Neuropsychological consequences of chronic bilateral stimulation of the subthalamic nucleus in Parkinson's disease

The aim of this study was to examine possible neuropsychological changes in patients with advanced idiopathic Parkinson's disease treated with bilater al deep brain stimulation (DBS) of the subthalamic nucleus (STN). Eleven pa tients (age = 67 +/- 8 years, years with Parkinson's disease = 15 +/- 3, ve rbal IQ = 114 +/- 12) were evaluated (in their best 'on state') with tests assessing processes reliant on the functional integrity of frontal striatal circuitry, prior to the procedure (n = 11), at 3-6 months (n = 11) and at 9-12 months (n =10) postoperatively. Six of these patients were older than 69 years. Despite clinical motor benefits at 3-6 months postoperative, sign ificant declines were noted in working memory, speed of mental processing, bimanual motor speed and co-ordination, set switching, phonemic fluency, lo ng-term consolidation of verbal material and the encoding of visuospatial m aterial. Declines were more consistently observed in patients who were olde r than 69 years, leading to a mental state comparable with progressive supr anuclear palsy. 'Frontal' behavioural dyscontrol without the benefit of ins ight was also reported by half (three of six) of the caregivers of the elde rly subgroup. At 9-12 months postoperative, only learning based on multiple trials had recovered. Tasks reliant on the integrity of frontal striatal c ircuitry either did not recover or gradually worsened over time. Bilateral STN DBS can have a negative impact on various aspects of frontal executive functioning, especially in patients older than 69 years. Future studies wil l evaluate a larger group of patients and examine the possible reversibilit y of these effects by turning the DBS off.