The link between the risk of dementia and hypertension is well established. What about higher-than-normal blood pressure in the absence of a hypertension diagnosis, however?
But what is "middle age," exactly? And does blood pressure need to become so high as to be diagnosed as hypertension for it to impact a person's risk of neurodegeneration?
This is what scientists spanning three countries — France, the United Kingdom, and Hungary — have strived to answer in a study, the results of which are now published in the European Heart Journal.
"Previous research has not been able to test the link between raised blood pressure and dementia directly by examining the timing in sufficient detail," notes first author Jessica Abell, of the French National Institute of Health and Medical Research in Paris, France, as well as University College London (UCL) in the U.K.
"In our paper," she notes, "we were able to examine the association at age 50, 60, and 70, and we found different patterns of association. This will have important implications for policy guidelines, which currently only use the generic term 'midlife.'"
'Increased risk for people at age 50'
They analyzed the medical data of 8,639 people — 32.5 percent of whom were women — recruited using a larger, ongoing, population study: Whitehall II, or the "Stress and Health Study," which is based at UCL.
Recruitees were aged 33–55 at baseline in 1985. Their blood pressures were measured every 6 years — in 1985, 1991, 1997, and 2003 — which allowed the team to see how higher blood pressures impacted brain health in time.
By 2017, 385 of the participants had developed a form of dementia, and the average age at which this happened was 75.
Looking at the participants' medical information, the research team noted that those individuals who had a systolic blood pressure — the blood pressure measured as the heart beats — of 130 millimeters of mercury (mm Hg) or over as they reached the age of 50 had a 45 percent higher risk of dementia than people with the same age and a lower systolic blood pressure.
The same did not hold true for people aged 60–70, and Abell and colleagues posit that this may be due to the fact that the period of risk exposure may be shorter at that point.
No association was found between the risk of dementia and diastolic blood pressure, which is the blood pressure measured as the heart is at rest between beats.
"Our analysis suggests that the importance of mid-life hypertension on brain health is due to the duration of exposure. So we see an increased risk for people with raised blood pressure at age 50, but not 60 or 70, because those with hypertension at age 50 are likely to be 'exposed' to this risk for longer."
High blood pressure and 'silent' strokes
The National Institute for Health and Care Excellence — in the U.K. — suggest that a hypertension diagnosis should be given when a person's "clinic blood pressure is 140/90 mm Hg or higher."
The American College of Cardiology (ACC), meanwhile, advise that this diagnosis be considered for a lower threshold: 130 mm Hg and over. The ACC define "normal blood pressure" as under 120/80 mm Hg.
In the new study, the scientists found that even people who presented no cardiovascular disease diagnoses still had a 47 percent higher risk of dementia if they had elevated systolic blood pressure at age 50.
"Our work," notes lead study author Prof. Archana Singh-Manoux, "confirms the detrimental effects of midlife hypertension for risk of dementia, as suggested by previous research."
"It also suggests that at age 50, the risk of dementia may be increased in people who have raised levels of systolic blood pressure below the threshold commonly used to treat hypertension," she adds.
But why does high blood pressure raise the risk of neurodegenerative conditions? The team believes that this may be due to the fact that elevated blood pressure could lead to "silent" strokes, which result in white matter damage and impair the blood supply (and therefore oxygen) to the brain without, however, having any visible symptoms.
This makes such strokes more difficult to identify and might mean that they can increase the risk of brain damage, unbeknownst to the individuals experiencing them.
Still, Abell cautions that "this is observational, population-level research and so these findings do not translate directly into implications for individual patients."
"Furthermore," she continues, "there is considerable discussion on the optimal threshold for the diagnosis of hypertension."
Her best advice? Look after your cardiovascular health, and do not neglect your medical exams if worried.
"There is plenty of evidence to suggest that maintaining a healthy blood pressure in middle age is important for both your heart and your brain later in life. Anyone who is concerned about their blood pressure levels should consult their [doctor]," Abell urges.