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The Speed of Sound

The Speed of Sound

Subcutaneous Air on US Unearths Soft Tissue Infections

Butts, Christine MD

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doi: 10.1097/01.EEM.0000530449.44582.56
    ultrasound, subcutaneous air, soft tissue infection
    ultrasound, subcutaneous air, soft tissue infection:
    Fig. 2. A larger focus of subcutaneous air with visible dirty shadowing (arrow) emanating from the bright, hyperechoic air pockets. In contrast to the clean shadowing produced by objects such as gallstones, dirty shadows have a hazy appearance and an indistinct margin.
    Figure
    Figure

    Using ultrasound to evaluate suspected soft tissue infection is not a new idea. Much has been written on the ability (or inability) of the clinician to determine whether a drainable fluid collection is present based on history and physical exam alone. Ultrasound can definitely be helpful in this regard, possibly preventing a painful and time-consuming procedure that will produce no pus. It can also assist in evaluating the underlying anatomy.

    Necrotizing soft tissue infections can progress quickly and insidiously. What appears to be a simple cellulitis at first glance can hide a brewing disaster. Many patients may appear ill and have a fever or tachycardia, but some of them may initially have only skin erythema, induration, or tenderness, which can easily be written off as a superficial process.

    So what does ultrasound contribute to the evaluation of these patients? Certainly clinical judgment can identify those suspected of necrotizing infection and prompt a quick surgical evaluation. Many of these patients may linger, await further imaging or testing, and clinically deteriorate in that time.

    Figure
    Figure:
    Fig. 1. Subcutaneous air is visualized in this image as two bright, hyperechoic areas (one highlighted by the arrow, the other just to the right). Shadowing is not as clear in this image due to the small amount of air present.

    Ultrasound has played a central role in diagnosing several necrotizing infections at our institution over the past several months. A complex infection was a consideration in three of the four cases because of clinical clues such as history of diabetes, presence of tachycardia, and severe pain on exam. Ultrasound was able to identify subcutaneous air in each of these cases and expedite surgical consultation and subsequent management. In the fourth case, ultrasound was originally employed to assess for a drainable fluid collection, and subsequently found a small area of subcutaneous air. The patient in this case did not appear initially toxic, and ultrasound increased the level of concern.

    It's helpful to take an organized approach when assessing soft tissue infections, especially when the affected area is large. Small air or fluid collections can easily be missed. A high-frequency transducer is essential to gaining adequate resolution. Beginning outside of the affected area, the transducer should be moved in a grid-type fashion until the entire area has been examined. Focusing on areas of greatest tenderness is often helpful in identifying fluid collections. A complete discussion of ultrasound of the soft tissue is outside the scope of this article, but the appearance of subcutaneous air deserves special mention. Depending on the amount of air present, it typically appears as bright, hyperechoic areas within the soft tissue. (Fig. 1.) Dirty shadowing with an indistinct margin is often present and is best appreciated when larger amounts of air are present. (Fig. 2.)

    Necrotizing soft tissue infections can progress rapidly, and quick recognition is essential. Consider adding ultrasound when examining these patients. It can make a huge difference and expedite your diagnosis.

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