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. 2021 May;11(5):e02115.
doi: 10.1002/brb3.2115. Epub 2021 Mar 26.

Entorhinal cortex and parahippocampus volume reductions impact olfactory decline in aged subjects

Natsuko Iizuka  1   2 , Yuri Masaoka  1 , Satomi Kubota  2 , Haruko Sugiyama  3 , Masaki Yoshida  4 , Akira Yoshikawa  1 , Nobuyoshi Koiwa  5 , Motoyasu Honma  1 , Keiko Watanabe  2 , Shotaro Kamijo  1 , Sawa Kamimura  1 , Masahiro Ida  6 , Kenjiro Ono  2 , Masahiko Izumizaki  1
Affiliations

Entorhinal cortex and parahippocampus volume reductions impact olfactory decline in aged subjects

Natsuko Iizuka et al. Brain Behav. 2021 May.

Abstract

Introduction: Pathological abnormalities first appear in the medial temporal regions including entorhinal cortex and parahippocampus in patients with Alzheimer's disease. Previous studies showed that olfactory decline in elderly subjects was associated with volume reductions in the left hippocampus and left parahippocampus without cognitive impairment. The aim of this study is to investigate the link between olfaction and volume reductions in the medial temporal regions including the parahippocampus, entorhinal cortex, and hippocampal subfields.

Method: 27 elderly subjects and 27 young controls were measured olfaction acuity, cognitive function, and structural magnetic resonance imaging. Image processing and gray matter volumetric segmentation were performed with FreeSurfer. Volume data were analyzed with SPSS Statistics software.

Results: Interesting results of this study were that volume reduction in the entorhinal cortex was not directly linked with declining olfactory ability. Volume reduction in the left entorhinal cortex was correlated with volume reduction in the left parahippocampus and dentate gyrus. However, left parahippocampus volume reduction had the greatest impact on olfactory decline, and the entorhinal cortex and dentate gyrus might additionally contribute to olfactory decline.

Conclusion: Our results indicate that olfactory decline may be directly reflected in the medial temporal regions as reduced parahippocampus volumes, rather than as morphological changes in the entorhinal cortex and hippocampus. The parahippocampus may play an important role in the association between memory retrieval and olfactory identification.

Keywords: brain volume; entorhinal cortex; memory; olfactory decline; parahippocampus.

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Conflict of interest statement

The authors declare that this study was funded by Kao Corporation. HS was employed by Kao Corporation and was involved for MRI measurements and analysis of olfaction and MoCA data.

Figures

FIGURE 1
FIGURE 1
Left: Visualization of brain segmentation in a single subject with FreeSurfer. The entorhinal cortex (ENT) is indicated in red, and the parahippocampus (para‐HI) is indicated in green. Right: Visualization of hippocampal subfield segmentation in a single subject with each subfield represented by a different color. DG, dentate gyrus, GC‐DG, granule cell layer of the dentate gyrus, HATA, HI‐AMG transition area, GC‐ML‐DG, granule cells in the molecular layer of the dentate gyrus
FIGURE 2
FIGURE 2
Path diagram and standardized path coefficients of elderly subjects. The solid lines indicate significant standardized direct effects. The dotted lines indicate significant standardized indirect effects. *p < .05, **p < .01, ***p < .0001 Statistical details were indicated in Table 3. DG, dentate gyrus, ENT, entorhinal cortex, para‐HI, parahippocampus
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