Dizzy spells experienced in middle age when standing up have been linked to an increased risk of dementia.
The brief drop in blood pressure that causes the dizziness may result in lasting damage to the brain, scientists believe.
A study found that the effect, known as orthostatic hypotension, upped the chances of developing dementia in old age by 40%.
Lead researcher Dr Andreea Rawlings, from the Johns Hopkins Bloomberg School of Public Health in the US, said: "Even though these episodes are fleeting, they may have impacts that are long lasting.
"We found that those people who suffered from orthostatic hypotension in middle age were 40% more likely to develop dementia than those who did not. It's a significant finding and we need to better understand just what is happening."
The team tested 11,503 people with an average age of 54 who were taking part in a large study looking at the risk of artery disease in four US communities.
After lying down for 20 minutes, each participant was asked to stand and had his or her blood pressure taken.
Orthostatic hypotension was defined as a drop of at least 20 millimetres of mercury (mmHg) in systolic blood pressure and at least 10 mmHg in diastolic blood pressure.
The first reading measures blood pressure with each pump of the heart and the second pressure between beats.
Roughly 6% of the participants met the criteria for orthostatic hypotension. After following their progress for 20 years, the scientists found they were 40% more likely to develop dementia than those who had not experienced orthostatic hypotension at the clinic.
The findings were presented at the American Heart Association's Epi/Lifestyle 2017 meeting in Portland, Oregon.
The researchers said it was not possible to say for certain whether orthostatic hypotension was directly linked to dementia or an indicator of some other underlying disease.
However, it was likely even a temporary reduction in blood flow to the brain could have a lasting effect.
Nor was it known whether the patients had been troubled by repeated episodes of orthostatic hypotension since they were not re-tested over time.
"Identifying risk factors for cognitive decline and dementia is important for understanding disease progression, and being able to identify those most at risk gives us possible strategies for prevention and intervention," said Dr Rawlings.
"This is one of those factors worth more investigation."
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