How do you differentiate flu-induced muscle soreness from myositis?

Differentiating Flu-Induced Muscle Soreness from Myositis

The key distinction is timing and severity: typical flu myalgia occurs during the acute febrile illness affecting the back and limbs, while influenza-associated myositis develops 2-4 days after respiratory symptoms begin to subside, predominantly affects the calf muscles with extreme tenderness, causes profound exhaustion and difficulty walking, and is associated with markedly elevated CPK levels (often >1000 U/L). 1

Clinical Timing: The Most Critical Differentiator

Flu-induced muscle soreness:

• Occurs during the acute illness, within the first 24 hours of fever onset 2
• Affects primarily the back and limbs as part of the systemic viral syndrome 2
• Resolves as the acute respiratory symptoms improve 2

Influenza-associated myositis:

• Develops during early convalescence, typically 2-4 days after onset of influenza symptoms 2, 1, 3
• Occurs as acute respiratory symptoms are subsiding, not during peak illness 1, 4
• Most commonly affects school-aged children (5-9 years), with 82% being male 5, 3

Anatomic Distribution

Flu-induced muscle soreness:

• Generalized myalgia affecting back and limbs diffusely 2
• Non-specific distribution without focal tenderness 2

Influenza-associated myositis:

• Gastrocnemius and soleus muscles (calves) are involved in 69% of cases, either alone or with other muscle groups 3
• Marked focal tenderness to palpation of affected muscles 2, 1
• Bilateral calf pain is the stereotypical presentation 5

Severity of Symptoms

Flu-induced muscle soreness:

• Mild to moderate discomfort that doesn't impair function significantly 2
• Patient remains ambulatory without difficulty 2

Influenza-associated myositis:

• Extreme exhaustion and profound fatigue due to muscle involvement 1
• Difficulty walking or refusal to walk, particularly in children 1
• Pain severe enough to cause functional impairment 2, 3

Laboratory Findings: The Definitive Test

Flu-induced muscle soreness:

• CPK levels normal or minimally elevated 2
• No specific laboratory abnormalities 2

Influenza-associated myositis:

• Check total CPK level, which should be markedly elevated, often >1000 U/L 1
• Mean CPK value is 3579 U/L in confirmed cases 5
• CPK is invariably elevated in myositis 3
• Elevation normalizes within 3 days as symptoms resolve 6

Diagnostic Algorithm

When evaluating muscle pain in influenza patients:

1. Assess timing relative to respiratory symptoms 1, 3

   • During acute illness = typical myalgia
   • 2-4 days after onset, as respiratory symptoms improve = suspect myositis

2. Examine for focal calf tenderness 1, 3

   • Diffuse tenderness = typical myalgia
   • Focal calf tenderness with pain on palpation = myositis

3. Evaluate functional status 1

   • Normal ambulation = typical myalgia
   • Difficulty walking or extreme exhaustion = myositis

4. Order CPK if myositis suspected 1

   • CPK >1000 U/L confirms myositis
   • Also check urine for myoglobinuria, renal function, and electrolytes 1

Critical Pitfalls to Avoid

• Do not miss rhabdomyolysis: While rare (3% of myositis cases), it occurs more commonly with influenza A (86% of rhabdomyolysis cases) and in girls (80%), leading to renal failure in 80% of those cases 3, 7
• Monitor renal function daily with BUN, creatinine, and electrolytes until CPK levels trend down if CPK is extremely elevated (>5-10 times normal) 1
• Avoid aspirin in children with influenza due to association with Reye's syndrome 1
• Avoid NSAIDs if renal function is compromised; prefer acetaminophen for pain management 1
• Do not assume cardiac disease based on CPK-MB alone; troponin is the definitive test for myocardial injury 1

Prognosis and Management

Influenza-associated myositis is typically benign:

• Complete recovery usually occurs within 3 days 2, 6, 3
• Most cases are self-limiting and require only supportive care 8, 5
• Aggressive hydration is essential if CPK is extremely elevated to prevent acute kidney injury 1
• No cases in recent series showed renal impairment despite elevated CPK 5