Traumatic Brain Injury Increases Risk of Psychiatric Disorders Following Neurological Disease: Evidence from the UK Biobank Cohort

Background: Traumatic brain injury (TBI) is common in older adults and frequently co-occurs with neurological conditions such as dementia, stroke, and Parkinson's disease. Both TBI and neurological disease independently increase vulnerability to psychiatric disorders, yet the long-term impact of lifetime TBI exposure on mental health outcomes within these neurological populations remains poorly understood. Methods: Using data from the UK Biobank cohort (N = 502,154), we examined associations between TBI and subsequent psychiatric diagnoses in individuals diagnosed with dementia, stroke, or Parkinson's disease. Participants were assigned to one of four exposure groups (no TBI and no neurological disease, TBI only, neurological disease only, TBI plus neurological disease), and mixed-effects logistic regression estimated psychiatric risk across these groups and within each neurological condition. Kaplan-Meier curves and Cox proportional hazards models assessed 10-year cumulative risk. All analyses were adjusted for demographic and socioeconomic factors. Results: The combined presence of TBI and neurological disease produced the greatest psychiatric risk, exceeding that observed for either condition alone. Across dementia, stroke, and Parkinson's disease, TBI most reliably increased the odds of mood disorders, while associations for anxiety were less consistent. Ten-year cumulative incidence analyses demonstrated higher risk of mood and overall mental health disorders across all three neurological cohorts. Conclusions: Lifetime TBI exposure is associated with increased likelihood and long-term risk of psychiatric disorders in individuals with dementia, stroke, and Parkinson's disease, with mood disorders being the most consistent outcome across conditions. These findings highlight the importance of recognising TBI history when assessing and managing psychiatric risk in ageing neurological populations and underscore the need for integrated clinical pathways that address both neurological and mental health needs.