Epidemiology of multiple sclerosis in a healthcare area of the Basque Country, Northern Spain: A population-based study (2009–2023)

Background Multiple sclerosis (MS) prevalence has increased across Europe, yet contemporary population-based data from southern Europe remain scarce. Objectives In this retrospective population-based cohort study conducted in a healthcare area of the Basque Country (Barrualde-Galdakao Integrated Healthcare Organisation [BGIHO]), northern Spain, we estimated prevalence, incidence, and clinical characteristics of MS. Prevalence was estimated on 31 December 2023 using the 2017 McDonald criteria, and incidence was assessed from 1 January 2009 to 31 December 2023 using both symptom-onset and diagnosis-based definitions. Results Prevalence was 131.8 per 100,000 inhabitants (95% CI 119.6–145.2); age-adjusted prevalence (European Standard Population [ESP2013]) was 125.4 (95% CI 113.3–138.1). Mean annual incidence (2009–2023) was 4.4 per 100,000 person-years (95% CI 3.9–5.1) by symptom onset and 5.3 (95% CI 4.7–6.0) by diagnosis date. Relapsing–remitting MS (RRMS) accounted for 67.1% of cases; secondary progressive MS (SPMS) for 18.6%; and primary progressive MS (PPMS) for 14.2%. Median EDSS was 2 (IQR 0–5). Conclusions MS prevalence in this healthcare area is high. The findings reflect an aging MS population with evolving clinical characteristics. The use of complementary incidence definitions provides insight into disease occurrence and healthcare detection. Continued epidemiological monitoring is warranted.