What it is: Vascular dementia accounts for 17.4 percent of all cases of dementia. It happens when a stroke interferes with blood flow to the brain. Usually the culprit is multiple small strokes (infarcts) caused by blood clots or thickened or ruptured small arteries that connect to the center of the brain. (This is called multi-infarct dementia.) It may also be caused by one big stroke (which would be referred to as post-stroke dementia).

How the symptoms compare to Alzheimer's: Vascular dementia may appear to be Alzheimer's because it, too, involves memory problems, confusion, disorientation, and trouble following directions. In this condition, however, recall of day-to-day events (episodic memory) becomes impaired, but recognition -- of people, for example -- doesn't. Alzheimer's generally affects both.

Unlike Alzheimer's, vascular dementia often begins abruptly. Memory loss may progress to hallucinations, agitation, or withdrawal. Symptoms may clearly worsen after each successive stroke.

Other signs of possible stroke may be observed, such as garbled speech, dizziness or loss of coordination, or weakness on one side of the body (face or limbs). These signs may not be apparent in very small strokes. Some people have both Alzheimer's and vascular dementia.

How it's diagnosed and treated: It's relatively easy for a physician to determine whether dementia has a cerebrovascular cause. An MRI or CT scan will show evidence of a stroke. A history of stroke or cardiovascular problems, as well as smoking, high cholesterol, high blood pressure, and diabetes are major risk factors. Treating these factors can slow the progress of dementia symptoms.

There are no medications approved for vascular dementia, although those used for Alzheimer's are sometimes prescribed to help cognitive symptoms, with mixed results. In 2006, donepezil (Aricept) was linked to 11 deaths in a clinical trial evaluating its use for vascular dementia, compared with none in the control group.