Parkinson's treatment may impair judgment

By Anthony J. Brown, MD

NEW YORK (Reuters Health) - Deep brain stimulation is known to improve the motor symptoms of Parkinson's disease, but it can also cause impulsivity. Now, new research suggests that this is because deep brain stimulation interferes with the brain's normal ability to slow down when making a difficult decision.

Deep brain stimulation is conducted using a surgically implanted, battery-operated neurostimulator that delivers electrical stimulation to areas of the subthalamic nucleus that control movement and blocks the abnormal nerve signals that cause symptoms such as tremor, rigidity, slow movement and walking problems. Currently, the procedure is used only for patients whose symptoms cannot be controlled with drugs

The treatment is widely known to cause loss of inhibitions and other cognitive deficits "and as a result, does not always improve patients' quality of life despite dramatic improvements in motor symptoms," Dr. Michael J. Frank, from the University of Arizona in Tucson, told Reuters Health.

"Our findings point to a mechanistic basis for these observations, and suggest that patients should be monitored for their susceptibility to impaired decision making, and that these factors should be given greater attention preoperatively," he added.

Frank and his team's results, which appear in the current online issue of Science, also show that while drugs used to treat Parkinson's disease do not impair the slowing down process, they do interfere with a patient's ability to learn from bad decisions.

In the study, Parkinson's disease patients and a comparison group of "controls" completed various computerized decision-making tasks. The tasks typically involved making easy decisions or difficult decisions, which would normally take longer to make than easy ones.

Patients were evaluated while deep brain stimulation was on or off and while they were on and off their normal dose of levodopa medication. The investigators found that with deep brain stimulation on, the speed with which difficult decisions were made increased, and was actually faster than the speed with which easy decisions were made. With deep brain stimulation off, the patients' response speeds were comparable to those of controls.

Areas for future research on the effects of deep brain stimulation on behavior, according to Frank, include looking at "whether there is a dose-response relationship (i.e., are the same effects produced at 50 percent voltage?)" and whether there are specific regions within the subthalamic nucleus that can be targeted by deep brain stimulation to effect motor function but not decision-making processes.

SOURCE: Science, October 25, 2007, online.