Magnetic Resonance Imaging of Human Olfactory Dysfunction

Analysis of Pleasure and Displeasure in Harmony Between Colored Light and Fragrance by the Left and Right OFC Response Differences

Daily actions are influenced by sensory information. Several studies have investigated the multisensory integration of multiple sensory modalities, known as crossmodal perception. Recently, visual-olfactory crossmodal perception has been studied using objective physiological measures rather than subjective evaluations. This study focused on sensing in the orbitofrontal cortex (OFC), which responds to visual and olfactory stimuli, and may serve as a physiological indicator of perception. Using near-infrared spectroscopy (NIRS), we analyzed the emotions evoked by combinations of colored light and fragrance with a particular focus on the lateralization of brain function. We selected pleasant and unpleasant fragrances from some essential oils, paired with colored lights that were perceived as either harmonious or disharmonious with the fragrances. NIRS measurements were conducted under the four following conditions: fragrance-only, colored light-only, harmonious crossmodal, and disharmonious crossmodal presentations. The results showed that the left OFC was activated during the crossmodal presentation of a harmonious color with a pleasant fragrance, thereby evoking pleasant emotions. In contrast, during the crossmodal presentation of a disharmonious color with an unpleasant fragrance, the right OFC was activated, suggesting increased displeasure. Additionally, the lateralization of brain function between the left and right OFC may be influenced by 'pleasure-displeasure ' and 'crossmodal perception-multimodal perception'.

Null cross-modal effects of olfactory training on visual, auditory or olfactory working memory in 6- to 9-year-old children

Systematic exposure to odours (olfactory training, OT) is a method of smell loss treatment. Due to olfactory system projections to prefrontal brain areas, OT has been hypothesized to enhance cognitive functions, but its effects have been studied predominantly in adults. This study tested OT effects on working memory (WM), i.e., the ability to store and manipulate information for a short time, in healthy children aged 6-9 years. We expected OT to improve olfactory WM and establish cross-modal transfer to visual and auditory WM. Participants performed 12 weeks of bi-daily OT with either 4 odours (lemon, eucalyptus, rose, cloves; OT group) or odourless propylene glycol (placebo group). Pre- and post-training, participants' WM was measured utilizing odours (olfactory WM) or pictures (visual WM) and a word-span task (auditory WM). 84 children (40 girls) completed the study. The analyses revealed no changes in the WM performance following OT. The olfactory WM task was the most difficult for children, highlighting the need to include olfactory-related tasks in educational programmes to improve children's odour knowledge and memory, just as they learn about sounds and pictures. Further neuroimaging research is needed to fully understand the impact of OT on cognitive functions in children.

Altered internetwork functional connectivity and graph analysis of occipital regions in patients with chronic rhinosinusitis accompanied by olfactory dysfunction

This study assessed whole-brain functional connectivity and network graph theory indices in patients with chronic rhinosinusitis with (CRSwOD) and without (CRSsOD) olfactory dysfunction. We also analyzed correlations between the abnormal network metrics and clinical indices. We acquired resting-state functional magnetic resonance images from 31 patients with CRSsOD, 26 with CRSwOD, and 25 healthy controls (HCs). Functional connectivity was computed and graph theory metrics were evaluated based on the Dosenbach-160 Atlas; relationships between neuroimaging indicators and clinical scales were assessed using Pearson correlation analysis. The results showed that CRSsOD patients had 11 edges with greater strength than HCs, CRSwOD patients had 1 greater edge than HCs, and CRSsOD patients had 5 greater edges than CRSwOD patients. Nodal degree centrality and efficiency in the right posterior occipital region were significantly altered in patients with CRSsOD compared with those in CRSwOD and in HCs. Five and two edges correlated with clinical scales in patients with CRSsOD and CRSwOD, respectively, whereas no correlations in global and nodal indicators were found. These results imply that distinct brain network patterns, particularly in the occipital cortex, could be a valid neuroimaging marker for related diagnosis and prognosis of CRSsOD and CRSwOD patients, and contribute to our better understanding of the central neural mechanisms of CRSwOD, providing new ideas for the clinical management of CRSwOD.

Which is superior, the University of Pennsylvania Smell Identification Test or the threshold, Discrimination and Identification Test for testing human olfaction? A systematic review

BACKGROUND

The awareness and prevalence of olfactory dysfunction (OD) has increased significantly in recent years, with the SARS-CoV-2 pandemic being a major catalyzer. Consequently, demands for reliable OD tests have also risen.

AIMS

This systematic review compares two commonly used tests for olfactory assessment: the University of Pennsylvania Smell Identification Test (UPSIT) and the Threshold, Discrimination, and Identification test (TDI). The correlation between UPSIT, TDI, and the Visual Analogue Scale (VAS) for olfaction is also examined.

MATERIAL AND METHODS

A systematic search of PubMed and EMBASE identified articles validating UPSIT or TDI and comparing these to self-reported OD via VAS. The outcome of interest was test-retest validity and correlation.

RESULTS

The search identified 1536 studies, with nine meeting inclusion criteria. UPSIT showed a 'very strong' test-retest correlation, while TDI showed a 'strong' correlation. One study addressed VAS in relation to UPSIT and five for TDI. Correlation coefficients varied between VAS and TDI/UPSIT.

CONCLUSIONS AND SIGNIFICANS

UPSIT and TDI exhibited strong to very strong test-retest reliability. Due to the limited number of studies, a meta-analysis was not feasible, and neither test was found to be superior in reliability. VAS did not correlate significantly with either TDI or UPSIT.

Long COVID-19-related and non-COVID-19 postviral olfactory dysfunction a comparative MRI study focusing on the olfactory cleft and bulbs

Objective

To compare the magnetic resonance imaging (MRI) features of the olfactory cleft (OC) and olfactory bulbs (OBs) in patients with long COVID-19-related (LCOD) and non-COVID-19 postviral olfactory dysfunction (NCPVOD) to explore mechanisms underlying persistent olfactory dysfunction.

Methods

This retrospective analysis included patients diagnosed with LCOD or NCPVOD at the China-Japan Friendship Hospital between February 2023 and July 2024. All patients underwent olfactory psychophysical testing (Sniffin' Sticks), a visual analogue scale (VAS) for olfactory function, and high-resolution MRI scans of the olfactory pathway. MRI features, including OC opacity, OB morphology, OB volume, and olfactory sulcus depth, were compared between groups. Correlations between MRI findings and olfactory test scores were assessed.

Results

Seventy patients were included (35 LCOD, 35 NCPVOD). LCOD patients had significantly higher OC opacity scores than NCPVOD patients (p < 0.001). No significant differences were found in OB morphology, abnormal OB signals, OB volume reduction, or distances between OBs and surrounding structures (p > 0.05). LCOD patients had significantly greater right olfactory sulcus depth than NCPVOD patients (p = 0.026), with negative correlation to age (r = -0.25, p = 0.04). OB volumes positively correlated with TDI and VAS scores.

Conclusion

LCOD patients exhibited greater OC opacity than NCPVOD patients, suggesting OC inflammation may contribute to persistent olfactory dysfunction. Treating inflammation in the OC could improve long-term olfactory outcomes. OB volume reduction was common in both groups.

Volumetric MRI correlates of persistent auditory verbal hallucinations and olfactory identification impairment in chronic schizophrenia: A cross-sectional study

BACKGROUND

Olfactory impairments are often observed in schizophrenia (SCZ) patients experiencing persistent auditory verbal hallucinations (pAVHs), yet it remains unclear whether these symptoms share a common neural mechanism with specific brain regions' gray matter volume (GMV) abnormalities. This study aimed to preliminarily elucidate olfactory impairment differences between SCZ patients with and without pAVHs and their correlation with GMV abnormalities in relevant brain regions.

METHODS

A total of 75 SCZ patients with pAVHs (pAVH group), 56 SCZ patients without AVHs (non-AVH group), and 83 healthy controls (HC group) were examined. Voxel-based morphometry is useful for comparing and analyzing the differences in GMV among three groups. The Odor Stick Identification Test for Japanese (OSIT-J) was harnessed to gauge olfactory abilities.

RESULTS

Olfactory impairments are notably significant across entire SCZ patients compared to HC, with no significant differences in olfactory performance among SCZ subgroups. Notably, the pAVH group demonstrated a significant GMV diminution in the frontal-temporal cortex, starkly contrasting with the non-pAVH and HC groups. Intriguingly, stepwise regression analysis confirmed a strong positive relation between OSIT-J scores and a GMV reduction in the right medial orbitofrontal cortex (mOFC), although this correlation was only observed in the overall SCZ patient group (P < 0.0036, Bonferroni correction).

CONCLUSIONS

The GMV perturbations within the mOFC, distinctive to SCZ, may underpin the neuroimaging substrates linked to heightened vulnerability to olfactory impairments in this population. This exploration underscores the imperative of delving into the neural underpinnings of sensory impairments within SCZ, propelling a nuanced understanding of its heterogeneity.

Olfactory dysfunction as potential biomarker in neurodegenerative diseases: a narrative review

Neurodegenerative diseases represent a group of disorders characterized by progressive degeneration of neurons in the central nervous system, leading to a range of cognitive, motor, and sensory impairments. In recent years, there has been growing interest in the association between neurodegenerative diseases and olfactory dysfunction (OD). Characterized by a decline in the ability to detect or identify odors, OD has been observed in various conditions, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and Amyotrophic Lateral Sclerosis (ALS). This phenomenon often precedes the onset of other clinical symptoms, suggesting its potential utility as an early marker or prodromal symptom of neurodegenerative diseases. This review provides a vast literature overview on the current knowledge of OD in PD, AD, ALS, and HD in order to evaluate its potential as a biomarker, particularly in the early and prodromal stages of these diseases. We summarize the most common methods used to measure olfactory function and delve into neuropathological correlations and the alterations in neurotransmitter systems associated with OD in those neurodegenerative diseases, including differences in genetic variants if applicable, and cater to current pitfalls and shortcomings in the research.

Normative Parameters of Olfactory Bulbs Based on Magnetic Resonance Imaging and Olfactory Function

BACKGROUND

Olfactory dysfunction is a frequently encountered sensory disorder that increases with aging, assessed magnetic resonance imaging (MRI); however, reference quantitative values for associated anatomical structures have rarely been suggested. The aim of this study was to assess the parameters of the olfactory bulbs (OBs) and olfactory sulcus (OS) in Korean adults according to age, along with their olfactory function.

METHODS

We retrospectively evaluated 217 consecutive patients (104 men, 113 women; mean age, 52.4 ± 15.6 years) who underwent sellar MRI and olfactory function testing before transsphenoidal approach at a single tertiary center from March 2022 to December 2023. Based on the T2-weighted MRI, we evaluated the quantitative size parameters and morphological features of patients' OB and OS, along with their olfactory function test scores. We assessed the relationship between OB volume and age in pairwise correlations.

RESULTS

The mean OB volume was 45.6 ± 15.3 mm³ in all patients. The patients' mean Korean version of the Sniffin' Sticks (KVSS) test II score was 26.8 ± 4.1. OB volume (P < 0.001), height (P < 0.001), and anteroposterior diameter (APD) (P < 0.001) differed significantly among the different age groups. Reduced OB volume was significantly associated with aging (r = -0.58, P < 0.001) and a decline in olfactory function scores (r = 0.34, P < 0.001).

CONCLUSION

Based on MRI, we proposed reference OB and OS values in adults of different age groups, highlighting the reduction in OB parameters, especially height and APD along with volume associated with aging and olfactory decline. These values can be useful for evaluating adult patients undergoing MRI for olfactory dysfunction.

Hippocampus and olfactory impairment in Parkinson disease: a comparative exploratory combined volumetric/functional MRI study

INTRODUCTION

Patients with Parkinson's Disease (PD) commonly experience Olfactory Dysfunction (OD). Our exploratory study examined hippocampal volumetric and resting-state functional magnetic resonance imaging (rs-fMRI) variations in a Healthy Control (HC) group versus a cognitively normal PD group, further categorized into PD with No/Mild Hyposmia (PD-N/MH) and PD with Severe Hyposmia (PD-SH).

METHODS

We calculated participants' relative Total Hippocampal Volume (rTHV) and performed Spearman's partial correlations, controlled for age and gender, to examine the correlation between rTHV and olfactory performance assessed by the Odor Stick Identification Test for the Japanese (OSIT-J) score. Mann-Whitney U tests assessed rTHV differences across groups and subgroups, rejecting the null hypothesis for p < 0.05. Furthermore, a seed-based rs-fMRI analysis compared hippocampal connectivity differences using a one-way ANCOVA covariate model with controls for age and gender.

RESULTS

Spearman's partial correlations indicated a moderate positive correlation between rTHV and OSIT-J in the whole study population (ρ = 0.406; p = 0.007), PD group (ρ = 0.493; p = 0.008), and PD-N/MH subgroup (ρ = 0.617; p = 0.025). Mann-Whitney U tests demonstrated lower rTHV in PD-SH subgroup compared to both HC group (p = 0.013) and PD-N/MH subgroup (p = 0.029). Seed-to-voxel rsfMRI analysis revealed reduced hippocampal connectivity in PD-SH subjects compared to HC subjects with a single cluster of voxels.

CONCLUSIONS

Although the design of the study do not allow to make firm conclusions, it is reasonable to speculate that the progressive involvement of the hippocampus in PD patients is associated with the progression of OD.

A Multivariate Exploration of Resting-State Networks and Sensory Measures of Olfaction and Taste

In neurodegenerative conditions such as Alzheimer's and Parkinson's diseases, a common symptom is the impairment of senses, particularly olfaction and gustation. More comprehensive neuroimaging studies are needed to elucidate the links between these sensory functions and neural circuit alterations. In this study, we used a subset of the Human Connectome Project (HCP) S1200 data containing both the neuroimaging and sensory test data (n = 50). Using high-order Group Independent Component Analysis (ICA) in combination with Multivariate Analysis of Covariance (MANCOVA), we studied the interplay between odor and taste sensory measures and spatiotemporal features of the resting-state networks. The findings revealed that a decline in olfaction was associated with decreased connectivity within the posterior default mode network, central executive network, and dorsal attention network. Similarly, a lower capacity for taste detection was linked to changes in the connectivity within the dorsal attention network. Conversely, enhanced taste detection was correlated with increased connectivity in the thalamus. We also found that olfaction was associated with the connectivity between the olfactory network and a cognitive network, which includes the inferior frontal orbital gyrus and Rolandic operculum. Taste, on the other hand, was associated with functional connectivity between the central executive network and the cerebellar network. Our study brings new insights into the relationship between olfaction, taste, and various resting-state networks.

Machine Learning Exploration of Brain Morphological Features and Sensory Measures

Previous investigations have implicated the neuroanatomical basis of sensory systems; however, definitive neuroimaging biomarkers remain elusive. The present study employs machine learning techniques to probe the relationship between brain morphological features and sensory measures of audition, olfaction, taste, and visual contrast sensitivity using a large dataset from the publicly available Human Connectome Project (n = 874). Applying both 5-fold cross-validation and leave-one-out cross-validation methods, performance of several machine learning models was evaluated. Feature selection methods including Random Forest, SelectKBest, Recursive Feature Elimination, and SHapley Additive exPlanations (SHAP) were utilized to identify the most significant neuroanatomical features for sensory performance. Binary classification via machine learning was also conducted to distinguish individuals with high vs. low sensory test scores based on brain morphological features, achieving a satisfactory accuracy of 67% and 71% for olfaction and visual contrast sensitivity, respectively. By integrating machine learning with high-dimensional neuroimaging data, this preliminary study offers new insights into the neural correlates of sensory performance.

Nature and human well-being: The olfactory pathway

The world is undergoing massive atmospheric and ecological change, driving unprecedented challenges to human well-being. Olfaction is a key sensory system through which these impacts occur. The sense of smell influences quality of and satisfaction with life, emotion, emotion regulation, cognitive function, social interactions, dietary choices, stress, and depressive symptoms. Exposures via the olfactory pathway can also lead to (anti-)inflammatory outcomes. Increased understanding is needed regarding the ways in which odorants generated by nature (i.e., natural olfactory environments) affect human well-being. With perspectives from a range of health, social, and natural sciences, we provide an overview of this unique sensory system, four consensus statements regarding olfaction and the environment, and a conceptual framework that integrates the olfactory pathway into an understanding of the effects of natural environments on human well-being. We then discuss how this framework can contribute to better accounting of the impacts of policy and land-use decision-making on natural olfactory environments and, in turn, on planetary health.

Changes in Structural Covariance among Olfactory-related Brain Regions in Anosmia Patients

Anosmia, characterized by the loss of smell, is associated not only with dysfunction in the peripheral olfactory system but also with changes in several brain regions involved in olfactory processing. Specifically, the orbitofrontal cortex is recognized for its pivotal role in integrating olfactory information, engaging in bidirectional communication with the primary olfactory regions, including the olfactory cortex, amygdala, and entorhinal cortex. However, little is known about alterations in structural connections among these brain regions in patients with anosmia. In this study, high-resolution T1-weighted images were obtained from participants. Utilizing the volumes of key brain regions implicated in olfactory function, we employed a structural covariance approach to investigate brain reorganization patterns in patients with anosmia (n=22) compared to healthy individuals (n=30). Our structural covariance analysis demonstrated diminished connectivity between the amygdala and entorhinal cortex, components of the primary olfactory network, in patients with anosmia compared to healthy individuals (z=-2.22, FDR-corrected p=0.039). Conversely, connectivity between the orbitofrontal cortex-a major region in the extended olfactory network-and amygdala was found to be enhanced in the anosmia group compared to healthy individuals (z=2.32, FDR-corrected p=0.039). However, the structural connections between the orbitofrontal cortex and entorhinal cortex did not differ significantly between the groups (z=0.04, FDR-corrected p=0.968). These findings suggest a potential structural reorganization, particularly of higher-order cortical regions, possibly as a compensatory effort to interpret the limited olfactory information available in individuals with olfactory loss.

Olfactory deficit: a potential functional marker across the Alzheimer's disease continuum

Alzheimer's disease (AD) is a prevalent form of dementia that affects an estimated 32 million individuals globally. Identifying early indicators is vital for screening at-risk populations and implementing timely interventions. At present, there is an urgent need for early and sensitive biomarkers to screen individuals at risk of AD. Among all sensory biomarkers, olfaction is currently one of the most promising indicators for AD. Olfactory dysfunction signifies a decline in the ability to detect, identify, or remember odors. Within the spectrum of AD, impairment in olfactory identification precedes detectable cognitive impairments, including mild cognitive impairment (MCI) and even the stage of subjective cognitive decline (SCD), by several years. Olfactory impairment is closely linked to the clinical symptoms and neuropathological biomarkers of AD, accompanied by significant structural and functional abnormalities in the brain. Olfactory behavior examination can subjectively evaluate the abilities of olfactory identification, threshold, and discrimination. Olfactory functional magnetic resonance imaging (fMRI) can provide a relatively objective assessment of olfactory capabilities, with the potential to become a promising tool for exploring the neural mechanisms of olfactory damage in AD. Here, we provide a timely review of recent literature on the characteristics, neuropathology, and examination of olfactory dysfunction in the AD continuum. We focus on the early changes in olfactory indicators detected by behavioral and fMRI assessments and discuss the potential of these techniques in MCI and preclinical AD. Despite the challenges and limitations of existing research, olfactory dysfunction has demonstrated its value in assessing neurodegenerative diseases and may serve as an early indicator of AD in the future.

fNIRS a novel neuroimaging tool to investigate olfaction, olfactory imagery, and crossmodal interactions: a systematic review

Olfaction is understudied in neuroimaging research compared to other senses, but there is growing evidence of its therapeutic benefits on mood and well-being. Olfactory imagery can provide similar health benefits as olfactory interventions. Harnessing crossmodal visual-olfactory interactions can facilitate olfactory imagery. Understanding and employing these cross-modal interactions between visual and olfactory stimuli could aid in the research and applications of olfaction and olfactory imagery interventions for health and wellbeing. This review examines current knowledge, debates, and research on olfaction, olfactive imagery, and crossmodal visual-olfactory integration. A total of 56 papers, identified using the PRISMA method, were evaluated to identify key brain regions, research themes and methods used to determine the suitability of fNIRS as a tool for studying these topics. The review identified fNIRS-compatible protocols and brain regions within the fNIRS recording depth of approximately 1.5 cm associated with olfactory imagery and crossmodal visual-olfactory integration. Commonly cited regions include the orbitofrontal cortex, inferior frontal gyrus and dorsolateral prefrontal cortex. The findings of this review indicate that fNIRS would be a suitable tool for research into these processes. Additionally, fNIRS suitability for use in naturalistic settings may lead to the development of new research approaches with greater ecological validity compared to existing neuroimaging techniques.

Trajectories of olfactory identification preceding incident mild cognitive impairment and dementia: a longitudinal study

BACKGROUND

The pattern of olfactory identification change in the early phases of dementing disorders is unclear. We aimed to assess olfactory identification trajectories preceding incident mild cognitive impairment (MCI) and dementia and explore the role of brain pathologies in these trajectories.

METHODS

Within the Rush Memory and Aging Project, 1318 dementia-free older adults were followed annually for up to 11 years. Olfactory identification was assessed using the Brief Smell Identification Test annually. Of 900 cognitively intact participants, incident MCI and dementia were diagnosed following standard criteria. Over follow-up, 518 participants died and underwent brain autopsies for neuropathological assessment. Data were analyzed using mixed-effect models with backward timescales.

FINDINGS

Compared to participants who remained cognitively intact, olfactory identification declined faster among those who developed MCI (β -0.09 [95% CI -0.13, -0.05]), leading to a significantly lower olfactory identification starting from five years preceding MCI diagnosis (mean difference at year -5: -0.39 [-0.71, -0.07]). Among participants with incident MCI, olfactory identification declined faster in those who developed dementia compared to those who did not (β -0.19 [-0.36, -0.01]), leading to a significantly lower olfactory identification starting from three years preceding dementia diagnosis (mean difference at year -3: -0.95 [-1.67, -0.23]). A faster decline in olfactory identification was associated with higher burdens of global Alzheimer's disease pathology, neurofibrillary tangles, and amyloid beta load.

INTERPRETATION

Olfactory identification declined faster preceding dementia disorders and Alzheimer's pathology may underlie these faster declines.

FUNDING

This study was funded by the National Institutes of Health (R01AG17917) and Swedish Research Council (2021-01647).

Association of Olfaction and Microstructural Integrity of Brain Tissue in Community-Dwelling Adults: Atherosclerosis Risk in Communities Neurocognitive Study

BACKGROUND AND OBJECTIVES

Research on olfaction and brain neuropathology may help understand brain regions associated with normal olfaction and dementia pathophysiology. To identify early regional brain structures affected in poor olfaction, we examined cross-sectional associations of microstructural integrity of the brain with olfaction in the Atherosclerosis Risk in Communities Neurocognitive Study.

METHODS

Participants were selected from a prospective cohort study of community-dwelling adults; selection criteria included the following: evidence of cognitive impairment, participation in a previous MRI study, and a random sample of cognitively normal participants. Microstructural integrity was measured by 2 diffusion tensor imaging (DTI) measures, fractional anisotropy (FA) and mean diffusivity (MD), and olfaction by a 12-item odor identification test at the same visit. Higher FA and MD values indicate better and worse microstructural integrity, respectively, and higher odor identification scores indicate better olfaction. We used brain region-specific linear regression models to examine associations between DTI measures and olfaction, adjusting for potential confounders.

RESULTS

Among 1,418 participants (mean age 76 ± 5 years, 41% male, 21% Black race, 59% with normal cognition), the mean olfaction score was 9 ± 2.3. Relevant to olfaction, higher MD in the medial temporal lobe (MTL) regions, namely the hippocampus (β -0.79 [95% CI -0.94 to -0.65] units lower olfaction score per 1 SD higher MD), amygdala, entorhinal area, and some white matter (WM) tracts connecting to these regions, was associated with olfaction. We also observed associations with MD and WM FA in multiple atlas regions that were not previously implicated in olfaction. The associations between MD and olfaction were particularly stronger in the MTL regions among individuals with mild cognitive impairment (MCI) compared with those with normal cognition (e.g., βhippocampus -0.75 [95% CI -1.02 to -0.49] and -0.44 [95% CI -0.63 to -0.26] for MCI and normal cognition, respectively, p interaction = 0.004).

DISCUSSION

Neuronal microstructural integrity in multiple brain regions, particularly the MTL (the regions known to be affected in early Alzheimer disease), is associated with odor identification ability. Differential associations in the MTL regions among cognitively normal individuals compared with those with MCI may reflect the earlier vs later effects of the dementia pathogenesis. It is likely that some of the associated regions may not have any functional relevance to olfaction.

Olfactory training effects in children after mild traumatic brain injury

OBJECTIVE

Mild traumatic brain injury (mTBI) might impair the sense of smell and cognitive functioning. Repeated, systematic exposure to odors, i.e., olfactory training (OT) has been proposed for treatment of olfactory dysfunctions, including post-traumatic smell loss. Additionally, OT has been shown to mitigate cognitive deterioration in older population and enhance selected cognitive functions in adults. We aimed to investigate olfactory and cognitive effects of OT in the pediatric population after mTBI, likely to exhibit cognitive and olfactory deficits.

METHODS

Our study comprised 159 children after mTBI and healthy controls aged 6-16 years (M = 9.68 ± 2.78 years, 107 males), who performed 6-months-long OT with a set of 4 either high- or low-concentrated odors. Before and after OT we assessed olfactory functions, fluid intelligence, and executive functions.

RESULTS

OT with low-concentrated odors increased olfactory sensitivity in children after mTBI. Regardless of health status, children who underwent OT with low-concentrated odors had higher fluid intelligence scores at post-training measurement, whereas scores of children performing OT with high-concentrated odors did not change.

CONCLUSION

Our study suggests that OT with low-concentrated odors might accelerate rehabilitation of olfactory sensitivity in children after mTBI and support cognitive functions in the area of fluid intelligence regardless of head trauma.

Brain structural analysis in patients with post-traumatic anosmia: Voxel-based and surface-based morphometry

PURPOSE AND BACKGROUND

Voxel-based morphometry (VBM) and surfaced-based morphometry (SBM) investigate the characteristics of gray matter (GM) in various diseases such as post-traumatic anosmia (PTA). This study uses SBM and VBM to examine neuroanatomical measurements of GM and its functional correlates in patients with PTA.

METHODS

MRI images and olfactory test results were collected from 39 PTA patients and 39 healthy controls. Sniffin' Sticks test was used to assess olfactory function. GM structure was analyzed using CAT12 and FreeSurfer, and olfactory bulb (OB) volume and olfactory sulcus (OS) depth were calculated using 3D-Slicer.

RESULTS

Anosmic patients showed lower scores in the Sniffin' Sticks olfactory test, as well as reduction of OB volume and OS depth compared to control subjects. In these patients, overlapping changes were found between the VBM and SBM findings in the areas with significant effects, in particular, orbitofrontal cortex, superior and middle frontal gyrus, superior and middle temporal gyrus, anterior cingulate cortex, and insular cortex. Using SBM, decreased cortical thickness clusters were located in inferior and superior parietal gyrus. Further analysis in the region of interest demonstrated correlations between the orbitofrontal cortex and odor threshold score as well as the middle frontal gyrus and smell loss duration.

CONCLUSION

These findings show that the morphological alterations in the OB, OS, and the central olfactory pathways might contribute to the pathogenic mechanism of olfactory dysfunction after head injury.

Altered structural connectivity in olfactory disfunction after mild COVID-19 using probabilistic tractography

We aimed to investigate changes in olfactory bulb volume and brain network in the white matter (WM) in patients with persistent olfactory disfunction (OD) following COVID-19. A cross-sectional study evaluated 38 participants with OD after mild COVID-19 and 24 controls, including Sniffin' Sticks identification test (SS-16), MoCA, and brain magnetic resonance imaging. Network-Based Statistics (NBS) and graph theoretical analysis were used to explore the WM. The COVID-19 group had reduced olfactory bulb volume compared to controls. In NBS, COVID-19 patients showed increased structural connectivity in a subnetwork comprising parietal brain regions. Regarding global network topological properties, patients exhibited lower global and local efficiency and higher assortativity than controls. Concerning local network topological properties, patients had reduced local efficiency (left lateral orbital gyrus and pallidum), increased clustering (left lateral orbital gyrus), increased nodal strength (right anterior orbital gyrus), and reduced nodal strength (left amygdala). SS-16 test score was negatively correlated with clustering of whole-brain WM in the COVID-19 group. Thus, patients with OD after COVID-19 had relevant WM network dysfunction with increased connectivity in the parietal sensory cortex. Reduced integration and increased segregation are observed within olfactory-related brain areas might be due to compensatory plasticity mechanisms devoted to recovering olfactory function.

Regional homogeneity alterations of resting-state functional magnetic resonance imaging of chronic rhinosinusitis with olfactory dysfunction

Objectives

The aim of this study was to assess the brain functional changes of patients with chronic rhinosinusitis with olfactory dysfunction (CRSwOD) using regional homogeneity (ReHo) of resting-state functional magnetic resonance imaging (MRI) scans, and to better explain the occurrence and development of olfactory decline in patients with chronic sinusitis provides a new idea for the study of more advanced olfactory therapy modalities.

Methods

A total of 28 CRSwOD patients, 24 patients with CRS without olfactory dysfunction (CRSsOD), and 25 healthy controls (HCs) were recruited. All subjects underwent olfactory testing, clinical and brief psychological assessments, and MRI scans. A two-sided two-sample t test with AlphaSim correction (voxel-p < 0.001, cluster size >54 voxels) was used to detect differences between CRSwOD, CRSsOD, and HC groups.

Results

Compared with HCs, the ReHo values in traditional olfactory regions (e.g., parahippocampal gyrus (PHG), hippocampal, olfactory cortex) were increased, and ReHo values in the frontal gyrus, middle temporal gyrus, precuneus, and posterior cingulate gyrus were decreased in CRSwOD patients. The ReHo values in the precuneus and posterior cingulate gyrus of CRSwOD patients were negatively correlated with Questionnaire of Olfactory Disorders-Negative Statements (QOD-NS) scores. Compared with CRSsOD patients, the ReHo values in cerebellar regions were increased and those in the inferior temporal gyrus, precuneus, postcentral, and paracentral gyrus were decreased in CRSwOD patients. The receiver operating characteristic (ROC) curve showed that the mean ReHo values significantly differed between the CRSwOD and CRSsOD groups.

Conclusion

Synchronization of regional brain activity in the regions of the secondary olfactory cortex orbitofrontal cortex (OFC), temporal gyrus, precuneus, and cerebellum may be closely related to the development of olfactory dysfunction. Precuneus and posterior cingulate gyrus may be critical brain areas of action for emotional dysfunction in CRSwOD patients.

Effect of Post-COVID-19 on Brain Volume and Glucose Metabolism: Influence of Time Since Infection and Fatigue Status

Post-COVID-19 syndrome (PCS) fatigue is typically most severe <6 months post-infection. Combining magnetic resonance imaging (MRI) and positron emission tomography (PET) imaging with the glucose analog [¹⁸F]-Fluorodeoxyglucose (FDG) provides a comprehensive overview of the effects of PCS on regional brain volumes and metabolism, respectively. The primary purpose of this exploratory study was to investigate differences in MRI/PET outcomes between people < 6 months (N = 18, 11 female) and > 6 months (N = 15, 6 female) after COVID-19. The secondary purpose was to assess if any differences in MRI/PET outcomes were associated with fatigue symptoms. Subjects > 6 months showed smaller volumes in the putamen, pallidum, and thalamus compared to subjects < 6 months. In subjects > 6 months, fatigued subjects had smaller volumes in frontal areas compared to non-fatigued subjects. Moreover, worse fatigue was associated with smaller volumes in several frontal areas in subjects > 6 months. The results revealed no brain metabolism differences between subjects > 6 and < 6 months. However, both groups exhibited both regional hypo- and hypermetabolism compared to a normative database. These results suggest that PCS may alter regional brain volumes but not metabolism in people > 6 months, particularly those experiencing fatigue symptoms.

Liu D, A. Müller C, A. Stuck B, Welge-Lüssen A, Hähner A. Olfactory Dysfunction: Etiology, Diagnosis, and Treatment

BACKGROUND

Disorders of the sense of smell have received greater attention because of the frequency with which they occur as a symptom of SARS-CoV-2 infection. Olfactory dysfunction can lead to profound reduction in quality of life and may arise from many different causes.

METHODS

A selective literature review was conducted with consideration of the current version of the guideline issued by the Association of the Scientific Medical Societies in Germany.

RESULTS

The cornerstones of diagnosis are the relevant medical history and psychophysical testing of olfactory function using standardized validated tests. Modern treatment strategies are oriented on the cause of the dysfunction. While treatment of the underlying inflammation takes precedence in patients with sinunasal dysosmia, olfactory training is the primary treatment option for other forms of the disorder. The prognosis is determined not only by the cause of the olfactory dysfunction and the patient's age, but also by the olfactory performance as measured at the time of diagnosis.

CONCLUSION

Options for the treatment of olfactory dysfunction are available but limited, depending on the cause. It is therefore important to carry out a detailed diagnostic work-up and keep the patient informed of the expected course and prognosis.

Two-year follow-up of brain structural changes in patients who recovered from COVID-19: A prospective study

The long-term effects of COVID-19 on brain structure remain unclear. A prospective study was conducted to explore the changes in brain structure in COVID-19 survivors at one and two years after discharge (COVID-19_one, COVID-19_two). The difference in gray matter volume (GMV) was analyzed using the voxel-based morphometry method, and correlation analyses were conducted. The dynamic changes in clinical sequelae varied. The GMVs in the cerebellum and vermis were reduced in COVID-19_one and COVID-19_two, positively correlated with lymphocyte count, and negatively correlated with neutrophil count, neutrophil/lymphocyte ratio (COVID-19_one), and systemic immune-inflammation index (COVID-19_two). The decreased GMVs in the left middle frontal gyrus, inferior frontal gyrus of the operculum, right middle temporal gyrus, and inferior temporal gyrus returned to normal in COVID-19_two. The decreased GMV in the left frontal lobe was negatively correlated with the Athens Insomnia Scale (AIS). The GMV in the left temporal lobe was aggravated in COVID-19_two and positively correlated with C-reactive protein. In conclusion, GMV recovery coexisted with injury, which was associated with AIS and inflammatory factors. This may shed some light on the dynamic changes in brain structure and the possible predictors that may be related to GMV changes in COVID-19_two.

Investigations and Outcomes for Olfactory Disorders

Purpose of Review

To provide a detailed overview of the investigations and core outcome measures for olfactory disorders.

Recent Findings

Olfactory disorders can have a detrimental impact to the quality of life of patients. There are a wide range of causes of olfactory loss including sinonasal conditions, idiopathic, post-head trauma or infection. This review highlights the key investigations and reasoning for their use to clinically assess and research patients with olfactory disorders. In addition, this review outlines the core outcome measures for olfaction that will help inform future research in olfactory disorders.

Summary

A systematic approach with history taking and examination particularly with nasal endoscopy can determine the cause of the olfactory disorder in most cases. Specific olfactory disorder questionnaires can demonstrate the impact on quality of life, while psychophysical testing can objectively assess and monitor olfaction over time. Olfactory-evoked potentials and functional MRI are reserved for research, whereas CT and MRI imaging are used depending on history and examination. A core outcome set for olfaction has been developed that will help standardise the outcome measures used in olfaction and olfactory disorders research.

Larger gray matter volumes in neuropsychiatric long-COVID syndrome

Neuropsychiatric symptoms are the most common sequelae of long-COVID. As accumulating evidence suggests an impact of survived SARS-CoV-2-infection on brain physiology, it is necessary to further investigate brain structural changes in relation to course and neuropsychiatric symptom burden in long-COVID. To this end, the present study investigated 3T-MRI scans from long-COVID patients suffering from neuropsychiatric symptoms (n = 30), and healthy controls (n = 20). Whole-brain comparison of gray matter volume (GMV) was conducted by voxel-based morphometry. To determine whether changes in GMV are predicted by neuropsychiatric symptom burden and/or initial severity of symptoms of COVID-19 and time since onset of COVID-19 stepwise linear regression analysis was performed. Significantly enlarged GMV in long-COVID patients was present in several clusters (spanning fronto-temporal areas, insula, hippocampus, amygdala, basal ganglia, and thalamus in both hemispheres) when compared to controls. Time since onset of COVID-19 was a significant regressor in four of these clusters with an inverse relationship. No associations with clinical symptom burden were found. GMV alterations in limbic and secondary olfactory areas are present in long-COVID patients and might be dynamic over time. Larger samples and longitudinal data in long-COVID patients are required to further clarify the mediating mechanisms between COVID-19, GMV and neuropsychiatric symptoms.

The Effects of Brain Magnetic Resonance Imaging Indices in the Association of Olfactory Identification and Cognition in Chinese Older Adults

Background

Olfactory identification dysfunction frequently occurs in individuals with cognitive decline; however, a pathological mechanism linking the two has not been discovered. We aimed to study the association between olfactory identification and cognitive function, and determine the effects of brain regions atrophy therein.

Methods

A total of 645 individuals (57.5% were female) from the Taizhou Imaging Study, who underwent cognitive and olfactory identification measurements, were included. A subsample of participants underwent brain magnetic resonance imaging (n = 622). Cognition was assessed with a neuropsychological battery. Olfactory identification was measured using a 12-item Sniffin’ Sticks test. Beta and logistic regressions were used to elucidate the association between olfactory identification and cognition, and the effects of brain regions atrophy in this association.

Results

Dementia was diagnosed in 41 (6.4%) individuals (mean age = 64.8 years), and mild cognitive impairment (MCI) in 157 (24.3%) individuals (mean age = 64.4 years). Olfactory identification was associated with MMSE and MoCA (both P < 0.001) and specific cognitive domains (memory, executive function, visuospatial function, and language; all P < 0.05). Higher olfactory identification was associated with lower likelihood of MCI and dementia (P < 0.05). The amygdala volume was significantly related to olfactory identification, MMSE, MoCA, and language, and could attenuate the association between olfactory identification and cognitive function.

Conclusion

The association between olfactory identification and cognition can be partly attributable to differences in amygdala volume, suggesting that the amygdala could be a shared neural substrate that links olfactory identification and cognitive function. Limitations of this study include that all these results were based on a cross-sectional study.

Functional Connectivity of the Chemosenses: A Review

Functional connectivity approaches have long been used in cognitive neuroscience to establish pathways of communication between and among brain regions. However, the use of these analyses to better understand how the brain processes chemosensory information remains nascent. In this review, we conduct a literature search of all functional connectivity papers of olfaction, gustation, and chemesthesis, with 103 articles discovered in total. These publications largely use approaches of seed-based functional connectivity and psychophysiological interactions, as well as effective connectivity approaches such as Granger Causality, Dynamic Causal Modeling, and Structural Equation Modeling. Regardless of modality, studies largely focus on elucidating neural correlates of stimulus qualities such as identity, pleasantness, and intensity, with task-based paradigms most frequently implemented. We call for further "model free" or data-driven approaches in predictive modeling to craft brain-behavior relationships that are free from a priori hypotheses and not solely based on potentially irreproducible literature. Moreover, we note a relative dearth of resting-state literature, which could be used to better understand chemosensory networks with less influence from motion artifacts induced via gustatory or olfactory paradigms. Finally, we note a lack of genomics data, which could clarify individual and heritable differences in chemosensory perception.

[Application research and development of objective examination of olfactory function]

The sense of smell is one of the five most primitive human sensory functions, and it plays a very important role in our daily lives. Despite numerous methods for evaluating olfactory function, there is still a lack of standardization of olfactory tests and the results are often inconsistent. Furthermore, the related research on objective evaluation started relatively late. Along with the deciphering of the olfactory pathway, the technical level of olfactory objective inspection has been greatly improved and significant progress has also been made in terms of clinical application, such as: olfactory pathway MRI and fMRI imaging, OERPs, BEAM for various olfactory disorders and early diagnosis of neurodegenerative disorders, as well as related research based on bionic olfactory sensing technology. This article mainly introduces the recent research progress of several commonly used objective olfactory tests and provides reference for more accurate evaluation of olfactory function.

Magnetic Resonance Imaging as a Diagnostic and Research Tool in Patients with Olfactory Dysfunction: A Systematic Review

BACKGROUND

Patients with acquired, idiopathic olfactory dysfunction (OD) commonly undergo magnetic resonance imaging (MRI) evaluation to rule out intracranial pathologies. This practice is highly debated given the expense of MRI relative to the probability of detecting a treatable lesion. This, combined with the increasing use of MRI in research to investigate the mechanisms underlying OD, provided the impetus for this comprehensive review.

OBJECTIVE

The purpose of this systematic review was to both assess the utility of MRI in diagnosis of idiopathic OD and to describe MRI findings among mixed OD etiologies to better understand its role as a research tool in this patient population.

METHODS

A literature search of PubMed, Embase, Cochrane, Web of Science, and Scopus for studies with original MRI data for patients with OD was completed. Studies exclusively investigating patients with neurocognitive deficits or those studying traumatic or congenital etiologies of OD were excluded.

RESULTS

From 1758 candidate articles, 33 studies were included. Four studies reviewed patients with idiopathic OD for structural pathologies on MRI, of which 17 of 372 (4.6%) patients had a potential central cause identified, and 3 (0.8%) had an olfactory meningioma or olfactory neuroblastoma. Fourteen studies (42.4%) reported significant correlation between olfactory bulb volume and olfactory outcomes, and 6 studies (18.8%) reported gray matter volume reduction, specifically in the orbitofrontal cortex, anterior cingulate cortex, insular cortex, parahippocampal, and piriform cortex areas, in patients with mixed OD etiologies. Functional MRI studies reported reduced brain activation and functional connectivity in olfactory network areas.

CONCLUSION

MRI uncommonly detects intracranial pathology in patients with idiopathic OD. Among patients with mixed OD etiologies, reduced olfactory bulb and gray matter volume are the most common abnormal findings on MRI. Further research is required to better understand the role of MRI and its cost-effectiveness in patients with acquired, idiopathic OD.

Olfaction and Executive Cognitive Performance: A Systematic Review

Objective tests of olfaction are widely available to aid in the assessment of olfaction. Their clearest role is in the characterization of olfactory changes, either reported by or suspected in a patient. There is a rapidly growing literature concerned with the association of olfactory changes with certain neuropsychiatric conditions and the use of olfactory testing to supplement conventional assessments in clinical and research practice is evolving. Neural pathways important for olfactory processing overlap extensively with pathways important for cognitive functioning, and especially those important for executive functioning, many of which are concentrated in the frontal lobes. Previous work has identified associations between performance on certain olfactory tests (most frequently olfactory identification) and executive functioning and behavioral measures (e.g. of impulsivity). More recently, similar associations have also been identified in non-clinical samples, raising new questions as to the utility of olfactory test scores as proxy measures for non-olfactory phenomena. In this systemic review, we sought to identify studies, both clinical and non-clinical, that investigated the associations of olfaction with performance on tasks sensitive to frontal lobe functioning. Our search criteria led to the identification of 70 studies published in English. We examined in detail and tabulated the data from these studies, highlighted each study's key findings, and critically evaluated these studies. We use the results of this review to reflect on some of the current and future challenges concerning the use of olfactory testing in clinical neuropsychiatric practice and research and speculate on the potential benefits of administering phonemic fluency in combination with olfactory testing to enhance its predictive value.

Magnetic Resonance Imaging Confirmed Olfactory Bulb Reduction in Long COVID-19: Literature Review and Case Series

An altered sense of smell and taste was recognized as one of the most characteristic symptoms of coronavirus infection disease (COVID-19). Despite most patients experiencing a complete functional resolution, there is a 21.3% prevalence of persistent alteration at 12 months after infection. To date, magnetic resonance imaging (MRI) findings in these patients have been variable and not clearly defined. We aimed to clarify radiological alterations of olfactory pathways in patients with long COVID-19 characterized by olfactory dysfunction. A comprehensive review of the English literature was performed by analyzing relevant papers about this topic. A case series was presented: all patients underwent complete otorhinolaryngology evaluation including the Sniffin’ Sticks battery test. A previous diagnosis of SARS-CoV-2 infection was confirmed by positive swabs. The MRIs were acquired using a 3.0T MR scanner with a standardized protocol for olfactory tract analysis. Images were first analysed by a dedicated neuroradiologist and subsequently reviewed and compared with the previous available MRIs. The review of the literature retrieved 25 studies; most cases of olfactory dysfunction more than 3 months after SARS-CoV-2 infection showed olfactory bulb (OB) reduction. Patients in the personal case series had asymmetry and a reduction in the volume of the OB. This evidence was strengthened by the comparison with a previous MRI, where the OBs were normal. The results preliminarily confirmed OB reduction in cases of long COVID-19 with an altered sense of smell. Further studies are needed to clarify the epidemiology, pathophysiology and prognosis.

Olfactory loss and brain connectivity after COVID-19

To address the impact of COVID‐19 olfactory loss on the brain, we analyzed the neural connectivity of the central olfactory system in recently SARS‐CoV‐2 infected subjects with persisting olfactory impairment (hyposmia). Twenty‐seven previously SARS‐CoV‐2 infected subjects (10 males, mean age ± SD 40.0 ± 7.6 years) with clinically confirmed COVID‐19 related hyposmia, and eighteen healthy, never SARS‐CoV‐2 infected, normosmic subjects (6 males, mean age ± SD 36.0 ± 7.1 years), were recruited in a 3 Tesla MRI study including high angular resolution diffusion and resting‐state functional MRI acquisitions. Specialized metrics of structural and functional connectivity were derived from a standard parcellation of olfactory brain areas and a previously validated graph‐theoretic model of the human olfactory functional network. These metrics were compared between groups and correlated to a clinical index of olfactory impairment. On the scanning day, all subjects were virus‐free and cognitively unimpaired. Compared to control, both structural and functional connectivity metrics were found significantly increased in previously SARS‐CoV‐2 infected subjects. Greater residual olfactory impairment was associated with more segregated processing within regions more functionally connected to the anterior piriform cortex. An increased neural connectivity within the olfactory cortex was associated with a recent SARS‐CoV‐2 infection when the olfactory loss was a residual COVID‐19 symptom. The functional connectivity of the anterior piriform cortex, the largest cortical recipient of afferent fibers from the olfactory bulb, accounted for the inter‐individual variability in the sensory impairment. Albeit preliminary, these findings could feature a characteristic brain connectivity response in the presence of COVID‐19 related residual hyposmia.

A Comparative Olfactory MRI, DTI and fMRI Study of COVID-19 Related Anosmia and Post Viral Olfactory Dysfunction

Rationale and Objective

To evaluate how COVID-19 anosmia imaging findings resembled and differed from postinfectious olfactory dysfunction (OD).

Material and Methods

A total of 31 patients presenting with persistent COVID-19 related OD and 97 patients with post-infectious OD were included. Olfactory bulb MRI, DTI and olfactory fMRI findings in both groups were retrospectively assessed.

Results

All COVID-19 related OD cases were anosmic, 18.6% of post-infectious OD patients were hyposmic and remaining 81.4% were anosmic. Mean interval between onset of OD and imaging was 1.5 months for COVID-19 related OD and 6 months for post-infectious OD.

Olfactory bulb volumes were significantly higher in COVID-19 related OD than post-infectious OD. Deformed bulb morphology and increased olfactory bulb signal intensity was seen in 58.1% and 51.6% with COVID-19 related OD; and 63.9% – 46.4% with post-infectious OD; without significant difference. Significantly higher rate of olfactory nerve clumping and higher QA values at orbitofrontal and entorhinal regions were observed in COVID-19 related OD than post-infectious OD.

Absence of orbitofrontal and entorhinal activity showed no statistically significant difference between COVID-19 related OD and post-infectious OD, however trigeminosensory activity was more robust in COVID-19 related OD cases.

Conclusion

Olfactory bulb damage may play a central role in persistent COVID-19 related anosmia. Though there is decreased olfactory bulb volume and decreased white matter tract integrity of olfactory regions in COVID-19 related anosmia, this is not as pronounced as in other post-infectious OD. Trigeminosensory activity was more robust in COVID-19 related OD. These findings may reflect better preserved central olfactory system in COVID-19 related OD compared to COVID-19 related OD.

[Olfactory disorders as a multidisciplinary problem]

Olfactory dysfunction is a serious symptom that requires careful differential diagnosis. The article presents convincing evidence that dysosmia is not only a symptom of rinological pathology, but also a manifestation of various neurodegenerative diseases. Some patients with SARS-CoV-2 have neurological symptoms. Modern studies show that olfactory and gustatory dysfunctions are significant symptoms in the clinical presentation of the COVID-19 infection. The importance of olfactory diagnostics in relatives of patients with hereditary neurodegenerative diseases for the purpose of early detection of pathology is noted. We consider the possibility of introducing new methods for the diagnosis of olfactory dysfunction, which is a promising task both in the field of neurology and otorhinolaryngology, in order to prevent the development of neurodegenerative diseases at an early stage, improve the quality of life and social adaptation of patients.

A systematic review of olfactory-related brain structural changes in patients with congenital or acquired anosmia

Brain structural features of healthy individuals are associated with olfactory functions. However, due to the pathophysiological differences, congenital and acquired anosmia may exhibit different structural characteristics. A systematic review was undertaken to compare brain structural features between patients with congenital and acquired anosmia. A systematic search was conducted using PubMed/MEDLINE and Scopus electronic databases to identify eligible reports on anosmia and structural changes and reported according to PRISMA guidelines. Reports were extracted for information on demographics, psychophysical evaluation, and structural changes. Then, the report was systematically reviewed based on various aetiologies of anosmia in relation to (1) olfactory bulb, (2) olfactory sulcus, (3) grey matter (GM), and white matter (WM) changes. Twenty-eight published studies were identified. All studies reported consistent findings with strong associations between olfactory bulb volume and olfactory function across etiologies. However, the association of olfactory function with olfactory sulcus depth was inconsistent. The present study observed morphological variations in GM and WM volume in congenital and acquired anosmia. In acquired anosmia, reduced olfactory function is associated with reduced volumes and thickness involving the gyrus rectus, medial orbitofrontal cortex, anterior cingulate cortex, and cerebellum. These findings contrast to those observed in congenital anosmia, where a reduced olfactory function is associated with a larger volume and higher thickness in parts of the olfactory network, including the piriform cortex, orbitofrontal cortex, and insula. The present review proposes that the structural characteristics in congenital and acquired anosmia are altered differently. The mechanisms behind these changes are likely to be multifactorial and involve the interaction with the environment.

Structural and Functional Abnormalities of Olfactory-Related Regions in Subjective Cognitive Decline, Mild Cognitive Impairment, and Alzheimer's Disease

BACKGROUND

Odor identification (OI) dysfunction is an early marker of Alzheimer's disease (AD), but it remains unclear how olfactory-related regions change from stages of subjective cognitive decline (SCD) and mild cognitive impairment (MCI) to AD dementia.

METHODS

Two hundred and sixty-nine individuals were recruited in the present study. The olfactory-related regions were defined as the regions of interest, and the grey matter volume (GMV), low-frequency fluctuation, regional homogeneity (ReHo), and functional connectivity (FC) were compared for exploring the changing pattern of structural and functional abnormalities across AD, MCI, SCD, and normal controls.

RESULTS

From the SCD, MCI to AD groups, the reduced GMV, increased low-frequency fluctuation, increased ReHo, and reduced FC of olfactory-related regions became increasingly severe, and only the degree of reduced GMV of hippocampus and caudate nucleus clearly distinguished the 3 groups. SCD participants exhibited reduced GMV (hippocampus, etc.), increased ReHo (caudate nucleus), and reduced FC (hippocampus-hippocampus and hippocampus-parahippocampus) in olfactory-related regions compared with normal controls. Additionally, reduced GMV of the bilateral hippocampus and increased ReHo of the right caudate nucleus were associated with OI dysfunction and global cognitive impairment, and they exhibited partially mediated effects on the relationships between OI and global cognition across all participants.

CONCLUSION

Structural and functional abnormalities of olfactory-related regions present early with SCD and deepen with disease severity in the AD spectrum. The hippocampus and caudate nucleus may be the hub joining OI and cognitive function in the AD spectrum.

The Effect of Olfactory Training on Olfaction, Cognition, and Brain Function in Patients with Mild Cognitive Impairment

BACKGROUND

The olfactory system is affected very early in Alzheimer's disease and olfactory loss can already be observed in patients with mild cognitive impairment (MCI), an early stage of AD.

OBJECTIVE

The aim of this randomized, prospective, controlled, blinded study was to evaluate whether olfactory training (OT) may have an effect on olfactory function, cognitive impairment, and brain activation in MCI patients after a 4-month period of frequent short-term exposure to various odors.

METHODS

A total of 38 MCI outpatients were randomly assigned to OT or a control training condition, which were performed twice a day for 4 months. Olfactory testing, comprehensive neuropsychological assessment, and magnetic resonance imaging were performed before and after training.

RESULTS

The results suggested that OT exhibited no significant effect on olfaction and cognitive function. However, OT exhibited a positive effect on frontal lobe activation (left middle frontal gyrus and orbital-frontal cortex) but exhibited no effect on grey matter volume. Moreover, the change of olfactory scores was positively associated with the change of frontal activation.

CONCLUSION

OT was found to have a limited effect on olfaction and cognition in patients with MCI compared to a non-OT condition but increased their functional response to odors in frontal area.

A Novel Spectrum Contrast Mapping Method for Functional Magnetic Resonance Imaging Data Analysis

Many studies reported that spontaneous fluctuation of the blood oxygen level-dependent signal exists in multiple frequency components and changes over time. By assuming a reliable energy contrast between low- and high-frequency bands for each voxel, we developed a novel spectrum contrast mapping (SCM) method to decode brain activity at the voxel-wise level and further validated it in designed experiments. SCM consists of the following steps: first, the time course of each given voxel is subjected to fast Fourier transformation; the corresponding spectrum is divided into low- and high-frequency bands by given reference frequency points; then, the spectral energy ratio of the low- to high-frequency bands is calculated for each given voxel. Finally, the activity decoding map is formed by the aforementioned energy contrast values of each voxel. Our experimental results demonstrate that the SCM (1) was able to characterize the energy contrast of task-related brain regions; (2) could decode brain activity at rest, as validated by the eyes-closed and eyes-open resting-state experiments; (3) was verified with test-retest validation, indicating excellent reliability with most coefficients > 0.9 across the test sessions; and (4) could locate the aberrant energy contrast regions which might reveal the brain pathology of brain diseases, such as Parkinson’s disease. In summary, we demonstrated that the reliable energy contrast feature was a useful biomarker in characterizing brain states, and the corresponding SCM showed excellent brain activity-decoding performance at the individual and group levels, implying its potentially broad application in neuroscience, neuroimaging, and brain diseases.

Clinicoradiological features in amyotrophic lateral sclerosis patients with olfactory dysfunction

Objective: Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disorder characterized by motor neuron involvement. Although olfactory dysfunction has been described in ALS, clinicoradiological features associated with the olfactory dysfunction remain poorly understood. Methods: We enrolled 30 patients with ALS and age- and sex-matched 53 healthy controls (HCs). All participants underwent the odor stick identification test for Japanese (OSIT-J) and clinical assessments, including disease duration, ALSFRS-R, site of onset, forced vital capacity, and cognitive examinations that reflected the general, executive, memory and language function. We investigated the associations between OSIT-J score and clinical features and examined atrophic changes by voxel-based morphometry (VBM) analysis to MRI. Results: The OSIT-J score was significantly lower in ALS patients than HCs (6.9 ± 3.2 vs. 9.8 ± 1.9, p < 0.001). In ALS, there were significant relationships between OSIT-J score and age at examination, frontal assessment battery, word fluencies, digit span forward, and ADAS-Jcog recognition, but not education, disease type, duration, ALSFRS-R and, %VC. Multiple regression analysis with stepwise method showed the only ADAS-Jcog recognition substantially predicted OSIT-J score. VBM analysis with age, sex, total intracranial volume, and ADAS-Jcog recognition as covariates showed OSIT-J scores were substantially correlated with atrophic changes of left orbital cortex consisting of gyrus rectus and medial orbital gyrus and right hippocampus in ALS. Conclusion: ALS patients could show substantial olfactory dysfunction in association with orbital cortex and hippocampus involvements. The olfactory examination could be a useful marker for screening of frontotemporal alteration in ALS.

Medical aromatherapy in geriatric syndrome

Geriatric syndromes are symptoms and signs, such as falls, incontinence, delirium, pressure ulcers, dysphagia and so on, that often threaten the independence of older adults, rather than the disease itself. Although the syndromes are very common in older people, it is difficult to treat those by modern medicine due to their complexity. To mitigate the intractable geriatric symptoms, we review the efficacy of aromatherapy, especially for dysphagia, dyspnea, cognitive dysfunction and falls in geriatric syndrome. Olfactory stimulation using a volatile black pepper oil on institutional residents improved the swallowing reflex, which is a crucial risk factor of aspiration pneumonia. Brain imaging study showed that olfactory stimulation using volatile black pepper oil activated cerebral regions of the anterior cingulate and the insular cortex, which play a role in controlling appetite and swallowing. Also, aromatherapy with volatile l-menthol decreased the sense of dyspnea and improved the efficacy of exercise therapy. The fragrance of the combination of rosemary and lemon oils in the morning, and the combination of lavender and orange oils in the night-time were reported to improve cognition and behavioural and psychological symptoms of dementia, respectively. Also, the combination of lavender and lemon balm oils was reported to be effective for irritability-related agitation in older adults. Furthermore, aromatherapy with lavender fragrance could improve both static and dynamic balance, resulting in a reduction in the number of fallers and the incidence rate in older people. Thus, aromatherapy is a promising remedy for geriatric syndrome. Geriatr Gerontol Int 2021; 21: 377-385.

Altered glucose metabolism of the olfactory-related cortices in anosmia patients with traumatic brain injury

PURPOSE

Impaired brain cortices contribute significantly to the pathophysiological mechanisms of post-traumatic olfactory dysfunction (PTOD). This study aimed to use ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) to measure cerebral cortices' metabolism activity and then to explore their associations with olfaction in patients with PTOD.

METHODS

Ethics committee-approved prospective studies included 15 patients with post-traumatic anosmia and 11 healthy volunteers. Olfactory function was assessed using the Sniffin' Sticks. Participants underwent ¹⁸F-FDG PET/CT scan and the image data were collected for the voxel-based whole brain analysis. Furthermore, the standardized uptake value ratio (SUVR) of the whole brain regions was measured and correlated with olfactory function.

RESULTS

Patients with post-traumatic anosmia showed significantly reduced glucose metabolism in bilateral rectus, bilateral superior and medial orbitofrontal cortex (OFC), bilateral thalamus, left hippocampus and parahippocampus and left superior temporal pole (all p < 0.001). In contrast, patients with post-traumatic anosmia had significantly increased glucose metabolism in the bilateral insula (all p < 0.001). SUVR values among a total of 17 cerebral cortices including frontal, limbic, and temporal regions were significantly and positively correlated with olfactory function. The cerebral cortices with the top three correlations were the right middle frontal OFC (r = 0.765, p = 0.001), right caudate (r = 0.652, p = 0.010) and right putamen (r = 0.623, p = 0.002).

CONCLUSION

Patients with post-traumatic anosmia presented with distinct patterns of brain metabolism and key cortices that highly associated with the retained olfactory function were identified. The preliminary results further support the potential use of PET imaging for precisely assessing brain metabolism in patients with PTOD.

The additive effect of late-life depression and olfactory dysfunction on the risk of dementia was mediated by hypersynchronization of the hippocampus/fusiform gyrus

Early detection of patients with late-life depression (LLD) with a high risk of developing dementia contributes to early intervention. Odor identification (OI) dysfunction serves as a marker for predicting dementia, but whether OI dysfunction increases the risk of dementia in LLD patients remains unclear. The present study aimed to explore the interactive effect of LLD and OI dysfunction on the risk of dementia and its underlying neuroimaging changes. One hundred and fifty-seven LLD patients and 101 normal controls were recruited, and data on their OI, cognition, activity of daily living (ADL), and resting-state functional magnetic resonance imaging were collected. Two × two factorial analyses were used to analyze the interactive effects of LLD and OI dysfunction on neuropsychological and neuroimaging abnormalities. Mediation analyses were used to explore whether abnormalities detected by neuroimaging mediated the the associations between OI and cognition/ADL. The results suggested that LLD and OI dysfunction exhibited additive effects on reduced ADL, global cognition and memory scores, as well as neuroimaging variables including (i) increased fractional amplitude of low-frequency fluctuation (fALFF) in the right orbitofrontal cortex and right precentral cortex, and (ii) increased regional homogeneity (ReHo) in the left hippocampus/fusiform gyrus, etc. In addition, these increased fALFF and ReHo values were associated with reduced neuropsychological scores (ADL, global cognition, memory, and language). Moreover, ReHo of the left hippocampus/fusiform gyrus completely mediated the relationship between OI and ADL, and partially mediated the relationship between OI and global cognition. Overall, mediated by the hypersynchronization of the left hippocampus/fusiform gyrus, OI dysfunction may increase the risk of dementia in LLD patients.

Improved Odor Identification Ability and Increased Regional Gray Matter Volume After Olfactory Training in Patients With Idiopathic Olfactory Loss

Idiopathic olfactory loss (IOL) is thought as an early marker for neurodegenerative disease. This study investigated the effect of olfactory training (OT) on regional gray matter volume (GMV) among patients with IOL. A total of 24 patients (mean age 64.6 years, 11 male) with IOL and 30 control participants with normal olfaction (mean age 62.6 years, 13 males) were included in the study. Voxel-based morphometry was performed to compare the GMV between patient and control groups. Only the patients received OT (averaged duration 7 months), and a longitudinal approach was used to examine the GMV change from pre- to post-OT. Moreover, the effect of OT on GMV change was explored for patients with different severity of olfactory loss (anosmia vs. hyposmia). Olfactory performance was measured alongside using the "Sniffin' Sticks." Patients had improved odor identification and larger GMV in the bilateral cerebellum, bilateral thalamus, left precentral gyrus, right gyrus rectus, and medial orbitofrontal cortex after OT. However, no correlation was found between changes of odor identification and increased regional GMV. Besides, patients with anosmia, compared with patient with hyposmia, demonstrated increased GMV in the left precuneus, left superior frontal medial cortex, and left midcingulate cortex after OT. The study showed improved odor identification ability among patients with IOL after OT, which is unlikely related to spontaneous recovery. In this specific patient group, the GMV alterations may be associated with factors not directly predicted by the currently performed measurements, but possibly higher order olfactory-related functional changes.

Post-traumatic olfactory loss and brain response beyond olfactory cortex

Olfactory impairment after a traumatic impact to the head is associated with changes in olfactory cortex, including decreased gray matter density and decreased BOLD response to odors. Much less is known about the role of other cortical areas in olfactory impairment. We used fMRI in a sample of 63 participants, consisting of 25 with post-traumatic functional anosmia, 16 with post-traumatic hyposmia, and 22 healthy controls with normosmia to investigate whole brain response to odors. Similar neural responses were observed across the groups to odor versus odorless stimuli in the primary olfactory areas in piriform cortex, whereas response in the frontal operculum and anterior insula (fO/aI) increased with olfactory function (normosmia > hyposmia > functional anosmia). Unexpectedly, a negative association was observed between response and olfactory perceptual function in the mediodorsal thalamus (mdT), ventromedial prefrontal cortex (vmPFC) and posterior cingulate cortex (pCC). Finally, connectivity within a network consisting of vmPFC, fO, and pCC could be used to successfully classify participants as having functional anosmia or normosmia. We conclude that, at the neural level, olfactory impairment due to head trauma is best characterized by heightened responses and differential connectivity in higher-order areas beyond olfactory cortex.

Parosmia is Associated with Relevant Olfactory Recovery After Olfactory Training

OBJECTIVE/HYPOTHESIS

This study aims to determine the association between parosmia and clinically relevant recovery of olfactory function in patients with post-infectious olfactory dysfunction (PIOD) receiving olfactory training.

STUDY DESIGN

Retrospective cohort study.

METHODS

This was a retrospective cohort study of patients with PIOD that received olfactory training. Adult patients with the major complaint of quantitative smell loss were recruited and treated at several ENT clinics in German between 2008 and 2018. The outcome was based on the association between smell-loss related factors (including parosmia and phantosmia) and clinically relevant changes in overall and subdimension olfactory function of threshold, discrimination, and identification using binary logistic regression analysis.

RESULTS

A total of 153 participants with PIOD were included. Clinically relevant improvements in overall olfactory function were more likely in those that had lower baseline olfactory function. Relevant improvements in discrimination function were more likely in those that had lower baseline olfactory function and those that had parosmia at the initial visit. Similarly, relevant improvements in odor identification were more likely in those that had a lower baseline olfactory function and in those who had parosmia at the first visit. Clinically significant improvements in odor threshold were more likely in those who were older in age.

CONCLUSIONS

This study demonstrated that the presence of parosmia is associated with clinically relevant recovery in olfactory discrimination and identification function in patients with PIOD receiving olfactory training.

LEVEL OF EVIDENCE

4 Laryngoscope, 131:618-623, 2021.

Olfaction as a Marker for Dystonia: Background, Current State and Directions

Dystonia is a heterogeneous group of hyperkinetic movement disorders. The unifying descriptor of dystonia is the motor manifestation, characterized by continuous or intermittent contractions of muscles that cause abnormal movements and postures. Additionally, there are psychiatric, cognitive, and sensory alterations that are possible or putative non-motor manifestations of dystonia. The pathophysiology of dystonia is incompletely understood. A better understanding of dystonia pathophysiology is highly relevant in the amelioration of significant disability associated with motor and non-motor manifestations of dystonia. Recently, diminished olfaction was found to be a potential non-motor manifestation that may worsen the situation of subjects with dystonia. Yet, this finding may also shed light into dystonia pathophysiology and yield novel treatment options. This article aims to provide background information on dystonia and the current understanding of its pathophysiology, including the key structures involved, namely, the basal ganglia, cerebellum, and sensorimotor cortex. Additionally, involvement of these structures in the chemical senses are reviewed to provide an overview on how olfactory (and gustatory) deficits may occur in dystonia. Finally, we describe the present findings on altered chemical senses in dystonia and discuss directions of research on olfactory dysfunction as a marker in dystonia.

Prefrontal Responses to Odors in Individuals With Autism Spectrum Disorders: Functional NIRS Measurement Combined With a Fragrance Pulse Ejection System

Individuals with autism spectrum disorders (ASD) are impaired not only in social competencies but also in sensory perception, particularly olfaction. The olfactory ability of individuals with ASD has been examined in several psychophysical studies, but the results have been highly variable, which might be primarily due to methodological difficulties in the control of odor stimuli (e.g., the problem of lingering scents). In addition, the neural correlates of olfactory specificities in individuals with ASD remain largely unknown. To date, only one study has investigated this issue using functional magnetic resonance imaging (fMRI). The present study utilized a sophisticated method-a pulse ejection system-to present well-controlled odor stimuli to participants with ASD using an ASD-friendly application. With this advantageous system, we examined their odor detection, identification, and evaluation abilities and measured their brain activity evoked by odors using functional near-infrared spectroscopy (fNIRS). As the odor detection threshold (DT) of participants with ASD was highly variable, these participants were divided into two groups according to their DT: an ASD-Low DT group and an ASD-High DT group. Behavioral results showed that the ASD-High DT group had a significantly higher DT than the typically developing (control) group and the ASD-Low DT group, indicating their insensitivity to the tested odors. In addition, while there was no significant difference in the odor identification ability between groups, there was some discrepancy between the groups' evaluations of odor pleasantness. The brain data identified, for the first time, that neural activity in the right dorsolateral prefrontal cortex (DLPFC) was significantly weaker in the ASD-High DT group than in the control group. Moreover, the strength of activity in the right DLPFC was negatively correlated with the DT. These findings suggest that participants with ASD have impairments in the higher-order function of olfactory processing, such as olfactory working memory and/or attention.