Many doctors, observing how their depressed patients appear to be more likely to develop dementia, have wondered: Which comes first, the depression or the dementia? Do both problems share the same cause?

Now, a new study from Rush University Medical Center shows depression to be independent of dementia-related brain changes. "These findings are exciting because they suggest depression truly is a risk factor for dementia,” said Dr. Robert S. Wilson, a neuropsychiatrist and lead investigator in the study, “and if we can target and prevent or treat depression and causes of stress we may have the potential to help people maintain their thinking and memory abilities into old age."

The study involved 1,764 people from the Rush Memory and Aging Project and the Religious Orders Study, which includes the participation of more than 1,100 older nuns, priests, and brothers, who have agreed to medical and psychological evaluation each year and brain donation after death. Participants, whose average age was 77, had no thinking or memory problems at the start of the study. Once the experiment began, they were screened annually for symptoms of depression, such as loneliness and lack of appetite, while also taking tests on their memory skills and thinking.

During the study, which lasted on average for eight years, 315 people, or 18 percent, developed dementia, while 922 people, or 52 percent, developed mild cognitive impairment (MCI), the kind of mild problems with memory and thinking that often appears as a precursor to Alzheimer's disease. Another 680 people died during the study, with the researchers able to perform autopsies on 582 of them. Unexpectedly, the researchers found no relationship between how much damage was found in the brain and the level of depression symptoms people had or in the change in depression symptoms over time.

The people who had developed mild cognitive impairment were more likely to have a higher level of symptoms of depression before they were diagnosed, but they were no more likely to have any change in their symptoms of depression after the diagnosis than people without MCI. At the same time, people with dementia were also more likely to have a higher level of depression symptoms before the dementia started, but they had a more rapid decrease in depression symptoms after dementia developed. Analyzing all of the data, the researchers found having a higher level of depression symptoms was associated with more rapid decline in thinking and memory skills.

Source: Wilson RS, Capuano AW, Boyle PA, et al. Clinical-pathologic study of depressive symptoms and cognitive decline in old age. Neurology. 2014.