High blood pressure is known to be the strongest risk factor for dementia in elderly people.

But a major pan-European study has found that treatment with antihypertensive drugs can reduce the risk.

More than 2,400 patients have taken part in the 10-year Systolic Hypertension in Europe trial.

The patients were at least 60 years old at the start of the trial and had a blood pressure, when sitting down, of 160-219 mm Hg systolic (when the heart contracts) and below 95 mm Hg diastolic (when the heart is relaxed). None had signs of dementia.

The patients were treated with the antihypertensive drug nitrendipine.

Their mental ability was then assessed using a test known as the "mini mental state examination," which measures recall, short term memory, attention and ability to calculate.

If the patient's score was 23 or less, diagnostic tests were done for dementia.

50% cut

The incidence of dementia among patients who received drug therapy fell by 50% compared with patients who were not treated from 7.7 to 3.7 cases per 1,000 patient years.

This means that if a 1,000 hypertensive patients were treated with antihypertensive drugs for five years, an estimated 19 cases of dementia would be prevented.

It is thought that antihypertensive drugs may protect against dementia because they improve blood flow to the brain.

But nitrendipine may have a direct effect on the brain.