To investigate the clinical characteristics of patients with polymyositis (PM) and dermatomyositis (DM) to provide a basis for clinical diagnosis and treatment of these conditions.

Three hundred and eight patients with PM and DM admitted to the Department of Rheumatology and Immunology of the Third Hospital Affiliated to Soochow University were collected from January 1998 to March 2018, and the clinical manifestations, muscle enzymes, electromyogram (EMG), muscle biopsy and treatment outcome were retrospectively analyzed.

The most common initial symptom of PM was muscle weakness, accounting for 92.8% of all cases, while rash was the most common initial presentation in DM cases, accounting for 66.7%. The incidence of interstitial lung disease (ILD) (69.8% vs 37.5%, χ²=20.790, P=0.000) and concomitant tumors (95.6% vs 4.3%, χ²=6.956891, P=0.008350) was significantly higher in DM patients than in PM patients. The percentages of PM patients with elevated CK (t=-6.684, P=0.000) and CK-MB (t=-6.964, P=0.000) were significantly higher than those of DM patients. The pathological changes in PM patients mainly consisted of myofiber degeneration and connective tissue hyperplasia, while those in DM patients were mainly characterized by peripapillary atrophy, vacuolar degeneration and inflammatory cell infiltration between the intermuscular and perivascular tissues. Follow-up was performed in 243 patients. The mortality of DM was higher than that of PM (18.7% vs 7.1%, χ²=4.392, P=0.036). The main causes of death were pulmonary infections and tumors.

PM and DM patients are different in clinical manifestations, muscle enzymes, pathology and mortality. Infection secondary to ILD and tumor are more common in DM, with a higher mortality rate, than in PM.