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.
Highlights
The olfactory bulb (OB) plays an important role in the processing of olfactory information
Classification of the olfactory dysfunction was based on the underlying etiological conditions, i.e., olfactory dysfunction secondary to chronic rhinosinusitis (CRS), post-infectious olfactory dysfunction (PIOD), posttraumatic olfactory dysfunction (PTOD), olfactory dysfunction associated with neurological disease, and idiopathic olfactory dysfunction
We found that an irregular OB shape was significantly more often observed in post-traumatic olfactory loss (n = 45, 12/45 = 26.7%) than in other etiologies of olfactory loss
Summary
The olfactory bulb (OB) plays an important role in the processing of olfactory information. The OB is one of the earliest structures of the vertebrate brain to develop [1,2]. The OB has been suggested to serve as a repository of resident progenitor cells in the mature human brain, receiving neuroblasts migrating to it via a lateral ventricular extension [3,4]. Results based on longitudinal studies showed how changes in OB volumes associate with improved olfactory function secondary to treatment [10]. The assessment of OB structure is considered to provide diagnostic and prognostic information on a morphological basis. All the previous studies focused on the size and volume of the OB. They neglected the relation between OB shape and olfactory function
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