Are chronic blues a prelude to dementia?

Is there a causal relationship between depression and dementia? The association exists, according to a study just published in the journal Archives of General Psychiatry. The question, however, is whether depression can actually cause dementia.

To investigate the association between these two conditions, researchers at Kaiser Permanente of Northern California examined data from over 13,500 long-term Kaiser Permanente members. These individuals had been enrolled in a larger study from 1964 to 1973, at the age of 40 to 55 years old. Health information, including a survey that asked about depression, was collected during that period. Researchers combined this initial data with a 1994 to 2000 follow-up that looked again at depression in this cohort, followed by a look at diagnoses of dementia or Alzheimer's disease in 2003. The average age of study participants at this final follow-up was 81; 58 percent were women.

Ultimately, the researchers found two significant correlations between depression and dementia or Alzheimer's disease. The strongest correlation between depression and dementia was in those participants who were depressed at both mid- and late life: 31.5 percent developed some form of dementia, compared to 20.7 percent of participants who never reported depression. When the researchers then separated the participants who developed Alzheimer s from those who developed what s known as vascular dementia, they found that people who were depressed later in life were more likely to develop Alzheimer's, while those depressed at both mid- and late life were three times as likely to develop vascular dementia. Those who experienced depression at midlife but not later in life, however, had no increased risk of developing either condition. Thus, say the researchers, while late-onset depression seems to be an early sign of Alzheimer's, chronic depression may very well increase the risk of vascular dementia.

Still, much remains to be resolved about these associations. It has long been established that Alzheimer's disease is the leading cause of dementia, while the second-leading cause is impaired blood supply to the brain which results in the condition known as vascular dementia. But can depression actually lead to one or both of these conditions, or are the symptoms of depression just a response to the early cognitive impairment that is the hallmark of developing dementia? While the Kaiser Permanente study strongly suggests that chronic depression increases the risk of vascular of dementia, any interpretation must take into account that these results are based on only about 550 participants who reported chronic depression. Replication would bolster the plausibility of a causal relationship.

"A prospective placebo-controlled study of the impact of treatment for depression on subsequent development of Alzheimer's or vascular dementia is necessary before any conclusions can be drawn," ACSH's Dr. Gilbert Ross says. He also believes that a preliminary study to correlate depression with PET scan changes and biomarkers in the blood and spinal fluid could prove helpful.

It will be important to watch for the results of further investigations, says ACSH's Dr. Elizabeth Whelan. "If these early studies are on to something, then adequate treatment of depression important in its own right could turn out to be a means of reducing a person's risk of dementia and Alzheimer's disease."