Abstract
Parkinson’s disease (PD) is a progressive neurological disorder that influences a wide range of functions, including cognition. Over the past 30 years, deep brain stimulation (DBS) of the subthalamic nucleus (STN) has been successful in managing the neurological symptoms of PD. However, less is known about the cognitive consequences of this treatment method. This thesis aimed to investigate changes in cognition following STN DBS, and to identify pre-operative characteristics influencing these changes. The current study is part of the NORSTIM study, a self-controlled single-center study of n = 55 PD patients treated at Oslo University Hospital (OUH) - Rikshospitalet. This thesis is mainly based on results from neuropsychological assessments that were carried out before, 1 year after and 5 years after STN DBS surgery. The significance of changes in cognitive functions across measurement points were analyzed using ANOVA for repeated measures, and pairwise comparisons were made using the paired-samples t-test. Exploratory correlation analysis and linear simple regression were used to identify predictors of change in cognitive function. Overall, the decline of function in the years following DBS surgery was more rapid than that of the normative population. A more challenging question to answer was the degree to which the observed decline differed from the cognitive changes in normally developing PD. The results indicate that processing speed, executive functions (inhibition and switching), word generation and delayed verbal recall were negatively affected by STN DBS. The relationships between these functions are dynamic and complex, raising the question of which functions are directly affected by STN DBS, and which are indirectly influenced by the decline of other functions. It seems that executive functions play a central role in these interactions, consistent with theories of underlying mechanisms of change of cognition in PD. In addition, the current study supports the recurring finding of reduced verbal fluency following STN DBS. Age and duration of disease were the strongest predictors of cognitive outcome, affecting change in motor function, attention/working memory, visual learning/memory and executive function in the year following STN DBS. In conclusion, the results of the current study indicate that STN DBS treatment adversely affects some, but not all, domains of cognitive function. Further, results indicate that age and duration of disease are the most important pre-operative characteristics to take into account when considering patients for STN DBS treatment, from a cognitive point of view. However, further research is needed to separate the effects of STN DBS on cognitive function from normal progression of PD.