Hearing Loss and Dementia: Why Audiologists Need to Be Paying Attention



Author: Nicholas S. Reed, Au.D.

Hearing loss impacts 38 million Americans1 and increases with age such that two-thirds of adults over the age of 70 years have a clinically meaningful hearing loss2. In the eyes of an audiologist, these numbers are meaningful because audiologists witness the consequences of hearing loss on a daily basis in clinics across the United States. However, to the public, including other healthcare practitioners and policy makers, these numbers don’t stir as much of a reaction. This may be because hearing loss is often viewed as a benign inconsequential aspect of aging.

This perception stems, in part, from a historical focus on hearing loss as the outcome of interest in auditory research and, in part, from hearing loss getting lost in the relatively high number of comorbid chronic conditions experienced by older adults – many of which are more costly with higher mortality rates than hearing loss. Therefore, studying the causes of hearing loss (noise, toxins, cardiovascular risk factors, etc.), which is often the focus of epidemiologic auditory research, generates little attention in the greater healthcare community. However, if we consider how hearing loss affects other areas of healthcare, it offers a more meaningful role of hearing loss to the general medical community.

To that extent, the association between hearing loss and dementia has received much press in recent years. Interestingly, the hypothetical relationship is not completely new. As far back as the mid-1980s, Barbara Weinstein described this relationship in a small cohort while Uhlmann and colleagues leveraged epidemiologic methods to produce a 1989 JAMA published case-control study revealing dementia patients had 2-times the odds of having hearing loss compared t to those without dementia3,4. In the past decade, a significant number of studies leveraging epidemiologic methodology have begun to further describe this relationship.

5.4 million Americans have dementia, a number that is projected to increase to 13.8 million over the next ~30 years given the aging demographics of the United States population. Importantly, dementia has substantial negative outcomes including declines in functionality, increased healthcare resource utilization and cost, and high caregiver burden. The Alzheimer’s Association reports the estimated lifetime cost of care for persons with dementia is as high as $341,840.

In media, terms such as cognition and dementia are often used synonymously; however each has a specific meaning. Cognition is characterized as a singular or collection of mental process (working memory, processing speed, language, attention, etc.). Dementia is considered impairment in two or more cognitive domains with significant interference in daily functioning5. Dementia has numerous causes including, but not limited to, vascular disorders, degenerative neurological diseases such as Alzheimer’s, and traumatic brain injury.

Hearing loss may also have a causal association with dementia. Biologic plausibility and appropriate measurement are needed to consider a causal relationship. The mechanistic pathways through which hearing loss may contribute to poorer outcomes includes increased cognitive load due to degraded auditory signal processing in the cochlea, changes to brain structure and function, social isolation due to communication difficulties, and loss of environmental sound cues6. Moreover, it is important to a priori identify cognitive tasks that do not require hearing to access the task. For example, a task such as the Mini-Mental State Exam7 requires conversation to complete the tasks that could be impacted by hearing loss while a measure such as the digit symbol substitution test does not require auditory input to complete the task.

Early research from Lin and colleagues established a cross-sectional association between hearing loss and individual cognitive measures among 605 adults aged 60-69 years in the National Health and Nutrition Examination Survey (NHANES)8 and 347 adults over 60 years of age in the Baltimore Longitudinal Study on Aging (BLSA)9. This research revealed that adults with hearing loss performed significantly poorer on cognitive tasks than individuals without hearing loss and this difference exacerbated as degree of hearing loss increased.

While cross sectional relationships are important methods to explore associations, longitudinal exploration is needed to establish temporal relationships. In a follow up longitudinal study of 1966 adults in the HealthABC study, Lin and colleagues revealed that adults with hearing loss experienced a 32 percent faster rate of decline on digit symbol substitution scores over a six-year period compared to individuals without hearing loss10.

While the studies noted above used measured performance on an individual task, two studies from Johns Hopkins measured incidence of dementia among persons with hearing loss. In a study of 639 adults without dementia at baseline in the BLAS, incidence of dementia was recorded over a 10-16 year period11. Survival analyses revealed, that over time, adults with hearing loss were at higher risk for developing dementia and risk increased with degree of hearing loss. Compared to normal hearing adults persons with mild, moderate, and severe hearing loss had 1.89-times (Hazard Ratio [HR] = 1.89, Confidence Interval [CI] = 1.00-3.58, P=0.05), 3.00-times (HR=3.00, CI=1.43-6.30, P=0.004), and 4.94-times (HR=4.94, CI=1.09-22.4, P=0.04) the risk of developing dementia, respectively. Similarly, in a study of 1889 adults in the HealthABC study, hearing loss was associated with higher risk for developing dementia and risk increased with higher PTA12.

In 2017, the Lancet commission on dementia prevention and care published their findings which featured multiple significant focus of the relationship between hearing loss and dementia. Firstly, authors reported hearing loss had greatest attributable risk among modifiable risk factors for dementia. Specifically, the commission concluded that 35% of risk factors for dementia is potentially modifiable (e.g. social isolation, physical activities, education, etc.) and largest attributable risk for dementia among those modifiable risk factors was hearing loss (9%). Secondly, the commission conducted a meta-analysis of the two aforementioned studies and one other, hearing loss was associated with 1.94 times the risk (Risk Ratio [RR]=1.94, CI=1.10-219) of incidence dementia14.

Importantly, hearing aid use may modify this relationship as the pathways outlined above are theoretically amendable to hearing care. Impact on cognitive load could be reduced by a clearer amplified signal and improved communication could decrease social isolation. Secondary analyses in the literature have noted that hearing aid use may be a protective of cognitive decline15. However, several of the of factors associated with hearing aid use (i.e. higher socioeconomic status) are known protective factors of cognitive decline. It is difficult to statistically tease these confounding factors apart. To that extent, methodologically rigorous randomized control trials are required to offer a definitive answer as to whether treating hearing loss could delay cognitive decline and/or prevent dementia.

At the moment, such a randomized control trial is taking place16. The Aging and Cognitive Health Evaluation in Elders (ACHIEVE) trial is a large randomized control trial nested within the ARIC study that will examine the influence of best practice hearing care compared to a healthy aging control on cognitive decline. However, results will not be available from another 3 to 5 years. Until that time, definitive claims that hearing aid use prevents dementia should be avoided as they are not scientifically factual. Understanding limitations to the literature is an important aspect of being an evidenced-based clinician.

In conclusion, recent epidemiologic literature has coalesced around the conclusion that hearing loss is an independent risk factor for dementia that may be modifiable. This relationship has catapulted hearing loss into the public eye as evidenced by recent national media coverage, attention from multidisciplinary academic bodies, and government legislation. Research is underway to better understand whether this relationship is amendable to treatment. This relationship is another aspect supporting hearing loss as a public health concern – a cause audiologists should continue to champion.    
Nicholas Reed, AuD, CCC-A is an Instructor of Audiology in the Department of Otolaryngology-Head and Neck Surgery at the Johns Hopkins School of Medicine. He received his clinical doctorate in audiology (AuD) from Towson University and completed his clinical fellowship at Georgetown University Hospital. He holds a Certificate of Clinical Competence in Audiology (CCC-A) from the American Speech Language Hearing Association. He has clinical experience with diagnostic audiology and amplification management across the lifespan (pediatric to older adults).
References
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