What are the causes of fistulas?

Causes of Fistulas

Fistulas are abnormal communications between two epithelial surfaces that can occur throughout the body, with the most common causes being trauma (particularly surgical or obstetric), inflammatory conditions (especially Crohn's disease), infections, malignancies, and radiation injury. 1

Types of Fistulas

Anatomical Classification

• Based on location:
  • Gastrointestinal fistulas (between portions of intestine)
  • Enterocutaneous fistulas (between intestine and skin)
  • Rectovaginal/anovaginal fistulas (between rectum/anus and vagina)
  • Rectovesicular fistulas (between rectum and bladder)
  • Thoracic fistulas (involving lung, tracheobronchial tree, pleural space, etc.) 2

Physiological Classification

• Based on output:
  • Low output
  • Moderate output
  • High output 1

Major Causes of Fistulas

1. Trauma and Iatrogenic Causes

• Surgical complications (most common cause of gastrointestinal fistulas)
• Obstetric trauma (accounts for 88% of rectovaginal fistulas) 3, 4
• Vaginal trauma
• Postoperative leaks from enteric anastomoses 3

2. Inflammatory Conditions

• Crohn's disease (major cause, accounting for approximately 9% of rectovaginal fistulas) 3, 4
  • In patients with Crohn's disease with colonic and rectal involvement, the prevalence of fistulizing anal disease can reach 92% 3, 4
  • Fistulas in Crohn's disease typically arise from within or just proximal to a stricture with active inflammation 3

3. Infections

• Pelvic infections including:
  • Diverticulitis
  • Tuberculosis
  • Lymphogranuloma venereum
  • Human papillomavirus
  • HIV
  • Cytomegalovirus
  • Schistosomiasis 3, 4
• Perianal abscesses (can develop into fistulas) 3

4. Malignancies

• Colorectal cancer
• Pelvic malignancies
• Gynecologic cancers
• Approximately 11% of colovesical and colovaginal fistulae are caused by malignancy 4

5. Radiation Injury

• Radiation therapy to the pelvic area can lead to fistula formation 3, 4

6. Congenital

• Less common cause in adults but important in pediatric populations 1

Specific Types of Fistulas and Their Causes

Perianal Fistulas

• Often develop after rupture or drainage of a perianal abscess 5
• In Crohn's disease, perianal fistulas may present at or before the time of diagnosis in about one-quarter of cases 3
• Can be classified using Parks' or St James' Classification systems 3

Rectovaginal Fistulas

• Primary causes:
  • Obstetric/vaginal trauma (88%)
  • Crohn's disease (9%)
  • Radiation
  • Pelvic infections
  • Malignancies 3, 6

Rectovesicular Fistulas

• Characterized by pneumaturia or fecaluria (pathognomonic)
• Primary causes:
  • Diverticulitis
  • Crohn's disease
  • Colorectal or pelvic malignancies
  • Radiation
  • Iatrogenic injury 3

Clinical Implications

• Fistula formation can cause serious complications including:

  • Fluid and electrolyte imbalances
  • Malnutrition
  • Sepsis
  • Mortality rates up to 30% in complicated cases 1

• The presence of active rectal Crohn's disease significantly complicates management and worsens prognosis 4

Diagnostic Approach

For suspected fistulas, the diagnostic approach depends on the location:

• Perianal/anorectal fistulas: MRI with intravenous contrast is the preferred imaging modality 3, 4
• Rectovaginal fistulas: Examination under anesthesia remains the gold standard when performed by an experienced surgeon 4
• Gastrointestinal fistulas: CT with intravenous contrast can detect fistulas with 76.5% sensitivity 4

Understanding the underlying etiology is crucial for appropriate management, as different causes require different treatment approaches.