A fistula is an abnormal tunnel between two organs in the body (such as the bowel and bladder). In an anal fistula a tunnel forms between the anal canal at the end of your colon and the skin around your anus (the part of your bottom where poo leaves).
They are also referred to as perianal fistulas, with peri meaning ‘around’.
What causes an anal fistula?
Most anal fistulas develop after an anal abscess that hasn’t healed or drained away properly.
Anal abscesses can form when small glands inside the anus get clogged. This can cause an infection and an abscess can develop.
Certain things can make you more likely to develop an anal abscess or anal fistula. These include:
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Different types of anal fistula and perianal fistula
Anal fistulas can be a single tunnel (simple fistula) or branch off into several tunnels (complex fistula). Some fistulas may form through muscles in your bottom which can be responsible for controlling your bowels.
Anal fistulas are given different names depending on where they are in relation to parts of the anal sphincter (the muscle that controls our bowel movements). These are:
The most common type is intersphincteric and the least common is extraphincteric.
What are the symptoms of an anal fistula?
Some of the common symptoms of an anal fistula include:
Throbbing pain around your bottom that may get worse when you sit or move
Swelling and redness around your bottom
Fever and chills
Feeling tired and sick
Redness, soreness or itching of the skin around the anal opening
Discharge coming out of the skin near your bottom. This discharge may be poo, blood or pus
When should you go to the doctor for anal fistula?
If you have symptoms of an anal fistula you should speak to a doctor as soon as you can, especially if you know you’ve previously had an anal abscess.
This may be your general practitioner or a specialist consultant like a gastroenterologist if you are under their care for a condition such as Crohn’s disease.
How is an anal fistula diagnosed?
The doctor will talk to you about your symptoms and any relevant medical history, such as if you have Crohn’s disease or have recently had a known anal abscess.
They should also carry out a physical examination which will involve looking at the area around your bottom and your anus. They may press the area to see if it’s sore or if a discharge comes out and insert a finger into your bottom (anus) to see if they can feel anything.
If it’s thought you have an anal fistula then you may be referred for more investigations. Some of these may be done under general anaesthetic and can include:
Proctoscopy - a camera inserted into your bottom to look inside your bowel
Fistula probe - this is inserted through the outer opening of the fistula to investigate it
Anoscope - a special device that’s inserted into your bottom to look inside your anal canal
Pelvic MRI - images are taken in an MRI scanner to look at the fistula
Endoanal ultrasound - an ultrasound scanner is inserted into your bottom to create an image of the muscles in the anal canal
Fistulography - the fistula is injected with a contrast dye and then an X-ray is taken