To characterize the presence of hyponatremia associated with autoimmune or infectious encephalitis subtypes.

Hyponatremia has been commonly depicted with bacterial and fungal meningitis but its presence in autoimmune or viral etiologies seems less understood. The literature supports a correlation of hyponatremia with subtypes of encephalitides, namely anti-LGI-1, anti-VGKC, and HSV; however, data are limited. This highlights that there may be identifiable patterns to help aide in faster diagnosis of suspected encephalitis subtypes, namely autoimmune. In this study, our aim was to identify hyponatremia patterns or lack thereof with various etiologies of encephalitis. 

A retrospective cohort study was conducted using electronic health records in Greater Houston and Baltimore areas between 2005 and 2022. Encephalitis patients were dichotomized by presence of hyponatremia (serum sodium < 135mmol/L). A bivariate analysis was used for dichotomous clinical features, while median was used for continuous variables. 

172 patients were identified to have encephalitis of autoimmune etiology and 91 patients were identified with infectious etiology. Patients were grouped into their identified subtypes based on antibody presence, identified pathogen, or noted to be seronegative. Anti-LGI-1, anti-VGKC, and HSV encephalitides were noted to have a statistically significant (P<0.05) association with hyponatremia. Cryptococcal encephalitis was noted to have a statistically significant (P<0.05) association with hyponatremia however only 2 cases were reported in this cohort. Hyponatremia also was associated with higher age (>50), female gender, and abnormal MRI findings.

Anti-LGI-, anti-VGKC, and HSV encephalitis showed a significant association with low serum sodium levels, which agrees with existing literature. This predictive pattern can be used in corroboration with additional features to develop a risk score in helping identify encephalitis subtypes, namely of autoimmune etiology. Given the limited number of cases in certain subtypes however, further data are needed to better characterize this correlation with hyponatremia.