What is the most common type of internal fistula that develops in patients with Crohn's disease (CD)?

Internal Fistulas in Crohn's Disease

The most common type of internal fistula that develops due to Crohn's disease is enteroenteric (small bowel to small bowel) fistulation. 1

Types and Prevalence of Fistulas in Crohn's Disease

Internal fistulas occur in approximately 15% of patients with Crohn's disease 2. The distribution of fistulas varies based on disease location:

• Enteroenteric fistulas (small bowel to small bowel): Most common type of internal fistula
• Enterocolic fistulas: Connect small bowel to colon
• Enterovesical fistulas: Connect intestine to bladder
• Rectovaginal fistulas: Connect rectum to vagina

The prevalence of fistulas correlates strongly with disease location:

• 12% in patients with ileal Crohn's disease
• 15% in patients with ileocolonic disease
• 41% in patients with colonic disease with rectal sparing
• 92% in patients with colonic disease with rectal involvement 1

Clinical Presentation and Diagnosis

Internal fistulas in Crohn's disease typically present with the following characteristics:

• Enteroenteric fistulas are often asymptomatic 1, 3
• They usually arise from within or just proximal to a stricture 1
• They do not typically present with florid sepsis or during disease flares
• They often form complex, asterisk-shaped or "clover-leaf" patterns when multiple tracts are present 1

Diagnostic Approach

Diagnosis of internal fistulas requires multi-modal assessment:

1. Cross-sectional imaging:

   • MR enterography and CT enterography have similar and moderately high accuracy for detecting penetrating Crohn's disease 1
   • These can detect unsuspected fistulas in patients without obvious symptoms 1

2. Endoscopy: Important to assess for concomitant inflammation, especially rectal involvement 4

Management Principles

Management depends on the type of fistula and associated symptoms:

1. Asymptomatic enteroenteric fistulas:

   • Usually require no treatment 1, 3, 5
   • Can be monitored without immediate intervention

2. Symptomatic internal fistulas (especially enterovesical, duodenocolic, gastrocolic):

   • Often require surgical intervention when causing persistent or severe symptoms 3, 5
   • Medical therapy may be attempted first, particularly for enterovesical fistulas 2

3. Medical therapy options:

   • Anti-TNF agents have shown the most promising results 2
   • Azathioprine/6-mercaptopurine may be beneficial 3
   • Limited high-quality evidence exists specifically for internal fistulas 5

4. Surgical considerations:

   • Joint medical and surgical discussion is required following treatment of associated abscesses 1
   • Interval surgical resection is not always necessary 1

Key Points to Remember

• Enteroenteric fistulas are the most common type of internal fistula in Crohn's disease
• Many enteroenteric fistulas are asymptomatic and can be managed conservatively
• Fistulas with more serious complications (obstruction, abscess formation, connection to critical organs) require more aggressive intervention
• Management should involve multidisciplinary input from gastroenterologists and surgeons
• The presence of active rectal Crohn's disease significantly complicates management and worsens prognosis 4

Unlike perianal fistulas, which have been extensively studied, internal fistulas have less robust evidence guiding their management, requiring individualized assessment of risks and benefits when determining treatment approach.