The incidence of anti-neutrophil cytoplasmic antibodies (ANCA) associated vasculitis (AAV) shows disparate results due to variable classification criteria and heterogeneous-population series. We aimed to estimate the incidence of AAV in a well-defined population with standardized classification criteria. Population-based study of AAV patients diagnosed from January 2000 to December 2023 in Cantabria, Northern Spain. Patients were classified according to ACR/EULAR 2022 into granulomatosis with polyangiitis (GPA), microscopic polyangiitis (MPA), or unclassified vasculitis if the criteria were not met. Eosinophilic granulomatosis with polyangiitis (EGPA) patients were not included. The annual incidence rates were estimated by cases over 1,000 000 (10⁶) (95% CI) including overall AVV, type of AAV, sex, and year of diagnosis. A literature review was also performed. We included 152 (80/72 men; mean age; 70.6 ± 13.18 years) patients. They were classified as MPA (67; 44%), GPA (64; 42.2%), and unclassified vasculitis (21; 13.8%). Annual incidence was 13.4 (10–16.8)/10⁶ [male 14.5 (10.5–18.5); female 12.1 (8.7–15.6)]. The Annual incidence of MPA was 5.9 (4–7.8)/10⁶ and GPA 5.6 (3.9–7.3)/10⁶. The mean Annual incidence increased from 6.1 (4.5–7.7)/106–16.5 (5.6–27.4)/10⁶ in the last three years, particularly, in GPA from 2.3 (0.3–4.9)/106–8.2 (2–14.5)/10⁶. The prevalence of AAV was 184.7 (181–188)/10⁶. During a 20-year period we found that the incidence of AAV (GPA and MPA) in Northern Spain is higher than Southern Spain, but lower than Northern European countries. An increase in the incidence was observed in the last years.

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.