The drugs -- called anticholinergic agents -- slow electrical impulses in nerve cells. They're used to help Parkinson's disease patients control unwanted movement such as tremors. These drugs also help with bladder control and dizziness. But other common drugs have anticholinergic activities, too. These include older allergy drugs and tricyclic antidepressants.
The report in the August issue of Annals of Neurology comes from Elaine K. Perry, PhD, senior scientist at University of Newcastle upon Tyne and colleagues. Perry's team examined the brains of deceased Parkinson's patients for the plaques and tangles seen in the brains of Alzheimer's patients.
The result: Those who took anticholinergic drugs for more than two years had significantly more plaque and tangles than those who never took the drugs. Taking the drugs for less than two years had no effect.
"What we saw were very, very low levels of Alzheimer's disease pathology, nothing like what you would see in Alzheimer's disease," Perry tells WebMD. "The drugs might increase this pathology, but they are not taking people into Alzheimer's disease."
But Allan Levey, MD, PhD, chair of the neurology department at Emory University School of Medicine, says these patients may have been on the way to Alzheimer's. Levey's commentary appears in the same issue of Annals of Neurology.
"If these people with Parkinson's disease had lived longer, they might have developed Alzheimer's disease," Levey tells WebMD.
Alzheimer's Risk in Widely Used Older Drugs
The category of drugs linked to Alzheimer's changes aren't now the front-line drugs for Parkinson's disease. One reason is that they can have terrible side effects. Medical students remember these side effects by learning a series of similes:
- "Mad as a hatter." Anticholinergic drugs can make patients delirious.
- "Blind as a bat." The drugs can dilate the pupils of the eye, making it hard to see.
- "Red as a beet." Flushing is a common side effect.
- "Dry as a bone." Dry skin is another side effect.
- "Hot as a hare." The drugs can cause overheating or hyperthermia.
Elderly patients are particularly likely to become confused when taking anticholinergic medicines. But the drugs are still widely used. And sometimes doctors aren't aware that other drugs their patients are taking have the same effects.
"The uglier perspective is all the drugs that have anticholinergic side effects," Levey says. "These are a lot of common medicines like antihistamines and the older antidepressants. And also we are talking about a worldwide problem, where many poorer nations don't have access to the more expensive new antidepressants."
Levey says the new findings don't prove anything. But they add another reason to prescribe these drugs with caution.
Perry also warns against jumping to conclusions.
"We cannot make any change to clinical practice based on one study," she says. "We need more studies before we can say, 'Be cautious about these drugs.' It is not right to start a panic in people taking these medicines."
But Levey says it can't hurt to talk this over with your doctor.
"Talk to your doctor about avoiding medicine with anticholinergic properties," he advises. "That's more because of the immediate effects, not because of new concerns about Alzheimer's disease. And if you need the medicines, don't worry about this."
Silver Lining: A Window on New Alzheimer's Treatments
There are also drugs that have the opposite effect of anticholinergic drugs. These compounds speed electrical impulses in nerve cells. Animal studies show that they can reduce plaques and tangles in the brain.
Perry and colleagues are the first to link human brain tangles to the cholinergic system. There are several candidate drugs for reducing Alzheimer's plaques -- but none for reducing Alzheimer's tangles. Now there's a new hope.
"This research is supportive about finding new and exciting approaches to treating Alzheimer's disease," Perry says.
"The Perry study raises new possibilities for how we might begin to treat tangle disorders," he says. "We have nothing now for these diseases. This at least may get people thinking about trying available drugs. We certainly don't expect they will have a big impact, but even a modest effect would be a start. And maybe there is no effect at all -- but this will start people looking in a new direction."