Glucocorticoids in high dosage is the accepted treatment for severe dermatomyositis-polymyositis, though there is no controlled trial to prove its effectiveness. Prednisone is generally started at a dose of 1 to 2 mg/kg body weight per day (60 to 100 mg/d for adults). Improvement may begin within 1 to 4 weeks, though in some patients treatment may need to be continued for 3 months before improvement occurs. When improvement is noted, the daily dose may be reduced by 5 mg every 4 weeks. Repeated manual muscle testing and serum CK determinations should be performed to ensure that the myositis does not relapse. At about 40 mg/d, the schedule is changed gradually to 80 mg every other day in order to reduce the incidence of glucocorticoid side effects. There is some evidence that the use of alternate-day glucocorticoids from the outset may be effective, particularly in patients with milder disease.
Adults with acute to subacute dermatomyositis-polymyositis tend to improve more rapidly than those with chronic polymyositis; children also respond in most cases. If the dose is reduced too rapidly, or to too low a level, relapse will occur, necessitating return to high dosage. Prednisone therapy may have to be continued for several years, but an attempt should be made every year to withdraw the therapy from patients who are clinically stable in order to determine if the disease is still active.
Cytotoxic drugs should be tried when the response to glucocorticoids is inadequate after 1 to 3 months or when relapses are frequent. The combined use of glucocorticoids and a cytotoxic drug usually allows a lower dose of glucocorticoids to be used. Azathioprine (2.5 to 3.5 mg/kg body weight per day in divided doses) is the most commonly used cytotoxic drug in this disease; preliminary studies have shown a benefit of azathioprine as adjunctive therapy in glucocorticoid-treated patients. The aim of therapy with azathioprine is to lower the total lymphocyte count to about 750/mcL, while maintaining the hemoglobin level above 12 g/dL, the total white cell count above 3000/mcL, and the platelet count about 125,000/mcL. Periodic blood counts are required to monitor the cytotoxic drug therapy. Uncontrolled studies have shown some benefit with methotrexate, cyclophosphamide, and cyclosporine. Methotrexate is effective at doses that do not produce lymphopenia (usual dose about 0.5 mg/kg body weight per week achieved by slowly increasing from a starting dose of 10 mg). In a small controlled study, high-dose intravenous immunoglobulin (IVIg) therapy (0.4 g/kg daily for 5 consecutive days) produced improvement in patients with steroid-resistant dermatomyositis and probably should be used in severe or refractory cases. Anecdotal reports have suggested that IVIg may also be effective in polymyositis resistant to other forms of therapy.
Total-body irradiation has been successfully used in some patients with disease refractory to glucocorticoids and immunosuppressants, but long-term follow-up and controlled studies are lacking for this potentially dangerous therapy. A controlled trial of plasma exchange and leukapheresis in a small number of patients with dermatomyositis-polymyositis resistant to glucocorticoids showed no benefit over sham pheresis. Bed rest has been recommended in the acute phase of the disease but is harmful in the long term. Physiotherapy and rehabilitative devices are important in the long-term treatment of patients with dermatomyositis-polymyositis.
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