Deep Brain Stimulation for Parkinson's Disease

Deep brain stimulation is one of the most effective therapies to treat Parkinson's disease. The subthalamic nucleus (STN), the internal part of globus pallidum (GPi) and the nucleus ventrointermedius (VIM) of the thalamus are common stimulation sites. A comprehensive neuropsychological test-battery should be applied prior to surgery to exclude dementia and to specifically examine executive functioning, because executive functions can worsen after surgery. Whereas neuropsychological changes are not common after stimulation of the VIM and the GPi, STN stimulation may lead to mild worsening in executive functioning. These changes have been examined extensively for STN-DBS. A decline in verbal fluency (Cohens' d-0.5) and Stroop task performance (Cohens' d-0.4) has been shown. However, these changes have no impact on the improvement of quality of life after surgery. Patients with an older age, suffering from axial motor symptoms and taking a higher levo-dopa equivalence dosage are at risk to decline in executive functions after surgery. If the electrode trajectories hit the caudate nucleus, patients showed a worsening in global cognition and working memory abilities. A worsening of verbal fluency and Stroop test performance are the consequence of the electrode placement itself, either due to a micro-lesional effect or an effect of electrode stimulation. As it can be switched off, DBS can may serve as a reversible, intra-individual lesion model of the STN. The modulation of the stimulation settings is a useful method to assess the impact of the STN on different cognitive domains such as spatial orientation, recognition of emotional stimuli and decision making to explore the role of the STN in the cognitive and the affective domain. Changes in these domains are only detectable using experimental designs and these changes have only minor consequences for patients' daily routine.