What investigation is best for suspected fistula-in-ano with symptoms of intermittent colicky pain, loose stool, perineal discharge, and a perineal to anal canal tract connection?

MRI is the Best Investigation for Suspected Fistula-in-Ano

For a patient with intermittent colicky pain, loose stool, perineal discharge, and suspected tract connection from perineum to anal canal, MRI is the most appropriate investigation as it provides superior visualization of fistulous tracts and surrounding tissue planes, which is essential for proper surgical planning and reducing recurrence rates. 1

Diagnostic Approach for Fistula-in-Ano

Clinical Presentation Analysis

• Intermittent colicky pain: Suggests active inflammation
• Loose stool: May indicate irritation of anal canal
• Perineal discharge: Classic sign of fistula with external opening
• Tract connection from perineum to anal canal: Defines the fistula-in-ano

Why MRI is Superior (Option B)

• MRI provides excellent soft tissue contrast for visualizing:

  • Fistulous tracts (primary and secondary)
  • Internal openings (critical for surgical planning)
  • Relationship to sphincter complex
  • Extensions and associated abscesses 1

• MRI has been shown to:

  • Influence surgical approach in 10% of cases 2
  • Reduce recurrence rates by identifying hidden tracts
  • Accurately classify fistulas according to surgical relevance 1

Why Other Options Are Less Appropriate

1. Colonoscopy (Option A):

   • Limited to visualizing the mucosal surface only
   • Cannot assess fistulous tracts through tissue planes
   • May identify internal opening but provides no information about tract course 3

2. Sigmoidoscopy (Option C):

   • Similar limitations to colonoscopy but with even more restricted examination area
   • Cannot evaluate the anatomical course of fistula or relationship to sphincters 3

3. Barium Enema (Option D):

   • Not recommended for fistula evaluation
   • Water-soluble contrast is preferred over barium when contrast studies are needed
   • Risk of barium spilling into peritoneal cavity or extraperitoneal spaces 1
   • Poor sensitivity for detecting fistulous tracts

MRI Protocol Considerations

• T2-weighted sequences: Best for identifying fluid-filled tracts
• Fat-suppressed T2-weighted sequences: Improve visualization of inflammatory changes
• IV contrast-enhanced fat-suppressed T1-weighted sequences: Help differentiate active from inactive tracts 1
• Diffusion-weighted imaging: Improves fistula conspicuity (100% sensitivity vs 91.2% for T2-weighted sequences) 1

Alternative Imaging Options (When MRI Unavailable)

1. Endoanal Ultrasound:

   • Reported sensitivity of 92%, specificity of 100% for perianal fistula 1
   • Limited field of view and depth penetration
   • Less accurate for extrasphincteric and suprasphincteric tracts
   • Practical limitations due to patient discomfort in acute setting 1

2. CT with IV Contrast:

   • Useful in acute setting for abscess detection (77% sensitivity) 1
   • Limited by poor spatial resolution in pelvis
   • Difficulty differentiating between fistula tract and inflammation 1

Clinical Implications

• Accurate preoperative imaging reduces recurrence rates by identifying all tracts
• Proper classification of fistula type guides surgical approach
• Identification of secondary extensions prevents incomplete treatment
• Detection of underlying conditions (e.g., Crohn's disease) that may affect management 1

MRI remains the gold standard for evaluating perianal fistulas due to its superior soft tissue contrast, multiplanar capabilities, and ability to detect occult sepsis that might be missed during surgical exploration 1, 4.