Viral Myositis

How viruses cause muscle inflammation

Some viruses invade muscle fibers directly, while others trigger an immune response that inflames the muscles. Either way, muscle cells are damaged and release the enzyme creatine kinase (CK) into the blood. In severe cases, large amounts of muscle break down (rhabdomyolysis), releasing myoglobin, which can block and damage the kidneys.

Benign acute childhood myositis (BACM)

BACM is the most common form, typically in children aged 5 to 9, more often boys, during flu season.

• begins 1 to 5 days after fever, cough or sore throat, often as these are improving
• sudden pain and tenderness in both calves
• the child refuses to walk, walks stiffly with a wide gait, or walks on tiptoes
• strength, reflexes and sensation are normal on examination (pain limits walking)
• CK is raised, often markedly
• resolves completely within 3 to 10 days with rest and fluids

It is important to distinguish BACM from conditions such as Guillain-Barré syndrome, which causes true weakness with reduced reflexes, and from bone or joint infections.

Viral myositis in adults

• generalized muscle aches and tenderness
• weakness, particularly in the thighs and shoulders
• fatigue
• in severe cases, rhabdomyolysis with dark urine
• COVID-19 and influenza can occasionally cause severe myositis, sometimes involving the heart muscle (myocarditis)

Rhabdomyolysis

Rhabdomyolysis is the main serious complication. Signs include:

• severe muscle pain, swelling and weakness
• dark, tea- or cola-colored urine
• reduced urine output
• nausea, confusion

It needs hospital care with intravenous fluids to protect the kidneys and monitoring of potassium, which can rise to dangerous levels.

Diagnosis

• History and examination, including strength, reflexes and gait
• Blood tests: CK (raised), kidney function, electrolytes, blood count
• Urine test: blood on dipstick without red cells suggests myoglobin
• Viral testing: flu, COVID-19 or other viral swabs
• Further tests if weakness persists or the picture is unusual: MRI of muscles, autoantibodies, or muscle biopsy to rule out inflammatory myopathies such as dermatomyositis

Treatment

• rest until pain settles
• fluids to keep well hydrated and protect the kidneys
• pain relief: acetaminophen (paracetamol) or ibuprofen, following the label; avoid NSAIDs if there is concern about the kidneys. Never give aspirin to children or teenagers with a viral illness because of the risk of Reye's syndrome.
• antiviral treatment (oseltamivir) may be given for influenza, especially early or in high-risk patients
• IV fluids in hospital for rhabdomyolysis or if CK is very high
• repeat CK to confirm improvement in some cases

Recovery

• children with BACM usually walk normally within a week
• adults often recover over 1 to 3 weeks
• persistent weakness beyond a few weeks should be investigated for other muscle diseases
• recurrence is uncommon but possible with future infections

Prevention

• yearly flu vaccination for everyone aged 6 months and older
• staying up to date with COVID-19 vaccination
• handwashing and staying home when sick

Frequently asked questions

Why does my child have calf pain after the flu?

Calf pain with difficulty walking a few days after flu is usually benign acute childhood myositis, a temporary inflammation of the calf muscles. It typically resolves within a week. Check urine color: dark urine needs emergency care.

Is viral myositis dangerous?

Usually not. Most cases resolve fully with rest and fluids. The main danger is rhabdomyolysis, where muscle breakdown products can damage the kidneys. Dark urine, very little urine or severe weakness need urgent medical attention.

How long does viral myositis last?

In children, benign acute childhood myositis usually resolves within 3 to 10 days. In adults, muscle aches and weakness often improve over 1 to 3 weeks. Symptoms lasting longer should be checked for other muscle conditions.

Can COVID-19 cause myositis?

Yes. COVID-19 can cause muscle aches and, less commonly, true muscle inflammation with raised CK, and occasionally rhabdomyolysis. Most people recover as the infection clears.