US - Pyomyositis: Ultrasound of the Month

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The case presentation...

Our patient was a 19-year-old male with past medical history of IVDU who presented to the emergency department with complaints of left elbow pain. He noted associated swelling and redness and said that this was his second visit in three days for this problem. At his first visit the providers noted an erythematous area on the proximal forearm with central fluctuance in the same area clinically consistent with abscess. They performed incision and drainage at bedside with purulence expressed. The borders of the surrounding cellulitis were marked with ink and he was discharged with prescriptions for trimethoprim-sulfamethoxazole and cephalexin. Since discharge, he noticed the redness extended beyond the original borders and he had no improvement in his pain despite taking his antibiotics, although he denied any systemic symptoms such as nausea or fevers. Of particular concern was that he reported new pain with extension of the digits of his left hand. On exam the providers noted that the left proximal forearm was status post incision and drainage with a small amount residual purulent material draining from the wound. There was light erythema extending approximately one centimeter beyond the previously marked border with a small amount of swelling and warmth.  He had pain with passive extension on his digits but was neurovascularly intact.

And now for the ultrasound images...

Left forearm, transverse plane

Left forearm, sagittal plane

Another orthogonal plane demonstrating fluid within the muscle belly

Note the hypoechoic region in the center of the muscle belly

Note the hypoechoic region in the center of the muscle belly

Bedside soft tissue ultrasound was used to evaluate for residual abscess versus cellulitis, with the consideration that the patient could have a retained foreign body. Images were attained with a linear probe. After evaluating at a superficial depth initially, the depth was increased to evaluate the deeper structures. The subcutaneous tissue demonstrated cobblestoning and increased subcutaneous tissue thickness. There was a very small anechoic area within the subcutaneous tissue consistent with a small residual abscess pocket. There is an additional finding of an irregular hypoechoic area within the muscle belly.

+ What is your diagnosis?

Although initially treated for cutaneous abscess and cellulitis, the clinicians at the bedside were ultimately able to ascertain the presence of a deep space infection using bedside ultrasound as a first-line investigation. The images demonstrated an area of hypoechogenicity within the skeletal muscle of the forearm and subsequent hand surgery consult and follow-up CT scan confirmed the presence of a fluid collection within the muscle belly of the flexor digitorum superficialis. This led to the diagnosis of pyomyositis.

Pyomyositis is a purulent infection of skeletal muscle usually arising from hematogenous spread. For this patient, a significant risk factor was his IVDU. The differential in this case should also include necrotizing fasciitis, gas gangrene, and other types of myositis (e.g. viral and parasitic). The most common culprit is Staphylococcus aureus followed by group A streptococcus, and so antibiotic therapy should be targeted appropriately. The clinical course of disease typically progresses through three stages starting with localized cramping, followed by fevers and marked tenderness, and finally with systemic toxicity including septic shock and rhabdomyolysis. This patient’s presentation was most consistent with stage I pyomyositis as evidenced by his lack of systemic signs and improvement without need for operative intervention.

+ Ultrasound Pearls

Studies have shown that ultrasound is a useful adjunct in distinguishing uncomplicated cellulitis from cutaneous abscess and has been shown to change management in a significant proportion of cases. A systematic literature review by Sathyaseelan et al. showed that POCUS significantly aids in diagnosis of abscess and is more accurate than clinical exam alone in both adults and children (1). Two studies referenced in the review by Marian et al. and Adams et al. showed that the greatest effect of POCUS was noted in clinically equivocal cases (2,3). This diagnostic utility of ultrasound is also of particular significance in pediatric populations in which incision and drainage often requires an extra level of procedural sedation not needed in most adults and in such cases ultrasound may be able to rule out abscess and allow clinicians to spare their patients the risk and pain of a procedure.

In cases of pymomyositis ultrasound can further differentiate cutaneous abscess from deeper tissue infection without the need for more time and resource intensive modalities such as CT and MRI and can expedite treatment. On ultrasound, initial findings can include either homogenously or heterogenously hyperechoic muscle tissue which is secondary to muscle tissue edema and also loss of normal muscle architecture (4). A hypoechoic or anechoic fluid pocket is not routinely visualized, but may be. In our case, an anechoic fluid collection was observed without adjacent hyperechoic muscle tissue or loss of normal sonographic appearance of the surrounding muscle. It is likely that this is because our patient's pyomyositis had been partially treated with antibiotics at the time of presentation.

There are a few pro-tips to use when scanning soft tissues. Be sure to scan the contralateral side. The contralateral side is a readily-available normal comparison for your patient and can help you more readily appreciate abnormalities. The use of a standoff pad (improvised with a bag of saline or water-filled glove) or a water bath can greatly enhance image acquisition, particularly when scanning very superficial structures (e.g. hands). Also be sure to scan in 2 orthogonal planes to fully assess dimensions. Also, increase the depth to evaluate the deep tissues including muscle. Knowing the sonographic characteristics of normal soft tissue structures such as subcutaneous tissue, fascia, muscle, tendon, vessels and nerves is imperative. Applying color to the window will reveal details about vascular supply and can be crucial in differentiating infectious fluid collections from other entities such as lymph nodes or even vascular aneurysms that could produce disastrous outcomes if erroneously incised. Lastly, using the probe to compress the tissue can give valuable clues as the to nature of the entity. Abscess contents would be expected to swirl or move under compression, a finding colloquially referred to as “squish sign.”

Post by Jared Ham, MD

Expert Review by Lori Stolz, MD RDMS