Objective: To investigate verbal fluency following unilateral subthalamic nucleus (STN) deep brain stimulation (DBS) in patients with Parkinson’s disease (PD).

Background: Verbal fluency declines in people with PD who undergo bilateral DBS is widely observed. Here we test the hypothesis that implant hemisphere differentially impacts longitudinal changes in verbal fluency in 19 PD patients who underwent unilateral STN DBS in the more affected hemisphere.

Method: SUNDIAL is an ongoing randomized, double-blind crossover study of circular versus directional unilateral STN DBS for PD.  In this interim analysis, left (n=14) and right (n=7) STN patients completed a validated verbal fluency task (letter fluency: FAS/CFL) preoperatively and at 2, 4, and 6-months post-surgery. Repeated-measure analysis of variance evaluated changes in verbal fluency outcomes including both demographically adjusted scores, and percent-change from baseline. We remain blinded to the allocation of directional or circular stimulation until study conclusion.

Results: Preoperatively, left STN patients performed worse than right STN patients (M=37.3±9.2 versus 49.1±11.1; t=-2.5, p=0.022, d=1.17). Following unilateral STN DBS, patients with left hemisphere implants showed declines in verbal fluency, whereas right hemisphere implants showed significant improvements, reflected by significant time x STN target interactions for demographically adjusted scores, F(1,19)=3.31, p=.033, and percent change from baseline, F(1,19)=4.14, p=0.013. Overall, left STN patients experienced a 12% decline in verbal fluency following DBS, while verbal fluency improved in right STN patients by 20%.

Conclusion: Here we found greater baseline dysfluency and declines in verbal fluency following unilateral left versus right STN stimulation, with preservation or even improvements in verbal fluency following right hemisphere implants. Our findings suggest that declines in verbal fluency after bilateral surgery arise primarily from the left hemisphere electrode, or else from combined effects of the bilateral intervention. Unilateral right STN DBS is a viable option for initial therapy in patients with prominent left hemibody motor asymmetry, particularly in more elderly patients or in those with specific concerns about potential declines in verbal fluency after surgery.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/subthalamic-nucleus-deep-brain-stimulation-implant-hemisphere-differentially-changes-verbal-fluency-in-patients-with-parkinson-disease/