Anal Fistula (Fistula-in-Ano): Complete Overview

Definition

Pathogenesis

• Crohn's disease (complex perianal fistulas in ~20-40% of patients)
• Trauma, obstetric injury
• Tuberculosis (extraintestinal TB can mimic cryptoglandular disease)
• Radiation
• Malignancy, actinomycosis, lymphogranuloma venereum

Classification: Parks System (1976)

• Simple: intersphincteric or low transsphincteric, single tract, no associated inflammatory bowel disease or prior surgery
• Complex: high transsphincteric, suprasphincteric, extrasphincteric, horseshoe tract, multiple tracts, or associated with Crohn's/radiation/malignancy

Diagnosis

• Perianal discharge (pus, blood, feces), intermittent swelling, discomfort
• Digital rectal examination (DRE) can identify the internal opening as an indurated nodule at the dentate line
• EUA (Examination Under Anesthesia) is often required to define complex anatomy

• MRI is the gold standard for complex fistulas - provides multiplanar views of sphincter anatomy and secondary extensions. Using MRI to guide surgical strategy is now routinely recommended pre-operatively for complex cases.
• Endoanal Ultrasound (EAUS/3D-EAUS) - useful, particularly with hydrogen peroxide enhancement; comparable to MRI for transsphincteric fistulas, but more operator-dependent
• CT - limited role; useful for suprasphincteric/extrasphincteric fistulas with suspected pelvic source

Surgical Principles

1. Drain and eradicate infection
2. Obliterate the fistula tract
3. Prevent recurrence
4. Preserve anal sphincter function and continence

Surgical Techniques

1. Fistulotomy (Laying Open)

• A grooved probe is passed from external to internal opening
• All tissue between the two openings is divided
• Wound is curetted, edges trimmed, and left open to granulate
• Marsupialisation (suturing wound edges to the base) reduces wound size and speeds healing

2. Fistulectomy

3. Seton Techniques

• No tension; does NOT cut tissue
• Used to: control sepsis before definitive repair, or as long-term palliation (e.g., in Crohn's disease)
• Prevents recurrent abscess formation; allows sphincter assessment and patient counselling
• Can be definitive treatment in Crohn's when radical surgery is not appropriate

• Tied with tension; gradually cuts through the encircled muscle over weeks
• Fibrosis forms around the seton, preventing sphincter retraction and maintaining continence as the muscle is divided
• High efficacy and low recurrence for high perianal fistulas; comparable outcomes to mucosal advancement flap
• Requires multiple tightening clinic visits; can cause pain and prolonged healing - Bailey & Love's 28th Ed.

4. Advancement Flap (Mucosal/Anodermal Advancement Flap)

• Internal opening is excised and covered by a full-thickness or partial-thickness flap of mucosa/submucosa/internal sphincter drawn down from above
• The fistula tract is cored out or curetted; the external opening is left open to drain
• Rectal advancement flap (RAF): success rates 60-80%; risk of flap necrosis; requires healthy well-vascularized tissue
• Contraindicated in active Crohn's disease proctitis; prior failed flaps reduce success

5. LIFT (Ligation of Intersphincteric Fistula Tract) - 2007 to present

• Sphincter preserved entirely - no muscle divided
• Suitable for transsphincteric fistulas of any height
• Reproducible, technically less demanding than flap procedures

6. Fibrin Glue Injection

• Obliterates the tract with a fibrin sealant; no sphincter involvement
• Simple, repeatable, minimal morbidity
• Short-term success only: healing rates 50-85% initially, but long-term recurrence high (50-70%)
• Useful as an adjunct or in patients unfit for more invasive surgery

7. Anal Fistula Plug (AFP)

• Scaffold for tissue ingrowth; no sphincter division
• Success rates variable: 20-80% in literature; less consistent than initially hoped
• Useful in high/complex fistulas where sphincter preservation is paramount
• Being revisited with newer synthetic and extracellular matrix materials

Minimally Invasive and Novel Techniques (Recent Advances)

8. VAAFT (Video-Assisted Anal Fistula Treatment) - 2011 to present

• An 8-degree angled fistuloscope is introduced through the external opening
• Direct visualization of the entire tract - identifies secondary extensions, blind tracts
• Working channel allows lavage, curettage, diathermy ablation of granulation tissue and internal opening
• External opening is then closed; the preparation stage alone is called VAAFT; definitive closure is typically with stapler or advancement flap

9. FiLaC (Fistula Tract Laser Closure) - First described 2011

• A radial-emitting diode laser probe (FiLaC, Biolitec) is introduced into the tract
• Delivers circumferential ablation of fistulous and granulation tissue; the shrinkage effect obliterates the tract
• The internal opening is typically covered with an advancement flap (or in a revised technique by Giamundo et al., the laser's shrinkage effect seals the opening directly - eliminating the need for flap)
• Precise, sphincter-sparing, no wound to heal externally

10. OTSC (Over-The-Scope Clip) - FISCLOSE Technique

• A nitinol clip (bear-claw mechanism) is deployed endoscopically over the internal opening from inside the anal canal
• Mechanically seals the internal opening; the external tract is disconnected and left to heal
• Sphincter-sparing, no incision required

11. PERFACT Procedure

Biological / Regenerative Therapies (Frontier)

12. Mesenchymal Stem Cell (MSC) Therapy

• First cell therapy approved for complex perianal fistulas in Crohn's disease (EMA approval 2018; not universally available)
• Injected into the fistula wall after tract preparation
• Mechanism: immunomodulation (suppression of TNF-alpha, IL-1, IL-6), promotion of tissue regeneration
• ADMIRE-CD trial: combined remission rate significantly higher vs. placebo at week 24; real-world Spanish data (2025) confirmed consistent clinical effectiveness and safety with no treatment-related adverse events
• A 2025 systematic review and meta-analysis (Frontiers in Medicine) of RCTs found adipose-derived MSC therapy superior to placebo for complex perianal fistulas, though heterogeneity was high
• MSC-derived exosomes are now being investigated as a non-cellular alternative with similar immunomodulatory properties

13. Tissue Engineering and Smart Setons

Special Situations

Crohn's Disease

• Multidisciplinary approach mandatory
• Anti-TNF agents (infliximab) first-line medical therapy; adalimumab as alternative
• Vedolizumab and ustekinumab have emerging evidence
• Seton drainage to control sepsis while medical therapy is optimized
• After biologic failure, darvadstrocel is now an option; surgical options are same as cryptoglandular but with higher recurrence and poorer healing
• Proctectomy only as last resort

Horseshoe Fistula

• A high transsphincteric or suprasphincteric tract with posterior midline internal opening that extends bilaterally into the ischiorectal fossae
• Deep postanal space drainage through posterior midline incision + bilateral counter-incisions (Penrose drains through ischiorectal fossae)
• Loose seton through the posterior sphincter tract
• VAAFT is particularly useful for debriding horseshoe tracts endoscopically

Immunocompromised Patients

• Avoid aggressive sphincter division
• Prolonged antibiotics (broad-spectrum) where surgery is not feasible
• Profoundly neutropenic patients (ANC <500/mm³): often NO surgery - antibiotics only until neutropenia resolves

Current Guidelines

• MRI recommended for complex fistulas pre-operatively
• Fistulotomy remains standard for simple low fistulas
• LIFT is recommended as a sphincter-sparing first choice for transsphincteric fistulas
• Advancement flap is an alternative for complex fistulas
• Newer techniques (VAAFT, FiLaC, OTSC) show promise but are not yet first-line due to limited long-term RCT data

Summary: Choosing the Right Operation

Key Recent Evidence

• ESCP 2024 Guidelines (Meta-Analysis) - 42 recommendations; LIFT preferred sphincter-sparing technique
• Network meta-analysis 2023, Bhat et al. - LIFT ranked #1 for continence preservation across 52 RCTs
• Anal Abscess and Fistula review, 2026 Surg Clin North Am - MSC injections highlighted as emerging therapy
• Darvadstrocel real-world outcomes 2025 (Spain): consistent remission, no adverse events, outcome-based payment model feasible
• Adipose MSC meta-analysis 2025 - stem cell therapy superior to placebo for complex fistulas in RCTs