The most significant brain regions implicated in olfactory dysfunction in Parkinson’s disease

Olfactory dysfunction is observed in over 95% of patients with Parkinson’s disease (PD). This study examines the relationship between gray matter volume (GMV) and olfactory impairment in a cohort of 182 subjects, including PD patients and healthy controls (HCs). Using the Iran Smell Identification Test, which is a standardized 24-item olfactory identification assessment, to evaluate the olfactory performance, PD patients were divided into two groups (scores ranging from 0 to 18 indicate olfactory dysfunction, while scores from 19 to 24 indicate normal olfaction): those with normal smell (PD-NS, n = 23) and those with smell disorders (PD-SD, n = 69). Differences in GMV were analyzed using voxel-based morphometry. Statistical analysis was conducted using SPSS 26. The results revealed that the PD-NS group exhibited reduced GMV in the right thalamus and the left parahippocampal gyrus compared to the HCs. Furthermore, the HC group demonstrated no statistically significant olfactory dysfunction. In contrast, the PD-SD group showed significant decreases in GMV in the right entorhinal cortex and both the right and left hippocampus compared to both the HC and PD-NS groups. These findings indicate that PD patients experience more severe olfactory dysfunction in hippocampal regions than the HC group, likely attributed to the initial pathological loss of gray matter in both the right and left hippocampus.