What imaging modalities are recommended for patients with anal fissures when imaging is warranted?

Imaging for Anal Fissures

For typical anal fissures, no imaging is needed—diagnosis is clinical. For atypical fissures (lateral location, multiple fissures, or suspicious features), perform endoscopy, CT scan, MRI, or endoanal ultrasound only when you suspect inflammatory bowel disease, colorectal cancer, or occult perianal sepsis. 1

When Imaging is NOT Indicated

• Typical acute anal fissures require no imaging investigations whatsoever—the diagnosis is made by history and physical examination alone 1
• Approximately 90% of anal fissures occur in the posterior midline, and these typical presentations do not warrant any radiological workup 1, 2
• Anterior fissures (10% in women, 1% in men) are still considered within the typical spectrum and generally do not require imaging 1, 2

When Imaging IS Indicated: The Atypical Fissure

Atypical features that should trigger investigation include: 1, 2

• Lateral location of the fissure (not posterior or anterior midline)
• Multiple fissures present simultaneously
• Failure to heal with 8 weeks of appropriate conservative management
• Associated symptoms suggesting systemic disease (weight loss, fever, diarrhea, constitutional symptoms)

Specific Imaging Modalities Based on Clinical Suspicion

When you encounter an atypical fissure, your imaging choice depends on what underlying pathology you suspect: 1

For Suspected Inflammatory Bowel Disease (IBD)

• Endoscopy (colonoscopy) is the primary investigation to visualize mucosal disease and obtain biopsies 1, 2
• MRI pelvis can identify perianal complications of Crohn's disease, including occult fistulas and abscesses 1

For Suspected Colorectal or Anal Cancer

• Endoscopy with biopsy is essential for tissue diagnosis 1, 2
• MRI pelvis provides superior soft tissue resolution for staging anal canal carcinoma 1, 3

For Suspected Occult Perianal Sepsis

• MRI pelvis is the gold standard, with sensitivity ranging from 81-100% and specificity from 67-100% for detecting perianal abscesses and fistulas 1
• CT scan offers advantages of rapid acquisition and widespread availability, though it has lower spatial resolution in the pelvis compared to MRI 1
• Endoanal ultrasound can be considered but requires special expertise and is often precluded by severe anal pain in the acute setting 1

MRI Technical Considerations (When Ordered)

If MRI is indicated for atypical fissures with suspected complications: 1

• Multi-channel phased array body coil is preferred over endoanal coil for better visualization of extent and patient tolerance
• Gadolinium-based IV contrast is preferred because active inflammation enhances avidly and abscesses show rim enhancement
• Diffusion-weighted sequences increase conspicuity of fistulae (100% sensitivity) and discriminate between inflammatory mass and abscess (100% sensitivity, 90% specificity)

Critical Clinical Pitfall

Do not order imaging reflexively for every anal fissure—this represents unnecessary healthcare utilization and patient expense 1. The key is recognizing the red flags: lateral or multiple fissures, failure to heal, or systemic symptoms suggesting IBD, sexually transmitted infections (HIV, syphilis, herpes), tuberculosis, or malignancy 1, 2. Only then should you pursue targeted investigations based on your specific clinical suspicion.