Exploring prior diseases associated with incident late-onset Alzheimer's disease dementia

Vestibular dysfunction: a hidden risk factor for Alzheimer's disease

BACKGROUND

London taxi drivers' navigationally challenged hippocampi are known to be enlarged, and reduced Alzheimer's disease (AD)-related mortality has recently been shown in similarly well-versed drivers, implying a neuroprotective effect through hippocampal engagement. Vestibular function has been linked to hippocampal size, suggesting that vestibular input may influence AD risk.

METHODS

Including 16 known modifiable lifestyle factors as covariates, we analyzed UK Biobank (UKB) volunteers aged over 55 years and without dementia at baseline to assess how peripheral vestibular dysfunction (PVD) influences the likelihood of an AD diagnosis.

RESULTS

4684 AD and 2133 PVD cases were identified based on their ICD diagnoses; even accounting for other risk factors, PVD increased the risk of AD 1.7 times in UKB volunteers.

DISCUSSION

Vestibular loss, linked to hippocampal atrophy and default mode network disruption, appears to increase AD risk. Consequently, active vestibular stimulation by balance training or neuromodulation could offer potential for modifying AD progression.

Mapping morbidity 10 years prior to a diagnosis of young onset Alzheimer's disease

INTRODUCTION

Early symptoms in young onset Alzheimer's disease (YOAD) may be misinterpreted, causing delayed diagnosis. This population-based study aimed to map morbidity prior to YOAD diagnosis.

METHODS

In a register-based incidence density matched nested case-control study, we examined hospital-diagnosed morbidity for people diagnosed with YOAD in Danish memory clinics during 2016-2020 compared to controls in a 10-year period. Conditional logistic regression produced incidence rate ratios (IRRs).

RESULTS

The study included 1745 cases and 5235 controls. YOAD patients had a higher morbidity burden in the year immediately before dementia diagnosis, for certain disorders up to 10 years before. This was especially evident for psychiatric morbidity with the highest increased IRRs throughout the entire period and IRR 1.43 (95% confidence interval 1.14-1.79) in the 5-10-years before dementia diagnosis.

DISCUSSION

YOAD patients display a different pattern of morbidity up to 10 years prior to diagnosis. Awareness of specific alterations in morbidity may improve efforts toward a timely diagnosis.

HIGHLIGHTS

Retrospective, nested case-control study of young onset Alzheimer's disease (YOAD). YOAD cases had a higher morbidity burden than controls. YOAD cases had a higher psychiatric morbidity burden up to 10 years before diagnosis. Altered morbidity patterns could serve as an early warning sign of YOAD.

Aging of the vestibular system and its relationship to dementia

PURPOSE OF REVIEW

Since October 2022, substantial new information has been published on age-related effects on the vestibular system. Since much of this evidence relates to the risk of dementia, the purpose of this review will be to provide an overview of this new information and critically evaluate it.

RECENT FINDINGS

This review will address studies published since October 2022 regarding age-related effects on the vestibular system and their relationship to cognition and dementia. There has been a particular increase in the last year in the number of studies relating aging of the vestibular system to Alzheimer's disease (AD), further supporting the view that vestibular dysfunction is associated with an increased risk of dementia.

SUMMARY

The conclusion of these recent studies is that, consistent with previous studies, vestibular function declines with age, and that this age-related decline is associated with cognitive impairment and an increased risk of dementia. Efforts are being made to consider these implications for cognition in the treatment of vestibular disorders.

Interpreting the meaning of changes in hippocampal volume associated with vestibular loss

Many studies have documented cognitive deficits, especially spatial cognitive deficits, in patients with some form of vestibular loss. Almost 20 years ago, hippocampal (HPC) atrophy was reported to be correlated with spatial memory deficits in such patients and the idea has gradually emerged that HPC atrophy may be causally responsible for the cognitive deficits. However, the results of studies of HPC volume following vestibular loss have not always been consistent, and a number of studies have reported no evidence of HPC atrophy. This paper argues that HPC atrophy, if it does occur following vestibular loss, may not be directly, causally responsible for the cognitive deficits, and that it is more likely that rapid functional changes in the HPC are responsible, due to the interruption of the transmission of vestibular information to the HPC. The argument presented here rests on 3 tranches of evidence: (1) Cognitive deficits have been observed in humans even in the absence of HPC atrophy; (2) HPC atrophy has not been reported in animal studies following vestibular loss, despite cognitive deficits; and (3) Animal studies have shown that the interruption of the transmission of vestibular information to the HPC has immediate consequences for HPC place cells, far too quickly to be explained by HPC atrophy. It is possible that HPC atrophy, when it does occur, is related to the longer-term consquences of living with vestibular loss, which are likely to increase circulating cortisol.

Deep phenotyping of Alzheimer's disease leveraging electronic medical records identifies sex-specific clinical associations

Alzheimer's Disease (AD) is a neurodegenerative disorder that is still not fully understood. Sex modifies AD vulnerability, but the reasons for this are largely unknown. We utilize two independent electronic medical record (EMR) systems across 44,288 patients to perform deep clinical phenotyping and network analysis to gain insight into clinical characteristics and sex-specific clinical associations in AD. Embeddings and network representation of patient diagnoses demonstrate greater comorbidity interactions in AD in comparison to matched controls. Enrichment analysis identifies multiple known and new diagnostic, medication, and lab result associations across the whole cohort and in a sex-stratified analysis. With this data-driven method of phenotyping, we can represent AD complexity and generate hypotheses of clinical factors that can be followed-up for further diagnostic and predictive analyses, mechanistic understanding, or drug repurposing and therapeutic approaches.

Vision impairment and cognitive decline among older adults: a systematic review

OBJECTIVES

There has been increasing epidemiological research examining the association between vision impairment (VI) and cognitive impairment and how poor vision may be a modifiable risk factor for cognitive decline. The objective of this systematic review is to synthesise the published literature on the association of VI with cognitive decline, cognitive impairment or dementia, to aid the development of interventions and guide public policies pertaining to the relationship between vision and cognition.

METHODS

A literature search was performed with Embase, Medline and Cochrane library databases from inception to March 2020, and included abstracts and articles published in peer-reviewed journals in English. Our inclusion criteria included publications that contained subjective/objective measures of vision and cognition, or a diagnosis of VI, cognitive impairment or dementia. Longitudinal or cross-sectional studies with ≥100 participants aged >50 years were included. The search identified 11 805 articles whose abstracts underwent screening by three teams of study authors. Data abstraction and quality assessment using the Effective Public Health Practice Project Quality Assessment Tool were performed by one author (NN). 10% of the articles underwent abstraction and appraisal by a second author (LA/VV), results were compared between both and were in agreement.

RESULTS

110 full-text articles were selected for data extraction, of which 53 were cross-sectional, 43 longitudinal and 14 were case-control studies. The mean age of participants was 73.0 years (range 50-93.1). Ninety-one (83%) of these studies reported that VI was associated with cognitive impairment.

CONCLUSION

Our systematic review indicates that a majority of studies examining the vision-cognition relationship report that VI is associated with more cognitive decline, cognitive impairment or dementia among older adults. This synthesis supports the need for additional research to understand the mechanisms underlying the association between VI and cognitive impairment and to test interventions that mitigate the cognitive consequences of VI.

Temporal Subtyping of Alzheimer's Disease Using Medical Conditions Preceding Alzheimer's Disease Onset in Electronic Health Records

Subtyping of Alzheimer's disease (AD) can facilitate diagnosis, treatment, prognosis and disease management. It can also support the testing of new prevention and treatment strategies through clinical trials. In this study, we employed spectral clustering to cluster 29,922 AD patients in the OneFlorida Data Trust using their longitudinal EHR data of diagnosis and conditions into four subtypes. These subtypes exhibit different patterns of progression of other conditions prior to the first AD diagnosis. In addition, according to the results of various statistical tests, these subtypes are also significantly different with respect to demographics, mortality, and prescription medications after the AD diagnosis. This study could potentially facilitate early detection and personalized treatment of AD as well as data-driven generalizability assessment of clinical trials for AD.

Hearing loss versus vestibular loss as contributors to cognitive dysfunction

In the last 5 years, there has been a surge in evidence that hearing loss (HL) may be a risk factor for cognitive dysfunction, including dementia. At the same time, there has been an increase in the number of studies implicating vestibular loss in cognitive dysfunction. Due to the fact that vestibular disorders often present with HL and other auditory disorders such as tinnitus, it has been suggested that, in many cases, what appears to be vestibular-related cognitive dysfunction may be due to HL (e.g., Dobbels et al. Front Neurol 11:710, 2020). This review analyses the studies of vestibular-related cognitive dysfunction which have controlled HL. It is suggested that despite the fact that many studies in the area have not controlled HL, many other studies have (~ 19/44 studies or 43%). Therefore, although there is certainly a need for further studies controlling HL, there is evidence to suggest that vestibular loss is associated with cognitive dysfunction, especially related to spatial memory. This is consistent with the overwhelming evidence from animal studies that the vestibular system transmits specific types of information about self-motion to structures such as the hippocampus.

Association between Late-Onset Ménière's Disease and the Risk of Incident All-Cause Dementia

Studies reported an association between impaired hearing and vestibular function with the risk of dementia. This study investigated the association between Ménière’s disease (MD) and the risk of dementia using a nationwide cohort sample of data obtained from the South Korea National Health Insurance Service. The MD group (n = 496) included patients aged over 55 years and diagnosed between 2003 and 2006. The comparison group was selected using propensity score matching (n = 1984). Cox proportional hazards regression models were used to calculate incidence and hazard ratios for dementia events. The incidence of dementia was 14.3 per 1000 person–years in the MD group. After adjustment for certain variables, the incidence of dementia was higher in the MD group than in the comparison group (adjusted hazard ratio (HR) = 1.57, 95% confidence interval = 1.17–2.12). Subgroup analysis showed a significantly increased adjusted HR for developing Alzheimer’s disease (1.69, 95% confidence interval = 1.20–2.37) and vascular dementia (1.99, 95% confidence interval = 1.10–3.57) in the MD group. Patients with dementia experienced a higher frequency of MD episodes than those without dementia. Our findings suggest that late-onset MD is associated with an increased incidence of all-cause dementia, and it might be used as a basis for an earlier diagnosis of dementia.

Altered Gut Microbial Load and Immune Activation in a Drosophila Model of Human Tauopathy

Tau is a microtubule-associated protein that stabilizes the neuronal cytoskeleton. In the family of neurodegenerative diseases known as tauopathies, including Alzheimer's disease (AD), frontotemporal dementia (FTD), and chronic traumatic encephalopathy (CTE), abnormal tau aggregation destabilizes microtubule structure, contributing to a cascade of cellular processes leading to neuronal cell death. The gut microbiome has increasingly become a target of neurodegenerative disease research since gut microbiome imbalances have been linked to protein aggregation and inflammation through a bidirectional axis linking the gut and brain. Accordingly, the present study examined tau-mediated changes to gut microbiome composition and immune activation in a Drosophila melanogaster model of human mutant tauopathy. Fecal deposit quantification and gastric emptying time courses suggested an abnormal food distribution and reduced gut motility in tau transgenic flies compared to controls. Tau transgenic flies also showed an increase in gut bacteria colony forming units (CFUs) from diluted fly homogenate, indicating an increased bacterial load. Finally, we showed that tau transgenic flies have a trend towards elevated systemic levels of antimicrobial peptides targeting gram-negative bacteria using qPCR, suggesting an enhanced innate immune response to bacterial insult. These data demonstrate qualifiable and quantifiable gut microbial and innate immune responses to tauopathy. Furthermore, these results provide a framework for future studies targeting the gut microbiome as a modifier of neurodegenerative disease.

Does Mild Traumatic Brain Injury Increase the Risk for Dementia? A Systematic Review and Meta-Analysis

Background: Mild traumatic brain injury (mTBI) is a putative risk factor for dementia; however, despite having apparent face validity, the evidence supporting this hypothesis remains inconclusive. Understanding the role of mTBI as a risk factor is becoming increasingly important given the high prevalence of mTBI, and the increasing societal burden of dementia. Objective: Our objective was to use the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) format to determine if an association exists between mTBI and dementia and related factors, and to quantify the degree of risk. Methods: In this format, two authors conducted independent database searches of PubMed, PsycInfo, and CINAHL using three search blocks to find relevant papers published between 2000 and 2020. Relevant studies were selected using pre-defined inclusion/exclusion criteria, and bias scoring was performed independently by the two authors before a subset of studies was selected for meta-analysis. Twenty-one studies met the inclusion criteria for this systematic review. Results: The meta-analysis yielded a pooled odds ratio of 1.96 (95% CI 1.698–2.263), meaning individuals were 1.96 times more likely to be diagnosed with dementia if they had a prior mTBI. Most studies examining neuropsychiatric and neuroimaging correlates of dementia found subtle, persistent changes after mTBI. Conclusion: These results indicate that mTBI is a risk factor for the development of dementia and causes subtle changes in performance on neuropsychiatric testing and brain structure in some patients.