Can depression increase Alzheimer's risk? by Emmanuel Fabella

Answer by Emmanuel Fabella:

The relationship between Depression and Alzheimer's is not conclusive, but there is evidence of increased risk, which I present below.
As alluded to by other responders, depression can itself cause Pseudodementia, which has its own attendant declines in memory and cognition.
Depression and Alzheimer's often coexist, with each condition likely exacerbating the other.
A 2008 study published in Neurology found increased risk of Alzheimer's Disease in people with previous episodes of depression, although the authors pointed out that their data could not differentiate between causation and mere association.

The study involved 486 people age 60 to 90 who had no dementia. Of those, 134 people had experienced at least one episode of depression that prompted them to seek medical advice.
The participants were followed for an average of six years. During that time 33 people developed Alzheimer's disease. People who had experienced depression were 2.5 times more likely to develop Alzheimer's disease than people who had never had depression. The risk was even higher for those whose depression occurred before the age of 60; they were nearly four times more likely to develop Alzheimer's than those with no depression.    

"We don't know yet whether depression contributes to the development of Alzheimer's disease or whether another unknown factor causes both depression and dementia,"  said study author Monique M.B. Breteler, MD, PhD, with the Erasmus University Medical Center in Rotterdam, the Netherlands.  "We'll need to do more studies to understand the relationship between depression and dementia."
One theory was that depression leads to loss of cells in two areas of the brain, the hippocampus and the amygdala, which then contributes to Alzheimer's disease. But this study found no difference in the size of these two brain areas between people with depression and people who had never had depression.
From: Depression Increases Risk of Alzheimer's Disease

Can depression increase Alzheimer's risk?