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Anal Fistula: Symptoms, Causes, MRI, Seton, Laser & Surgery

  • Prof. Dr. Zahid Mahmood
  • August 10, 2026

Anal Fistula: Symptoms, Causes, MRI, Seton, Laser & Surgery

Medically reviewed by Prof. Dr. Zahid Mahmood, MBBS, FCPS — General, Laparoscopic & Laser Surgeon, Lahore.

An anal fistula, also called a fistula-in-ano, is an abnormal tunnel between the anal canal and the skin near the anus. It most often develops after an anal abscess. The outer opening may repeatedly discharge pus or blood, temporarily close, and then become painful and swollen again.

An anal fistula rarely heals permanently by itself. Treatment is planned according to the route of the tunnel, the amount of anal sphincter muscle involved, previous surgery and the patient’s existing bowel control. The central aim is to cure the fistula while protecting continence.

How does an anal fistula develop?

Small glands inside the anal canal can become infected. Pus collects to form an anal abscess. When the abscess drains naturally or is opened surgically, a tunnel may remain between the infected gland and the skin. This is the common cryptoglandular anal fistula.

Less common causes include:

  • Crohn’s disease
  • Tuberculosis or another chronic infection
  • Hidradenitis suppurativa
  • Previous anal or pelvic surgery, injury or radiotherapy
  • Rarely, a tumour or other underlying disease

Recurrent, multiple, unusually placed or non-healing fistulas may require investigation for an underlying condition.

Symptoms of an anal fistula

  • Repeated pus, fluid or blood-stained discharge from a small opening near the anus
  • Pain or pressure that improves after discharge
  • Skin irritation, itching, moisture or staining of underwear
  • A small lump or opening near the anus
  • Repeated abscesses in the same area
  • Fever, increasing pain and swelling if a new abscess develops

Some fistulas have only mild intermittent discharge. Others branch into several tunnels or extend deeply around the sphincter muscles.

When should you seek urgent medical help?

Seek prompt assessment if there is severe or rapidly increasing anal pain, fever, chills, spreading redness, marked swelling, difficulty passing urine, heavy bleeding or feeling generally unwell. These features may indicate an abscess or spreading infection that needs urgent drainage.

People with diabetes, impaired immunity, Crohn’s disease or pregnancy should seek advice early.

How is an anal fistula diagnosed?

Diagnosis begins with the history and examination of the skin around the anus. A rectal examination and proctoscopy may help locate the internal opening and identify other disease. Forceful probing in the clinic is avoided because it can cause pain or create a false passage.

An examination under anaesthesia (EUA) allows careful assessment when the area is too painful or surgery is planned.

Is MRI always required?

Not every straightforward fistula needs a scan. MRI is the preferred imaging test for complex, recurrent or uncertain fistulas because it maps the main tract, sphincter muscles, abscesses and hidden side branches. Endoanal ultrasound is another useful test in selected patients. Imaging is especially valuable after previous unsuccessful surgery or when Crohn’s disease is suspected.

Simple and complex anal fistulas

A simple fistula is generally a single low tunnel involving little sphincter muscle. A complex fistula may pass through a substantial amount of muscle, lie high around the anus, branch, form a horseshoe, recur after surgery, have multiple openings, connect to the vagina, or occur with Crohn’s disease or impaired continence.

This distinction matters because opening a low fistula may be safe, whereas cutting through too much sphincter muscle can weaken bowel control.

Can medicines cure an anal fistula?

Antibiotics can help treat surrounding infection in selected patients but usually do not remove the fistula tunnel. Pain medicines, warm baths, careful hygiene and keeping the stool soft may relieve symptoms while treatment is arranged. Crohn’s-related fistulas may also require specialist medical therapy.

Definitive treatment is usually a procedure or operation. The best method cannot be chosen from the skin opening alone; the complete anatomy and continence risk must be assessed.

Anal fistula operations

Fistulotomy

In a fistulotomy, a suitable low fistula is opened along its length so it heals gradually from the base. It is a highly effective treatment for many simple fistulas but is not appropriate when too much sphincter muscle would need to be divided.

Seton placement

A seton is a soft surgical thread or loop placed through the fistula. A loose draining seton keeps the tunnel open, allows infection to drain and reduces repeated abscess formation. It may remain for weeks or months before another operation, or longer in selected complex or Crohn’s-related fistulas. A loose seton controls drainage but does not necessarily cure the fistula by itself.

Cutting setons are designed to pass gradually through tissue. Because continence risk varies, their use requires careful specialist selection and discussion.

LIFT procedure

Ligation of the intersphincteric fistula tract (LIFT) treats the tract between the sphincter muscles without deliberately dividing the external sphincter. It is a sphincter-preserving option for selected trans-sphincteric fistulas. Healing is not guaranteed, and recurrence may require further treatment.

Advancement flap

An advancement flap closes the internal opening with healthy tissue from inside the rectum. It may be used for a complex fistula when fistulotomy would carry an unacceptable continence risk.

Fistulectomy

Fistulectomy removes or cores out the tract. It may help define the anatomy but can create a larger wound than fistulotomy. Its suitability depends on the tract and sphincter involvement.

Laser treatment (FiLaC)

During fistula-tract laser closure, a laser fibre is passed through the tract to shrink and seal it while aiming to preserve the sphincter. It can be considered for selected fistulas with suitable anatomy. However, laser is not automatically the best treatment for every fistula; success varies, side branches or active abscesses can lead to failure, and another operation may be needed. The continence-preserving advantage must be balanced against the possibility of recurrence.

VAAFT, glue and fistula plugs

Video-assisted treatment, fibrin glue and biological plugs are sphincter-preserving options used in selected cases. Their results are variable and they may be less reliable than fistulotomy for a simple low fistula.

How is the right procedure chosen?

The decision is individual and considers:

  • The internal and external openings
  • Whether the tunnel is low, high, single or branching
  • The amount of sphincter muscle involved
  • Any active abscess or side extension
  • Previous anal operations and recurrence
  • Existing bowel control, age and obstetric history
  • Crohn’s disease or another underlying condition
  • The relative importance of healing probability and continence protection

Sometimes the final anatomy becomes clear only during examination under anaesthesia. A staged approach—first draining infection with a seton and later performing definitive repair—is often safer than attempting everything at once.

What to expect after surgery

Many procedures are performed as day surgery. The wound may be deliberately left open so it heals from the inside out. Mild bleeding and drainage are common initially, and a pad can protect clothing. A seton may continue to drain mucus or pus.

Follow the surgeon’s instructions, which commonly include:

  • Take prescribed pain relief
  • Keep stool soft with fluids, fibre and any prescribed laxative
  • Avoid straining and prolonged sitting on the toilet
  • Wash gently with water and pat dry; avoid irritating perfumed products
  • Use a clean absorbent pad if there is drainage
  • Walk regularly and increase activity gradually
  • Attend follow-up so healing, drainage and continence can be assessed

Return to work depends on the operation, discomfort and type of job. Open fistulotomy wounds may take several weeks to heal; complex or staged treatment can take longer.

Possible complications

  • Recurrence or persistence of the fistula
  • A new abscess or wound infection
  • Bleeding, delayed healing or troublesome discharge
  • Temporary urgency or difficulty controlling gas
  • Uncommonly, significant bowel-control problems
  • Need for another procedure

Risk varies greatly with fistula anatomy, previous operations and the chosen procedure. Preserving continence is a major part of surgical planning.

Why does an anal fistula come back?

Recurrence can occur if the internal opening, a side branch or an abscess remains untreated; if a wound heals prematurely over ongoing infection; or if Crohn’s disease or another underlying condition remains active. Complex and previously operated fistulas have a higher risk. MRI and staged drainage can help define and control difficult disease, but no treatment can promise a 100% cure.

Common questions

Is anal fistula the same as piles or fissure?

No. A fistula is an infected tunnel, a fissure is a painful tear, and piles are enlarged anal cushions. Symptoms may overlap, so examination is important.

Can an anal fistula become cancer?

Most fistulas are benign and do not become cancer. A very longstanding, atypical or non-healing fistula may rarely be associated with malignancy and may need biopsy.

Will an abscess operation cure the fistula?

Draining an abscess treats the urgent infection. A fistula may still remain and require later treatment. Trying to find and divide a fistula during severe acute inflammation is not always safe.

Does every fistula need laser surgery?

No. A simple low fistula may be best treated by fistulotomy, while a complex fistula may need a seton, LIFT, advancement flap, laser or another staged plan. Treatment should fit the anatomy, not a single preferred technology.

Can I live normally with a seton?

Most people can walk, work and perform usual daily activities once initial discomfort settles. Some drainage and the need for a small pad are expected. Follow the surgeon’s advice about strenuous exercise and wound care.

Key message

Anal fistula usually follows an anal abscess and causes repeated pus discharge, pain and swelling. MRI is especially helpful for complex or recurrent disease. Successful treatment requires accurate mapping, control of infection and a procedure that balances fistula healing with protection of the anal sphincter and continence.

Anal fistula treatment in Lahore

Prof. Dr. Zahid Mahmood assesses simple and complex anal fistulas and discusses fistulotomy, seton drainage, LIFT, laser closure and other sphincter-preserving options according to each patient’s fistula anatomy and continence risk.

For an appointment, call 0300 413 0159 or visit professorzahid.com.

This article provides general education and does not replace personal medical assessment. Fever, rapidly increasing pain, swelling, heavy bleeding or feeling seriously unwell requires urgent medical care.

Medical references

  1. Williams NS, O’Connell PR, McCaskie AW, editors. Bailey & Love’s Short Practice of Surgery. 28th ed. CRC Press; 2023. Chapter 80, “The anus and anal canal,” pp. 1436–1440.
  2. Williams G, et al. The treatment of anal fistula: second ACPGBI Position Statement. Colorectal Disease. 2018.
  3. NHS. Anal fistula: symptoms, causes, diagnosis and treatment.
  4. NHS. Anal fistula treatment.
  5. NICE. Radially emitting laser fibre treatment of an anal fistula.

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