Fifteen years of autoimmune encephalitis in Denmark: incidence, epidemiology, and trends in treatment

INTRODUCTION: Despite increasing awareness of autoimmune encephalitis (AE), real-world, population-based epidemiological data are scarce. Using 15 years of nationwide, centralized testing data, we aimed to (1) characterize all Danish patients with neuronal cell-surface antibodies (NSAbs), GAD65, and GFAP; (2) estimate AE incidence in Denmark; and (3) assess temporal trends in treatment strategies. METHODS: Nationwide testing data from 2009 to 2023 were obtained from the Danish centralized antibody test center, and the medical records for antibody-positive patients were reviewed. National and regional incidence rates were calculated using census data, and trends in testing, incidence, and treatment were analyzed. RESULTS: A total of 198 patients were included. The most prevalent antibody was NMDAR (n = 79), followed by LGI1 (n = 37), GAD65 (n = 20), and CASPR2 (n = 19). The national average crude incidence rate (IR) in 2019-2023 was 2.8 cases per million person-years. Incidence increased by 10% annually during the study period (IRR 1.10, 95% CI 1.04-1.17, P = 0.001). The highest regional IR was observed in the region hosting the national testing center. Second-line treatment in NMDARE increased from 24% in 2009-2013 to 48% in 2019-2023. In contrast, use of steroid-sparing therapy declined from 41 to 26% in NMDARE and from 39 to 0% in LGI1. DISCUSSION: In this nationwide cohort, Danish AE patients were comparable to previously reported cohorts and incidence estimates were similar to those reported in other national studies. Higher incidence in the testing region suggests that local expertise improves case ascertainment. Treatment patterns evolved over time, likely reflecting increasing clinical experience with AE.