Decline in Verbal Fluency After Subthalamic Nucleus Deep Brain Stimulation in Parkinson's Disease: A Microlesion Effect of the Electrode Trajectory?

Background: Decline in verbal fluency (VF) is frequently reported after chronic deep brain stimulation (DBS) of the subthalamic nucleus (STN) in Parkinson disease (PD). Objective: We investigated whether the trajectory of the implanted electrode correlate with the VF decline 6 months after surgery. Methods: We retrospectively analysed 59 PD patients (mean age, 61.9 +/- 7; mean disease duration, 13 +/- 4.6) who underwent bilateral STN-DBS. The percentage of VF decline 6 months after STN-DBS in the on-drug/on-stimulation condition was determined in respect of the preoperative on-drug condition. The patients were categorised into two groups (decline and stable) for each VF. Cortical entry angles, intersection with deep grey nuclei (caudate, thalamic or pallidum), and anatomical extent of the STN affected by the electrode pathway, were compared between groups. Results: A significant decline of both semantic and phonemic VF was found after surgery, respectively 14.9% +/- 22.1 (P < 0.05) and 14.2% +/- 30.3 (P < 0.05). Patients who declined in semantic VF (n = 44) had a left trajectory with a more anterior cortical entry point (56 +/- 53 versus 60 +/- 55 degree, P = 0.01) passing less frequently trough the thalamus (P = 0.03). Conclusions: Microlesion of left brain regions may contribute to subtle cognitive impairment following STN-DBS in PD.