What are the causes of perianal fistulas?

Causes of Perianal Fistulas

The most common cause of perianal fistulas is infection of the intersphincteric anal glands, which can lead to abscess formation and subsequent fistula development when the abscess ruptures or is inadequately drained. 1

Primary Causes

Cryptoglandular Infection

• Obstruction and infection of the intersphincteric anal glands is the predominant etiology
• The infection process typically begins with:
  • Obstruction of the draining duct of an anal gland
  • Formation of an intersphincteric abscess
  • Rupture through the external sphincter or inadequate drainage of the abscess
  • Development of a chronic fistulous tract 1, 2

Inflammatory Bowel Disease

• Crohn's disease is a major cause of perianal fistulas
  • Occurs in 13-38% of Crohn's disease patients 1
  • Higher prevalence in patients with colonic disease (41%) and rectal involvement (92%) 1
  • May precede or present simultaneously with intestinal disease in 36-81% of patients 1
  • Can be the only manifestation of Crohn's disease in some patients
• Fistulas in Crohn's disease may arise from:
  • Infected anal glands (similar to cryptoglandular disease)
  • Penetration of fissures or ulcers in the rectum or anal canal 1

Secondary Causes

Trauma and Iatrogenic Causes

• Obstetric trauma (88% of rectovaginal fistulas) 1
• Surgical complications
• Radiation therapy to the pelvis

Infections

• Pelvic infections including:
  • Diverticulitis
  • Tuberculosis
  • Lymphogranuloma venereum
  • Human papillomavirus
  • HIV
  • Cytomegalovirus
  • Schistosomiasis 1

Malignancy

• Anorectal cancer
• Perineal malignancies
• Gynecologic cancers
• Carcinomas may rarely arise in chronic fistula tracts 1

Classification of Perianal Fistulas

Anatomical Classification (Parks)

• Superficial (low): Involves distal anal canal without involving sphincters
• Intersphincteric (low or high): Between internal and external sphincter
• Transsphincteric (low or high): Crosses the anal sphincter muscle
• Suprasphincteric (high): Passes upward in intersphincteric plane above puborectalis
• Extrasphincteric (high): Passes directly from rectum to perineal skin 1

Clinical Classification

• Simple fistulas:

  • Low origin (superficial, low intersphincteric, low transsphincteric)
  • Single external opening
  • No pain or fluctuation suggesting abscess
  • No rectovaginal fistula
  • No anorectal stricture 1

• Complex fistulas:

  • High origin (high intersphincteric, high transsphincteric, extrasphincteric, suprasphincteric)
  • Multiple external openings
  • Associated with pain or abscess
  • Associated with rectovaginal fistula
  • Associated with anorectal stricture
  • Associated with active rectal disease 1

Clinical Implications

Diagnostic Approach

• Physical examination is often diagnostic but may be limited for deeper or complex fistulas
• Advanced imaging modalities for accurate classification:
  • Examination under anesthesia (EUA): 90% accuracy when performed by experienced surgeons
  • MRI with phased-array or endoanal coils
  • Endoanal ultrasound (EUS) 1

Treatment Considerations

• Simple cryptoglandular fistulas typically respond well to surgical treatment alone
• Complex fistulas, especially in Crohn's disease, often require combined medical and surgical approaches
• Antibiotics alone do not prevent fistula formation after perianal abscess drainage 3
• Persistent complex perianal fistulas without luminal Crohn's disease may represent early Crohn's disease or severe cryptoglandular disease 4

Pitfalls and Caveats

1. Failure to recognize underlying Crohn's disease in patients with complex or recurrent fistulas
2. Inadequate drainage of associated abscesses leading to recurrence
3. Overlooking malignancy as a potential cause or complication of chronic fistulas
4. Treating with antibiotics alone without appropriate surgical intervention
5. Missing deeper extensions or secondary tracts during initial assessment
6. Neglecting to evaluate for active proctitis, which significantly impacts treatment approach and outcomes

Understanding the etiology of perianal fistulas is crucial for appropriate management, as treatment strategies differ significantly based on the underlying cause, especially between cryptoglandular disease and inflammatory bowel disease.