Laboratory tests may show anemia, and the ESR sometimes may be elevated. ANA are present in up to 80% of patients. Creatine kinase is the most sensitive and reliable enzyme tested and may be elevated before muscle weakness. Levels increase in periods of disease activity and decrease with response to therapy.

Electromyography is a sensitive, yet nonspecific test for DM/PM. Findings include spontaneous fibrillations, complex repetitive discharges, and early recruitment. The electromyogram is abnormal at presentation in 90% of patients. A normal test makes the diagnosis unlikely. Electromyography may be helpful in the selection of a site for muscle biopsy.

Muscle biopsy is usually performed in all patients with suspected idiopathic inflammatory myopathies. Although percutaneous needle biopsy is less invasive, open surgical biopsy is usually recommended because a larger specimen can be obtained. Histologic features typically reveal chronic inflammatory cells in the perivascular and interstitial areas surrounding myofibrils. Degeneration and necrosis of myofibrils, phagocytosis of necrotic cells, and myofibril regeneration are common features. In DM, perifascicular myofibril atrophy, endothelial hyperplasia of vessels, deposition of immune complexes in intramuscular arteries, and vasculitis can be seen.

The clinical utility of MRI of the muscle has not been well established, but it may be helpful in the selection of muscle biopsy site and in the distinction between active muscle inflammation and fatty infiltration.

Extramuscular involvement should be evaluated by chest radiography, spirometry with diffusion capacity, and electrocardiography. Other tests are obtained if symptoms suggest specific abnormalities such as barium swallow in patients with dysphagia and high-resolution chest CT in patients with dyspnea. Age-appropriate screening for cancer is recommended.

TREATMENT

The severity and prognosis of PM and DM vary, ranging from mild disease to severe disease that may be resistant to multiple therapies. Features that may be associated with a worse prognosis include delay in onset of treatment, severe weakness at presentation, dysphagia, respiratory muscle weakness, and interstitial lung disease. The presence of anti–Jo-1 antibodies may be associated with a poorer response to treatment and prognosis.

Corticosteroids are first-line therapy and should be used in high doses for the first several months, with a very slow taper over a period of 9 to 12 months. Treatment with lower doses and shorter courses may result in incomplete response or disease recurrences. Initial dose is usually 1 mg/kg/day, and pulse intravenous corticosteroids can be given at 1 g/day for 3 days for severe disease.

Immunosuppressive agents such as methotrexate and azathioprine are used for patients who fail treatment with corticosteroids alone or are given at diagnosis in more severe cases. Earlier treatment may limit exposure to corticosteroids. Other medications include mycophenolate mofetil, tacrolimus, and cyclosporine. Cyclophosphamide is used in severe lung disease. Methotrexate is usually avoided in patients with interstitial lung disease because it may be difficult to recognize methotrexate-induced lung toxicity. Studies showed azathioprine use resulted in a lower requirement of prednisone, but response to therapy may take as long as 4 to 6 months. Methotrexate has been studied in retrospective trials, with response rates up to 80%, including patients who had failed corticosteroid therapy.

Most patients require a long course of therapy. An attempt to discontinue the prednisone before stopping other immunosuppressive therapies is preferred. A slow taper of methotrexate or azathioprine after a long period of disease remission can also be attempted. Most patients achieve sustained disease remission on therapy; however, disease recurrence after discontinuation of treatment is common.

Recurrent and resistant disease pose a major challenge to treatment. Patients who do not completely respond to initial therapy with prednisone, methotrexate, or azathioprine may respond to intravenous immunoglobulin or rituximab. Studies with rituximab are limited by its testing in a small number of patients. Controlled trials with larger number of patients are needed. Intravenous immunoglobulin is an effective short-term therapy for resistant myositis. Cost is an important limitation to this treatment. Cyclosporine, tacrolimus, and mycophenolate mofetil have been evaluated in retrospective studies of patients with resistant disease with positive results, including a reduction in chronic corticosteroid doses.

Response to treatment should be carefully monitored with clinical evaluation and creatine kinase levels. During later stages of disease, with severe muscle atrophy, levels of creatine kinase may not increase significantly and disease flares may be difficult to distinguish from corticosteroid myopathy.

Potential complications from immunosuppressive therapy must be monitored and avoided if possible. Long exposure to corticosteroids has well-known toxicities. Most patients with idiopathic inflammatory myopathies require prolonged exposure to prednisone and therefore should be on prophylactic treatment with bisphosphonates. Patients with new onset of dyspnea and/or cough during therapy should be evaluated for possible lung infection.