Apathy and verbal fluency in STN-stimulated PD patients - An observational follow-up study

Objective To evaluate apathy and its relation to verbal fluency tasks in a consecutive series of 19 patients with Parkinson's disease (PD) submitted to deep brain stimulation of the subthalamic nucleus (DBS of STN). Methods 19 consecutive PD patients submitted to bilateral DBS of STN were studied for apathy pre-operatively and 17 months after surgery. The PD patients underwent a battery of cognitive tests assessing reasoning, memory and frontal executive functions, including phonemic and categorial fluency tasks. The Beck Depression Inventory (BDI) was used for depression. Apathy was assessed by means of the Apathy Scale (AS). In order to quantify changes among individual patients, the clinical criterion of more or less than 1 SD (standard z-score) was used to register a patient as improved or worsened, respectively. Results After surgery, apathy scores did not change and mood improved (p < 0.02), while a significant worsening was found in the phonemic fluency (p < 0.001). The percentage of patients with an apathy score above the recommended cut-off value (14) was 42% both before and after DBS of STN. Individual outcomes on the apathy scale (1 SD criterion) evidenced that 53% of the patients remained stable, 16% improved, while 31% worsened. This last percentage reduced to 21% (4/19) when considering only the PD patients with an apathy score >= 14 after surgery. No significant correlation was found between the apathy scores variation and any of the neurological variables considered, and, in particular, no correlation was found between apathy and verbal fluency. Conclusions The results of the present study suggest that DBS of STN does not necessarily induce apathy even if individual patients show a moderate post-operative worsening of apathetic symptoms.