VOTW: Necrotizing Fasciitis

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: Lung Point

Hi all, this week's VOTW is presented by Drs Forrest, Yang and Schiller!

A 71 year old male w/ hx of COPD presented to the ED for altered mental status. He was found to be obtunded due to hypercapnia and was intubated in the ED. Several hours after admission to the MICU the patient suddenly desaturated to 64%. 

A POCUS was rapidly performed which showed…

Clip 1 shows a POCUS of the R anterior chest. On the left side of the screen, the pleural line has absent lung sliding. From the right of the screen, normal pleura with lung sliding is seen coming into the image with every breath. This is a “lung point” which is the exact point at which the pneumothorax starts. A chest x-ray confirmed a large R sided pneumothorax with mediastinal shift. A chest tube was placed by the ED team for a tension pneumothorax with improvement in vitals.

Image 1 shows an M-mode image obtained expertly by the team at the lung point which shows both "seashore sign" indicative of normal lung as well as "barcode sign" indicative of pneumothorax in one clip. You'll see this only if you use M-mode at the lung point.

M-mode showing areas of “sandy beach” alternating with “barcode sign” at the lung point

Lung sliding

In normal lung, the pleural line will appear to shimmer due to the movement of the visceral and parietal pleura sliding against each other. With a pneumothorax the contact between the two pleura are lost and the pleural line will appear still. 

*The presence of lung sliding rules out a pneumothorax at the location of the chest you are scanning. 

*Image the least dependent site (where air is most likely accumalate) to maximize sensitivity of the test (anterior chest in a supine patient).

*Reduce your image depth all the way! This way you don't have to squint while looking for lung sliding

Lung point

This is the point at which normal lung sliding and absent lung sliding are seen next to eachother simultaneosuly and is the exact point where the viseral pleural is peeling away from the parietal pleura. If found, this finding is highly specific for pneumothorax. It won't be seen with a large pneumothorax that envelops the entire lung.

Does absence of lung sliding always indicate pneumothorax?

No. Absence of lung sliding can be seen with many conditions including a bleb from COPD, right mainstem intubation (no left-sided lung sliding), patients w/ previous thoracic surgery (such as pleurodesis or VATS), pleural adhesions, ARDS, pulmonary fibrosis, atelectasis, and phrenic nerve paralysis. If the patient is stable, confirm the diagnosis with a chest x-ray or CT prior to placing a chest tube.

Which lung ultrasound artifacts rule out pneumothorax?

A-lines are reverberation artifacts that can be generated by air in normal lung tissue or air in the pleura so cannot be used to rule out pneumothorax.

B-lines indicate the presence of interstitial edema which can only be seen if the lung tissue is abutting the pleura. Even seeing one B-line is enough to rule out pneumothorax.
Happy sliding,

Your Sono Team

VOTW: DVT

This week’s VOTW is brought to you by Drs Kim, Nguyen and Sanghvi!

A patient with a previous history of DVTs no longer on anticoagulation presented with 4 days of right lower extremity pain, shortness of breath and chest pain. A POCUS of the lower extremities showed…

Clip 1 shows a non-compressible R common femoral vein containing echogenic material concerning for a DVT. The clot is seen extending into the saphenous vein as it takes off from the common femoral vein. Clip 2 shows the L common femoral vein also with a DVT extending into the saphenous vein. You can see that there is enough force applied with the probe to compress the artery completely, yet the vein is not fully compressed.

Chronic DVT

The appearance of these DVTs suggest that they are chronic. In general, chronic DVTs are more echogenic and have a more ragged appearing edge. Over time, DVTs tend to recanalize centrally. In image 1 below, you can see there is some areas that are recanalizing outlined in green. Image 2 shows an illustration of acute vs chronic DVTs.

DVT with area of recannalization

Acute vs Chronic DVTs

Acute DVT

An acute DVT generally has smoother edges and is less echogenic than a chronic DVT. Some acute DVTs cannot be seen with ultrasound and their presence can only be identified by the inability to collapse the vessel completely. If you are placing enough pressure to collapse the artery but the vein is not yet collapsed, this is concerning for a DVT.

Tips and tricks for lower extremity DVT studies

1. Use a linear transducer and choose the DVT setting

2. Squirt gel on the entire thigh instead of the probe so you don’t have to repeatedly re-gel the probe

3. Start in the inguinal crease and identify the take-off of the saphenous vein. This is a common site for a DVT and is the proximal starting point for our ED performed limited compression studies. Compress and take a clip here.

4. While the saphenous vein is considered a superficial vein, clots close to the sapheno-femoral junction should be treated with anticoagulation.

5. The common femoral vein bifurcates into the deep femoral vein (DFV) and superficial femoral vein (SFV). The DFV courses deep and is difficult to evaluate. The SFV is a mis-nomer and is actually a deep vein. Follow the SFV as far as you can down the thigh compressing every 2cm

6. Move onto compression of the popliteal region where the popliteal vein is on top of the artery (“pop on top”)

7. Compressing obliquely is a common reason the vein does not compress completely resulting in a false positive interpretation. Use your non-probe hand to assist in compressing the vein perpendicularly to the femur.

Back to the patient
A CTA chest was negative for pulmonary embolism and patient was discharged on oral anticoagulation and outpatient follow up.

Happy Compressing and De-compressing,

Your Sono Team