Epidemiology, autoimmune overlap and in-hospital outcomes of idiopathic inflammatory myopathies in Spain, 1997–2024

This is the authors' abstract. We don't add key points for this paper.

OBJECTIVES: To define the epidemiology, autoimmune overlap and in-hospital outcomes of idiopathic inflammatory myopathies (IIM) in Spain, separating true IIM from the symptomatic-myopathy code (G73.7), and to identify predictors of mortality and intensive-care (ICU) admission. METHODS: Population-based analysis of the Spanish hospital-discharge registries (CMBD and RAE-CMBD, 1997-2024). Rates are hospitalization rates, not incidence or prevalence. True IIM comprised the DM/PM group (dermatomyositis, polymyositis and paraneoplastic DM codes), immune-mediated necrotizing myopathy/other (IMNM/Other) and inclusion body myositis (IBM). Estimates were verified against patient-clustered standard errors. RESULTS: Of 36 732 episodes (8053 patients in 2016-2024, 9162 earlier), 34 228 (93.2%) were true IIM (DM/PM group 84.0%: 61.4% DM-coded, 35.7% PM-coded, 2.9% paraneoplastic; IMNM/Other 13.7%, IBM 2.4%). Hospitalization rose from 1.67 to 4.24 per 100 000/year, confined to ages ≥60. Coexisting autoimmune disease occurred in 13.7% (about one in seven). Mortality was 6.1% and stable. Predictors of death were advancing age (adjusted OR up to 7.97), malignancy, heart failure, interstitial lung disease (ILD), dysphagia and chronic kidney disease (C-statistic 0.77); ICU admission fell in the very elderly (OR 0.21), indicating ceiling-of-care de-escalation. Apparent protective effects of COPD and diabetes reflected collider bias. Within the DM/PM group, 29.4% were coded as polymyositis (M33.2), with more ILD, overlap and antisynthetase features but less malignancy, declining from 40% to 27%. CONCLUSION: IIM hospitalizations doubled over 28 years, driven by IMNM recognition and ageing, with outcomes governed by acute complications and comorbidity rather than subtype. Symptomatic myopathy should be analysed separately and paraneoplastic dermatomyositis unified with idiopathic DM.