US - Soft tissue deep abscess of the neck: Ultrasound of the Month

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The Case

The patient is a woman in her 40's with a past medical history of hypertension and hyperlipidemia who presents to the emergency department (ED) with concern for an abscess on her left shoulder.  She states that she began developing a dull pain around her left ear approximately one month prior to presentation, which was not accompanied by fevers, chills, headaches, or hearing changes.  As the pain progressed, she noticed and enlarging lump in her supraclavicular fossa that was increasingly painful and red.  She initially presented to urgent care for evaluation, was diagnosed with cellulitis, and provided with a prescription for antibiotics (cephalexin and trimethoprim-sulfamethoxazole).  Upon the urgent care provider's recommendation the patient returned for a wound check two days later and was referred to the ED due to concerns for interval worsening.  On interview in the ED she states that although she has been more fatigued over the last week, she had no nausea, vomiting, myalgias, or drainage from the area.  She denies any history of prior abscess or intravenous drug use.  Her vital signs are a temp 37.1 C, HR 85, BP 122/66 mmHg, RR 18, SpO2 95% on room air.  Her skin exam is notable for a circular erythematous, moderately tender nodule superior to the left mid-left clavicle, 2 cm in diameter with focal fluctuance centrally, surrounded by a ring of induration.  The remainder of the physical exam is unremarkable.  A soft tissue ultrasound is performed of the area.  

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Still frame showing depth of complex fluid collection

+ What do you see on Ultrasound?

There is a collection of hypoechoic fluid in the superficial subcutanous tissue and overlying a deeper, circular area of mixed echogenicity, extending at least 4 centimeters into the underlying tissues. These findings are concerning for a deep abscess. Due to concerns of deeper tissue involvement, a follow up CT of the neck with contrast was ordered, a representative image of which is displayed below:

The CT demonstrated a multilocular abscess, penetrating through the upper thoracic cavity from the left supraclavicular fossa, measuring up to 6.5 cm in the cephalocaudal dimension with associated inflammatory stranding. Due to concern for the extent of the abscess, the patient was given intravenous clindamycin and transferred to a tertiary care center for evaluation of potential operative management by otolaryngology.

Despite the diagnostic certainty, an ultrasound was performed that ended up changing the patient's clinical course for the better, allowing for delivery of appropriate care in a timely manner.

Historically, the diagnosis of abscess has been confirmed and treated by the same procedure: incision and drainage (I&D). In some cases of low pretest probability, the provider could perform a needle aspiration in an attempt to save the patient from the scalpel. As the role of diagnostic ultrasound in the ED continues to expand, the ability of soft tissue scanning to enhance our physical exam has been the subject of multiple investigations in the literature (1,2,4).

One way in which ultrasound can increase our accuracy is in differentiating simple cellulitis from an underlying fluid collection, suggesting drainable abscess. This is essential, as the management differs between these two infectious processes. Sonographically, cellulitis is classically described as "cobblestone" in its appearance, with hypo or isoechoic strands of fluid running between hyperechoic adipose lobules (top left image). Abscesses can be variable in their appearance (other images), but are generally described as masses of hypoechoic fluid in the subcutaneous tissue with variable echogenicity centrally and occasionally posterior acoustic shadowing (5).

Cellulitis Abscess
Abscess Abscess

At the bedside, ultrasound can often change management in cases of soft tissue infection. In a report of 126 patients presenting to an urban ED with concern for cellulitis, and no overt signs of abscess (fluctuance, drainage, skin elevation), treating physicians were asked to define their management plan prior to obtaining the results of a soft tissue ultrasound (2). After receiving the results, management changed in 56% of the cases, including 73% in the group that had already planned to undergo incision and drainage. In the group that was deemed low risk for abscess, discovery of a drainable fluid allowed for appropriate management. In the high pretest probability group, who had initially bought themselves and I&D based on exam, 16 of the 32 patients were spared a painful procedure due to the abscence of identifiable abscess on ultrasound.

These findings call into question the test characteristics of physical exam for identifying abscesses. In 2005, Squire et al. performed a prospective trial comparing clinical exam alone to exam with the addition of ultrasound in the detection of abscess (3). Using expression of purulence as the gold standard, clinical exam clocked in at a suboptimal 86% sensitivity and 70% specificity. Adding a bedside ultrasound exam increased sensitivity to 98% and the specificity to 88%. In this particular study, ultrasound examiners were given a 30 minute training session, suggesting that acceptable diagnostic accuracy can be achieved with minimal trainaing. Indeed, in a similarly constructed investigation, a two day ultrasound course with only 15 minutes dedicated to soft tissue imaging was sufficient to produce excellent diagnostic accuracy (area under the curve [AUC] = 0.85), which outperformed clinical exam alone (AUC = 0.75) (6).

While ultrasound appears to add value to the physical exam, sonography confers additional benefits in evaluation of soft tissue infection. Identification of structures that are not always obvious on physical exam, especially underlying vasculature, can prevent unintended complications from a blind I&D (7). Additionally, some populations that often present with abscess, specifically patients who use intravenous drugs, are at high risk for retained foreign bodies. Sonography has long been described as an alternative to plain radiography for identification of foreign bodies, and has a particularly high specificity in this application (8,9).

Returning to our case, after transfer, the patient was taken to the operating room by otolaryngology for incision and drainage of her abscess, with evacuation of copious amounts of purulent fluid. During the dissection, the surgeons broke up multiple loculations that tracked towards the superior portion of the patient's left lung, without violating the pleura. Cultures were sent that demonstrated pan-sensitive alpha-hemolytic strep as well as gram positive beading rods consistent with nocardia. She improved after operative management, leaving the hospital on postoperative day 3 with oral antibiotics and outpatient follow up with the otolaryngology team.

+ Learning Points

  • Soft tissue ultrasound is relatively simple to learn, quick to perform, and can identify abscess with a high degree of diagnostic accuracy.
  • Addition of ultrasound to physical exam yields an increase in both sensitivity and specificity for abscess.
  • Even when the diagnosis seems assured based on physical exam, ultrasound can offer further information that may change management and avoid undesired complications.

Authored by Chris Shaw, MD

Posted by Tim MUrphy, MD

References

  1. Adhikari, S., & Blaivas, M. (2012). Sonography first for subcutaneous abscess and cellulitis evaluation. Journal of Ultrasound in Medicine, 31(10), 1509-1512.

  2. Iverson, K., Haritos, D., Thomas, R., & Kannikeswaran, N. (2012). The effect of bedside ultrasound on diagnosis and management of soft tissue infections in a pediatric ED. The American journal of emergency medicine, 30(8), 1347-1351.

  3. Squire, B. T., Fox, J. C., & Anderson, C. (2005). ABSCESS: applied bedside sonography for convenient evaluation of superficial soft tissue infections. Academic Emergency Medicine, 12(7), 601-606.

  4. Tayal, V. S., Hasan, N., Norton, H. J., & Tomaszewski, C. A. (2006). The effect of softtissue ultrasound on the management of cellulitis in the emergency department. Academic emergency medicine, 13(4), 384-388.

  5. Loyer, E. M., DuBrow, R. A., David, C. L., Coan, J. D., & Eftekhari, F. (1996). Imaging of superficial soft-tissue infections: sonographic findings in cases of cellulitis and abscess. AJR. American journal of roentgenology, 166(1), 149-152.

  6. Berger, T., Garrido, F., Green, J., Lema, P. C., & Gupta, J. (2012). Bedside ultrasound performed by novices for the detection of abscess in ED patients with soft tissue infections. The American journal of emergency medicine, 30(8), 1569-1573.

  7. Blaivas, M., & Adhikari, S. (2011). Unexpected Findings on Point-of-Care Superficial Ultrasound Imaging Before Incision and Drainage. Journal of Ultrasound in Medicine, 30(10), 1425-1430.

  8. Oikarinen, K. S., Nieminen, T. M., Mäkäräinen, H., & Pyhtinen, J. (1993). Visibility of foreign bodies in soft tissue in plain radiographs, computed tomography, magnetic resonance imaging, and ultrasound: an in vitro study. International journal of oral and maxillofacial surgery, 22(2), 119-124.

  9. Davis, J., Czerniski, B., Au, A., Adhikari, S., Farrell, I., & Fields, J. M. (2015). Diagnostic accuracy of ultrasonography in retained soft tissue foreign bodies: a systematic review and meta-analysis. Academic Emergency Medicine, 22(7), 777-787.