Introduction to Supplement Issue: Sensory loss in Alzheimer's disease and related dementias

Introduction

There is growing evidence that impairments in sensory function may be linked to brain health and risk of dementia. Sensory function declines with age and impairments in individual and multiple sensory domains are common.^1,2 Recent evidence suggests that sensory impairments are associated with faster rates of cognitive decline, worse brain health, and a higher risk of Alzheimer's disease (AD) and related dementias (ADRD). ³ Although the mechanisms are not well understood, sensory impairments (especially hearing and vision) may augment the risk of cognitive impairment and dementia through increased cognitive load, brain atrophy, or indirectly through impacts on social isolation, psychological wellbeing, or physical activity and systemic health.^3,4 Some sensory losses, such as olfactory dysfunction, may also be an indicator of early brain changes or preclinical AD/ADRD. ⁵ However, associations could be due other aging factors such as shared vascular pathways or accelerated aging processes. ³ Sensory impairments also present challenges for accurate measurement of cognitive status and may exacerbate the challenges of living with dementia. ⁶

Sensory impairments represent a major public health challenge, but they are often detectable clinically and, in many cases, there are effective interventions (e.g., hearing aids, cochlear implants, cataract surgery, eyeglasses, etc.). Thus, there may be significant opportunities to improve sensory function in later life and to study the potential of such interventions to slow cognitive decline or prevent dementia. ⁷ Moreover, measurement of sensory functions may aid in earlier risk stratification, diagnosis, treatment and prevention of dementia. ³ This Supplement Issue brings together original research, narrative reviews, and thought papers on the connections of sensory health with brain health, cognitive impairment and decline, and AD/ADRD.

This issue begins with several papers focused on various aspects of the association between hearing loss and dementia. The first paper examines digits in noise testing in older adults with cognitive impairment, finding that it is a practical screening tool for identifying disabling levels of hearing loss. ⁸ A unique pilot study explores associations between environmental noise exposure, especially nighttime exposure, and subtle structural brain changes, as indicated by elevated gray-to-white matter signal intensity contrast values. ⁹ Finally, an editorial explores alternative epidemiologic explanations for an association between hearing loss and dementia and suggests future research directions to evaluate pathways. ¹⁰

Next, the issue includes several papers on vision loss and eye health and their associations with cognitive function and AD/ADRD. A case-control study using a nationwide administrative claims database from Japan demonstrates an association between open-angle glaucoma and early-onset AD. ¹¹ Hreha et al. report on the association between pre-stroke and stroke-related vision impairment(s) among stroke survivors and suggest that stroke-related vision impairment, but not pre-stroke vision impairment, is associated with higher odds of dementia. ¹² Lastly, a study conducted semi-structure interviews to explore eye test experiences and barriers to eye testing for clinicians, people with dementia, and their caregivers. ¹³

The issue then includes several reports from studies related to olfaction. A narrative review on olfactory evaluation in AD/ADRD explores different aspects of odor tests, focusing on odor recognition memory methods and their utility in dementia research. ¹⁴ Several papers explore the association of olfactory dysfunction among patients with subjective cognitive decline (SCD) and mild cognitive impairment (MCI), which are considered along a continuum of cognitive decline. One paper finds olfactory dysfunction is already evident in individuals with SCD and that classification of SCD compared to cognitively normal older adults is enhanced combined with anxiety and depression measures. ¹⁵ Another paper reports that the predictive value of olfactory identification and episodic memory scores can help discriminate individuals with MCI from those with SCD, and that odor identification was worse in MCI. ¹⁶ A paper on the functional connectivity of olfactory networks using functional magnetic resonance imaging lends further support that olfactory dysfunction is stronger among MCI participants than SCD. MCI participants had lower olfactory scores and alterations in functional connectivity, while SCD participants only showed differences in functional connectivity. ¹⁷

Finally, several papers focus on other senses or multiple sensory impairments. A narrative review discusses links between gustatory (taste) dysfunction, oral frailty, and AD, addressing possible underlying mechanisms. ¹⁸ A nationally representative study of older adults in India investigates cross-sectional associations between dual sensory loss (combined hearing and vision impairments) and cognitive performance. ¹⁹ Another paper investigates racial and ethnic differences in the associations of individual and multiple sensory impairments with cognition in older US adults, finding differences in how strongly smell and multiple sensory impairment predicts cognition. ²⁰ The issue ends with a paper estimating the fraction of dementia prevalence associated with treatable sensory loss (hearing and vision), which estimated that 23.8% of dementia prevalence in the US was associated with clinically significant sensory loss. ²¹

Together this works highlights the important links between sensory impairments and cognitive health. This Issue presents novel findings on the associations between sensory impairments and AD/ADRD outcomes; highlights the population impact of sensory loss and dementia; examines issues related to the measurement of sensory loss among people with dementia; and reviews the current state of knowledge on mechanisms. These papers also point to potential future directions: including interventions on sensory loss to slow cognitive decline, sensory care among people living with dementia, and use of olfaction and other senses as predictors of preclinical AD, and investigation of novel sensory markers such as environmental noise and gustatory frailty. These articles underscore the importance of sensory health in understanding, detecting, and potentially mitigating dementia risk.