Olfactory Function is Associated with Cognitive Performance: Results of the Heinz Nixdorf Recall Study

The current status and challenges of olfactory dysfunction study in Alzheimer's Disease

Olfactory functioning involves multiple cognitive processes and the coordinated actions of various neural systems. Any disruption at any stage of this process may result in olfactory dysfunction, which is consequently widely used to predict the onset and progression of diseases, such as Alzheimer's Disease (AD). Although the underlying mechanisms have not yet been fully unraveled, apparent changes were observed in olfactory brain areas form patients who suffer from AD by means of medical imaging and electroencephalography (EEG). Olfactory dysfunction holds significant promise in detecting AD during the preclinical stage preceding mild cognitive impairment (MCI). Owing to the strong specificity, olfactory tests are prevalently applied for screening in community cohorts. And combining olfactory tests with other biomarkers may further establish an optimal model for AD prediction in studies of specific olfactory dysfunctions and improve the sensitivity and specificity of early AD diagnosis.

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.

The Association of Allergy-Related and Non-Allergy-Related Olfactory Impairment with Cognitive Function in Older Adults: Two Cross- Sectional Studies

BACKGROUND

Evidence on the association of Olfactory Impairment (OI) with age-related cognitive decline is inconclusive, and the potential influence of allergy remains unclear.

OBJECTIVE

We aimed to evaluate the cross-sectional associations of allergy-related and non-allergy- related OI to cognitive function.

METHODS

We included 2,499 participants from the Health and Retirement Study (HRS)-Harmonized Cognitive Assessment Protocol (HCAP) sub-study and 1,086 participants from the English Longitudinal Study of Ageing (ELSA)-HCAP. The Olfactory Function Field Exam (OFFE) using Sniffin' Stick odor pens was used to objectively assess olfactory function and an olfactory score <6/11 indicated OI. Mini-Mental Status Examination (MMSE) was used to assess global cognitive function and define cognitive impairment (<24/30). A neuropsychologic battery was used to assess five cognitive domains.

RESULTS

Compared to non-OI participants, individuals with OI had lower MMSE z-score [βHRS = -0.33, 95% Confidence Interval (CI): -0.41 to -0.24; βELSA = -0.31, -0.43 to -0.18] and higher prevalence of cognitive impairment (Prevalence Ratio (PR)HRS = 1.46, 1.06 to 2.01; PRELSA = 1.63, 1.26 to 2.11). The associations were stronger for non-allergy-related OI (βHRS = -0.36; βELSA = -0.34) than for allergy-related OI (βHRS = -0.26; βELSA = 0.13). Similar associations were observed with domain- specific cognitive function measures.

CONCLUSION

OI, particularly non-allergy-related OI, was related to poorer cognitive function in older adults. Although the current cross-sectional study is subject to several limitations, such as reverse causality and residual confounding, the findings will provide insights into the OI-cognition association and enlighten future attention to non-allergy-related OI for the prevention of potential cognitive impairment.

Olfactory function, neurofilament light chain, and cognitive trajectory: A 12-year follow-up of the Shanghai Aging Study

This study aimed to determine whether blood neurofilament light chain (NfL) modifies the association of olfactory dysfunction (OD) with long-term cognitive decline. A total of 1125 non-demented older adults in the Shanghai Aging Study were evaluated for baseline olfaction (12-item Sniffin' Sticks Smell Test) and cognitive trajectory by a 12-year follow-up. Baseline blood NfL was quantified using Single Molecular Array assay, and dichotomized into low and high levels based on the median value of concentration. The Mini-Mental State Examination (MMSE) and Telephone Interview for Cognitive Status-40 were used to assess participants' cognitive function. Cognitive decline was ascertained when dementia was diagnosed or documented in the medical record during follow-up, or the MMSE declining rate (slope) was 1.0 SD larger than the group mean. OD participants presented a steeper trajectory of MMSE score (p interaction = 0.004) and a high risk of cognitive decline (adjusted HR [95% CI], 1.82 [1.11, 2.98]) only in those with high NfL. Participants with combined OD and high NfL showed the highest risk of cognitive decline (adjusted HR, 2.43 [1.20, 4.92]). OD, especially in combination with high blood NfL concentration, may be able to identify individuals who later incur cognitive deterioration.

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.

Association Between Olfactory Test Data with Multiple Levels of Odor Intensity and Suspected Cognitive Impairment: A Cross-Sectional Study

BACKGROUND

Olfactory function decline has recently been reported to be associated with a risk of cognitive impairment. Few population-based studies have included younger adults when examining the association between olfactory test data with multiple odor intensities and suspected cognitive impairment.

OBJECTIVE

We investigated the association between high-resolution olfactory test data with fewer odors and suspected cognitive impairments. We also examined the differences between older and younger adults in this association.

METHODS

The Japanese version of the Montreal Cognitive Assessment (MoCA-J) was administered to 1,450 participants, with three odor-intensity-level olfactometry using six different odors. Logistic regressions to discriminate suspected cognitive impairment were conducted to examine the association, adjusted for age, sex, education duration, and smoking history. Data were collected from the Program by Tohoku University Tohoku Medical Megabank Organization, with an additional olfactory test conducted between 2019 and 2021.

RESULTS

We generally observed that the lower the limit of distinguishable odor intensity was, the higher the MoCA-J score was. The combination of spearmint and stuffy socks contributed most to the distinction between suspected and unsuspected cognitive impairment. Furthermore, the association was significant in women aged 60-74 years (adjusted odds ratio 0.881, 95% confidence interval [0.790, 0.983], p = 0.024).

CONCLUSIONS

The results indicate an association between the limit of distinguishable odor intensity and cognitive function. The olfactory test with multiple odor intensity levels using fewer odors may be applicable for the early detection of mild cognitive impairment, especially in older women aged 60-74 years.

Fluctuation of depressive symptoms in cognitively unimpaired participants and the risk of mild cognitive impairment 5 years later: Results of the Heinz Nixdorf Recall study

BACKGROUND

Depression might be an independent risk factor for cognitive decline, a prodromal dementia symptom or a reaction to cognitive/functional impairment.

OBJECTIVE

To investigate the association between (1) depressive symptoms and (2) depressive symptom patterns over 13 years with incident mild cognitive impairment (MCI) 5 years later.

MATERIALS AND METHODS

We included 724/823 cognitively unimpaired men/women who participated in the population-based Heinz Nixdorf Recall study (t1: 2005-2008, ø62.9 years; t2: 2010-2015, ø68.1 years). Depressive symptoms were assessed in the study center and during six postal follow-ups using the short form of the Center for Epidemiologic Studies Depression Scale (CES-D). Relative risks (RR; 95% confidence intervals) for MCI at t2 (men/women: 71/76) were estimated for CES-D at t1 (linear and dichotomized at ≥17, cut-off for clinically relevant depressive symptoms) and CES-D fluctuations over 13 years (stable low, large fluctuations, stable high/stable around cut-off) using log-linear regression models with Poisson working likelihood adjusted for age, sex, education, diabetes mellitus, coronary heart disease, and stroke.

RESULTS

Fully adjusted risk for MCI at t2 (per CES-D point increase at t1) was elevated for the total cohort (1.053, 1.031-1.076), men (1.046, 1.012-1.081), and women (1.059, 1.029-1.090). Applying the dichotomized CES-D, risk for MCI was substantially increased for the total cohort [2.22 (1.38-3.58)] and in women [2.59 (1.46-4.58)]. Large CES-D fluctuations and stable high/stable around cut-off were associated with increased RR for MCI in the total cohort and in women compared to stable low symptoms.

CONCLUSION

Depressive symptoms predicted MCI in cognitively unimpaired participants of our population-based study. Adequate treatment of depression may therefore contribute to the maintenance of normal cognition and delay dementia onset.

Klotho KL-VS haplotype does not improve cognition in a population-based sample of adults age 55-87 years

The heterozygous human Klotho KL-VS haplotype has been associated with improved cognitive performance but results are inconsistent. Here we assessed Klotho KL-VS haplotype and cognition using data from the third examination of the population-based Heinz Nixdorf Recall Study. We analyzed cognition tests (immediate and delayed word list, Trail-Making Test [TMT] part A and B, Maze test, interference condition of the Stroop color-word test, verbal fluency) and their associations with Klotho KL-VS haplotype. The Klotho KL-VS haplotype is classified by the V-allele at SNP rs9536314 (F352V) and the S-allele at SNP rs9527025 (C370S). Heterozygotes for the KL-VS haplotype were compared with non-carriers. Analyses were performed in 1812 subjects (55-87 years). We found consistent but only slightly lower performance in heterozygous carriers of the KL-VS haplotype in all tasks with Z-scores ranging between Z = - 0.042 (verbal fluency) and - 0.17 (TMT part A). Differences between carriers and non-carriers were similar for men and women for all tests but TMT part B (interaction contrast = 8.4 s (95% CI - 2.3; 19.1)). While cognition declined with age, we found an effect modification by age (55-65 years, 66-75 years, > 75 years). In the 66-75 years KL-VS heterozygous age group, lower performance was seen in memory, visual attention and motor speed. Contrary to our hypothesis, heterozygous carriers of the KL-VS haplotype did not show enhanced performance in cognitive tests in our study.

Olfaction and Aging: A Review of the Current State of Research and Future Directions

Olfaction, the sense of smell, is characterized by a notable age-dependency such that aging individuals are more likely to have poor olfactory abilities. These impairments are considered to be mostly irreversible and as having potentially profound effects on quality of life and food behavior, as well as constituting warning signs of mortality, cognitive dysfunction, and dementia. Here, we review the current state of research on aging and olfaction, focusing on five topics which we regard to be of particular relevance for the field: nutrition and health, cognition and dementia, mortality, environment and genetics, and training-based enhancement. Under each of these headlines, we provide a state-of-the-art overview and discuss gaps in our knowledge which might be filled by further research. Understanding how olfactory abilities are diminished in aging, and how they may be alleviated or recovered, involves a set of challenging tasks for researchers in the years to come.

Trail Making Test Normative Data for the German Older Population

OBJECTIVE

We provide normative data for the Trail Making Test (TMT)-A and B and the derived scores B - A and B/A, for the German general population aged 57-84 years.

METHODS

Data were derived from the third examination of the population-based Heinz Nixdorf Recall study. We excluded participants with a history of dementia or stroke, a depression score above cutoff (CES-D Center for Epidemiologic Studies Depression Scale score ≥ 18), or mild cognitive impairment according to a neurocognitive test battery. The normative sample (n = 2,182) was stratified by age, using the interval superposition approach, and by three levels of educational attainment (up to 10 years of schooling; >10 years of schooling; and university degree).

RESULTS

We tabulated test performance scores at percentage rank thresholds 5, 10, 15, 20, 25, 50, 75, and 90. In multiple linear regression, TMT-A performance declined by 1 s each year of life, and TMT-B performance by 3 s; educational level had an impact of up to 30 s in TMT-B. TMT-B/A was only weakly associated with age and education. TMT-B and B - A correlated r = 0.96. Completion of the TMT-B within the time limit of 300 s was not achieved by 10.9% of participants >74 years, and especially by those >74 years who were on the lowest educational level (13.9%).

CONCLUSIONS

For TMT-A, TMT-B, and B - A, the narrow age categorization and distinction between three educational levels proved meaningful. The 300 s limit for the TMT-B impedes the identification of thresholds for very low performance in this age group and needs reconsideration.

Is There a Shared Etiology of Olfactory Impairments in Normal Aging and Neurodegenerative Disease?

As we age, our olfactory function declines. In addition to occurring in normal aging, more rapid decrement of olfactory decline has been associated with several neurodegenerative diseases including Alzheimer's disease (AD) and Parkinson's disease (PD). It has been argued that since olfactory deficits occur less frequently or are absent in diseases such as progressive supranuclear palsy, corticobasal degeneration, and multiple system atrophy, olfactory deficits can be used for differential diagnoses of AD and PD. The purpose of this review is to provide a survey of current knowledge about the molecular bases and differential patterns of olfactory deficits present in normal aging, AD, and PD. As substantial research has been conducted in this area, the majority of the content of this review focuses on articles published in the past decade. We hypothesize that olfactory deficits in normal aging, AD, and PD may have different underlying causes, and propose the use of model organisms with small, tractable nervous systems and/or easy to manipulate genomes to further investigate the cellular mechanisms responsible for these deficits.

Predictive Value of Odor Identification for Incident Dementia: The Shanghai Aging Study

OBJECTIVE

This study aimed to evaluate the value of odors in the olfactory identification (OI) test and other known risk factors for predicting incident dementia in the prospective Shanghai Aging Study.

METHODS

At baseline, OI was assessed using the Sniffin' Sticks Screening Test 12, which contains 12 different odors. Cognition assessment and consensus diagnosis were conducted at both baseline and follow-up to identify incident dementia. Four different multivariable logistic regression (MLR) models were used for predicting incident dementia. In the no-odor model, only demographics, lifestyle, and medical history variables were included. In the single-odor model, we further added one single odor to the first model. In the full model, all 12 odors were included. In the stepwise model, the variables were selected using a bidirectional stepwise selection method. The predictive abilities of these models were evaluated by the area under the receiver operating characteristic curve (AUC). The permutation importance method was used to evaluate the relative importance of different odors and other known risk factors.

RESULTS

Seventy-five (8%) incident dementia cases were diagnosed during 4.9 years of follow-up among 947 participants. The full and the stepwise MLR model (AUC = 0.916 and 0.914, respectively) have better predictive abilities compared with those of the no- or single-odor models. The five most important variables are Mini-Mental State Examination (MMSE) score, age, peppermint detection, coronary artery disease, and height in the full model, and MMSE, age, peppermint detection, stroke, and education in the stepwise model. The combination of only the top five variables in the stepwise model (AUC = 0.901 and sensitivity = 0.880) has as a good a predictive ability as other models.

CONCLUSION

The ability to smell peppermint might be one of the useful indicators for predicting dementia. Combining peppermint detection with MMSE, age, education, and history of stroke may have sensitive and robust predictive value for dementia in older adults.

Associations between self-reported sleep characteristics and incident mild cognitive impairment: The Heinz Nixdorf Recall Cohort Study

Associations of sleep characteristics with mild cognitive impairment (MCI) have been examined in cross-sectional, but rarely in longitudinal studies. Incident MCI and sleep characteristics were assessed in 1,890 participants of the first and second follow-up of the Heinz Nixdorf Recall study, a population-based cohort study in Germany (age at first follow-up 50-80 years, mean follow-up 5.2 years). MCI was assessed with extensive cognitive tests. Sleep questionnaires including PSQI (Pittsburgh Sleep Quality Index) were used to assess sleep quality, sleep disturbances, time asleep, and time in bed. Relative risks (RR) of developing MCI when exposed to sleep characteristics were assessed in regression models adjusted for sociodemographic and cardiovascular risk factors. Poor sleep quality (PSQI > 5) (RR = 1.43, 95% CI: 1.12-1.82, fully adjusted, reference: PSQI ≤ 5) and difficulties initiating sleep (almost nightly versus never) (RR = 1.40, 0.94-2.08) were associated with incident MCI. For time in bed, the risk of MCI was increased for ≤ 5 hours (RR = 2.86, 1.24─6.60, reference:7 to <8 hours). In this longitudinal study with older participants, MCI risk was increased in persons with poor sleep quality, difficulties initiating sleep, and short time in bed.

Olfactory function is associated with cognitive performance: results from the population-based LIFE-Adult-Study

Background

Studies in older adults or those with cognitive impairment have shown associations between cognitive and olfactory performance, but there are few population-based studies especially in younger adults. We therefore cross-sectionally analyzed this association using data from the population-based LIFE-Adult-Study.

Methods

Cognitive assessments comprised tests from the Consortium to Establish a Registry for Alzheimer’s Disease (CERAD): verbal fluency (VF), word list learning and recall (WLL, WLR), and the Trail Making Tests (TMT) A and B. The “Sniffin’ Sticks Screening 12” test was used to measure olfactory performance. Linear regression analyses were performed to determine associations between the number of correctly identified odors (0 to 12) and the five cognitive test scores, adjusted for sex, age, education, and the presence of depressive symptoms. Receiver operating characteristic (ROC) analysis was carried out to determine the discriminative performance of the number of correctly identified odors regarding identification of cognition impairment.

Results

A total of 6783 participants (51.3% female) completed the olfaction test and the VF test and TMT. A subgroup of 2227 participants (46.9% female) also completed the WLL and WLR tests. Based on age-, sex-, and education-specific norms from CERAD, the following numbers of participants were considered cognitively impaired: VF 759 (11.2%), WLL 242 (10.9%), WLR: 132 (5.9%), TMT-A 415 (6.1%), and TMT-B/A ratio 677 (10.0%). On average, score values for VF were higher by 0.42 points (p < 0.001), for WLL higher by 0.32 points (p = 0.001), for WLR higher by 0.31 points (p = 0.002), for TMT-A lower by 0.25 points (p < 0.001), and for TMT-B/A ratio lower by 0.01 points (p < 0.001) per number of correctly identified odors. ROC analysis revealed area under the curve values from 0.55 to 0.62 for the five cognitive tests.

Conclusions

Better olfactory performance was associated with better cognitive performance in all five tests in adults — adjusted for age, sex, education, and the presence of depressive symptoms. However, the ability of the smell test to discriminate between individuals with and without cognitive impairment was limited. The value of olfactory testing in early screening for cognitive impairment should be investigated in longitudinal studies.

Subjective cognitive decline, APOE ε4, and incident mild cognitive impairment in men and women

Introduction

Possible joint effects of subjective cognitive decline (SCD) and apolipoprotein E (APOE) ε4 genotype on incident mild cognitive impairment (MCI) were examined for men and women separately.

Methods

Cognitively normal participants with and without SCD were included from the first follow-up examination of the population-based Heinz Nixdorf Recall study. Sex-stratified logistic regression models estimated main effects and interactions (additive, multiplicative) of SCD at the first follow-up (yes+/no−) and APOE ε4 (positive+/negative−) groups for MCI 5 years later.

Results

Odds for MCI 5 years later were higher in SCD/APOE ε4 group +/+ than the sum of groups +/− and −/+ in women, with a trend for positive interaction. Odds for incident MCI in men was highest in group +/−, with no interaction effect.

Discussion

Our findings indicate that APOE ε4 may play an important role in the association of SCD and incident MCI, especially considering sex. Further studies need to examine these associations with larger sample sizes.