Even if your blood pressure is only a little above normal, you may still be at an increased risk of stroke, a new review suggests.

According to the Heart and Stroke Foundation, normal blood pressure is between 120/80 mm Hg and 129/84 mm Hg. "Abnormal" blood pressure is a top number (systolic pressure) above 140 mm Hg, and a bottom number (diastolic pressure) above 90 mm Hg.

But about one in five adults in Canada is somewhere in between, meaning they don't quite meet the definition of high blood pressure but their BP is not in the healthy range either.

When a patient's top number is between 120 and 139 mm Hg, or their bottom number is between 80 and 89 mm Hg, doctors often call this "prehypertension" or "high-normal." But this new study suggests there's nothing "normal" or safe about this range.

The study, in the journal Neurology, reviewed the results of 12 previous studies that looked at blood pressure and the risk of stroke. In all, more than 518,000 adults were included.

The review found that people with prehypertension were 50 per cent more likely to have a stroke compared to people with normal blood pressure levels -- even after the researchers took into account such factors such as age, diabetes, obesity, cholesterol and smoking.

Those who had blood pressure readings just a little above normal -- 120/80 to 129/ 85 -- they still had a 22 per cent higher risk of having a stroke.

Those with blood pressure between 130/ 80 to 139/ 89 had an 80 per cent higher risk of having a stroke.

Antoine Hakim of the Canadian Stroke Network, who was not involved with the study, says the review shows that just because someone doesn't meet the definition of high blood pressure doesn't mean they are safe from stroke.

"It is very clear that people whose blood pressure is in the range 120 to 139 are at high risk of stroke. So we can't just sit back and say, ‘Maybe you are upset or anxious.' We need to do something," he tells CTV News.

"If your systolic is above 130, something must be done about it because you are going to damage your brain," he adds.

High blood pressure is a serious condition. It is the number one risk factor for stroke and is a major risk factor for heart disease. Over time, high blood pressure damages blood vessel walls, causing scarring that promotes the build-up of fatty plaque. This plaque can eventually block arteries, leading to a heart attack or an ischemic stroke.

It also strains the heart and eventually weakens it. Very high blood pressure can cause blood vessels in the brain to burst, resulting in a hemorrhagic stroke.

Hakim's advice is not to put people with pre-hypertension on medication, but to urge them to measure their blood pressure at least once a month and to make lifestyle changes if it stays above normal.

"That means cut down on salt, increase fruit intake, exercise more, don't be a couch potato. If you are smoking, quit," he said.

The Heart and Stroke Foundation also recommends limiting alcohol to no more than one to two drinks a day, to a weekly maximum of 14 drinks for men and 9 drinks for women; and to find healthy ways to manage stress.

Only when all else fails to bring those numbers down should medications be considered, Hakim said.

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip