Association of olfactory training with olfactory bulb morphology in adults with post-viral long-lasting olfactory dysfunction: A COVID-19 related prospective study

Objective: In this prospective case-controlled study, we aimed to examine changes in olfactory bulb (OB) morphology due to COVID-19 infection and to examine the effects of olfactory training (OT). Methods: This study included 29 patients with anosmia or hyposmia due to COVID-19 infection (Group 1), 24 normosmic patients after COVID-19 infection (Group 2), and 25 normosmic subjects without COVID-19 infection (Group 3). OB volumes, thickness, length, sulcus depth, and signal intensity were evaluated using magnetic resonance imaging (MRI). Psychophysical tests (odor discrimination, thresholds, and odor identification) and a survey of olfactory symptoms were performed. After 12 weeks of OT, the MRI data and odor scores of Group 1 were compared with those of the other groups. Results: OB volumes were significantly smaller in Group 1 (54.01+2.92 mm3) compared with the other groups (group 2:56.7+3.2 mm3 and group 3:59.45+3.09 mm3). The OB thicknesses and lengths differed significantly between Group 1 and Groups 2 and 3. Group 1 had abnormalities in the OB signal intensity in the form of diffusely increased signal intensity compared with the others groups. Following OT, OB volume (right, p= 0.002; left, p=0.021) and Threshold Discrimination Identification score (p< 0.001) significantly increased in Group 1 patients. Conclusion: These findings suggest that post-COVID olfactory loss is associated with smaller OB volumes and increased OB signal intensity. Additionally, this study provides evidence supporting the effectiveness of OT in improving olfactory function and OB volume in patients with long-lasting post-COVID-19 symptoms.