It has taken the development of imaging techniques to confirm what clinical wisdom has long suggested: there is a type of major depression that develops in adults—generally those over the age of 60—and it is due to vascular disease affecting the brain as well as other parts of the body.

Depending on where in the brain the vascular disease occurs, even subtle changes in blood flow may damage crucial connection tracts and lead to depression. Often, the damage is done by so-called silent strokes that go undetected because they don't affect movement or sensation. They are very common, especially as people age.

Vascular depression is marked by lesions in the prefrontal cortex, and the more lesions, the more severe the depression, K. Ranga R. Krishnan, M.D., head of psychiatry at Duke University, reported at a meeting of the American Psychiatric Association.

Patients with this variety of depression are characteristically apathetic, and display psychomotor retardation and fatigue. The disorder also affects cognition. Among other changes, studies show that affected patients make some specific cognitive errors. For example, they are impervious to negative feedback about their performance; they will make the same mistake over and over again even when told what their error is.

Vascular depression is difficult to treat, and recovery time is measured in months, not weeks. However, the brain lesions tend to worsen over time, eventually leading to dementia. The disorder is especially likely to play out in suicide among older men.

Dr. Krishnan does not believe that every depressed person over 65 needs to undergo a brain scan to search for silent lesions. But he does recommend that older people with depression be screened and treated for high cholesterol levels and high blood pressure, as both conditions affect blood flow to the brain. Patients treated for high blood pressure, he reports, have lower rates of relapse of depression.

What's more, he suggested, some simple measures among the healthy may help stave off the subtle cardiovascular disease that can lead to vascular depression. One is the use of aspirin, widely recommended by cardiologists to prevent heart disease.

The other is by taking supplements of omega-3 fatty acids. "There's good data that they prevent cardiovascular disease," Dr. Krishnan reported. "Lot's of my patients take them. There's lots of evidence that they have implications for the way cardiovascular disease develops. Patients like taking natural products. But the evidence isn't in yet that they prevent depression."