A clinical, serological, and histopathological study of myositis patients with and without anti-RNP antibodies.

Twenty-nine patients with myositis, including 10 with polymyositis (PM), 6 with dermatomyositis (DM), and 13 with myositis associated with a connective tissue disease (CTD), were followed up for a mean observation time of 49 months. The 13 patients with CTD-associated myositis were further separated by the presence or absence of anti-RNP antibodies. The functional disability at diagnosis was pronounced without differences between the groups. The patients with anti-RNP antibodies did not differ from the other patients regarding initial muscle weakness, erythrocyte sedimentation rate, or creatinine phosphokinase values, but the histopathological muscle changes were generally milder. Rapid improvement of muscle strength on moderate doses of corticosteroids was seen in most patients. At the end of study, corticosteroid treatment had been withdrawn from 17 patients because of remission, including 6 of the 7 patients with anti-RNP antibodies. The presence of electromyographic changes compatible with myositis, pronounced muscle weakness before treatment, and a low erythrocyte sedimentation rate seemed to indicate a less favorable outcome unrelated to diagnostic subgroupings. However, the combination was rarely found among the anti-RNP-positive patients.