Fistula-in-Ano: Diagnosis and Treatment
What is Fistula-in-Ano?
Fistula-in-ano is an abnormal tract connecting the anal canal to the perianal skin, typically arising from a preceding anorectal abscess as part of the same disease spectrum. 1 The condition predominantly affects males (2:1 ratio) with a mean age of 40 years at presentation. 1
Pathophysiology
- The cryptoglandular hypothesis is the most accepted mechanism: infection of intersphincteric anal glands leads to abscess formation, which then ruptures to create a fistulous tract. 1
- Approximately one-third of anorectal abscesses will develop into fistulas. 1
- The fistula consists of three components: an internal opening (typically at the dentate line), a tract through tissue planes, and an external opening on the perianal skin. 2
Anatomical Classification
The Parks classification defines fistulas based on their relationship to the anal sphincter complex:
- Intersphincteric: tract passes between internal and external sphincters (most common)
- Trans-sphincteric: tract crosses the external sphincter
- Suprasphincteric: tract passes above the puborectalis muscle
- Extrasphincteric: tract passes outside the sphincter complex entirely 1
Clinical Presentation
Typical Symptoms
- Drainage of blood, pus, or fecal material from an external perianal opening 1
- Intermittent pain (often cyclical with abscess formation and spontaneous drainage) 1
- Perianal itching and irritation 1
- Visible external opening with granulation tissue 1
Important Differential Diagnoses to Exclude
You must actively screen for underlying conditions, particularly in recurrent or complex cases:
- Crohn's disease: occurs in 13-27% of CD patients and may be the initial manifestation in up to 81% of those who develop perianal disease 1
- Malignancy: carcinoma can rarely arise in chronic fistulas, particularly in Crohn's disease 1
- Tuberculosis, HIV, actinomycosis 1
- Radiation proctitis 1
Diagnostic Approach
Clinical Examination
A focused medical history and complete physical examination with digital rectal examination is the foundation of diagnosis. 1
Key examination findings to document:
- Location of external opening(s) relative to the anal verge
- Presence of induration along the tract
- Palpable internal opening on digital rectal examination
- Assessment for active proctitis (contraindication to definitive surgery)
- Signs of Crohn's disease: surgical scars, anorectal deformities, skin tags, other perianal lesions 1
- Presence of associated abscess (requires drainage before definitive treatment) 1
Laboratory Testing
Check serum glucose, hemoglobin A1c, and urine ketones to identify undetected diabetes mellitus (strong recommendation). 1
- Diabetes is a common comorbidity and affects surgical planning 3
- If systemic infection is present: obtain complete blood count, serum creatinine, and inflammatory markers (CRP, procalcitonin, lactate) 1
Imaging Studies
Imaging is not routinely required for simple, clinically obvious fistulas, but should be obtained for:
- Atypical presentations 1, 3
- Suspected occult supralevator abscesses 1, 3
- Complex or recurrent fistulas 1
- Suspected or known Crohn's disease 1, 3
MRI is the preferred imaging modality when indicated, offering superior soft tissue resolution for defining fistula anatomy and detecting abscesses. 1 However, CT with IV contrast is acceptable when MRI is unavailable or time-sensitive evaluation is needed, though it has lower sensitivity (77% for abscesses, only 24% accuracy for fistula classification). 1
Endoanal ultrasound is an alternative with reported accuracy of 82% for fistula classification, but requires specialized expertise and may be poorly tolerated in acute settings due to pain. 1
Treatment Strategy
Initial Management: Control Sepsis First
Any associated abscess MUST be drained before considering definitive fistula surgery. 1 More than two-thirds of patients have an abscess associated with their fistula. 1
Surgical drainage with incision as close as possible to the anal verge minimizes potential fistula tract length. 4
Definitive Surgical Treatment: Algorithm Based on Fistula Complexity
For Simple, Low Fistulas (Subcutaneous, Superficial, or Low Intersphincteric)
Fistulotomy (laying open the tract) is the procedure of choice, with healing rates exceeding 95% and low recurrence. 5
- This involves unroofing the entire tract with debridement of granulation tissue 1, 2
- Contraindications to fistulotomy: active proctitis, high trans-sphincteric involvement, or significant sphincter muscle at risk 1
For Complex Fistulas (High Trans-sphincteric, Suprasphincteric, or Involving Significant Sphincter)
Sphincter-preserving techniques are mandatory to prevent incontinence. 5
The most effective sphincter-saving approaches with optimal outcomes are:
Ligation of Intersphincteric Fistula Tract (LIFT): healing rates 60-90% 5
Rectal advancement flaps: healing rates 60-90% 5
- Requires absence of active proctitis 1
Loose seton drainage: may be definitive treatment when combined with medical therapy, with seton removal possible in up to 98% at median 33 weeks 1
- Particularly useful when moderate-to-severe proctitis complicates the fistula 1
Novel techniques under evaluation include:
- Fistula laser closure (FiLaC): healing rates 65-90% 5
- Video-assisted anal fistula treatment (VAAFT): healing rates 65-90% 5
- Transanal opening of intersphincteric space (TROPIS): still under evaluation 5
Critical Surgical Principles
If a low fistula not involving sphincter muscle is identified during abscess drainage, fistulotomy can be performed at the same time. 3
For fistulas involving sphincter muscle, place a loose draining seton rather than performing immediate fistulotomy to prevent incontinence. 3, 4
Avoid probing to search for a fistula if one is not obvious—this may cause iatrogenic complications. 3
Special Consideration: Crohn's Disease-Associated Fistulas
Indications for Surgery in CD Fistulas
Surgery should only be attempted when:
- Patient is symptomatic 1
- No concomitant abscess present 1
- Proctitis is medically controlled 1
- Fistula tract is anatomically defined (preferably by MRI) 1
Treatment Approach for CD Fistulas
Loose setons are the primary surgical intervention to establish drainage and prevent abscess recurrence while medical therapy controls inflammation. 1
Medical therapy is imperative:
- Thiopurines, infliximab, or adalimumab should be used as maintenance therapy (either alone or combined with seton drainage). 1
- Infliximab is indicated for reducing the number of draining enterocutaneous and rectovaginal fistulas and maintaining fistula closure in adults with fistulizing Crohn's disease. 7
Fistulotomy can be considered for subcutaneous, superficial, or low intersphincteric/trans-sphincteric fistulas in the lower third of the sphincter, but contraindications include CDAI >150 and evidence of perineal Crohn's involvement. 1
Surgical outcomes for high or complex Crohn's fistulas remain disappointing, with unpredictable recurrence and worse functional outcomes compared to cryptoglandular fistulas. 8 For refractory cases, diverting ostomy should be considered, with proctectomy as last resort. 1
Antibiotic Therapy
Antibiotics are NOT routinely indicated for adequately drained anorectal abscesses in immunocompetent patients. 3
Antibiotics ARE indicated for:
When antibiotics are needed, use empiric broad-spectrum coverage for Gram-positive, Gram-negative, and anaerobic bacteria. 3
Common Pitfalls and How to Avoid Them
Pitfall 1: Missing Underlying Crohn's Disease
Screen every patient with recurrent or complex fistulas for Crohn's disease through detailed history (diarrhea, abdominal pain, weight loss) and examination for perianal manifestations. 1 Consider colonoscopy if suspicion exists.
Pitfall 2: Operating on Active Proctitis
Active rectal inflammation is a contraindication to definitive fistula surgery and will lead to failure. 1 Control proctitis medically first, then proceed with surgery.
Pitfall 3: Inadequate Abscess Drainage
Incomplete drainage, missed loculations, or horseshoe extensions are major risk factors for recurrence (up to 44% recurrence rate after drainage alone). 4 Ensure complete drainage of all abscess cavities before addressing the fistula.
Pitfall 4: Overly Aggressive Initial Surgery
Immediate fistulotomy of complex fistulas risks incontinence. 3, 4 When in doubt, place a seton and reassess after inflammation subsides.
Pitfall 5: Iatrogenic Tract Creation
Do not probe aggressively searching for fistulas that are not clinically obvious—this creates false passages and complicates future management. 3