Bilateral Subthalamic Stimulation and Cognitive-Motor Performance in Parkinson's Disease Patients

Abstract & Commentary

By Panida Piboolnurak, M.D., Assistant Professor, Department of Neurology and Neuroscience, Weill Medical College, Cornell University. Dr. Piboolnurak reports no financial relationship relevant to this field of study.

Synopsis: Spreading of stimulation current to non-motor regions of the subthalamic nucleus (STN) or surrounding fiber tracts could be responsible for cognitive declines after STN deep brain stimulation (DBS), especially during bilateral stimulation and during complex cognitive-motor dual task.

Source: Alberts JL, Voelcker-Rehage C, Hallahan K, et al. Bilateral subthalamic stimulation impairs cognitive-motor performance in Parkinson's disease patients. Brain 2008;131:3348-3360.

Although bilateral subthalamic nucleus stimulation (STN DBS) significantly improves motor performance in patients with advanced Parkinson's disease (PD), it may be associated with cognitive decline. However, the incidence of neuropsychological effects appears to be less in unilateral STN DBS compared to bilateral STN DBS. For a better understanding of the effects of STN DBS on a performance requiring both cognitive and motor function resembling a real-life situation, Alberts et al conducted this study using cognitive-motor dual task.

Eight PD patients between the ages of 48 and 70 years (mean = 56.5) participated in this study. All of them underwent bilateral STN DBS surgery at least 12 months prior to the study and their stimulation parameters were stable for at least six months, with the active contacts in the dorsal border of the STN. They were evaluated in the off-medication state with three stimulation modes — bilateral stimulation, unilateral stimulation (in most affected side of the brain), and off-stimulation. In each state, they performed a cognitive (n-back task) and motor (force tracking) task under single- and dual-task conditions.

UPDRS-III scores (motor scores for PD) were improved by 33% with unilateral stimulation and by 46% with bilateral stimulation. Under simple dual-task conditions, there was no difference in cognitive or motor performance between unilateral and bilateral stimulation. However, as dual tasks became more complex, cognitive and motor performances were significantly worse, with bilateral stimulation compared to unilateral stimulation, and reached the level of the off-stimulation state with the most complex task.

This finding supports the hypothesis that bilateral STN DBS interferes with patients' ability to handle higher demands placed on cognitive processes, particularly in a dual task. Given the very small size of STN, even when the lead is located in the sensorimotor region, the stimulation current can spread to limbic and associative areas as well as adjacent structures that may also be involved in cognition. Since unilateral stimulation did not result in the same level of working memory decline, it indicates that the non-stimulated basal ganglia may be able to compensate for the disruption induced by stimulation, or that unilateral stimulation is not enough to disrupt the working memory function. Interestingly, one study showed that 44% of patients who were working prior to the DBS surgery did not return to work because of cognitive dysfunction. Given that more young patients are undergoing STN DBS surgery, detection and prevention of subtle cognitive decline may allow them to remain in the workforce longer.

Commentary

Bilateral STN DBS is very effective on the motor symptoms of Parkinson's disease. However, cognitive declines after STN DBS have been increasingly reported. In some circumstances, changes in cognitive functions are very subtle and cannot be detected when evaluating cognitive and motor functions separately. For this reason, cognitive-motor dual task can be more useful as it will reflect patient's ability to perform a more complex task that requires both mental and motor power. It is also a useful tool to help select a good candidate for bilateral STN DBS. If the future study shows that GPi DBS does not impair dual-task performance, GPi-DBS can be an alternative option for a patient with cognitive deficit.