How do you differentiate between viral myositis and autoimmune myositis in a teenager?

Differentiating Viral Myositis from Autoimmune Myositis in Teenagers

The key distinction is that viral myositis presents with acute, self-limited bilateral lower extremity pain following a respiratory infection with normal or mildly elevated CK, while autoimmune myositis develops subacutely over weeks to months with proximal muscle weakness (not just pain), persistently elevated muscle enzymes, and positive myositis-specific autoantibodies. 1, 2, 3

Clinical Presentation Differences

Viral Myositis Characteristics:

• Acute onset of bilateral leg pain and muscle tenderness, typically affecting the gastrocnemius and soleus muscles of the lower limbs specifically 3
• Pain is the primary symptom, not true weakness—patients experience difficulty walking due to pain rather than inability to generate force 1, 3
• Follows a recent viral illness (usually respiratory infection within 1-2 weeks), most commonly influenza A/B or enteroviruses 4, 3
• Self-limiting course resolving within 3-7 days without treatment 3
• Neurological examination remains completely normal 3
• Gait disturbance is pain-mediated, not weakness-mediated 3

Autoimmune Myositis Characteristics:

• Subacute onset developing over weeks to months 1, 2
• True proximal muscle weakness is the hallmark—difficulty standing from seated position, climbing stairs, lifting arms overhead 1, 2
• Symmetric involvement of proximal upper and lower extremities 1
• In juvenile dermatomyositis specifically: characteristic rash (heliotrope, Gottron papules), calcinosis cutis, cutaneous vasculitis, and potential gastrointestinal vasculopathy 1
• Progressive course without treatment 1, 2

Laboratory Differentiation

Creatine Kinase (CK) Levels:

• Viral myositis: Normal to mildly elevated (typically <1000 IU/L), though rhabdomyolysis can occur with extreme elevations 4, 3
• Autoimmune myositis: Persistently elevated, often markedly so (can exceed 10 times upper limit of normal in immune-mediated necrotizing myopathy) 1, 2
• Critical point: Monitor CK serially—viral myositis normalizes within days, autoimmune remains elevated 2, 3

Inflammatory Markers:

• Both conditions can show elevated ESR/CRP, so these are not discriminatory 1

Autoantibody Testing:

• Myositis-specific autoantibodies (anti-Jo-1, anti-Mi-2, anti-MDA5, anti-TIF1-gamma, anti-NXP2) are present in autoimmune myositis and define clinical phenotypes 1, 2
• These autoantibodies are absent in viral myositis 4
• Testing should include a comprehensive myositis panel 1, 2

Advanced Diagnostic Testing

Electromyography (EMG):

• Autoimmune myositis: Shows polyphasic motor unit action potentials of short duration and low amplitude, increased insertional activity, fibrillation potentials, and sharp waves 1
• Viral myositis: Should be normal or show only minimal nonspecific changes 1

MRI Imaging:

• Autoimmune myositis: T2-weighted sequences with fat suppression (STIR) show diffuse muscle edema in proximal muscle groups, useful for identifying biopsy sites 1, 2
• Viral myositis: May show scattered muscle hyperintensities but typically not required for diagnosis 5

Muscle Biopsy:

• Autoimmune myositis: Shows perivascular and perimysial inflammation, perifascicular atrophy (in dermatomyositis), or necrosis without significant inflammation (in immune-mediated necrotizing myopathy) 1
• Viral myositis: Biopsy is rarely indicated and typically not performed given the self-limited nature 3

Temporal Course as Diagnostic Tool

The timeline is perhaps the most practical differentiator:

• Viral myositis: Symptoms peak within 24-48 hours and resolve within 3-7 days 3
• Autoimmune myositis: Progressive worsening over weeks to months without immunosuppressive treatment 1, 2

Common Pitfalls to Avoid

• Do not mistake pain for weakness: Viral myositis causes pain that limits movement; autoimmune myositis causes true motor weakness 1, 3
• Do not rely solely on CK elevation: Extreme CK elevation can occur in viral myositis with rhabdomyolysis, but the clinical course and autoantibodies distinguish them 4
• Do not miss the rash: In teenagers with dermatomyositis, the characteristic photosensitive rash with Gottron papules and heliotrope sign may precede muscle symptoms 1
• Consider viral triggers for autoimmunity: Viral infections (including influenza and COVID-19) can trigger autoimmune myositis, so recent viral illness does not exclude autoimmune disease 6, 7

Practical Diagnostic Algorithm

1. Assess symptom onset and duration: Acute (<1 week) suggests viral; subacute (weeks-months) suggests autoimmune 1, 3
2. Distinguish pain from weakness: Test actual muscle strength against resistance, not just patient-reported difficulty 1, 3
3. Check CK and repeat in 3-5 days: Normalizing CK supports viral; persistent elevation supports autoimmune 2, 3
4. Order myositis-specific autoantibodies: Positive results confirm autoimmune etiology 1, 2
5. Examine for dermatomyositis rash: Heliotrope, Gottron papules, shawl sign, V-sign 1
6. If diagnosis remains unclear after 7 days: Proceed to EMG and MRI imaging 1