Tips for performing a soft tissue ultrasound for necrotizing fasciitis.
You are working in the emergency department when you see a 29-year-old obese female present with right groin pain and swelling beginning three days prior. She states she came to the emergency department as her pain was getting significantly worse. She denies fever, vomiting or drainage from the area. The patient has a history of smoking, but no other known medical problems.
On physical exam she is in moderate distress. Her temperature is 99.1°F, blood pressure is 127/85, and heart rate is 84 beats per minute. She is noted to have right inguinal redness, swelling and tenderness with palpation. The area is about 4 x 6 centimeters with associated induration. The area of tenderness extends about three centimeters beyond the area of induration. There is no fluctuance, and no active drainage.
After examining the patient, you are most concerned for cellulitis versus abscess. You decide to perform a bedside soft tissue ultrasound. You place the ultrasound probe over the area of redness and scan in two planes orthogonal to each other. The ultrasound shows a well circumscribed hypoechoic fluid collection with posterior acoustic enhancement, consistent with abscess. There is also evidence of surrounding cellulitis.
This ultrasound image shows a well circumscribed hypoechoic fluid collection (*) with posterior acoustic enhancement (arrow). This is consistent with an abscess.
In your mind you are already thinking about the management plan, which includes a simple incision and drainage procedure, and discharging the patient with oral antibiotics. But as you sweep through the rest of her inguinal region you notice multiple hyperechoic foci with posterior dirty shadow indicative of subcutaneous gas.
This ultrasound image shows a well circumscribed hypoechoic fluid collection (A) with posterior acoustic enhancement (B). In addition, there is subcutaneous thickening, and subcutaneous gas (arrow). Subcutaneous gas is visualized as highly echogenic (or bright) areas with posterior dirty shadowing (*). Notice how the dirty shadowing obscures the posterior acoustic enhancement. This ultrasound is consistent with necrotizing fasciitis.
Concerned for necrotizing fasciitis, you start intravenous antibiotics right away and consult surgery for operative debridement. The patient is taken immediately to the operating room and has 22 centimeters of tissue resected. She is continued on intravenous antibiotics throughout her hospital stay, and discharged in good condition nine days later.
Pearls and Pitfalls for performing a Soft Tissue Ultrasound for Necrotizing Fasciitis
- Necrotizing fasciitis is a rapidly progressing infection of the skin and soft tissue that results in significant morbidity and mortality. The presentation of necrotizing fasciitis can be similar to cellulitis or a simple abscess, as in this case.
- Bedside ultrasound has moderate sensitivity and high specificity for necrotizing fasciitis, reported at 88% and 93%, respectively. It can be performed immediately at the bedside and can expedite surgical intervention if necrotizing fasciitis is found.
- Classic findings on ultrasound include subcutaneous thickening, fascial irregularities, fascial fluid and subcutaneous air.
- With a sensitivity of 88% it is important to remember that ultrasound cannot rule out the diagnosis.
- Computed tomography (CT) and magnetic resonance imaging (MRI) may be used as adjuncts to clinical exam, however, these modalities are time consuming and may delay definitive treatment.
- Ultrasound should be performed first, and if equivocal, additional imaging and/or surgical consultation may be warranted.
Video 1:
This video shows necrotizing fasciitis. There is evidence of a subcutaneous hypoechoic fluid collection with a large amount of subcutaneous gas. Subcutaneous gas is visualized on this ultrasound as multiple highly echogenic foci with posterior dirty shadowing.
