Blood clots in the brain may be associated with a higher risk of dementia, scientists suggest.
The clots affect the brain
The clots, known as cerebral emboli, were seen in people who had Alzheimer's disease or another form of dementia.
The University of Manchester team who carried out the study say they could act as an early warning sign of the conditions.
Dementia experts said the British Medical Journal finding was a significant and valuable discovery.
The researchers monitored the occurrence of spontaneous blood clots in the brain in 170 patients - 85 with Alzheimer's disease and 85 with vascular dementia - compared them with groups of people of the same age.
The clots do not form in the brain and could have a range of causes, including a build up cholesterol crystals or blood clots which have travelled up from the vein system in the body.
Forty per cent (32) patients with Alzheimer's and 37% (31) with vascular dementia had clots in just one hour of monitoring, compared to just 14 and 15% in the healthy groups (12 patients in each).
The researchers, led by Professor Charles McCollum, said the finding suggested that the types of dementia have more in common than had been previously thought.
He said: "We found that 40% of patients with dementia who were monitored for just one hour had emboli in their brain."
His team will now study a number of patients with early-stage dementia to see if those with the most emboli deteriorate fastest, which would demonstrate a direct link between the emboli and dementia.
"We could then consider using medications which can prevent emboli forming."
Clive Ballard, director of research at the Alzheimer's Society which partially funded the study, said: "Blood clots in the brain can be detected in people at any age and their prevalence in people with Alzheimer's disease is a very significant finding.
"The strong correlation between Alzheimer's and blood clots could lead to a way of detecting and treating Alzheimer's disease much earlier."
He added: "It is also significant that the blood clots were present in people with vascular dementia and Alzheimer's.
"This is an important link between the two as it could be a valuable scientific discovery, creating a new and important target for treatment."