Objective: To investigate the association between traumatic brain injuries (TBIs) and the risk of young onset dementia (YOD), that is, dementia before 65 years of age.
Methods: The study cohort comprised 811,622 Swedish men (mean age 5 18 years) conscripted for military service between 1969 and 1986. TBIs, dementia, and covariates were extracted from national registers. Time-dependent exposures using Cox proportional hazard regression models were evaluated.
Results: During a median follow-up period of 33 years, there were 45,249 men with at least 1 TBI in the cohort. After adjustment for covariates, 1 mild TBI (hazard ratio [HR] 5 1.0, 95% confidence interval [CI] 5 0.5–2.0), at least 2 mild TBIs (HR 5 2.5, 95% CI 5 0.8–8.1), or 1 severe TBI (HR 5 0.7, 95% CI 5 0.1–5.2) were not associated with Alzheimer dementia (AD). Other types of dementia were strongly associated with the risk of 1 mild TBI (HR 5 3.8, 95% CI 5 2.8–5.2), at least 2 mild TBIs (HR 5 10.4, 95% CI 5 6.3–17.2), and 1 severe TBI (HR 5 11.4, 95% CI 5 7.4–17.5) in age-adjusted analysis. However, these associations were largely attenuated after adjustment for covariates (1 mild TBI: HR 5 1.7; at least 2 mild TBIs: HR 5 1.7; 1 severe TBI: HR 5 2.6; p < 0.05 for all).
Interpretation: In the present study, we found strong associations between YOD of non-AD forms and TBIs of different severity. These associations were, however, markedly attenuated after multivariate adjustment.