Treatment with prednisone is indicated. This patient's findings of weakness, elevated muscle enzymes, electromyogram and MRI abnormalities, and no skin involvement suggest polymyositis. A definitive diagnosis can only be made by muscle biopsy, and every effort should be done to obtain it quickly. Nonetheless, treatment should be started as soon as possible to prevent complications of active disease, improve symptoms, and have a better long-term prognosis. Treatment should not be withheld if the biopsy is delayed or cannot be obtained. The initial treatment of polymyositis or dermatomyositis with muscle involvement is systemic glucocorticoids, most commonly prednisone given at 1 mg/kg/d. Some patients with severe disease require treatment with intravenous methylprednisolone, and many physicians use methotrexate or azathioprine at onset for their glucocorticoid-sparing benefits. If refractory or recurrent disease is noted, additional agents such as mycophenolate mofetil, intravenous immune globulin, rituximab, cyclophosphamide, or tumor necrosis factor (TNF)-α inhibitors can also be considered.

The TNF-α inhibitor adalimumab is not recommended as initial therapy prior to a trial of prednisone with or without a glucocorticoid-sparing agent in patients with polymyositis or dermatomyositis. TNF-α inhibitors have been reported to be effective in some patients with refractory disease.

Cyclosporine is an immunosuppressant agent that preferentially targets T cells and demonstrates efficacy in several rheumatologic and autoimmune diseases, including rheumatoid arthritis, systemic lupus erythematosus, inflammatory myositis, psoriasis, pyoderma gangrenosum, and inflammatory bowel disease. Toxicity is relatively common (hypertension, nephrotoxicity, tremor, hirsutism); therefore, cyclosporine is mainly used as a third-line agent in rheumatologic diseases.

Leflunomide is approximately as effective as methotrexate for rheumatoid arthritis; its use in other diseases is less explored. Toxicities include liver and hematopoietic abnormalities, infection, and interstitial lung disease. This agent has an extremely long half-life (months) and undergoes enterohepatic circulation.