Olfactory Bulb Volume Reflects Olfactory Dysfunction and Network Organization: Insights From the Population-Based...

Olfactory dysfunction is common in aging and an early symptom of neurodegenerative diseases, but how structural (olfactory bulb [OB] volume) and functional (olfactory network [OFN] functional connectivity [FC]) brain features interact to shape odor identification ability remains unclear. Therefore, we assessed the interrelations among OB volume, OFN FC, and odor identification ability in a large population-based cohort.

Using cross-sectional data from 5605 participants of the Rhineland Study (age range: 30-95 years), we extracted OB volume and OFN FC from 3T MRI scans. Odor identification was examined with the 12-item "Sniffin' Sticks" test. Using linear regression, we examined the relations between OB volume, OFN FC, and odor identification.

A smaller OB was associated with worse odor identification (standardized β = 0.09, 95% confidence interval: 0.06-0.12). This association was stronger in men and strongest in older individuals of both sexes. Only in participants with a large OB, lower OFN FC was significantly associated with worse odor identification (standardized β between 0.03 and 0.12), especially among older participants (62-95 years) and in memory-related regions (hippocampus, amygdala, and orbitofrontal cortex).

Our findings demonstrate the importance of OB volume in detecting olfactory dysfunction. Moreover, they reveal that the OB contributes to odor identification both directly and by modulating central network function, offering new insights into olfactory dysfunction as a potential biomarker for neurodegeneration.