The other days, I was taught that old patients with depression looked like suffering from dementia. They have lesser attention, memory, and executive function. Family members sometimes misdiagnose them as dementia. But if properly treated, their cognitive function recovers to intact. These confusing symptoms are called "pseudo-dementia". Therefore, we were told that it is decisive to distinguish depression as treatable dementia from true dementia.
However, a considerable amount of evidence was found, suggesting that depressive symptoms prospect the onset of dementia, recently. There are similar biological basis between depression in older people and dementia. For example, the volume of hippocampus of depressive patients is reduced compared to age-matched healthy people. Hippocampus is a region in the brain, which has an important role in memory. Patients with dementia also have reduced volume of hippocampus.
Accepting such findings, we psychiatrists were forced to change the explanation to the patients and family. Formally, "you are not demented, but simply depressed. You should be cured." In contrast, we have to say, "You will be recovered, or will not. I hate to say that your symptoms can be due to dementia".
It is surprising that another hypothesis was published in an academic journal. The author insists that, although it is true that depressed patients have poorer cognitive function, however which is independent from dementia.
AAN.com: How Is Depression Related to Dementia?
The original article is shown below, although the complete content is available only for subscribers.
Neurology: Clinical-pathologic study of depressive symptoms and cognitive decline in old age
If this finding is true, we have to change again the explanation for the patients. In my clinical experience, some patients become demented gradually during treatment of depression. But others can recover completely, even if they showed severe impairment of cognitive function in the maelstrom of depression. It is difficult to distinguish one from the other in advance.
I guess this kind of controversy is originated from the vagueness of definition of depression. Depression includes extremely wide status, from stupor to heartbreak. Biological research is required to make more clear and trustworthy criteria of depression.