An anal fistula is a small tunnel that develops between the end of the bowel and the skin near the anus (where poo leaves the body).
They're usually the result of an infection near the anus causing a collection of pus (abscess) in the nearby tissue. When the pus drains away, it can leave a small channel behind.
Anal fistulas can cause unpleasant symptoms, such as discomfort and skin irritation, and won't usually get better on their own. Surgery is recommended in most cases.
Symptoms of an anal fistula
Symptoms of an anal fistula can include:
- skin irritation around the anus
- a constant, throbbing pain that may be worse when you sit down, move around, have a bowel movement or cough
- smelly discharge from near your anus
- passing pus or blood when you poo
- swelling and redness around your anus and a high temperature (fever), if you also have an abscess
- difficulty controlling bowel movements (bowel incontinence) in some cases
The end of the fistula might be visible as a hole in the skin near your anus, although this may be difficult for you to see yourself.
When to get medical advice
See your doctor if you have persistent symptoms of an anal fistula. They'll ask about your symptoms and whether you have any bowel conditions.
They may also ask to examine your anus and gently insert a finger inside it (rectal examination) to check for signs of a fistula.
If your doctor thinks you might have a fistula, they can refer you to a specialist called a colorectal surgeon for further tests to confirm the diagnosis and determine the most suitable treatment.
These may include:
- a further physical and rectal examination
- a proctoscopy – where a special telescope with a light on the end is used to look inside your anus
- an ultrasound scan, (MRI) scan or computerised tomography (CT) scan
What causes an anal fistula?
Most anal fistulas develop after an anal abscess. They can occur if the abscess doesn't heal properly after the pus has drained away.
It's estimated that between one in every two to four people with an anal abscess will develop a fistula.
Less common causes of anal fistulas include:
- Crohn's disease – a long-term condition in which the digestive system becomes inflamed
- diverticulitis – infection of the small pouches that can stick out of the side of the large intestine (colon)
- hidradenitis suppurativa – a long-term skin condition that causes abscesses and scarring
- infection with tuberculosis (TB) or HIV
- a complication of surgery near the anus
How to treat an anal fistula
Anal fistulas usually require surgery as they rarely heal if left untreated. The main options include:
- a fistulotomy – a procedure that involves cutting open the whole length of the fistula so it heals into a flat scar
- seton procedures – where a piece of surgical thread called a seton is placed in the fistula and left there for several weeks to help it heal before a further procedure is carried out to treat it
- other techniques – including filling the fistula with special glue, blocking it with a special plug, or covering it with a flap of tissue
All these procedures have different benefits and risks. You can discuss this with your surgeon.Many people don't need to stay in hospital overnight after surgery, although some may need to stay in hospital for a few days.Read more about treating an anal fistula.
Surgery is usually necessary to treat an anal fistula because very few will heal by themselves.
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.
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 that you do not feel any pain.
Some of the different types of anal fistula surgery are explained below. The type of surgery you have will depend on the position of your fistula. In all cases, your surgeon will be able to explain the procedure to you in more detail.
A fistulotomy is the most commonly used type of anal fistula surgery. 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 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.
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, often for several months, to keep the tract open. 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 if 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.
Advancement flap procedures are thought to be effective in around 70% of cases.
A bioprosthetic plug is a cone-shaped plug made from human 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 that the fistula can continue to drain. New tissue then grows around the plug to heal it.
However, this procedure can sometimes lead to a new abscess forming or the plug being pushed out of place.
Two trials that used bioprosthetic plugs have reported success rates of over 80%. However, there is still uncertainty over the reoccurrence rates and long-term outcomes.
Fibrin glue is currently the only non-surgical option for treating fistulae. The fibrin glue is injected into the fistula 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.