An anal fistula is a small channel that develops between the end of the bowel, known as the anal canal, and the skin near the anus (opening where waste leaves the body).

On the surface of the skin around the anus, one or more of the fistula ends may be seen as holes. An anal fistula is painful and can cause bleeding and discharge when passing stools.

There are many different types of fistula, ranging from a simple, single tract, to more complicated fistulae that are made up of several tracts that branch out. Some fistulae can be connected to the sphincter muscles (the rings of muscles that open and close the anus). See symptoms for more information about the different types of fistula.

How common are anal fistulae?

Anal fistulae are more common in men, and occur between 20 and 40 years of age. A study that was published in 2007 and looked at four different European countries, including England, found that there are between one and three cases of anal fistulae for every 10,000 people.

An anal fistula usually develops after an anal abscess (a collection of pus) bursts, or where an abscess has not been completely treated. A fistula can also be caused by a condition that affects the intestines, such as irritable bowel syndrome (IBS) or Crohn's disease.

Among people with Crohn’s disease, the incidence of anal fistulae may be as high as 30-50%. Up to 30% of people with HIV (a virus that attacks the body's immune system) will also develop an anal fistula.

Outlook

Most anal fistulae will require surgery because they rarely heal if they are left untreated. There are several different surgical methods depending on where the fistula is and how complicated it is. New research is also being carried out to investigate non-surgical methods.

Following surgery, there is a risk of the anal fistula reoccurring. For example, after the most common type of surgery for a fistula (known as a fistulotomy) the risk of an anal fistula reoccurring is around 21%. See the treatment section for more information.

The symptoms of an anal fistula can include:

• a throbbing, constant pain that may be worse when you sit down, move around, pass stools or cough
• pus or blood when passing stools
• irritation of the skin around the anus (the opening where waste leaves the body)
• itchiness around the anus
• a high temperature (fever) of 38°C (100.4°F) or over

If you have a fistula that is the result of a condition that causes inflammation (swelling) of the intestines (part of your digestive system), such as irritable bowel syndrome (IBS) or ulcerative colitis (see causes), you may experience other symptoms. These could include:

• abdominal pain
• diarrhoea
• loss of appetite 
• weight loss
• nausea (feeling sick)
• vomiting

Types of fistulae

Anal fistulae are classified as different types, depending on their position and how close they are to the sphincter muscles. The sphincter muscles are two rings of muscles that open and close your anus. They are described in more detail below.

• Your internal sphincter muscle is a ring of smooth muscle that keeps your anus closed until you are ready to pass waste out. You cannot control your internal sphincter.
• Your external sphincter muscle is a ring of muscle that goes around the outside of your internal sphincter. You can control your external sphincter.  

The most common types of anal fistulae are:

• Intersphincteric fistulae: the fistula tract (channel) crosses the internal sphincter and then opens on the surface of the skin next to the anus.
• Transsphincteric fistulae: the fistula tract passes through both the internal and external sphincters and opens on the surface of the skin next to the anus.

Other types of anal fistulae can:

• begin at a different part of the colon (large intestine)
• extend in a different direction (so that it does not open next to the anus)
• develop in a horseshoe shape, with two open ends either side of the anus 

An anal abscess is the most common cause of an anal fistula.

Anal abscess

An abscess is a collection of pus and infected fluid. An anal abscess usually develops after a small gland, just inside the anus, becomes infected with bacteria. The cause of the abscess is often unknown, although abscesses are more common in people with immune deficiencies, such as HIV and AIDS.

Abscesses are usually treated with a course of antibiotics (medication to treat infections that are caused by bacteria). In most cases, you will also need to have the infected fluid drained away from the abscess.

If an anal abscess bursts before it has been treated, it can sometimes lead to an anal fistula developing. A fistula may also occur if an abscess has not completely healed, or if the infected fluid has not been entirely drained away.

Approximately 40% of people with an anal abscess will go on to develop an anal fistula.

Other causes

An anal fistula may also develop as a result of:

• a growth or ulcer (painful sore)
• a complication from surgery
• a congenital abnormality (a health problem that you were born with)

Anal fistulae are also a common complication of conditions that result in inflammation (swelling) of the intestines (part of your digestive system). Some of these conditions include:

• Irritable bowel syndrome (IBS): a chronic (long-term) disorder that affects the digestive system, causing abdominal pain, diarrhoea and constipation.
• Diverticulitis: the formation of small pouches that stick out of the side of the large intestine (colon), which become infected and inflamed.
• Ulcerative colitis: a chronic condition that causes the colon to become inflamed and can cause ulcers to form on the lining of the colon.
• Crohn's disease: a chronic condition that causes inflammation of the lining of the digestive system.

Other infections or conditions that can result in the development of an anal fistula include:

• Cancer of the rectum: the rectum is an area at the end of the colon where faeces (stools) are stored.
• Tuberculosis (TB): a bacterial infection that primarily affects the lungs, but can also spread to many different parts of the body.
• HIV and AIDS: a virus that attacks the body's immune system (the body’s system of defence against disease and infection).
• Chlamydia: a sexually transmitted infection (STI) that often causes no symptoms.
• Syphilis: a bacterial infection that is passed on through sexual contact, injecting drugs or blood transfusions. 
   

To make a diagnosis, your GP will look at your medical history and carry out a physical examination. They will pay particular attention to any history of anal abscesses or conditions that affect your bowels, such as Crohn's disease, because these conditions can sometimes lead to an anal fistula developing.

Physical examination

When conducting a physical examination, your GP will examine your anus (the opening where waste leaves the body) and the surrounding area for any physical signs of a fistula. The opening of a fistula usually appears as a red, inflamed (swollen) spot, which often oozes pus.

If your GP is able to locate the opening of the fistula, they may be able to work out where the path of the fistula lies. Your GP will feel the skin around the external opening of the fistula. Sometimes, the path of the fistula can be felt as a hard, cord-like structure beneath the skin.

Digital rectal examination

Your GP will also perform a digital rectal examination (DRE). This involves placing a finger (digit) into your anus. While carrying out the DRE, your GP will wear gloves and will lubricate their finger with gel.

The DRE allows your GP to find out where the internal opening of the fistula is, and if there are any secondary tracts (channels) branching off the fistula.

During the DRE, your GP may ask you to squeeze your sphincter muscles (the rings of muscles that open and close the anus) around their finger, to assess how well they are working.

The DRE will help to determine what kind of treatment you need, or whether further tests are necessary.

Further tests

If you have a complicated fistula, for example, if there are several branches, you may need to have further tests in order to determine the exact position of the fistula tracts. This will help with the planning of your treatment.

Some further tests that may be recommended are described below.  

• Anal endosonography (ultrasound). This test uses high-frequency sound waves to create an image of the inside of your body. This is an accurate and frequently used way of locating the internal opening of a fistula.
• Magnetic resonance imaging (MRI) scan. An MRI scan uses strong magnetic fields and radio waves to produce a detailed image of the inside of your body. This type of scan is often used in cases of complex or reoccurring fistulae.
• Computerised tomography (CT) scan. A CT scan uses X-rays and a computer to create detailed images of the inside of your body. This may be used if you have an inflammatory bowel disease, such as Crohn’s disease, as it can be used to assess the extent of the inflammation (swelling). 

Surgery is usually necessary to treat an anal fistula because very few will heal by themselves.

The main aim of surgery is to heal the fistula while trying to avoid damaging the sphincter muscles (the ring of muscles that open and close the anus). Damage to the sphincter muscles could cause bowel incontinence (where you do not have control over your bowels).

Surgery

Surgery for anal fistulae is usually carried out under either general anaesthetic, where you are unconscious and cannot feel anything, or local anaesthetic, where you are conscious but the area being treated is numbed so that you do not feel any pain.

Some of the different types of surgery for treating anal fistulae are explained below. The type of surgery you have will depend on the position of your anal fistula. In all cases, your surgeon will be able to explain the procedure to you in more detail.

Fistulotomy

A fistulotomy is the main surgical procedure used to treat an anal fistula. It is used in 85-95% of cases of fistulae.

A fistulotomy involves cutting open the whole length of the fistula, from the internal opening to the external opening. The surgeon will flush out the contents of the fistula and flatten it out. After one to two months, the fistula will heal into a flat scar.

To access the fistula, your surgeon may have to cut a small portion of the anal sphincter muscle. However, this will depend on the position of the fistula. Your surgeon will make every attempt to reduce the likelihood of bowel incontinence.

Seton techniques

Your surgeon may decide to use a seton during your surgery. A seton is a piece of surgical thread that is left in the fistula tract. This may be considered if you are at high risk of developing incontinence, for example, because your fistula crosses your sphincter muscles.

A seton is often used for a number of reasons. For example, it may be used to:

• leave the fistula intact while encouraging it to drain continuously
• allow secondary tracts to heal around the main fistula tract before further surgery is carried out on the main tract
• divide the sphincter muscle into stages, and allow it to heal in between operations

If your surgeon is planning to use a seton, they will discuss this with you. In some cases, it may be necessary to have several operations in order to treat your fistula using seton techniques. 

Advancement flap procedures

Advancement flap procedures may be considered if your fistula is complex, or if there is a high risk of incontinence.

An advancement flap is a piece of tissue that is removed from the rectum (the storage area at the end of the colon that holds stools), or from the skin around the anus.

During surgery, the fistula tract is removed, rather than opened up as it is in a fistulotomy. The procedure to remove the fistula is called a fistulectomy. The advancement flap is then removed and reattached where the internal opening of the fistula was.

Advancement flap procedures are thought to be effective in around 70% of cases. 

Fibrin glue

Fibrin glue is currently the only non-surgical option for treating fistulae. The fibrin glue is injected into the fistula in order to seal the tract. The glue is injected through the opening of the fistula, and the opening is then stitched closed.

Fibrin glue may seem an attractive option as it is a simple, safe and painless procedure. However, the long-term results for this treatment method are poor. For example, one small study had an initial success rate of 77%, but after 16 months, only 14% of people were still successfully healed. 

Bioprosthetic plug

A bioprosthetic plug is a cone-shaped plug that is made from human tissue. It can be used to block the internal opening of the fistula. Stitches are then used to keep the plug in place. However, the external opening of the fistula is not completely sealed so that the fistula can continue to drain. New tissue will then grow around the plug to heal the fistula.

Two trials (medical tests) that used bioprosthetic plugs have reported success rates of over 80%. However, there is still uncertainty over the reoccurrence rates and long-term outcomes. At the moment, the procedure is only used in clinical trials (a type of research that tests one treatment against another).

Ongoing research

There are currently several clinical trials taking place to compare the different types of treatment for anal fistulae, and you may be asked to take part in one.

If you are interested, you will be given information about the particular trial and you will be asked for your consent. Before giving your consent, make sure that you are fully aware of everything that the trial involves, and feel free to decline if you do not wish to take part.

After the operation

After having surgery to remove an anal fistula, you should be able to move around and eat and drink after the effects of the anaesthetic (painkilling medication) have worn off.

If the fistula is relatively simple to operate on, you may be able to go home on the same day as having the surgery. However, if the fistula is particularly long or complicated you may need to stay in hospital for a few days or undergo a second stage of surgery to complete the procedure.

Looking after the wound

After having surgery, you will need to wear a dressing over the incision (surgical cut) until the wound has healed. A district nurse will visit you at home regularly to change the dressing and check how the wound is healing. Most wounds take around six weeks to heal. 

There may be some bleeding or a discharge from the wound for the first few weeks, particularly the first time you have a bath or open your bowels (go to the toilet). You may wish to wear a pad, such as a sanitary towel, inside your underwear to avoid staining your clothes. This advice applies to both men and women.

You should see you GP if you experience:

• heavy bleeding
• increasing pain, redness, swelling or discharge
• a high temperature (fever) of 38°C (100.4°F) or over
• nausea (feeling sick) or vomiting
• constipation (being unable to empty your bowels) for more than three days, despite using a laxative (see below)
• difficulty passing urine

Painkilling medication

After the anaesthetic has worn off, you may need to take some pain relief medication. Over-the-counter (OTC) painkillers, such as paracetamol or ibuprofen, can normally be used, although you should check with your surgeon before using them. Always read the manufacturer’s instructions.  

You may find that a 15-minute bath will also help to reduce the pain. The bath water should be as warm as you can comfortably sit in.

Antibiotics

You may be prescribed antibiotics (medication to treat infections that are caused by bacteria) to take before and after surgery. This will help to reduce the risk of an infection occurring. If you are prescribed antibiotics, you should make sure that you complete the course.

Laxatives

Laxatives are a type of medicine that can help you to empty your bowels. You may be prescribed laxatives to make it easier for you to go to the toilet after your operation.  

Returning to normal activities

You may need rest for a few days after your operation, but you should avoid sitting still for a long period of time. You should also avoid doing too much walking.

You can return to work and start to do some gentle exercise when you feel able to. Once you can comfortably perform an emergency stop, you can to start to drive again. You should not go swimming until the wound has completely healed.

Complications from an anal fistula are usually the result of fistula surgery.

Infection

Any type of surgery carries a risk of infection. If the fistula is not completely removed, for example, because you are having the surgery carried out in several stages, an infection in the tract (channel) can sometimes spread to other parts of the body.

If this happens, you may require a course of antibiotics (medication to treat infections that are caused by bacteria). If the infection is severe, you may need to be admitted to hospital so that antibiotics can be administered intravenously (through a drip in your arm).

Incontinence

In some cases, surgery can damage the anal sphincter muscles (the ring of muscles that open and close the anus). If the muscles are damaged, you may lose control of your bowels, leading to faeces (stools) leaking uncontrollably from your rectum (the area where stools are stored). This is known as faecal incontinence or bowel incontinence.

The likelihood of incontinence occurring after surgery will depend on the type of surgery that you had, and the position of your fistula. If you have previously had some bowel incontinence before having surgery, you may find that this is now worse.

Incontinence after a fistulotomy (surgery that opens up the fistula) is more common in women and in people with Crohn's disease, which is a condition that causes inflammation (swelling) of the lining of the digestive system. The rate of incontinence varies, although most studies report incontinence in between 3 and 7% of people.

After using seton techniques the incontinence rate rises to 17%, and after an advancement flap procedure the incontinence rate is around 6-8%. You should ask your surgeon about the risks that are associated with your procedure.

Reoccurrence of the anal fistula

In some cases, the fistula can reoccur despite having surgery. After having a fistulotomy, the reoccurrence rate rises to 21%. After an advancement flap procedure, the reoccurrence rate may be as high as 36%.