Cognitive abilities tend to decline with age. Although there is considerable variability in degree from one person to another, a large percentage of the very old develop dementia. By contrast, depression is not a normal part of aging, but many of the problems of aging—bereavement, illness, social isolation—can be risk factors for depression and affect cognitive abilities.
Depression is very prevalent in people with dementia, with estimates ranging as high as 50 percent. Depression combined with chronic stress can damage the brain. Some studies have shown that people with depression have a smaller than normal hippocampus, a brain structure associated with learning and memory.
One of the questions researchers are addressing is which comes first: Does dementia trigger depression or vice versa? It may be that depression is an early symptom of dementia; or that changes in the brain causing dementia in turn cause mood changes. Or it may be that depression causes a lower threshold in susceptible people. It is even possible that some people have a genetic factor that increases the risk for both depression and dementia.
Studies by scientists suggest that depression comes first and may be a risk factor for dementia. Researchers have been studying data from more than 13,000 long-time members of the Kaiser medical plan, looking at mid-life versus late-life depression as dementia risk. They have found that people with mid-life depression that abated before old age did not show an abnormal rate of dementia in old age. Those with depression that arose late in life, however, had twice the average risk for Alzheimer’s disease and a 50 percent increased risk for vascular dementia, which is dementia caused by impaired blood circulation in the brain, a possible result of a stroke.
Another important question is what treatments are available and how effective are they? A trial in which people with depression were treated with selective serotonin reuptake inhibitor (SSRI) antidepressants, including fluxetine (Prozac) or sertraline (Zoloft), showed improved if not completely normalized cognitive function. But whether or not treatment can delay the onset of dementia is unknown. The drugs currently available for dementia are not very effective. A number of drugs are under study currently, including drugs with different mechanisms of action. The goal is to provide longer, better relief or, ideally, to prevent the onset of dementia.