Fistula-in-Ano

Aetiology

1. Infection of an anal gland in the intersphincteric space
2. Perianal abscess formation
3. Spontaneous or surgical drainage - leaving a persistent fistulous track

Secondary causes

• Crohn's disease (most common secondary cause; suspect with multiple/complex tracts)
• Tuberculosis
• Lymphogranuloma venereum
• Actinomycosis
• Rectal duplication
• Foreign body
• Malignancy (rarely arising within a longstanding fistula)
• Trauma / obstetric injury
• Bailey and Love's Short Practice of Surgery 28th Ed, p. 1456-1457

Pathological Anatomy

Classification

Parks' Classification (most widely used)

AGA (American Gastroenterological Association) Classification

• Bailey and Love, Table 80.2

Clinical Features

Symptoms

• Intermittent purulent discharge from the external opening (may be blood-stained)
• Cyclical pain - builds up until drainage occurs, followed by relief
• History of prior anorectal abscess
• Passage of flatus or faeces through the external opening suggests a rectal (rather than anal canal) internal opening

Signs

• One or more chronic wounds or discharging openings on the perianal skin
• Palpable cord-like subcutaneous induration between the external opening and the anal margin (suggests superficial track)
• Supralevator induration - suggests high primary track or secondary extension
• Bilateral external openings - think deep postanal space involvement
• Multiple external openings ("watering-can perineum") - strongly suspect Crohn's disease

Goodsall's Rule

• External opening ANTERIOR to transverse anal line → track runs radially/directly into the anal canal (except when >3 cm from anal verge, which may be an anterior extension of a posterior horseshoe fistula)
• External opening POSTERIOR to transverse anal line → track curves in a curvilinear fashion to a posterior midline internal opening

Investigations

Clinical Examination

• Full history including obstetric, GI, and continence history
• Proctosigmoidoscopy
• Gentle palpation for subcutaneous cord; digital rectal examination for supralevator induration
• Identification of internal opening: palpable induration, enlarged papilla at the dentate line

Intraoperative Tools

• Anal fistula probe - gentle probing from external opening; care to avoid creating false passages
• Dilute hydrogen peroxide / methylene blue / milk - injected via the external opening to identify the internal opening
• Anoscopy - direct inspection of the dentate line for erythematous crypt or visible internal opening

Imaging

• MRI is especially valuable for complex or recurrent fistulae and high-level tracts

Principles of Surgery

1. Treat any undrained infection
2. Define fistula anatomy
3. Remove or ablate epithelialized tracts
4. Avoid or minimize the risk of fecal incontinence
5. Prevent recurrence

Surgical Options

1. Fistulotomy (Lay-Open)

• Probe is passed through the tract; overlying tissue is divided; wound heals by secondary intention
• Best for: Simple, low-lying fistulae (intersphincteric or low transsphincteric) involving less than one-third of the external sphincter
• Recurrence rate: 2-8%; clinically significant incontinence: <5% in patients with normal preoperative sphincter function
• Historically described by John of Arderne in De Arte Phisicali et de Cirurgia

2. Seton

• Draining seton (silastic vessel loop or rubber band): Controls the fistula, prevents cyclical symptoms, allows fibrosis and narrowing of the tract, may enable partial fistulotomy through soft tissue to the level of the sphincter
• Cutting/tightening seton: Progressively tightened to slowly divide the tract, allowing fibrosis before division - reduces incontinence risk; however, seton tightening has largely fallen out of favour due to unpredictable incontinence

3. Endorectal Advancement Flap

• Fistula tract is excised; the rectal/internal opening is closed with a vascularized mucosal flap advanced from proximal rectum
• Used for transsphincteric fistulae and complex fistulae
• Healing rates: 66-87%
• Risk: mild-moderate incontinence in up to 35% if sphincter fibers are included in the flap

4. LIFT (Ligation of Intersphincteric Fistula Tract)

• The intersphincteric plane is accessed, the fistula tract is identified, divided, and suture-ligated within the intersphincteric space; the sphincter is approximated
• Sphincter-preserving technique
• If it fails, the result is often an intersphincteric fistula amenable to simple fistulotomy

5. Fibrin Glue / Anal Fistula Plug

• The tract is obliterated with fibrin glue or plugged with a bioresorbable device
• Sphincter-preserving; however, long-term success rates are poor; not recommended as definitive treatment

6. VAAFT (Video-Assisted Anal Fistula Treatment)

• Fistulascope inserted into the external opening; tract and all branches are visualized under direct endoscopic vision
• Internal opening closed with suture/stapling; granulation tissue debrided; tract ablated with an electrode
• No sphincter muscle divided
• Reported healing rates: 71-85% at 1-2 years

7. FiLaC (Fistula Laser Closure)

• Radially emitting laser probe ablates the fistula tract from within under direct vision
• Similar principle to VAAFT but uses laser energy
• Recent meta-analysis: healing in ~65% at median 24 months

8. Stem Cell Therapy (Emerging)

• Mesenchymal stem cells transplanted into peri-fistular tissue
• Promising results in small studies and RCTs, particularly in Crohn's perianal fistulae (darvadstrocel/Alofisel has regulatory approval in some countries)

Special Situations

Crohn's-Related Fistula

• Often complex, multiple, and recurrent
• Medical management is first-line: anti-TNF agents (infliximab/adalimumab), setons for drainage
• Surgery is reserved for palliation; sphincter-sacrificing procedures are generally avoided

Horseshoe Fistula

• Deep postanal space abscess spreads circumferentially through the ischiorectal fossae
• Both ischiorectal fossae are involved with a common posterior communication
• Treatment: drain the deep postanal space (posterior fistulotomy) plus counter-incisions over each ischiorectal component (Hanley's procedure)

Fistula with Abscess

• The acute phase should be drained first; primary fistulotomy at the time of abscess drainage remains controversial (anatomy is distorted and sphincter involvement may be underestimated)

Continence Risk Factors

• Amount of sphincter divided (the dominant factor)
• Preoperative sphincter function
• Prior fistula surgery
• Female sex (shorter anterior sphincter)
• Anterior fistulae in women (highest risk)
• Obstetric injury history

Prognosis

• Simple fistulae treated by fistulotomy: low recurrence (2-8%), excellent continence outcomes
• Complex fistulae: higher recurrence rates, ongoing risk of continence impairment; often require staged or repeat procedures
• Crohn's fistulae have the highest rates of recurrence and complexity

• Sabiston Textbook of Surgery: The Biological Basis of Modern Surgical Practice, Ch. 97 (Fistula-in-Ano)
• Bailey and Love's Short Practice of Surgery 28th Edition, Ch. 80
• Sleisenger and Fordtran's Gastrointestinal and Liver Disease
• Harrison's Principles of Internal Medicine 22E (2025)