What is the management and treatment of viral myositis?

Viral Myositis: Essential Clinical Guide

What It Is

Viral myositis is a self-limited, benign inflammatory muscle condition most commonly caused by influenza A and B viruses, presenting with acute bilateral lower extremity pain and weakness in children, requiring only supportive care with monitoring for rhabdomyolysis. 1, 2

• Viral myositis presents as diffuse muscle involvement with sudden onset of bilateral muscle pain and tenderness, primarily affecting the lower limbs in preschool and school-aged children 2, 3
• The condition is transitory and self-limiting, typically resolving without specific antiviral treatment 2
• Influenza viruses (especially type B) are the most common causative agents, though coxsackievirus B can cause pleurodynia, and other viruses may trigger acute rhabdomyolysis 1, 2

Clinical Presentation

Look for these specific features:

• Acute onset of bilateral leg pain with difficulty walking, standing, or climbing stairs, occurring during or shortly after a flu-like illness 2, 3
• Muscle tenderness on palpation of the lower limb muscle groups, particularly the calves, with preserved neurological examination 2
• Gait abnormalities including limping or refusal to walk due to pain, not weakness 2, 3
• Fever, myalgia, weakness, and lethargy accompanying the muscle symptoms 3
• The clinical symptoms are sufficiently clear that diagnosis can be made based on examination alone without waiting for laboratory confirmation 3

Diagnostic Approach

Obtain these specific tests:

• Creatine phosphokinase (CPK) levels, which will be elevated in viral myositis and are essential for monitoring rhabdomyolysis risk 2, 3
• AST and ALT levels, which are significantly elevated in patients with myositis 3
• Influenza testing to confirm the viral etiology 3
• The diagnosis is primarily clinical based on the characteristic presentation of bilateral lower limb pain following viral illness with normal neurological examination 2, 3

Management and Treatment

Provide supportive care only—no immunosuppression:

• Supportive care with rest, hydration, and analgesics for pain control is the cornerstone of treatment, as the condition is self-limited 2
• Serial CPK monitoring is mandatory to detect rhabdomyolysis, the main complication requiring intervention 2
• No corticosteroids or immunosuppressive agents are indicated, as this is a benign, self-resolving condition fundamentally different from immune-mediated inflammatory myopathies 2
• Hospitalization is typically unnecessary unless rhabdomyolysis develops with significantly elevated CPK levels 2

Critical Distinction: Viral vs. Other Myositis Types

Do not confuse viral myositis with these conditions that require aggressive treatment:

• Infectious necrotizing myositis requires immediate surgical debridement and broad-spectrum antibiotics (vancomycin plus piperacillin-tazobactam), as mortality reaches 36.5% even with treatment 4
• Immune-mediated necrotizing myopathy requires high-dose corticosteroids (prednisone 1 mg/kg/day) plus steroid-sparing agents like methotrexate, azathioprine, or mycophenolate mofetil 5, 6
• Bacterial pyomyositis requires imaging-guided or surgical drainage plus antibiotics 7

Key Differentiating Features

Viral myositis is distinguished by:

• Benign, self-limited course over days to weeks versus progressive weakness in inflammatory myopathies 2
• Bilateral lower extremity involvement with preserved strength (pain-limited function) versus true proximal muscle weakness in polymyositis/dermatomyositis 8, 2
• Absence of rash, unlike dermatomyositis which presents with heliotrope rash, Gottron papules, and photosensitive erythema 8
• Normal or mildly elevated CPK (typically <10x normal) versus markedly elevated CPK (>10x normal) in necrotizing myopathy 8, 2
• Recent viral illness with flu-like symptoms versus subacute onset over weeks to months in autoimmune myositis 8, 2

Monitoring and Follow-Up

Track these parameters:

• Serial CPK measurements every 24-48 hours until trending downward to ensure rhabdomyolysis is not developing 2
• Urine output and color monitoring for myoglobinuria if CPK is significantly elevated 2
• Clinical reassessment within 3-5 days to confirm expected improvement 2
• Resolution typically occurs within 3-7 days without sequelae 2

Common Pitfalls to Avoid

• Do not order extensive immunologic workup (myositis-specific antibodies, EMG, muscle biopsy) for typical viral myositis presentations, as the clinical diagnosis is sufficient 3
• Do not initiate corticosteroids, which are contraindicated in viral myositis and could worsen outcomes 2
• Do not miss rhabdomyolysis by failing to check CPK levels, as this is the only serious complication requiring intervention 2
• Do not confuse pain-limited function with true weakness—children with viral myositis have normal strength when pain is controlled, unlike inflammatory myopathies 2