OBJECTIVES/HYPOTHESIS: Olfactory dysfunction is thought to be associated with reduced gray matter (GM) volume in olfactory-related brain areas. The aim of this study was to determine GM structural changes within olfactory-related regions of the brain in patients with smell loss due to upper respiratory tract infection (URTI) before and after olfactory rehabilitation.

STUDY DESIGN: Prospective intervention case-control study.

METHODS: Magnetic resonance imaging structural brain images were collected from 30 patients with smell loss due to URTI and 31 controls. Patients exposed themselves to odors (olfactory training [OT]) over 12 weeks and then were rescanned. Olfactory testing was performed using the validated Sniffin' Sticks test. GM was investigated with voxel-based morphometry.

RESULTS: GM volumes were found to be reduced in the limbic system and thalamus among pretraining patients compared to controls; in patients, OT was associated with a significant increase of GM volume in these two regions. The GM volume within other olfactory-related regions was not different between patients and controls. In addition, no relevant difference between the GM volume pre- and post-OT was observed in primary olfactory-related regions.

CONCLUSIONS: OT was associated with an increase in GM volume of the hippocampus and the thalamus, possibly pointing toward a strategy for more effective exploitation of olfactory signals based on a higher degree of attention toward odors and association of memories with olfactory input.

LEVEL OF EVIDENCE: 3b. Laryngoscope, 128:1531-1536, 2018.