Prehypertension, which is blood pressure at the higher end of the normal range, appears to be associated with a greater risk of stroke, according to a meta-analysis of medical studies.

The report was published Wednesday online in Neurology.

The analysis of 518,520 adults involved in 12 previous studies on blood pressure and stroke has found that people whose blood pressure was above normal were 55 percent more likely to have a stroke compared to those with normal blood pressure. The studies, which date back to 2003, included patients in the United States, China, Japan and India.

High blood pressure is a key factor that more than doubles the chance of stroke, while other factors such as diabetes, obesity, smoking, stress and depression also raise the risk.

The study also found that in groups whose average age was 65 or under, having a blood pressure between normal and high was linked to a 79 percent higher risk.

The researchers said young and middle-aged people with slightly high blood pressure should consider cutting down their salt intake and exercising more to reduce their risk.

At this point it is way too early to recommend blood pressure drugs, said lead author Bruce Ovbiagele, director of the stroke prevention program at the University of California, San Diego.

To reduce their risk of stroke, people with prehypertension should lower their blood pressure by losing weight, curbing their sodium intake and making other healthy lifestyle changes, says Bruce Ovbiagele.

Blood pressure is recorded in two measurements known as systolic and diastolic, with anything consistently lower than 130 millimetres of mercury (mmHg) systolic or 80mmHg diastolic considered normal. Prehypertension is defined as a systolic blood pressure between 120 and 139 or a diastolic blood pressure between 80 and 89.

Stroke is an often disabling or deadly disruption of blood flow to the brain. It occurs when blood flow is blocked or when a blood vessel in the brain bursts, depriving brain cells of oxygen

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