Polymyositis is a rare and favourable outcome subtype of idiopathic inflammatory myopathy in Chinese patients

Objectives: To investigate the prevalence and characteristics of typical polymyositis (PM) in Chinese patients with idiopathic inflammatory myopathy (IIM). Methods: Patients diagnosed with IIM according to the 2017 EULAR/ACR criteria were included. Serological aspects including myositis-specific antibodies (MSA) and pathological data were re-evaluated. The diagnosis of typical PM was strictly done using the pathological criteria, while excluding other IIM subtypes such as dermatomyositis (DM), immune-mediated necrotising myopathies (IMNM), anti-synthetase syndrome (ASS), and sporadic inclusion body myositis (sIBM), based on their respective diagnostic criteria. Results: A total of 544 IIM patients with muscle biopsy were involved, and 129 of them were diagnosed with initial PM according to the 2017 EULAR/ACR criteria. Only 6 (1.1%, 6/544) patients met the strict definition of typical PM after re-evaluation. Patients with typical PM were MSA-negative (100% vs. 35.7%, p=0.003) and had CD8+ T cells surrounding or invading non-necrotic muscle fibres in muscle biopsies (100% vs. 7.8%, p<0.001) compared to the initially diagnosed PM patients. All typical PM patients achieved clinical remission at the second-year follow-up. Typical PM patients had a favourable prognosis compared to MSA-negative IMNM and unspecific myositis patients. Conclusions: Strictly defined typical PM is a rare clinical subtype in Chinese IIM patients. Typical PM patients with classical pathology were MSA-negative and responded well to treatment and had a favourable prognosis. It is crucial for clinicians to combine clinical, serological, and pathological features to properly distinguish PM from other IIM subtypes.