Pyomyositis with intramuscular abscess formation has occurred worldwide especially in Africa in both adults and children; yet in 1971, Levin described the emergence of pyomyositis in North America, with over 600 cases having been reported since [3–7]. Pyomyositis exhibits an overall male predominance, with cases in milder climates affecting primarily adults and the elderly. In addition to other immunocompromised states, such as HIV infection, diabetes, steroid use, and certain hematologic malignancies, trauma has been noted to be an important risk factor, occurring in up to 40% of cases in North America . The emergence of HIV infection in North America has certainly contributed to the increased prevalence of pyomyositis in North America . Primary pyomyositis is due to transient bacteremia as the major cause; yet, secondary pyomyositis can occur from contiguous infectious spread from osteomyelitis or cellulitis [1, 2]. While Staphylococcus aureus remains the causative organism in the majority of cases, other gram-positive, gram-negative, atypical, parasitic, and viral etiologies have also been reported . Community-associated methicillin resistant S. aureus has also been reported to cause intramuscular abscess pyomyositis syndrome [8, 9]. Our patient’s recent chemotherapy and his myelodysplasia with lower neutrophil count certainly put him at a much higher risk for developing quadriceps intramuscular abscess and pyomyositis.
Progression of pyomyositis has been described as occurring in three discrete stages [10, 11]. Thought to arise via hematogenous spread from a focal source, the early or invasive stage is characterized by nonspecific symptoms, including fever, malaise, and anorexia, and generally lasts approximately 10 days. Examination findings may include minimal swelling and mild overlying tenderness. The second or purulent stage is characterized by progression of local symptoms, including erythema and more pronounced tenderness. Most patients will seek initial medical attention during this phase, when the diagnosis may be mistaken for cellulitis or deep vein thrombosis. The final or late stage involves more clinically overt infection, often meeting the criteria for systemic inflammatory response syndrome or sepsis, with the physical exam progressing to reveal fluctuance of the overlying skin [10, 11]. Treatment beyond the first stage of infection involves ultrasound-guided drainage or surgical debridement in addition to antibiotic therapy. Delayed treatment may result in pronounced systemic disease, including metastasis of abscesses to distant anatomical sites, sepsis, and septic shock [10, 11].
While magnetic resonance imaging (MRI) is considered the diagnostic gold standard for the diagnosis of pyomyositis, ultrasonography is most useful during the purulent stage of the infection, when it may reveal diffuse muscle hyperechogenicity with or without localized hypoechogenicity and diffuse hyperemia [12–17]. Despite the presence of fluid or pus, abscesses located within the muscle layer may not exhibit the typical sonographic features expected from more superficially located collections. The appearance of muscle isoechogenicity and a solid appearance on a static ultrasound exam may be falsely reassuring for the absence of an intramuscular abscess. A dynamic compression ultrasound exam may be required to aid in the identification of fluid and loculated pus, allowing for the definitive diagnosis of an intramuscular abscess or stage II pyomyositis and for the appropriate management of ultrasound-guided aspiration and drainage or open surgical drainage [13, 16]. Direct probe pressure with high-frequency linear array ultrasound probe applied to our patient’s quadriceps muscle produced swirling of the heterogeneous deep-muscle purulent material.
The ED diagnosis of pyomyositis can be challenging especially when patients present with atypical symptoms such as fever with hip, back, or flank pain, especially in patients who are difficult to manage such as intravenous drug users [18–21]. Chern reported on ten patients who presented to the ED with a final diagnosis of psoas intramuscular abscess and pyomyositis, and five or 50% of the patients complained of flank pain, and the triad of fever, flank pain, and a limitation of hip movement, which is specific for psoas muscle abscess, was present in only three or 30% of the patients . Emergency CT or MRI scanning of an ED patient’s psoas muscle would provide the definitive diagnosis of pyomyositis as even a lower frequency ED ultrasound probe may not penetrate to the patient’s retroperitoneal psoas muscle for a definitive diagnosis.