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

Area of Science:

• Neurology
• Neuroscience
• Speech Pathology

Background:

• Subthalamic nucleus deep brain stimulation (STN-DBS) is frequently associated with a decline in verbal fluency in Parkinson's disease patients.
• The precise mechanisms underlying this verbal fluency decline remain incompletely understood.

Purpose of the Study:

• To investigate if mental speed and executive dysfunction contribute to phonemic verbal fluency decline after STN-DBS in Parkinson's disease.
• To explore potential alternative mechanisms for verbal fluency changes post-STN-DBS.

Main Methods:

• A cohort of 94 Parkinson's patients (Galaxy Trial) underwent neuropsychological testing before and 6 months after STN-DBS.
• Tests included phonemic verbal fluency, Trail Making Test (TMT), and Stroop test to assess verbal fluency, mental speed, and executive functions.
• Reliable Change Index (RCI) was used to quantify changes in verbal fluency, with detailed analysis of verbal fluency content (clusters and switches).

Main Results:

• A significant decline in phonemic verbal fluency and switching was observed post-STN-DBS, while cluster size remained stable.
• Patients showed significant increases in the time to complete TMT-B and all Stroop subtests, indicating slower mental speed and executive dysfunction.
• Phonemic verbal fluency decline correlated only with changes in Stroop I and II, not other measures of mental speed or executive function.

Conclusions:

• The study could not confirm mental speed or executive dysfunction as the primary drivers of phonemic verbal fluency decline after STN-DBS.
• The correlation with specific Stroop subtests suggests that altered speech processing or other unmeasured factors may contribute to the observed decline.