Patients diagnosed with the heart rhythm condition suffered cognitive impairment sooner than others, and their condition progressed more quickly, a US study found.
Researchers believe many small strokes caused by AF may go unnoticed, leading to damaged brain tissue.
In the study, researchers followed 5,150 participants, aged 65 or older and without AF or a history of stroke, for around seven years. These patients received a 100-point cognitive test up to nine times a year.
During the study, 552 participants, around 11%, developed AF.
An analysis showed that people with AF were more likely to have lower memory and thinking scores at earlier ages than those without the condition.
Researchers calculated that, between the ages of 80 and 85, a person with AF would decline by 10.3 points on the cognitive score, compared with a fall of 6.4 points among those without the condition.
The findings suggest participants with AF would develop dementia about two years earlier than others. Further decline would also be faster among these patients.
Researchers said reduced blood flow to the brain caused by low cardiac output during AF may also explain the loss of brain function.
Study author Evan Thacker PhD from the University of Alabama at Birmingham said heart health was ‘an important factor related to brain health'.
He added: 'If there is indeed a link between AF and memory and thinking decline, the next steps are to learn why that decline happens and how we can prevent that decline.'
Dr Doug Brown, director of research and development at the Alzheimer’s Society, said: 'We need more research into how an irregular heartbeat might cause cognitive decline and whether taking treatments to combat this cardiac problem could have an impact on the development of dementia.’