Epidemiology of Dermatomyositis and Other Idiopathic Inflammatory Myopathies in Northern Spain

Background/Objectives: The epidemiology of dermatomyositis (DM) and other idiopathic inflammatory myopathies (IIMs) remains not well established, especially in the Mediterranean region. We aimed to estimate the prevalence and incidence of IIM in a well-defined population of South Europe using standardized classification criteria. Methods: This population-based study included all IIM patients diagnosed from January 2000 to December 2022 in Cantabria, Northern Spain. IIM diagnosis was confirmed by fulfillment of the 2017 EULAR/ACR classification criteria or, alternatively, by European Neuro Muscular Center criteria for immune-mediated necrotizing myopathy (IMNM) and Connors' criteria for antisynthetase syndrome (ASyS). Prevalence and incidence were expressed in cases per 100,000. A literature review was also performed. Results: A total of 60 patients (41 women, 19 men; mean age 52.6 ± 18.8 years) were included. The prevalence of IIM was 20 cases per 100,000 population [95% CI 14.5-25.1], and the annual incidence rate was 0.9 cases per 100,000 person-years [95% CI 0.6-1.14]. A significant upward trend in IIM incidence was observed with an estimated annual percentage change of 5.74% (95% CI: 2.16%-9.44%, p = 0.0015). The most common subtype was DM (n = 31, 51.7%), followed by ASyS (n = 17, 24%), IMNM (n = 9, 14.6%), and polymyositis (PM) (n = 3, 4.7%). No inclusion body myositis (IBM) cases were identified. Conclusions: Incidence and prevalence of IIM align with prior reports. We observed an increase in IIM incidence and a shift in subtype distribution, with ASyS and IMNM becoming more frequent. These findings have clinical relevance, as each IIM subtype carries distinct prognostic and therapeutic implications.