• Marie Holm Eliasen, Janne Petersen, Michael Eriksen Benros, and Merete Osler.
• Center for Clinical Research and Prevention, Bispebjerg and Frederiksberg Hospital, Frederiksberg, Denmark.
• Acta Psychiatr Scand. 2021 Jul 7.

ObjectiveTo explore the association of the number of traumatic brain injuries (TBIs) and temporal associations with the subsequent risk of depression in the population.MethodsNational register-based cohort study on all individuals registered with TBI (ICD-10: S06, ICD-8: 85.0-85.5) from 1977 to 2015 in Denmark (n = 494,216) and a sex- and age-matched reference population (n = 499,505). The associations with the number of TBIs and time to depression (0-6, 7-12 and more than 12 months following TBI) were analyzed using Cox proportional hazard regression.ResultsDuring a follow-up of mean 14.5 (SD 11.3) years, a total of 27,873 (5.6%) individuals who had at least one TBI and 15,195 (3.0%) in the reference population were diagnosed with a depression. First-time TBI was associated with a higher risk of depression in both men (HR = 1.73 [95% CI:1.67-1.79]) and women (HR = 1.66 [95% CI:1.61-1.70]) after multiple adjustments for educational status and comorbidities including previous depression, and the association became stronger in a dose-response association with the number of TBIs (test for trend p < 0.01). The HRs for depression were highest the first 6 months after the TBI in both men (HR = 5.69 [95% CI:4.66-6.94]) and women (HR = 4.55 [95% CI:3.93-5.26]) and decreased gradually the following year but remained elevated from one year after TBI until end of follow-up independent of the number of TBIs (p < 0.01). The associations did not vary with age or calendar time.ConclusionTraumatic brain injury is associated with a higher risk of depression, especially in the first months after TBI and the risk increases with the number of TBIs.© 2021 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.

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