What is the best management for a patient with Crohn's disease on infliximab (chimeric monoclonal antibody against tumor necrosis factor-alpha) presenting with a perianal discharging fistula, induration, and low-grade fever for 3 weeks?

Management of Perianal Fistula in Crohn's Disease on Infliximab

MRI is the best initial management for a patient with Crohn's disease on infliximab presenting with a perianal discharging fistula, induration, and low-grade fever for 3 weeks. 1

Rationale for MRI as First-Line Management

Pelvic MRI is essential in this case for several reasons:

1. Abscess Detection: The patient's presentation with induration and low-grade fever strongly suggests a possible perianal abscess complicating the fistula. MRI is crucial to identify any abscesses that require drainage before adjusting medical therapy 1, 2

2. Fistula Tract Mapping: MRI provides detailed anatomical information about the fistula tract(s), which is essential for planning appropriate surgical intervention 1

3. Gold Standard Imaging: The British Society of Gastroenterology guidelines strongly recommend pelvic MRI as "an important adjunct to clinical assessment" in evaluating fistulizing perianal Crohn's disease 1

4. Preventing Sepsis: Starting or adjusting anti-TNF therapy without ruling out and draining abscesses could lead to overwhelming septicemia 2

Management Algorithm

Step 1: Imaging and Assessment

• Perform pelvic MRI to assess fistula anatomy and identify any abscesses 1
• Combine with proctosigmoidoscopy to evaluate rectal inflammation 2

Step 2: Based on MRI Findings

• If abscess present:

  • Surgical drainage under antibiotics is required 1, 2
  • Statement 57 emphasizes: "Intra-abdominal abscesses complicating Crohn's disease may be treated initially with intravenous antibiotics and where possible radiological drainage" 1
  • Anti-TNF therapy should only be started after abscesses have been treated 1

• If no abscess but active fistula:

  • Examination under anesthesia (EUA) by an experienced colorectal surgeon for possible seton placement 1
  • Setons improve healing rates and reduce recurrence when placed prior to anti-TNF therapy 1

Step 3: Medical Management Optimization

• After abscess drainage (if present):
  • Check infliximab trough levels and optimize dosing 1
  • Higher infliximab doses (target levels >10 μg/mL) are associated with better response in perianal fistulizing disease 1
  • Consider combination therapy with immunomodulators (azathioprine or 6-mercaptopurine) to improve efficacy 2

Why Other Options Are Less Appropriate

1. Increasing infliximab dose without imaging (Option A): Potentially dangerous as an undrained abscess could lead to sepsis when anti-TNF therapy is intensified 1, 2

2. Antibiotics then assess infliximab level (Option B): While antibiotics are part of treatment, they should not delay proper imaging and potential surgical drainage. Antibiotics alone have high relapse rates for complex fistulas 1

3. Swab from discharge (Option C): Insufficient for detecting deeper abscesses or mapping fistula tracts, which are essential for proper management 1

Important Considerations

• Multidisciplinary approach: After MRI, management should involve gastroenterologists and colorectal surgeons 1, 2

• Rectal inflammation: Assessment of rectal mucosa is crucial as outcomes are worse for perianal fistulizing disease associated with rectal inflammation 1

• Long-term management: Complete healing may not be achieved in all patients despite optimal management, with studies showing persistent fistulas in up to 58% of cases 2

• Caution with skin tags: Never excise perianal skin tags in Crohn's disease as this can lead to chronic, non-healing ulcers 2

By prioritizing MRI as the first step, you ensure proper assessment of the fistula and any associated abscess, which is critical before making decisions about adjusting medical therapy or performing surgical interventions.