The greater the degree of depression, the more likely the impairment, the researchers found.
"This is important, because mild cognitive impairment often precedes dementia," notes lead author Deborah Barnes, PhD, MPH, a mental health researcher at SFVAMC. Approximately 50 percent of patients diagnosed with MCI go on to develop dementia within three years, according to the study authors.
The study also found no correlation between depression and vascular disease - a significant finding, say the authors, because other researchers have hypothesized that vascular disease might lead to both depression and cognitive impairment by causing inadequate blood flow to different brain structures. "We found no evidence to support that hypothesis," reports Barnes, who is also an assistant professor of psychiatry at UCSF.
The study appears in the March, 2006 issue of Archives of General Psychiatry.
The researchers looked at 2,220 participants in the Cardiovascular Health Study, a longitudinal prospective study of adults 65 and older living in four American communities that is sponsored by the National Heart, Lung, and Blood Institute. The researchers measured the subjects' depressive symptoms using a standard depression scale. Six years later, the subjects were assessed for MCI by a team of dementia experts.
Ten percent of subjects with no depressive symptoms went on to develop MCI, and 13.3 percent of subjects with low depressive symptoms did. In contrast, 19.7 percent of subjects with moderate to high depression developed MCI after six years - nearly twice the rate of subjects with no depressive symptoms.
The findings were consistent among all subgroups in the study - men and women, younger and older, with and without vascular disease, and regardless of education level.
One major implication of the study, according to Barnes, is that family members and health care providers should pay attention when an older person seems newly depressed. "Even if they don't have cognitive impairment at that time, our study suggests that you probably want to keep an eye on them," she says. "Depression might be an early sign of neurodegeneration - in fact, it might be the first symptom that a family member notices."
Kristine Yaffe, MD, chief of geriatric psychiatry at SFVAMC and the principal investigator of the study, says the next step is to investigate whether treating older adults with newly diagnosed depressive symptoms might be effective in preventing the development of MCI. "Perhaps getting a family member in for early treatment would make a difference," she speculates. "We don't know the answer yet, but I think it's important to evaluate." Yaffe is also associate professor of psychiatry, neurology, and epidemiology at UCSF.
Barnes says she would also like to see other researchers investigate the reasons for the association between depression and MCI. "If vascular disease is not the mechanism, what is?" she asks. "One theory is that people who are undergoing stress or experiencing depression often have elevated levels of the stress hormone cortisol. There is growing evidence that this may lead directly to brain damage in the hippocampus," a part of the brain that plays a significant role in memory and Alzheimer's disease. "It would be good to find out," she concludes.
Co-authors of the study were George S. Alexopoulos, MD, of Cornell University; Oscar L. Lopez, MD, of the University of Pittsburgh School of Medicine; and Jeff D. Williamson, MD, MHS, of Wake Forest University School of Medicine.
The research was supported by grants from the National Heart, Lung, and Blood Institute, the National Institute on Aging, and the National Alliance for Research on Schizophrenia and Depression.
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