A wide spectrum of physicians evaluate patients with skin and soft tissue infections. These infections include cellulitis, abscesses, necrotizing fasciitis, and pyomyositis. The diagnosis of cellulitis by physical examination alone is usually straightforward when classic signs such as erythema, warmth, and tenderness are present. Similarly, determining that a subcutaneous abscess exists is simple when fluctuance or focal skin necrosis is present.
Physical examination findings can be misleading, however, when a cutaneous abscess is small, deep to the skin surface, early in its formation, in an area of thick subcutaneous fat, or where there is induration or preexisting scar tissue. An occult abscess may be present when the clinical presentation seems consistent with simple cellulitis. In the case of pyomyositis, with the abscess formation in the muscle belly itself, palpation alone for diagnosis is not sufficient. Ambiguous clinical findings may direct the clinician away from performing a much-needed drainage procedure or obtaining surgical consultation, making point-of-care ultrasound (POCUS) an immensely valuable tool for assessing soft tissue infections at the bedside.
Ultrasound is increasingly used in emergency and ambulatory care settings to detect occult abscesses, facilitate abscess drainage, and help providers avoid an unnecessary procedure in patients without true abscess. Ultrasound may allow timely diagnosis of deeper and/or more rapidly progressive infections, which necessitate prompt intervention.
Other imaging modalities may be helpful in characterizing soft tissue infections, but they are not available at the patient’s bedside. Both computed tomography (CT) and magnetic resonance imaging (MRI) provide higher sensitivity for detection of soft tissue gas in necrotizing infections,1 but they are more time consuming to perform and require potentially unstable patients to leave the emergency department (ED).
The clinical indications for the use of ultrasound in the management of soft tissue infections include the following:
Detection of occult subcutaneous, deep tissue, perivascular, or intravascular abscesses when clinical findings are uncertain.
Localization of the optimal site for incision and drainage or aspiration of an abscess.
Assessment of a soft tissue inflammatory process to determine if it is the result of an adjacent arthritis or tendonitis rather than cellulitis.
Detection of rapidly progressive soft tissue infection that necessitates prompt surgical consultation.
DETECTION OF OCCULT SUBCUTANEOUS ABSCESS
The ability of ultrasound to detect soft tissue abscesses and guide subsequent incision and drainage has been appreciated since the 1980s.2–6 In reports focusing on injection drug users, particularly those with inflammatory lesions of the groin, ultrasound has been shown to successfully differentiate cellulitis from abscess, in addition to being useful for detecting adenitis, septic thrombophlebitis, and pseudoaneurysm.2,5 Ultrasound has also been found to be of value in the diagnosis and treatment of odontogenic facial abscesses7 and in the assessment of patients with buccal space swelling.8