Antibody-Positive Autoimmune Encephalitis and Paraneoplastic Neurological Syndrome: Epidemiology and Outcome of Neuronal Antibody Testing in Sweden

Objective. To estimate the 5-year incidence rate of autoimmune encephalitis (AE) and paraneoplastic neurological syndrome (PNS) in Sweden. Methods. All patients who were tested for a neuronal antibody in Sweden between 2015 and 2019 were included. Patients in Healthcare region Mid Sweden (population 2.1 million) were invited to participate in a case ascertainment substudy. AE and PNS cases were defined using established diagnostic criteria. Crude and age-adjusted incidence rates of AE and PNS in Healthcare region Mid Sweden were estimated. Results. The number of tests for neuronal antibodies in Sweden increased between 2015 and 2019 from 1867 to 2505 (serum) and 863 to 1376 (CSF) per annum. The frequencies of positive results were stable over the entire study period, and the mean value was 6.1% for serum (CI95% 5.5–6.7) and 4.8% for CSF (CI95% 4.0–5.6). In total, 125 patients tested positive for neuronal antibodies in Healthcare region Mid Sweden between 2015 and 2019. Of these, 94 were included, and after case ascertainment, thirty-one cases of definite AE or PNS could be identified. The 5-year incidence rate of AE and PNS was 3.0 per million person-years (95% CI 1.9-4.1). The yearly incidence rates increased in the study period, from 1.5 per million person-years in 2015 (95% CI 0.0-3.2) to 4.3 per million person-years in 2019 (95% CI 1.5-7.1). Conclusions. In this first epidemiological study of AE and PNS in Sweden, the number of cases doubled from 2015 to 2019. This likely reflects increased availability of testing and awareness of these conditions.