UC Davis

Nearly 3 million Americans sustain a traumatic brain injury (TBI) annually and 5.3 million are currently living with a TBI-related disability, with depression being the most prevalent consequence. While past studies linked TBI and dementia risk and depression and dementia risk, this is the first study that we know of that investigates the association between depression and dementia post-TBI in a non-military cohort. The objectives of this study are to characterize the incidence of depression and dementia post-TBI in a cohort representative of the U.S., to assess the role of depression in the risk of developing dementia, and to assess the timing of depression post-TBI to dementia to determine if depression is a consequence of a TBI or a prodromal symptom of dementia. This retrospective cohort comprises over 55,000 patients diagnosed with a TBI via International Classification of Diseases, Ninth Revision (ICD-9) and Tenth Revision (ICD-10) in Merative Marketscan from January 1, 2000 to December 31, 2019. Dementia diagnosis, medical, and psychiatric comorbidities were identified via ICD-9 and ICD-10. Depression diagnosis was defined via ICD-9 and ICD-10 coding as well as the prescription of common antidepressants. Of those participants, 30737 (56%) individuals were diagnosed with depression post-TBI and 8493 (15%) developed dementia. The adjusted hazard ratio for dementia was 3.021 (95% CI 2.857-3.185) for depressed individuals. In this cohort, depression diagnosis was associated with over a 3-fold increase in the risk of dementia diagnosis. The median (IQR) time from incident TBI to depression diagnosis was 8.86 (1.86-60.86) weeks, while time from depression diagnosis to dementia was 121.5 (65.43 -211.29) weeks, showing that depression is a consequence of TBI rather than a prodrome of dementia. Future research on timing, mechanisms, and treatment of TBI-related depression and dementia is needed.