(CNN) Older people with higher-than-average blood pressure have more markers of brain disease than their average-pressure peers, according to a study published Wednesday in the journal Neurology. In particular, the researchers saw increased signs of brain infarcts, or areas of dead tissue caused by a block in the blood supply to the brain, when looking at postmortem tissue under a microscope.
Autopsied brains also revealed that higher-than-average blood pressure is associated with one marker of Alzheimer's disease.
"We know that blood pressure, especially if high, is related to stroke and also to dementia," said Dr. Zoe Arvanitakis, lead author of the study and a professor of neurology at Rush University Medical Center in Chicago.
"In this study, we wanted to examine the relationship of blood pressure across a range of values -- not just high but also normal and low -- to the two most common causes of stroke and dementia," Arvanitakis said. Those causes are brain infarcts (also called brain lesions) and the signature biomarkers of Alzheimer's disease: the plaques and tangles, both made of different proteins, in the brain.
Blood pressure is a measure of the force at which blood flows through our veins, arteries and capillaries, according to the American Heart Association. Too great a force means high pressure; too weak a force means low.
In numbers, high blood pressure is considered anything above 140/90. The first number, the systolic pressure, reflects the pressure in vessels when the heart beats. The second number, diastolic, measures when the heart is at rest. Low blood pressure is purposely left undefined because no number is considered too low as long as there are no troubling symptoms such as dizziness, nausea and problems concentrating.
To understand how blood pressure relates to stroke and dementia, Arvanitakis and her colleagues enlisted the help of 1,288 people, all 65 or older and about two-thirds of them women. Participation included yearly physical exams, some neuropsychological testing and records of their medical histories and medications.
The enrollees also consented to a brain autopsy when they died; just before age 89 was the average.
"What we wanted to do is look at the actual brain tissue to see whether we saw the underlying changes in the brain that cause stroke or the underlying changes in the brain that cause dementia," Arvanitakis said.
The participants' data showed an average systolic blood pressure of 134 and an average diastolic blood pressure of 71. Two-thirds had a history of high blood pressure, and 87% had been taking high blood pressure drugs.
"We looked at blood pressure in several different ways," including how changes in blood pressure -- such as declining blood pressure -- "might be associated with the brain lesions at the time of death," Arvanitakis said.
The research found that "the higher the average blood pressure, the more likely a person was to have brain lesions," Arvanitakis said.
For example, a systolic blood pressure of 147 translated to a 46% increased risk of having one or more brain lesions, specifically infarcts, the analysis showed. This same higher-than-average systolic pressure (147 vs. the group average of 134) also meant a 46% greater chance of large lesions plus a 36% greater risk of very small lesions, the results indicated.
Separately, higher-than-average diastolic blood pressure also showed a relationship with brain lesions, the researchers found. People with higher-than-average diastolic pressure (79, for example, compared with the group average of 71) had a 28% greater risk of one or more lesions, the researchers found.
Arvanitakis and her colleagues also discovered that higher systolic blood pressure was associated with a higher number of tangles in the brain tissue, a common sign of Alzheimer's disease -- but not amyloid plaques, another common feature of the neurodegenerative disease.
Arvanitakis said this finding is difficult to interpret and requires more research.
Dr. Joe Verghese, a professor of neurology and medicine and director of the Resnick Gerontology Center at the Albert Einstein College of Medicine, said strong points of the new study include the large participation rate and the evaluations of blood pressure and cognition "done using standardized procedures."
Verghese, who was not involved in the research, added that its key weakness was the somewhat "limited" data available. Still, he says the study took a "different approach" in its examination of brain health in the elderly.
"In previous studies, the focus was on dementia diagnosed clinically" -- in a doctor's office, based on tests while patients were still alive, Verghese wrote in an email. However, Arvanitakis and her colleagues showed a direct relationship between blood pressure and signs of the causes of stroke and dementia: "the presence of vascular pathology in post-mortem brains."
"The study further supports treatment of blood pressure in late life to prevent cerebrovascular disease," Verghese said. "The story regarding Alzheimer risk is less clear."
Still, past research, including Verghese's own, has shown that abnormal blood pressure in both mid- and late life was associated with dementia risk.
"A recent Institute of Medicine review concluded that there was supportive evidence to recommend blood pressure management to reduce risk of cognitive decline and risk of dementia," he said.
Arvanitakis said that anyone concerned about stroke or dementia "should continue to follow the current medical recommendations for good neurological health, and these include doing regular exercise and following a good nutrition plan."
"Since blood pressure is an important risk factor for disease, we really need to understand how it affects the brain," she said. "Research really needs the participation of volunteers and needs the ongoing scientific funding to continue making important discoveries."