New research reveals that almost half of people with Parkinson's disease who take dopamine agonists for their condition go on to develop impulse control disorders.
Parkinson's disease is characterized by a deficiency of a key brain chemical called dopamine.
Dopamine plays a crucial role in learning, but it is also known as the "sex, drugs, and rock 'n' roll" neurotransmitter because our brains release it when we experience pleasure.
The production of dopamine can be excessively stimulated by taking drugs such as alcohol, cocaine, or heroin.
So, the neurotransmitter is at the heart of addictions and impulse control disorders ranging from substance abuse to sex addiction and gambling.
Such impulse control issues have been found to be common in people with Parkinson's disease. Pathological gambling and compulsive shopping, as well as compulsive eating and sexual behavior, have all been documented among patients with Parkinson's.
The drugs often prescribed to people with Parkinson's are the main risk factor for such compulsive behavior. Because dopamine is deficient in Parkinson's, the go-to treatment is dopamine agonists — which are drugs that activate the brain's dopamine receptors — or the well known levodopa, which turns itself into dopamine.
However, until now, researchers have not been able to establish a clear dose-effect relationship between Parkinson's drugs and impulse control disorders. As the authors of the new research write, some studies found such an association, while others did not.
So, researchers led by Dr. Jean-Christophe Corvol — of the ICM Brain and Spine Institute at the Pitié-Salpêtrière Hospital in Paris, France — set out to investigate whether such a relationship existed in a large, longitudinal cohort of patients.
Having a larger sample size and longer follow-up period in the new research leads to more reliable results, which can settle the discrepancies of previous studies, explain Dr. Corvol and colleagues.
The findings were published in the journal Neurology.
Pramipexole, ropinirole have highest risk
The researchers investigated 411 people who had received a Parkinson's disease diagnosis 5 years or under before the study, and who were clinically followed for at least 3 years.
Dr. Corvol and his colleagues interviewed the participants about any symptoms of impulse control disorders, such as compulsive shopping, eating, gambling, or sexual behaviors.
Of the 411 participants, 356 (or almost 87 percent) had taken dopamine agonists at least once since their Parkinson's diagnosis. At baseline, 81 participants (almost 20 percent) reported an impulse control disorder.
Specifically, 11 percent reported binge eating, 9 percent reported compulsive sexual behavior, 5 percent said that they shopped compulsively, and 4 percent admitted to having a gambling problem.
Of the 306 participants who did not report having any impulse control problems at baseline, 94 developed such a problem during the study. According to the scientists, this amounts to a "5-year cumulative incidence" of impulse control disorders of 46 percent.
By comparison, those who had never taken the drugs had a 5-year incidence of 12 percent. What is more, 30 participants with compulsive behaviors stopped taking the drugs during the study, which put an end to their symptoms.
Finally, higher doses of dopamine agonists and the duration of the treatment correlated directly with the risk of developing impulse control disorders.
Of all the drugs studied, pramipexole and ropinirole were linked with the highest risk of developing compulsive behavior.
The lead researcher comments on the significance of the findings.
"Our study suggests that impulse control disorders are even more common than we thought in people who take dopamine agonists [...] These disorders can lead to serious financial, legal and social and psychological problems."
Dr. Jean-Christophe Corvol
In an editorial accompanying the article, Dr. Laura S. Boylan — of New York University in New York City — writes, "These disorders can be challenging for neurologists to discover."
"People might be ashamed to tell their doctor about their problems," she adds, "they may think these issues are not related to their Parkinson's disease, or they may not even consider the disorders a problem," says Dr. Boylan.