What is the differential diagnosis and approach for a child presenting with fever, cough, difficulty walking, and bilateral thigh pain, suspected to have viral myositis?

Differential Diagnosis and Approach to Pediatric Fever, Cough, and Bilateral Thigh Pain

Your Clinical Suspicion is Correct

Benign acute childhood myositis (viral myositis) is the most likely diagnosis in this child with fever, cough for 7 days, and bilateral thigh pain with difficulty walking, particularly given the absence of trauma or joint involvement. 1, 2

Key Differential Diagnoses to Consider

Most Likely: Viral Myositis

• Classic presentation: Lower extremity pain during or following recovery from viral respiratory illness, with symmetric bilateral calf/thigh pain, refusal to walk or tiptoe gait 1
• Typically occurs in children around age 7 years (median), more common in boys 2
• Self-limited course with symptom resolution usually within 3-4 days 1, 2
• Associated with influenza and other respiratory viruses 2, 3

Critical Diagnoses to Rule Out

Pyomyositis 4, 5

• Look for: Focal muscle tenderness, fever, single muscle group involvement (not typically bilateral symmetric)
• S. aureus causes 90% of cases; community-acquired MRSA increasingly common 4
• Requires imaging (MRI preferred) and drainage if abscess present 4

Bacterial Meningitis 4, 6

• Look for: Bulging fontanelle (if infant), irritability, high-pitched cry, poor feeding, altered mental status 6
• Critical pitfall: Presence of viral respiratory symptoms does NOT exclude bacterial co-infection 7
• Requires lumbar puncture if suspected 4

Kawasaki Disease 7, 8

• Look for: Fever ≥5 days (this child has 7 days), conjunctival injection, oral changes, rash, extremity changes, cervical lymphadenopathy 7
• Time-sensitive: Risk of coronary artery aneurysms increases significantly after 10 days of fever 7
• Requires urgent echocardiography if suspected 7

Osteomyelitis/Septic Arthritis 4

• Look for: Localized bone/joint tenderness, refusal to bear weight, elevated inflammatory markers 4
• Less likely given bilateral symmetric presentation and absence of joint pain

Influenza-Related Encephalopathy 4, 6

• Look for: Lethargy, altered mental status, seizures, decreased consciousness 4, 6
• Can present with diarrhea and hyperventilation in children <2 years 6
• Myositis typically develops after subsidence of acute respiratory symptoms 4

Diagnostic Approach

History - Specific Details to Elicit

• Timing: When did leg pain start relative to respiratory symptoms? (Viral myositis typically occurs during or after recovery from URI) 1, 2
• Gait pattern: Tiptoe walking? Crawling? Complete refusal to walk? 1
• Muscle weakness vs. pain: Can the child move legs when not bearing weight? 1
• Urine color: Dark/tea-colored urine suggests myoglobinuria (rare but serious complication) 1
• Sick contacts: Recent influenza or respiratory illness in family members 6
• Fever pattern: High fever (>39°C) with purulent nasal discharge from onset suggests bacterial process 4

Physical Examination - Critical Findings

• Calf/thigh palpation: Symmetric bilateral tenderness without focal swelling or erythema (supports viral myositis) 1, 2
• Mental status: Any lethargy, irritability, or altered consciousness (suggests CNS involvement or severe systemic illness) 6
• Skin examination: Petechiae, purpura, rash (suggests meningococcemia, Kawasaki disease, or other serious bacterial infection) 7, 6
• Joint examination: Swelling, warmth, limited range of motion (suggests septic arthritis or osteomyelitis) 4
• Cardiovascular: Capillary refill, pulse quality, blood pressure (assess for shock) 6

Laboratory Investigations

For Typical Viral Myositis (Mild Cases)

• No investigation needed if classic presentation with mild symptoms 1

For Severe Cases or Atypical Features

• Creatine phosphokinase (CPK): Expect significant elevation (mean 5507 U/L in viral myositis series) 2
• Lactate dehydrogenase (LDH): Typically elevated (mean 827 U/L) 2
• AST/ALT: May be elevated (mean AST 199 U/L) 2
• Complete blood count: Tendency toward leukopenia (mean 4590/mm³) in viral myositis 2
• Urine myoglobin: Screen for rhabdomyolysis if severe pain or dark urine 1
• Inflammatory markers (CRP, ESR): Elevated in bacterial infections, Kawasaki disease 7, 8
• Blood cultures: If fever >38°C or concern for bacterial infection 8

Additional Testing Based on Clinical Concern

• Lumbar puncture: If any signs of meningismus, altered mental status, or ill appearance 4, 6
• Echocardiography: If fever ≥5 days with any features of Kawasaki disease 7
• MRI of affected muscles: If focal tenderness, fever persists, or concern for pyomyositis 4

Imaging

• Generally not needed for typical viral myositis 1
• MRI with contrast: Gold standard if pyomyositis suspected (demonstrates muscle inflammation and abscess formation) 4
• Radiographs: Consider if trauma history unclear or localized bone tenderness 4

Management Algorithm

If Classic Viral Myositis (Most Likely in This Case)

1. Reassurance: Self-limited, resolves in 3-4 days 1, 2
2. Symptomatic treatment: Rest and analgesia (acetaminophen or ibuprofen) 1
3. Hydration: Encourage oral fluids 1
4. Follow-up: Recheck in 24-48 hours if not improving 1
5. Return precautions: Dark urine, worsening weakness, inability to walk, altered mental status 1

Admission Criteria

• Myoglobinuria or highly elevated CPK (risk of acute kidney injury) 1
• Inability to walk with severe pain 1
• Altered mental status or lethargy 6
• Signs of shock or severe dehydration 6
• Concern for bacterial infection requiring IV antibiotics 4

If Pyomyositis Suspected

1. Empiric antibiotics: Vancomycin (covers MRSA) pending cultures 4
2. Imaging: MRI to identify abscess 4
3. Surgical drainage: If abscess present 4

If Kawasaki Disease Suspected (Fever ≥5 Days)

1. Urgent cardiology consultation 7
2. Echocardiography 7
3. IVIG and aspirin if criteria met 7

Common Pitfalls to Avoid

1. Anchoring bias: Don't assume viral myositis without considering serious bacterial infections 8
2. False reassurance from viral symptoms: Viral URI does NOT exclude bacterial co-infection 7
3. Missing Kawasaki disease: This child has 7 days of fever—actively look for other criteria 7
4. Delaying lumbar puncture: If any concern for meningitis, perform LP before antibiotics 4
5. Ignoring rhabdomyolysis: Check urine for myoglobin in severe cases 1
6. Assuming unilateral pathology: Bilateral symmetric pain is classic for viral myositis but can rarely occur with bilateral pyomyositis in immunocompromised hosts 4

Expected Clinical Course for Viral Myositis

• Respiratory symptoms: Mean duration 4.3 days 2
• Musculoskeletal symptoms: Mean duration 3.6 days 2
• Laboratory normalization: Complete recovery in 30 days (median) 2
• Recurrence: Rare but documented (one case after 10 months in series of 35) 2