What is the best imaging modality to diagnose a fistula from an anal fissure?

MRI is the Best Imaging Modality for Diagnosing Fistulas from Anal Fissures

MRI is the superior imaging modality for diagnosing fistulas from anal fissures due to its high sensitivity (81-100%) and specificity (67-100%), with the ability to accurately identify fistula tracts, extensions, and associated abscesses. 1

Comparison of Imaging Modalities

MRI

• Diagnostic accuracy: Sensitivity 81-100%, specificity 67-100% 1
• Advantages:
  • Superior visualization of fistula tracts and extensions
  • Excellent detection of associated abscesses
  • Accurate identification of the internal opening (97% accuracy) 2
  • Better visualization of surrounding soft tissues
  • Ability to differentiate between active and inactive tracts 2
  • Superior for complex, supralevator, and extrasphincteric fistulas 1

Endoanal Ultrasound (EAUS)

• Diagnostic accuracy: Sensitivity 87-92%, specificity 43-64% 2, 3
• Advantages:
  • Good for intersphincteric and transsphincteric fistulas (88-93% accuracy) 2
  • Lower cost and greater availability than MRI
  • Can be performed in outpatient setting 4
• Limitations:
  • Limited field of view and depth of penetration
  • Lower accuracy for extrasphincteric and suprasphincteric tracts (50-67%) 2
  • Patient discomfort, especially in acute settings 2
  • Highly operator-dependent 2

Other Modalities

• CT scan: Limited by poor spatial resolution in the pelvis and difficulty differentiating between fistula tracts and inflammation 2
• Fistulography: Very low accuracy (as low as 16%), now largely obsolete 2
• Radiography: Not useful for fistula assessment 2

Optimal MRI Protocol

For best results, MRI for anal fistula should include:

• 1.5T or 3T scanner with multichannel phased array body coil 1
• T2-weighted fast spin-echo (FSE) sequences with fat suppression
• T1-weighted sequences with IV contrast and fat suppression
• Diffusion-weighted imaging (DWI) - increases fistula detection to 100% 1, 2

The combination of T2-weighted sequences and IV contrast-enhanced T1-weighted sequences provides the highest concordance with surgical findings 2, 1.

Clinical Decision Algorithm

1. First-line imaging: MRI with IV contrast

   • Provides comprehensive assessment of fistula anatomy
   • Helps in surgical planning by identifying all tracts and extensions
   • Particularly valuable for suspected complex fistulas

2. When MRI is unavailable or contraindicated:

   • Endoanal ultrasound is a reasonable alternative
   • Consider 3D acquisitions for better visualization 2
   • May be supplemented with hydrogen peroxide injection into the external opening

3. Special considerations:

   • For suspected Crohn's disease-related fistulas: MRI is strongly preferred 2
   • For recurrent fistulas: MRI provides better assessment of scarring and active disease 1
   • For simple, superficial fistulas with clear external opening: Clinical examination may be sufficient before proceeding to surgery

Common Pitfalls to Avoid

• Relying solely on clinical examination: Up to 25-40% of fistulas have extensions that are not apparent on physical examination
• Using radiography or conventional fistulography: These modalities have very limited value in fistula assessment 2
• Failing to use IV contrast with MRI: IV gadolinium enhances visualization of active inflammation and helps differentiate abscesses from granulation tissue 2, 1
• Not considering patient factors: In acute settings with severe pain, endoanal ultrasound may be poorly tolerated 2
• Missing supralevator extensions: These are better visualized with MRI than endoanal ultrasound 2

In conclusion, while both MRI and endoanal ultrasound have roles in diagnosing fistulas from anal fissures, MRI offers superior comprehensive assessment, particularly for complex cases, and should be considered the first-line imaging modality when available.