What are the causes of an anal fistula?

Causes of Anal Fistulas

Anal fistulas are most commonly caused by infection of the intersphincteric anal glands, which can lead to abscess formation and subsequent development of fistulous tracts connecting the anorectal canal to the perianal skin or other adjacent structures. 1

Primary Causes

Cryptoglandular Infection

• The majority of anal fistulas are cryptoglandular in origin, resulting from infection of the anal glands located in the intersphincteric space 1, 2
• Obstruction of the draining duct may produce an intersphincteric abscess that can rupture through the external sphincter, forming fistulous tracts 1

Inflammatory Bowel Disease

• Crohn's disease is a major cause of anal fistulas, accounting for approximately 9% of rectovaginal fistulas 1
• In patients with Crohn's disease with colonic involvement and rectal disease, the prevalence of fistulizing anal disease can be as high as 92% 1
• Crohn's-associated fistulas result from transmural inflammation and deep ulceration in the bowel wall 2
• These fistulas are often more complex than cryptoglandular fistulas and have lower healing rates 2

Other Significant Causes

Trauma and Iatrogenic Causes

• Obstetric or vaginal trauma (accounts for 88% of rectovaginal fistulas) 1
• Surgical complications following anorectal procedures 1
• Radiation therapy to the pelvic area 1

Infections

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

Malignancy

• Approximately 11% of colovesical and colovaginal fistulae are caused by malignancy 1
• Carcinomas can rarely arise in chronic fistula in the anorectum 1
• Malignancies of the anorectum, perineum, and gynecologic organs 1

Foreign Bodies

• Internalization of foreign bodies into the gastrointestinal tract can lead to perforation and fistula formation, though this is rare 3

Classification of Anal Fistulas

Anal fistulas are typically classified as:

1. Simple vs. Complex:

   • Simple fistulas: Low (superficial or low intersphincteric or low transsphincteric), single external opening, no pain/fluctuation suggesting abscess, no rectovaginal fistula, no anorectal stricture 1
   • Complex fistulas: High (high intersphincteric, high transsphincteric, extrasphincteric, or suprasphincteric), possibly multiple external openings, may be associated with abscess, rectovaginal fistula, anorectal stricture, or active rectal disease 1

2. Parks Classification:

   • Superficial (low)
   • Intersphincteric (low or high)
   • Transsphincteric (low or high)
   • Suprasphincteric (high)
   • Extrasphincteric (high) 1

Diagnostic Considerations

When evaluating a patient with suspected anal fistula, it's important to:

• Perform proctosigmoidoscopy to assess for concomitant rectal inflammation, which has prognostic and therapeutic relevance 1
• Consider contrast-enhanced pelvic MRI or endoscopic anorectal ultrasound for assessment of perianal fistulas 1
• Examination under anesthesia (EUA) remains the gold standard for accurate classification when performed by an experienced surgeon 1
• Rule out malignancy when imaging shows soft mass, mass-like thickening of the anorectal wall, or malignant-appearing lymphadenopathy 1

Clinical Implications

Understanding the cause of anal fistulas is crucial for appropriate management, as different etiologies require different treatment approaches. Cryptoglandular fistulas may respond well to surgical intervention alone, while Crohn's-associated fistulas often require a combination of medical and surgical management 2. The presence of active rectal Crohn's disease significantly complicates management and worsens prognosis 1.