Effects of bilateral deep brain stimulation of the subthalamic nucleus on olfactory function in Parkinson's disease patients

Objective: The goal of the present study was to evaluate the effects of bilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) on olfaction in patients with Parkinson's disease (PD). Methods: 15 patients suffering from sporadic PD- related dysosmia were implanted with bilateral electrodes aimed at the STN. One week before the surgery, odor detection threshold (DT) and identification threshold (IT) were evaluated in all patients using the 'five odor olfactory detection arrays' in both medication-off and medication-on conditions. 15 healthy age-matched controls also received the same olfactory evaluation. Patient evaluations were repeated at 6 and 12 months postoperatively in a medicationoff/stimulator-on or medication-off/stimulator-off condition. Odor DT and IT scores were compared pre- and postoperatively, as well as between the medication-off/stimulator-on or -off conditions. Results: The motor symptoms of all 15 PD patients, including rigidity, tremor, bradykinesia, postural instability, and gait were significantly improved after stimulator implantation. The UPDRS motor (UPDRS III) scores decreased significantly in the medication-off/stimulator-on condition (p < 0.01). The odor DT and IT scores of PD patients were higher than those of healthy controls (p < 0.01). In the medication-off/stimulator-off condition, there was no significant difference in the odor DT and IT scores in PD patients pre- vs. postoperatively (p > 0.05). Notably, there were no significant alterations to DT scores in the stimulator-on and -off conditions at the 6- and 12-month follow-up (p > 0.05), whereas IT scores were significantly improved in the stimulation-on relative to the stimulation-off condition at the 6- and 12-month follow-up. Conclusions: STN DBS can significantly improve olfactory cognitive function in PD patients. The possible mechanisms include an improvement in striatal metabolism and neuronal activity in the orbitofrontal cortex mediated by STN DBS, as well as increased glucose metabolism in the striatum, midbrain, cin-gulate gyrus, and motor and higher-order somatosensory association cortices.