The Case
A 64 year old male with a history of ESRD and diabetes mellitus presents with 1 day of progressively worsening right calf pain. He reports fevers, nausea, and fatigue. He has noticed a small amount of swelling and an area of erythema to the posterior lateral aspect of his right lower extremity. He is hemodynamically stable with a blood pressure of 124/82 and a heart rate of 93. His body temperature measures 99.3.
Exam of the patient demonstrated a non toxic appearing male with a well demarcated 10cm x 5cm area of erythema on his right lower extremity. There is warmth to the area, but no fluctuance or crepitus. POCUS obtained the images below.
Ultrasound of the area demonstrates a large area of abscess extending from just below the knee down into the achilles. There are areas of shadowing cast by the air within the tissues and necrotic debris can be seen swirling within the abscess cavity when downward pressure is applied by the operator through the probe. CT of the extremity confirmed US findings and the patient was emergentely taken to the OR for debridement and washout. The necrotizing soft tissue infection was found to have invaded the achilles, with extension into the gastroc and soleus muscle beds. The achilles was extensively involved and ruptured during the debridement of the infection due to the weakened necrotic tissue.
Discussion
Necrotizing cellulitis, myositis, and fasciitis are all versions of necrotizing soft tissue infections, or NSTI. The case presented above demonstrates the challenges of relying on physical exam alone in diagnosing a NSTI or drainable collection. The visible area of the infection will often appear relatively benign and may easily be mistaken for a simple cellulitis. Squire et al. (2005) found that US was superior to physical exam for the detection of abscess. The study’s data showed physical exams to have a sensitivity of 86% and specificity of 70% in identifying an underlying abscess, while US achieved a 98% sensitivity and 88% specificity. In a study by Tayal et al (2006), published in Academic Emergency Medicine, use of ultrasound changed the management in 56% of soft tissue infection cases. This included the identification of an underlying abscess in 48% of the cases in which the lesion was judged as low probability for having a drainable collection present. In regards to the most sinister type of soft tissue infections, a 2018 systematic review and meta analysis by Fernando et al (2018) found that physical exam and plain films were unreliable in identifying NSTI, with the latter demonstrating only 49% sensitivity. While CT is sensitive and specific for the disease, unfortunately it is not a feasible option for every patient presenting with a soft tissue infection. As a safe, effective, and fast alternative to both PE or CT, this author suggests that bedside ultrasound should be used in the evaluation of all soft tissue infections.
Infected tissue can be identified on ultrasound through a variety of characteristic findings. The classic "cobblestoning" that can be visualized when surveying infected tissue is a result of edematous subcutaneous tissues. Abscesses can be identified by their hypoechoic appearance, often nesting between the subcutaneous tissues and the fascial plane, and air is characterized by a echogenic dirty acoustic shadowing effect that can be seen in any plane of tissue that is infected. NSTI's can be associated with subcutaneous emphysema that tracks along the fascia. (Ma and Mateer, 2014).
Though the above case did not show the classic emphysematous fascial gas pattern associated often associated with necrotizing fasciitis cases, it did illustrate the importance of using ultrasound on all soft tissue infections as a means of identifying patients that need incision and drainage procedures or emergent surgical intervention. Physical exam had mis-identified this patient as simple cellulitis and he would have been dispositioned incorrectly.
References
Fernando, S., Tran, A., Cheng, W., Rochwerg, B., Kyeremanteng, K., Seely, A., . . . Perry, J. (n.d.). Necrotizing Soft Tissue Infection: Diagnostic Accuracy of Physical Examination, Imaging, and LRINEC Score: A Systematic Review and Meta-Analysis. Annals of surgery, 2018.
Ma, O. J. (2014). Ma and Mateers emergency ultrasound. New York: McGraw-Hill Education Medical.
Tayal, V. S., Hasan, N., Norton, H. J., & Tomaszewski, C. A. (2006). The Effect of Soft-tissue Ultrasound on the Management of Cellulitis in the Emergency Department. Academic Emergency Medicine,13(4), 384-388. doi:10.1197/j.aem.2005.11.074
Torrejón MDC, Celi E, Cancho D, Knox A, Henriquez-Camacho C. Necrotizing Myositis in a Neutropenic Patient: The Use of Ultrasound in the Diagnosis of Atypical Presentations. Case Reports in Emergency Medicine. 2014;2014:685263. doi:10.1155/2014/685263.