SonoApp & Study: Cellulitis vs Abscess; US more sensitive than CT for soft tissue abscess

A patient comes into your emergency department or outpatient clinic that has a painful red area on their skin:

 -from Medicineo blog

…and you wonder whether its a superficial cellulitis, or if it’s a pus-filled abscess – and if it is an abscess, then how deep is it? how long is it? how loculated is it? Physical exam goes just so far with this, and I have been surprised before when I performed a needle aspirate, got nothing, then an I&D done the next day after I wrongly discharged the patient that had a huge loculated abscess (this, of course, was a long time ago before ultrasound saved my life). The great thing is that Romolo Gaspari and friends recently published a study on how ultrasound is more sensitive than CT for soft tissue abscesses. They state that : out of 65 patients that received both an US and a CT for suspected abscess, “US demonstrated a sensitivity and specificity for the diagnosis of abscess of 96.7% (87.0% to 99.4%) and 85.7% (77.4% to 88.0%) respectively. The overall sensitivity and specificity of CT for the diagnosis of an abscess was 76.7% (65.5% to 82.8%) and 91.4% (81.8% to 96.7%) respectively.” US more sensitive, CT more specific – but also that US was better in the details of the abscess than CT. Now I know what you’re thinking – I didn’t believe it either – CT MUST be more sensitive, it shows everything – with all that radiation…..

Well, ultrasound is good enough – without the radiation – and you should know how to tell the difference between cellulitis and an abscess. Ben Squire, Chris Fox, and others did a great study discussing this a few years back that has been the quoted literature, called the ABSCESS study (clever, I know) in that they showed that after a quick training session, physicians were able to correctly detect abscess by bedside ultrasound.

Using the linear probe, you slide over and visualize the entire length of the region of interest, ensuring adequate depth (which is very important, as an abscess can be quite deeper than you think and the default setting for the linear probes are more shallow than sometimes required). While scanning through the area, you apply graded compression over the area.

Cellulitis by bedside ultrasound looks like a bunch of rocks bunched up next to each other – termed the cobblestoning appearance of cellulitis without movement in between. If there is an abscess, this pressure will make the pus within the tissue move around, which helps in diagnosing an abscess – We call it Pus-stalsis.  If there are loculations, it can easily be mistaken for cellulitis as the loculations will have echogenicity (brightness) and may look like cobblestoning, but instead, loculations are more of a cluster of grapes appearance with small movements of pus in between the loculations.

The images below shows the cobblestoning appearance of cellulitis.

The below image shows cellulitis cobblestoning, but also more of a cluster of grapes appearance to suggest loculations. There was no compression done to evaluate whether the dark areas have “pus-stalsis” but I sure would want to….

The below image is of a superficial small abscess without the superficial cellulitis pattern of cobblestoning – so just an abscess – drain and done.

The below image is of an abscess showing pus-stalsis with compression within the abscess walls, minimal loculations

The below image shows another abscess but with more loculations shown as bright hyperechoic linear areas within the abscess wall and with narrow areas of pus-stalsis in between when compression applied:

So, when you have a patient with a red and painful lesion and you are evaluating for abscess, dont hesitate to look with your bedside ultrasound, ensure adequate depth to know how deep it really is if you see an abscess, look for cobblestoning to suggest cellulitis, and apply pressure over the site to evaluate for pus-stalsis (even if it looks like cobblestoning).