What are the causes of perianal fistulas?

Causes of Perianal Fistulas

The majority of perianal fistulas originate from infection of the intersphincteric anal glands, which can progress to abscess formation and subsequent fistula development. 1

Primary Causes

Cryptoglandular Infection (Most Common)

• Infection of the intersphincteric anal glands is the predominant cause 1, 2
• Obstruction of the draining duct leads to abscess formation
• Inadequately drained anorectal abscesses or spontaneously burst abscesses commonly progress to fistulas 2

Inflammatory Bowel Disease

• Crohn's disease is a major cause of perianal fistulas 1
  • Occurs in 13-38% of Crohn's disease patients 1
  • Higher prevalence in patients with colonic disease (41-92%) compared to ileal disease (12-15%) 1
  • Can be the initial or only manifestation of Crohn's disease in some patients 1
  • Mechanism: penetration of fissures/ulcers in rectum or anal canal 1

Secondary Causes

Infections

• Tuberculosis 1, 3
• HIV 1
• Actinomycosis (rare) 4

Trauma and Iatrogenic Causes

• Prior anal surgery 1
• Foreign body 1

Other Conditions

• Radiation proctitis 1
• Malignancy (rare) 1
  • Carcinoma can develop in chronic fistulas, particularly in Crohn's disease 1

Classification of Perianal Fistulas

Anatomical Classification (Parks)

• Based on the external sphincter as reference point 1:
  • Superficial (low)
  • Intersphincteric (low or high)
  • Transsphincteric (low or high)
  • Suprasphincteric (high)
  • Extrasphincteric (high)

Clinical Classification

• Simple fistulas:

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

• Complex fistulas:

  • High origin (high intersphincteric, high transsphincteric, extrasphincteric, or suprasphincteric)
  • May have multiple external openings
  • May be associated with perianal abscess
  • May have rectovaginal component
  • May have anorectal stricture
  • Often associated with active rectal disease 1

Clinical Considerations

Diagnostic Approach

• Physical examination is the initial step but may miss deep or complex fistulas
• Advanced imaging is often required:
  • Examination under anesthesia (EUA) - approximately 90% accurate 1
  • MRI with phased-array or endoanal coils 1
  • Endoanal ultrasound 1
  • CT with IV contrast for suspected abscesses 1

Important Clinical Pitfalls

• Isolated perianal fistulas may be the first or only manifestation of Crohn's disease in some patients 1, 5
• Recurrent complex fistulas without obvious cause should raise suspicion for underlying Crohn's disease 5
• Malignancy should be considered in chronic, non-healing fistulas 1
• Fistulography and non-contrast CT have poor diagnostic accuracy and should not be relied upon 1
• Despite optimal management, complex perianal fistulas often persist, with studies showing only 21% achieving complete closure even with combined medical-surgical approaches 5

Treatment Considerations

• Treatment approach varies based on fistula type, underlying cause, and presence of active inflammation
• Multidisciplinary approach combining surgical and medical management is often required, especially for Crohn's disease-related fistulas 6
• Simple fistulas may be amenable to fistulotomy, while complex fistulas often require seton placement and medical therapy 3, 6
• The balance between fistula healing and preservation of continence remains challenging 3