Importance: Incidence of traumatic brain injury (TBI) is rising in older adults. Dementia is a common comorbidity amongst older adults and may worsen post-TBI outcomes, but this has not been systematically studied. Objective: To compare all-cause mortality following TBI or non-TBI trauma (NTT), and quantify the impacts of age, deprivation, and pre-injury dementia. Design: Population-based retrospective cohort study Setting: Electronic health records (EHRs) from primary and secondary care Participants: Adult residents in Wales aged 18-100 with TBI or NTT from January 2000 to December 2022 Exposures: Traumatic (intracranial) brain injury (TBI) or non-TBI trauma (NTT) recorded in secondary care. Pre-injury diagnosis of dementia within primary or secondary care. Main Outcomes and Measures: All-cause mortality within 1, 6, and 12 months of hospitalised TBI/NTT in those with and without pre-injury dementia. Cox proportional hazard models were used to estimate survival probability across groups. Groups were propensity-matched by age, sex, and health conditions. Models were stratified by sex, age, and deprivation. Results: 23,428 TBIs (n=18,940) and 589,169 NTTs (n=421,259) were identified. TBIs were associated with higher mortality than NTT at all timepoints and age bands. Older age was associated with high mortality after TBI, with 16.9% of patients aged 65-79 (Hazard Ratio (HR)=4.52, 95% CI=[4.05,5.05], p<0.0001, relative to patients aged 18-39) and 31% of patients aged 80-100 dead within one month (9.08 [8.17,10.1], p<0.0001). The impact of pre-injury dementia on post-TBI mortality was specifically during the chronic phase, 6 and 12 months after injury. Using NTT without dementia as a comparator, 12-month mortality was significantly higher in TBI patients with pre-injury dementia (2.15 [2.03,2.28], p<0.0001) than those without (1.52 [1.44,1.60], p<0.0001). Conversely, 30-day mortality was significant after TBI irrespective of dementia: TBI with (2.67 [2.42, 2.95], p<0.0001) or without pre-injury dementia (2.87 [2.65, 3.12], p<0.0001) had more than twice the mortality risk relative to NTT without dementia. Conclusions and relevance: TBI is associated with higher all-cause mortality than NTT, particularly in older age. Pre-injury dementia was associated with particularly high mortality at 6 and 12 months. There is an urgent need to understand the reasons for poor outcomes in older adult TBI populations, especially those with pre-injury dementia.

DJS has received an honorarium from the Rugby Football Union (RFU) for participation in an expert concussion panel, and received payment by the RFU, the Football Association, and Premiership Rugby for private clinical services at the Institute of Sports Exercise and Health; no other relationships or activities that could appear to have influenced the submitted work.

This work is supported by Imperial College London and the UK Dementia Research Institute, which receives its funding from UK DRI Ltd, funded by the UK Medical Research Council, Alzheimer's Society, and Alzheimer's Research UK. The work was also supported by the Imperial College London NIHR Biomedical Research Centre. The funding sources were not involved in the study design, or in the preparation and submission of the article for publication. HL is supported by the President's PhD Scholarship at Imperial College London, and LML is supported by an NIHR clinical lectureship.

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