Declining verbal fluency after DBS in Parkinson's disease Mental speed or executive dysfunction?

Background: Despite the frequent decline in verbal fluency after STN-DBS surgery, the cause of decline remains unclear.
Objective: This study investigates whether mental speed and executive dysfunctions are underlying mechanisms of the phonemic verbal fluency decline after STN-DBS in Parkinson's patients.
Methods: Neuropsychological assessments consisting of phonemic verbal fluency, Trail Making Test (TMT), and Stroop test were conducted before and 6 months after STN-DBS in 94 Parkinson's patients in the Galaxy Trial. A Reliable Change Index (RCI) pre- versus postoperative of verbal fluency was calculated, representing the magnitude of change. Content assessment of phonemic verbal fluency resulted in content profiles (clusters and switches).
Results: A statistically significant decline in postoperative phonemic verbal fluency was found, with statistically significant declined switching, while cluster size remained stable. Postoperative, statistically significant increases in time to perform TMT-B and all Stroop subtests was found. However, phonemic verbal fluency decline was correlated to change scores of Stroop I and II only. Comparisons of declined (N  = 23) and improved (N = 11) patients, according to RCI's, revealed that decliners had higher preoperative phonemic verbal fluency scores.
Conclusions: Postoperative phonemic verbal fluency decline could not be explained by the proposed underlying mechanisms of mental speed and executive dysfunction. However, the significant correlation of phonemic verbal fluency decline to Stroop I and II change scores suggests other possible underlying mechanisms, like slower speech.