Viral Myositis Diagnosis and Management

Introduction to Viral Myositis

Infectious necrotizing myositis requires immediate surgical debridement and broad-spectrum antibiotics, with a mortality rate of 36.5% even with treatment, according to the American College of Surgeons 1

Immune-mediated necrotizing myopathy requires high-dose corticosteroids plus steroid-sparing agents, as recommended by the American College of Rheumatology 2
Viral myositis is distinguished from inflammatory myopathies by its benign, self-limited course, bilateral lower extremity involvement, and preserved strength, as noted by the Mayo Clinic 3
The absence of rash and normal or mildly elevated CPK levels are key differentiating features of viral myositis, according to the American Academy of Pediatrics 3
Recent viral illness with flu-like symptoms is a characteristic feature of viral myositis, unlike the subacute onset of autoimmune myositis, as described by the Infectious Diseases Society of America 3

Normal or mildly elevated CPK levels (typically <10x normal) are characteristic of viral myositis, whereas markedly elevated CPK (>10x normal) is seen in necrotizing myopathy, as reported by the American Association for Clinical Chemistry 4

The American College of Physicians recommends pain management with acetaminophen or NSAIDs for myalgias and calf pain in patients with viral myositis, with no contraindications 5

The European League Against Rheumatism suggests that asymmetric weakness or true muscle weakness, not just pain-related limitation, indicates inflammatory myopathy, and presence of rash indicates dermatomyositis requiring immunosuppression, and systemic signs require urgent evaluation for more severe myositis 6

Immune‑mediated necrotizing myopathy presents with creatine kinase (CK) levels > 10 × upper‑limit‑of‑normal and requires immediate high‑dose corticosteroids together with steroid‑sparing agents; untreated cases have a high mortality rate. 7
Myositis induced by immune checkpoint inhibitors can progress rapidly to rhabdomyolysis and myocardial involvement; urgent high‑dose corticosteroid therapy is indicated. 8
Checkpoint‑inhibitor‑related myositis may be accompanied by myocarditis; serial troponin measurement and cardiology consultation are recommended. 8

Development of respiratory muscle weakness or dysphagia signals severe myositis and mandates hospital admission plus high‑dose corticosteroids. 9
Appearance of myoglobinuria or laboratory evidence of rhabdomyolysis necessitates aggressive intravenous hydration and close monitoring of renal function to prevent acute kidney injury. 9
Persistence or worsening of myopathic symptoms beyond one week, or a rising CK level, should trigger referral to rheumatology for evaluation of possible inflammatory myopathy. 9

In patients receiving statin therapy who develop muscle pain with elevated CK, temporary discontinuation of the statin should be undertaken during the diagnostic work‑up to exclude statin‑induced myopathy. 10