Estimate the incidence and prevalence of autoimmune encephalitis (AE) among patients seen at two major health systems serving approximately 70% of residents in the state of Utah. 

The detection of AE may be increasing over time, owing to increased awareness of the diagnosis and an increasing number of validated antibody biomarkers available to aid in diagnosis. 

Adult and pediatric cases of AE diagnosed from January 1, 2014 to December 31, 2024 at one of two major health systems in Utah were identified via ICD codes and confirmed by manual chart review in accordance with the 2016 adult and 2020 pediatric consensus diagnostic criteria. Incidence rates (new cases per 100,000 person-years) and point prevalence (cases per 100,000 persons as of December 31, 2024) were calculated for AE and infectious encephalitis. 95% confidence intervals were calculated using the exact Poisson method. Data from the second major health system in Utah will be integrated in a subsequent analysis. 

Of 1264 patients seen at one major health system with an encephalitis-related ICD code and a residential address in the state of Utah, 76 patients had AE (54 definite and 22 probable; 54% male, median age 55 years [range 4-80]) and 48 had infectious encephalitis (pathogen confirmed). The incidence of AE was 1.96 per 100,000 person years (95% CI 1.49-2.49) and point prevalence was 17.68 per 100,000 people (95% CI 13.54-22.38).  The incidence of infectious encephalitis was 1.31 per 100,000 person years (95% CI 0.93-1.74) and point prevalence was 9.39 per 100,000 people (95% CI 6.35-12.98).  

The incidence and prevalence of AE exceeded those of infectious encephalitis in this cohort and were higher than prior US population-based estimates, likely reflecting increased disease awareness and expanded neural antibody testing. Ongoing work to include data from a second major health system in Utah will enable population-based estimates.