To compare the epidemiology of autoimmune and infectious encephalitis cases in a large tertiary center during the recent decade.

All encephalitis cases, aged 18 and above, admitted to our medical facility between the years 2010-2020 were thoroughly reviewed for demographic, clinical, laboratory, and imaging data and categorized based on etiology.

225 patients with encephalitis were identified. The most common identifiable cause was viral (42%), followed by autoimmune encephalitis (35%), bacterial (18%), and fungal/parasitic (5%). The incidence of AIE cases out of the yearly admitted cases increased substantially, from 3.8/100K in 2010 to 18.8/100K in 2020. The incidence of viral cases also increased while those of bacterial and fungal/parasitic infections remained stable. Patients with AIE were younger compared to infectious patients (p-value<0.001) and had significantly lower markers of systemic and cerebrospinal fluid inflammation (p-value for all <0.001). Seizures were more common among AIE patients (p-value<0.001), yet one-year mortality rates were significantly higher among infectious patients (p-value<0.001).

AIE incidence has risen significantly in our institution during the past decade, with current rates comparable to those of all infectious causes combined. Based on this cohort, clinical clues for an autoimmune etiology include a non-inflammatory cerebrospinal fluid profile, the presence of seizures, and temporal lobe imaging abnormalities (also common in herpetic encephalitis). In light of its rising incidence and the importance of early treatment, AIE should be considered in the differential diagnosis of all encephalitis cases.