There are thousands of existential questions. To be or not to be? Coke or Pepsi? Crunchy or creamy? But these questions often boil down to just one for emergency physicians in clinical practice: to drain or not to drain?
Patients with evidence of soft tissue infection, such as induration, pain, and overlying erythema, can be difficult to assess clinically. Differentiating between cellulitis and abscess based on physical exam alone is often unreliable. Performing an incision and drainage of a suspected abscess in many cases may be a major undertaking requiring sedation and a large number of resources.
Ultrasound is a powerful tool in evaluating the soft tissue and can easily identify signs of cellulitis versus abscess. Normal soft tissue is uniform, as I mentioned last month, with clear delineations between the layers. Noting deviations from this pattern will allow for recognition of soft tissue infection. The progression from cellulitis to abscess is not linear, and the entire area that is involved should be scanned carefully because small pockets can be missed.
There is frequently only thickening of the subcutaneous layer in early cellulitis. (Image 1.) This may be subtle or dramatic depending on the amount of swelling present. This is especially notable when comparing an area of pain, swelling, or erythema with the unaffected areas.
Soft tissue edema will then progress in more advanced cellulitis, with anechoic (black) or hypoechoic (gray) areas visible within the superficial layers. (Image 2.) This is often referred to as cobblestoning, a reference to its appearance like a cobblestone street, with the edema representing the cement between the stones (thickened bands of tissue).
After this point, a localized fluid collection (or more than one) may develop and can be seen within the area of soft tissue thickening and cobblestoning. (Image 3.) It may appear anechoic, but purulent collections are typically more hypoechoic or appear to have a mixed density. Collections can appear irregular (as in most abscesses), but they may also appear rounded or regular, particularly if a cyst that was initially present became secondarily infected.
Determining whether a discrete fluid collection is present is straightforward, but a few pitfalls may cause a false-negative scan. Failure to thoroughly scan the entire area is a common mistake. Covering the entire area in a systematic way is helpful to avoid overlooking a small collection. Another technique that can be helpful is to focus on the area of greatest pain, taking care to avoid increasing the discomfort for the patient. My experience is that this is where you will find a localized fluid collection.
Next month: Ultrasound of the tendons and muscles.
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