Polymyositis Diagnosis and Management

Diagnostic Criteria

The American College of Rheumatology recommends that polymyositis diagnosis requires a combination of clinical weakness, elevated muscle enzymes, characteristic electromyography findings, and inflammatory changes on muscle biopsy, with muscle biopsy serving as the gold standard for definitive diagnosis 1
Proximal muscle weakness is the hallmark symptom, affecting shoulders and hips more than distal muscles, with patients experiencing difficulty standing up, lifting arms, and moving around 1
Weakness, not pain, is the primary feature distinguishing true myositis from polymyalgia-like syndromes 1
Myalgia occurs in severe cases but is not the dominant feature 1

Elevated creatine kinase (CK) levels are present in approximately 64% of cases and serve as the primary screening test 2
Additional muscle enzymes to assess include aspartate aminotransferase (AST) and alanine aminotransferase (ALT) 3, 4
Lactate dehydrogenase (LDH) and aldolase are also elevated in some cases 4, 5
Inflammatory markers (ESR and CRP) are elevated in approximately 55% of cases 2, 3

Initial clinical assessment should document pattern and severity of weakness, focusing on proximal muscles 3, 2
Laboratory screening should include CK, AST, ALT, LDH, aldolase, ESR, CRP, and autoantibody panel (ANA, RF, anti-CCP, myositis-specific antibodies) 3, 2, 5
Cardiac evaluation should include troponin I and ECG (mandatory cardiac screening) 6
EMG of affected muscles can demonstrate active myopathy 3, 2
MRI of affected muscle groups can identify inflammation and guide biopsy 4
Muscle biopsy is necessary for definitive diagnosis when diagnosis is uncertain or to exclude mimics 3, 2

High-dose corticosteroids (prednisone 0.5-1 mg/kg/day) should be initiated concurrently with a steroid-sparing immunosuppressive agent such as methotrexate, azathioprine, or mycophenolate mofetil 5
For severe disease, consider methylprednisolone 1-2 mg/kg IV or higher-dose pulse therapy, IVIG therapy, or plasmapheresis 5, 6
If myocarditis is confirmed or suspected, permanently discontinue immune checkpoint inhibitors and initiate emergent high-dose corticosteroids (1 mg/kg methylprednisolone IV) 6