Variability and Coupling of Olfactory Identification and Episodic Memory in Older Adults

Poor olfaction prior to cardiac surgery: Association with cognition, plasma neurofilament light, and post-operative delirium

OBJECTIVES

Post-operative delirium (POD) affects up to 50% of cardiac surgery patients, with higher incidence in older adults. There is increasing need for screening tools that identify individuals most vulnerable to POD. Here, we examined the relationship between pre-operative olfaction and both incident POD and POD severity in patients undergoing cardiac surgery. We also examined cross-sectional relationships between baseline olfaction, cognition, and plasma neurofilament light (NfL).

METHODS

Individuals undergoing cardiac surgery (n = 189; mean age = 70 years; 75% men) were enrolled in a clinical trial of cerebral autoregulation monitoring. At baseline, odor identification performance (Brief Smell Identification Test), cognitive performance, and plasma concentrations of NfL levels (Simoa™ NF-Light Assay) were measured. Delirium was assessed with the Confusion Assessment Method (CAM) or CAM-ICU, and delirium severity was assessed using the Delirium Rating Scale-Revised-98. The association of baseline olfaction, delirium incidence, and delirium severity was examined in regression models adjusting for age, duration of cardiopulmonary bypass, logistic European System for Cardiac Operative Risk Evaluation (EuroSCORE), and baseline cognition.

RESULTS

Olfactory dysfunction was present in 30% of patients, and POD incidence was 44%. Pre-operative olfactory dysfunction was associated with both incident POD (OR = 3.17, p = 0.001) and greater severity of POD after cardiac surgery (OR = 3.94 p < 0.001) in models adjusted for age, duration of bypass, and a surgical risk score. The addition of baseline cognition attenuated the strength of the association, but it remained significant for incident POD (OR = 2.25, p = 0.04) and POD severity (OR 2.10, p = 0.04). Poor baseline olfaction was associated with greater baseline cognitive dysfunction (p < 0.001) and increased baseline plasma NfL concentrations (p = 0.04). Neither age, cognition, nor baseline NFL concentration modified the association of impaired olfaction and delirium outcomes.

CONCLUSIONS

Olfactory assessment may be a useful pre-surgical screening tool for the identification of patients undergoing cardiac surgery at increased risk of POD. Identifying those at highest risk for severe delirium and poor cognitive outcomes following surgery would allow for earlier intervention and pre-operative rehabilitation strategies, which could ultimately impact the functional disability and morbidity associated with POD.

Olfaction and declarative memory in aging: a meta-analysis

Olfactory and declarative memory performances are associated, as both functions are processed by overlapping medial-temporal and prefrontal structures and decline in older adults. While a decline in olfactory identification may be related to a decline in declarative memory, the relationship between olfactory detection threshold and declarative memory remains unclear. In this meta-analysis, we assessed (i) the relationship between olfactory identification/detection threshold and verbal declarative memory in cognitively normal older adults, and (ii) the effect of age on these relationships. We included articles from PsychNet, PubMed, and Academic Search Complete according to the following criteria: (i) inclusion of cognitively normal older adults; (ii) assessment of episodic or semantic memory; and (iii) assessment of olfactory identification or detection threshold. Seventeen studies and 22 effect sizes were eligible and included in this meta-analysis. Olfactory identification was associated with episodic (small effect size: r = 0.19; k = 22) and semantic memory (small effect size: r = 0.16; k = 23). Similarly, the olfactory detection threshold was associated with both episodic (small to medium effect size: r = 0.25; k = 5) and semantic memory (small effect size: r = 0.17; k = 7). Age was found to moderate the relationship between olfactory detection threshold and memory performance. Both olfactory identification and detection threshold performances are associated with declarative memory in older adults, and age only moderates the relationship between olfactory detection threshold and declarative memory performances.

Assessment of olfactory fluctuations in a clinical context

PURPOSE

The aim of the study was to investigate whether olfactory fluctuations (OF) are pronounced in patients with sinonasal olfactory dysfunction (OD).

METHODS

The retrospective investigation included patients aged 18 years or older, who consulted a tertiary referral center for olfactory loss. Patients with normal smell function were excluded. Patients answered a structured questionnaire about their olfactory symptoms, with specific questions related to the presence of OF and its average frequency, amplitude, duration, time since most recent OF, and associated symptoms of self-reported OF. Patients also underwent clinical evaluation including a structured medical history and physical examination including nasal endoscopy. In addition, we assessed orthonasal olfactory function using Sniffin' Sticks, and gustatory function using "taste sprays".

RESULTS

Participants included 131 men and 205 women (n = 336), aged 18 to 86 years (mean 50, SD 16). Patient-reported fluctuations occurred most frequently in sinonasal (38%), idiopathic (29%), and postviral (29%) OD. Amplitude of OF was highest in postviral OD (p = 0.009). Average frequency, duration, and the time since the most recent fluctuation were not significantly different between groups (all p's > 0.42). Odor discrimination (p = 0.002) and identification (p = 0.017) scores were higher among those individuals with OF.

CONCLUSION

Amplitude of OF may help distinguish postviral from other causes of OD, especially in patients presenting with equivocal symptoms of sinonasal disease.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

DISCUSSION

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

HIGHLIGHTS

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

The potential utility of smell testing to screen for neurodegenerative disorders

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Joint trajectories of episodic memory and odor identification in older adults: patterns and predictors

Emerging evidence suggests that olfactory function is closely linked to memory function. The aims of this study were to assess whether olfactory and episodic memory functions follow similar age-related decline trajectories, to identify different patterns of decline, as well as predictors of the patterns. 1023 participants from the Memory and Aging Project were followed for up to 8 years with annual episodic memory and odor identification assessments. Trajectories were modelled using growth mixture models. Multivariate logistic regression was used to identify pattern predictors. Three patterns of joint trajectories were identified; Class 1- stable average performance in both functions (n=690, 67.4%); Class 2- stable average episodic memory and declining odor identification (n=231, 22.6%); and Class 3- decline in both functions (n= 102, 10.0%). Class predictors included age, sex, APOE ε4 status, cognitive activity level and BMI. Participants in Class 3 were most likely to develop dementia. Episodic memory and olfactory function show similar trajectories in aging. Such classification can contribute to a better understanding of the factors related to cognitive decline and dementia.