The Shape of the Olfactory Bulb Predicts Olfactory Function

Xiaoguang Yan; Akshita Joshi; Yunpeng Zang; Francisca Assunção; Henrique M Fernandes; Thomas Hummel

doi:10.3390/brainsci12020128

The Shape of the Olfactory Bulb Predicts Olfactory Function

Brain Sci. 2022 Jan 18;12(2):128. doi: 10.3390/brainsci12020128.

Authors

Xiaoguang Yan¹, Akshita Joshi¹, Yunpeng Zang¹, Francisca Assunção², Henrique M Fernandes^{2

3

4}, Thomas Hummel¹

Affiliations

¹ Smell and Taste Clinic, Department of Otorhinolaryngology, TU Dresden, Fetscherstrasse 74, 01307 Dresden, Germany.
² Flavour Institute, Department of Clinical Medicine, Aarhus University, Noerrebrogade 44, 1A, 8000 Aarhus, Denmark.
³ Center for Music in the Brain, Department of Clinical Medicine, Aarhus University, Noerrebrogade 44, 1A, 8000 Aarhus, Denmark.
⁴ Hedonia Research Group, Department of Psychiatry, University of Oxford, Oxford OX1 2JD, UK.

Abstract

The olfactory bulb (OB) plays a key role in the processing of olfactory information. A large body of research has shown that OB volumes correlate with olfactory function, which provides diagnostic and prognostic information in olfactory dysfunction. Still, the potential value of the OB shape remains unclear. Based on our clinical experience we hypothesized that the shape of the OB predicts olfactory function, and that it is linked to olfactory loss, age, and gender. The aim of this study was to produce a classification of OB shape in the human brain, scalable to clinical and research applications. Results from patients with the five most frequent causes of olfactory dysfunction (n = 192) as well as age/gender-matched healthy controls (n = 77) were included. Olfactory function was examined in great detail using the extended "Sniffin' Sticks" test. A high-resolution structural T2-weighted MRI scan was obtained for all. The planimetric contours (surface in mm²) of OB were delineated manually, and then all surfaces were added and multiplied to obtain the OB volume in mm³. OB shapes were outlined manually and characterized on a selected slice through the posterior coronal plane tangential to the eyeballs. We looked at OB shapes in terms of convexity and defined two patterns/seven categories based on OB contours: convex (olive, circle, and plano-convex) and non-convex (banana, irregular, plane, and scattered). Categorization of OB shapes is possible with a substantial inter-rater agreement (Cohen's Kappa = 0.73). Our results suggested that non-convex OB patterns were significantly more often observed in patients than in controls. OB shapes were correlated with olfactory function in the whole group, independent of age, gender, and OB volume. OB shapes seemed to change with age in healthy subjects. Importantly, the results indicated that OB shapes were associated with certain causes of olfactory disorders, i.e., an irregular OB shape was significantly more often observed in post-traumatic olfactory loss. Our study provides evidence that the shape of the OB can be used as a biomarker for olfactory dysfunction.

Keywords: MRI; anosmia; deformation; olfaction; olfactory bulb shape; plasticity; smell.

Grants and funding

n/TU Dresden