Among older adults with dementia, those who are also depressed tend to have more rapid cognitive decline, according to a new study.
"Many studies have found that older people with some depression are more likely to develop cognitive decline or dementia," Robert S. Wilson said.
But there has been disagreement over whether depression actually contributes to cognitive decline or whether both are the result of other underlying problems, he told Reuters Health.
Wilson worked on the study at the Rush Alzheimer's Disease Center in Chicago.
He and his coauthors analysed data on 1,700 people over age 50 who initially had no cognitive problems and were followed for an average of eight years. Each year doctors evaluated participants' depression symptoms and tested their thinking and memory skills.
About half of the original group developed mild cognitive impairment, which often comes before dementia, and 18 per cent were diagnosed with dementia.
Participants who had more depression symptoms on their initial exams were more likely to experience cognitive decline.
Depression symptoms were also linked to dementia and to more rapid decline after a dementia diagnosis, according to results published in Neurology.
Almost 600 of the participants died during the study and their brains were examined postmortem.
Among those participants, depression was still associated with faster mental decline when amyloid plaques - considered the hallmarks of Alzheimer's disease - and other neuropathological problems found on autopsy were taken into account.
"Late life dementia is a very complex disorder and there are many factors that contribute to it, and depression is one of those factors," Wilson said.
Depression accounted for five per cent of the variability in mental decline that couldn't be attributed to other problems with the brain, he said.
Depression symptoms did not increase once cognitive decline started, and actually decreased once dementia was diagnosed. Depression itself is an organised process of the brain, and as brain function declines it is common for people to experience periods of unhappiness but it is rare that those periods are consistent, Wilson said.
Dr. Amos D. Korczyn, professor emeritus of neurology at Tel Aviv University in Israel, called the study an "important contribution" to scientists' understanding of how cognitive decline, depression and changes in the brain may be linked.
"Obviously as the authors note, more work needs to be done to better understand the relationship of depression to dementia which is so important when trying to modify the disease course," he told Reuters Health in an email.
Depression symptoms are common among people with dementia, especially early on in the disease, he said.
It's still unclear exactly how depression might lead to cognitive decline and dementia, Wilson said.
"Depression is probably doing something to your brain if it's affecting cognition," Wilson said. "We think in the meantime there ought to be thought given to the importance of treating depression for these people."
Treating depression with medication or behavioural therapy might enhance mental function among people with cognitive decline or dementia, he said.
There isn't much scientific evidence that antidepressants are effective for people with dementia but most doctors prescribe them and the impression is that they provide some relief, Korczyn said.
"People with depression, as well as those who are demented, frequently tend to avoid exposure to others, even to friends and this may exacerbate the depression," Korczyn added. "They should be encouraged to be socially integrated."
Caregivers often become depressed as well, and as they are usually spouses, are neither young nor necessarily healthy themselves, he noted. They should be offered help and moral support as well.