Subthalamic Nucleus Stimulation Does Not Have Any Acute Effects on Verbal Fluency or on Speed of Word Generation in Parkinson's Disease

Effects of thalamic and basal ganglia deep brain stimulation on language-related functions - Conceptual and clinical considerations

Deep Brain Stimulation (DBS) is a therapy for various neurological movement disorders. It acts predominantly on motor symptoms, but may unfold a number of mostly subtle cognitive effects. In this regard, reports on particular language-related DBS sequels are comparably frequent, but difficult to overlook, given the heterogeneity of targeted structures in the brain, treated diseases, assessment methods and results reported. Accordingly, available knowledge was organized with respect to important aspects, such as the main DBS loci and surgical versus neuromodulatory therapy actions. Current views of biolinguistic underpinnings of the reviewed data, their clinical relevance and potential implications are discussed.

Deep brain stimulation effects on verbal fluency dissociated by target and active contact location

OBJECTIVE

Deep brain stimulation (DBS) improves motor symptoms in Parkinson's disease (PD), but it can also disrupt verbal fluency with significant costs to quality of life. The current study investigated how variability of bilateral active electrode coordinates along the superior/inferior, anterior/posterior, and lateral/medial axes in the subthalamic nucleus (STN) or the globus pallidus interna (GPi) contribute to changes in verbal fluency. We predicted that electrode location in the left hemisphere would be linked to changes in fluency, especially in the STN.

METHODS

Forty PD participants treated with bilateral DBS targeting STN (n = 23) or GPi (n = 17) completed verbal fluency testing in their optimally treated state before and after DBS therapy. Normalized atlas coordinates from left and right active electrode positions along superior/inferior, anterior/posterior, and lateral/medial axes were used to predict changes in fluency postoperatively, separately for patients with STN and GPi targets.

RESULTS

Consistent with prior studies, fluency significantly declined pre- to postsurgery (in both DBS targets). In STN-DBS patients, electrode position along the inferior to superior axis in the left STN was a significant predictor of fluency changes; relatively more superior left active electrode was associated with the largest fluency declines in STN. Electrode coordinates in right STN or GPi (left or right) did not predict fluency changes.

INTERPRETATION

We discuss these findings in light of putative mechanisms and potential clinical impact.

Dissociable effects of subthalamic nucleus deep brain stimulation surgery and acute stimulation on verbal fluency in Parkinson's disease

OBJECT

Verbal fluency (VF) is the cognitive test which shows the most consistent and persistent post-operative decline after subthalamic deep brain stimulation (STN-DBS) in Parkinson's disease (PD). However, the reasons are not completely understood, and the debate has focused on two hypotheses: a surgical effect or an acute STN-DBS effect.

METHODS

We recruited 3 PD samples: (1) a group assessed before and after STN-DBS surgery (2) a group assessed On vs. Off STN-DBS and (3) an unoperated PD control group. All groups performed letter, category and switching category VF tasks. The total number of correct words generated were noted and measures of clustering and switching were also obtained.

RESULTS

We found a significant effect of STN-DBS surgery on all VF tasks which was associated with a post-operative decline in the total number of words generated, and a reduction of phonemic switching during the letter and category VF tasks, and a reduction of semantic clustering for category VF. By contrast to the effects of surgery, acute On vs. Off stimulation did not influence the number of words generated on any of the VF tasks. Acute stimulation only produced two effects on the category VF task: increased semantic cluster size and decreased number of semantic switches when STN-DBS was switched On.

CONCLUSIONS

This study differentiates between the effects of STN-DBS surgery and acute stimulation on VF performance. Our findings indicate that the STN-DBS effect on VF are a surgical and not an acute STN stimulation effect.