Predictive value of indirect ultrasound signs for low risk of acute appendicitis in paediatric patients without visualisation of the appendix on ultrasound

Have you ever ordered an ultrasound on a patient and had the report come back stating “appendix not visualised”? This seems to be a common and frustrating occurrence. We have historically considered this to be a result that doesn’t change our thought process, and we throw the report in the bin.

But what about the indirect signs of appendicitis that may be seen on ultrasound? If the appendix is inflamed, often so are the surrounding structures. Can the presence or absence of indirect signs change how we think about the post-test probability of disease?

Researchers from a single centre in beautiful Valladolid, Spain conducted a retrospective study of paediatric patients aged 3-13 who were thought possibly to have appendicitis and had a diagnostic workup.

There were 1,756 patient encounters. They used the Alvarado score to risk stratify the kids into low, intermediate or high risk. The positive appendicitis rate was ultimately found to be 18%.

A total of 60% of children had an ultrasound performed. About half the time the appendix was not visualised. In this cohort of patients, they went further to look at the ultrasound report for possible indirect signs of appendicitis.

Indirect ultrasound findings independently associated with appendicitis were a small amount of free fluid (OR 5), abundant free fluid (OR 30), and inflammation of the periappendiceal fat (OR 7).

The authors conclude that the absence of these indirect signs ruled out appendicitis in patients with low or intermediate suspicion with a sensitivity of 85% and a negative predictive value of 99% even when the appendix was not visualised. The missed cases tended to be patients that presented early after onset of pain as they likely had less inflammation present.

Some might argue that 85% sensitivity is not good enough. But appendicitis is not aortic dissection. I think a lower sensitivity is acceptable so that we are not causing harm by over investigation and admission.  A short delay to diagnosis of appendicitis is not likely to be harmful especially in early presenters that don’t have ultrasound signs of inflammation. Obviously, we must give good discharge instructions and return precautions for those that we send home.  

To be complete, there were a lot of limitations to this retrospective study. But there are other studies out there with similar results and conclusions.

Next time you get an ultrasound report of “appendix not visualised” make sure you read further. Indirect signs might be your friend.

Covering:

Pernía J, Cancho T, Segovia I, de Ponga P, Granda E, Velasco R. Predictive values of indirect ultrasound signs for low risk of acute appendicitis in paediatric patients without visualisation of the appendix on ultrasound. Emerg Med J. 2024 Jul 22;41(8):475-480. doi: 10.1136/emermed-2023-213466. PMID: 38729752. [link to article]