Olfactory performance acts as a cognitive reserve in non-demented patients with Parkinson's disease

Objective: To explore whether olfactory performance acts as a cognitive reserve in non-demented patients with Parkinson's disease (PD). Methods: Patients with non-demented PD (n=119) underwent T1-weighted MRI and olfactory identification tests. According to their olfactory performance, PD patients were subdivided into three groups of high score (PD-H, n=38), middle score (PD-M, n=48), and low score (PD-L, n=33). We investigated the pattern of gray matter (GM) density according to olfactory performance using voxel-based morphometry (VBM) and analyzed the correlation between GM density and olfactory performance. Results: No significant differences in demographic characteristics were observed among the groups. A neuropsychological test showed that cognitive deficits in verbal memory function were more severe in the PD-L group than in the PD-H group. However, a VBM analysis revealed that patients in the PD-H group possessed significantly decreased GM density in the bilateral temporal areas, orbitofrontal areas, mesiofrontal areas extending into the cingulate gyrus, and prefrontal areas, compared with patients in the PD-L group. No areas exhibiting a significant difference in GM density were observed between the PD-H and PD-M groups. Olfactory performance in patients with PD was negatively correlated with both the brain GM volume and intracerebral volume; in particular, GM density in the caudate nucleus and putamen exhibited a negative correlation with olfactory performance. Conclusions: Our data show that a high olfactory performance may compensate GM volume loss in order to minimize the exhibition of cognitive impairment and thus may act as a cognitive reserve in non-demented patients with PD.