People with prehypertension, where blood pressure is at the high end of normal, have a 55% higher risk of having a stroke than people without prehypertension, according to a new study published online this week in the journal Neurology. Researchers from the University of California, San Diego (UCSD) School of Medicine arrived at this finding after pooling data from studies involving over 518,000 participants.
Prehypertension is where blood pressure is at the high end of normal, defined by experts in 2003 as a systolic (the top number) pressure reading between 120 and 139 and a diastolic (the bottom number) reading between 80 and 89 mm of mercury (Hg).
We already know that abnormally high blood pressure or hypertension, which lies above this range, is a major risk factor for cardiovascular disease and stroke, but we don’t know much about the size of risk that might be linked to prehypertension.
Senior author Dr Bruce Ovbiagele, professor of neurosciences at UCSD School of Medicine said it seems reasonable to assume that risk of death and disease goes up in line with blood pressure, perhaps starting within the normal range, but conclusive evidence on this is lacking, so:
” … we decided to compile all the published studies in the scientific literature to date, and using statistical techniques find out if there is indeed a higher risk of future stroke in people with prehypertension, the extent of that risk, and whether particular characteristics were associated with higher stroke risk.”
They found 12 studies covering more than 518,000 participants in cohorts representative of the general population: four from the US, five from Japan, two from China and one from India. The period covered by the studies ranged from 2.7 to 32 years and all incidents of stroke were documented.
The rate of prehypertension in the cohorts ranged from 25 to 46%: this is comparable to the estimated rate in the US, where it is thought about 1 in 3 adults has prehypertension.
Ovbiagele said they found that overall, the participants who had prehypertension had a 55% higher risk of experiencing a future stroke than participants without prehypertension.
“This result held regardless of sex, race-ethnicity, blood pressure type (systolic or diastolic) or the type of stroke (ischemic or hemorrhagic),” he added.
This risk was measurably higher, 79%, for those whose blood pressure was in the high end of prehypertension.
Ovbiagele said their findings should clarify understanding of the health risk of prehypertension, a subject that has caused much controversy since its inception.
One criticism that has been levelled agains the prehypertension category is that it could unjustifiably lead to otherwise healthy people being labelled unhealthy. Another is that it could be used as an excuse to put people on medication to satisfy the pharmaceutical companies.
Neurologist Dr Thomas Hemmen, director of UCSD’s Stroke Program, was not involved in the study: he described the findings as “ground-breaking.”
He said as the years have gone by, the blood pressure threshold for identifying increased stroke and cardiovascular risk has come down.
“Now anything that’s above 115 is thought to increase risk,” said Hemmen.
“But we need more tools for diagnosing prehypertension. We need to learn more so that we can adjust risk and develop therapies. Hopefully, this study will lead to more research,” he added.
He and Ovbiagele point out there is little evidence that taking drugs to reduce blood pressure actually prevents future strokes. This does not mean that they don’t, it just means evidence is lacking (a good moment to remember the empiricist’s adage “absence of evidence does not equal evidence of absence”).
“There just haven’t been any large studies,” said Hemmen said.
But both Hemmen and Ovbiagele hope these new findings will encourage people whose blood pressure is on the high side of normal to adopt some healthier habits. This is a relatively safe thing to do and could do more than lower risk of stroke: it may also reduce risk of heart attack, heart failure and kidney disease, which can result from prolonged elevated blood pressure.
“Young and middle-aged persons should check their blood pressure regularly. If they do fall into the higher range of prehypertension, they should take specific steps to modify their lifestyle, such as reducing salt intake and maintaining a normal weight,” said Ovbiagele.
Written by Catharine Paddock PhD