Anal Fistula

Definition & Epidemiology

An anal fistula is an abnormal tract connecting the internal opening at the dentate line (from an infected anal gland) to the perianal skin. It represents the chronic form of a perianal abscess — when the infected gland fails to heal after drainage, a persistent tract remains. Approximately 30–50% of perianal abscesses lead to fistula formation. The incidence is ~8.6 per 100,000, with men affected 2–3× more than women. — Goldman-Cecil Medicine

Pathogenesis (Cryptoglandular Theory)

Most fistulas arise from infection in the anal glands (cryptoglandular origin), located at the level of the dentate line. The gland becomes infected → forms an abscess → tracks through the sphincter → discharges onto perianal skin, leaving a persistent epithelialized tract.

Parks Classification

Type	Description	Frequency
Intersphincteric	Tract in the intersphincteric space; external opening near anal verge	~70% (most common)
Transsphincteric	Traverses both internal and external sphincters; classified as low (distal EAS) or high (proximal EAS)	~20–25%
Suprasphincteric	Ascends above entire sphincter complex then descends through levator ani to ischioanal fossa	Uncommon
Extrasphincteric	Internal opening above dentate line (rectum), perforates levator ani; associated with Crohn disease, trauma, or pelvic abscess	Rare

A horseshoe fistula has bilateral external openings with a single posterior midline internal opening. — Goldman-Cecil Medicine; Mulholland & Greenfield's Surgery

Clinical Features

Recurrent perianal abscess at the same site (pathognomonic clue)
Persistent intermittent purulent discharge from perianal skin
External opening visible as erythematous nodule with granulation tissue
Palpable fibrous tract coursing toward the anal canal
The internal opening at the dentate line may or may not be visible on anoscopy

Differential diagnoses: hidradenitis suppurativa (superficial tracts, no anal canal connection), pilonidal disease (gluteal cleft, foreign body reaction to hair).

Evaluation

History: duration, prior anorectal surgery, continence, bowel habits (screen for Crohn disease)
Examination under anesthesia (EUA): probe passed along tract; assess sphincter involvement
Goodsell's rule: Posterior external openings → internal opening in posterior midline; anterior external openings → nearest anal crypt
Hydrogen peroxide injection ± endoanal ultrasound to identify smaller tracts
MRI pelvis (modality of choice for complex fistulas): high signal on fat-suppressed T2W/STIR sequences; shows tracts, abscesses, supralevator extensions; reduces recurrence rates when used preoperatively
Endoanal ultrasound: good for intraluminal/internal opening assessment
Fistulography (water-soluble contrast): useful when anatomy is unclear

Treatment

Treatment of anal fistulae is fundamentally surgical. The key principle: eliminate the internal opening while preserving sphincter function (continence).

1. Fistulotomy (Lay-Open)

Unroofing (laying open) of the fistula tract; heals by secondary intention
Indicated for intersphincteric and low transsphincteric fistulas with normal continence
Safe when < 30% of sphincter is involved
Healing rates up to 90%
⚠ Caution: Anterior fistulotomy in women (short anterior sphincter; obstetric injury risk)

2. Seton Placement

A suture or Silastic vessel loop threaded through the tract and secured
Draining seton: keeps external opening patent, prevents abscess; allows maturation of the tract before definitive repair
Cutting seton: tightened at 2–4 week intervals to gradually erode through muscle with fibrosis; success rates up to 94%, but incontinence risk 23–22%
Often the first step for high or complex fistulas

3. Advancement Flap (Endoanal/Mucosal Advancement Flap)

A flap of mucosa + internal sphincter is raised; tract is curetted; internal opening is sutured; flap advanced to cover it
Used for complex fistulas where sphincter division is unsafe
Success rate: 66–87%
Failed flaps can be repeated after inflammation resolves

4. LIFT Procedure (Ligation of Intersphincteric Fistula Tract)

After seton-induced fibrosis, incision in intersphincteric groove → tract identified → both ends ligated and divided
Sphincter-sparing
Success rates: 40–95% (wide range)

5. Fibrin Glue & Anal Fistula Plug

Biological sealant injected into tract (fibrin glue) or bioabsorbable plug inserted
High recurrence rates — not recommended as primary treatment

6. Newer/Emerging Techniques (Recent Evidence)

TROPIS (transanal opening of intersphincteric space): recent meta-analysis (2025, PMID 40693967) supports its use for high complex fistulas
Stem cell therapy (adipose-derived MSCs): 2025 meta-analysis (PMID 40205247) shows promising healing rates, especially in Crohn-associated fistulas
Autologous platelet-rich plasma (PRP): 2024 meta-analysis (PMID 39127896) supports its efficacy as an adjunct
Anal fistula clips (OTSC): 2025 meta-analysis (PMID 40929949) evaluating outcomes

Fistula in Crohn Disease

Affects 13–54% of Crohn disease patients
Asymptomatic fistulas: no treatment needed
Complex multi-tract fistulas: unroof branching tracts first → achieve single tract
Anti-TNF agents (e.g., infliximab): effective for symptomatic control and healing
Once quiescent → standard surgical options applicable
Severe cases: long-term seton + medical therapy; temporary diversion; proctectomy for refractory perianal disease

Summary Table: Surgical Options

Procedure	Best For	Healing Rate	Continence Risk
Fistulotomy	Simple/low fistulas, <30% sphincter	~90%	Low if <30% muscle
Cutting seton	High fistulas	~94%	Moderate (~23%)
Advancement flap	Complex/high	66–87%	Low
LIFT	Transsphincteric	40–95%	Very low
Fibrin glue/plug	—	Poor	None
TROPIS	High complex	Emerging evidence	Low
Stem cells	Crohn-related	Promising	Minimal

Sources: Mulholland and Greenfield's Surgery, 7e; Goldman-Cecil Medicine; Grainger & Allison's Diagnostic Radiology; Recent meta-analyses (PubMed 2024–2025).

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