VOTW: Necrotizing Fasciitis

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This week’s VOTW is brought to you by Dr. Aaron Ryoo!!

17 yom presented to the Bay Ridge (Free-standing) ED with 5 days of right lower extremity pain that started as a scab. Two days later he had an I&D of his right calf by his PMD. Over the day prior to presentation, the patient became febrile with worsening pain and erythema. On exam, he had a large erythematous indurated region along his calf with pain out of proportion to exam and purulent/bloody drainage from the previous I&D site. A POCUS was performed which showed…

Clip 1 shows a soft tissue image of the calf area, with cobblestoning of the subcutaneous tissues consistent w/ cellulitis. Deep to the subcutaneous tissue there is edema along the muscle/fascial layers and several hyperechoic foci of air with “dirty shadowing” concerning for necrotizing soft tissue infection (NSTI) by gas-forming bacteria.

Necrotizing soft tissue infection

POCUS is a quick and easy way to evaluate for the presence of soft tissue gas when there is a concern for NSTI. Other findings include fluid collections along the fascial plane and findings of overlying cellulitis. Overall, POCUS has a sensitivity of 85-100% and specificity of 44-98% for NSTI1. Fluid accumalation along the fascial planes is most sensitive finding while subcutaneous emphysema is most specific (100%) based on a recent meta-analysis1

You can use the acronym "STAFF" to remember the findings:

ST = subcutaneous thickening

A = air or emphysema

FF = fascial fluid layer greater than 2mm

An I&D can also introduce air into the area but in a septic patient this should be assumed to be necrotizing fasciitis until proven otherwise.

Image 1. Dirty shadowing is caused by sound wave-reflecting objects like gas (think of shadowing from bowel gas). The shadow created is not unformily anechoic. Clean shadowing is caused by sound wave-absorbing objects (think gallstones and bones). The shadow created is unformily anechoic. 

Image 1. Dirty shadowing

Findings of cellulitis on POCUS

  • Skin and subcutaneous tissue appears thickened and diffusely hyperechoic

  • Area becomes “hazy” with loss of clear borders between epidermis, dermis and hypodermis (subcutaneous tissue)

  • Cobblestoning- hyperechoic fat lobules in the subcutaneous tissue become separated by edema giving the appearance of cobble stones

Back to the patient:

CT imaging showed “deep perifascial bubbles of gas and edema along the lateral head of the gastrocnemius". Patient was transferred emergently to Maimo and was taken to the OR by general surgery the next morning for an I&D. Purulent material was found along the fascial planes confirming the diagnosis of necrotizing fasciitis.

References:

Marks et al. Ultrasound for the diagnosis of necrotizing fasciitis: A systematic review of the literature, The American Journal of Emergency Medicine, Volume 65, 2023, Pages 31-35

Happy Shadowing,

Your Sono Team

VOTW: Soft Tissue Foreign Body

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This week’s VOTW is brought to you by Dr. DeStefano and Dr. Wong!

A 3 year old female was brought into the ED a week after a she slid down a wooden pillar and suffered a splinter into her right thigh. A POCUS of the area showed…

Clip 1 is a POCUS of the posterior thigh that shows a small echogenic object with posterior acoustic shadowing. As the they scan through the area, we can tell that the object is linear, about 1cm in length and that its trajectory courses from the dermis to the subcutaneous layer and ends just before entering the muscle. There is no reverbration artifact which is consistent with wood. There is no surrounding signs of abscess of cellulitis.

POCUS for Foreign Bodies

Soft tissue foreign bodies can be imaged by x-ray, CT or ultrasound. Many of us reach for X-rays first but is that really the right move?

X-rays have poor sensitivity for foreign bodies especially for radiolucent objects such as plastic and wood(1). 

Ultrasound on the other hand is highly sensitive for foreign bodies, regardless of what the composition, and has the following advantages over X-rays including:

  1. No radiation

  2. Can map out the shape, trajectory, depth of the object at bedside

  3. Evaluate for involvement of tendons, muscles, joints

  4. Evaluate or complications such as cellulitis or abscess

  5. Guide removal of the object in real time (see videos below)

Characteristics of common foreign bodies on US

Glass: hyperechoic, + shadow, + reverb artifact

Metal: hyperechoic, + shadow, + reverb artifact

Wood: hyperechoic, + shadow, - reverb artifact

Plastic: hyperechoic, + shadow, - reverb artifact

Here is an example of metal which is hyperechoic with reverberation artifact (repeated hyperechoic horizontal lines extending deep to the object)

Metal foreign body with reverberation artifact

Technique

  1. Use a linear probe.

  2. Scan the area of interest in both transverse and sagittal.

  3. Look for a hyperechoic structure with posterior shadowing +/- reverbration artifact.

  4. Identify the shape, length, trajectory and surrounding structures.

  5. For very supericial foregin bodies, try using a water bath to increase the distance between the probe and foreign body (this brings the object closer to the "focal point", the part of image with the best "two-point discrimination" or resolution, which is closer to midway down the screen). Water also provides a great acoustic window.

Foreign body removal using ultrasound-guidance

Check out these great videos on how to use ultrasound to assist w/ foregin body removal

  1. https://www.youtube.com/watch?v=x80NrSUNRrI

  2. https://www.youtube.com/watch?v=OeFDg1hZRDk

  3. https://www.youtube.com/watch?v=h1YQY7guUb0

Back to the patient:

The team identified the splinter in the soft tissue with no evidence of celluitis or abscess. The team approrpiately did not order an x-ray and saved the patient from unecessary radiation! The patient was referred to outpatient general surgery for evaluation for removal of the object.

References:

  1. Pattamapaspong N et al. Accuracy of radiography, computed tomography and magnetic resonance imaging in diagnosing foreign bodies in the foot. Radiol Med. 2013 

  2. https://rebelem.com/pocus-and-soft-tissue-foreign-bodies/

  3. https://sjrhem.ca/detection-of-foreign-bodies-in-soft-tissue-a-pocus-guided-approach/