Perianal fistula

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Excellent data. I now have comprehensive information from Sabiston, Bailey & Love, and Harrison's. Let me compile the full answer.

Perianal Fistula (Fistula-in-Ano)

Definition and Pathophysiology

A perianal fistula is a chronic abnormal communication extending from the anorectal lumen (the internal opening) to an external opening on the perianal skin, perineum, or buttock (or rarely to the vagina in women). The vast majority are cryptoglandular in origin - arising from infection of anal glands that open at the dentate line. Fistulae are lined by granulation tissue, not epithelium.

The sequence: anal gland infection → perianal abscess → spontaneous or surgical drainage → persistent communication = fistula. Approximately 30-50% of perianal abscesses will ultimately develop into a fistula. - Sabiston Textbook of Surgery, p. 2162; Harrison's Principles, p. 2628

Aetiology

Category	Examples
Cryptoglandular (majority)	Idiopathic anal gland sepsis
Inflammatory bowel disease	Crohn's disease (most common IBD cause)
Infections	Tuberculosis, lymphogranuloma venereum, actinomycosis
Malignancy	Can arise within a longstanding fistula
Miscellaneous	Foreign body, rectal duplication, radiation

Bailey & Love's Short Practice of Surgery, 28th ed., p. 1457

Classification - Parks' System

The most widely used classification is based on the relationship of the primary tract to the external anal sphincter:

Parks' Classification of Fistula-in-Ano showing Types 1-4

Type	Description	Frequency
Type 1 - Intersphincteric	Track confined to the intersphincteric plane; does not cross the external sphincter	45%
Type 2 - Transsphincteric	Crosses both internal and external sphincters; passes through ischiorectal fossa to perianal skin	30-40%
Type 3 - Suprasphincteric	Track rises above puborectalis, curves down through levators and ischioanal fossa	20%
Type 4 - Extrasphincteric	No specific relation to sphincters; usually from pelvic disease or trauma	5%

The American Gastroenterology Association (AGA) classification simplifies this into:

Simple fistula: low (superficial, low intersphincteric or low transsphincteric), single external opening
Complex fistula: high tract, extra- or suprasphincteric, associated abscess, anovaginal, anal stricture, or prior radiation/IBD
Bailey & Love's, p. 1457-1458

Clinical Features

Intermittent purulent (sometimes bloody) discharge from the external opening
Cyclical pain and swelling that temporarily resolves upon drainage
History of prior anorectal abscess in most cases
Passage of flatus/faeces through external opening suggests a rectal (rather than anal) internal opening
Bilateral external openings → suspect a fistula involving the deep postanal space
Multiple external openings ("watering can perineum") → strongly suggests perianal Crohn's disease

Goodsall's Rule

This rule predicts the internal opening based on the location of the external opening:

Anterior external opening (anterior to transverse anal line): track runs radially (directly) into the nearest anal crypt
Posterior external opening: track runs curvilinearly to a posterior midline internal opening
Exception: A fistula opening >3 cm from the anal verge anteriorly may be an anterior extension of a horseshoe fistula originating posteriorly
Sabiston, p. 2163; Harrison's, p. 2629

Investigations

Clinical Assessment

Full history including obstetric, GI, anal surgical, and continence history
Proctosigmoidoscopy to exclude IBD, malignancy
Key goals: identify internal opening, external opening(s), primary track course, secondary extensions

Intraoperative

Gentle fistula probe passed from external to internal opening
Hydrogen peroxide / methylene blue / milk injected through external opening to identify the internal opening
Examination under anaesthesia (EUA) is the gold standard for fistula evaluation

Imaging

MRI (pelvic/anal fistula protocol): Best modality for complex fistulae; identifies tracts with ~80% accuracy. Preferred for preoperative planning of high/complex fistulae
Endoanal ultrasound (EAUS): Useful for sphincter integrity assessment, especially with hydrogen peroxide instillation; comparable to MRI for most fistulae
Fistulography and CT have limited accuracy and are not routinely recommended
Sabiston, p. 2162-2163; Bailey & Love's, p. 1458-1459

Treatment

The goals of treatment are to:

Treat undrained infection
Define fistula anatomy
Remove or ablate epithelialised tracts
Avoid/minimize fecal incontinence (the primary risk)
Prevent recurrence

Simple / Low Fistulae (involving <1/3 of external sphincter)

Fistulotomy (lay-open) - the standard treatment

The entire fistula tract is unroofed and left to heal by secondary intention
Success rate: ~94%
Risk of mild fecal incontinence: ~13% (systematic review); <5% in patients with normal preoperative sphincter function and simple low fistulae
Recurrence rate: 2-8%

Seton Placement (all complex / high fistulae, uncertain sphincter involvement)

A silastic vessel loop or rubber band is threaded through the fistula tract. Uses:

Draining seton - maintains tract patency, reduces recurrent infection; allows anatomy to be defined once inflammation subsides (4-6 weeks), then definitive repair
Cutting seton - progressively tightened to slowly divide the sphincter in a controlled manner; largely fallen out of favour due to incontinence risk

Sphincter-Sparing Procedures (high / complex fistulae)

Procedure	Description	Success Rate
Anorectal advancement flap	Mobilization of a flap of rectal mucosa/submucosa/muscle to cover the internal opening	Up to 78%
LIFT procedure (Ligation of Intersphincteric Fistula Tract)	Dissection into intersphincteric plane; fistula tract divided and suture-ligated; sphincters preserved	~65-70%
Fibrin glue / Cyanoacrylate glue	Biological plug injected into tract	Lower success (~50%)
Anal fistula plug (AFP)	Bioabsorbable plug placed in the tract	Variable; ~50%
FiLaC (Fistula Laser Closure)	Radially emitting laser ablates the fistula tract from within	~65% healing at 24 months
Video-Assisted Anal Fistula Treatment (VAAFT)	Endoscopic identification and ablation under direct vision	Promising short-term results

Sphincter-sparing procedures achieve up to 78% success with no incontinence risk. - Harrison's, p. 2629; Sabiston, p. 2163-2165

Crohn's-Associated Perianal Fistula

This is a particularly challenging subset requiring a multidisciplinary approach:

Anti-TNF agents (infliximab, adalimumab) are first-line medical therapy; infliximab has the best evidence (ACCENT II trial)
Adalimumab biosimilars show similar efficacy per a 2025 systematic review and meta-analysis (PMID 41267606)
Mesenchymal stem cell therapy (darvadstrocel - Alofisel): The ADMIRE trial (n=212) showed fistula remission at 52 weeks in 59% vs 42% placebo; now an established option for complex perianal Crohn's fistulae where anti-TNFs have failed
Seton drainage is used to control sepsis before medical therapy
Caution with any sphincter-cutting surgery due to already-impaired anorectal function in Crohn's disease
A 2025 consensus statement also highlights the risk of fistula-associated anorectal cancer in long-standing Crohn's fistulizing disease
Harrison's Principles, 22nd ed., p. 2629-2630

Prognosis and Complications

Main risk of all surgical treatment: fecal incontinence (ranges from minor soiling to complete loss; dependent on sphincter involvement)
Recurrence more common with complex/high fistulae
Untreated fistulae can lead to recurrent abscesses, sepsis, and rarely malignant transformation in very long-standing cases

Key sources: Sabiston Textbook of Surgery, 21st ed.; Bailey & Love's Short Practice of Surgery, 28th ed.; Harrison's Principles of Internal Medicine, 22nd ed. (2025)

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