The effect of deep brain stimulation of the subthalamic nucleus on language function in Parkinson’s disease: A systematic review

Abstract

Purpose: This systematic review focuses on the effect of bilateral Deep Brain Stimulation (DBS) of the Subthalamic Nucleus (STN) on language function in Parkinson’s Disease (PD). It fills an important gap in recent reviews, by considering other language tasks in addition to verbal fluency. Methods: We critically and systematically reviewed the literature on studies that investigated the effect of bilateral STN-DBS on language function in PD. All studies included a matched PD control group who were on Best Medical Treatment (BMT), with language testing at similar baseline and follow-up intervals as the DBS PD group. Results: Thirteen identified studies included a form of a verbal fluency task, seven included picture naming, and only two studies included more language-oriented tasks. We found that verbal fluency was negatively affected after DBS, whereas picture naming was unaffected. Studies investigating individual change patterns using Reliable Change Indices (RCIs) showed that individual variability is larger for picture naming than for verbal fluency. Conclusions: Verbal fluency is the most frequently investigated aspect of language function. Our analysis showed a pattern of decline in verbal fluency across multiple studies after STN-DBS, whereas picture naming was unaffected. Data on more language-oriented tests in a large DBS sample and BMT control group are sparse. The investigation of language function in PD after DBS requires sensitive language tests (with and without time pressure) and experimental designs as used in the studies reviewed here. RCI statistics are a promising tool for investigating individual differences in performance after DBS.