What is the possible diagnosis for a 14-year-old boy with influenza, presenting with extreme exhaustion and elevated Creatine Phosphokinase (CPK) MB, but normal Troponin I and echocardiogram results?

Influenza-Associated Myositis

The most likely diagnosis is influenza-associated myositis (IAM), a well-recognized complication of influenza infection in school-aged children that typically presents with extreme exhaustion, muscle pain, and elevated CPK levels after respiratory symptoms begin to subside. 1

Clinical Reasoning

Why Myositis Fits This Presentation

• Timing is characteristic: Myositis typically develops 2-4 days after the onset of influenza symptoms, occurring as the acute respiratory symptoms begin to subside 1
• Age group matches perfectly: IAM predominantly affects school-aged children with a 2:1 male predominance 2
• CPK elevation is universal: All cases of IAM demonstrate elevated serum creatine phosphokinase levels 2
• Extreme exhaustion is expected: The muscle involvement causes profound fatigue and weakness, particularly affecting the gastrocnemius and soleus muscles (calf muscles in 69% of cases) 1, 2

Why Cardiac Involvement is Unlikely

The normal troponin I and echocardiogram effectively rule out clinically significant myocarditis 1. While minor ECG abnormalities can occur in up to 81% of hospitalized influenza patients, and post-mortem evidence of myocarditis has been reported, clinically significant myocarditis with normal troponin and normal echocardiogram is essentially excluded 1.

Understanding the CPK-MB Elevation

The elevated CPK-MB in this context does NOT indicate myocardial infarction—this is a critical clinical pitfall 1. CPK-MB can be elevated from:

• Skeletal muscle injury and breakdown during severe myositis 1
• Rhabdomyolysis, which occurs in approximately 3% of IAM cases 2
• The normal troponin I is the key discriminator here, as troponin is far more specific for myocardial injury than CPK-MB 1

Diagnostic Approach

Confirm the Diagnosis

• Check total CPK level: Should be markedly elevated (often >1000 U/L in myositis) 1, 2
• Assess for rhabdomyolysis: Check urine for myoglobinuria, monitor renal function (BUN, creatinine), and electrolytes 1, 2
• Physical examination: Look for calf muscle tenderness and pain on palpation, difficulty walking or refusing to walk 1, 2

Monitor for Complications

Rhabdomyolysis with renal failure occurs in 3% of IAM cases and is more common in girls and with influenza A 2. If CPK is extremely elevated (>5-10 times normal), aggressive hydration is essential to prevent acute kidney injury 1, 2.

Management Strategy

Supportive Care

• Hydration: Aggressive IV fluids if rhabdomyolysis is suspected (CPK >5000 U/L or myoglobinuria present) 1, 2
• Pain management: Avoid NSAIDs if renal function is compromised; acetaminophen is preferred 1
• Rest: Complete muscle rest until CPK levels normalize 2
• Monitor renal function: Daily BUN, creatinine, and electrolytes until CPK trending down 1, 2

Expected Course

• Clinical recovery typically occurs within 3 days (range 1-30 days) 2
• CPK levels should begin declining within 24-48 hours with appropriate hydration 2
• Complete recovery is the rule in uncomplicated cases 1, 2

Critical Pitfalls to Avoid

1. Do not assume cardiac disease based on CPK-MB alone—troponin is the definitive test for myocardial injury 1
2. Do not miss rhabdomyolysis—check urine for myoglobin and monitor renal function closely 1, 2
3. Do not overlook other serious complications: Consider meningococcal disease (increased risk post-influenza), encephalitis, or Reye's syndrome if neurological symptoms develop 1
4. Avoid aspirin: Given the association with Reye's syndrome in children with influenza 1