Some research has suggested that depression might increase the risk of developing dementia. Researchers in Denmark looked at that question using hospitalization records.

What the researchers wanted to know: Are people hospitalized with depression or bipolar disorder likely to be hospitalized later with dementia?

What they did: For this study, the researchers looked at about 40,000 patients who were hospitalized with depression or bipolar disorder for the first time between 1970 and 1999. Of those, nearly 23,000 were admitted to psychiatric hospitals at least once between 1986 and 1999 and were used in the study.

What they found: The more times a person was hospitalized with depression, the higher his or her chances of being hospitalized later for dementia. This was also true for bipolar hospitalizations, although less so. Each hospitalization for depression increased the rate of hospitalization for dementia by, on average, 13 percent; for bipolar disorder, it was 6 percent.

What the study means to you: This supports the hypothesis that depression and dementia are somehow related. Maybe the brain changes that come with depression and bipolar disorder are permanent—if so, that suggests those disorders ought to be taken seriously and treated early.

Caveats: Some patients were probably diagnosed with dementia but not hospitalized for it, so the researchers didn't know about them. Also, they said, brain imaging would have helped—a direction for more research, perhaps.

Find out more: The common signs of dementia from the American Academy of Family Physicians.

Read the article: Kessing, L.V. and P.K. Andersen. "Does the Risk of Developing Dementia Increase with the Number of Episodes in Patients with Depressive Disorder and in Patients With Bipolar Disorder?" Journal of Neurology, Neurosurgery, and Psychiatry. December 2004, Vol. 75, pp. 1662–1666.

Abstract online: http://jnnp.bmjjournals.com