Atrial fibrillation causes a chaotic heartbeat
A common heart disorder has been linked to a raised risk of Alzheimer's disease by US researchers.
Atrial fibrillation causes the heart to beat chaotically, increasing the risk of blood clots and, if the condition is left untreated, stroke.
It has previously been linked to some types of dementia - but not Alzheimer's, the most common form.
The study, by Intermountain Medical Center in Utah, was based on more than 37,000 patients.
The study found atrial fibrillation patients under the age of 70 had a 187% greater risk of all types of dementia compared with the general population.
But their specific risk of Alzheimer's disease was also up - by 130%.
However, the overall risk of Alzheimer's for all patients remained low.
Lead researcher Dr Jared Bunch said: "Previous studies have shown that patients with atrial fibrillation are at higher risk for some types of dementia, including vascular dementia.
"But to our knowledge, this is the first large-population study to clearly show that having atrial fibrillation puts patients at greater risk for developing Alzheimer's disease."
Alzheimer's, which accounts for up to 80% of all dementia cases, is known to be linked to age and genetics.
It has long been suspected that poor heart health may also play a role.
The researchers said more research was needed to explain why atrial fibrillation may raise the risk of Alzheimer's.
They put forward several theories for a possible link. They suspect atrial fibrillation damages the small blood vessels, potentially reducing blood flow to the brain.
Alternatively, the condition is linked to tiny micro-strokes, the damage from which may accumulate over time, making Alzheimer's more likely.
Thirdly, atrial fibrillation is associated with a general increase in inflammation throughout the body, again a possible risk factor for Alzheimer's.
Researcher Dr John Day said: "Now that we've established this link, our focus will be to see if early treatment of atrial fibrillation can prevent dementia or the development of Alzheimer's disease."
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said: "If research helps us understand the relationship between the two conditions, we will be in a better position to develop desperately needed new treatments.
"We can all lower our dementia risk by maintaining a balanced diet and taking regular exercise, which is also good advice for protecting your heart."
Alasdair Little, cardiac nurse at the British Heart Foundation, said: "This is an interesting study which adds weight to previous data indicating a link between atrial fibrillation - a common disturbance of heart rhythm - and dementia.
"The risk factors for heart disease and dementia are very similar so it is not surprising that in some people they co-exist.
"Further research into both conditions is needed to understand whether there is a causal link between the two."
Details of the study were presented to the Heart Rhythm Society in Boston.