Fistula

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What is a Fistula?

What is Perianal Fistula?

A perianal fistula is a small, abnormal tunnel that forms between the inside of the anus and the skin around the anal area. It usually develops after an anal infection or abscess that does not heal completely. This condition can cause pain, swelling, itching, and continuous or occasional discharge near the anus. Without proper care, symptoms often worsen over time.

Anatomical Illustration:
A simple, patient-friendly diagram showing the anal canal, fistula tract, and external opening on the skin.

A fistula does NOT heal on its own — expert medical treatment is essential.

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Types of Fistula

Parks Classification (Surgical Gold Standard)

The Parks Classification is the most widely used system by surgeons worldwide. It categorizes fistulas based on their relationship to the anal sphincter muscles.

Intersphincteric (45–70%) – Most Common

  • The fistula tract runs within the intersphincteric plane between the internal and external sphincters.

  • Typically has a single tract with a straightforward course

Transsphincteric (25–40%)

  • The tract passes through the external sphincter

  • Extends into the ischioanal (ischiorectal) fossa

  • Higher risk of muscle damage if not managed carefully

Suprasphincteric (3–5%)

  • The tract ascends above the puborectalis sling after traversing the intersphincteric space

  • Curves around the sphincter complex before opening externally

Extrasphincteric (1–2%) – Rare

  • Originates above the sphincter complex, usually from pelvic pathology (e.g. trauma, Crohn’s disease, prior surgery)

  • Bypasses the anal canal entirely

St. James’s University Hospital MRI Classification

This MRI-based classification helps surgeons understand fistula complexity and hidden extensions before surgery.

Simple Intersphincteric

  • Single, straight tract

  • No abscess or secondary branches

Intersphincteric with Abscess or Secondary Tract

  • Presence of pus collection or branching

  • Requires careful drainage and planning

Transsphincteric

  • Tract crosses the external sphincter

  • No abscess or secondary extension

Transsphincteric with Abscess or Secondary Tract

  • More extensive disease

  • Higher recurrence risk if not fully treated

Supralevator / Extrasphincteric Disease

  • Fistula extends above the levator ani

  • May bypass the anal canal

  • Requires expert evaluation and advanced surgical planning

Simple vs Complex Fistula Classification

This practical classification helps determine treatment intensity and risk.

Simple Fistula

  • Low intersphincteric or low transsphincteric tract

  • Single, non-branching tract

  • No associated Crohn’s disease or tuberculosis

  • No history of recurrent or previously operated fistula

Complex Fistula

  • High transsphincteric, suprasphincteric, or extrasphincteric fistula

  • Multiple tracts, horseshoe extensions, or supralevator spread

  • Associated with Crohn’s disease, tuberculosis, radiation, or malignancy

  • Recurrent fistula

Visual Representation (Essential)

  • Anatomical diagrams for each fistula type

  • MRI-based illustrations for complex extensions

  • Helps patients understand their condition and treatment plan

Causes of Perianal Fistula

A perianal fistula develops when a chronic infection near the anus fails to heal completely, leading to formation of an abnormal tunnel between the anal canal and the skin. Understanding the cause is important for choosing the right treatment and preventing recurrence.

Primary Cause – Cryptoglandular Origin :

Accounts for ~75% of cases

  • Small glands (anal crypts) inside the anal canal help with lubrication

  • Blockage of these glands allows bacteria to multiply, leading to an anal abscess

  • If the abscess drains spontaneously or is inadequately treated, a persistent fistula tract may form

Crohn’s Disease

  • Crohn’s disease is a chronic inflammatory bowel disease.

  • Chronic inflammation weakens intestinal and perianal tissues

  • Leads to:

    • Multiple fistula tracts

    • Recurrent abscesses

    • Poor wound healing

Tuberculosis (TB) Indian Context

  • Tuberculosis can affect organs outside the lungs, including the perianal region

  • TB-related fistulas:

    1. Heal poorly

    2. Often recur after surgery if TB is untreated

    3. May have multiple external openings

    4. Require medical treatment alongside surgical care

Trauma or Previous Surgery

  • Injury to the anal canal or perianal tissues due to:

    1. Previous anorectal surgeries

    2. Obstetric trauma

    3. Accidental or iatrogenic injury

      Can disrupt normal anatomy, leading to infection and fistula formation

  • Can disrupt normal anatomy and lead to infection and fistula formation.

Other Less Common Causes

  • Radiation therapy: Causes tissue damage and poor healing

  • Hidradenitis suppurativa: Chronic inflammatory skin disease involving sweat glands

  • Sexually transmitted infections (STIs): Rare cause, more likely in immunocompromised individuals

Risk Factors (Increase Likelihood, Not Direct Causes)

Certain conditions make individuals more prone to developing fistulas or impair healing:

  • Diabetes mellitus – poor immunity and delayed wound healing

  • Immunocompromised states – HIV, chemotherapy, long-term steroid use

  • Smoking: Impairs wound healing and increases recurrence risk

Diagnosis of Perianal Fistula

Accurate diagnosis is the most critical step in the successful treatment of perianal fistula. Missed tracts, hidden abscesses, or secondary extensions can lead to recurrence even after surgery. Therefore, diagnosis usually involves a combination of careful clinical examination and advanced imaging, especially MRI.

Clinical Examination :

Visual Inspection

  • The doctor inspects the area around the anus.

  • Looks for:

    1. External opening (small hole on skin)

    2. Discharge of pus or stool

    3. Redness, swelling, or induration

    4. Scar marks from previous abscess or surgery

Digital Rectal Examination (DRE)

  • A gloved finger is gently inserted into the rectum.

  • Helps assess:

    1. Possible internal opening

    2. Tenderness

    3. Induration along the fistula tract

    4. Anal sphincter tone

Probing

  • A thin, flexible probe may be gently inserted into the external opening.

  • Helps identify:

    1. Direction of the fistula tract

    2. Approximate location of internal opening

MRI Pelvis (Gold Standard Investigation) :

Why MRI is Important:

  • Provides a three-dimensional map of the fistula

  • Identifies:

    1. Primary tract

    2. Secondary tracts

    3. Horseshoe extensions

    4. Abscesses

What MRI Shows:

  • T2-weighted images: Bright signals indicate pus or active infection

  • Axial views: Relationship with internal and external sphincters

  • Coronal views: Height of tract and supralevator extension

Endoanal Ultrasound (EAUS) :

Uses:

  • Evaluates:

    1. Internal and external anal sphincter integrity

    2. Sphincter defects (important in recurrent or post-surgery cases)

Limitations:

Less effective in detecting:

  • Complex tracts

  • High or supralevator fistulas

  • Multiple extensions

Anoscopy :

Purpose:

Identifies:

  • Internal opening of fistula

  • Associated hemorrhoids

  • Anal fissures or inflammation

Examination Under Anesthesia (EUA) :

Why EUA is Important:

  • Pain-free, thorough examination

  • Allows:

    1. Gentle probing

    2. Injection of dye (methylene blue or hydrogen peroxide)

    3. Confirmation of internal opening

    4. Detection of hidden tracts

Successful Treatments

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Success Rate

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AI Powered Wound Monitoring

Years Experience
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Frequently Asked Questions

Can fistula heal without surgery?

No – medicine alone cannot cure

Chronic infection, extension, rare cancer risk

Modern techniques minimize pain

Sphincter-sparing techniques protect continence

Most patients recover within 2–4 weeks, while complex cases may take up to 3 months.

While fistula recurrence can happen, careful evaluation, MRI-based planning, and personalized treatment greatly improve long-term outcomes.

Anal fissure is a small tear in the lining of the anal canal that causes severe pain and bleeding during bowel movements and often heals with medicines and lifestyle changes.
Anal fistula is an abnormal tunnel between the anal canal and skin, usually causing pus discharge and recurrent infection, and always requires surgery for cure.

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