Feeling light-headed and dizzy when standing up suddenly may be a warning sign of Alzheimer's, research has shown.
Scientists who studied the effect in more than 6,000 healthy individuals found that it increased the risk of dementia by up to 39%.
The sensation is caused by a temporary drop in blood pressure, which is thought to trigger brain damage if it occurs repeatedly.
Read more: Air pollution and lack of vitamin D could increase risk of dementia
The dizzy spell phenomenon has the medical name orthostatic hypotension (OH) and is especially common among the elderly, affecting 20% to 30% of this population.
For a brief period of time it can result in a shortage of blood flowing through the brain, depriving nerve cells of oxygen and nutrients.
The study tracked 6,204 men and women aged 55 and over for 15 years on average and recorded how many of them developed any type of dementia, including Alzheimer's.
Almost a fifth of participants regularly experienced a significant drop in blood pressure within three minutes of standing from a resting position. They were classified as being prone to OH.
Read more: Air pollution and lack of vitamin D could increase risk of dementia
Compared with those who did not feel dizzy when standing up, this group was 15% more likely to develop dementia.
The association was especially strong when a person's heart rate did not speed up to compensate for the momentary drop in blood pressure, the Dutch study showed
For individuals in this category, the long-term risk of dementia was raised by 39%.
The research, led by Dr Arfan Ikram, from Erasmus Medical Centre, is reported in the online journal Public Library of Science Medicine.
Summing up their results, the authors wrote: "We found OH to be associated with long-term risk of dementia on continuous follow-up, independent of various other risk factors.
Read more: Air pollution and lack of vitamin D could increase risk of dementia
"The most apparent explanation for our findings is that OH causes brain damage due to recurrent transient cerebral hypoperfusion (low blood flow).
"Brief episodes of hypoperfusion elicited by sudden blood pressure drops may lead to hypoxia (oxygen deprivation), with detrimental effects on brain tissue via, for instance, neuro-inflammation and oxidative stress."
Of the study participants who developed dementia, almost 80% had Alzheimer's disease and 8% had a vascular form of the condition linked to impaired blood supply.
The relative risk of both was increased to the same extent by recurrent episodes of OH, the scientists said.
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