Over the years, it has become increasingly clear that having higher-than-normal blood pressure for a sustained amount of time can impact the brain.
More than 100 million people in the United States have hypertension, and worldwide, it impacts almost a third of all adults.
Given the size of the affected population, understanding the risks associated with raised blood pressure is paramount.
Hypertension and the brain
Recently, researchers from the Rush Alzheimer's Disease Center at Rush University Medical Center in Chicago, IL, set up a study to look for links between blood pressure and physical markers of brain health in older adults.
The findings are published this week in the journal Neurology. Study co-author Dr. Zoe Arvanitakis explains the types of pathology they were searching for.
"We researched whether blood pressure in later life was associated with signs of brain aging that include plaques and tangles linked to Alzheimer's disease."
They also looked for a type of brain lesion called an infarct. These are "areas of dead tissue caused by a blockage of the blood supply, which can increase with age, often go undetected, and can lead to stroke."
Included in the study were almost 1,300 people who were followed until their deaths, which was an average of 8 years from the beginning of the study. In total, two thirds of the group had a history of high blood pressure, and 87 percent were taking drugs to manage hypertension.
Each year, the participants had their blood pressure assessed, and, after death, their brains were autopsied. Almost half were found to have at least one infarct.
High blood pressure is considered to be anything above 140/90 millimeters of mercury (mmHg). The first number is known as systolic blood pressure, which measures pressure in the blood vessels as the heart contracts.
The second reading is diastolic blood pressure — that is, the pressure in the arteries when the heart is at rest between beats.
Increased lesion risk
As expected, the researchers did find links between hypertension and brain health. They found that for every standard deviation above the group's average systolic blood pressure, there was a 46 percent increased chance of having at least one brain lesion.
To put that into perspective, that is the equivalent of around 9 years of brain aging. In this study, an example of one standard deviation above average would be something like 147 mmHg compared with 134 mmHg.
Similarly, there was a 46 percent increased risk of large lesions and a 36 percent increased risk of smaller lesions with each standard deviation increase in systolic blood pressure.
The results were similar when they studied diastolic blood pressure; one standard deviation above the group average produced a 28 percent increased risk of developing one or more lesions.
On a slightly different note, the authors found that having a diastolic blood pressure that declined over time was also associated with an increased risk of lesions.
Declining blood pressure over time has previously been linked with increased mortality risk.
Hypertension and Alzheimer's
When the researchers investigated possible links between hypertension and the neural features of Alzheimer's, the picture was less clear. They looked at two neurological features: tangles, or twisted fibers within neurons; and plaques, or protein buildup between nerve cells.
Although higher blood pressure readings were associated with a higher number of tangles, they did not predict increased numbers of plaques.
Why this disparity between the two hallmarks of Alzheimer's disease and blood pressure exists will need to be unpicked in future research.
The study authors are quick to note the study's shortcomings. For instance, they only had access to blood pressure readings during the participants' later life. Building a picture of how blood pressure changes throughout an individual's lifespan would provide deeper insight.
Furthermore, their blood pressure readings were only taken once every year and therefore do not offer an accurate picture of how someone's blood pressure may fluctuate over months, weeks, or days. As for conclusions, Dr. Arvanitakis is cautious.
"While our findings may eventually have important public health implications for blood pressure recommendations for older people, further studies will be needed to confirm and expand on our findings before any such recommendations can be made."
Dr. Zoe Arvanitakis
Studies looking further into the relationship between hypertension and brain health are already ongoing, so more answers are sure to follow.