Anal Fistulae (Fistula-in-Ano)

Definition

• Pfenninger and Fowler's Procedures for Primary Care, p. 693

Aetiology and Pathophysiology

• Crohn's disease (fistulas occur in 13-54% of Crohn's patients)
• Immunocompromise (steroids, HIV)
• Diabetes mellitus
• Prior perianal radiation
• Tuberculosis, malignancy, actinomycosis (less common)

• Harrison's Principles of Internal Medicine 22E, p. 2629

Classification: Parks Classification

Clinical Features

• Constant or intermittent purulent discharge from the perianal area - this is the hallmark symptom
• Perianal pain and tenderness
• History of prior perianal abscess (often recurrent)
• A visible external opening on the perianal skin, often with granulation tissue; may be inflamed or may close over superficially

Evaluation

History

Physical Examination

Goodsell's Rule

• External openings posterior to a transverse line through the anus → internal opening in the posterior midline
• External openings anterior to the transverse line → internal opening at the nearest crypt
• Exception: an opening >3 cm from the anal verge may have upward extension and not follow the rule

Investigations

• Examination under anaesthesia (EUA) - the gold standard; allows probing the tract, injection of hydrogen peroxide through the external opening to identify the internal opening
• MRI - the imaging modality of choice for complex fistulae; achieves ~80% accuracy in delineating tracts. Fat-suppressed T2W (STIR) sequences show the fistula tract as high signal against the lower-signal sphincter and adjacent fat. Also detects associated abscesses and supralevator collections that are clinically occult. Preoperative MRI reduces recurrence rates in complex disease.

• Endoanal ultrasound (EUS) - useful for identifying the tract and internal opening, particularly within the sphincter complex
• Fistulography (water-soluble contrast with fluoroscopy) - for complex or multi-tract disease
• Grainger & Allison's Diagnostic Radiology, p. 593
• Mulholland and Greenfield's Surgery 7e, p. 3561

Differential Diagnosis

• Hidradenitis suppurativa - inflammation of apocrine sweat glands; tracts are superficial and do not connect to the anal canal
• Pilonidal disease - origin in the gluteal cleft, foreign body reaction to hair; does not have an internal opening at the dentate line
• Crohn's disease (may coexist)
• Rectovaginal or rectourethral fistula

Management

1. Fistulotomy (unroofing)

• The tract is laid open by incising the overlying tissue.
• Suitable for simple, low fistulas where less than ~30% of the sphincter is involved.
• Healing rates up to 90%.
• Risk: incontinence if too much sphincter is divided; women are particularly at risk for anterior fistulotomy.
• Post-op: wound heals by secondary intention over 4-6 weeks; must be kept open from inside out.
• A 2025 meta-analysis (Quinn et al., ANZ J Surg, PMID 40125894) directly compared fistulotomy vs. fistulectomy in RCTs.

2. Seton Placement

• A vessel loop or silk tie is passed through the tract.
• Two uses:
  • Draining seton: maintains patency, reduces inflammation; used as a bridge before definitive repair.
  • Cutting seton: progressively tightened every 2-4 weeks to slowly erode through the sphincter, inducing fibrosis. Success rates up to 94%, but incontinence rates of 23% or more are reported.
• Required before most sphincter-preserving procedures (LIFT, advancement flap) to allow fibrosis and reduce inflammation.

3. Endoanal Advancement Flap

• A flap of mucosa with underlying internal sphincter is raised, the tract is curetted, the internal opening sutured, and the flap is advanced to cover it.
• Used for complex fistulas. Success rate: 66-87%.
• Can be repeated after an interval if it fails.

4. LIFT Procedure (Ligation of Intersphincteric Fistula Tract)

• After seton-induced fibrosis, an incision is made in the intersphincteric groove; both ends of the tract are ligated and the tract is divided.
• Reported success: 40-95% (wide range across studies).
• Sphincter-preserving.

5. Fibrin Glue and Fistula Plug

• Associated with high recurrence rates and are not recommended as primary treatment.

6. Medical Management (Crohn's-related fistulae)

• Anti-TNF agents (infliximab, adalimumab) are the cornerstone.
• Asymptomatic Crohn's fistulae do not need treatment.
• Long-term seton placement + medical therapy for complex Crohn's fistulae.
• Allogeneic adipose-derived mesenchymal stem cell therapy (darvadstrocel / ADMIRE study): 59% remission at 52 weeks vs. 42% placebo - statistically significant but cost is prohibitive.
• Temporary fecal diversion may facilitate healing; proctectomy is reserved for severe refractory perianal Crohn's disease.

7. Emerging: Laser-based therapies

• A 2025 systematic review (Duda et al., Int J Colorectal Dis, PMID 40936062) evaluated laser-based techniques with and without adjunctive measures.

Complications of Surgery

• Anal incontinence - the major concern; risk increases with amount of sphincter divided
• Fistula recurrence - especially if the wound closes prematurely before the deep tract has healed, or if complex tracts are missed
• Creating a false tract with aggressive probing
• Missing a concurrent occult perianal abscess
• Missing underlying Crohn's disease (requires specific medical approach)

Key Points Summary