Any sudden drop in blood pressure in middle-aged people, which makes them feel faint, dizzy or light-headed while standing up, could be a sign of increased risk of dementia or stroke later, say researchers.

The findings suggested that people who experienced the blood pressure drop -- known as orthostatic hypotension -- had a 54 per cent  higher risk of developing dementia. They were also found to be at two times higher risk of developing ischemic stroke, which occurs when a blood vessel carrying blood to the brain is blocked by a blood clot.

"Orthostatic hypotension has been linked to heart disease, fainting and falls, so we wanted to conduct a large study to determine if this form of low blood pressure was also linked to problems in the brain, specifically dementia," said Andreea Rawlings, from the Johns Hopkins University in Maryland, US.

For the study, published in the journal Neurology, the team analysed data from 11,709 people with an average age of 54 years, who were followed for an average of 25 years. The participants were instructed to lie down for 20 minutes and then stand up in a smooth, swift motion.

A total of 999 of the 11,156 without orthostatic hypotension, or 9 per cent, developed dementia, compared to 69 of the 552 people with orthostatic hypotension, or 12.5 percent. In addition, a total of 15.2 per cent, or 84 of 552 people, with orthostatic hypotension had an ischemic stroke, compared to 6.8 per cent, or 758 of 11,157 people without orthostatic hypotension. "Measuring orthostatic hypotension in middle-age may be a new way to identify people who need to be carefully monitored for dementia or stroke," Rawlings explained.