ScienceDaily (Apr. 20, 2008) — Compared to older drugs for Parkinson disease, a newer class of medications called dopamine agonists might be better at preventing some of the disabling muscle control problems associated with the disease and its treatment, a new review of recent studies concludes.

However, patients who take dopamine agonists suffer from an increase in numerous side effects -- from sleepiness to nausea to hallucinations in some cases -- and are more likely to drop out of treatment than those who take the older treatment levodopa or no drugs at all.

"Patients taking dopamine agonists were more than twice as likely to quit treatment, suggesting that the side effects were severe enough to have a meaningful impact on patients' quality of life, outweighing the muscle control problems," said Rebecca Stowe of the University of Birmingham, the review's lead author.

The review of studies appears in a recent issue of The Cochrane Library, a publication of The Cochrane Collaboration, an international organization that evaluates medical research. Systematic reviews like this one draw evidence-based conclusions about medical practice after considering both the content and quality of existing medical trials on a topic.

Parkinson's disease is a degenerative disease of the nervous system that can impair a person's movements and speech. Dopamine agonists, drugs that stimulate the production of the important neurotransmitter dopamine in the brain, are increasingly used as first-line therapies for the disease.

Levodopa, an older drug that can be metabolized in the brain to produce dopamine, is also used widely in Parkinson's treatment. However, patients who use levodopa over long periods can develop painful, distorting, involuntary muscle spasms and repetitive movements.

Stowe and colleagues reviewed 29 studies that included 5,247 patients who were in the early stages of Parkinson's disease and did not show any significant signs of muscle and movement problems. Some of the studies compared dopamine agonists only with levodopa, while some used a combination of dopamine agonists and levodopa.

There was no significant difference in the death rates between patients using dopamine agonists and those who did not take the drugs, the researchers found.

"Importantly, the review highlights that the balance of risks and benefits of dopamine agonists remains unclear," said Stowe, who called for further studies on patients' overall quality of life and the economic costs of the treatments.

Researchers are tracking another troubling effect of dopamine agonists -- their potential link to impulsive behaviors such as uncontrolled gambling and hypersexuality. Dr. Joseph Jankovic, a Parkinson's disease and motor disorders expert at the Baylor College of Medicine, said dopamine agonists "play an important role in triggering these nonmotor symptoms" that appear in some patients with Parkinson's disease, particularly men and those who develop the disease as younger adults.

Reference: Stowe RL, et al. Dopamine agonist therapy in early Parkinson disease. The Cochrane Database of Systematic Reviews 2008, Issue 2.

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