Diagnostic Approach to Fistulae
Imaging Modalities
- MRI is the most accurate imaging modality for diagnosis and classification of fistulae, with a sensitivity of 81-100% and specificity of 67-100% for visualizing complete fistula anatomy, extensions, and abscesses, and should be the first-line diagnostic test, as recommended by the European Crohn's and Colitis Organisation and the American College of Radiology 1, 2, 3
- MRI with IV contrast is essential for comprehensive assessment, providing superior visualization of fistula tracts, extensions, and associated abscesses, and helping to differentiate between active and inactive tracts, as recommended by the European Crohn's and Colitis Organisation and the American College of Radiology 1, 2, 3
- A multichannel phased array body coil is recommended for better visualization of extent, rather than an endoanal coil, as suggested by the American College of Radiology 3
- Transrectal/Endoanal Ultrasound is a reasonable alternative when MRI is unavailable or contraindicated, but has limitations, including lower sensitivity and specificity compared to MRI, and is inferior for complex or deep-seated fistulae, as recommended by the European Crohn's and Colitis Organisation and the American College of Radiology 1, 3
- CT scan and fistulography have limited value in fistula assessment, with CT scan limited by poor spatial resolution in the pelvis and difficulty differentiating between fistula tracts and inflammation, and fistulography having very low accuracy (as low as 16%), as stated by the American College of Radiology 3, 4
- The combination of T2-weighted sequences and IV contrast-enhanced T1-weighted sequences provides the highest concordance with surgical findings, with diffusion-weighted imaging increasing fistula detection to 100%, as recommended by the American College of Radiology 3
Clinical Evaluation
- A complete physical assessment of the fistula should be performed and findings documented, as recommended by the American Society of Nephrology 5
- Examination Under Anesthesia (EUA) complements imaging findings and provides opportunity for both staging and initial treatment, as suggested by the European Crohn's and Colitis Organisation 2
- Combining MRI with EUA improves diagnostic accuracy, as recommended by the European Crohn's and Colitis Organisation 1
- Initial assessment should include clinical evaluation and endoscopic examination of the rectum, followed by imaging with MRI and IV contrast, and surgical planning with EUA and potential immediate therapeutic intervention, as recommended by the Gut guideline 6
Management
- Medical therapy (anti-TNF agents) is often needed in conjunction with surgical drainage, as recommended by the European Crohn's and Colitis Organisation 1
- Placement of a non-cutting seton for complex fistulae is recommended to establish drainage, as suggested by the European Crohn's and Colitis Organisation 2
- Reassessment with clinical evaluation plus MRI and endoscopic examination of rectum is recommended, as suggested by the European Crohn's and Colitis Organisation 1
- MRI significantly impacts treatment decisions and outcomes by providing preoperative planning, identifying complex cases, and reducing recurrence rates, with complete identification of all tracts and extensions helping to ensure complete treatment, as recommended by the American College of Radiology and the European Crohn's and Colitis Organisation 3, 1