Infectious Myositis

Infectious myositis refers to infection of skeletal muscle, ranging from localized pyomyositis (a discrete bacterial abscess within a muscle, most commonly caused by Staphylococcus aureus) to more diffuse or systemic infectious causes of muscle inflammation (viral myositis, and, in the tropics, various parasitic causes including trichinellosis and, rarely, cysticercosis, see that entry).

Pyomyositis was historically considered a tropical disease (also termed "tropical pyomyositis"), but is increasingly recognized in temperate settings, particularly in the context of diabetes, immunosuppression, IV drug use, or preceding trauma or muscle strain that may seed hematogenous bacteria into an area of localized muscle injury.

• Pyomyositis: presents in three overlapping stages, an early invasive stage with localized, dull, cramping muscle pain, mild fever, and woody induration without fluctuance (often misdiagnosed initially as a muscle strain or hematoma given the absence of overt abscess features), followed by a suppurative stage (days to weeks later) with a more clearly localized, tender, sometimes fluctuant mass and systemic toxicity, and, if untreated, a late stage with systemic sepsis and risk of metastatic infection elsewhere. Most commonly affects the large muscles of the thigh, calf, or gluteal region
• Viral myositis (commonly influenza, particularly in children, and less commonly other respiratory viruses or coxsackievirus) presents with acute, often bilateral, symmetric muscle pain and tenderness, classically affecting the calves, alongside the systemic features of the underlying viral illness, and is typically self-limiting
• Both can cause elevated creatine kinase, and severe cases (of either etiology) carry a risk of rhabdomyolysis with resulting acute kidney injury

• MRI is the imaging investigation of choice for suspected pyomyositis, showing the characteristic muscle edema and, where a discrete abscess has formed, a rim-enhancing fluid collection, with substantially higher sensitivity than ultrasound or CT for early-stage disease
• Blood cultures should be sent given the frequency of accompanying bacteremia, alongside FBC and inflammatory markers (typically significantly elevated)
• CK should be checked in any suspected myositis, both to support the diagnosis and to screen for rhabdomyolysis
• Image-guided or surgical aspiration/drainage of any identified collection provides both source control and definitive microbiological diagnosis via Gram stain and culture
• Viral myositis diagnosis is largely clinical in the context of a compatible viral prodrome, with specific viral testing (e.g. influenza PCR) supporting the diagnosis where relevant to management decisions

• Pyomyositis: drainage of any discrete abscess (image-guided percutaneous or open surgical) is central to management, particularly once frank pus has formed, combined with empirical IV antibiotics covering Staphylococcus aureus (including MRSA cover where locally prevalent or risk factors are present): IV flucloxacillin 2 g qds, or, where MRSA is a concern, IV vancomycin (dosed and monitored per local protocol) or clindamycin, adjusted once culture results are available; total duration typically 3 to 4 weeks, guided by clinical and radiological response, with an initial IV course transitioning to oral therapy as improvement is established
• Early-stage disease without a discrete drainable collection may respond to antibiotics alone with close monitoring, but a low threshold for repeat imaging and drainage should be maintained if response is inadequate
• Viral myositis: supportive management (rest, analgesia, hydration), with monitoring of CK and renal function given the rhabdomyolysis risk, particularly in more severe cases; specific antiviral treatment (e.g. oseltamivir for influenza, see Influenza entry) may be indicated based on the underlying viral cause and its own treatment criteria
• Rhabdomyolysis, if present, is managed with aggressive IV fluid resuscitation and monitoring of renal function and electrolytes (see Acute Kidney Injury entry)

Referral: interventional radiology or surgery for drainage of pyomyositis abscess; infectious disease for antibiotic selection and duration guidance, particularly in atypical or refractory cases.