A LARGE US electronic health records study has found that encephalitis is associated with a markedly increased long-term risk of dementia, with the strongest signal in non-infectious and viral cases and no significant association for bacterial encephalitis.
Researchers conducted a retrospective cohort study using TriNetX US electronic health records covering 2014 to 2024. Patients diagnosed with encephalitis were compared with propensity score-matched controls for a composite outcome of dementia and related diagnoses, its individual diagnostic categories, epilepsy as a positive control, and mortality.
Analyses were stratified by four age groups (under 20, 20-40, 40-60, and over 60 years) and by encephalitis cause, including non-infectious/post-infectious inflammatory, viral, bacterial, fungal/parasitic, and unspecified subtypes.
To assess specificity, the encephalitis cohort was also compared against sepsis, meningitis, and stroke cohorts, with risk ratios and risk differences calculated after accounting for confounders.
The overall cumulative incidence and prevalence of encephalitis were 74.3 and 90.2 per 100,000 people, respectively, over the study period. Encephalitis was associated with an increased risk of the composite dementia outcome across all age groups. When restricted to specific dementia diagnoses, the association remained robust in adults aged 40-60 and over 60, but was no longer evident in younger cohorts, suggesting the signal in younger individuals may reflect post-encephalitic neurological sequelae rather than true dementia.
Non-infectious/post-infectious inflammatory encephalitis conferred the highest relative risk, followed by viral, fungal/parasitic, and unspecified encephalitis, while bacterial encephalitis showed no significant association.
Compared with sepsis, meningitis, and stroke, the encephalitis cohort showed higher risk for the composite outcome and individual categories, with vascular dementia the sole exception, higher in the stroke cohort.
Epilepsy risk was significantly increased across all encephalitis cohorts, and mortality was significantly higher in the encephalitis group versus all comparators except sepsis.
These findings have demonstrated that encephalitis is associated with a markedly elevated long-term risk of dementia and related diagnoses. As this was a retrospective observational cohort study, causality cannot be established. The authors have emphasised the need for long-term monitoring of encephalitis survivors regardless of age or underlying cause.
Reference
Aditi et al. Long-term risk of dementia following encephalitis: a large-scale retrospective cohort study of electronic health records. J Neurol. 2026;273:511.
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