What are the differences between myopathy, myositis, and myalgia in terms of diagnosis and treatment?

Myopathy versus Myositis versus Myalgia: Differential Diagnosis and Treatment

Myopathy, myositis, and myalgia represent a spectrum of muscle disorders with distinct diagnostic features and treatment approaches that require careful differentiation for optimal patient outcomes.

Comparison Table

Feature	Myopathy	Myositis	Myalgia
Definition	Broad term for any disorder affecting skeletal muscle [1]	Inflammation within skeletal muscle [1]	Muscle pain without weakness or inflammation [2]
Primary Symptoms	Muscle weakness (often proximal)	Weakness with inflammation; may include pain	Pain without true weakness [2]
CK Levels	May be normal or elevated	Significantly elevated (median 2650 IU/L) [2]	Usually normal [2]
Inflammatory Markers	May be normal	Highly elevated (ESR, CRP) [2]	May be elevated in some conditions
Diagnostic Tests	EMG, muscle biopsy, MRI	EMG (shows fibrillations), MRI (shows inflammation), muscle biopsy, autoantibodies	Clinical evaluation, normal CK
MRI Findings	Variable depending on type	Muscle edema, enhancement	Usually normal
EMG Findings	Myopathic pattern	Myopathic pattern with fibrillations	Usually normal
Associated Conditions	Muscular dystrophies, metabolic disorders, drug-induced	Autoimmune disorders, immune checkpoint inhibitor therapy, infections	Polymyalgia rheumatica, fibromyalgia, drug-induced
Life-threatening Potential	Generally low	High (especially with myocarditis) [2]	Low
First-line Treatment	Depends on cause	High-dose corticosteroids [3]	NSAIDs, sometimes low-dose corticosteroids [2]

Diagnostic Approach

Myopathy

• Broad category including any disorder affecting skeletal muscle 1
• Diagnostic workup includes:
  • Blood tests: CK, transaminases (AST, ALT), LDH, aldolase 3
  • EMG: Shows polyphasic motor unit action potentials of short duration 3
  • MRI: Helps identify appropriate biopsy sites 3
  • Muscle biopsy: Gold standard for differentiating types of myopathy 3

Myositis

• Defined by inflammation within skeletal muscle tissue 1
• Key diagnostic features:
  • Significantly elevated CK (median 2650 IU/L) 2
  • Proximal muscle weakness is the primary symptom 2
  • May present with myalgia in severe cases 2
  • Cardiac evaluation is essential due to risk of myocarditis 2
  • Diagnostic tests include:
    • Autoantibody panels (myositis-specific and myositis-associated antibodies) 2
    • EMG showing muscle fibrillations 2
    • MRI showing muscle inflammation 2
    • Muscle biopsy confirming inflammatory changes 2

Myalgia

• Muscle pain without true weakness 2
• Diagnostic features:
  • Normal CK levels (key differentiator from myositis) 2
  • Pain primarily in proximal muscles 2
  • May have elevated inflammatory markers in conditions like polymyalgia rheumatica 2
  • Normal EMG and MRI findings 2

Treatment Approaches

Myopathy

• Treatment depends on underlying cause:
  • For genetic myopathies: Supportive care, physical therapy
  • For drug-induced myopathies: Discontinuation of offending agent 4
  • For metabolic myopathies: Specific metabolic interventions

Myositis

• First-line treatment is high-dose corticosteroids (prednisone 0.5-1 mg/kg/day) 3
• Add steroid-sparing agents early:
  • Methotrexate (15-25 mg weekly) 3
  • Azathioprine (2 mg/kg ideal body weight) 3
  • Mycophenolate mofetil (target dose 1000 mg twice daily) 3
• For severe or refractory cases:
  • IVIG (1-2g/kg divided over 1-2 days) 3
  • Rituximab for severe cases 3
• For immune checkpoint inhibitor-induced myositis:
  • Discontinuation of immunotherapy is mandatory with life-threatening manifestations 2
  • High-dose corticosteroids, IVIG, and/or plasma exchange for severe cases 2

Myalgia

• NSAIDs alone are often insufficient for symptom control 2
• For polymyalgia rheumatica-like symptoms:
  • Low-dose corticosteroids (10-20 mg/day) 2
  • Steroid-sparing agents for steroid-dependent cases 2
• For drug-induced myalgia:
  • Consider discontinuation of offending agent (e.g., statins)

Critical Distinctions and Pitfalls

1. Myositis vs. Myalgia: The key differentiator is CK level - elevated in myositis, normal in myalgia 2

2. Life-threatening associations:

   • Myositis may be associated with myocarditis and myasthenia gravis, requiring urgent cardiac evaluation 2
   • Early recognition is critical as mortality is higher in immune checkpoint inhibitor-related myositis (20%) compared to idiopathic inflammatory myositis (<10%) 2

3. Diagnostic pitfalls:

   • Inflammation can be present in some muscular dystrophies, complicating diagnosis 5
   • "Treatment-resistant myositis" may actually be inclusion body myositis or genetic muscular dystrophy 5
   • Always perform muscle biopsy before initiating immunosuppressive therapy 5

4. Treatment monitoring:

   • Regular assessment of muscle strength and enzymes
   • Monitor for medication-specific side effects
   • Adjust therapy based on clinical response and laboratory parameters

By carefully distinguishing between myopathy, myositis, and myalgia through appropriate diagnostic testing and clinical evaluation, clinicians can implement targeted treatments that address the specific pathophysiology of each condition.