March 26, 2003 -- Alzheimer's disease and other forms of dementia may be more than twice as likely to occur in people who have had "silent" brain infarcts -- a stroke that causes no apparent symptoms and afflicts as many as 11 million Americans each year.
This finding, by Danish researchers and published in the March 27 issue of TheNew England Journal of Medicine, is the latest to suggest an increased risk of dementia later and a more severe and rapid decline in memory loss from the brain damage caused by silent strokes.
"I was surprised, when we did the study, to see how large a proportion of elderly people do have small infarcts in their brain that were never clinically recognized," lead researcher Monique M.B. Breteler, MD, PhD, tells WebMD. "One in five people over the age of 60 had at least one silent brain infarct. I was also impressed by the size of the risk increase, which suggests that we should advocate stronger control of vascular risk factors in an attempt to help prevent dementia."
"It's been observed for years that people who had a stroke and later develop Alzheimer's typically don't do as well with their Alzheimer's, and have a faster rate of decline," says Bill Thies, PhD, vice president of medical and scientific affairs for the Alzheimer's Foundation. "This study is high-quality confirmation of data we've seen before that suggests the development of dementia is, in many cases, perhaps a combination of effects" -- partly from the brain changes that are associated with Alzheimer's and partly from those that result from problems with circulation and blood vessels.
The take-home message: "Take care of your blood vessels if you want to retain as much function as possible in your later years," Thies tells WebMD. That means following the same "heart-smart" regimen of eating a healthy diet, getting regular exercise, not smoking, and controlling blood pressure, cholesterol, weight, and blood sugar as you would to prevent heart disease, cancer, diabetes, and other health threats.
This strategy may be especially important if you consider that these silent strokes occur nearly 15 times more often than classic strokes -- yet many people are completely unaware they have had them. According to data presented two years ago to the American Stroke Association, silent strokes afflict about 4% of the U.S. population at some point in their lives.
Although rare before age 30, the chance of having silent strokes doubles every 10 years, say researchers. By the time people reach their 70s, one in three has experienced at least one silent stroke per year -- usually showing no obvious initial symptoms and detectable only through a brain scan. But the injury accumulates over time, leading to symptoms of dementia such as memory and thinking impairments, as well as difficulty in walking.
In her study of 1,015 elderly patients, Breteler and her colleagues found that those who had silent strokes at the beginning of their study, as measured on an MRI, and who also showed no signs of dementia were more than twice as likely to demonstrate symptoms of mental decline over the course of the four-year study. Those with a history of silent strokes at the beginning of the study also had a faster rate of decline in their thinking and memory abilities compared with those who were stroke-free.
Their results are strikingly similar to a 1997 study, in which American researchers first suggested that silent strokes may increase the likelihood of later developing Alzheimer's. In reviewing autopsy reports and brain scans of deceased elderly nuns, they noted that Alzheimer's was found in nearly all of those with a history of silent strokes, compared with only 57% of stroke-free nuns.
What's more, one-third of the nuns whose brains showed definite signs of Alzheimer's hadn't exhibited any symptoms of the disease before their deaths. The reason: "They most likely had not suffered strokes," says that study's lead researcher, David Snowdon, PhD, MPH, of the Sanders-Brown Center on Aging at the University of Kentucky. Meanwhile, those with a history of silent strokes typically had the most severe Alzheimer's symptoms and a faster rate of functional decline.
"This study is really great news because there isn't any drug available today to slow down the lesions of Alzheimer's growing in your brain. But clearly, you can work on preventing stroke," Snowdon tells WebMD. "It's not rocket science that if you have one brain-damaging condition like stroke, another brain-damaging condition like Alzheimer's would add to your worries. Except in this case, it looks like the brain-damaging effects of stroke may actually trigger the Alzheimer's symptoms."