To determine the incidence of anti-NMDAR encephalitis in a large multi-ethnic patient cohort.

Anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis is a potentially fatal, increasingly recognized cause of acute encephalitis. Prior studies have been unable to estimate the disease’s incidence or examine racial/ethnic differences.

We conducted a retrospective cohort study of the multi-ethnic, community-dwelling members of Kaiser Permanente Southern California (KPSC) between January, 2011, and December, 2022. The complete electronic health records of individuals with text-string mention of ‘NMDA’ and ‘encephalitis’ were reviewed to identify persons who met the diagnostic criteria for definite or probable anti-NMDAR encephalitis. Crude and adjusted age and sex-standardized incidences stratified by race and ethnicity were estimated according to the 2020 US Census population. Risk ratios comparing incidence rates between racial/ethnic groups were calculated using Poisson regression.

We identified 70 patients who met the diagnostic criteria for anti-NMDAR encephalitis. The median age at onset was 23 years (IQR 14-30y) and 45 (64%) were females. The female preponderance was more pronounced in Black (83%) and Asian (80%) than White persons (33%) and intermediate in Hispanic persons (59.5%). The crude incidence of anti-NMDAR encephalitis per 1 million person-years was higher in Black (2.95, CI 1.28–4.61), Hispanic (2.09, CI 1.46–2.72) and Asian persons (1.86, CI 0.71–3.01) than White persons (0.38, CI 0.08–0.68). Teratoma or dermoid cyst was the most commonly identified underlying etiology, present in 22 (31.4%) patients, including 21 females and was more common in Black persons but uncommon in patients <12 years of age. A viral trigger was reported in 4 (5.7%) individuals. Most patients (n=43, 61.4%) had no identifiable etiology.