Most patients who develop atrial fibrillation an irregular heart beat and condition affecting about 2.7 million Americans are prescribed an anticoagulant to prevent a stroke. The most common anticoagulant used worldwide is warfarin.
It has been a lifesaver, preventing heart failure and strokes in countless atrial fibrillation patients, said Bunch. But it carries risks, including brain hemorrhages and other serious bleeding.
Warfarin, for example, has been known to increase the risk of fatal falls in the elderly. And the Intermountain study suggests that, if not well managed, warfarin can lead to dementia, which in turn increases a person's chances of falling.
"It gets into the chicken-and-egg of which comes first," said Bunch. "We know patients who have dementia have more trouble with warfarin management. And now we know poorly managed warfarin contributes to dementia."
The findings, presented at the 2014 Annual Heart Rhythm Society Scientific Session on Friday, follow an earlier study in which Intermountain researchers found patients with atrial fibrillation were at higher risk for developing dementia.
"The thought was maybe micro strokes are causing repetitive injury to the brain, something the use of a blood thinner should help," said Bunch.
But they discovered that too much warfarin also raised the risk of dementia.
"This points to the possibility that dementia in atrial fibrillation patients is partly due to small repetitive clots or bleeds in the brain," Bunch said.
Researchers followed 2,693 patients of Intermountain Medical Center's coumadin clinic who had no history of dementia or stroke. Such clinics are common at hospitals where patients with erratic warfarin levels are closely monitored, sometimes weekly, Bunch said.
Of those, 4.1 percent were diagnosed with dementia.
Patients who were within the optimal dosing range less than 25 percent of the time were 4.5 times more likely to develop dementia. Patients whose dosages were within the therapeutic range 51-75 percent of the time were only 2.5 percent more likely to develop dementia.
The risk remained significant even when adjusting for common risks of stroke and bleeding, said, Bunch.
The results underscore the importance of very close monitoring of some patients. In most patients, warfarin levels are stable, but in about 25 to 30 percent those with dietary restrictions or certain genetic factors levels are erratic, he said.
Patients in whom "the variability is so high it can't be managed" should probably consider alternative therapies, he added.