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Health condition · Clinically reviewed

Anal fistula, MRI planning and sphincter-preserving surgery.

A chronic tract between the anal canal and skin, usually born from a blocked anal gland. Modern UK care starts with MRI and protects continence at every step.

Jump to treatment
A radiographer guides a patient onto the bed of an advanced 3 Tesla MRI scanner in a London imaging suite

Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a registered UK clinician before publication.

  • 02

    Sourced from guidance

    Checked against ACPGBI, NICE and peer-reviewed colorectal sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK colorectal practice, including MRI-first imaging, sphincter-preserving surgery and biologics for Crohn’s fistulas.

Key facts

Anal fistula at a glance.

The essentials, in plain English: what it is, how it forms, and how UK colorectal teams classify and treat it today.

  • What it is

    An abnormal tract linking the anal canal to the perianal skin, most often the sequel of a drained or spontaneously discharging anal-gland abscess.

  • Why it forms

    Cryptoglandular theory: a blocked anal gland at the dentate line becomes infected, forms an abscess, then epithelialises into a tract.

  • Classification

    Park’s classification (intersphincteric, transsphincteric, suprasphincteric, extrasphincteric) plus the MRI-based St James score.

  • Simple vs complex

    Low fistulas with little sphincter involvement are simple. High, multiple, anterior in women, or Crohn’s-related tracts are complex.

  • Gold-standard imaging

    MRI of the pelvis maps the tract, secondary extensions and sphincter involvement before any definitive surgery.

  • Cornerstone of care

    Cure the sepsis, define the anatomy, preserve continence. Simple tracts respond to fistulotomy; complex tracts need sphincter-preserving techniques.

Why this guide matters

Cure the sepsis, protect the sphincter.

Anal fistula is common, treatable and, with the right plan, curable in most patients. Three principles shape everything else on this page.

  • MRI before definitive surgery

    MRI of the pelvis maps the tract, secondary extensions and sphincter involvement before any operation that could risk continence.

  • Simple vs complex matters

    Simple low fistulas can be laid open with a small continence risk. Complex tracts need sphincter-preserving techniques and specialist input.

  • Crohn’s changes the plan

    Crohn’s-related fistulas need medical therapy with biologics alongside surgical drainage. Darvadstrocel offers a stem-cell option for refractory disease.

How the diagnosis is made

From first symptoms to a clear operative plan.

The steps a UK GP or colorectal surgeon will normally follow, in order, so you know what to expect and why each one matters.

  1. 01

    Assessing

    Focused history

    Recurrent perianal discharge, intermittent swelling, pain that eases when pus drains, and any previous drained abscess or bowel symptoms.

  2. 02

    Assessing

    Careful perianal examination

    Look for the external opening, palpate a cord-like tract towards the anal verge, and note skin excoriation or satellite openings.

  3. 03

    Assessing

    DRE and proctoscopy

    Digital rectal examination and proctoscopy help identify the internal opening and assess sphincter tone before any intervention.

  4. 04

    Confirming

    Goodsall’s rule at the bedside

    A useful guide: anterior external openings usually track straight to the anal canal; posterior openings curve to the posterior midline.

  5. 05

    Confirming

    MRI pelvis (gold standard)

    Maps the primary tract, secondary extensions, abscess collections, sphincter involvement and any Crohn’s-related changes. Scored with the St James system.

  6. 06

    Preparing

    Examination under anaesthesia

    EUA with gentle probing and methylene blue injection confirms internal opening and anatomy at the moment of definitive surgery.

  7. 07

    Preparing

    Colonoscopy if Crohn’s suspected

    Complex, multiple, recurrent or aggressive fistulas warrant colonoscopy and small-bowel imaging to look for underlying Crohn’s disease.

Typical timeline: from first colorectal review to definitive surgery in weeks, occasionally months for complex disease.

Symptoms

What an anal fistula actually feels like.

The classic mix of recurrent discharge, intermittent swelling and pain, often following a previously drained abscess, plus the features that mean it is time to escalate.

  • Recurrent perianal discharge

    Persistent or intermittent leakage of pus, blood or mucus from a small perianal opening staining underwear.

  • Intermittent perianal swelling

    A tender lump that swells, discharges and settles in cycles as the tract blocks and drains.

  • Perianal pain

    Throbbing pain that worsens as pressure builds and eases when pus escapes through the external opening.

  • External opening on the skin

    One or more small openings on the perianal skin, sometimes with a visible bead of granulation tissue.

  • Skin excoriation and irritation

    Chronic moisture and discharge cause macerated, itchy, inflamed perianal skin.

  • History of drained abscess

    Many patients recall a previous perianal abscess that was drained or discharged spontaneously.

  • Palpable cord on examination

    A firm cord may be felt running from the external opening towards the anal verge, marking the tract.

  • Red flag — systemic sepsis

    Fever, spreading cellulitis or severe pain suggests undrained sepsis and needs urgent surgical assessment.

Treatment

How anal fistula is treated in the UK.

Fistulotomy cures simple low tracts. Complex disease needs sphincter-preserving techniques such as loose setons, LIFT, VAAFT or advancement flaps, with biologics and stem-cell therapy reserved for Crohn’s.

  • Fistulotomy (lay open)

    Best cure for simple low fistulas with minimal sphincter involvement. Small continence risk, so patient selection matters.

  • Loose seton drainage

    A soft suture threaded through the tract to keep it draining and settle sepsis before a sphincter-preserving definitive procedure.

  • LIFT procedure

    Ligation of the intersphincteric fistula tract for transsphincteric fistulas. Preserves sphincter function with reasonable healing rates.

  • VAAFT

    Video-assisted anal fistula treatment uses a fine endoscope to visualise and ablate the tract while sparing the sphincter.

  • Endorectal advancement flap

    A flap of healthy rectal wall is mobilised to cover the internal opening, useful for high or recurrent complex tracts.

  • Fibrin glue and fistula plug

    Sphincter-sparing options with limited long-term efficacy, sometimes tried in selected simple or recurrent tracts.

  • Biologics for Crohn’s fistulas

    Infliximab or adalimumab combined with seton drainage remains the backbone of care for Crohn’s-related perianal disease.

  • Darvadstrocel (Cx601, Alofisel)

    Allogeneic stem-cell therapy licensed by the FDA and EMA for complex perianal fistulas in Crohn’s disease unresponsive to standard care.

What this guide is based on

The sources behind every claim on this page.

UK national and specialist colorectal society guidance, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP or colorectal surgeon knows your history and imaging, and can tell you which parts apply to you. If in doubt, get seen.

  • Association of Coloproctology of Great Britain and Ireland (ACPGBI). Guidelines on the management of anal fistula.

  • NICE. Crohn’s disease: management (NG151) and related technology appraisals for biologics and darvadstrocel.

  • European Society of Coloproctology (ESCP). Consensus statements on anal fistula and perianal Crohn’s disease.

  • St James’s University Hospital. MRI-based classification of anal fistulas (St James score).

Red flags

When anal fistula needs urgent attention.

Most fistulas can be managed on a planned colorectal pathway. These are the situations that need faster review, imaging or specialist input.

  • Undrained perianal sepsis

    Severe pain, spreading redness, fever or systemic upset suggests an ongoing abscess that needs urgent surgical drainage.

  • Fournier’s gangrene

    Rapidly spreading necrotising infection of the perineum is a surgical emergency, especially in diabetes or immunosuppression.

  • Recurrent or multiple tracts

    Complex, branching or recurrent fistulas warrant specialist colorectal review with MRI before any further surgery.

  • Suspected Crohn’s disease

    Aggressive, atypical or complex fistulas, especially with diarrhoea or weight loss, should trigger colonoscopy and small-bowel imaging.

  • Tuberculosis or immunosuppression

    Consider TB in high-risk groups and in atypical, indolent perianal disease that fails to heal as expected.

  • Hidradenitis suppurativa overlap

    Chronic perianal, buttock and groin sinuses may reflect hidradenitis suppurativa and need combined dermatology and colorectal input.

  • Suspected malignancy

    A chronic, non-healing or indurated tract, or one with unusual bleeding, needs biopsy to exclude anal or fistula-tract cancer.

  • Continence at risk

    Anterior fistulas in women, previous obstetric injury or prior sphincter surgery raise the threshold for laying open.

  • Recurrent complex disease

    Multiply operated fistulas belong in a specialist colorectal centre with MRI, EUA and access to sphincter-preserving techniques.

Living with it

A treatable condition, with a clear surgical ladder.

Four things that make the biggest difference day to day: gentle perianal care, soft bowels, sticking with the plan and knowing when to escalate.

A quiet reminder

Healing is a marathon, not a sprint.

A loose seton can sit quietly for months while the tissues settle. That waiting is doing real work.

  1. 01 Hygiene

    Gentle perianal care

    Warm sitz baths after opening bowels, gentle drying and barrier creams protect skin from ongoing discharge.

  2. 02 Bowels

    Keep stool soft and formed

    Adequate fibre, fluids and, if needed, a stool softener reduce trauma to the anal canal and healing wounds.

  3. 03 Follow-up

    Stick with the plan

    A loose seton may need to stay in place for weeks or months before definitive surgery. Regular reviews matter.

  4. 04 Escalate

    Report new pain or fever

    Increasing pain, swelling or systemic symptoms usually mean the tract has blocked and needs prompt review.

Frequently asked

Everything we get asked about anal fistula.

Quick answers on MRI, setons, sphincter-preserving surgery and Crohn’s-related disease.

  • What is an anal fistula?

    An anal fistula is an abnormal tract that connects the inside of the anal canal to the skin around the anus. It most often develops after an anal-gland infection forms an abscess that then drains and leaves a chronic epithelialised tract behind.

  • How is an anal fistula different from a perianal abscess?

    A perianal abscess is an acute collection of pus that needs urgent drainage. A fistula is the chronic tract that can be left behind once the abscess has drained. Roughly a third of drained perianal abscesses go on to form a fistula.

  • Why is MRI so important?

    MRI of the pelvis is the gold-standard investigation. It maps the primary tract, any secondary extensions, sphincter involvement and hidden collections, and helps distinguish simple from complex disease. Surgery based on accurate MRI planning has better outcomes and fewer recurrences.

  • Will surgery affect my continence?

    Simple low fistulas can usually be laid open with a small, acceptable continence risk. Complex, high or anterior tracts, particularly in women, need sphincter-preserving techniques such as loose setons, LIFT, VAAFT or advancement flaps to protect continence.

  • What is a seton?

    A seton is a soft thread passed along the tract and tied loosely to keep it draining. It settles infection, prevents further abscesses and stabilises the anatomy before a sphincter-preserving definitive procedure. Some setons are cutting, but loose draining setons are used most often today.

  • What if my fistula is caused by Crohn’s disease?

    Crohn’s-related fistulas are often complex and aggressive. Standard care combines a loose seton for drainage with biologic therapy such as infliximab or adalimumab. Darvadstrocel (Cx601, Alofisel), an allogeneic stem-cell therapy approved by the FDA and EMA, is an option for complex perianal fistulas that have not responded to conventional treatment.

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