What is fistula in ano and how is it treated?

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What is Fistula-in-Ano

Fistula-in-ano is an abnormal tract connecting the anal canal or rectum to the perianal skin, typically resulting from cryptoglandular infection of the intersphincteric anal glands. 1

Pathophysiology and Clinical Features

• The condition develops when obstruction of the draining duct from infected intersphincteric anal glands leads to abscess formation, which then ruptures to create a fistulous tract. 1
• Approximately one-third of patients with anorectal abscesses will subsequently develop fistulas. 2
• The fistula typically includes an internal opening (in the anal canal), a tract, and an external opening (on the perianal skin). 3

Risk Factors

• Male sex is a significant risk factor, with men affected twice as commonly as women (2:1 ratio), and males having a 15.8% incidence compared to 11.6% in females. 2
• Peak incidence occurs in young adults aged 20-40 years, with a mean age at presentation of 40 years. 2
• Crohn's disease is associated with perianal fistulas in 13-27% of patients, with complex and multifocal fistulae being more common in this population. 2, 4
• Other risk factors include diabetes mellitus, immunocompromised states (HIV), prior anal surgery, and pelvic radiation. 2

Treatment Approach

Initial Assessment and Imaging

MRI of the pelvis is the preferred initial imaging modality to define fistula anatomy and identify occult abscesses, with higher diagnostic accuracy compared to CT scans. 4

• Examination under anesthesia (EUA) by an experienced surgeon combined with MRI provides the highest diagnostic accuracy. 4
• Proctosigmoidoscopy should be performed to evaluate for concomitant rectal inflammation, particularly to exclude Crohn's disease. 4
• Endoanal ultrasound can be used as an alternative to MRI if rectal stenosis is excluded, but has lower diagnostic accuracy. 4

Surgical Management Algorithm

For simple fistulas (low, intersphincteric, or low transsphincteric without significant sphincter involvement):

• Fistulotomy remains the most commonly used treatment, involving laying open the fistula tract. 3
• This approach has high success rates but carries risk of incontinence depending on the amount of sphincter muscle involved.

For complex fistulas (high transsphincteric, suprasphincteric, extrasphincteric, or horseshoe configuration):

1. Initial management requires loose seton placement after drainage of any associated abscess. 4
2. More than two-thirds of complex fistulas have an associated abscess that must be drained before definitive intervention, with emergent drainage required in cases of sepsis, immunosuppression, diabetes, or diffuse cellulitis. 4
3. The seton establishes drainage, prevents abscess recurrence, and allows time for medical optimization before definitive sphincter-sparing procedures. 4
4. Draining seton alone can achieve complete symptom resolution in 73.7% of patients and significant amelioration in an additional 18.4%, with a recurrence rate of only 7.1%. 5

Sphincter-Preserving Techniques

After inflammation is controlled with seton drainage, definitive sphincter-sparing options include:

• Advancement flaps (mucosa-submucosa, rectal wall, or anocutaneous) with success rates around 75-88% for horseshoe fistulas. 6
• Ligation of the intersphincteric fistula tract (LIFT). 3
• Fibrin glue, collagen fistula plug, or stem cell therapy, though these lack consistent long-term results. 3

Special Considerations for Crohn's Disease

Medical therapy to control inflammation is imperative before attempting definitive surgical closure in Crohn's patients. 4

• Initiate antibiotics (metronidazole and/or ciprofloxacin) in combination with seton drainage. 4
• Add thiopurines or anti-TNF therapy (infliximab or adalimumab) as second-line treatment for refractory disease. 4
• Maintenance therapy with thiopurines, infliximab, or adalimumab combined with seton drainage is required after surgery. 4
• Concomitant perianal skin tags should not be surgically treated, as this leads to chronic non-healing ulcers. 4
• Diverting ostomy or proctectomy may be required for refractory cases. 4

Critical Pitfalls to Avoid

• Inadequate drainage of associated abscesses leads to recurrence rates as high as 44%. 1
• Patients requiring one or more additional operative procedures to unroof collections before planned seton removal have a 7-fold increased risk of treatment failure. 5
• Always exclude underlying Crohn's disease, especially with recurrent abscesses or complex fistulas, as surgical outcomes are markedly reduced without appropriate medical management. 4, 7
• Approximately 11% of colovesical and colovaginal fistulae are caused by malignancy, so maintain suspicion for underlying neoplasia when imaging shows soft tissue mass or malignant-appearing lymphadenopathy. 1, 2