Structural Abnormalities of Olfactory-Related Brain Regions in Mild Cognitive Impairment and Subjective Cognitive Decline Individuals.

BackgroundOlfactory impairment might be already present at the subjective cognitive impairment (SCD) individuals, and deepens with disease severity in the Alzheimer's disease (AD) spectrum. However, the neuroanatomical correlates of olfactory impairment in SCD individuals are not fully elucidated.MethodsA hundred and twenty enrolled older adults without dementia (25 healthy controls (HCs), 45 SCD individuals and 50 mild cognitive impairment (MCI) individuals) completed olfactory assessment and structural magnetic resonance imaging (MRI) scanning. Olfactory function was evaluated by the 16-item Sniffin' Sticks odor identification test (SSIT). Region of interest (ROI) analysis was conducted for the gray matter volume (GMV) of 8 olfactory-related brain regions.ResultsIn ROI analysis, from HC, SCD to MCI group, smaller GMV of olfactory-related regions and olfactory impairment became increasingly severe. For HC group, olfactory impairment was only associated with smaller entorhinal cortex (P < 0.05). In SCD individuals, reduced GMVs of entorhinal cortex and hippocampus were associated with olfactory impairment (P < 0.05). In MCI individuals, decreased GMVs of piriform cortex, amygdala, entorhinal cortex, orbitofrontal cortex, hippocampus and parahippocampus were significantly associated with olfactory impairment (P < 0.05).ConclusionsThe atrophy of olfactory-related brain regions gradually increased and the corresponding olfactory function gradually decreased in older adults of HC, SCD and MCI. The olfactory regions associated with olfactory impairment in SCD individuals were mainly in entorhinal cortex and hippocampus.