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.
This page covers:
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 GP 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 GP 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, magnetic resonance imaging (MRI) scan, or computerised tomography (CT) scan
Causes of anal fistulas
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
Treatments for 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 as very few heal by themselves.
The best option for you will depend on the position of your fistula and whether it's a single channel or branches off in different directions.
Sometimes you may need to have an initial examination of the area under general anaesthetic (where you're asleep) to help determine the best treatment.
Your surgeon will talk to you about the options available and which one they feel is the most suitable for you.
Surgery for an anal fistula is usually carried out under general anaesthetic. In many cases, it's not necessary to stay in hospital overnight afterwards.
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, which could potentially result in loss of bowel control bowel incontinence).
The main options are outlined below.
The most common type of surgery for anal fistulas is a fistulotomy. This involves cutting along the whole length of the fistula to open it up so it heals as a flat scar.
A fistulotomy is the most effective treatment for many anal fistulas, although it's usually only suitable for fistulas that don't pass through much of the sphincter muscles, as the risk of incontinence is lowest in these cases.
If your surgeon has to cut a small portion of anal sphincter muscle during the procedure, they will make every attempt to reduce the risk of incontinence.
In cases where the risk of incontinence is considered too high, one of the procedures below may be recommended instead.
If your fistula passes through a significant portion of anal sphincter muscle, your surgeon may initially recommend inserting a seton.
A seton is a piece of surgical thread that is left in the fistula for several weeks to keep it open. This allows it to drain and helps it heal, while avoiding the need to cut the sphincter muscles.
Loose setons allow fistulas to drain, but don't cure them. To cure a fistula, tighter setons may be used to cut through the fistula slowly. This may require several procedures that your surgeon can discuss with you.
Alternatively, they may suggest carrying out several fistulotomy procedures, carefully opening up a small section of the fistula each time, or one of the treatments described below.
Advancement flap procedure
An advancement flap procedure may be considered if your fistula passes through the anal sphincter muscles and having a fistulotomy carries a high risk of causing incontinence.
This involves cutting or scraping out the fistula and covering the hole where it entered the bowel with a flap of tissue taken from inside the rectum, which is the final part of the bowel.
This has a lower success rate than a fistulotomy, but avoids the need to cut the anal sphincter muscles.
Another option in cases where a fistulotomy carries a high risk of incontinence is the insertion of a bioprosthetic plug.
This is a cone-shaped plug made from animal tissue that is used to block the internal opening of the fistula.
Some studies have suggested this may be an effective treatment for anal fistulas, but more evidence is needed to be certain.
The National Institute for Health and Care Excellence (NICE) currently recommends carrying out the procedure as part of medical research.
The ligation of the intersphincteric fistula tract (LIFT) procedure is a relatively new treatment for anal fistulas.
It's designed as a treatment for fistulas that pass through the anal sphincter muscles, where a fistulotomy would be too risky.
During the treatment, a cut (incision) is made in the skin above the fistula and the sphincter muscles are moved apart. The fistula is then sealed at both ends and cut open so it lies flat.
This procedure has had some promising results so far, but it's only been around for a few years, so more research is needed to determine how well it works in the short and long term.
Treatment with fibrin glue is currently the only non-surgical option for anal fistulas.
It involves your surgeon injecting a special glue into your fistula while you're under a general anaesthetic. The glue helps seal the fistula and encourages it to heal.
It's generally less effective than fistulotomy for simple fistulas and the results may not be long-lasting, but it may be a useful option for fistulas that pass through the anal sphincter muscles because they don't need to be cut.