An anal fistula is most commonly caused by an anal abscess. It can also be caused by conditions that affect the intestines (part of your digestive system).

Anal abscess

An abscess is a painful collection of pus. 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 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 cause an anal fistula to develop. A fistula may also occur if an abscess has not completely healed, or if the infected fluid has not been entirely drained away.

Approximately 30-50% of people with an anal abscess will develop an anal fistula. Around 80% of all anal fistulas develop from an infection in the anus.

Inflammation of the intestines

An anal fistula can be a complication of conditions that cause the intestines to become inflamed, including:

• Crohn’s disease – a chronic condition that causes inflammation of the lining of the digestive system
• diverticulitis – infection of the small pouches that can stick out of the side of the large intestine (colon)

Crohn’s disease is a type of inflammatory bowel disease (IBD).

Other causes

Other infections or conditions that can lead to the development of an anal fistula include:

• cancer of the anorectum – the rectum is an area at the end of the colon where faeces are stored
• tuberculosis – a bacterial infection that mainly 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 (its defence against disease and infection)
• chlamydia – a sexually transmitted infection that often causes no symptoms
• syphilis – a bacterial infection that is passed on through sexual contact, injecting drugs or blood transfusions

An anal fistula may also develop as a result of:

• a growth or ulcer (painful sore)
• a complication of surgery
• a health problem you were born with

Complications from an anal fistula are usually the result of fistula surgery. They can include infection, bowel incontinence or the fistula returning.

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 caused by bacteria).

If the infection is severe, you may need to be admitted to hospital so antibiotics can be administered through a drip in your arm (intravenously).

Incontinence

In rare 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 some control of your bowels, leading to faeces leaking from your rectum (the area where they are stored). This is known as faecal or bowel incontinence.

The likelihood of incontinence occurring after surgery will depend on the type of surgery you had and the position of your fistula. If you had some bowel incontinence before surgery, this may get worse.

Incontinence after a fistulotomy (surgery that opens up the fistula) is more common in women and people with Crohn’s disease (a condition that causes inflammation of the lining of the digestive system).

Rates of incontinence vary, although most studies report incontinence in 3-7% of people.

The incontinence rate is 17% for seton techniques and around 6-8% after an advancement flap procedure. Ask your surgeon about the risks associated with your procedure.

Recurrence of the anal fistula

In some cases, the fistula can recur despite surgery. After having a fistulotomy, the recurrence rate can vary from 7-21%, depending on factors such as whether it is simple or complex.

After an advancement flap procedure, the recurrence rate may be as high as 36%.

To diagnose an anal fistula, your GP will look at your medical history and carry out a physical examination.

If there are several fistula tracts (channels), further tests may be needed to determine their position.

Your GP will also pay particular attention to any history of anal abscesses or conditions that affect your bowels, such as Crohn’s disease. These conditions can lead to an anal fistula developing.

You will usually be referred to a specialist in bowel conditions, known as a colorectal surgeon, for further investigation.

Physical examination

The specialist 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 the opening of the fistula is found, the specialist may be able to work out where the path of the fistula lies. The path of the fistula can sometimes be felt as a hard cord-like structure beneath the skin.

Rectal examination

Your specialist may need to perform a rectal examination to find out where the internal opening of the fistula is and if there are any secondary tracts branching off it. 

A rectal examination involves placing a finger into your anus and then up into your rectum (back passage). The doctor’s finger will be covered with a glove and lubricated with gel.

During the examination, the doctor 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.

This can help to determine what kind of treatment you need, or whether further tests are necessary.

Proctoscopy

Your specialist may also need to use a proctoscope (special telescope with a light on the end) to see inside your rectum.

They may also use a fistula probe, which is a tiny instrument inserted through the fistula.

These examinations may be performed under general anaesthetic, where you are asleep.

Further tests

If you have a complicated fistula with several branches, you may need further tests to determine the exact position of the fistula tracts. This will help guide the treatment you have.

Some further tests that may be recommended include:

• anal endosonography (ultrasound) – this test uses high-frequency sound waves to create an image of the inside of your body, and 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, and 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; it may be used if you have an inflammatory bowel disease, such as Crohn’s disease, as it can assess the extent of the inflammation

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.

The end of the fistula can appear as a hole in the skin around the anus. The anus is the opening where waste leaves the body.

Anal fistulas are usually classed as either:

• simple or complex – depending on whether there is a single fistula tract or interlinking connections
• low or high – depending on its position and how close it is to the sphincter muscles (the rings of muscles that open and close the anus)

When should I see my GP?

The common symptoms of an anal fistula include:

• skin irritation around the anus
• a throbbing, constant pain that may be worse when you sit down, move around, have a bowel movement or cough
• a discharge of pus or blood when having a bowel movement (rectal bleeding)

You should see your GP if you have any of these symptoms. You may be referred to a specialist in bowel conditions, known as a colorectal surgeon, for further investigation.

Read more about diagnosing an anal fistula.

What causes an anal fistula?

An anal fistula usually develops after an anal abscess (a collection of pus) bursts, or when an abscess has not been completely treated.

A fistula can also be caused by conditions that affect the intestines, such as inflammatory bowel disease (IBD) or diverticulitis.

An anal fistula affects:

• as many as 50% of people with Crohn’s disease
• up to 30% of people with HIV (a virus that attacks the body’s immune system)
• approximately 30-50% of people with an anal abscess (this is slightly more common in women than men)

Read more information about the causes of an anal fistula.

Treating an anal fistula

Most anal fistulas require surgery because they rarely heal if they are not treated. Several surgical methods are available, depending on where the fistula is and whether it is classed as simple or complex.

You may be able to go home on the day of surgery. However, you may need to stay in hospital for a few days if the fistula is difficult to treat. 

Read more information about treating an anal fistula and recovering from anal fistula surgery.

There is a risk of complications after anal fistula surgery, including:

• infection
• bowel incontinence 
• the anal fistula coming back

For example, after the most common type of surgery for a fistula (known as a fistulotomy), the risk of an anal fistula coming back is around 21%.

The risks vary depending on the type of procedure. You can discuss this with your surgeon.

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 the surgery. However, if the fistula is complicated, you may need to stay in hospital for a few days or have further surgery to complete the procedure.

Looking after the wound

After the operation you will need to wear a dressing over the surgical cut until the wound has healed. Your dressings will need to be changed regularly and you will usually be shown how to do this at home. 

However, you may need to visit the hospital or GP surgery so they can check how the wound is healing or change the dressing for you. 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 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 your GP if you have:

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

Washing

The following tips may help keep the area around the fistula clean and prevent infection or irritation:

• use warm water and cotton wool to wash the skin, rather than a towel or sponge – pat the skin dry rather than rubbing it, or use a hairdryer on a low setting
• avoid perfumed products and talcum powder as these can irritate the skin around the fistula
• you may be prescribed a barrier cream, which can be applied to stop irritants reaching the skin

Medication

Painkilling medication

After the anaesthetic has worn off, you may need to take some pain relief medication.

Over-the-counter 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.

A 15-minute bath may also help reduce the pain. The bath water should be as warm as you can comfortably sit in.

Laxatives

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

Antibiotics

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

Resting

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

When you are resting, the following tips may help make you more comfortable:

• wear loose-fitting clothes and underwear
• lie on your side when on the sofa or in bed
• pillows or cushions may help make sitting more comfortable – some pharmacies sell cushions designed to relieve pressure when sitting

Returning to normal activities

You can return to work and start to do some gentle exercise when you feel able to.

Ask your surgeon for advice on when you can drive again. This is usually after a minimum of 48 hours.

You should not go swimming until the wound has completely healed.

There are several common symptoms of an anal fistula.

Symptoms include:

• skin irritation around the anus (the opening where waste leaves the body)
• a throbbing, constant pain that may be worse when you sit down, move around, have a bowel movement or cough
• a discharge of pus or blood when you have a bowel movement – read more information about rectal bleeding

If your fistula was caused by an abscess that you still have, you may have:

• a high temperature of 38ºC (100.4ºF) or over
• fatigue
• a general feeling of being unwell

If your fistula was caused by inflammation of the intestines (part of your digestive system) – for example, because of a condition such as Crohn’s disease – you may also have:

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

Read more information about the causes of an anal fistula.

Types of fistula

An anal fistula is usually classified as:

• simple or complex – depending on whether there is a single fistula tract or interlinking connections
• low or high – depending on its position and how close it is to the sphincter muscles

The sphincter muscles are two rings of muscles that open and close the anus. They are known as the internal and external sphincter muscles.

The most common types of anal fistula are:

• intersphincteric fistula – the fistula tract (channel) crosses the internal sphincter and opens on the surface of the skin next to the anus
• transsphincteric fistula – 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 fistula can:

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

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

The type of surgery you have will depend on the position of your fistula and whether it is classed as simple or complex. Your surgeon will be able to explain the procedure to you in more detail.

Surgery

The aim of surgery is to heal the fistula while avoiding damage to the sphincter muscles (the ring of muscles that open and close the anus). Damage to the sphincter muscles could lead to bowel incontinence, where you do not have control over your bowels.

Read more about complications of an anal fistula.

Surgery for an anal fistula is usually carried out under a general anaesthetic, where you are unconscious and cannot feel anything. In some cases a local anaesthetic is used, where you are conscious but the area being treated is numbed so you do not feel any pain.

Some of the different types of anal fistula surgery are explained below.

Fistulotomy

A fistulotomy is the most commonly used type of anal fistula surgery, used in 85-95% of cases.

It involves cutting open the whole length of the fistula, from the internal opening to the external opening. The surgeon will flush out the contents 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 to keep the tract open, often for several months. This allows it to drain properly before it heals.

This may be considered if you are at high risk of developing incontinence – for example, because your fistula crosses your sphincter muscles.

It is also sometimes used to allow secondary tracts to heal before further surgery is carried out on the main tract. It can also be used to divide the sphincter muscle, which allows it to heal 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 to treat your fistula using seton techniques. 

Advancement flap procedures

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

An advancement flap is a piece of tissue that is removed from the rectum or from the skin around the anus.

During surgery, the fistula tract is removed (a procedure called fistulotomy). The advancement flap is then attached to where the internal opening of the fistula was.

Bioprosthetic plug

A bioprosthetic plug is a cone-shaped plug made from animal tissue. It can be used to block the internal opening of the fistula.

Stitches are used to keep the plug in place, but the external opening is not completely sealed so the fistula can continue to drain. New tissue then grows around the plug to heal it.

However, this procedure can sometimes lead to complications, such as:

• pain and increased drainage – this may require treatment with antibiotics
• a new abscess forming
• the plug being pushed out of place

Read the National Institute for Health and Care Excellence (NICE) guidance on closure of anal fistula using a suturable bioprosthetic plug.

Two trials using bioprosthetic plugs have reported success rates of more than 80%. However, there is still uncertainty over the recurrence rates and long-term outcomes.

Non-surgical treatments

Fibrin glue

Fibrin glue is currently the only non-surgical option for treating an anal fistula. The fibrin glue is injected into the fistula to seal the tract. It 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.

Ongoing research

Several clinical trials are currently taking place to compare the different types of treatment for an anal fistula. You may be asked to take part in one.

Read more information about taking part in a clinical trial.

If you are interested, you will be given information about the particular trial and asked for your consent.

Before giving your consent, make sure that you are fully aware of everything the trial involves. Feel free to decline if you do not wish to take part.

Read a list of current clinical trials for anal fistula.