What is the recommended workup and treatment for myositis?

Myositis Workup and Treatment

The recommended workup for myositis should include a comprehensive rheumatologic and neurologic evaluation, muscle enzyme testing (CK, transaminases, LDH, aldolase), inflammatory markers, and appropriate imaging, followed by treatment with corticosteroids (prednisone 0.5-1 mg/kg/day) as first-line therapy with early addition of steroid-sparing agents. 1, 2

Diagnostic Workup

Initial Laboratory Tests

• Muscle enzymes: CK, transaminases (AST, ALT), LDH, aldolase
• Inflammatory markers: ESR, CRP
• Complete blood count (CBC)
• Comprehensive metabolic panel (CMP)
• Thyroid function tests: TSH, free T4
• 25-OH vitamin D levels
• Cardiac evaluation: Troponin (to evaluate myocardial involvement)
• Autoantibody testing:
  • ANA, RF, anti-CCP
  • Myositis-specific antibodies panel

Specialized Testing

• Electromyography (EMG) when diagnosis is uncertain
• MRI of affected proximal limbs
• Muscle biopsy (gold standard) if diagnosis remains uncertain
• Consider paraneoplastic autoantibody testing when myasthenia gravis is suspected

Clinical Evaluation

• Assess distribution, symmetry, and progression of muscle weakness
• Examine for skin findings suggestive of dermatomyositis
• Evaluate for dysphagia and respiratory compromise
• Check for symptoms of temporal arteritis (headache, visual disturbances)

Treatment Algorithm

Grade 1 (Mild) Myositis

• Continue normal activities
• Analgesia with acetaminophen or NSAIDs if no contraindications
• Monitor CK, ESR, CRP

Grade 2 (Moderate) Myositis

1. If CK is elevated (≥3x normal) and muscle weakness is present:

   • Initiate prednisone 0.5-1 mg/kg/day
   • Refer to rheumatologist or neurologist
   • NSAIDs for pain management if no contraindications

2. Consider early addition of steroid-sparing agents:

   • Methotrexate (first-line)
   • Azathioprine (alternative)
   • Mycophenolate mofetil (particularly useful for skin disease)

3. Continue treatment until CK normalizes before tapering steroids

   • Slow taper of corticosteroids after CK normalizes
   • Target CK levels in the low-normal range

Grade 3-4 (Severe) Myositis

1. Hospitalize for severe weakness

2. Urgent referral to rheumatologist or neurologist

3. Higher-dose corticosteroids:

   • Prednisone 1 mg/kg/day or
   • IV methylprednisolone 1-2 mg/kg or higher-dose bolus for severe compromise (weakness severely limiting mobility, cardiac, respiratory involvement, dysphagia)

4. Consider additional therapies:

   • IVIG therapy (especially helpful for skin manifestations)
   • Plasmapheresis in severe cases
   • Immunosuppressants if no improvement after 4-6 weeks:
     • Methotrexate, azathioprine, mycophenolate mofetil
     • Rituximab for refractory cases (may take up to 26 weeks to work)
     • TNF-α or IL-6 receptor inhibitors (caution with IL-6 inhibitors in patients with colitis)

5. Permanently discontinue immune checkpoint inhibitors if myositis is related to immunotherapy and there is evidence of myocardial involvement

Monitoring and Follow-up

• Regular assessment of muscle strength
• Serial CK measurements (target low-normal range)
• Functional status improvement
• Ability to taper corticosteroids
• Annual assessment of disease damage using validated indices

Special Considerations

Inclusion Body Myositis (IBM)

• Corticosteroids are generally not effective for IBM
• Consider trial of prednisone with methotrexate in select patients
• Early physical therapy to prevent joint contractures
• Early referral to physiatrist for assistive devices

Steroid Myopathy vs. Active Inflammatory Myopathy

Parameter	Steroid Myopathy	Active Inflammatory Myopathy
Onset	Insidious	May be acute or subacute
Pain	Usually absent	Often present
Muscle enzymes	Normal or minimally elevated	Significantly elevated
Response to steroids	Worsens with continued high-dose therapy	Improves with therapy

Common Pitfalls

• Tapering corticosteroids too quickly before CK normalizes often leads to relapse 3
• Failure to monitor for cardiac complications (conduction defects, tachyarrhythmias)
• Overlooking respiratory complications which can be life-threatening
• Not addressing dysphagia, which can lead to aspiration pneumonia
• Delaying immunosuppressive therapy in patients not responding to corticosteroids alone

Multidisciplinary Care

• Include physiotherapist and specialist nurse in care team
• Implement supervised exercise program to improve muscle performance
• Consider early physical therapy to prevent joint contractures and muscle atrophy
• Assess for need of assistive devices

Recent evidence suggests that early add-on IVIG to standard glucocorticoid therapy may lead to faster improvement and better outcomes in newly diagnosed myositis patients 4, though this approach requires further validation in larger studies.