A perianal fistula is a small channel or tunnel that forms between the anal canal and the skin around the anus, where none should be.
A fistula usually has an internal opening on the bowel side and an external opening in the skin around the anus. Some are a single short channel; others branch, pass between or through the anal muscles, and have a more complicated shape.
Why does a perianal fistula form?
The commonest cause is infection of one of the small glands around the anus. That infection can lead to a collection of pus called a perianal abscess.
The abscess may drain on its own or be drained surgically. But in some patients, after the abscess has drained, a thin channel remains between the anal gland where the infection started and the skin. If that channel becomes permanent, a perianal fistula forms.
In short, the sequence is usually:
Infection in an anal gland → abscess → the abscess drains → the fistula channel remains
But not every perianal abscess leaves a fistula behind.
Is a perianal fistula related to Crohn’s disease?
Yes.
Most perianal fistulas are related to infection of the anal glands, but some can be a sign of Crohn’s disease. Assessment for underlying Crohn’s disease may be needed particularly with;
- Recurrent fistulas
- Fistulas with more than one external opening
- Complex, branching fistulas
- Recurrent perianal abscesses
- Fistulas that do not heal
If the patient also has long-standing diarrhoea, abdominal pain, weight loss or other bowel symptoms, this must be reported to the doctor.
Treatment of fistulas caused by Crohn’s disease can differ from ordinary perianal fistulas, and may need gastroenterology and colorectal surgery to assess the patient together.
What are the symptoms of a perianal fistula?
One of the commonest complaints is recurrent discharge from around the anus. The discharge may be;
- Yellow or purulent
- Foul-smelling
- Bloody at times
There may also be;
- Pain around the anus
- Swelling
- Redness
- Irritation and itching of the skin
- Underwear constantly soiled
- Swelling that comes back in the same place at intervals
The external opening of the fistula can sometimes be seen as a small hole the patient notices themselves.
Why does it swell, then drain and settle?
This is quite typical of perianal fistulas.
If the external opening closes for a time, inflammation and pus can build up again inside. Pain and swelling follow. When the channel opens again and the pus drains, the pressure falls and the patient may feel better.
But this does not mean the fistula has healed. As long as the channel is there, the same episodes can come back.
Are a perianal fistula and an abscess the same thing?
No.
A perianal abscess is a collection of pus around the anus — usually a painful, swollen, acute infection. A perianal fistula is a chronic connection that has formed between the anal canal and the skin.
The two are closely related. A patient may first develop an abscess, and after it drains a fistula may be left behind. If the fistula channel becomes blocked, an abscess can develop again.
Is a perianal fistula a haemorrhoid?
No. Haemorrhoids and perianal fistulas are entirely different conditions.
Haemorrhoids are structures related to the blood vessels and supporting tissue of the anal region. A fistula is an abnormal channel that develops between the anal canal and the skin. Their treatments differ accordingly.
How is the diagnosis made?
The patient’s complaints and the doctor’s examination come first. The external opening of the fistula may be visible around the anus. Discharge, and the scars of any earlier abscess or operation, can be assessed.
In some simple fistulas the examination is enough for the diagnosis. But imaging may be needed in some patients to establish the fistula’s relation to the anal muscles and whether it has side branches.
Is an MRI needed?
Not every perianal fistula needs an MRI. In simple, superficial fistulas the examination may be enough. But a pelvic MRI (fistula MRI) can be very useful in;
- Recurrent fistulas
- Complex fistulas
- More than one fistula opening
- Suspected deep abscess
- Fistulas already operated on
- Fistulas related to Crohn’s disease
MRI helps show where the fistula channel starts, its relation to the anal muscles, whether it branches, and any hidden pockets of abscess.
Why does the fistula’s relation to the anal muscles matter?
Around the anus are the sphincter muscles that hold stool and wind. The fistula channel can pass between them, through them, or around them.
When treatment is planned, preserving these muscles matters as much as removing the fistula, because damaging the sphincter muscles more than necessary can lead to problems controlling wind or stool.
This is why the same operation is not done for every fistula.
Does a perianal fistula heal on its own?
Most perianal fistulas are not expected to heal completely on their own.
The discharge may stop for a while and the external opening may close. But the fistula channel can continue inside. So symptoms disappearing for a time does not always mean the fistula has gone.
Do antibiotics cure a fistula?
Antibiotics alone do not usually remove an established fistula channel.
Antibiotics may be needed in particular situations where there is active infection. But the main aim of treating a fistula is to treat the channel itself, and any abscess, properly. Where there is an abscess, the mainstay is draining it appropriately.
How is a perianal fistula treated?
Treatment depends on the fistula’s;
- Position
- Relation to the anal sphincter muscles
- Being simple or complex
- Whether it has been treated before
- Whether Crohn’s disease is present
Treatment has two aims: to heal the fistula, and to preserve as far as possible the anal muscles that hold stool.
What is a fistulotomy?
In suitable fistulas that are simple and involve only a small part of the anal sphincter muscles, a fistulotomy may be performed. The fistula channel is laid open along its length into a flat wound, which is then allowed to heal from its base over time.
It is an effective treatment in simple, suitable fistulas. But if the fistula passes through a significant part of the anal sphincter muscles, a fistulotomy may risk stool control, and other methods may be preferred.
What is a seton?
A seton is a special surgical thread or similar material passed through the fistula channel. It can be used particularly in fistulas that pass through the anal muscles, or in complex fistulas.
A seton keeps the fistula channel open so that inflammation can drain, and can reduce the risk of another abscess forming. In some patients the seton is the first stage of treatment, with another surgical procedure to follow.
In fistulas caused by Crohn’s disease, draining setons can also be an important part of treatment.
Are there other treatments?
Yes. Depending on the shape of the fistula;
- The LIFT procedure
- An advancement flap
- Various sphincter-preserving surgical techniques
- In selected cases, endoscopic or laser-based methods
may be used.
A new method is not the best method for every patient
With laser and some minimally invasive methods in particular, short-term results can be promising, but more data are needed on long-term success and recurrence rates compared with the classical methods.
Is treatment different for fistulas caused by Crohn’s disease?
Yes.
In perianal fistulas caused by Crohn’s disease, surgery directed at the fistula channel alone is often not enough. Treatment may be planned to combine;
- Control of the infection and any abscess
- A seton where needed
- Drug treatment directed at the Crohn’s disease
- Surgical or endoscopic procedures in suitable patients
This is why these patients should be assessed together by gastroenterology and colorectal surgery.
Do fistulas come back?
Yes, some fistulas recur after treatment. The risk of recurrence can be affected by;
- The fistula being complex
- The presence of side channels
- Previous fistula surgery
- Active infection
- An underlying condition such as Crohn’s disease
In recurrent fistulas, reassessment — and where needed, MRI to set out the fistula anatomy in detail — may be required.
Does a perianal fistula turn into cancer?
An ordinary perianal fistula is not expected to turn into cancer.
Malignant change associated with very long-standing chronic fistulas has been reported extremely rarely, and matters more in particular situations such as long-standing Crohn’s disease.
So it is not right for patients to worry that “I have a fistula, it will turn into cancer”. That said, a long-standing fistula that develops unusual firmness, a mass, increasing pain, unexplained bleeding, or a marked change in character should be reassessed.
When is urgent assessment needed?
A new abscess or active infection may be present
In someone with a fistula, rapidly increasing severe anal pain, new marked swelling, fever and shivering, rapidly spreading redness, marked weakness or deterioration in general condition all call for medical attention without delay.
In short
- A perianal fistula Is an abnormal channel between the anal canal and the skin around the anus.
- Most fistulas Follow an earlier perianal abscess and infection.
- Fistulas are not all alike Some are superficial and simple; others pass through the anal muscles, branch, and have more complicated shapes.
- The principle of treatment Map the channel correctly, control the infection, treat the fistula, and preserve the anal sphincters and stool control.
- In recurrent or complex fistulas And where Crohn's disease is suspected, detailed assessment matters.
This information is not enough to make a diagnosis
The type of perianal fistula and its treatment vary from person to person. The right treatment is decided after the examination and, where needed, imaging results have been assessed.
Sources
- Prof. Ali Tüzün İnce, MD