What are the diagnosis and management options for myositis?

Myositis Diagnosis and Management

Diagnosis

Myositis requires a systematic diagnostic approach centered on measuring creatine kinase (CK), which is elevated in the majority of patients (median 2650 IU/L), combined with assessment for life-threatening complications including myocarditis and myasthenia gravis. 1

Clinical Presentation

• Proximal muscle weakness is the hallmark symptom, with patients experiencing difficulty standing up, lifting arms, and moving around 1
• Myalgia may be present but is not the primary feature; pain without weakness suggests polymyalgia rheumatica rather than myositis 1
• Ptosis and diplopia occur commonly and may indicate associated myasthenia gravis (present in 12.5% of cases) 1
• Dropped head syndrome can occur in some patients 1
• Typical dermatomyositis rash is usually absent in most cases 1

Essential Laboratory Testing

• Creatine kinase (CK) is the most important test, elevated in the majority of patients with myositis (range 335-20,270 IU/L); normal CK is typical in patients with myalgia alone 1, 2
• Aldolase, AST, ALT, and LDH may also be elevated 1, 2
• Inflammatory markers (ESR and CRP) should be measured 1
• Myositis-specific and myositis-associated antibodies are mostly negative but can include anti-Jo1, anti-SRP, anti-PL-7, anti-PL12, anti-PM/Scl, anti-TIF1 gamma, or anti-SM 1, 2

Critical Life-Threatening Assessments

Cardiac evaluation must be systematic for any patient with suspected myositis because myocarditis significantly increases mortality (20% vs <10% in idiopathic inflammatory myositis): 1

• Cardiac troponin (troponin I is more specific than troponin T in skeletal muscle disease) 1
• Electrocardiography 1
• Cardiac MRI if clinical syndrome, elevated troponin, or ECG abnormalities are present 1
• Screen for dyspnea, palpitations, chest pain, or syncope as these indicate possible myocarditis 1

Evaluate for myasthenia gravis with anti-acetylcholine receptor (AChR) and antistriational antibodies, especially if ptosis, diplopia, or bulbar symptoms (dysphagia, dysarthria, dysphonia) are present 1, 2

Additional Diagnostic Studies

• Muscle MRI with T2-weighted/STIR sequences is highly sensitive for detecting muscle inflammation and edema 1, 2
• Electromyography (EMG) typically reveals myopathic pattern with muscle fibrillations 1, 2
• Muscle biopsy confirms diagnosis with variable degrees of inflammatory and necrotic changes when diagnosis remains uncertain 1, 2
• Urinalysis to assess for rhabdomyolysis 1

Key Diagnostic Pitfalls

• Normal cardiac enzymes cannot always rule out myocarditis, so maintain high clinical suspicion 1
• CK levels are usually normal in patients with myalgia alone, helping differentiate from true myositis 1
• Distinguish from polymyalgia rheumatica (pain without true weakness, normal CK) and statin-induced myopathy 1

Management

High-dose glucocorticoids (prednisone 1-2 mg/kg/day or equivalent) are the first-line treatment for myositis, with mandatory immunotherapy withdrawal and consideration of IVIG and/or plasma exchange in the presence of life-threatening manifestations. 1

Treatment Algorithm by Severity

Grade 1 (Mild weakness with or without pain)

• Continue monitoring with close observation 1
• Prednisone 0.5 mg/kg/day may be offered if CK/aldolase are elevated with muscle weakness 1
• Acetaminophen or NSAIDs for myalgia if no contraindications 1
• Consider holding statins 1

Grade 2 (Moderate weakness limiting instrumental activities of daily living)

• Hold immunotherapy temporarily if applicable; may resume upon symptom control with normal CK and prednisone <10 mg 1
• Prednisone 0.5-1 mg/kg/day if CK is elevated (≥3× upper limit of normal) 1
• Referral to rheumatologist or neurologist 1
• May require permanent discontinuation of immunotherapy if symptoms persist 1

Grade 3-4 (Severe weakness, life-threatening manifestations)

Immediate aggressive treatment is imperative for severe myositis: 1

• Permanently discontinue immunotherapy if applicable 1
• High-dose systemic glucocorticoids: prednisone 1-2 mg/kg/day (median 70 mg/day) or intravenous methylprednisolone pulses 1, 3
• Intravenous immunoglobulins (IVIG) used in up to 20% of severe cases 1
• Plasma exchange performed in approximately 10% of patients, especially with poor response to corticosteroids or life-threatening situations 1

Life-Threatening Indications for Aggressive Therapy

Mandatory aggressive treatment with high-dose glucocorticoids, IVIG, and/or plasma exchange when: 1

• Bulbar symptoms (dysphagia, dysarthria, dysphonia)
• Dyspnea or respiratory failure
• Myocarditis
• Severe muscle weakness

Second-Line and Steroid-Sparing Agents

Consider adding immunosuppressive agents early to control disease and reduce glucocorticoid-related side effects: 4, 5

• Methotrexate 1, 4
• Mycophenolate mofetil 1, 4
• Azathioprine 1, 4
• Hydroxychloroquine 1
• Rituximab shows evidence of effect in patients with certain myositis-specific autoantibodies 4

Corticosteroid Tapering

• If improvement occurs, slow taper over 4-6 weeks according to response 1
• If unable to lower corticosteroid dose below 10 mg/day after 6-8 weeks, consider disease-modifying antirheumatic drugs (DMARDs) 1
• Early consideration of steroid-sparing agents is recommended due to likely prolonged treatment requirements 1

Adjunctive Therapy

Individualized and supervised exercise should be combined with pharmacological treatment based on evidence showing improved muscle performance 4

Special Considerations

• Prednisone is FDA-approved for systemic dermatomyositis (polymyositis) 3
• Subgrouping patients into clinical and serological subtypes may identify biomarkers for response to specific agents 4
• Long-term monitoring is necessary as skin changes can develop months after initial myopathy presentation 6
• Malignancy screening should be performed, particularly in dermatomyositis 7