1. There is a significantly increased risk of dementia in patients with previously diagnosed depression, with the highest risk observed in patients diagnosed with severe depression
Evidence Level: 2 (Good)
The prevalence of dementia is on the rise. With a scarcity of effective treatment options, increased emphasis has been placed on the identification of risk factors and effective prevention of the disease. Depression has previously been found to be associated with greater dementia risk, though the pre-existing literature examining the relationship between the two conditions has been inconclusive. Specifically, previous research has been characterized by short follow up times, making it unclear as to whether there was a true association, or if depression could be manifesting as a prodromal phase or early indicator of dementia. To address this knowledge gap, researchers of the present study sought to better characterize the relationship between the two conditions by performing a nationwide cohort study using the data between 1964 through 2016 from the Swedish National Patient Register. The study consisted of a cohort of individuals diagnosed with depression (n=119,386) with matched controls without depression diagnosis and a second cohort of siblings (n=50,644) consisting of same-sex full sibling pairs with discordant depression status. Researchers found an increased risk of dementia in the population matched cohort study, with 9802 individuals being diagnosed with dementia during a mean follow-up period of 10.41 years (OR 2.47, 95% CI 2.35 to 2.58, p<0.001). In examining specific diagnoses, researchers noted even stronger associations between depression and vascular dementia (OR 2.68, 95% CI 2.44 to 2.95, p<0.001) and Alzheimer’s disease (OR 1.79, 95% CI 1.68 to 1.92). The association was strongest within the first 6 months after depression diagnosis (OR 15.20, 95% CI 11.85 to 19.50, p<0.001), and remained significant even 20 years after follow-up (OR 1.58, 95% CI 1.27 to 1.98, p<0.001). Similar findings were elicited in the sibling cohort study, suggesting that the association exists despite accounting for some familial factors. This study therefore shows that, although a causal relationship cannot be established, there is a positive, enduring relationship between depression and dementia. In addition to risk identification, these findings also warrant investigation of successful prevention and treatment of depression as a means of decreasing dementia risk.
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