Neuropsychological functioning before and after unilateral thalamic stimulating electrode implantation in Parkinson's disease

One theoretical advantage of chronic thalamic stimulation compared with thalamotomy for the treatment of refractory Parkinson's disease (PD) entails the avoidance and reversibility of potential cognitive morbidity. Support for the cognitive safety of thalamic stimulation remains largely anecdotal; empirical data are limited to the neuropsychological findings published for one small series of patients. The purpose of this study was to supplement those published findings that pertain to the mean changes in neuropsychological test scores and to extend previous findings by evaluating cognitive changes in individual cases from preoperative baseline to 4 months after electrode implantation. Nine patients with tremor-dominant, refractory PD underwent unilateral implantation of a deep brain stimulating electrode in the ventralis intermedius thalamic nucleus (five patients on their left and four patients on their right sides). A neuropsychological test battery was administered to each patient to evaluate attention, language, memory, and visuoperceptual and executive functions during their best “on” state before surgery, while on a medication regimen, and with the stimulator turned on after surgery. As a group, the patients attained significantly higher scores on word list recognition (discriminability) and delayed recall of prose passages after surgery than before surgery. In addition, there was a trend toward higher scores on a visual confrontation naming test after surgery. Examination of individual patient data indicated gains and losses in test scores exceeding two standard deviations to be very rare. Changes of one standard deviation were also relatively rare, but gains were more likely to occur than losses. These observations provide preliminary support for the cognitive safety of thalamic stimulation for PD.