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Can Some Parkinson's Drugs Cause Alzheimer's-like Symptoms?

Finding shouldn't discourage use of the medicines, expert says

(HealthDay is the new name for HealthScoutNews.)

FRIDAY, July 25, 2003 (HealthDayNews) -- Some drugs used to treat Parkinson's disease might just be responsible for Alzheimer's-like dementia in those same patients.

A study appearing in the Aug. 2 issue of Annals of Neurology found Parkinson's patients who took drugs to control tremors and bladder function also had the hallmark "plaques" and "tangles" of Alzheimer's disease, albeit at a much lower density than Alzheimer's patients and without clinical symptoms.

Both Parkinson's and Alzheimer's are neurodegenerative diseases, meaning that cells in particular areas of the brain die off and give rise to various symptoms, explains William Thies, vice president of medical and scientific affairs at the Alzheimer's Association in Chicago.

Both of these processes also give rise to specific changes in neurotransmitters so that, in the case of Parkinson's disease, drugs that block acetylcholine receptors can help ease some symptoms. These same drugs could, however, exacerbate Alzheimer's, which is marked by a loss of acetylcholine, the study says.

The findings -- by researchers at Newcastle General Hospital and University College London, both in England -- make sense in light of current knowledge.

"We know that a fair amount of dementia is attached to Parkinson's," Thies says. "It's not the first symptoms but often people get demented and, for that matter, people with Alzheimer's often develop motor problems late in the course of the disease. So it is perhaps not entirely surprising that these drug which block a particular type of acetylcholine receptor would lead to some exacerbation of thinking processes."

According to the study authors, people with Parkinson's have a six times higher rate of dementia compared with healthy individuals, although this is not always due to Alzheimer-related pathology.

The drugs that block acetylcholine receptors in Parkinson's patients are called "antimuscarinic" agents and are usually prescribed to control tremors and bladder dysfunction. Tricyclic antidepressants have the same effects and are also sometimes prescribed for these individuals.

The authors of this study looked at the brains of 120 people over the age of 70 who had died of Parkinson's. Specifically, they were looking for evidence of the protein buildups -- or plaques and tangles -- that are hallmarks of Alzheimer's disease.

Those who had taken acetylcholine-blocking drugs for more than two years had more than twice as many plaques and tangles as patients who took the drugs for less than two years or not at all. The density of the structures did not approach that found in Alzheimer's patients, however, and could not even be considered "definite Alzheimer's disease." Also, none of the Parkinson's patients ever had symptoms severe enough to warrant an Alzheimer's diagnosis, the researchers say.

People who had taken tricyclic antidepressants also seemed to have more plaques and tangles but the increase was not seen as statistically significant, the study says.

Although use of this type of drug has decreased in recent years -- thanks largely to the availability of new drugs -- they are still prescribed.

This study is an observational one, meaning it showed an association but no definite cause-and-effect relationship. Nevertheless, Thies says, "it certainly should be something that physicians who are treating people with Parkinson's are aware of and are paying attention to. Whether it revolutionizes the care, I think, is unknown."

Dr. Allan I. Levey is the author of an accompanying editorial in the journal and chairman of the department of neurology at Emory University School of Medicine. "It [the possible link] probably doesn't change the immediate landscape, [but] it should make people more cautious. One good outcome would be if people's awareness was heightened so that they think twice about prescribing this mediation," he says.

"There's still a lot of ignorance in the medical community. These drugs are still prescribed way too widely," he adds. "We see it on a daily basis. People are prescribed so many medicines and when there's no clear benefit, people hope for a benefit and they just keep taking them, and that's the real risk. If it turned out to be that long-term use enhanced the likelihood of developing Alzheimer's disease, that would be tragic."

On the research side, however, the findings may provide some clues as to the kinds of medications that could prevent Alzheimer's symptoms, Levey says.

"There are other medications that act in opposing ways so instead of blocking acetylcholine, they promote it. The drugs that promote acetylcholine may add some benefit in reducing the chance of getting Alzheimer's," he says. In fact, some clinical trials testing that theory are already under way. The current study just "adds more fuel," Levey adds.

However, Thies says the findings aren't enough to discourage use of the drugs. "If you were using these drugs and saw a very rapid deterioration in people's cognitive function, you might want to examine whether drugs were a component of that," he says.

More information

For more on Alzheimer's disease, visit the Alzheimer's Association or the Parkinson's Disease Foundation.

SOURCES: William Thies, Ph.D., vice president, medical and scientific affairs, Alzheimer's Association, Chicago; Allan I. Levey, M.D., Ph.D., professor and chairman, department of neurology, Emory University School of Medicine, Atlanta; Aug. 2, 2003, Annals of Neurology
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