The primary aim of this thesis was to investigate the effect of STN DBS-treatment on neuropsychological functioning. A central part of this investigation was describing the study group’s changes in cognitive functioning one and five years after DBS surgery compared with baseline levels. Already at baseline, is was apparent that the study group performed somewhat lower than the normative average, despite their high educational level. 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. When looking at the timing of the observed changes, in combination with knowledge from other research about normal development of PD and effects of STN DBS on cognition, the results of the current study indicate that processing speed, executive functions (inhibition and switching), word generation and delayed verbal recall were affected by STN DBS. As discussed, 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. The pattern of results in the current study indicates that decline in executive function plays 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. Working memory and verbal/visuospatial memory functions other than delayed verbal recall appeared to be relatively well-preserved following DBS surgery.
51 The other central question of this thesis was to identify pre-operative characteristics
predicting cognitive outcome of STN DBS treatment. The results showed that age and duration of disease were the strongest predictors of cognitive outcome, including changes in motor function, attention/working memory, visual learning/memory and executive function in the year following STN DBS. In addition, measures of pre-operative neurological symptom severity and stability of treatment effects predicted outcome of executive function and verbal learning/memory, respectively. Between one and five years post-operatively, when normal progression of PD is more likely to influence results, age and duration of disease were still important predictors, together with baseline global cognitive function.
In conclusion, the results of the current study indicate that STN DBS treatment adversely affects some, but not all, domains of cognitive function. The results further 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.
52
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