Prevalence and clinical characteristics of polymyositis in a Chinese cohort / 中华风湿病学杂志

ABSTRACT

Objective:To analyze the frequency and characteristics of polymyositis (PM) in idiopathic inflammatory myopathy (IIM), and to investigate whether PM is over-diagnosed.Methods:Patients diagnosed as IIM according to the Bohan & Peter criteria of IIM hospitalized in the Department of Rheumatology of China-Japan Friendship Hospital from 2008 to 2019 were involved in the study. Definite PM (dPM) was defined as typical clinical and pathological features including elevated creatine kinase (CK) level, muscle weakness and muscle biopsy findings with endomysial CD8 + T cell infiltration and expression of MHC-1 on sarcolemma. Meanwhile, dermatomyositis (DM), anti-synthase syndrome(ASS), immune-mediated necrotic myopathy(IMNM), sporadic inclusion body myositis(sIBM) and other myopathies were excluded according to the new classification criteria of IIM subtypes respectively. Statistical analysis was performed using SPSS software 24.0. The Kruskal-Wallis test and χ2 test were used to compare the clinical characteristics between the dPM group and other IIM subtypes. Results:A total of 1 259 patients with IIM including 1 015 (80.6%) DM and 244(19.4%) PM were enrolled in this study. According to the strict definition of PM criteria, only 0.5% of patients (6/1 259) in IIM could be diagnosed as dPM. Most PM patients were IMNM and ASS according to the new IIM subtypes criteria, of which 48.0% (117/244) were IMNM and 32.0% (78/244) were ASS. 66.7%(4/6) of dPM patients were women. One complicated with RA, and one was dPM overlaped with systemic sclerosis. All of them had muscle weakness, mild elevation of CK level [611(391,1 451) U/L], and were myositis-specific autoantibodies negative. Except one dPM patients who did not receive immunoregulatory therapy due to chronic obstructive pulmonary disease, the others were administrated with low or medium dose prednisone combined with or without immunosuppressive agents. After a median follow-up of (38±26) months, the muscle strength of dPM patients were improved.Conclusion:dPM is a very rare clinical subtype of IIM. PM is an over-diagnosed entity in clinical practice. Patients with dPM have mild symptoms and good outcome.