Cognitive outcomes after deep brain stimulation for Parkinson's disease: a review of initial studies and recommendations for future research

Modern ablative surgery for movement disorders probably results in less frequent and severe cognitive morbidity than seen in early surgical series. Nonetheless, recent studies indicate that neurobehavioral functions commonly compromised in Parkinson's disease (PD) (e.g., executive functions, verbal fluency, and memory) are negatively impacted in some patients by lesion placement. The potential reversibility of cognitive dysfunction after chronic electrical deep brain stimulation (DBS) for PD has lead some to favor this treatment modality over ablation. This paper reviews the initial studies of the cognitive effects of thalamic, pallidal, and subthalamic DBS. These studies suggest that DBS is relatively safe from a cognitive standpoint and that the benefits of motor improvements probably outweigh the cost of minimal cognitive morbidity. This conclusion must be offered with caution, however, given the small numbers of studies to date and their methodological limitations. Neurobehavioral research has yet to adequately address (1) outcome relative to appropriate control groups; (2) effects of electrode placement versus stimulation; (3) laterality- and site-specific effects of DBS; (4) long-term effects of DBS; (5) effects of stimulation parameters; (6) risk factors for cognitive dysfunction with DBS; (7) whether cognitive dysfunction associated with DBS is reversible; and (8) comparative neurobehavioral outcome after DBS and ablation. DBS affords an exciting opportunity to clarify the neurobehavioral role of the basal ganglia.