Myalgia vs. Myositis: Key Differences and Clinical Implications
Myalgia refers to muscle pain without inflammation, while myositis is an inflammatory condition of the muscles characterized by muscle weakness, elevated muscle enzymes, and inflammatory changes on diagnostic testing. 1
Key Distinguishing Features
Myalgia
- Primary symptom: Pain in muscles without true weakness
- Laboratory findings:
- Normal creatine kinase (CK) levels
- May have elevated inflammatory markers (ESR, CRP)
- Imaging/EMG: No evidence of myopathy or muscle inflammation on MRI or EMG
- Clinical presentation: Often presents as proximal muscle pain resembling polymyalgia rheumatica
- Functional impact: Pain without significant weakness or functional limitation
Myositis
- Primary symptom: Muscle weakness, primarily in proximal extremities
- Difficulties standing up, lifting arms, and moving around
- May also have myalgia in severe cases
- Laboratory findings:
- Markedly elevated CK (median 2650 IU/L, range 335-20,270 IU/L) 1
- Elevated inflammatory markers
- Imaging/EMG:
- MRI shows increased intensity and edema in affected muscles
- EMG shows muscle fibrillations indicative of myopathy
- Clinical presentation:
- Weakness more prominent than pain
- Can have fulminant necrotizing course with rhabdomyolysis
- May involve vital muscles including myocardium
- Potential complications: Can be life-threatening, especially when associated with myocarditis or bulbar involvement 1
Diagnostic Approach
For Suspected Myalgia
- Check inflammatory markers (ESR, CRP)
- Verify normal CK levels
- Rule out other causes of muscle pain:
- Fibromyalgia
- Statin-induced myopathy
- Soft tissue rheumatic disorders
For Suspected Myositis
- Comprehensive laboratory evaluation:
- Muscle enzymes (CK, AST, ALT, LDH, aldolase)
- Inflammatory markers (ESR, CRP)
- Consider myositis-associated autoantibodies
- Advanced diagnostics:
- EMG
- MRI of affected muscles
- Consider muscle biopsy in unclear cases
- Cardiac evaluation:
- Cardiac troponin (troponin I more specific than troponin T in muscle disease)
- Electrocardiography
- Consider cardiac MRI if abnormalities detected 1
Treatment Implications
Myalgia
- NSAIDs may be sufficient for mild cases
- Corticosteroids for moderate to severe cases
- Physical therapy and pain management strategies
Myositis
- Immediate discontinuation of triggering agents (e.g., immune checkpoint inhibitors if iatrogenic)
- High-dose corticosteroids (1-2 mg/kg/day, sometimes IV pulse therapy)
- Consider additional immunosuppressive therapy:
- IVIG (intravenous immunoglobulin)
- Plasma exchange in severe or refractory cases
- Steroid-sparing agents (methotrexate, azathioprine, mycophenolate mofetil) 2
- Close monitoring for life-threatening complications
Clinical Pitfalls to Avoid
- Misdiagnosis: Mistaking myositis for simple myalgia can delay critical treatment
- Incomplete evaluation: Failing to check CK levels in patients with muscle pain
- Overlooking complications: Not screening for cardiac involvement in myositis
- Inadequate treatment: Using NSAIDs alone for true inflammatory myositis
- Delayed referral: Not involving specialists (rheumatology, neurology) promptly for suspected myositis
Special Considerations
- Immune checkpoint inhibitor-related myositis can be particularly severe with higher mortality rates (20% vs <10% for idiopathic inflammatory myositis) 1
- Bulbar symptoms (dysphagia, dysarthria, dysphonia) or respiratory failure may indicate severe myositis or associated myasthenia gravis 1
- Infectious myositis should be considered in appropriate clinical contexts, especially with localized symptoms 3
By understanding these key differences, clinicians can appropriately diagnose and manage these distinct muscle conditions, potentially preventing serious complications in cases of true myositis.