Researchers have long seen a correlation between depression and dementia. A new study shows that older adults with Alzheimer's who also suffer from depression are more likely to see rapid decline than adults with Alzheimer's who are not depressed. The study was published in Neurobiology.

Robert S. Wilson, who led the study, said that, "Many studies have found that older people with some depression are more likely to develop cognitive decline or dementia."

However, he added that scientists could not say for certain whether depression led to Alzheimer's or whether both depression and Alzheimer's are caused by the same underlying things.

Wilson and his team studied over 1,700 people over the age of 50. They chose people who did not have Alzheimer's at the start of the study and followed them for about eight years, testing them for depression and for cognitive skills. The team performed this research at the Rush Alzheimer's Disease Center in Chicago.

Of the participants, about half showed signs of mild cognitive decline, which is often a precursor to dementia. About 18 percent were diagnosed with dementia. Individuals who showed more signs of depression at the start of the study were more likely to develop cognitive decline than those without depression.

The study also found that of those who had dementia, those with depression had a more rapid cognitive decline than those without depression.

About 600 people in the study died during the course of the research. An autopsy of their brains showed that depression was associated with a more rapid cognitive decline even considering the development of amyloid plaques, one of the biggest signs of Alzheimer's.

Wilson said, "Late life dementia is a very complex disorder and there are many factors that contribute to it, and depression is one of those factors."

The study showed a five percent increase in mental decline associated with depression. They found that depression did not increase with Alzheimer's; individual depression levels stayed constant before and after diagnosis of Alzheimer's, and in fact, depression symptoms decreased after diagnosis of Alzheimer's. It is common for people to be sad after being diagnosed with Alzheimer's, but it is rare for that sadness to turn into depression, as per Wilson.

However, Wilson is still not sure why depression might worsen the symptoms of Alzheimer's disease. He said, "Depression is probably doing something to your brain if it's affecting cognition. We think in the meantime there ought to be thought given to the importance of treating depression for these people."

He added that treating depression among older people with dementia or cognitive decline might help preserve mental function. Depression can be treated with medication or with therapy.

Although there is not currently proof that treating depression helps prevent the worsening of Alzheimer's, it may help patients to function with the disease and feel better.

Dr. Amos D. Korczyn, professor emeritus of neurology at Tel Aviv University in Israel, 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. They should be encouraged to be socially integrated."