WEDNESDAY, Sept. 28 (HealthDay News) -- Having blood pressure readings that are just slightly above normal -- a condition known as prehypertension -- appears to raise the risk of stroke, new research finds.

Normal blood pressure is a systolic blood pressure (top number) below 120 mmHg and a diastolic blood pressure (bottom number) below 80 mmHg.

Prehypertension is slightly above that -- systolic blood pressure between 120 and 139 mmHg or diastolic blood pressure between 80 and 89 mmHg.

For this study, researchers reviewed 12 previous studies about blood pressure and stroke involving more than half a million adults from the United States, Japan, China and India.

The investigators found that people with prehypertension and no prior history of cardiovascular disease were 55 percent more likely to have a stroke than people with normal blood pressure, even after taking into account factors such as age, gender, diabetes, obesity, cholesterol and smoking.

When the researchers split the people with prehypertension into two groups -- those at the lower end of the prehypertensive range and those at the upper end -- they found those in the upper range (130 to 139 mmHg systolic and 85 to 89 mmHg diastolic) had a 79 percent increased risk of stroke.

The risk of stroke was not shown to be significantly increased in the lower end of the prehypertensive group.

"The message for patients is that stroke is the fourth leading cause of death and a leading cause of disability in the United States, and if you do fall into [the prehypertensive] category you should take it very seriously and strongly consider a change in lifestyle to try and reduce your risk of stroke," said senior study author Dr. Bruce Ovbiagele, a professor of neurosciences at University of California, San Diego.

Ovbiagele and his colleagues also found that the impact of higher than normal blood pressure on stroke was the most pronounced among those under age 65. Their risk of stroke was nearly 80 percent higher than people in that age bracket with normal blood pressure.

In people older than 65, researchers believe other factors obscure the impact of prehypertension. "Age is such a powerful factor putting people at risk of stroke that we think it overwhelms any added contribution from the slightly higher blood pressure," Ovbiagele said.

The study is published in the Sept. 28 online edition and the Oct. 4 print issue of the journal Neurology.

Between 25 percent and 46 percent of the study participants were prehypertensive. Prior research has found that about 25 percent of U.S. adults have prehypertension, according to background information in an accompanying editorial.

Experts said it was premature to suggest that everyone with slightly elevated blood pressure be put on medications.

Instead, current recommendations call for people with prehypertension to make changes such as quitting smoking, exercising at least 30 minutes daily, limiting alcohol, reducing salt intake, and maintaining a normal body mass index.

Medications are recommended when lifestyle changes fail to bring down blood pressure, and in people with diabetes or kidney disease, said Dr. Amytis Towfighi, an assistant professor of neurology at University of Southern California's Keck School of Medicine, who co-wrote the accompanying editorial.

"Lifestyle changes have been shown to lower blood pressure in individuals with prehypertension," Towfighi said.

More information

The American Academy of Family Physicians has tips on lowering high blood pressure.