Size matters - The olfactory bulb as a marker for depression

Background: Major Depression is mainly related to structural and functional alterations in brain networks involving limbic and prefrontal regions. Reduced olfactory sensitivity in depression is associated with reduced olfactory bulb (OB) volume. We determined if the OB volume reduction is a specific biomarker for depression and whether its diagnostic accuracy allows its use as a valid biomarker to support its diagnosis. Methods: 84 in-patients with mixed mental disorders and 51 age-matched healthy controls underwent structural MR imaging with a spin- echo T2- wheighted sequence. Individual OB volume was calculated manually (interrater- reliability =. 81, p < .001) and compared between groups. Multiple regression analysis with OB volume as dependent variable and Receiver Operator Characteristic analysis to obtain its diagnostic accuracy for depression were ruled out. Results: Patients exhibited a 13.5% reduced OB volume. Multiple regression analysis showed that the OB volume variation was best explained by depression (beta = -.19), sex (beta = -.31) and age (beta = -.29), but not by any other mental disorder. OB volume attained a diagnostic accuracy of 68.1% for depression. Limitations: The patient group mainly contained highly comorbid patients with mostly internalizing disorders which limits the generalisability of the results of the regression analysis. Conclusion: The OB may serve as a marker for depression. We assume that reduced neural olfactory input to subsequent limbic and salience processing structures moderates this relation. However, the OB was in an inferior position compared to conventional questionnaires for diagnosis of depression. Combination with further structural or functional measurements is suggested.