Middle-aged people who experience rapid drop in blood pressure often causing dizziness may be at an increased risk of developing cognitive decline and dementia 20 years later, a study said.

The findings suggest that these temporary episodes -- known as orthostatic hypotension -- may cause lasting damage, possibly because they reduce needed blood flow to the brain. 

Orthostatic hypotension is defined as a drop of 20 mmHg or more in systolic blood pressure -- the top number which refers to the amount of pressure in arteries during contraction of the heart muscle -- or 10 mmHg or more in diastolic blood pressure -- the bottom number which refers to blood pressure when the heart muscle is between beats. 

"We found that those people who suffered from orthostatic hypotension in middle age were 40 per cent more likely to develope dementia than those who did not," said lead author Andreea Rawlings, post-doctoral student at the Johns Hopkins University in Maryland, US.

Rawlings siad it was a significant finding and it required better understanding of what was happening.

"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," Rawlings added.

For the study, presented at the American Heart Association's EPI|LIFESTYLE 2017 Scientific Sessions in Oregon, US, the researchers analysed 703 people with an average age of 54 years. 

However, it is not possible to tease out for certain whether the orthostatic hypotension was an indicator of some other underlying disease or whether the drop in blood pressure itself is the cause, Rawlings said.

Though it was likely that the reduction in blood flow to the brain, however temporary, could have lasting consequences, the author added.