What is the treatment approach for influenza myositis, considering patient factors like age and symptom severity?

Influenza-Associated Myositis: Treatment Approach

Influenza-associated myositis is primarily managed with aggressive hydration and supportive care, with oseltamivir (75 mg twice daily for adults, weight-based dosing for children) initiated within 48 hours of symptom onset if the patient presents during the acute phase. 1, 2

Immediate Assessment and Risk Stratification

Key Diagnostic Features

• Timing is critical: Myositis typically develops 2-4 days after influenza symptoms begin, as respiratory symptoms start to subside 2
• Check CPK levels immediately: Expect marked elevation, often >1000 U/L in true myositis 2
• Assess for rhabdomyolysis: Check urine for myoglobinuria, monitor renal function (BUN, creatinine), and electrolytes 2
• Physical examination: Look for focal calf muscle tenderness, pain on palpation, and difficulty or refusal to walk 2

Age-Specific Considerations

• School-aged children are predominantly affected, with calf muscles most commonly involved 1, 2
• Children under 1 year and those at high risk require direct GP or A&E assessment 1
• Adults can develop myositis but it is less common; maintain high suspicion during influenza outbreaks 3

Treatment Protocol

Antiviral Therapy

• Initiate oseltamivir if presenting within 48 hours of symptom onset: 75 mg twice daily for adults; weight-based dosing for children (30-75 mg twice daily depending on weight) 1, 4
• Benefits include: 24-hour reduction in illness duration and possible reduction in complications 1
• Oseltamivir is NOT required if the patient presents after respiratory symptoms have resolved and myositis is already established, as the condition is self-limiting 5, 6

Hydration Management

• Aggressive IV hydration is essential if CPK is extremely elevated (>5-10 times normal) to prevent acute kidney injury 2
• Monitor daily: BUN, creatinine, electrolytes, and CPK levels until CPK trends downward 2
• Oral fluids are sufficient for mild cases with CPK <5000 U/L and normal renal function 1

Pain Management

• First-line: Acetaminophen (paracetamol) for fever and muscle pain 1, 2
• Avoid NSAIDs if renal function is compromised or CPK is markedly elevated 2
• Never use aspirin in children under 16 years due to Reye's syndrome risk 1, 2

Supportive Care

• Rest until muscle pain and weakness resolve 1
• Avoid ambulation if severe calf pain or weakness is present 2
• Monitor for complications: Daily assessment of walking ability and pain severity 7

Critical Monitoring Parameters

Laboratory Surveillance

• Daily CPK monitoring until levels decline consistently 2
• Renal function (BUN, creatinine) daily if CPK >5000 U/L 2
• Electrolytes, particularly potassium, to detect hyperkalemia from muscle breakdown 2
• Urinalysis for myoglobinuria if rhabdomyolysis is suspected 2

Clinical Red Flags Requiring Escalation

• CPK >10,000 U/L: High risk for acute kidney injury 2
• Rising creatinine: Indicates developing renal failure 2
• Dark urine (tea-colored): Suggests myoglobinuria 2
• Altered mental status: May indicate severe metabolic derangement 1
• Persistent fever >4-5 days: Consider secondary bacterial infection 1

Common Pitfalls to Avoid

Diagnostic Errors

• Do not assume cardiac disease based on CPK-MB elevation alone; troponin is the definitive test for myocardial injury 2
• Do not miss rhabdomyolysis: Always check urine for myoglobin when CPK is markedly elevated 2
• Do not confuse with simple myalgias: True myositis presents with focal tenderness, functional impairment, and occurs AFTER respiratory symptoms improve 2

Treatment Errors

• Do not withhold hydration in patients with elevated CPK, even if they appear well-hydrated clinically 2
• Do not use NSAIDs indiscriminately: They can worsen renal function in the setting of rhabdomyolysis 2
• Do not discharge without follow-up plan: Patients need reassessment within 48 hours to ensure CPK is declining 2

Expected Clinical Course and Follow-Up

Prognosis

• Most cases are self-limiting with complete recovery within 3-7 days 2, 5, 3
• Rhabdomyolysis and renal failure are rare but can occur, particularly with influenza A 1, 8
• Improvement should begin within 48 hours of supportive care initiation 1, 3

When to Re-Consult

• No improvement after 48 hours of treatment 1
• Worsening muscle pain or weakness 1
• Development of dark urine 2
• New confusion or altered mental status 1
• Shortness of breath or chest pain 1