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But it might lower the risk of slight declines in thinking and memory, a condition known as mild cognitive impairment (MCI), the researchers added.
The clinical trial is the "first study in history to show that any intervention can reduce your risk of developing mild cognitive impairment, an early form of dementia," said lead researcher Dr. Jeff Williamson. He is a professor of gerontology and geriatric medicine at Wake Forest School of Medicine, in Winston-Salem, N.C.
"What is good for your heart in terms of blood pressure-lowering is also good for your brain," Williamson added. He noted that the trial ended early, which likely affected the dementia result. "We just didn't have enough dementia cases develop over time" in the group with less-restricted blood pressure, he explained.
High blood pressure affects more than three-fourths of people over the age of 65, and it has been identified as a potential risk factor for MCI and dementia in observational studies, the study authors said in background notes.
The new clinical trial focused on nearly 9,400 people, average age 68, who had been randomly assigned to treatment that would keep their systolic blood pressure at or below a goal of either 120 or 140 mm Hg.
Participants were all at high risk for heart disease, and the clinical trial initially was aimed at seeing whether a more stringent blood pressure goal would save lives.
The planned five-year trial ended early, after a little over three years, because the group kept at 120 systolic blood pressure fared so much better than the other group, the researchers said.
The lower blood pressure goal "prevented death, stroke, heart attack and heart failure so much more quickly and so much more powerfully," that people in the 140 systolic group were allowed to go on the same therapy as the lower blood pressure group, Williamson said.
But an offshoot of the trial continued to track the participants, to see whether tight blood pressure control had any effect on risk of developing dementia or mild cognitive impairment.
After five years of follow-up, the investigators found that tighter blood pressure control reduced the risk of MCI by 19 percent, and the risk of MCI or probable dementia combined by 15 percent.
The 120 systolic group also had a 17 percent reduced risk of dementia, but that result was not statistically significant, according to the report.
The Alzheimer's Association has agreed to provide funding to keep tracking the patients for another couple of years, Williamson said.
"We feel additional follow-up of this cohort will provide the last piece of the puzzle we need," he said.
There are a couple of ways blood pressure could influence brain health, Williamson suggested. High blood pressure could damage the tiny blood vessels in the brain, harming brain tissue. It also could affect the amount of toxic substances that enter and lodge in the brain.
Dr. Kristine Yaffe is chief of neuropsychiatry and director of the Memory Disorders Clinic at the San Francisco Veteran's Affairs Medical Center. She said that even though the trial didn't meet its primary endpoint of significantly reducing dementia, it's still "exciting because it reminds us how important cardiovascular disease and vascular health is to the brain and to cognition."
Yaffe, who wrote an editorial accompanying the results, added, "I think you can't throw the baby out with the bathwater. You have to look at the whole picture here. They found an effect on MCI, which is the first time anybody's shown a way to reduce MCI. Now we need to study this more. I think there's a lot more we can try and understand."
The clinical trial was published online Jan. 28 in the Journal of the American Medical Association.
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