Viral Myositis

Viral myositis is inflammation of skeletal muscle caused directly or indirectly by a viral infection. It ranges from a mild, self-limited illness (most common) to severe rhabdomyolysis, and can also occur as a chronic myopathy in the setting of retroviral infection.

Causes

Many viruses have been implicated:

Influenza A and B - the classic and most common cause, especially in children ("benign acute childhood myositis")
Enteroviruses: coxsackievirus, echovirus
Adenovirus, parainfluenza virus, Epstein-Barr virus, cytomegalovirus, arbovirus, dengue virus
Hepatitis B and C
HIV and HTLV-1 (retroviruses) - cause a more chronic, polymyositis-like syndrome rather than an acute illness
COVID-19 - reported to cause myositis, possibly via direct viral effect or triggering autoimmunity

(Sources: - Goldman-Cecil Medicine, p. 4105; - Bradley and Daroff's Neurology in Clinical Practice; - Rheumatology, 2-Volume Set)

Clinical presentation

The most classic pattern is acute influenza-associated myositis in children:

Occurs a few days after the initial respiratory/flu symptoms begin to improve, not during the acute febrile phase
Sudden onset of severe, focal calf pain and tenderness ("severe calf myalgia") causing difficulty walking or refusal to walk
This is distinct from ordinary myalgia (body aches) that accompanies flu itself - it comes on later, is more focal (calves), and is more intense
Usually self-limited, resolving within a few days with supportive care

General features regardless of virus:

Preceding or concurrent viral prodrome: fever, cough, sore throat, rhinorrhea, GI symptoms - though these may have resolved before myositis appears
Muscle pain, tenderness, and weakness (proximal in some infections, focal/calf in influenza)
Elevated creatine kinase (CK), sometimes markedly
EMG may show myopathic changes
Muscle biopsy in influenza myositis typically shows fiber degeneration/necrosis with relatively little inflammatory infiltrate (Rheumatology, 2-Volume Set, p. 1936-1943)

HIV-associated myopathy behaves differently - a slowly progressive, polymyositis-like illness with proximal weakness, elevated CK, and endomysial/perimysial mononuclear infiltrates on biopsy. A key clinical challenge is distinguishing this from myopathy caused by nucleoside reverse transcriptase inhibitor (NRTI) drugs used to treat HIV, which often requires a trial off the medication (Rheumatology, 2-Volume Set).

Severe end of the spectrum

Rarely, viral myositis can progress to rhabdomyolysis with markedly elevated CK, myoglobinuria, and risk of acute kidney injury - this warrants urgent evaluation, especially if a child cannot walk or has extreme tenderness.

Diagnosis

Clinical picture in the setting of recent viral illness
Elevated CK (can be markedly elevated)
Urinalysis/renal function to exclude myoglobinuria-related kidney injury in severe cases
EMG if diagnosis is unclear (myopathic pattern)
Muscle biopsy is not usually needed in typical influenza-associated cases but helps exclude autoimmune inflammatory myopathy (dermatomyositis, polymyositis, inclusion body myositis, necrotizing autoimmune myopathy) when the picture is atypical, prolonged, or the CK/weakness is disproportionate

Treatment

Management is largely supportive, since most cases are self-limited:

Rest, hydration (important to prevent renal complications from myoglobinuria), and analgesia (NSAIDs can help myalgia)
Monitor CK and renal function in more severe cases
Antiviral therapy for the underlying infection if indicated (e.g., oseltamivir for influenza, though this doesn't necessarily shorten the myositis itself)
IV fluids and closer monitoring if rhabdomyolysis develops
For HIV myopathy, treatment addresses the underlying HIV and may require adjusting antiretroviral regimen if NRTI toxicity is suspected

When to worry / red flags

Seek urgent evaluation if a patient (particularly a child) develops:

Inability to walk or bear weight
Extreme muscle tenderness or swelling
Dark urine (suggesting myoglobinuria)
Signs that suggest an alternative diagnosis (e.g., ongoing high fever, focal warmth/erythema suggesting bacterial pyomyositis, or a rash/systemic features suggesting an autoimmune myopathy)

These findings help differentiate benign viral myositis from more serious conditions like bacterial pyomyositis, compartment syndrome, or an emerging autoimmune inflammatory myopathy.

Recent literature

Recent PubMed case reports (2024-2025) continue to describe classic pediatric post-influenza viral myositis presenting with sudden calf pain and gait difficulty (PMID 39529758, 38420103, 38659543), plus a 2025 report of an atypical presentation with isolated distal upper-extremity weakness (PMID 41331971) - a reminder that presentations can occasionally be atypical (proximal vs. distal, upper vs. lower limb). A 2025 Pediatrics in Review article on infectious myositis broadly (PMID 41173301) reinforces the same supportive-care approach. No recent systematic reviews or meta-analyses have emerged that contradict this classic teaching.

If you have a specific clinical scenario (e.g., a particular virus, pediatric vs. adult presentation, or distinguishing from autoimmune myositis) I can go deeper on that.

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