Odor identification as a biomarker of preclinical AD in older adults at risk

Odor identification dysfunction in late-life depression with suicidal ideation

BACKGROUND

Suicide is more prevalent among older adults compared to younger individuals. Late-life depression (LLD) poses the highest risk for suicide. However, early recognition of suicidal ideation is challenging. Dysfunction in odor identification (OI), a characteristic of LLD, may hold potential for early identification of suicidal ideation. This study aims to compare OI between LLD patients with suicidal ideation (LLD-S) and LLD patients without suicidal ideation (LLD-NS), and examine its relationship with cognitive function.

METHODS

A total of 262 LLD-NS patients, 63 LLD-S patients, and 316 healthy normal older adults (HOAs) underwent OI testing, standardized clinical interviews, and comprehensive neuropsychological assessments.

RESULTS

(1) LLD-S patients exhibited lower OI scores and poorer cognitive performance (including global cognition, information processing speed, memory, language, executive function, and visuospatial ability) compared to LLD-NS patients and HOAs. (2) There were interactive effects between suicidal ideation and OI dysfunction, leading to lower scores in information processing speed and visuospatial ability. (3) OI dysfunction mediated the differences in cognition between the LLD-NS and LLD-S groups.

LIMITATIONS

The present study was a cross-sectional design.

CONCLUSIONS

LLD-S patients had worse odor identification than LLD-NS patients and HOAs, suggesting that OI testing could be a valuable approach for identifying suicidal ideation in LLD and screening for suicide risk. The presence of both OI impairment and suicidal ideation was associated with poorer cognitive performance in LLD.

AROMHA Brain Health Test: A Remote Olfactory Assessment as a Screen for Cognitive Impairment

Cost-effective, noninvasive screening methods for preclinical Alzheimer's disease (AD) and other neurocognitive disorders remain an unmet need. The olfactory neural circuits develop AD pathological changes prior to symptom onset. To probe these vulnerable circuits, we developed the digital remote AROMHA Brain Health Test (ABHT), an at-home odor identification, discrimination, memory, and intensity assessment. The ABHT was self-administered among cognitively normal (CN) English and Spanish speakers (n=127), participants with subjective cognitive complaints (SCC; n=34), and mild cognitive impairment (MCI; n=19). Self-administered tests took place remotely at home under unobserved (among interested CN participants) and observed modalities (CN, SCC, and MCI), as well as in-person with a research assistant present (CN, SCC, and MCI). Olfactory performance was similar across observed and unobserved remote self-administration and between English and Spanish speakers. Odor memory, identification, and discrimination scores decreased with age, and olfactory identification and discrimination were lower in the MCI group compared to CN and SCC groups, independent of age, sex, and education. The ABHT revealed age-related olfactory decline, and discriminated CN older adults from those with cognitive impairment. Replication of our results in other populations would support the use of the ABHT to identify and monitor individuals at risk for developing dementia.

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.

SARS-CoV-2-Induced Type I Interferon Signaling Dysregulation in Olfactory Networks Implications for Alzheimer's Disease

Type I interferon signaling (IFN-I) perturbations are major drivers of COVID-19. Dysregulated IFN-I in the brain, however, has been linked to both reduced cognitive resilience and neurodegenerative diseases such as Alzheimer's. Previous works from our group have proposed a model where peripheral induction of IFN-I may be relayed to the CNS, even in the absence of fulminant infection. The aim of our study was to identify significantly enriched IFN-I signatures and genes along the transolfactory route, utilizing published datasets of the nasal mucosa and olfactory bulb amygdala transcriptomes of COVID-19 patients. We furthermore sought to identify these IFN-I signature gene networks associated with Alzheimer's disease pathology and risk. Gene expression data involving the nasal epithelium, olfactory bulb, and amygdala of COVID-19 patients and transcriptomic data from Alzheimer's disease patients were scrutinized for enriched Type I interferon pathways. Gene set enrichment analyses and gene-Venn approaches were used to determine genes in IFN-I enriched signatures. The Agora web resource was used to identify genes in IFN-I signatures associated with Alzheimer's disease risk based on its aggregated multi-omic data. For all analyses, false discovery rates (FDR) <0.05 were considered statistically significant. Pathways associated with type I interferon signaling were found in all samples tested. Each type I interferon signature was enriched by IFITM and OAS family genes. A 14-gene signature was associated with COVID-19 CNS and the response to Alzheimer's disease pathology, whereas nine genes were associated with increased risk for Alzheimer's disease based on Agora. Our study provides further support to a type I interferon signaling dysregulation along the extended olfactory network as reconstructed herein, ranging from the nasal epithelium and extending to the amygdala. We furthermore identify the 14 genes implicated in this dysregulated pathway with Alzheimer's disease pathology, among which HLA-C, HLA-B, HLA-A, PSMB8, IFITM3, HLA-E, IFITM1, OAS2, and MX1 as genes with associated conferring increased risk for the latter. Further research into its druggability by IFNb therapeutics may be warranted.

Associations between olfactory dysfunction and cognition: a scoping review

INTRODUCTION

Strong evidence suggests that olfactory dysfunction (OD) can predict additional neurocognitive decline in neurodegenerative conditions such as Alzheimer's and Parkinson's diseases. However, research exploring olfaction and cognition in younger populations is limited. The aim of this review is to evaluate cognitive changes among non-elderly adults with non-COVID-19-related OD.

METHODS

We performed a structured comprehensive literature search of PubMed, Ovid Embase, Web of Science, and Cochrane Library in developing this scoping review. The primary outcome of interest was the association between OD and cognitive functioning in adults less than 60 years of age.

RESULTS

We identified 2878 studies for title and abstract review, with 167 undergoing full text review, and 54 selected for data extraction. Of these, 34 studies reported on populations of individuals restricted to the ages of 18-60, whereas the remaining 20 studies included a more heterogeneous population with the majority of individuals in this target age range in addition to some above the age of 60. The etiologies for smell loss among the included studies were neuropsychiatric disorders (37%), idiopathic cause (25%), type 2 diabetes (7%), trauma (5%), infection (4%), intellectual disability (4%), and other (18%). Some studies reported numerous associations and at times mixed, resulting in a total number of associations greater than the included number of 54 studies. Overall, 21/54 studies demonstrated a positive association between olfaction and cognition, 7/54 demonstrated no association, 25/54 reported mixed results, and only 1/54 demonstrated a negative association.

CONCLUSION

Most studies demonstrate a positive correlation between OD and cognition, but the data are mixed with associations less robust in this young adult population compared to elderly adults. Despite the heterogeneity in study populations and outcomes, this scoping review serves as a starting point for further investigation on this topic. Notably, as many studies in this review involved disorders that may have confounding effects on both olfaction and cognition, future research should control for these confounders and incorporate non-elderly individuals with non-psychiatric causes of smell loss.

Odor identification score as an alternative method for early identification of amyloidogenesis in Alzheimer's disease

A simple screening test to identify the early stages of Alzheimer's disease (AD) is urgently needed. We investigated whether odor identification impairment can be used to differentiate between stages of the A/T/N classification (amyloid,  tau, neurodegeneration) in individuals with amnestic mild cognitive impairment or AD and in healthy controls. We collected data from 132 Japanese participants visiting the Toranomon Hospital dementia outpatient clinic. The odor identification scores correlated significantly with major neuropsychological scores, regardless of apolipoprotein E4 status, and with effective cerebrospinal fluid (CSF) biomarkers [amyloid β 42 (Aβ42) and the Aβ42/40 and phosphorylated Tau (p-Tau)/Aβ42 ratios] but not with ineffective biomarkers [Aβ40 and the p-Tau/total Tau ratio]. A weak positive correlation was observed between the corrected odor identification score (adjusted for age, sex, ApoE4 and MMSE), CSF Aβ42, and the Aβ42/40 ratio. The odor identification score demonstrated excellent discriminative power for the amyloidogenesis stage , according to the A/T/N classification, but was unsuitable for differentiating between the p-Tau accumulation and the neurodegeneration stages. After twelve odor species were analyzed, a version of the score comprising only four odors-India ink, wood, curry, and sweaty socks-proved highly effective in identifying AD amyloidogenesis, showing promise for the screening of preclinical AD.

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.

Preliminary study on early diagnosis of Alzheimer's disease in APP/PS1 transgenic mice using multimodal magnetic resonance imaging

Alzheimer's disease (AD) has an insidious onset and lacks clear early diagnostic markers, and by the time overt dementia symptoms appear, the disease is already in the mid-to-late stages. The search for early diagnostic markers of AD may open a critical window for Alzheimer's treatment and facilitate early intervention to slow the progression of AD. In this study, we aimed to explore the imaging markers for early diagnosis of AD through the combined application of structural magnetic resonance imaging (sMRI), resting-state functional magnetic resonance imaging (rs-fMRI), and 1H-magnetic resonance spectroscopy (1H-MRS) multimodal magnetic resonance imaging (MRI) techniques at the animal experimental level, with the aim to provide a certain reference for early clinical diagnosis of AD. First, sMRI scans were performed on 4-month-old amyloid beta precursor protein/presenilin 1 (APP/PS1) transgenic AD model mice and wild type mice of the same litter using a 7.0 T animal MRI scanner to analyze the differential brain regions with structural changes in the gray matter of the brain by voxel-based morphometry (VBM). Next, rs-fMRI scans were performed to analyze the differential brain regions between groups for local spontaneous brain activity and functional connectivity (FC) between brain regions. Finally, 1H-MRS scans were performed to quantify and analyze intergroup differences in the relative concentrations of different metabolites within regions of interest (cortex and hippocampus). Compared with wild type mice, the volume of the left hippocampus, and right olfactory bulb of APP/PS1 transgenic AD model mice were reduced, the functional activity of the bilateral hippocampus, right piriform cortex and right caudate putamen was reduced, the functional network connectivity of the hippocampus was impaired, and the relative content of N-acetylaspartate (NAA)in the hippocampus was decreased. In addition, this study found that imaging changes in olfactory-related brain regions were closely associated with AD diagnosis, and these findings may provide some reference for the early diagnosis of AD.

The first quick olfactory test specific for Alzheimer's disease and French culture

PURPOSE

Olfactory identification disorder is considered a promising early biomarker of Alzheimer's disease (AD). The QUICK TODA2 can be used as a short olfactory screening tool specific for French AD patients. The selection of AD specific odorants and the design of this screening were the main objectives of this twofold study.

METHODS

In study 1, the TODA2 olfactory test was administered to 43 mild-AD patients and 45 healthy controls (HC) in five memory centres in France. The selection of AD specific odorants was based on the differences in the proportion of correct answers and in the threshold means between AD and HC groups. In study 2, another set of 19 mild-AD patient were included at the memory centre of Nice Hospital. All participants completed the olfactory assessment pipeline including the QUICK TODA2, TODA2 and Sniffin' Sticks Identification sub-Test (SST-i). The individual scores of the three tests were correlated.

RESULTS

In study 1, ten TODA2 odorants could significantly differentiate AD participants from controls. We selected the six most AD-sensitive items to design the QUICK TODA2. In study 2, we reported strong significant correlations between QUICK TODA2 and TODA2 (ρ(17) = 0.68, p = 0.001**), SST-i and QUICK TODA2 (ρ(17) = 0.65, p = 0.002**), SST-i and TODA2 (ρ(17) = 0.57, p = 0.01*).

CONCLUSION

 QUICK TODA2 is a 5-min non-invasive olfactory AD screening tool dedicated to French culture. Its results converge with those of longer, validated olfactory tests. It could be used as a quick screening tool in the general daily practice before an extensive assessment in memory centres.

Cerebral hypometabolism mediates the effect of stroke volume on cognitive impairment in heart failure patients

AIMS

The present study aimed to phenotype the cerebral structural and glucose metabolic alterations in patients with heart failure (HF) using simultaneous positron emission tomography (PET)/magnetic resonance (MR) and to investigate their relationship to cardiac biomarkers and cognitive performance.

METHODS AND RESULTS

Forty-two HF patients caused by ischaemic heart disease (mean age 67.2 ± 10.4, 32 males) and 32 age- and sex-matched healthy volunteers (mean age 61.3 ± 4.8, 18 males) were included in this study. Participants underwent simultaneous cerebral fluorine-18 (18 F) fluorodeoxyglucose PET/MR followed by cardiac MR scan, and neuropsychological scores were obtained to assess cognitive performance. The grey matter volume (GMV) and standardized uptake value ratio (SUVR) were calculated to examine cerebral structural and metabolic alterations. Cardiac biomarkers included cardiac MR parameters and cardiac serum laboratory tests. Mediation analysis was performed to explore the associations among cerebral alterations, cardiac biomarkers, and cognitive performance. HF patients demonstrated notable cognitive impairment compared with normal controls (P < 0.001). Furthermore, HF patients exhibited regional brain hypometabolism in the bilateral calcarine cortex, caudate nucleus, thalamus, hippocampus, precuneus, posterior cingulate cortex, lingual and olfactory cortex, and GMV reduction in bilateral thalamus and hippocampus (cluster level at P < 0.05, Gaussian random field correction). The SUVR of the hypometabolic brain regions was correlated with the Montreal Cognitive Assessment (MoCA) scores (r = 0.55, P = 0.038) and cardiac stroke volume (r = 0.49, P = 0.002). Cerebral hypometabolism played a key role in the relationship between the decreased stroke volume and MoCA scores, with a mediation effect of 33.2%.

CONCLUSIONS

HF patients suffered cerebral metabolic and structural alterations in regions associated with cognition. The observed correlation between cardiac stroke volume and cognitive impairment underscored the potential influence of cerebral hypometabolism, suggesting that cerebral hypometabolism due to chronic systemic hypoperfusion may significantly contribute to cognitive impairment in HF patients.

Evaluation of the NIH Toolbox Odor Identification Test across normal cognition, amnestic mild cognitive impairment, and dementia due to Alzheimer's disease

INTRODUCTION

Olfactory decline is associated with cognitive decline in aging, amnestic mild cognitive impairment (aMCI), and amnestic dementia associated with Alzheimer's disease neuropathology (ADd). The National Institutes of Health Toolbox Odor Identification Test (NIHTB-OIT) may distinguish between these clinical categories.

METHODS

We compared NIHTB-OIT scores across normal cognition (NC), aMCI, and ADd participants (N = 389, ≥65 years) and between participants positive versus negative for AD biomarkers and the APOE ε4 allele.

RESULTS

NIHTB-OIT scores decreased with age (p < 0.001) and were lower for aMCI (p < 0.001) and ADd (p < 0.001) compared to NC participants, correcting for age and sex. The NIHTB-OIT detects aMCI (ADd) versus NC participants with 49.4% (56.5%) sensitivity and 88.8% (89.5%) specificity. NIHTB-OIT scores were lower for participants with positive AD biomarkers (p < 0.005), but did not differ based on the APOE ε4 allele (p > 0.05).

DISCUSSION

The NIHTB-OIT distinguishes clinically aMCI and ADd participants from NC participants.

HIGHLIGHTS

National Institutes of Health Toolbox Odor Identification Test (NIHTB-OIT) discriminated normal controls from mild cognitive impairment. NIHTB-OIT discriminated normal controls from Alzheimer's disease dementia. Rate of olfactory decline with age was similar across all diagnostic categories. NIHTB-OIT scores were lower in participants with positive Alzheimer's biomarker tests. NIHTB-OIT scores did not differ based on APOE genotype.

A Review of the Role of Estrogens in Olfaction, Sleep and Glymphatic Functionality in Relation to Sex Disparity in Alzheimer's Disease

Several risk factors contribute to the development of Alzheimer's disease (AD), including genetics, metabolic health, cardiovascular history, and diet. It has been observed that women appear to face a higher risk of developing AD. Among the various hypotheses surrounding the gender disparity in AD, one pertains to the potential neuroprotective properties of estrogen. Compared to men, women are believed to be more susceptible to neuropathology due to the significant decline in circulating estrogen levels following menopause. Studies have shown, however, that estrogen replacement therapies in post-menopausal women do not consistently reduce the risk of AD. While menopause and estrogen levels are potential factors in the elevated incidence rates of AD among women, this review highlights the possible roles estrogen has in other pathways that may also contribute to the sex disparity observed in AD such as olfaction, sleep, and glymphatic functionality.

Periphery Biomarkers Predicting Conversion of Type 2 Diabetes to Pre-Alzheimer-Like Cognitive Decline: A Multicenter Follow-Up Study

Background

The prevalence of Alzheimer's disease (AD) is increasing, therefore, identifying biomarkers to predict those vulnerable to AD is imperative. Type 2 diabetes (T2D) serves as an independent risk factor for AD. Early prediction of T2D patients who may be more susceptible to AD, so as to achieve early intervention, is of great significance to reduce the prevalence of AD.

Objective

To establish periphery biomarkers that could predict conversion of T2D into pre-AD-like cognitive decline.

Methods

A follow-up study was carried out from 159 T2D patients at baseline. The correlations of cognitive states (by MMSE score) with multi-periphery biomarkers, including APOE genotype, plasma amyloid-β level, platelet GSK-3β activity, and olfactory score were analyzed by logistic regression. ROC curve was used for establishing the prediction model. Additionally, MRI acquired from 38 T2D patients for analyzing the correlation among cognitive function, biomarkers and brain structure.

Results

Compared with the patients who maintained normal cognitive functions during the follow-up period, the patients who developed MCI showed worse olfactory function, higher platelet GSK-3β activity, and higher plasma Aβ42/Aβ40 ratio. We conducted a predictive model which T2D patients had more chance of suffering from pre-AD-like cognitive decline. The MRI data revealed MMSE scores were positively correlated with brain structures. However, platelet GSK-3β activity was negatively correlated with brain structures.

Conclusions

Elevated platelet GSK-3β activity and plasma Aβ42/Aβ40 ratio with reduced olfactory function are correlated with pre-AD-like cognitive decline in T2D patients, which used for predicting which T2D patients will convert into pre-AD-like cognitive decline in very early stage.

Human-derived air-liquid interface cultures decipher Alzheimer's disease-SARS-CoV-2 crosstalk in the olfactory mucosa

BACKGROUND

The neurological effects of the coronavirus disease of 2019 (COVID-19) raise concerns about potential long-term consequences, such as an increased risk of Alzheimer's disease (AD). Neuroinflammation and other AD-associated pathologies are also suggested to increase the risk of serious SARS-CoV-2 infection. Anosmia is a common neurological symptom reported in COVID-19 and in early AD. The olfactory mucosa (OM) is important for the perception of smell and a proposed site of viral entry to the brain. However, little is known about SARS-CoV-2 infection at the OM of individuals with AD.

METHODS

To address this gap, we established a 3D in vitro model of the OM from primary cells derived from cognitively healthy and AD individuals. We cultured the cells at the air-liquid interface (ALI) to study SARS-CoV-2 infection under controlled experimental conditions. Primary OM cells in ALI expressed angiotensin-converting enzyme 2 (ACE-2), neuropilin-1 (NRP-1), and several other known SARS-CoV-2 receptor and were highly vulnerable to infection. Infection was determined by secreted viral RNA content and confirmed with SARS-CoV-2 nucleocapsid protein (NP) in the infected cells by immunocytochemistry. Differential responses of healthy and AD individuals-derived OM cells to SARS-CoV-2 were determined by RNA sequencing.

RESULTS

Results indicate that cells derived from cognitively healthy donors and individuals with AD do not differ in susceptibility to infection with the wild-type SARS-CoV-2 virus. However, transcriptomic signatures in cells from individuals with AD are highly distinct. Specifically, the cells from AD patients that were infected with the virus showed increased levels of oxidative stress, desensitized inflammation and immune responses, and alterations to genes associated with olfaction. These results imply that individuals with AD may be at a greater risk of experiencing severe outcomes from the infection, potentially driven by pre-existing neuroinflammation.

CONCLUSIONS

The study sheds light on the interplay between AD pathology and SARS-CoV-2 infection. Altered transcriptomic signatures in AD cells may contribute to unique symptoms and a more severe disease course, with a notable involvement of neuroinflammation. Furthermore, the research emphasizes the need for targeted interventions to enhance outcomes for AD patients with viral infection. The study is crucial to better comprehend the relationship between AD, COVID-19, and anosmia. It highlights the importance of ongoing research to develop more effective treatments for those at high risk of severe SARS-CoV-2 infection.

Differences in olfactory functional connectivity in early-onset depression and late-onset depression

Background

Late-onset depression (LOD) and early-onset depression (EOD) exhibit different pathological mechanisms and clinical phenotypes, including different extents of olfactory dysfunction. However, the brain abnormalities underlying the differences in olfactory dysfunction between EOD and LOD remain unclear.

Objective

The aim of this study was to compare the functional connectivity (FC) patterns of olfactory regions between EOD patients and LOD patients and examine their relationship with cognitive function.

Methods

One hundred and five patients with EOD, 101 patients with LOD and 160 normal controls (NCs) were recruited for the present study. Participants underwent clinical assessment, olfactory testing, cognitive assessments, and magnetic resonance imaging. Eight regions of the primary and secondary olfactory regions were selected to investigate olfactory FC.

Results

Patients with LOD exhibited decreased odor identification (OI) compared with patients with EOD and NCs. The LOD group exhibited decreased FC compared with the EOD and NC groups when primary and secondary olfactory regions were selected as the regions of interest (the piriform cortex, lateral entorhinal cortex, and orbital-frontal cortex). Additionally, these abnormal olfactory FCs were associated with decreased cognitive function scores and OI, and the FC between the left orbital-frontal cortex and left amygdala was a partial mediator of the relationship between global cognitive scores and OI.

Conclusion

Overall, patients with LOD exhibited decreased FC in both the primary and secondary olfactory cortices compared with patients with EOD, and abnormal olfactory FC was associated with OI dysfunction and cognitive impairment. The FC between the orbital-frontal cortex and amygdala mediated the relationship between global cognitive function and OI.

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).

Smaller grey matter volume in the central olfactory system in mild cognitive impairment

One of the major challenges in the diagnosis of Alzheimer's disease (AD) is to increase the specificity of the early diagnosis. While episodic memory impairment is a sensitive AD marker, other measures are needed to improve diagnostic specificity. A promising biomarker might be a cerebral atrophy of the central olfactory processing areas in the early stages of the disease since an impairment of olfactory identification is present at the clinical stage of AD. Our goal was therefore, (1) to evaluate the grey matter volume (GMV) of central olfactory processing regions in prodromal AD and (2) to assess its association with episodic memory. We included 34 cognitively normal healthy controls (HC), 92 individuals with subjective cognitive decline (SCD), and 40 with mild cognitive impairment (MCI). We performed regions of interest analysis (ROI) using two different approaches, allowing to extract GMV from (1) atlas-based anatomical ROIs and from (2) functional and non-functional subregions of these ROIs (olfactory ROIs and non-olfactory ROIs). Participants with MCI exhibited smaller olfactory ROIs GMV, including significant reductions in the piriform cortex, amygdala, entorhinal cortex, and left hippocampus compared to other groups (p ≤ 0.05, corrected). No significant effect was found regarding anatomical or non-olfactory ROIs GMV. The left hippocampus olfactory ROI GMV was correlated with episodic memory performance (p < 0.05 corrected). Limbic/medial-temporal olfactory processing areas are specifically atrophied at the MCI stage, and the degree of atrophy might predict cognitive decline in AD early stages.

Association of Performance on Olfactory and Cognitive Screening Tests With Conversion to Dementia in a Biracial Cohort of Older Adults

BACKGROUND AND OBJECTIVES

Odor identification deficits are associated with transition to dementia, whereas intact odor identification and global cognition test performance may identify lack of transition. The purpose of this study was to examine intact odor identification and global cognition as prognostic indicators of lack of transition to dementia in a biracial (Black and White) cohort.

METHODS

In a community-dwelling sample of older adults from the Health, Aging, and Body Composition study, odor identification was measured using the Brief Smell Identification Test (BSIT), and global cognition was measured using the Teng Modified Mini-Mental State Examination (3MS). Survival analyses for dementia transition over 4 and 8 years of follow-up used Cox proportional hazards models.

RESULTS

A total of 2,240 participants had an average age of 75.5 years (SD 2.8). Approximately 52.7% were female individuals. Approximately 36.7% were Black and 63.3% were White individuals. Impaired odor identification (hazard ratio [HR] 2.29, 95% CI 1.79-2.94, p < 0.001) and global cognition (HR 3.31, 95% CI 2.26-4.84, p < 0.001) were each independently associated with transition to dementia (n = 281). Odor identification remained robustly associated with transition to dementia for Black (HR 2.02, 95% CI 1.36-3.00, p < 0.001, n = 821) and White participants (HR 2.45, 95% CI 1.77-3.38, p < 0.001, n = 1,419), whereas global cognition was associated with transition among Black participants only (HR 5.06, 95% CI 3.18-8.07, p < 0.001). ApoE genotype was consistently associated with transition among White participants only (HR 1.75, 95% CI 1.20-2.54, p < 0.01). Among participants with intact performance on both odor identification (BSIT ≥9/12 correct) and global cognition (3MS ≥ 78/100 correct), 8.8% transitioned to dementia over 8 years. Intact performance on both measures had high positive predictive value for identifying individuals who did not transition to dementia over 4 years (0.98 for ages 70-75 years with only 2.3% transitioning, 0.94 for ages 76-82 years with only 5.8% transitioning).

DISCUSSION

Odor identification testing paired with a global cognitive screening test identified individuals at low risk of transition to dementia in a biracial community cohort with a pronounced effect in the eighth decade of life. Identification of such individuals can reduce the need for extensive investigation to establish a diagnosis. Odor identification deficits showed utility in both Black and White participants, unlike the race-dependent utility of a global cognitive test and ApoE genotype.

Olfactory Dysfunction Is Associated with Cerebral Amyloid Deposition and Cognitive Function in the Trajectory of Alzheimer's Disease

Olfactory dysfunction is consistently observed in individuals with Alzheimer's disease (AD), but its association with beta-amyloid (Aβ) deposition remains unclear. This study aimed to investigate the relationship among olfactory function, cerebral Aβ deposition, and neuropsychological profiles in individuals with no cognitive impairment (NCI), mild cognitive impairment (MCI), and AD dementia. A total of 164 participants were included, and olfactory function was assessed using the brief smell identification test (B-SIT). Cerebral Aβ deposition was measured using [18F]-flutemetamol PET imaging (A-PET). The results show a significant group difference in olfactory function, with the highest impairment observed in the Aβ-positive MCI and AD dementia groups, and the impairment was the lowest in Aβ-negative NCI. Olfactory dysfunction was positively associated with cognitive impairments across multiple domains. Furthermore, individuals with Aβ deposition had lower olfactory function compared to those without Aβ, even within the same neuropsychological stage. The association between olfactory dysfunction and Aβ deposition was observed globally and in specific cortical regions. These findings suggest that olfactory dysfunction is associated with both cognitive function and cerebral Aβ pathology in the trajectory of AD. Olfactory deficits may serve as an additional marker for disease progression and contribute to understanding the underlying mechanisms of AD.

Gender-Related Differences in the Correlation between Odor Threshold, Discrimination, Identification, and Cognitive Reserve Index in Healthy Subjects

BACKGROUND

Many studies suggested that olfactory function could be associated with semantic memory, executive function, and verbal fluency. However, the gender-related association between olfactory function and the cognitive domain is not well investigated. The aim of this study was to estimate gender-related differences in the relationship between olfactory function and each specific cognitive domain of the Cognitive Reserve Index (CRI) questionnaire, such as education, working activity, and leisure time in healthy subjects.

METHODS

Two hundred and sixty-nine participants were recruited (158 women and 111 men), with a mean age of 48.1 ± 18.6 years. The CRI questionnaire and Sniffin' Sticks test were used to evaluate the cognitive reserve and the olfactory function, respectively.

RESULTS

In all subjects, significant associations between the odor threshold versus CRI-Education, between the odor discrimina-tion and identification versus CRI-Working activity and CRI-Leisure Time, were found. In women, odor threshold, discrimination, and identification were associated with CRI-Leisure Time, while in men, only a significant association between odor threshold and CRI-Education was observed.

CONCLUSIONS

Our data, showing significant gender-related associations between olfactory function and CRI scores, suggested the use of olfactory evaluation and cognitive reserve as an important screening tool for the early detection of mild cognitive impairment.

Assessing how Age, Sex, Race, and Education Affect the Relationships Between Cognitive Domains and Odor Identification

BACKGROUND

The associations between cognitive domains and odor identification are well established, but how sociodemographic variables affect these relationships is less clear.

PURPOSE

Using the survey-adapted Montreal Cognitive Assessment instrument (MoCA-SA), we assess how age, sex, race, and education shape these relationships.

METHODS

We first used cluster analysis and multidimensional scaling to empirically derive distinct cognitive domains from the MoCA-SA as it is unclear whether the MoCA-SA can be disaggregated into cognitive domains. We then used ordinal logistic regression to test whether these empirically derived cognitive domains were associated with odor identification and how sociodemographic variables modified these relationships.

STUDY POPULATION

Nationally representative sample of community-dwelling US older adults.

RESULTS

We identified 5 out of the 6 theoretical cognitive domains, with the language domain unable to be identified. Odor identification was associated with episodic memory, visuospatial ability, and executive function. Stratified analyses by sociodemographic variables reveal that the associations between some of the cognitive domains and odor identification varied by age, sex, or race, but not by education.

CONCLUSIONS

These results suggest that (1) the MoCA-SA can be used to identify cognitive domains in survey research and (2) the performance of smell tests as a screener for cognitive decline may potentially be weaker in certain subpopulations.

Association between cognition and olfaction-specific parameters in patients with chronic rhinosinusitis

BACKGROUND

Patients with chronic rhinosinusitis (CRS) have reported significantly cognitive and olfactory dysfunction. This study aimed to explore the relationship between cognitive function and olfaction-specific parameters in patients with CRS.

METHODS

A cross-sectional survey method was used to investigate 98 participants, including 75 patients with CRS and 23 healthy controls. Cognitive function and psychophysical olfactory tests were performed. Olfactory cleft endoscopy scale and olfactory cleft computed tomography (CT) scores were obtained. Multivariate logistic regression was used to analyze the risk factors of Mild Cognitive Impairment (MCI) in patients with CRS.

RESULTS

There are significant differences in age, Montreal Cognitive Assessment (MoCA) scores, number of MCI, Lund-Mackay olfactory cleft (LM-OC) score, and blood eosinophil count between CRS with and without olfactory dysfunction groups (all P < 0.05). Total MoCA scores were positively correlated with thresholds-discrimination-identification (TDI) score (r = 0.541, P < 0.001), olfactory threshold (OT) (r = 0.440, P < 0.001), olfactory discrimination (OD) (r = 0.541, P < 0.001), and olfactory identification (OI) (r = 0.382, P = 0.001) scores. Furthermore, total MoCA scores were negatively correlated with LM-OC scores (r =  - 0.351, P = 0.002). After adjusting for patient demographics, only the OD score was an independent risk factor for MCI among patients with CRS (odds ratio = 0.792; P = 0.039). The OD scores less than 11.5 were the best predictor of MCI in patients with CRS.

CONCLUSION

Olfaction-specific clinical parameters were highly correlated with cognitive function in patients with CRS and the OD score was an independent risk factor for MCI in patients with CRS.

Neurons, Nose, and Neurodegenerative Diseases: Olfactory Function and Cognitive Impairment

Olfactory capacity declines with aging, but increasing evidence shows that smell dysfunction is one of the early signs of prodromal neurodegenerative diseases such as Alzheimer's and Parkinson's disease. The study of olfactory ability and its role in neurodegenerative diseases arouses much interest in the scientific community. In neurology, olfactory impairment is a potential early marker for the onset of neurodegenerative diseases, but the underlying mechanism is poorly understood. The loss of smell is considered a clinical sign of early-stage disease and a marker of the disease's progression and cognitive impairment. Highlighting the importance of biological bases of smell and molecular pathways could be fundamental to improve neuroprotective and therapeutic strategies. We focused on the review articles and meta-analyses on olfactory and cognitive impairment. We depicted the neurobiology of olfaction and the most common olfactory tests in neurodegenerative diseases. In addition, we underlined the close relationship between the olfactory and cognitive deficit due to nasal neuroepithelium, which is a direct extension of the CNS in communication with the external environment. Neurons, Nose, and Neurodegenerative diseases highlights the role of olfactory dysfunction as a clinical marker for early stages of neurodegenerative diseases when it is associated with molecular, clinical, and neuropathological correlations.

Synergistic effects of hIAPP and Aβ1-42 impaired the olfactory function associated with the decline of adult neurogenesis in SVZ

According to many in the field，the prevalence of Alzheimer's disease (AD) in type II diabetes (T2DM) populations is considerably higher than that in the normal population. Human islet amyloid polypeptide (hIAPP) is considered to be a common risk factor for T2DM and AD. Preliminary observations around T2DM animal model show that the decrease of adult neural stem cells (NSCs) in the subventricular zone (SVZ) is accompanied by olfactory dysfunction. Furthermore, impaired olfactory function could serve as to an early predictor of neurodegeneration，which is associated with cognitive impairment. However, the synergistic effects between hIAPP and amyloid-beta (Aβ) _1-42 in the brain and the neurodegeneration remains to be further clarified. In this study, olfactory capacity, synaptic density, status of NSC in SVZ, and status of newborn neurons in olfactory bulb (OB) were assessed 6 months after stereotactic injection of oligomer Aβ_1-42 into the dens gyrus (DG) of hIAPP-/+ mice or wild-type homogenous mice. Our results set out that Aβ₄₂ and amylin co-localized into OB and raised Aβ₄₂ deposition in hIAPP^-/+ mice compared with wild-type brood mice. In addition, 6 months after injection of Aβ_1-42 in hIAPP^-/+ mice, these mice showed increased olfactory dysfunction, significant loss of synapses, depletion of NSC in SVZ, and impaired cell renewal in OB. Our present study suggested that the synergistic effects between hIAPP and Aβ_1-42 impairs olfactory function and was associated with decreased neurogenesis in adults with SVZ.

Olfactory Dysfunction and Incidence of Motoric Cognitive Risk Syndrome: A Prospective Clinical-Pathologic Study

BACKGROUND AND OBJECTIVES

To examine associations between olfactory dysfunction, Alzheimer disease (AD) pathology, and motoric cognitive risk syndrome (MCR), a predementia syndrome characterized by cognitive complaints and slow gait that is associated with risk for AD and other dementias.

METHODS

We conducted a retrospective analysis of a prospective cohort study to examine whether baseline olfactory function was associated with the risk of incident MCR in 1,119 adults aged 60 years and older (75.1% female). The association between performance on the Brief Smell Identification Test (BSIT) and incident MCR risk was computed using Cox models and reported as the hazard ratio (HR) with 95% CIs adjusted for demographic, comorbidity, and cognitive factors. Furthermore, we assessed the relationship between postmortem AD pathology and non-AD pathology and olfactory function at the time of MCR diagnosis using linear regression models adjusted for sex, education, age at death, and time from diagnosis to death.

RESULTS

There were 544 (48.6%) incident cases of MCR over a median follow-up of 3.94 years. Lower BSIT scores (poor olfaction) at baseline were associated with an increased risk of incident MCR (HR for a 1-point increase in BSIT score 0.92; 95% CI 0.88-0.96) in fully adjusted models. Those with hyposmia (scores of ≤8 on the BSIT) at baseline (26.6%) were at an increased risk of MCR (HR 1.44; 95% CI 1.19-1.74) compared with those with normal olfactory function. Higher levels of the composite measure of global AD pathology and presence of Lewy body pathology were associated with lower BSIT scores at the time of incident MCR diagnosis (n = 118). τ tangle density, a specific component of AD pathology, was inversely associated with olfactory function, and the correlation remained after controlling for mild cognitive impairment syndrome and the presence of Lewy body pathology.

DISCUSSION

The results provide evidence that olfactory dysfunction precedes MCR incidence and is related to Alzheimer pathology, providing a clinical approach to risk stratify and subtype MCR.

Particulate matter and Alzheimer's disease: an intimate connection

The environmental role in disease progression has been appreciated for decades; however, understanding how airborne toxicant exposure can affect organs beyond the lungs is an underappreciated area of scientific inquiry. Particulate matter (PM) includes various gases, liquids, and particles in suspension and is produced by industrial activities such as fossil fuel combustion and natural events including wildfires and volcanic eruptions. Although agencies have attempted to reduce acceptable airborne particulate levels, with urbanization and population growth, these policies have been only moderately effective in mitigating disease progression. A growing area of research is focused on the role of PM exposure in the progression of Alzheimer's disease (AD). This review will summarize the knowns and unknowns of this expanding field.

Enhancement of Impaired Olfactory Neural Activation and Cognitive Capacity by Liraglutide, but Not Dapagliflozin or Acarbose, in Patients With Type 2 Diabetes: A 16-Week Randomized Parallel Comparative Study

OBJECTIVE

The comparative neuroprotective effects of different antidiabetes drugs have not been characterized in randomized controlled trials. Here, we investigated the therapeutic effects of liraglutide, dapagliflozin, or acarbose treatment on brain functional alterations and cognitive changes in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

Thirty-six patients with type 2 diabetes inadequately controlled with metformin monotherapy were randomized to receive liraglutide, dapagliflozin, or acarbose treatment for 16 weeks. Brain functional MRI (fMRI) scan and a battery of cognitive assessments were evaluated pre- and postintervention in all subjects.

RESULTS

The 16-week treatment with liraglutide significantly enhanced the impaired odor-induced left hippocampal activation with Gaussian random field correction and improved cognitive subdomains of delayed memory, attention, and executive function (all P < 0.05), whereas dapagliflozin or acarbose did not. Structural equation modeling analysis demonstrated that such improvements of brain health and cognitive function could be partly ascribed to a direct effect of liraglutide on left hippocampal activation (β = 0.330, P = 0.022) and delayed memory (β = 0.410, P = 0.004) as well as to the metabolic ameliorations of reduced waist circumference, decreased body fat ratio, and elevated fasting insulin (all P < 0.05).

CONCLUSIONS

Our head-to-head study demonstrated that liraglutide enhanced impaired brain activation and restored impaired cognitive domains in patients with type 2 diabetes, whereas dapagliflozin and acarbose did not. The results expand the clinical application of liraglutide and provide a novel treatment strategy for individuals with diabetes and a high risk of cognitive decline.

Association of Olfactory Impairment With All-Cause Mortality: A Systematic Review and Meta-analysis

Importance

Olfactory impairment is highly prevalent and associated with multiple comorbidities, including neurodegenerative, cardiovascular, nutritional, and immune disorders. However, epidemiologic associations between olfactory impairment and mortality are discordant.

Objective

To systematically clarify the epidemiologic associations between olfactory impairment and mortality.

Data Sources

The PubMed, Embase, and Cochrane Library databases were searched from inception to August 13, 2021.

Study Selection

Two blinded reviewers selected observational studies published as full-length, English-language articles in peer-reviewed journals that reported the presence or severity of chronic olfactory impairment, whether objectively measured or self-reported, in association with any mortality estimate, among adults aged 18 years or older.

Data Extraction and Synthesis

Two reviewers independently extracted data, evaluated study bias using the Newcastle-Ottawa Scale, and appraised the quality of the evidence using the Grading of Recommendations Assessment, Development and Evaluation framework, following Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) and Meta-analysis of Observational Studies in Epidemiology (MOOSE) guidelines and a PROSPERO-registered protocol. Maximally adjusted estimates were pooled using mixed-effects models, heterogeneity was measured using I2 statistics, sources of heterogeneity were investigated using meta-regression and subgroup meta-analyses, and publication bias was qualitatively and quantitatively assessed.

Main Outcomes and Measures

Hazard ratios for all-cause mortality.

Results

One retrospective cohort study and 10 prospective cohort studies (with a total of 21 601 participants) from 1088 nonduplicated records were included. Ten studies had a low risk of bias, whereas 1 study had a moderate risk; exclusion of the latter did not alter conclusions. Nine studies were included in the meta-analysis. Olfactory loss was associated with a significantly higher pooled hazard of all-cause mortality (hazard ratio, 1.52; 95% CI, 1.28-1.80; I2 = 82%). Meta-regression sufficiently explained heterogeneity, with longer mean follow-up duration weakening the pooled association, accounting for 91.3% of heterogeneity. Self-reported and objective effect sizes were similar. Associations were robust to trim-and-fill adjustment and the Egger test for publication bias. The overall quality of evidence was moderate.

Conclusions and Relevance

The findings of this systematic review and meta-analysis suggest that olfactory impairment is associated with all-cause mortality and may be a marker of general health and biological aging. Further research is required to establish the underlying mechanisms and the scope for interventions.

Olfactory identification disorders due to Alzheimer's disease: A new test from France to Quebec

Olfactory identification disorder is regarded as an early marker of Alzheimer's disease (AD) and of similar diagnostic significance of biological or cognitive markers. Premature damage of the entorhinal olfactory cortex, the hippocampus and the orbitofrontal cortex characterize AD and suggest a specific impairment of olfactory identification. The use of psychophysical olfactory identification tests in clinical diagnostic practice is therefore strongly recommended, but not required. As these widespread tests are rarely used, an innovative test, adapted to this target group has been developed. It has been used and validated in a routine care protocol at different Memory Centers in France and in Quebec, Canada. A total of 157 participants were recruited: including 63 Alzheimer's patients and 94 healthy controls. The test was composed of 14 odorants diluted into 4 different concentrations. A computer interface generated randomization of 6 odors per participant and the automatic calculation of identification scores, of perceptual thresholds and of composite scores. All participants underwent a Mini Mental Scale Examination within the previous three months or on the same day of the olfactory test. The Alzheimer's patients had a score between 20 and 30 and healthy controls participants had a score above 28 without any loss of points on recalled items. The results show that our olfactory identification test is able to significantly differentiate Alzheimer's patients from healthy controls (p < 0.001), and to distinguish the French population tested from the Quebec population (p < 0.001). This study highlights an olfactory identification disorder as a target for early diagnosis of AD. Its cultural qualities make it a potential candidate for differentiated calibration between France and Quebec.

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.

Differences in Odor Identification in Early-Onset and Late-Onset Depression

(1) Background: Odor identification (OI) dysfunction is a potential predictor of developing dementia in late life depression (LLD). However, it is not clear whether patients with early onset depression (EOD) and late onset depression (LOD) may exhibit different OI dysfunctions. The aim of this study was to compare OI between EOD patients and LOD patients and its relationship with cognitive function. (2) Methods: A total of 179 patients with LLD and 189 normal controls were recruited. Participants underwent clinical assessment, olfactory testing, and comprehensive neuropsychological assessment. The OI scores of EOD patients and LOD patients were compared, and correlation analyses and mediation analyses were used to explore the relationship between OI and cognition. (3) Result: LOD patients exhibited lower OI scores than EOD patients and normal controls (NCs). Additionally, the LOD patients exhibited a higher percentage of OI dysfunction than the EOD patients. Moreover, OI scores were associated with global cognition, memory, language, and visuospatial ability in the EOD group (p < 0.05) but were not associated with any cognitive score in the LOD patients (p > 0.05). Finally, the scores of the Auditory Verbal Learning Test Immediate recall and Boston Naming Test exhibited a partially mediating effect on the difference in OI scores between the EOD and LOD patients. (4) Conclusions: LOD patients exhibited worse OI than EOD patients, and their difference in OI was mediated by their memory and language function.

Neuropsychiatric Symptoms Mediated the Relationship Between Odor Identification and Cognition in Alzheimer's Disease Spectrum: A Structural Equation Model Analysis

Background: Odor identification dysfunction is an early predictor of the development of Alzheimer's disease (AD), but neuropsychiatric symptoms (NPS), which are common in AD and mild cognitive impairment (MCI), are also associated with odor identification dysfunction. Whether NPS affect the specificity of using odor identification dysfunction to predict cognitive decline in AD and MCI remains unclear.

Methods: Patients (233 with MCI and 45 with AD) and 45 healthy controls (HCs) underwent assessments of odor identification (Sniffin' Sticks), NPS (Neuropsychiatric Inventory-12), and cognitive function (global cognition, memory, language, executive function, visual-spatial skill, and attention). Structural equation modeling (SEM) with bootstrapping estimation was conducted to explore the relationships between odor identification, NPS, and cognition.

Results: Patients with NPS showed significantly worse performance in odor identification and cognition than patients without NPS and HCs. The SEM showed odor identification to be positively associated with cognition, and cognition had special indirect effects on odor identification through affective and psychosis symptoms (two factors extracted from Neuropsychiatric Inventory-12). Additionally, affective and psychosis symptoms partially mediated the effect of cognition on odor identification.

Conclusion: Neuropsychiatric symptoms are associated with odor identification dysfunction in patients with AD and MCI. Studies exploring the relationship between odor identification dysfunction and cognitive decline in patients with AD and MCI should include an assessment of affective and psychosis symptoms, and adjust their confounding effects.

Olfactory dysfunction is associated with cognitive impairment in patients with obstructive sleep apnea: a cross-sectional study

PURPOSE

Obstructive sleep apnea (OSA) increases the risk for olfactory dysfunction. However, the relationship between olfactory function and cognition in OSA patients is unclear. The present study aimed to investigate the relationship between cognition and olfactory dysfunction (OD) in patients with OSA.

METHOD

This was a cross-sectional study in which 74 patients with OSA and 22 controls were recruited. All subjects completed polysomnography, Sniffin' Sticks, and -neurocognitive assessments. According to results of Sniffin' Sticks, OSA patients were divided into two groups: OSA with OD (53 cases) and OSA without OD (21 cases). Neurocognitive function was assessed by Montreal Cognitive Assessment (MoCA), Memory and Executive Screening (MES), and Shape Trail Test (STT). Cognition was compared between OSA with and without OD. Correlation between olfactory parameters and respiratory sleep parameters and neurocognitive assessments was analyzed.

RESULTS

Compared with OSA without OD, OSA with OD showed significantly decreased neurocognitive scores of MoCA (29-27 vs 27-23, p < 0.01), MES-5R (45-40.1 vs 43-33.5, p < 0.01) and increased consuming time of STT-B (91.66 vs 121.63, p < 0.01). A positive correlation was found between the scores of MoCA and MES-5R and all olfactory parameters. In addition, a negative correlation was present between the time consumed for STT-B and odor thresholds (r = - 0.344, p < 0.01), odor identification (r = - 0.335, p < 0.01), and threshold-discrimination-identification scores (r = - 0.448, p < 0.01).

CONCLUSION

Olfactory function is associated cognitive function in patients with OSA and may provide a new direction for early treatment interventions in OSA patients at risk for cognitive impairment.

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.

Interactive Effects of Agitation and Cognitive Impairment on Odor Identification in Patients With Late-Life Depression

BACKGROUND

Late-life depression (LLD) is a risk factor for cognitive decline in older adults, and odor identification (OI) deficits are an early indicator of cognitive decline with LLD. However, neuropsychiatric symptoms (NPSs) are common in LLD and are associated with OI deficits. In subjects with LLD, when OI deficits forecast cognitive decline, whether and how NPS affects the relationship between OI and cognition still must be further explored.

OBJECTIVE

To comprehensively explore the potential effects of various NPSs on the relationship between OI and cognition in participants with LLD.

METHODS

There were 167 patients with LLD and 105 normal elderly (NE) participants. The odor identification test (Sniffin' Sticks), cognitive function assessments (global cognition, memory, executive function, attention, language, visual space), and an NPS assessment (the neuropsychiatric inventory questionnaire) were performed on the subjects. In patients with LLD, the relationship among OI, cognition and NPSs was examined using correlation analysis and moderation analysis.

RESULTS

In patients with LLD, OI was positively correlated with cognition (global cognition, memory, executive function, attention, language) and negatively associated with NPSs (agitation and aberrant motor behavior). In NE group, OI was correlated with executive function. Moderation analysis showed that there was an interactive effect of agitation and cognitive impairment (language deficit or attention deficit) on OI in patients with LLD.

CONCLUSION

The coexistence of agitation and language or attention deficit was associated with worse OI in subjects with LLD. Agitation should be considered since OI predicts cognitive decline in patients with LLD.

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.

A potential biomarker of preclinical Alzheimer's disease: The olfactory dysfunction and its pathogenesis-based neural circuitry impairments

The olfactory dysfunction can signal and act as a potential biomarker of preclinical AD. However, the precise regulatory mechanism of olfactory function on the neural pathogenesis of AD is still unclear. The impairment of neural networks in olfaction system has been shown to be tightly associated with AD. As key brain regions of the olfactory system, the olfactory bulb (OB) and the piriform cortex (PCx) have a profound influence on the olfactory function. Therefore, this review will explore the mechanism of olfactory dysfunction in preclinical AD in the perspective of abnormal neural networks in the OB and PCx and their associated brain regions, especially from two aspects of aberrant oscillations and synaptic plasticity damages, which help better understand the underlying mechanism of olfactory neural network damages related to AD.

Increased oscillatory power in a computational model of the olfactory bulb due to synaptic degeneration

Several neurodegenerative diseases impact the olfactory system, and in particular the olfactory bulb, early in disease progression. One mechanism by which damage occurs is via synaptic dysfunction. Here, we implement a computational model of the olfactory bulb and investigate the effect of weakened connection weights on network oscillatory behavior. Olfactory bulb network activity can be modeled by a system of equations that describes a set of coupled nonlinear oscillators. In this modeling framework, we propagate damage to synaptic weights using several strategies, varying from localized to global. Damage propagated in a dispersed or spreading manner leads to greater oscillatory power at moderate levels of damage. This increase arises from a higher average level of mitral cell activity due to a shift in the balance between excitation and inhibition. That this shift leads to greater oscillations depends critically on the nonlinearity of the activation function. Linearized analysis of the network dynamics predicts when this shift leads to loss of oscillatory activity. We thus demonstrate one potential mechanism involved in the increased gamma oscillations seen in some animal models of Alzheimer's disease, and we highlight the potential that pathological olfactory bulb behavior presents as an early biomarker of disease.

ODor-evoked Autobiographical Memory in Alzheimer's disease?

OBJECTIVE

Because memory decline is the hallmark of Alzheimer's disease (AD), an important endeavor for both clinicians and researchers is to improve memory performances in AD. This can be pursued by olfactory stimulation of memory in patients with AD and by studying the effects of olfactory stimulation on autobiographical memory (i.e., memory for personal information). The effects of olfactory stimulation on autobiographical memory in patients with mild AD have been reported by recent research. We thus provide the first comprehensive overview of research on odor-evoked autobiographical memory in AD. We also establish the basis for solid theoretical analysis concerning the memory improvement reported by research on odor-evoked autobiographical memory in AD.

METHOD

We examined literature on odor-evoked autobiographical memories in AD and propose the "OdAMA" (Odor-evoked Autobiographical Memory in Alzheimer's disease) model.

RESULTS AND DISCUSSION

According to OdAMA model, odor exposure activates involuntary access to specific autobiographical memories, which promotes enhanced experience subjective of retrieval in patients with AD and improves their ability to construct not only recent and remote events but also future ones. The OdAMA model could serve as a guide for researchers and clinicians interested in odor-evoked autobiographical memory in AD.

Olfaction as an early marker of Parkinson's disease and Alzheimer's disease

Olfactory impairment is a common and early sign of Parkinson's disease (PD) and Alzheimer's disease (AD), the two most prevalent neurodegenerative conditions in the elderly. This phenomenon corresponds to pathologic processes emerging in the olfactory system prior to the onset of typical clinical manifestations. Clinically available tests can establish hyposmia through odor identification assessment, discrimination, and odor detection threshold. There are significant efforts to develop preventative or disease-modifying therapies that slow down or halt the progression of PD and AD. Due to the convenience and low cost of its assessment, olfactory impairment could be used in these studies as a screening instrument. In the clinical setting, loss of smell may also help to differentiate PD and AD from alternative causes of Parkinsonism and cognitive impairment, respectively. Here, we discuss the pathophysiology of olfactory dysfunction in PD and AD and how it can be assessed in the clinical setting to aid in the early and differential diagnosis of these disorders.

Handgrip strength as a moderator of the influence of age on olfactory impairment in US adult population ≥ 40 years of age

The aim of this study was to determine whether handgrip strength attenuates the negative relationship between age and olfactory function in a representative US population sample 40 years old and over. A cross-sectional study was performed with 2861 adults from the US National Health and Nutrition Examination Survey NHANES (2013–2014). An 8-item odor identification test was applied to determine olfactory function. Muscle strength was determined through a handgrip dynamometer (defined as the sum of the largest handgrip strength reading from right and left hands). Moderation analysis was performed to test whether the association between age and olfactory impairment was moderated by handgrip strength. Moderation analysis highlighted two regions of significance: the first region was found at < 56.6 kg, indicating that the adverse influence of age on olfactory function may be greater for the participants in this area; the second region was found at ≥ 56.6 kg, indicating that the negative impact of age on olfactory function disappeared for adults who were above this estimate point. In conclusion, handgrip strength, a general indicator of muscle strength, moderates the relationship between age and olfactory ability in a US adult population aged 40 years and older. Our findings are clinically relevant, since they emphasize the importance of muscular fitness in adulthood and old age by diminishing the deleterious effect of aging on olfactory performance.

Open science datasets from PREVENT-AD, a longitudinal cohort of pre-symptomatic Alzheimer's disease

To move Alzheimer Disease (AD) research forward it is essential to collect data from large cohorts, but also make such data available to the global research community. We describe the creation of an open science dataset from the PREVENT-AD (PResymptomatic EValuation of Experimental or Novel Treatments for AD) cohort, composed of cognitively unimpaired older individuals with a parental or multiple-sibling history of AD. From 2011 to 2017, 386 participants were enrolled (mean age 63 years old ± 5) for sustained investigation among whom 349 have retrospectively agreed to share their data openly. Repositories are findable through the unified interface of the Canadian Open Neuroscience Platform and contain up to five years of longitudinal imaging data, cerebral fluid biochemistry, neurosensory capacities, cognitive, genetic, and medical information. Imaging data can be accessed openly at https://openpreventad.loris.ca while most of the other information, sensitive by nature, is accessible by qualified researchers at https://registeredpreventad.loris.ca. In addition to being a living resource for continued data acquisition, PREVENT-AD offers opportunities to facilitate understanding of AD pathogenesis.

Altered White Matter Microstructures in Type 2 Diabetes Mellitus: A Coordinate-Based Meta-Analysis of Diffusion Tensor Imaging Studies

Objective

Type 2 diabetes mellitus (T2DM) is often accompanied by cognitive decline and depressive symptoms. Numerous diffusion tensor imaging (DTI) studies revealed microstructural white matter (WM) abnormalities in T2DM but the findings were inconsistent. The present study aimed to conduct a coordinate-based meta-analysis (CBMA) to identify statistical consensus of DTI studies in T2DM.

Methods

We performed a systematic search on relevant studies that reported fractional anisotropy (FA) differences between T2DM patients and healthy controls (HC). The anisotropic effect size seed-based d mapping (AES-SDM) approach was used to explore WM alterations in T2DM. A meta-regression was then used to analyze potential influences of sample characteristics on regional FA changes.

Results

A total of eight studies that comprised 245 patients and 200 HC, along with 52 coordinates were extracted. The meta-analysis identified FA reductions in three clusters including the left inferior network, the corpus callosum (CC), and the left olfactory cortex. Besides, FA in the CC was negatively correlated with body mass index (BMI) in the patients group.

Conclusions

T2DM could lead to subtle WM microstructural alterations, which might be associated with cognitive deficits or emotional distress symptoms. This provides a better understanding of the pathophysiology of neurodegeneration and complications in T2DM.

Systematic Review Registration

Registered at PROSPERO (http://www.crd.york.ac.uk/PROSPERO), registration number: CRD42020218737.

Narrative Review of Sensory Changes as a Biomarker for Alzheimer's Disease

Early recognition of Alzheimer's disease (AD) in the prodromal period has not been robust yet will be necessary if effective disease-modifying drugs are to be useful in preventing or delaying the condition. The objective of this narrative review was to describe the current, evidenced based understanding of alterations in sensory data as potential biomarkers for AD. Review of empirical studies that tested senses as biomarkers for AD and were published in English within the past 50 years was completed. Eighteen empirical studies were identified that met the strict criteria for inclusion, with 12 of these studies being related to the olfactory system. Two studies examined auditory, two examined vision, one examined proprioception, and one examined taste. Thus, only olfaction has been studied to any extent, leaving a clear gap in the literature for the use of other senses. A promising area of research has begun to be reported concerning differences in responses to pain stimuli in AD relative to cognitively normal subjects. Pain is not a single sense like the others but integrates several senses and may allow for use as an early biomarker for AD, as it integrates several brain areas and pathways. Unlike the other senses, simple devices can be used to measure changes in pain perception in cognitively normal adults with genetic predispositions for possible AD, making this potentially useful for clinicians in the future.

Clinical Implications of Psychophysical Olfactory Testing: Assessment, Diagnosis, and Treatment Outcome

Purpose of Review

Olfactory dysfunction dramatically impairs quality of life with a prevalence of 20% in the general adult population. Psychophysical olfactory testing has been widely used to evaluate the ability to smell due to its validated utility and feasibility in clinic. This review summarizes the current literature regarding psychophysical olfactory testing and the clinical relevance of the olfactory testing with different components. Furthermore, the review highlights the diagnosis and treatment value of olfactory subtests in patients with olfactory dysfunction.

Recent Findings

With the accumulation of studies of psychophysical olfactory testing in olfactory disorders, the clinical relevance of olfactory testing with different components is expanding. Different olfactory domains present with distinct olfactory processing and cortical activity. Psychophysical assessment of olfaction with three domains reveals different levels of olfactory processing and might assist with analyzing the pathophysiologic mechanism of the various olfactory disorders. Furthermore, olfactory thresholds provided the largest amount of non-redundant information to the olfactory diagnosis. Sinonasal olfactory dysfunction and non-sinonasal-related olfactory dysfunction are emerging classifications of smell disorders with certain characteristics of olfactory impairment and different responses to the therapy including steroids, sinus surgery, and olfactory training.

Summary

These recent advancements should promote the understanding of psychophysical olfactory testing, the association between individual subcomponents and neurophysiological processes, and pave the way for precision assessment and treatment of the olfactory dysfunction.

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.

Enhancing GABAergic signaling ameliorates aberrant gamma oscillations of olfactory bulb in AD mouse models

Background

Before the deposition of amyloid-beta plaques and the onset of learning memory deficits, patients with Alzheimer’s disease (AD) experience olfactory dysfunction, typified by a reduced ability to detect, discriminate, and identify odors. Rodent models of AD, such as the Tg2576 and APP/PS1 mice, also display impaired olfaction, accompanied by aberrant in vivo or in vitro gamma rhythms in the olfactory pathway. However, the mechanistic relationships between the electrophysiological, biochemical and behavioral phenomena remain unclear.

Methods

To address the above issues in AD models, we conducted in vivo measurement of local field potential (LFP) with a combination of in vitro electro-olfactogram (EOG), whole-cell patch and field recordings to evaluate oscillatory and synaptic function and pharmacological regulation in the olfactory pathway, particularly in the olfactory bulb (OB). Levels of protein involved in excitation and inhibition of the OB were investigated by western blotting and fluorescence staining, while behavioral studies assessed olfaction and memory function.

Results

LFP measurements demonstrated an increase in gamma oscillations in the OB accompanied by altered olfactory behavior in both APP/PS1 and 3xTg mice at 3–5 months old, i.e. an age before the onset of plaque formation. Fewer olfactory sensory neurons (OSNs) and a reduced EOG contributed to a decrease in the excitatory responses of M/T cells, suggesting a decreased ability of M/T cells to trigger interneuron GABA release indicated by altered paired-pulse ratio (PPR), a presynaptic parameter. Postsynaptically, there was a compensatory increase in levels of GABA_AR α1 and β3 subunits and subsequent higher amplitude of inhibitory responses. Strikingly, the GABA uptake inhibitor tiagabine (TGB) ameliorated abnormal gamma oscillations and levels of GABA_AR subunits, suggesting a potential therapeutic strategy for early AD symptoms. These findings reveal increased gamma oscillations in the OB as a core indicator prior to onset of AD and uncover mechanisms underlying aberrant gamma activity in the OB.

Conclusions

This study suggests that the concomitant dysfunction of both olfactory behavior and gamma oscillations have important implications for early AD diagnosis: in particular, awareness of aberrant GABAergic signaling mechanisms might both aid diagnosis and suggest therapeutic strategies for olfactory damage in AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00434-7.

The utility of olfactory function in distinguishing early-stage Alzheimer's disease from HIV-associated neurocognitive disorders

OBJECTIVES

Given the rising number of older people with HIV (PWH) and the overlap in cognitive dysfunction profiles in HIV-associated neurocognitive disorders (HAND) and Alzheimer's disease and its precursor, amnestic mild cognitive impairment (aMCI), methods are needed to distinguish aMCI/Alzheimer's disease from HAND. As an early indicator of Alzheimer's disease, we examined whether olfactory dysfunction could help to distinguish between aMCI/Alzheimer's disease and HAND among PWH.

DESIGN

An observational cohort study.

METHODS

Eighty-one older (≥50 years) PWH (83% men, 65% white) from the California NeuroAIDS Tissue Consortium completed the University of Pennsylvania Smell Identification Test (UPSIT; higher scores = better smell identification) and a comprehensive seven-domain neuropsychological test battery and neuromedical evaluation. HAND was classified via Frascati criteria. High aMCI risk was defined as impairment (>1.0 SD below normative mean) on two of four delayed recall or recognition outcomes (at least one recognition impairment required) from the Hopkins Verbal Learning Test-Revised and the Brief Visuospatial Memory Test-Revised. We examined UPSIT scores in relation to aMCI risk and HAND status, and continuous memory scores considering adjustments for demographics and relevant clinical or HIV disease characteristics.

RESULTS

Fifty-seven participants were classified with HAND (70%) and 35 participants were classified as high aMCI risk (43%). UPSIT scores were lower (worse) in the high versus low aMCI risk group [F (1,76) = 10.04, P = 0.002], but did not differ by HAND status [F (1,76) = 0.62, P = 0.43]. UPSIT scores positively correlated with all memory outcomes (Ps < 0.05).

CONCLUSION

Olfactory assessments may help in detecting early aMCI/Alzheimer's disease among PWH and allow for appropriate and early disease intervention.

Development of the Spanish Version of Sniffin's Sticks Olfactory Identification Test: Normative Data and Validity of Parallel Measures

The Sniffin' Sticks Olfactory Identification Test is a tool for measurement of olfactory performance developed in Germany and validated in several countries. This research aims to develop the Spanish version of the Sniffin' Sticks Olfactory Identification Test and obtain normative values for the Spanish population. The parameters are free recall and subjective intensity of odorants are included. The influence of possible demographic covariates such as sex, age, smoking, or educational level are analyzed, and the items that best discriminate are studied. In addition, the internal structure validity of the blue and purple versions is studied as a parallel measure, and a cultural adaptation of the purple version is carried out. For this, three independent samples of normosmic healthy volunteers were studied. To obtain normative values, the sample was of 417 participants (18-89 years). For the internal structure validity study of both versions, the sample was 226 (18-70 years), and for familiarity of the purple version, the sample was 75 participants (21-79 years). Results indicated that men and women and smokers and non-smokers perform equally. However, differences were found as age progresses, being more pronounced after 60 years old in all three measurements of the identification test. This research also provides the items that best discriminate in the blue version and a cultural adaptation for the purple version. In conclusion, the Sniffin' Sticks Odor Identification Test is a suitable tool for olfactory assessment in the Spanish population. The instrument has been expanded with two new scores, and normative data as a function of age are provided. Its parallel version also seems appropriate for testing, as items have been culturally adapted and evidence of internal structure validity for both versions is reported.