly into your bloodstream
• Recurrence of the fistula — anal fistulas can return even after surgery

Your level of risk after anal fistula surgery depends on the position of your fistula and the specific procedure used to treat it.

If an anal fistula is not treated correctly, you may experience anal abscesses more often and develop a complex network of anal fistulas. This may cause:

• Bleeding
• Bowel incontinence
• Pain 
• Recurring anal fistulas