Is retrocaecal appendicitis better visualized with ultrasound (USS) or computed tomography (CT) scan?

Visualization of Retrocaecal Appendicitis on USS vs CT

CT scan is superior to ultrasound for visualizing retrocaecal appendicitis, with significantly higher sensitivity and specificity compared to ultrasound, which often fails to visualize the retrocaecal appendix due to its position behind the caecum. 1

Diagnostic Performance of Imaging Modalities for Appendicitis

Ultrasound (USS) Limitations

• USS has variable sensitivity and specificity for appendicitis in general, but particular limitations for retrocaecal appendicitis:
  • Limited visualization of retrocaecal appendix due to its anatomical position behind the caecum 1
  • High rates of non-visualization (up to 473/682 or 69% in one study) 2
  • Operator-dependent results 3
  • Particularly challenging in obese patients 3

CT Scan Advantages

• Superior diagnostic performance for retrocaecal appendicitis:
  • Overall sensitivity of 95-97% and specificity of 94% for appendicitis 4, 3
  • Particularly valuable for retrocaecal appendicitis with sensitivity approaching 96-98% 1
  • Able to visualize the appendix regardless of its anatomical position 1
  • Can detect complications such as perforation, abscess formation 4

Evidence Supporting CT for Retrocaecal Appendicitis

In a dedicated study examining retrocaecal appendicitis specifically, researchers found that:

• Of 12 patients requiring CT scan after equivocal USS findings, 7 had retrocaecal appendicitis that was missed on ultrasound 1
• When USS was combined with CT for select cases (including retrocaecal position), diagnostic accuracy improved significantly from 83% to 93% (p=0.0484) 1
• Sensitivity improved from 81% with USS alone to 96% (p=0.0014) when CT was added for difficult cases like retrocaecal appendicitis 1

Recommended Diagnostic Algorithm

Based on the most recent guidelines:

1. Initial imaging with USS for all patients with suspected appendicitis 4, 3

   • If appendix is clearly visualized and normal → exclude appendicitis
   • If appendix is clearly visualized and inflamed → diagnose appendicitis
   • If appendix is not visualized or findings are equivocal → proceed to CT

2. Proceed to CT scan when:

   • USS is equivocal or non-diagnostic 4
   • Clinical suspicion for retrocaecal appendicitis exists (based on exam findings) 1
   • Patient characteristics make USS technically challenging (obesity) 3

3. Consider MRI as an alternative to CT in:

   • Pregnant patients 4, 3
   • Children where radiation exposure is a concern 4, 3

Common Pitfalls and Caveats

• Relying solely on USS when the appendix is not visualized can lead to missed diagnoses, particularly with retrocaecal appendicitis 1
• Delaying CT when USS is equivocal increases the risk of perforation and complications 3
• The position of the appendix should be considered when interpreting negative USS results, with lower confidence for excluding retrocaecal appendicitis 1

In summary, while USS remains the recommended initial imaging modality for suspected appendicitis due to lack of radiation, CT scan is significantly superior for visualizing retrocaecal appendicitis and should be promptly utilized when USS is non-diagnostic or when retrocaecal position is suspected clinically.