The Best Sonographer for Acute Appendicitis

The Master is none other than the Patient herself, of course.

We passed the probe to her, asked her to put it at the point of maximal tenderness, and watched the ultrasound monitor. When she stopped, we recognized the offending lesion in its short axis. The red arrow points to the peritoneum; while the round structure (yellow arrow) measuring just over 1cm was most likely the inflammed appendix.

We took over the probe to visualize the entire structure, both in the short and long axis, to confirm that it is a blind ending tube (i.e. its the appendix)

The appendix can be traced from tip to caecum. The first criteria for acute appendicitis is size: diameter ≥6mm. Secondly, it must also be non-compressible, as seen here:

Similar to DVT scans, compression should be done in short axis; least you run the possibility of “sliding off” rather than “compressing” in the long axis. With these two criteria, appendicitis is ruled in. She has other supporting signs, such as increased flow in the appendiceal wall with colour Doppler.

Caveat: Note that the hyperemia will disappear as the appendix becomes more ischemic.


The patient is the best (in fact, the only) person to know where it hurts most. It is fascinating to let go of the probe, into the hands of the patient, as they scan to locate the inflamed appendix for you.

Once the point of maximal tenderness is identified, all you need to do is to rotate the probe about this point to identify the appendix, both in long and short axis.

But we know that the sensitivity is only 83%.

Why do we miss acute appendicitis on ultrasound? Operator, machine and patient factors aside, there are 5 "appendiceal" reasons for a false negative scan. The first 3 may be correctable with good ultrasound techniques; you'll need CT for the rest.

  1. Appendiceal tip appendicitis. Localised inflammation at the tip. So make sure you scan the entire structure.

  2. Gross distension. A hugely distended appendix can mimic the small bowel: again, important to scan the entire structure, from base to blind ending tip.

  3. Retrocecal location. Turn the patient to the left lateral decubitus and hunt for it from the coronal plane, behind the caecum.

  4. Gas-filled. Difficult to visualize, as air will obscure the distal wall.

  5. Perforated. ​With perforation, it loses is usual appearance, may be “compressible” (i.e. pus can be decompressed through the perforation) and got lost in surrounding tissue with marked inflammatory changes.


  1. Give the probe to your patient with suspected appendicitis; he or she knows exactly where it hurts.

  2. The two sonographic criteria for appendix (i.e. a blind ending tube) that is inflamed are:

  • ≥6mm diameter

  • Noncompressible​

  1. Be aware of false negative scans. Always use good ultrasound techniques; CT when necessary.


Doria AS, Moineddin R, Kellenberger CJ, Epelman M, Beyene J, Schuh S, Babyn PS, Dick PT. US or CT for Diagnosis of Appendicitis in Children and Adults? A Meta-Analysis. Radiology. 2006 Oct;241(1):83-94.


Dr. Toh Hong Chuen

Acute and Emergency Care Centre, Khoo Teck Puat Hospital. Singapore



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