Association of impaired olfactory identification with prevalent mild cognitive impairment and regional brain atrophy: the Hisayama Study.

Aim: To investigate the association of olfactory identification ability with both the prevalence of mild cognitive impairment (MCI) and regional brain atrophy in a community-dwelling older population.

Methods: A total of 1293 participants without dementia aged 65 years or older underwent a Japanese version of the 4-Item Pocket Smell Test, an assessment of cognitive function, and brain magnetic resonance imaging scanning in 2017-2018. Regional gray matter volumes (GMVs) and white matter lesions volume (WMLV) were estimated using FreeSurfer software. The association of impaired olfactory identification with the presence of MCI and regional GMVs or WMLV was estimated by logistic regression analysis and ANCOVA, respectively. We also assessed the association between impaired olfactory identification and GMVs without regions of interest, using a voxel-based morphometry (VBM) analysis.

Results: Participants with impaired olfactory identification had a significantly higher likelihood of MCI than those without (odds ratio, 1.99 [95% confidence interval, 1.36-2.91]). In the FreeSurfer analysis, participants with impaired olfactory identification had significantly lower GMVs in the entorhinal cortex, inferior temporal gyrus, amygdala, thalamus, hippocampus, and cingulate gyrus, and higher WMLV than those without. The VBM analysis also showed that impaired olfactory identification was significantly associated with lower volume of the left entorhinal cortex, left amygdala, left hippocampus, bilateral thalami, and bilateral subcallosal areas.

Conclusion: Our findings suggest that cognitive function and brain changes may need to be evaluated in older individuals with impaired olfactory identification, which may reflect the extent of neurodegeneration, WMLV, and subsequent cognitive impairment.