Diagnostic Approach to Acute Myositis

Initial Assessment and Cardiac Involvement

The American College of Cardiology recommends immediately assessing for cardiac involvement with troponin and ECG in patients with suspected myositis, as cardiac involvement would require permanent discontinuation of any causative agents and dramatically alter management 1, 3
Cardiac involvement changes the entire treatment paradigm and requires urgent cardiology consultation, with consideration of echocardiogram or cardiac MRI if troponin is elevated 1, 3

Muscle weakness is more typical of myositis than pain alone, which is the key distinguishing feature, according to the American College of Rheumatology 1, 2, 3
The American College of Rheumatology recommends assessing for proximal symmetric weakness, examining skin carefully for dermatomyositis findings, and documenting if weakness limits instrumental activities of daily living 1, 2, 3
A muscle enzyme panel, including aldolase, AST, ALT, and LDH, should be ordered to differentiate myositis types, as recommended by the American Association for Clinical Chemistry 1, 2, 4
Inflammatory markers, such as CRP, should be ordered to complement the ESR, as recommended by the American College of Rheumatology 1, 4

The American College of Cardiology recommends reviewing for drug-induced causes, including statins, before diagnosing inflammatory myositis, and holding any immune checkpoint inhibitors or statins immediately if the patient is taking them 1, 2, 3
The American Heart Association recommends considering other medications, such as fibrates, colchicine, and hydroxychloroquine, as potential causes of myositis 4

The American College of Rheumatology recommends considering EMG, MRI, or muscle biopsy based on diagnostic uncertainty, as these tests can help differentiate myopathic from neurogenic patterns and guide biopsy site 1, 2, 3
The American Cancer Society recommends ordering age-appropriate cancer screening, such as CT chest/abdomen/pelvis, colonoscopy, and PSA, in patients with suspected paraneoplastic myositis 3

The American College of Rheumatology recommends initiating prednisone 0.5-1 mg/kg/day given CK elevation ≥3x ULN with weakness, and providing NSAIDs for symptomatic relief if no contraindications 2, 3, 4
The American Society of Nephrology recommends ensuring hydration with urine output >0.5 mL/kg/hour to prevent myoglobin-induced acute kidney injury, and monitoring serum creatinine twice daily if any concern for rhabdomyolysis 4
The American College of Rheumatology recommends rechecking CK, ESR, CRP weekly initially, and considering adding immunosuppressants if no improvement after 2 weeks on corticosteroids 2, 3, 4