Individual variability of olfactory fMRI in normosmia and olfactory dysfunction

Abstract

Purpose The diagnosis of olfactory dysfunction is mainly based on psychophysical measurements. The aim of the current study was to investigate how well the olfactory functional magnetic resonance imaging (fMRI) can effectively distinguish between normosmic people and subjects with olfactory dysfunction. Methods Thirty-eight participants were recruited for the study. Group 1 consisted of 22 subjects with olfactory dysfunction (mean age = 44.3 years, SD = 18.6), and Group two consisted of 16 participants with normal olfactory function (mean age = 49.6 years, SD = 11.6). Olfactory functions were assessed in great detail for all participants, and brain activation in response to odorous stimulation was assessed using fMRI. Results The between-group comparison showed stronger odor induced brain activation of the primary olfactory area and the insular cortex among the normosmic group as compared to the dysosmic group. As indicated by the individual analysis, positive responses in the primary olfactory cortex were significantly higher in normosmic people (94%) than in subjects with olfactory dysfunction (41%). However, there was no association between individual fMRI parameters (including the percentage of BOLD signal change, activated cluster size and peak z value), and psychophysical olfactory test scores. Receiver operating characteristic analysis suggested the subjects could not be differentiated from normosmics based on their BOLD signal from the primary olfactory area, orbitofrontal cortex, or the insular cortex. Conclusion There are large inter-individual variabilities for odor-induced brain activation among normosmic subjects and subjects with olfactory dysfunction, due to this variation, at present it appears problematic to diagnose olfactory dysfunction on an individual level using fMRI.

International consensus statement on allergy and rhinology: Olfaction

“…1202 Another study investigated patients with idiopathic olfactory loss, considered as at risk for developing PD, in comparison to patients with PD and normosmic controls. 1203 This study found that, on a group level, fractional anisotropy measured at the level of the substantia nigra was decreased in idiopathic patients and patients with PD in comparison to controls. This finding suggests a reduced integrity of the substantia nigra in patients with idiopathic smell loss, supporting their PD at-risk status.…”

“…One study investigated patients with congenital anosmia and found that these patients have network dysfunction but intact structural integrity 1202 . Another study investigated patients with idiopathic olfactory loss, considered as at risk for developing PD, in comparison to patients with PD and normosmic controls 1203 . This study found that, on a group level, fractional anisotropy measured at the level of the substantia nigra was decreased in idiopathic patients and patients with PD in comparison to controls.…”

“…One prospective cohort study evaluated brain metabolism using fluorodeoxyglucose‐positron emission tomography under olfactory stimulation 1203 . It showed that brain metabolism in certain brain regions is significantly different between patients with IOD and controls, 1204 with a correlation between disease duration and fluorodeoxyglucose uptake.…”

“…1201 Diffusion MRI has been investigated in two prospective cohort studies. 1202,1203 One study investigated patients with congenital anosmia and found that these patients have network dysfunction but intact structural integrity. 1202 Another study investigated patients with idiopathic olfactory loss, considered as at risk for developing PD, in comparison to patients with PD and normosmic controls.…”

Functional Imaging in Olfactory Disorders

“…This method offers the possibility to study the processing of olfactory information and constitutes an objective assessment of olfactory functioning. Although fMRI has significantly increased our knowledge of human olfactory processing, its clinical value on an individual level remains somewhat debatable [ 1 ••].…”