Neuroanatomical correlates of olfactory loss in normal aged subjects

The Bulb, the Brain and the Being: New Insights into Olfactory System Anatomy, Organization and Connectivity

BACKGROUND/OBJECTIVES

Olfaction is in many ways the least understood sensory modality. Its organization and connectivity are still under debate. The aim of this study was to investigate the anatomy of the olfactory system by using a cadaver fiber dissection technique and in vivo tractography to attain a deeper understanding of the subcortical connectivity and organization.

METHODS

Ten cerebral hemispheres were used in this study for white matter dissection according to Klingler's technique. Measurements of different cortical structures and interhemispheric symmetry were compared. Diffusion tensor imaging sequences from twenty-five healthy individuals from the Human Connectome Project dataset were used to explore the connectivity of the olfactory system using DSI Studio. White matter connectivity between the following were reconstructed in vivo: (1) Olfactory bulb to primary olfactory cortices; (2) Olfactory bulb to secondary olfactory cortices; (3) Primary to secondary olfactory cortices. The DTI metrics of the identified major associative, projection and commissural pathways were subsequently correlated with olfactory function and cognition in seventy-five healthy individuals with Spearman's rank correlation and the Benjamini-Hochberg method for false discoveries (CI 95%, p < 0.05) using R.

RESULTS

1. The dissection showed that the lateral stria was significantly longer on the left side and projected towards the amygdala, the entorhinal and piriform cortex. 2. The medial stria was not evident as a consistent white matter structure. 3. Both dissection and tractography showed that major associative white matter pathways such as the uncinate fasciculus, the inferior fronto-occipital fasciculus and cingulum supported the connectivity between olfactory areas together with the anterior commissure. 4. No significant correlation was found between DTI metrics and sensory or cognition test results.

CONCLUSIONS

We present the first combined fiber dissection analysis and tractography of the olfactory system. We propose a novel definition where the primary olfactory network is defined by the olfactory tract/bulb and primary olfactory cortices through the lateral stria only. The uncinate fasciculus, inferior fronto-occipital fasciculus and cingulum are the associative pathways supporting the connectivity between primary and secondary olfactory areas together with the anterior commissure. We suggest considering these structures as a secondary olfactory network. Further work is needed to attain a deeper understanding of the pathological and physiological implications of the olfactory system.

Cerebrovascular burden and neurodegeneration linked to 15-year odor identification decline in older adults

Background

The mechanisms underlying olfactory decline in aging need further investigation. Noticeably, the longitudinal relationship of biological markers with olfaction remains underexplored. We investigated whether baseline levels and progression of microvascular lesions and brain atrophy are associated with odor identification (OID) decline.

Methods

The association between structural MRI markers and OID decline was examined in participants from the SNAC-K MRI study who were free from dementia at baseline (n = 401, mean age = 70.2 years, 60% females). OID was repeatedly assessed over 15 years. Presence of lacunes, white matter hyperintensities (WMH), perivascular spaces (PVS), and lateral ventricular, hippocampal, amygdalar, and total gray matter (GM) volumes were assessed up to 6 years, concurrent with the first 6 years of olfactory assessments.

Results

Higher PVS count and lower hippocampal and GM volumes at baseline were associated with accelerated OID decline (pFWE  < 0.05). Longitudinally (n = 225), presence of lacunes at follow-up, faster WMH volume and PVS count increases, faster lateral ventricular enlargement, and faster hippocampal, amygdalar, and GM atrophy were associated with accelerated OID decline (p FWE < 0.05).

Conclusion

Olfactory decline is related to both increased cerebrovascular burden and accelerated brain atrophy over time.

Brain-wide pleiotropy investigation of alcohol drinking and tobacco smoking behaviors

To investigate the pleiotropic mechanisms linking brain structure and function to alcohol drinking and tobacco smoking, we integrated genome-wide data generated by the GWAS and Sequencing Consortium of Alcohol and Nicotine use (GSCAN; up to 805,431 participants) with information related to 3935 brain imaging-derived phenotypes (IDPs) available from UK Biobank (N = 33,224). We observed global genetic correlation of smoking behaviors with white matter hyperintensities, the morphology of the superior longitudinal fasciculus, and the mean thickness of pole-occipital. With respect to the latter brain IDP, we identified a local genetic correlation with age at which the individual began smoking regularly (hg38 chr2:35,895,678-36,640,246: rho = 1, p = 1.01 × 10-5). This region has been previously associated with smoking initiation, educational attainment, chronotype, and cortical thickness. Our genetically informed causal inference analysis using both latent causal variable approach and Mendelian randomization linked the activity of prefrontal and premotor cortex and that of superior and inferior precentral sulci, and cingulate sulci to the number of alcoholic drinks per week (genetic causality proportion, gcp = 0.38, p = 8.9 × 10-4, rho = -0.18 ± 0.07; inverse variance weighting, IVW beta = -0.04, 95%CI = -0.07--0.01). This relationship could be related to the role of these brain regions in the modulation of reward-seeking motivation and the processing of social cues. Overall, our brain-wide investigation highlighted that different pleiotropic mechanisms likely contribute to the relationship of brain structure and function with alcohol drinking and tobacco smoking, suggesting decision-making activities and chemosensory processing as modulators of propensity towards alcohol and tobacco consumption.

Olfactory Function and Depression: A Meta-Analysis

OBJECTIVES

We investigated the relationship between olfactory function and depression and suggested future research in this area from rhinology.

METHODS

We independently searched 5 databases (PubMed, SCOPUS, Embase, the Web of Science, and the Cochrane database) for recent studies published from December 2019 to the present. From the obtained studies, we reviewed the findings on olfactory function and depression using a questionnaire to measure depression and olfactory tests.

RESULTS

The olfactory function test score based on the UPSIT-40 (standardized mean difference = -.37 [-.66; -.08], P = .0123) was significantly lower in the depression group than in the control group. The olfactory function score based on the Sniffin' sticks test for identification, discrimination, and threshold was lower in the depression group than in the control. A meta-analysis of the studies showed that depressed patients showed lower olfactory function than the control group.

CONCLUSIONS AND SIGNIFICANCE

The findings revealed that an understanding of the relationship between olfaction and depression can be determined using an analysis methodology and a standardized olfactory test. Olfactory functioning and processing are highly integrated with emotion and memory through projections from the olfactory bulb to the central areas.

Olfactory Identification as a Biomarker for Cognitive Impairment: Insights from Healthy Aging, Subjective Cognitive Decline, and Mild Cognitive Impairment

Introduction: This study aims to investigate the relationship between olfactory identification (OI) and cognitive impairment by examining OI abilities across various stages of cognitive deterioration. Methods: A total of 264 participants were divided into three groups based on cognitive status: cognitively healthy, subjective cognitive, and mild cognitive impairment. All participants were assessed using the Sniffin' Sticks Olfactory Identification test and a comprehensive neuropsychological test battery. Results: Our results highlight the main effects of age and cognitive status on OI scores. Regarding cognitive abilities, OI is associated with measures of short-term memory, long-term, working memory, and selective attention. Finally, logistic regression models showed that OI is a significant predictor for discriminating SCD from CH, MCI from CH, and MCI from SCD. Discussion: These findings suggest the addition of olfactory identification measures in neuropsychological assessments could improve the early detection of individuals at risk for cognitive impairment.

Inflammation and olfactory loss are associated with at least 139 medical conditions

Olfactory loss accompanies at least 139 neurological, somatic, and congenital/hereditary conditions. This observation leads to the question of whether these associations are correlations or whether they are ever causal. Temporal precedence and prospective predictive power suggest that olfactory loss is causally implicated in many medical conditions. The causal relationship between olfaction with memory dysfunction deserves particular attention because this sensory system has the only direct projection to memory centers. Mechanisms that may underlie the connections between medical conditions and olfactory loss include inflammation as well as neuroanatomical and environmental factors, and all 139 of the medical conditions listed here are also associated with inflammation. Olfactory enrichment shows efficacy for both prevention and treatment, potentially mediated by decreasing inflammation.

Lateral Entorhinal Cortex Dysfunction in Alzheimer's Disease Mice

In Alzheimer's disease (AD), the formation of amyloid beta and neurofibrillary tangles (NFTs) leads to neuronal loss in entorhinal cortex (EC), a crucial brain region involved in memory and navigation. These pathological changes are concurrent with the onset of memory-related issues in AD patients with symptoms of forgetfulness such as misplacing items, disorientation in familiar environments etc. The lateral EC (LEC) is associated with non-spatial memory processing including object recognition. Since in LEC, neurons fire in response to objects (object cells) and at locations previously occupied by objects (trace cells), pathology in this region could lead to dysfunction in object location coding. In this paper we show that a transgenic mouse model, EC-App/Tau, which expresses both APP and tau primarily in the EC region, have deficits in LEC-specific memory tasks. Using in vivo single-unit electrophysiology recordings we show that the LEC neurons are hyperactive with low information content and high sparsity compared to the controls indicating poor firing fidelity. We finally show that object cells and trace cells fire less precisely in the EC-App/Tau mice compared to controls indicating poor encoding of objects. Overall, we show that AD pathology causes erratic firing of LEC neurons and object coding defects leading to LEC-specific memory impairment.

Tau propagation in the brain olfactory circuits is associated with smell perception changes in aging

The direct access of olfactory afferents to memory-related cortical systems has inspired theories about the role of the olfactory pathways in the development of cortical neurodegeneration in Alzheimer's disease (AD). In this study, we used baseline olfactory identification measures with longitudinal flortaucipir and PiB PET, diffusion MRI of 89 cognitively normal older adults (73.82 ± 8.44 years; 56% females), and a transcriptomic data atlas to investigate the spatiotemporal spreading and genetic vulnerabilities of AD-related pathology aggregates in the olfactory system. We find that odor identification deficits are predominantly associated with tau accumulation in key areas of the olfactory pathway, with a particularly strong predictive power for longitudinal tau progression. We observe that tau spreads from the medial temporal lobe structures toward the olfactory system, not the reverse. Moreover, we observed a genetic background of odor perception-related genes that might confer vulnerability to tau accumulation along the olfactory system.

Brain-wide pleiotropy investigation of alcohol drinking and tobacco smoking behaviors

To investigate the pleiotropic mechanisms linking brain structure and function to alcohol drinking and tobacco smoking, we integrated genome-wide data generated by the GWAS and Sequencing Consortium of Alcohol and Nicotine use (GSCAN; up to 805,431 participants) with information related to 3,935 brain imaging-derived phenotypes (IDPs) available from UK Biobank (N=33,224). We observed global genetic correlation of smoking behaviors with white matter hyperintensities, the morphology of the superior longitudinal fasciculus, and the mean thickness of pole-occipital. With respect to the latter brain IDP, we identified a local genetic correlation with age at which the individual began smoking regularly (hg38 chr2:35,895,678-36,640,246: rho=1, p=1.01×10 -5 ). This region has been previously associated with smoking initiation, educational attainment, chronotype, and cortical thickness. Our genetically informed causal inference analysis using both latent causal variable approach and Mendelian randomization linked the activity of prefrontal and premotor cortex and that of superior and inferior precentral sulci, and cingulate sulci to the number of alcoholic drinks per week (genetic causality proportion, gcp=0.38, p=8.9×10 -4 , rho=-0.18±0.07; inverse variance weighting, IVW beta=-0.04, 95%CI=-0.07 - -0.01). This relationship could be related to the role of these brain regions in the modulation of reward-seeking motivation and the processing of social cues. Overall, our brain-wide investigation highlighted that different pleiotropic mechanisms likely contribute to the relationship of brain structure and function with alcohol drinking and tobacco smoking, suggesting decision-making activities and chemosensory processing as modulators of propensity towards alcohol and tobacco consumption.

Age-related differences in perception and coding of attractive odorants in mice

Hedonic perception deeply changes with aging, significantly impacting health and quality of life in elderly. In young adult mice, an odor hedonic signature is represented along the antero-posterior axis of olfactory bulb, and transferred to the olfactory tubercle and ventral tegmental area, promoting approach behavior. Here, we show that while the perception of unattractive odorants was unchanged in older mice (22 months), the appreciation of some but not all attractive odorants declined. Neural activity in the olfactory bulb and tubercle of older mice was consistently altered when attraction to pleasant odorants was impaired while maintained when the odorants kept their attractivity. Finally, in a self-stimulation paradigm, optogenetic stimulation of the olfactory bulb remained rewarding in older mice even without ventral tegmental area's response to the stimulation. Aging degrades behavioral and neural responses to some pleasant odorants but rewarding properties of olfactory bulb stimulation persisted, providing new insights into developing novel olfactory training strategies to elicit motivation even when the dopaminergic system is altered as observed in normal and/or neurodegenerative aging.

Normative Data for the 12-Item Sniffin' Sticks Odor Identification Test in Older Adults

OBJECTIVE

Quantitative olfactory assessment has demonstrated clinical utility for the evaluation of a range of neurologic, psychiatric, and sinonasal conditions. Here, we provide age, sex, race, and education-specific normative data for the 12-item Sniffin Sticks Odor Identification Test (SSOIT-12) in older Black and White U.S. adults without preclinical or clinical dementia or sinonasal disease.

METHOD

A sample of 2,224 Atherosclerosis Risk in Communities study participants aged 66-89 years were included. A normative regression equation was developed using a linear model for the association of age, sex, race, and education with odor identification score. Regression-based normative mean scores and percentiles were generated by age, sex, race, and education groups.

RESULTS

Participants (mean age = 74 years, 59% women, 20% Black, 48% > high school education) had a mean SSOIT-12 score of 9.8. Age, sex, race, and education were all associated with odor identification performance (all ps < .05). A linear regression model for the predicted SSOIT-12 score was developed for use with an individual's actual SSOIT-12 score in order to calculate the Z-score and corresponding percentile for a specific age, sex, race, and education group. Data are also reported in tabular format.

CONCLUSIONS

Our study provides SSOIT-12 normative data obtained from a large population of White and Black older adults without preclinical or clinical dementia or sinonasal disease living in the USA. These findings can aid clinicians in assessing the degree of olfactory loss, establishing concordance with a person's perception of olfactory difficulties and quantitatively monitoring changes in olfactory performance over time.

Does Olfactory Training Improve Brain Function and Cognition? A Systematic Review

Olfactory training (OT), or smell training,consists of repeated exposure to odorants over time with the intended neuroplastic effect of improving or remediating olfactory functioning. Declines in olfaction parallel declines in cognition in various pathological conditions and aging. Research suggests a dynamic neural connection exists between olfaction and cognition. Thus, if OT can improve olfaction, could OT also improve cognition and support brain function? To answer this question, we conducted a systematic review of the literature to determine whether there is evidence that OT translates to improved cognition or altered brain morphology and connectivity that supports cognition. Across three databases (MEDLINE, Scopus, & Embase), 18 articles were identified in this systematic review. Overall, the reviewed studies provided emerging evidence that OT is associated with improved global cognition, and in particular, verbal fluency and verbal learning/memory. OT is also associated with increases in the volume/size of olfactory-related brain regions, including the olfactory bulb and hippocampus, and altered functional connectivity. Interestingly, these positive effects were not limited to patients with smell loss (i.e., hyposmia & anosmia) but normosmic (i.e., normal ability to smell) participants benefitted as well. Implications for practice and research are provided.

Sniffing out meaning: Chemosensory and semantic neural network changes in sommeliers

Wine tasting is a very complex process that integrates a combination of sensation, language, and memory. Taste and smell provide perceptual information that, together with the semantic narrative that converts flavor into words, seem to be processed differently between sommeliers and naïve wine consumers. We investigate whether sommeliers' wine experience shapes only chemosensory processing, as has been previously demonstrated, or if it also modulates the way in which the taste and olfactory circuits interact with the semantic network. Combining diffusion-weighted images and fMRI (activation and connectivity) we investigated whether brain response to tasting wine differs between sommeliers and nonexperts (1) in the sensory neural circuits representing flavor and/or (2) in the neural circuits for language and memory. We demonstrate that training in wine tasting shapes the microstructure of the left and right superior longitudinal fasciculus. Using mediation analysis, we showed that the experience modulates the relationship between fractional anisotropy and behavior: the higher the fractional anisotropy the higher the capacity to recognize wine complexity. In addition, we found functional differences between sommeliers and naïve consumers affecting the flavor sensory circuit, but also regions involved in semantic operations. The former reflects a capacity for differential sensory processing, while the latter reflects sommeliers' ability to attend to relevant sensory inputs and translate them into complex verbal descriptions. The enhanced synchronization between these apparently independent circuits suggests that sommeliers integrated these descriptions with previous semantic knowledge to optimize their capacity to distinguish between subtle differences in the qualitative character of the wine.

Olfactory Dysfunction and Depression Trajectories in Community-Dwelling Older Adults

BACKGROUND

We examined the relationship between baseline olfactory performance and incident significant depressive symptoms and longitudinal depression trajectories in well-functioning older adults. Inflammation and cognitive status were examined as potential mediators.

METHODS

Older adults (n = 2 125, 71-82 years, 51% female, 37% Black) completed an odor identification task at Year 3 (our study baseline) of the Health, Aging, and Body Composition study. Cognitive assessments, depressive symptoms, and inflammatory markers were ascertained across multiple visits over 8 years. Discrete-time complementary log-log models, group-based trajectory models, and multivariable-adjusted multinomial logistic regression were employed to assess the relationship between baseline olfaction and incident depression and longitudinal depression trajectories. Mediation analysis assessed the influence of cognitive status on these relationships.

RESULTS

Individuals with lower olfaction had an increased risk of developing significant depressive symptoms at follow-up (hazard ratio = 1.04, 95% confidence interval [CI]: 1.00, 1.08). Of the 3 patterns of longitudinal depression scores identified (stable low, stable moderate, and stable high), poorer olfaction was associated with a 6% higher risk of membership in the stable moderate (relative risk ratio [RRR] = 1.06, 95% CI: 1.02, 1.10)/stable high (RRR = 1.06, 95% CI: 1.00, 1.12) groups, compared to the stable low group. Poor cognitive status, but not inflammation, partially mediated the relationship between olfactory performance and incident depression symptom severity.

CONCLUSIONS

Suboptimal olfaction could serve as a prognostic indicator of vulnerability for the development of late-life depression. These findings underscore the need for a greater understanding of olfaction in late-life depression and the demographic, cognitive, and biological factors that influence these relationships over time.

Associations of olfactory function with brain structural and functional outcomes. A systematic review

In aging, olfactory deficits have been associated with lower cognition and motor function. Olfactory dysfunction is also one of the earliest features of neurodegenerative disease. A comprehensive review of the neural correlates of olfactive function may reveal mechanisms underlying the associations among olfaction, cognition, motor function, and neurodegenerative diseases. Here, we summarize existing knowledge on the relationship between brain structural and functional measures and olfaction in older adults without and with cognitive impairment, including Alzheimer's disease. We identified 33 eligible studies (30 MRI/DTI,3 fMRI); 31 were cross-sectional, most assessed odor identification, and few examined multiple brain areas. Lower olfactory function was associated with smaller volumes in the temporal lobe (hippocampus,parahippocampal gyrus,fusiform gyrus), olfactory-related regions (piriform cortex,amygdala,entorhinal cortex), pre- and postcentral gyri, and globus pallidus. During aging, olfactory impairment may be associated with pathology in brain areas important for motor function and cognition, especially memory. Future longitudinal studies that include neuroimaging across different brain areas are warranted to determine the neurobiological changes underlying olfactory changes in the aging brain and the progression of neurodegeneration.

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.

Overnight olfactory enrichment using an odorant diffuser improves memory and modifies the uncinate fasciculus in older adults

OBJECTIVE

Cognitive loss in older adults is a growing issue in our society, and there is a need to develop inexpensive, simple, effective in-home treatments. This study was conducted to explore the use of olfactory enrichment at night to improve cognitive ability in healthy older adults.

METHODS

Male and female older adults (N = 43), age 60-85, were enrolled in the study and randomly assigned to an Olfactory Enriched or Control group. Individuals in the enriched group were exposed to 7 different odorants a week, one per night, for 2 h, using an odorant diffuser. Individuals in the control group had the same experience with de minimis amounts of odorant. Neuropsychological assessments and fMRI scans were administered at the beginning of the study and after 6 months.

RESULTS

A statistically significant 226% improvement was observed in the enriched group compared to the control group on the Rey Auditory Verbal Learning Test and improved functioning was observed in the left uncinate fasciculus, as assessed by mean diffusivity.

CONCLUSION

Minimal olfactory enrichment administered at night produces improvements in both cognitive and neural functioning. Thus, olfactory enrichment may provide an effective and low-effort pathway to improved brain health.

Olfactory dysfunction: A plausible source of COVID-19-induced neuropsychiatric symptoms

Olfactory dysfunction and neuropsychiatric symptoms are commonly reported by patients of coronavirus disease 2019 (COVID-19), a respiratory infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Evidence from recent research suggests linkages between altered or loss of smell and neuropsychiatric symptoms after infection with the coronavirus. Systemic inflammation and ischemic injury are believed to be the major cause of COVID-19-related CNS manifestation. Yet, some evidence suggest a neurotropic property of SARS-CoV-2. This mini-review article summarizes the neural correlates of olfaction and discusses the potential of trans-neuronal transmission of SARS-CoV-2 or its particles within the olfactory connections in the brain. The impact of the dysfunction in the olfactory network on the neuropsychiatric symptoms associated with COVID-19 will also be discussed.

The Characteristics of Entorhinal Cortex Functional Connectivity in Alzheimer's Disease Patients with Depression

BACKGROUND

Depression is one of the most common neuropsychiatric symptoms of Alzheimer's disease (AD) which decreases the life quality of both patients and caregivers. There are currently no effective drugs. It is therefore important to explore the pathogenesis of depression in AD patients.

OBJECTIVE

The present study aimed to investigate the characteristics of the entorhinal cortex (EC) functional connectivity (FC) in the whole brain neural network of AD patients with depression (D-AD).

METHODS

Twenty-four D-AD patients, 14 AD patients without depression (nD-AD), and 20 healthy controls underwent resting-state functional magnetic resonance imaging. We set the EC as the seed and used FC analysis. One-way analysis of variance was used to examine FC differences among the three groups.

RESULTS

Using the left EC as the seed point, there were FC differences among the three groups in the left EC-inferior occipital gyrus. Using the right EC as the seed point, there were FC differences among the three groups in the right EC-middle frontal gyrus, -superior parietal gyrus, -superior medial frontal gyrus, and -precentral gyrus. Compared with the nD-AD group, the D-AD group had increased FC between the right EC and right postcentral gyrus.

CONCLUSION

Asymmetry of FC in the EC and increased FC between the EC and right postcentral gyrus may be important in the pathogenesis of depression in AD.

Associations of Olfaction With Longitudinal Trajectories of Brain Volumes and Neuropsychological Function in Older Adults

BACKGROUND AND OBJECTIVES

Olfactory function declines with aging, and olfactory deficits are one of the earliest features of neurodegenerative diseases, such as Parkinson disease and Alzheimer disease. Previous studies have shown that olfaction is associated with brain volumes and cognitive function, but data are exclusively cross-sectional. We aimed to examine longitudinal associations of olfaction with changes in brain volumes and neuropsychological function.

METHODS

In the Baltimore Longitudinal Study of Aging, we chose the first assessment of olfaction to examine the associations with retrospective and prospective changes in neuropsychological performance and brain volumes in participants aged 50 years or older using linear mixed-effects models, adjusted for demographic variables and cardiovascular disease. Olfaction was measured as odor identification scores through the 16-item Sniffin' Sticks.

RESULTS

We analyzed data from 567 (58% women, 42% men, 27% Black, 66% White, and 7% others) participants who had data on odor identification scores and brain volumetric MRI (n = 420 with retrospective repeats over a mean of 3.7 years, n = 280 with prospective repeats over a mean of 1.2 years). We also analyzed data from 754 participants (56% women, 44% men, 29% Black, 65% White, and 6% others) with neuropsychological assessments (n = 630 with retrospective repeats over a mean of 6.6 years, n = 280 with prospective repeats over a mean of 1.5 years). After adjustment, higher odor identification scores were associated with prior and subsequent slower brain atrophy in the entorhinal cortex (β ± SE = 0.0093 ± 0.0031, p = 0.0028 and β ± SE = 0.0176 ± 0.0073, p = 0.0169, respectively), hippocampus (β ± SE = 0.0070 ± 0.0030, p = 0.0192 and β ± SE = 0.0173 ± 0.0066, p = 0.0089, respectively), and additional frontal and temporal areas (all p < 0.05). Higher odor identification scores were also associated with prior slower decline in memory, attention, processing speed, and manual dexterity and subsequent slower decline in attention (all p < 0.05). Some associations were attenuated after exclusion of data points at and after symptom onset of cognitive impairment or dementia.

DISCUSSION

In older adults, olfaction is related to brain atrophy of specific brain regions and neuropsychological changes in specific domains over time. The observed associations are driven, in part, by those who developed cognitive impairment or dementia. Future longitudinal studies with longer follow-ups are needed to understand whether olfactory decline precedes cognitive decline and whether it is mediated through regionally specific brain atrophy.

Anosmia, mild cognitive impairment, and biomarkers of brain aging in older adults

Olfactory impairment is a potential marker for prodromal dementia, but the underlying mechanisms are poorly understood. This population-based study included 4214 dementia-free participants (age ≥65 years). Olfaction was assessed using the 16-item Sniffin' Sticks identification test. In the subsamples, we measured plasma amyloid beta (Aβ)40, Aβ42, total tau, and neurofilament light chain (NfL; n = 1054); and quantified hippocampal, entorhinal cortex, and white matter hyperintensity (WMH) volumes, and Alzheimer's disease (AD)-signature cortical thickness (n = 917). Data were analyzed with logistic and linear regression models. In the total sample, mild cognitive impairment (MCI) was diagnosed in 1102 persons (26.2%; amnestic MCI, n = 931; non-amnestic MCI, n = 171). Olfactory impairment was significantly associated with increased likelihoods of MCI, amnestic MCI, and non-amnestic MCI. In the subsamples, anosmia was significantly associated with higher plasma total tau and NfL concentrations, smaller hippocampal and entorhinal cortex volumes, and greater WMH volume, and marginally with lower AD-signature cortical thickness. These results suggest that cerebral neurodegenerative and microvascular lesions are common neuropathologies linking anosmia with MCI in older adults.

Pathophysiology of SARS-CoV-2 Infection of Nasal Respiratory and Olfactory Epithelia and Its Clinical Impact

PURPOSE OF REVIEW

While the predominant cause for morbidity and mortality with SARS-CoV-2 infection is the lower respiratory tract manifestations of the disease, the effects of SARS-CoV-2 infection on the sinonasal tract have also come to the forefront especially with the increased recognition of olfactory symptom. This review presents a comprehensive summary of the mechanisms of action of the SARS-CoV-2 virus, sinonasal pathophysiology of COVID-19, and the correlation with the clinical and epidemiological impact on olfactory dysfunction.

RECENT FINDINGS

ACE2 and TMPRSS2 receptors are key players in the mechanism of infection of SARS-CoV-2. They are present within both the nasal respiratory as well as olfactory epithelia. There are however differences in susceptibility between different groups of individuals, as well as between the different SARS-CoV-2 variants. The sinonasal cavity is an important route for SARS-CoV-2 infection. While the mechanism of infection of SARS-CoV-2 in nasal respiratory and olfactory epithelia is similar, there exist small but significant differences in the susceptibility of these epithelia and consequently clinical manifestations of the disease. Understanding the differences and nuances in sinonasal pathophysiology in COVID-19 would allow the clinician to predict and counsel patients suffering from COVID-19. Future research into molecular pathways and cytokine responses at different stages of infection and different variants of SARS-CoV-2 would evaluate the individual clinical phenotype, prognosis, and possibly response to vaccines and therapeutics.

Olfactory loss is a predisposing factor for depression, while olfactory enrichment is an effective treatment for depression

The loss of olfactory stimulation correlates well with at least 68 widely differing neurological disorders, including depression, and we raise the possibility that this relationship may be causal. That is, it seems possible that olfactory loss makes the brain vulnerable to expressing the symptoms of these neurological disorders, while daily olfactory enrichment may decrease the risk of expressing these symptoms. This situation resembles the cognitive reserve that is thought to protect people with Alzheimer’s neuropathology from expressing the functional deficit in memory through the cumulative effect of intellectual stimulation. These relationships also resemble the functional response of animal models of human neurological disorders to environmental enrichment, wherein the animals continue to have the induced neuropathology, but do not express the symptoms as they do in a standard environment with restricted sensorimotor stimulation.

Rapid olfactory decline during aging predicts dementia and GMV loss in AD brain regions

INTRODUCTION

Longitudinal multivariable analyses are needed to determine if the rate of olfactory decline during normal cognition predicts subsequent Alzheimer's disease (AD) diagnoses and brain dysmorphology.

METHODS

Older adults (n = 515) were assessed annually for odor identification, cognitive function and dementia clinical diagnosis (max follow-up 18 years). Regional gray matter volumes (GMV) were quantified (3T MRI) in a cross-sectional subsample (n = 121). Regression models were adjusted for APOE-ε4 genotype, dementia risk factors and demographics.

RESULTS

Faster olfactory decline during periods of normal cognition predicted higher incidence of subsequent MCI or dementia (OR 1.89, 95% CI: 1.26, 2.90, p < 0.01; comparable to carrying an APOE-ε4 allele) and smaller GMV in AD and olfactory regions (β = -0.11, 95% CI -0.21, -0.00).

DISCUSSION

Rapid olfactory decline during normal cognition, using repeated olfactory measurement, predicted subsequent cognitive impairment, dementia, and smaller GMVs, highlighting its potential as a simple biomarker for early AD detection.

HIGHLIGHTS

Rate of olfactory decline was calculated from olfactory testing over ≥3 time points. Rapid olfactory decline predicted impaired cognition and higher risk of dementia. Neurodegeneration on 3T magnetic resonance imaging was identical in those with olfactory decline and Alzheimer's disease.

Alterations of Cerebral Blood Flow and Its Connectivity in Olfactory-Related Brain Regions of Type 2 Diabetes Mellitus Patients

To investigate the alteration of cerebral blood flow (CBF) and its connectivity patterns in olfactory-related regions of type 2 diabetes mellitus (T2DM) patients using arterial spin labeling (ASL). Sixty-nine patients with T2DM and 63 healthy controls (HCs) underwent ASL scanning using 3.0T magnetic resonance imaging. We compared the CBF values of the olfactory-related brain regions between the two groups and analyzed the correlation between their changes and clinical variables. We also used these regions as seeds to explore the differences in CBF connectivity patterns in olfactory-related brain regions between the T2DM patients and HCs. Compared with the HC group, the CBF of the right orbital part of the inferior frontal gyrus (OIFG), right insula, and bilateral olfactory cortex was decreased in the T2DM patients. Moreover, the duration of the patients was negatively correlated with the CBF changes in the right OIFG, right insula, and right olfactory cortex. The CBF changes in the right OIFG were positively correlated with the Self-Rating Depression Scale scores, those in the right insula were negatively correlated with the max blood glucose of continuous glucose, and those in the right olfactory cortex were negatively correlated with the mean blood glucose of continuous glucose. In addition, the T2DM patients also showed decreased CBF connectivity between the right OIFG and the left temporal pole of the middle temporal gyrus and increased CBF connectivity between the right medial orbital part of the superior frontal gyrus and the right orbital part of the superior frontal gyrus and between the right olfactory cortex and the bilateral caudate and the left putamen. Patients with T2DM have decreased CBF and altered CBF connectivity in multiple olfactory-related brain regions. These changes may help explain why olfactory dysfunction occurs in patients with T2DM, thus providing insights into the neuropathological mechanism of olfactory dysfunction and cognitive decline in T2DM patients.

Impact of Systemic Diseases on Olfactory Function in COVID-19 Infected Patients

Background

COVID-19 (SARS-CoV-2/2019-nCoV) is now a major public health threat to the world. Olfactory dysfunctions (ODs) are considered potential indicating symptoms and early case identification triaging for coronavirus disease 2019 (COVID-19). The most common reported comorbidities are diabetes mellitus, chronic lung disease, and cardiovascular disease. The objective of this study was to evaluate prevalence of different types of smell disorders in patients with laboratory-confirmed COVID-19 infection and impact of involved systemic diseases.

Methodology

A cross-sectional retrospective study has been done for patients with laboratory-confirmed COVID-19 infection (mild-to-moderate). The data collected from patient's files and developed online electronic questionnaire (WhatsApp) based on the patients most common and recurrent reported data including: a) symptoms of olfactory dysfunction and associated covid19 symptoms fever and headache, cough, sore throat, pneumonia, nausea, vomiting and diarrhea, arthralgia and myalgia and taste dysfunction. b) Associated systemic diseases including: diabetes, hypertension, asthma, chronic renal disease, chorionic liver disease and hypothyroidism.

Results

Of 308 patients confirmed with Covid-19 infection, (72.4%) developed OD distributed as follows; complete anosmia (57.8%), troposmia (8.4%), hyposmia (2.9%), partial anosmia (2.6%) and euosmia (0.6%). Significantly increased prevalence of diabetes, hypertension asthma in the group with olfactory dysfunction (p < 0.001), chronic liver disease (p = 0.005), and hypothyroidism (p = 0.03).

Conclusion

The development of ODs after Covid-19 infection was associated with mild disease form and lower hospitalization. In addition, it showed significant relationship with preexisting systemic diseases. Anosmia is the common modality of ODs.

International consensus statement on allergy and rhinology: Olfaction

BACKGROUND

The literature regarding clinical olfaction, olfactory loss, and olfactory dysfunction has expanded rapidly over the past two decades, with an exponential rise in the past year. There is substantial variability in the quality of this literature and a need to consolidate and critically review the evidence. It is with that aim that we have gathered experts from around the world to produce this International Consensus on Allergy and Rhinology: Olfaction (ICAR:O).

METHODS

Using previously described methodology, specific topics were developed relating to olfaction. Each topic was assigned a literature review, evidence-based review, or evidence-based review with recommendations format as dictated by available evidence and scope within the ICAR:O document. Following iterative reviews of each topic, the ICAR:O document was integrated and reviewed by all authors for final consensus.

RESULTS

The ICAR:O document reviews nearly 100 separate topics within the realm of olfaction, including diagnosis, epidemiology, disease burden, diagnosis, testing, etiology, treatment, and associated pathologies.

CONCLUSION

This critical review of the existing clinical olfaction literature provides much needed insight and clarity into the evaluation, diagnosis, and treatment of patients with olfactory dysfunction, while also clearly delineating gaps in our knowledge and evidence base that we should investigate further.

Gray matter volume reduction in the emotional brain networks in adults with anosmia

Loss of olfaction, or anosmia, frequently accompanies emotional dysfunctions, partly due to the overlapping brain regions between the olfactory and emotional processing centers. Here, we investigated whether anosmia was associated with gray matter volume alterations at a network level, and whether these alterations were related to the olfactory-specific quality of life (QOL) and depressive symptoms. Structural brain magnetic resonance imaging was acquired in 22 individuals with postinfectious or idiopathic anosmia (the anosmia group) and 30 age- and sex-matched controls (the control group). Using independent component analysis on the gray matter volumes, we identified 10 morphometric networks. The gray matter volumes of these networks were compared between the two groups. Olfactory-specific QOL and depressive symptoms were assessed by self-report questionnaires and clinician-administered interviews, respectively. The anosmia group showed lower gray matter volumes in the hippocampus-amygdala and the precuneus networks, relative to the control group. Lower gray matter volumes in the hippocampus-amygdala network were also linearly associated with lower olfactory-specific QOL and higher depressive symptom scores. These findings suggest a close relationship between anosmia and gray matter volume alterations in the emotional brain networks, albeit without determined causal relations.

The potential utility of smell testing to screen for neurodegenerative disorders

INTRODUCTION

Loss of smell is a common early feature of neurodegenerative diseases including Alzheimer's and Parkinson's disease. Identifying these conditions in their early stages is important to understand more about early pathophysiological events and the development of disease modifying therapies. Smell testing may be an effective future tool for screening large populations for early neurodegeneration.

AREAS COVERED

: In this review, we appraise the evidence for, and discuss the likelihood of, the use of smell testing in large screening programs to detect early neurodegeneration. We evaluate the predictive power of smell tests for neurodegenerative disease, compare performance to other established screening programs, and discuss ethical and practical considerations and limitations.

EXPERT OPINION

: Even if disease modifying therapies were available for neurodegenerative disease, smell tests alone are unlikely to have high enough predictive power to be used in a future screening program. However, we believe they could be a valuable component of a short battery of tests or part of a stepwise process that together could more accurately identify early neurodegeneration in large populations.

The Neuroanatomic Correlates of Olfactory Identification Impairment in Healthy Older Adults and in Persons with Mild Cognitive Impairment

BACKGROUND

Olfactory identification (OI) impairment appears early in the course of Alzheimer's disease dementia (AD), prior to detectable cognitive impairment. However, the neuroanatomical correlates of impaired OI in cognitively normal older adults (CN) and persons with mild cognitive impairment (MCI) are not fully understood.

OBJECTIVE

We examined the neuroanatomic correlates of OI impairment in older adults from the Atherosclerosis Risk in Communities Neurocognitive Study (ARIC-NCS).

METHODS

Our sample included 1,600 older adults without dementia who completed clinical assessment and structural brain imaging from 2011 to 2013. We characterized OI impairment using the 12-item Sniffin' Sticks odor identification test (score ≤6). We used voxel-based morphometry (VBM) and region of interest (ROI) analyses to examine the neuroanatomic correlates of impaired OI in CN and MCI, after adjusting for potential confounders. Analyses were also separately stratified by race and sex.

RESULTS

In CN, OI impairment was associated with smaller amygdala gray matter (GM) volume (p < 0.05). In MCI, OI impairment was associated with smaller GM volumes of the olfactory cortex, amygdala, entorhinal cortex, hippocampus, and insula (ps < 0.05). Differential associations were observed by sex in MCI; OI impairment was associated with lower insular GM volumes among men but not among women (p-interaction = 0.04). There were no meaningful interactions by race.

CONCLUSION

The brain regions associated with OI impairment in individuals without dementia are specifically those regions known to be the primary targets of AD pathogenic processes. These findings highlight the potential utility of olfactory assessment in the identification and stratification of older adults at risk for AD.

Diffusion Tensor Imaging of the Olfactory System in Older Adults With and Without Hyposmia

Objectives: To compare gray matter microstructural characteristics of higher-order olfactory regions among older adults with and without hyposmia.

Methods: Data from the Brief Smell Identification Test (BSIT) were obtained in 1998–99 for 265 dementia-free adults from the Health, Aging, and Body Composition study (age at BSIT: 74.9 ± 2.7; 62% White; 43% male) who received 3T diffusion tensor imaging in 2006–08 [Interval of time: mean (SD): 8.01 years (0.50)], Apolipoprotein (ApoEε4) genotypes, and repeated 3MS assessments until 2011–12. Cognitive status (mild cognitive impairment, dementia, normal cognition) was adjudicated in 2011–12. Hyposmia was defined as BSIT ≤ 8. Microstructural integrity was quantified by mean diffusivity (MD) in regions of the primary olfactory cortex amygdala, orbitofrontal cortex (including olfactory cortex, gyrus rectus, the orbital parts of the superior, middle, and inferior frontal gyri, medial orbital part of the superior frontal gyrus), and hippocampus. Multivariable regression models were adjusted for total brain atrophy, demographics, cognitive status, and ApoEε4 genotype.

Results: Hyposmia in 1998–99 (n = 57, 21.59%) was significantly associated with greater MD in 2006–08, specifically in the orbital part of the middle frontal gyrus, and amygdala, on the right [adjusted beta (p value): 0.414 (0.01); 0.527 (0.01); respectively].

Conclusion: Older adults with higher mean diffusivity in regions important for olfaction are more likely to have hyposmia up to ten years prior. Future studies should address whether hyposmia can serve as an early biomarker of brain microstructural abnormalities for older adults with a range of cognitive functions, including those with normal cognition.

Olfactory dysfunction in aging and neurodegenerative diseases

Alterations in olfactory functions are proposed to be early biomarkers for neurodegeneration. Many neurodegenerative diseases are age-related, including two of the most common, Parkinson's disease (PD) and Alzheimer's disease (AD). The establishment of biomarkers that promote early risk identification is critical for the implementation of early treatment to postpone or avert pathological development. Olfactory dysfunction (OD) is seen in 90% of early-stage PD patients and 85% of patients with early-stage AD, which makes it an attractive biomarker for early diagnosis of these diseases. Here, we systematically review widely applied smelling tests available for humans as well as olfaction assessments performed in some animal models and the relationships between OD and normal aging, PD, AD, and other conditions. The utility of OD as a biomarker for neurodegenerative disease diagnosis and future research directions are also discussed.

Olfaction and Physical Functioning in Older Adults: A Longitudinal Study

BACKGROUND

Poor olfaction is associated with worse functional performance in older adults, but longitudinal evidence is lacking. We investigated poor olfaction in relation to longitudinal changes in physical functioning among community-dwelling older adults.

METHODS

The analysis included 2,319 participants from the Health, Aging and Body Composition Study (aged 71-82 years, 47·9% men, and 37·3% blacks) who completed the Brief Smell Identification Test in 1999-2000. Olfaction was defined as good (test score 11-12), moderate (9-10), or poor (0-8). Physical functioning was assessed up to four times over 8 years, using the Short Physical Performance Battery (SPPB) and the Health ABC Physical Performance Battery (HABCPPB). We conducted joint model analyses and reported the differences in annual declines across olfaction groups.

RESULTS

During the follow-up, compared to those with good olfaction, older adults with poor olfaction had greater annual declines in both the SPPB score (-0.137, 95%CI: -0.186, -0.088) and all its subscales: standing balance (-0.068, 95%CI:-0.091, -0.044), chair stand (-0.046, 95%CI: -0.070, -0.022), and gait speed (-0.022, 95%CI: -0.042, -0.001). A similar observation was made for the HABCPPB score (difference in annual decline: -0.032, 95%CI:-0.042, -0.021). These findings are robust and cannot be explained by measured confounding from demographics, lifestyle factors, chronic diseases, nor by potential biases due to death and loss of follow-up. Similar associations were observed across subgroups of sex, race, and self-reported general health status.

CONCLUSION

This study provides the first epidemiological evidence that poor olfaction predicts a faster decline in physical functioning. Future studies should investigate potential mechanisms.

The Olfactory System as Marker of Neurodegeneration in Aging, Neurological and Neuropsychiatric Disorders

Research studies that focus on understanding the onset of neurodegenerative pathology and therapeutic interventions to inhibit its causative factors, have shown a crucial role of olfactory bulb neurons as they transmit and propagate nerve impulses to higher cortical and limbic structures. In rodent models, removal of the olfactory bulb results in pathology of the frontal cortex that shows striking similarity with frontal cortex features of patients diagnosed with neurodegenerative disorders. Widely different approaches involving behavioral symptom analysis, histopathological and molecular alterations, genetic and environmental influences, along with age-related alterations in cellular pathways, indicate a strong correlation of olfactory dysfunction and neurodegeneration. Indeed, declining olfactory acuity and olfactory deficits emerge either as the very first symptoms or as prodromal symptoms of progressing neurodegeneration of classical conditions. Olfactory dysfunction has been associated with most neurodegenerative, neuropsychiatric, and communication disorders. Evidence revealing the dual molecular function of the olfactory receptor neurons at dendritic and axonal ends indicates the significance of olfactory processing pathways that come under environmental pressure right from the onset. Here, we review findings that olfactory bulb neuronal processing serves as a marker of neuropsychiatric and neurodegenerative disorders.

Reorganizing brain structure through olfactory training in post-traumatic smell impairment: An MRI study

PURPOSE AND BACKGROUND

Post-traumatic olfactory dysfunction (PTOD), mostly caused by head injury, is thought to be associated with changes in the structure and function of the brain olfactory processing areas. Training and repeated exposure to odorants lead to enhanced olfactory capability. This study investigated the effects of a 16-weeks olfactory training (OT) on olfactory function and brain structure.

METHODS

Twenty-five patients with PTOD were randomly divided in three groups: (1) 9 control patients who did not receive any training, (2) 9 patients underwent classical OT by 4 fixed odors, and (3) 7 patients underwent modified OT coming across 4 sets of 4 different odors sequentially. Before and after the training period, all patients performed olfactory function tests and structural magnetic resonance imaging (MRI). Sniffin' Sticks test was used to assess olfactory function. MRI data were analyzed using voxel-based morphometry and surface-based morphometry.

RESULTS

Both trained groups showed a considerable recovery of olfactory function, especially in odor identification. MRI data analysis revealed that the classical OT leads to increases in cortical thickness/density of several brain regions, including the right superior and middle frontal gyrus, and bilateral cerebellums. In addition, the modified OT yielded a lower extent of cortical measures in the right orbital frontal cortex and right insular. Following modified OT, a positive correlation was observed between the odor identification and the right orbital frontal cortex.

CONCLUSION

Both olfactory training methods can improve olfactory function and that the improvement is associated with changes in the structure of olfactory processing areas of the brain.

The Prevalence of Olfactory Dysfunction in the General Population: A Systematic Review and Meta-analysis

BACKGROUND

Olfactory dysfunction (OD) is common and has been reported as an early indicator of COVID-19. However, the reported prevalence of OD in the general population varies widely depending upon the metric used to assess olfaction.Methodology/Principal: To perform a systematic review and meta-analysis of the prevalence of OD in the healthy general population, review the various assessment metrics used, and compare pooled OD prevalence rates.

RESULTS

A total of 175,073 subjects were identified (mean age 63.5 years, range 18 to 101) with an overall OD prevalence of 22.2% (95% CI 14.8-30.6). OD prevalence was significantly greater using objective olfactory assessments, compared to subjective measures (28.8%, CI 20.3-38.2 versus 9.5%, CI 6.1-13.5, p < 0.001). The prevalence of OD was greater using expanded identification tests (>8 items) compared to brief test with ≤8 items (30.3%, CI 16.2-46.5 versus 21.2%, CI 12.3-31.8). Prevalence was higher in studies with a mean age greater than 55 years compared to those with a mean age of 55 years or less (34.5%, CI 23.4-46.5 versus 7.5%, CI 2.6-14.5, p < 0.001).

CONCLUSIONS

The reported prevalence of OD in the general population depends on the testing method and population age. OD prevalence was greater in studies using objective tests, expanded identification tests, and in those with older subjects.

Fimbria-Fornix Volume Is Associated With Spatial Memory and Olfactory Identification in Humans

White matter pathways that surround the hippocampus comprise its afferent and efferent connections, and are therefore crucial in mediating the function of the hippocampus. We recently demonstrated a role for the hippocampus in both spatial memory and olfactory identification in humans. In the current study, we focused our attention on the fimbria-fornix white matter bundle and investigated its relationship with spatial memory and olfactory identification. We administered a virtual navigation task and an olfactory identification task to 55 young healthy adults and measured the volume of the fimbria-fornix. We found that the volume of the right fimbria-fornix and its subdivisions is correlated with both navigational learning and olfactory identification in those who use hippocampus-based spatial memory strategies, and not in those who use caudate nucleus-based navigation strategies. These results are consistent with our recent finding that spatial memory and olfaction rely on similar neural networks and structures.

Magnetic Resonance Imaging of Human Olfactory Dysfunction

Olfactory dysfunctions affect a larger portion of population (up to 15% with partial olfactory loss, and 5% with complete olfactory loss) as compared to other sensory dysfunctions (e.g. auditory or visual) and have a negative impact on the life quality. The impairment of olfactory functions may happen at each stage of the olfactory system, from epithelium to cortex. Non-invasive neuroimaging techniques such as the magnetic resonance imaging (MRI) have advanced the understanding of the advent and progress of olfactory dysfunctions in humans. The current review summarizes recent MRI studies on human olfactory dysfunction to present an updated and comprehensive picture of the structural and functional alterations in the central olfactory system as a consequence of olfactory loss and regain. Furthermore, the review also highlights recent progress on optimizing the olfactory functional MRI as well as new approaches for data processing that are promising for future clinical practice.

Impaired olfaction is associated with cognitive decline and neurodegeneration in the brain

Objective

We aimed to examine whether impaired olfaction is associated with cognitive decline and indicators of neurodegeneration in the brain of dementia-free older adults.

Methods

Within the Rush Memory and Aging Project, 380 dementia-free participants (mean age = 78 years) were followed for up to 15 years, and underwent MRI scans. Olfactory function was assessed using the Brief Smell Identification Test (B-SIT) at baseline, and categorized as anosmia (B-SIT <6), hyposmia (B-SIT 6–10 in men and 6–10.25 in women), and normal (B-SIT 10.25–12 in men and 10.5–12 in women). Cognitive function was annually assessed with a battery of 21 tests, from which composite scores were derived. Structural total and regional brain volumes were estimated. Data were analyzed using linear regression and mixed-effects models.

Results

At study entry, 138 (36.3%) had normal olfactory function, 213 (56.1%) had hyposmia, and 29 (7.6%) had anosmia. In multiadjusted mixed-effects models, hyposmia (β = −0.03, 95% confidence interval [CI] −0.05 to −0.02) and anosmia (β = −0.13, 95% CI −0.16 to −0.09) were associated with faster rate of cognitive decline compared to normal olfaction. On MRI, impaired olfaction (hyposmia or anosmia) was related to smaller volumes of the hippocampus (β = −0.19, 95% CI −0.33 to −0.05), and in the entorhinal (β = −0.16, 95% CI −0.24 to −0.08), fusiform (β = −0.45, 95% CI −0.78 to −0.14), and middle temporal (β = −0.38, 95% CI −0.72 to −0.01) cortices.

Conclusion

Impaired olfaction predicts faster cognitive decline and might indicate neurodegeneration in the brain among dementia-free older adults.

Smell training improves olfactory function and alters brain structure

Training and repeated exposure to odorants leads to enhanced olfactory sensitivity. So far, the efficacy of intensive olfactory training on olfactory function in a healthy population and its underlying neurobiological basis remain poorly known. This study investigated the effects of a 6-week intensive and well-controlled olfactory training on olfactory function and brain structure/neuroplasticity. Thirty-six healthy young individuals were recruited and randomly distributed in three groups: (1) 12 participants underwent daily intensive olfactory training of at least 20 min that included an (a) odor intensity classification task, an (b) odor quality classification task and an (c) target odor detection task, (2) 12 participants underwent an equivalent visual control training, and (3) 12 control individuals did not participate in any training. Before and after the training period, all participants performed a series of olfactory tests and those from groups 1 and 2 underwent structural magnetic resonance (MR) imaging, from which we obtained measures such as cortical thickness and tissue density. Participants improved in the respectively trained tasks throughout the 6-weeks training period. Those who underwent olfactory training improved general olfactory function compared to control participants, especially in odor identification, thus showing intramodal transfer. Further, MR imaging analysis revealed that olfactory training led to increased cortical thickness in the right inferior frontal gyrus, the bilateral fusiform gyrus and the right entorhinal cortex. This research shows that intensive olfactory training can generally improve olfactory function and that this improvement is associated with changes in the structure of olfactory processing areas of the brain.

Heightened olfactory dysfunction and oral irritation among chronic smokers and heightened propylthiouracil (PROP) bitterness among menthol smokers

Chronic cigarette smoking may influence chemosensory function, which in turn, may affect cigarette usage. Because menthol in cigarettes can attenuate nicotine bitterness, choice of menthol/nonmenthol cigarettes may be influenced by ability to perceive bitterness. We examined chemosensory function of chronic smokers, hypothesizing they would show altered function in comparison to non-smokers and by menthol cigarette preference. In laboratory-based measures, chronic smokers (N = 135; 84 menthol smokers) self-reported their chemosensory function and participated in smell (identification task with perceived intensity) and taste (quinine and NaCl intensity on tongue-tip and whole mouth) testing. A taste genetics probe (propylthiouracil (PROP) bitterness) also was assessed. Self-reported and measured chemosensory function were compared with nationally-representative 2013-2014 National Health and Nutrition Examination Survey (NHANES) data generated with similar measures. The taste measures also were compared between smokers and age- and sex-matched non-smokers from a laboratory database. Frequencies of self-reported smell and taste alterations among smokers exceeded NHANES prevalence estimates for non-smokers. The rate of measured smell dysfunction also exceeded NHANES prevalence for hyposmia. Compared to non-smokers, smokers reported elevated tongue-tip and whole mouth intensities from 1 M NaCl, with no significant differences in whole mouth quinine or 0.32 M NaCl. Inconsistent with previous hypotheses, smokers were not more likely to report depressed PROP bitterness than non-smokers. However, as expected, menthol smokers reported greater PROP bitterness than non-menthol smokers. In conclusion, chemosensory alterations were more frequent among chronic smokers, including hyposmia and heightened intensity from NaCl at an oral-irritant concentration. PROP supertasters were most likely to prefer mentholated cigarettes.

An intrinsic association between olfactory identification and spatial memory in humans

It was recently proposed that olfaction evolved to aid navigation. Consistent with this hypothesis, olfactory identification and spatial memory are linked to overlapping brain areas which include the orbitofrontal cortex and hippocampus. However, the relationship between these two processes has never been specifically investigated. Here, we show that olfactory identification covaries with spatial memory in humans. We also found that the cortical thickness of the left medial orbitofrontal cortex, and the volume of the right hippocampus, predict both olfactory identification and spatial memory. Finally, we demonstrate deficits in both olfactory identification and spatial memory in patients with lesions of the medial orbitofrontal cortex. Our findings reveal an intrinsic relationship between olfaction and spatial memory that is supported by a shared reliance on the hippocampus and medial orbitofrontal cortex. This relationship may find its roots in the parallel evolution of the olfactory and hippocampal systems.

Olfaction, the sense of smell, may have originally evolved to aid navigation in space, but there is no direct evidence of a link between olfaction and navigation in humans. Here the authors show that olfaction and spatial memory abilities are correlated and rely on similar brain regions in humans.

Alzheimer's disease and alpha-synuclein pathology in the olfactory bulbs of infants, children, teens and adults ≤ 40 years in Metropolitan Mexico City. APOE4 carriers at higher risk of suicide accelerate their olfactory bulb pathology

There is growing evidence that air pollution is a risk factor for a number of neurodegenerative diseases, most notably Alzheimer's (AD) and Parkinson's (PD). It is generally assumed that the pathology of these diseases arises only later in life and commonly begins within olfactory eloquent pathways prior to the onset of the classical clinical symptoms. The present study demonstrates that chronic exposure to high levels of air pollution results in AD- and PD-related pathology within the olfactory bulbs of children and relatively young adults ages 11 months to 40 years. The olfactory bulbs (OBs) of 179 residents of highly polluted Metropolitan Mexico City (MMC) were evaluated for AD- and alpha-synuclein-related pathology. Even in toddlers, hyperphosphorylated tau (hTau) and Lewy neurites (LN) were identified in the OBs. By the second decade, 84% of the bulbs exhibited hTau (48/57), 68% LNs and vascular amyloid (39/57) and 36% (21/57) diffuse amyloid plaques. OB active endothelial phagocytosis of red blood cell fragments containing combustion-derived nanoparticles (CDNPs) and the neurovascular unit damage were associated with myelinated and unmyelinated axonal damage. OB hTau neurites were associated mostly with pretangle stages 1a and 1b in subjects ≤ 20 years of age, strongly suggesting olfactory deficits could potentially be an early guide of AD pretangle subcortical and cortical hTau. APOE4 versus APOE3 carriers were 6-13 times more likely to exhibit OB vascular amyloid, neuronal amyloid accumulation, alpha-synuclein aggregates, hTau neurofibrillary tangles, and neurites. Remarkably, APOE4 carriers were 4.57 times more likely than non-carriers to die by suicide. The present findings, along with previous data that over a third of clinically healthy MMC teens and young adults exhibit low scores on an odor identification test, support the concept that olfactory testing may aid in identifying young people at high risk for neurodegenerative diseases. Moreover, results strongly support early neuroprotective interventions in fine particulate matter (PM_2.5) and CDNP's exposed individuals ≤ 20 years of age, and the critical need for air pollution control.

Environmental Enrichment and Successful Aging

The human brain sustains a slow but progressive decline in function as it ages and these changes are particularly profound in cognitive processing. A potential contributor to this deterioration is the gradual decline in the functioning of multiple sensory systems and the effects they have on areas of the brain that mediate cognitive function. In older adults, diminished capacity is typically observed in the visual, auditory, masticatory, olfactory, and motor systems, and these age-related declines are associated with both a decline in cognitive proficiency, and a loss of neurons in regions of the brain. We will review how the loss of hearing, vision, mastication skills, olfactory impairment, and motoric decline accompany cognitive loss, and how improved functioning of these systems may aid in the restoration of the cognitive abilities in older adults. The human brain appears to require a great deal of stimulation to maintain its cognitive efficacy as people age and environmental enrichment may aid in its maintenance and recovery.

Olfactory Dysfunction in Neurodegenerative Diseases

PURPOSE OF REVIEW

The sense of smell is today one of the focuses of interest in aging and neurodegenerative disease research. In several neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease, the olfactory dysfunction is one of the initial symptoms appearing years before motor symptoms and cognitive decline, being considered a clinical marker of these diseases' early stages and a marker of disease progression and cognitive decline. Overall and under the umbrella of precision medicine, attention to olfactory function may help to improve chances of success for neuroprotective and disease-modifying therapeutic strategies.

RECENT FINDINGS

The use of olfaction, as clinical marker for neurodegenerative diseases is helpful in the characterization of prodromal stages of these diseases, early diagnostic strategies, differential diagnosis, and potentially prediction of treatment success. Understanding the mechanisms underlying olfactory dysfunction is central to determine its association with neurodegenerative disorders. Several anatomical systems and environmental factors may underlie or contribute to olfactory loss associated with neurological diseases, although the direct biological link to each disorder remains unclear and, thus, requires further investigation. In this review, we describe the neurobiology of olfaction, and the most common olfactory function measurements in neurodegenerative diseases. We also highlight the evidence for the presence of olfactory dysfunction in several neurodegenerative diseases, its value as a clinical marker for early stages of the diseases when combined with other clinical, biological, and neuroimage markers, and its role as a useful symptom for the differential diagnosis and follow-up of disease. The neuropathological correlations and the changes in neurotransmitter systems related with olfactory dysfunction in the neurodegenerative diseases are also described.

Neuroimaging abnormalities in individuals exhibiting Parkinson's disease risk markers

BACKGROUND

The concept of prodromal Parkinson's disease (PD) involves variable combinations of nonmotor features and subtle motor abnormalities as a result of ongoing neurodegeneration in the brain stem including substantia nigra (SN) and abnormal findings upon transcranial sonography and nuclear imaging. Except for nuclear imaging, the predictive value of risk markers for the conversion to overt PD is low.

OBJECTIVE

The objective of this study was to determine whether PD risk markers are associated with changes in brain structure and to what extent cognitive changes are risk markers for PD.

METHODS

Diffusion-weighted imaging, voxel-based morphometry, and cortical thickness analysis was performed in 29 individuals with hyposmia and/or an increased SN hyperechogenicity (SN+) upon transcranial sonography and 28 controls without these 2 risk markers. Classical parkinsonian signs were an exclusion criterion. All of the participants underwent a neuropsychological test battery addressing executive functions, learning ability, and verbal fluency.

RESULTS

In the PD risk group, diffusion-weighted imaging mean diffusivity was increased in 4 left hemisphere clusters (posterior thalamus, inferior longitudinal fasciculus, fornix, corticospinal tract). A negative relationship of mean diffusivity and smell function was present for the posterior thalamus and the corticospinal tract. There was a significant correlation of mean diffusivity values and SN+ in all clusters. Neither voxel-based morphometry nor cortical thickness analysis revealed any group differences. No relevant group differences were observed for cognitive tests included.

CONCLUSION

PD-free individuals with PD risk markers show microstructural changes of the white matter, including areas relevant for motor and limbic processes. In addition, our study provides for the first time a neuroanatomical correlate for SN hyperechogenicity. © 2018 International Parkinson and Movement Disorder Society.

The relationship between smell identification and neuropsychological domains: Results from a sample of community-dwelling adults suspected of dementia

OBJECTIVE

This study aimed to test the theoretical relationships between smell identification and cognitive tasks based on existing neuroimaging and anatomical findings.

METHOD

Utilizing data collected from a memory assessment clinic, theory-derived mediation and moderation models were tested. The sample used in this study consisted of 103 (39 male, 64 female) individuals referred for memory assessments. The sample's mean education was 12.4 years (SD = 3.2), and the mean age of the sample was 77.2 years (SD = 6.3).

RESULTS

The University of Pennsylvania Smell Identification Test (UPSIT) was a significant, partial mediator of the relationship between the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) Immediate and Delayed Memory indexes. Olfactory identification did not mediate list learning and retrieval; however, olfactory identification was a significant partial mediator of the relationship between story encoding and later memory retrieval of the story. Olfactory identification also fully mediated the relationship between a visuospatial construction task and its reconstruction from memory after a short delay. The relationship between processing speed and the olfactory identification was significantly mediated by semantic memory. Finally, the UPSIT moderated the relationship between a measure of premorbid ability, the Wechsler Test of Adult Reading, and current global cognitive functioning.

CONCLUSIONS

Our results support theoretical relationships between olfaction and neuropsychological domains. Additionally, our results suggest that the UPSIT may serve as a proxy for cerebral integrity and is likely related to the duration of neurodegeneration.

Relationship between smell identification testing and the neuropsychological assessment of dementia in community-dwelling adults

The objective of this research was to investigate the relationship of the University of Pennsylvania Smell Identification Test (UPSIT) with neuropsychological tests and identify the utility of the UPSIT in detecting cognitive impairment. This research was an archival study of neuropsychological test results obtained from 70 clients (30 male/40 female) of a community-based memory clinic. The sample had an average age of 69.7 (SD = 9.7) and education of 14.6 (SD = 2.8) years. Hypotheses were tested using correlations, receiver operating characteristic (ROC) curves, and logistic regression. The UPSIT showed significant, weak to moderate correlations with neuropsychological tests. The UPSIT raw score correlated significantly with all but one cognitive ability domain. The UPSIT T-score was significantly correlated with all cognitive domains. Obtained areas under the ROC curve (AUC) for the UPSIT ranged from .60 to .87. The AUCs of the UPSIT did not differ significantly from verbal semantic fluency tests, but the Repeatable Battery for the Assessment of Neuropsychological Status Total Scale and Delayed Memory index tended to produce larger AUCs than the UPSIT. Results from step-wise logistic regressions suggest that the UPSIT raw score provides unique information beyond its relationship to age. Olfaction relates broadly to cognitive ability and may be sensitive to early symptoms of cognitive decline. Further research is needed to explore the relationships between smell identification tests and neuropsychological assessment.