Olfactory changes in patients with Parkinson's disease undergoing deep brain stimulation in the subthalamic nucleus: a systematic review and meta-analysis

Deep brain stimulation in the subthalamic nucleus (STN-DBS) is a therapeutic intervention for patients with Parkinson's disease (PD) primarily aimed at improving motor symptoms. However, the effects of STN-DBS on non-motor symptoms (NMS), such as olfactory dysfunction (OD), remain poorly understood. We performed a systematic review and single-arm meta-analysis to evaluate the effects of STN-DBS on OD in PD patients. We searched Medline, Scopus, and Web of Science databases. Eligible studies included observational studies with >= 4 patients reporting the effects of STN-DBS on OD in PD patients. Mean differences (MD) between pre- and post-operative, along with the final mean pooled analysis, olfactory scores values with 95% confidence intervals (CI) with a random effects model were used. The statistical analysis was performed using the software R Studio. The heterogeneity was assessed with I-2 statistics, and leave-one-out sensitivity analysis was used to address high heterogeneity. A total of seven studies, encompassing 188 PD patients, with a mean age of 60.8 years, were included in our analysis. There was a statistically significant difference between pre- and post-operative value in Odor discrimination (ODI) (MD 2.16; 95% CI 1.37 to 2.96; p < 0.01; I-2 = 0%) and Unified Parkinson's Disease Rating Scale part III OFF medication (UPDRS III OFFMED) (MD -11.96; 95% CI -22.57 to -1.35; p = 0.03; I-2 = 88%), showing improvement in OD and motor function after DBS, compared to baseline. In contrast, there was no statistically significant difference between the initial and final value in Odor thresholds (OT) (MD 1.54; 95% CI -0.22 to 3.30; p = 0.09; I-2 = 90%), and the University of Pennsylvania Smell Identification Test (UPSIT) (MD -0.37; 95% CI -3.40 to 2.66; p = 0.81; I-2 = 0%). Additionally, the pooled analysis for Odor identification (OI) showed a final mean of 7.06 (95% CI 4.39 to 9.72; I-2 = 100%), and for the TDI score (composite score derived from the sum of OD, OI, and OT results), a final mean of 25.96 (95% CI 17.31 to 34.62; I-2 = 94%). Despite the prior indication of STN-DBS for motor symptoms, it may improve OD in PD patients. Clinical trial number Not applicable.