Perianal Fistula (Anal Fistula)

A perianal fistula, or anal fistula, is an abnormal tunnel that forms between the inside of the anal canal and the skin around the anus. In medical terminology it is also called fistula-in-ano.

A fistula usually has:

• An internal opening inside the anal canal,
• An external opening in the skin around the anus,
• A fistula tract connecting these two openings.

Most perianal fistulas arise from abscesses that develop as a result of inflammation of the small glands around the anal canal. If the tract left inside does not close after the abscess has drained on its own or been drained surgically, a fistula may form. Fistulas of this type are called cryptoglandular fistulas [1].

A perianal fistula is not cancer and is not contagious. It is generally not caused by poor personal hygiene. However, if left untreated it can lead to recurrent abscesses, pain and discharge.

The main aim of treatment is to eliminate the fistula tract while preserving the anal muscles that provide bowel control, that is, the anal sphincters. Because the relationship of the fistula to the muscles differs in every patient, the same operation is not performed on everyone [1].

What symptoms does a perianal fistula cause?

The most common symptoms are:

• Recurrent pus-like or bloody discharge from a small opening around the anus
• Staining of underwear
• Unpleasant odour
• Itching and skin irritation around the anus
• Pain when sitting or opening the bowels
• Recurrent swelling in the same area
• Pain easing once the discharge starts
• Occasional bleeding from the anus
• Fever, chills and severe pain when an abscess develops

The external opening may close from time to time. However, if the fistula tract persists inside, infection can build up again and a painful abscess may form. For this reason, a temporary stop in the discharge may not mean that the fistula has completely healed.

What is the relationship between a perianal abscess and a fistula?

A perianal abscess is a collection of infection and pus around the anus. A fistula is the permanent tract that remains between this focus of infection and the skin.

The process usually develops as follows:

1. A gland around the anal canal becomes infected.
2. An abscess forms under the skin or deeper.
3. The abscess drains on its own or is drained surgically.
4. In some patients a permanent tunnel remains between the anal canal and the skin.

Not every patient who has had an abscess develops a fistula. However, if there is recurrent swelling or continuous discharge in the same area after an abscess, a fistula should be suspected.

Why does a perianal fistula form?

The most common cause is a previous or ongoing perianal abscess. Less common causes are:

• Crohn’s disease
• Radiotherapy to the anal or rectal area
• Previous surgery or trauma
• Certain infections such as tuberculosis
• Hidradenitis suppurativa
• Suppression of the immune system
• Very rarely, tumours of the anus or rectum

In fistulas with more than one external opening, or that are recurrent, unusually located or resistant to treatment, Crohn’s disease and other specific causes may be investigated.

What do simple and complex fistula mean?

Fistulas are classified according to their relationship with the anal muscles and the branches they form.

A simple fistula usually:

• Is superficial or low-lying.
• Passes through a small portion of the anal muscles.
• Consists of a single tract.
• Has not previously been operated on many times.
• Is not associated with Crohn’s disease or radiotherapy.

A complex fistula, on the other hand:

• May pass through a significant portion of the anal muscles.
• May have more than one tract or abscess cavity.
• May extend around the anus in a horseshoe shape.
• May have recurred.
• May be located at the front of the anus in women.
• May develop in patients at risk of faecal incontinence.
• May be associated with Crohn’s disease, radiotherapy or another condition.

This distinction is important in determining the treatment method and the risk to bowel control [1].

Diagnosis begins with listening to the patient’s complaints and examining the anal area.

Your doctor will ask:

• Whether you have had an abscess before,
• Whether the abscess drained on its own or with surgery,
• How long the discharge has been going on,
• Whether the swelling and pain keep coming back,
• Whether you have had fistula surgery before,
• Whether you have any problems controlling stool or wind,
• About symptoms of Crohn’s disease such as diarrhoea, abdominal pain and weight loss.

During the examination, the external opening, discharge, a hardened fistula tract and signs of a new abscess are assessed. A digital rectal examination and, if necessary, anoscopy may be performed.

Care must be taken during the examination, because passing an instrument into the fistula tract in an uncontrolled way may create a new tract.

Is an MRI needed for every fistula?

No. Imaging is not always needed for simple fistulas whose external opening and course can be determined by examination.

A pelvic MRI may be particularly useful in the following situations:

• Suspected complex or high fistula
• Recurrent fistula
• More than one external opening
• Suspected horseshoe-shaped extension
• A deep abscess that cannot be found on examination
• Crohn’s disease
• A suppressed immune system
• Previous unsuccessful fistula operations
• Uncertainty about the relationship with the anal muscles

A pelvic MRI can show the main tract of the fistula, its branches, the internal opening, abscess cavities and the relationship with the anal muscles [1].

What other tests may be used?

Depending on the patient’s condition:

• Endoanal ultrasound: Can assess the fistula tract and the anal muscles.
• Examination under anaesthesia: The fistula tract and internal opening can be explored under anaesthesia; drainage or seton placement can be carried out in the same session.
• Colonoscopy: May be performed if Crohn’s disease, bowel inflammation, bleeding or another bowel disease is suspected.
• Blood and stool tests: May be used when investigating inflammatory bowel disease or infection.
• Biopsy: May rarely be needed for fistulas that have not healed for a long time or that are hard, irregular or unusual.

MRI and surgical findings complement each other. The surgeon decides on the treatment method by assessing the actual anatomy of the fistula together with the risk to bowel control.

Most perianal fistulas do not close permanently with medication alone. Treatment usually requires a surgical procedure.

No single method is suitable for all fistulas. Treatment must strike a balance between two goals:

1. Healing the fistula and preventing the abscess from recurring
2. Preserving the anal muscles that provide bowel control

While a procedure with a high success rate can be used for simple fistulas, complex fistulas may require muscle-sparing and sometimes staged treatment [1].

What is done if there is an active abscess?

If there is a painful swelling and abscess, the priority is to drain the abscess. This procedure is called drainage.

Antibiotics alone are often not enough to treat an abscess. Delayed drainage can lead to the spread of infection and tissue damage.

Antibiotics are usually added to treatment in the following situations:

• Widespread redness of the surrounding skin
• Fever or systemic infection
• A suppressed immune system
• High-risk conditions such as uncontrolled diabetes
• Certain specific infections

After a drained, uncomplicated abscess in a healthy person, routine antibiotics are not always necessary [1].

If the structure of the fistula is not clear when the abscess is drained, the definitive fistula operation may be left to a later session in order to protect the anal muscles.

What is a fistulotomy?

A fistulotomy is the opening of the fistula tract along its whole length, turning it into a flat wound. The wound heals from the base towards the surface.

It is one of the most effective treatments for simple, low fistulas that pass through only a very small portion of the anal muscles. However, if the fistula passes through a significant portion of the muscles, opening the tract completely can lead to impaired control of wind or stool.

For this reason, the decision to perform a fistulotomy is made not only on the basis of whether the fistula is short or long, but on its relationship with the anal muscles.

The risk to bowel control is assessed more carefully in the following situations:

• High or complex fistula
• A fistula at the front of the anus in women
• Previous anal surgery
• Pre-existing leakage of wind or stool
• Childbirth-related anal muscle damage
• Crohn’s disease
• Advanced age or sphincter weakness

What is a seton?

A seton is a thin thread, silicone loop or similar surgical material passed through the fistula tract.

It can be used for two main purposes:

• Draining seton: Keeps the fistula tract open and so prevents infection and the abscess from building up again.
• Cutting seton: A method intended to cut through the tissue in a controlled way over time. Because of the risk to bowel control, it is used only in selected patients.

A draining seton may be used particularly in complex fistulas, an active abscess or Crohn’s disease. A seton is not always the final procedure that heals the fistula completely; it may be used to control infection and to prepare for a later muscle-sparing operation.

In some patients with Crohn’s disease the seton may be kept in place for longer. The duration is determined according to the discharge and the medical treatment to be given [1,2].

What is the LIFT operation?

LIFT is a procedure in which the fistula tract is located in the part between the anal muscles, tied off and divided. The abbreviation stands for “ligation of the intersphincteric fistula tract”.

The aim of this method is to avoid cutting the anal muscles. For this reason it may be preferred for some fistulas that pass through a significant portion of the muscles.

Its advantage is that it is aimed at preserving bowel control. However, it is possible for the fistula not to heal or to recur. Success depends on the anatomy of the fistula, previous operations and whether there is active infection [1].

What is an advancement flap operation?

In this method, the opening of the fistula inside the anal canal is cleaned and closed with a piece of tissue prepared from the healthy inner lining of the rectum. It is a sphincter-sparing method that aims to reduce cutting of the anal muscles.

It may be used particularly in some high or complex fistulas in which a fistulotomy would be risky for bowel control.

The flap may fail to heal, the fistula may recur and, rarely, bowel control may be impaired. An active abscess or uncontrolled Crohn’s inflammation may reduce the likelihood of success [1,2].

What is laser treatment?

In fistula laser treatment, a thin laser probe is placed into the fistula tract. The aim is to seal the inner surface of the tract with laser energy. This method is also known as FiLaC.

An important advantage of laser treatment is that it can be performed without cutting the anal muscles. Pain and the wound after the operation may be more limited. However:

• It is not suitable for every fistula.
• Control of the internal opening and side tracts is important.
• Success rates vary between studies.
• The long-term risk of recurrence is not entirely clear.
• A repeat procedure may be needed.

The ASCRS guideline states that the short-term results of laser and endoscopic methods may be acceptable, but that the evidence on long-term healing and recurrence rates is limited [1]. NICE likewise reports that, although no major safety concerns have been identified, there is uncertainty about the efficacy of the method [3].

For this reason, it is not scientifically correct to give a guarantee of “definitive, recurrence-free treatment with laser”.

What are VAAFT and other closed methods?

In the VAAFT method, a thin camera is placed inside the fistula tract; the tract is viewed and cleaned from the inside and an attempt is made to close the internal opening.

Other muscle-sparing methods may include:

• Fibrin glue
• Biological fistula plug
• Endoscopic closure methods
• Various energy-based procedures.

Although these methods have the advantage of preserving the anal muscles, they do not achieve permanent healing in every patient. The success rates of fibrin glue and biological plugs vary, and some current guidelines do not consider them effective enough for routine use [1].

How is a fistula caused by Crohn’s disease treated?

The treatment of perianal fistulas caused by Crohn’s disease differs from that of typical abscess-related fistulas.

Treatment is usually planned jointly by:

• A colorectal surgeon,
• A gastroenterologist,
• A radiologist,
• An inflammatory bowel disease nurse.

If there is an active abscess, it is drained first and a draining seton is usually placed. Appropriate medical treatment is then given to control Crohn’s inflammation in the bowel and the perianal area.

Biological medicines, especially anti-TNF treatments, are an important part of fistula treatment in suitable patients. Antibiotics can help control infection but are not enough on their own to close a complex Crohn’s fistula permanently [2].

In a complex Crohn’s fistula, closing the external opening quickly while there is an uncontrolled abscess can cause infection to build up inside again. For this reason, infection control is achieved first.

In patients with Crohn’s disease who have active inflammation of the rectum, fistulotomy or operations aimed at closing the internal opening are chosen with care. In very severe disease that is resistant to treatment, a temporary stoma or larger operations in which the rectum is removed may rarely be needed.

Why does fistula treatment sometimes require more than one operation?

In complex fistulas, the first procedure may be performed to control infection and clarify the anatomy of the fistula. The definitive closure procedure may be carried out in a later session.

Staged treatment does not mean failure. The aim is to reduce the risk of causing unnecessary damage to the anal muscles while trying to close the fistula in a single attempt.

Recovery time depends on the method used, the structure of the fistula and whether there is an accompanying abscess.

After a simple fistulotomy, light daily activities can be resumed within a few days. Complete closure of the wound may take several weeks. In complex fistulas, in patients with a seton or in staged operations, the treatment process may be longer.

In general, after the operation:

• Painkillers should be taken as recommended.
• The area should be kept clean.
• The area can be cleaned with warm water after opening the bowels.
• The dressing routine recommended by the doctor should be followed.
• Constipation and excessive straining should be avoided.
• Adequate fluids should be taken and diet should be adjusted to the individual.
• If there is a seton, it should not be pulled or cut by the patient.

Some discharge and staining can be expected, especially in patients with a seton. However, it should be remembered that a reduction in discharge does not always mean that the fistula has closed.

Return to desk work may be possible in most patients within a few days to 1–2 weeks. Longer may be needed after major or complex operations.

You should contact your healthcare team in the following situations:

• High fever or chills
• Increasing pain and swelling
• Painful swelling forming again after the discharge has stopped
• Heavy bleeding
• Inability to pass urine
• New impairment of control of wind or stool
• The seton breaking or coming out of place
• The wound not healing

A perianal fistula can recur after treatment. The likelihood of recurrence depends on the complexity of the fistula, the presence of side tracts, control of the internal opening, Crohn’s disease and previous operations.

1. Gaertner WB, Burgess PL, Davids JS, et al. The American Society of Colon and Rectal Surgeons Clinical Practice Guidelines for the Management of Anorectal Abscess, Fistula-in-Ano, and Rectovaginal Fistula. Diseases of the Colon & Rectum. 2022;65(8):964–985. doi:10.1097/DCR.0000000000002473. ASCRS guideline
2. Adamina M, Minozzi S, Warusavitarne J, et al. ECCO Guidelines on Therapeutics in Crohn’s Disease: Surgical Treatment. Journal of Crohn’s and Colitis. 2024;18(10):1556–1582. doi:10.1093/ecco-jcc/jjae089. PubMed record
3. National Institute for Health and Care Excellence. Radially Emitting Laser Fibre Treatment of an Anal Fistula. Interventional Procedures Guidance IPG644. 2019. NICE guidance
4. Hanna LN, et al. Perianal Fistulizing Crohn’s Disease: Utilizing the TOpClass Classification System to Guide Management. Clinical Gastroenterology and Hepatology. 2025. Access the publication