Effects of Deep Brain Stimulation on Cognition, Mood and Behaviour in Parkinson’s Disease

Abstract

32Disappointed by the limitations of pharmacotherapy, emboldened by technological advances in surgery and radiology and armed with a better understanding of pathophysiology, physicians and scientists in the 1980s charted a renaissance of surgery for movement disorders such as Parkinson's disease (PD). The desire for a safer alternative to lesional or ablative neurosurgery, coupled with observations that intraoperative electrical stimulation used for target identification could alleviate abnormal movements, 1,2 prompted the exploration of fully Neurobehavioural Effects of Deep Brain Stimulation in Parkinson's DiseaseBy far the most attention to neurobehavioural outcomes of DBS has been devoted to PD, and the majority of these studies have examined the outcome of subthalamic (STN) rather than thalamic or pallidal (GPi) DBS. Probably greater controversy attends the neurobehavioural outcomes after STN DBS than after GPi or thalamic DBS, and this probably reflects, at least in part, differences among studies in the sample characteristics, selection and exclusion criteria, length of follow-up, surgical technique, post-operative DBS programming and pharmacotherapy protocols, and the thoroughness and timing of the neuropsychological evaluation protocol. In general, studies employing cognitive screening instruments fail to detect neurobehavioural morbidity. While some may argue that the lack of change on screening instruments suggests that neuropsychological changes detected by more extensive evaluations are not of clinical significance, a recent meta-analysis of the empirical data suggests that screening instruments may be insensitive even to clinically meaningful changes after DBS. 8 Consequently, cognitive screening measures are probably useful in helping decide which surgical candidates can be excluded from further evaluation (including full neuropsychological evaluation), but insufficient to adequately document neurobehavioural outcomes of DBS. Thalamic Deep Brain StimulationFour studies 9-12 have observed no widespread or significant changes in cognition, mood or behaviour after unilateral thalamic DBS, although one study suggested that statistically (but not necessarily clinically) significant declines in verbal memory are associated with left thalamic DBS. Few studies have examined mood after thalamic DBS, but one study 9 found an improvement in depressive symptoms four to 10 days after surgery. Pallidal Deep Brain StimulationUnilateral GPi DBS appears cognitively safe, although this conclusion is tempered by the limited number of small-sample studies published. [13][14][15] Although patients in one study showed statistically significant declines in visuoconstructional ability and verbal fluency, the changes were rarely of clinical significance. Even when using a liberal criterion of impairment (a test score falling one standard deviation below the mean of normative samples), another study 13 observed that only six of the 20 patients showed any increase, no matter how small, in the percentage of tests in the imp...