Individuals with dementia commonly experience depression as well, but the nature of this connection has remained unclear. Some have suggested that individuals develop depression in response to worsening cognitive abilities, while others believe that depression may be brought on by underlying biological mechanisms that contribute to Alzheimer's disease (AD) and the mild cognitive impairments (MCI) that preceded it. In a new study, however, researchers from Chicago's Rush University Medical Center question these theories, demonstrating that depression occurs independently of MCI and AD and may be a risk factor for cognitive decline rather than a symptom of it.
Specifically, the researchers looked for changes in depressive symptoms experienced by individuals who would go on to develop MCI or AD. With data from the Rush Religious Orders Study, an ongoing investigation of cognitive functioning in a cohort of religious clergy, the researchers examined 917 individuals who did not initially display signs of dementia. For up to 13 years, they were tested annually for signs of cognitive impairments and symptoms of depression. The researchers were able to observe the subjects for an average of four years before they received a diagnosis, and by the study's end, 190 subjects had developed AD. Subjects who had more symptoms of depression during their clinical assessments were more likely to develop MCI or AD, a result that has been supported by other recent evidence of a link between earlier depression and subsequent cognitive decline.
In looking at changes in depressive symptoms over time, the researchers also found evidence refuting earlier theories on the connection between depression and cognitive decline. If depression developed in response to an individual's awareness of cognitive decline, they reasoned, then an increase in depressive symptoms should occur at the first signs of dementia. Similarly, an increase in depressive symptoms should accompany the onset of dementia if the two are connected by biological mechanisms. This was not the case, however, as they found no increase in depressive symptoms either before or after a diagnosis of MCI or AD, an indication that depression, while it elevated the risk of MCI and AD, was not a result of mental decline.
In analyzing their findings, the researchers suggest that depression may cause changes in the brain that leave it vulnerable to biological processes that promote cognitive decline. Researchers should continue to study this connection and further clarify the relationship between depression and AD, seeking ways to treat the brain changes caused by these disorders. With a better understanding of how depression could contribute to AD, we may be better able to identify those at risk and and develop treatments to prevent symptoms of depression from contributing to cognitive decline later on.
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