What is a perianal fistula?
A perianal (anal) fistula is a chronic, abnormal tract connecting the inside of the anal canal with the skin around the anus. It usually develops as the sequel of a perianal abscess, when infection of an anal gland (cryptoglandular theory) opens a persistent communication to the surface.
It has an internal opening — typically at a crypt of the dentate line — and one or more external openings on the perianal skin. Fistulas are classified as simple (low) or complex (high, multiple, recurrent, or associated with Crohn's disease).
How common is it?
The incidence is estimated at roughly 1–2 cases per 10,000 population per year. It is more common in men (about 2:1) and presents mainly in the 3rd–5th decade of life.
About 30–50% of perianal abscesses go on to form a fistula, which is why correct initial management of an abscess matters for preventing chronicity.
How does it present?
The main symptoms of a perianal fistula are:
- Persistent or intermittent discharge (pus, fluid or blood) from an opening next to the anus.
- Local pain, swelling and redness, worse before drainage.
- Irritation, itching and moisture of the perianal skin.
- Recurrent perianal abscesses.
- Less commonly, passage of gas or fluid from the external opening.
How is it diagnosed?
Assessment combines clinical examination and, where needed, imaging:
- Clinical inspection to locate the external opening and palpate the tract.
- Digital rectal examination and proctoscopy to identify the internal opening.
- Pelvic MRI — the tool of choice for complex or recurrent fistulas, mapping the tract and its relationship to the sphincters.
- Endoanal ultrasound as an alternative imaging method.
- Examination under anaesthesia (EUA) for definitive staging and simultaneous treatment.
How are they classified (Parks)?
The Parks classification describes the relationship of the tract to the sphincter complex and guides treatment:
- Intersphincteric: the tract stays between the internal and external sphincter — the most common.
- Transsphincteric: the tract crosses the external sphincter.
- Suprasphincteric: the tract passes above the pelvic sphincter complex.
- Extrasphincteric: rare, bypassing the sphincter.
In practice, fistulas are divided into simple (low, low risk to continence) and complex (high, multiple, anterior in women, recurrent, or in Crohn's disease), which determines the choice of technique.
Modern treatment options
The principle is twofold: eradicate the infection and close the tract, while preserving continence. The technique is individualised according to the classification and the extent of sphincter involvement.
Fistulotomy (laying open)
The classic, high-success technique for simple, low fistulas. The tract is laid open and allowed to heal. Very low recurrence — used when minimal sphincter is involved, so continence is not affected.
Seton (loose / draining)
A thin thread is placed through the tract to control infection and let it mature, while preserving the sphincter. Used for complex fistulas or as a first stage before the definitive procedure.
LIFT — sphincter-preserving
Ligation of Intersphincteric Fistula Tract. For transsphincteric fistulas: the tract is ligated and divided at the intersphincteric plane, without dividing the sphincter and preserving continence.
Advancement flap
For high or complex fistulas. The internal opening is covered with a healthy mucosal flap, without dividing the sphincter. Good results in selected patients with preserved function.
Laser (FiLaC) / VAAFT
Modern minimally invasive, sphincter-preserving methods. Using a laser fibre (FiLaC) or endoscopically (VAAFT), the tract is closed from within, with minimal postoperative pain and rapid return to activity.
Frequently asked questions
Can a perianal fistula heal without surgery?
Rarely. Fistulas usually do not close on their own; conservative measures only control the inflammation temporarily, while the definitive treatment is surgical. The goal is to eradicate the infection while preserving continence.
Is continence (bowel control) at risk?
The risk depends on the height of the fistula and how much sphincter is involved. For high or complex fistulas, sphincter-preserving techniques (seton, LIFT, advancement flap, laser) are preferred precisely to protect continence.
What is the link with a perianal abscess?
Most fistulas are the chronic sequel of a perianal abscess. Once the abscess is drained, a persistent tract communicating with the anal canal may remain.
Is an MRI needed before surgery?
For complex, high or recurrent fistulas, pelvic MRI is the tool of choice to map the tract and its relationship to the sphincters, reducing the chance of recurrence.