Does Hearing Loss Cause Dementia?

Older adults are more likely to have chronic conditions including hearing loss, and dementia or cognitive decline. Since the number of older adults is rising globally in the next few decades, we expect to see a growing number of individuals with hearing loss and dementia. The number of individuals with Alzheimer’s disease, the most common type of dementia, in the US is expected to rise dramatically from 5 million in 2014 to 14 million by year 2060.¹ This growth will place significant strain on our healthcare system and increase the social and emotional burden that family members and caregivers of older adults with dementia must learn to navigate, making prevention and intervention of dementia a medical and public health priority.

Preventing or delaying the presentation of dementia symptoms by even a few years could have a profound impact on the overall burden of disease in our society. Collectively, the medical and public health communities could therefore make substantial contributions to the expected quality of life for many older adults with collaboration and identification of novel ways to approach dementia prevention and intervention.

A global commission on the state of the research for dementia care and prevention² released an updated consensus report in July 2020. In this report, they identified 12 key risk factors for dementia and cognitive decline. These risk factors were selected as they are considered the strongest risk factors that are also potentially modifiable (i.e., possible to change) including: less education, hypertension, smoking, obesity, depression, physical inactivity, diabetes, low social contact, and notably, hearing loss.

If all 12 risk factors combined could be eliminated, it was estimated up to 40% of dementia cases could be prevented.² This degree of prevention would have a profound effect on the overall burden of dementia care, care costs, and decreased quality of life for individuals and their families and caregivers. For hearing loss specifically, it was calculated that up to 8% of the total number of dementia cases could potentially be avoided with management of hearing loss, making hearing loss the single greatest contributor to dementia risk. While this estimate assumes that hearing loss causes dementia (something still to be confirmed by further investigations), the important role hearing loss and hearing rehabilitative therapies may play in dementia risk and prevention is substantial.

Why is hearing the largest contributor to dementia? There are two reasons. Hearing loss is very common in older adults: nearly two-thirds of adults over the age of 70 have a hearing loss.^3-4 Additionally, the overall risk of dementia comparing people with and without hearing loss is high compared to other risk factors – it is estimated that those with untreated hearing loss are 90% more likely to be diagnosed with dementia compared to those with normal hearing.² Comparatively, people with hypertension, another highly prevalent condition in older adults, have a 60% higher risk for dementia.²

What do we mean by hearing loss or dementia?

While the terms hearing loss and dementia are common conditions within health care and for older adults, what we mean by them is rather complex. When we say hearing loss, we are really referring to two processes that simultaneously are working together to allow an individual to hear in a given environment and for communication— what you might call peripheral hearing ability and speech-in-noise performance (SPIN). Similarly, dementia can be considered along a continuum and is diagnosed via clinical expertise, case history, neurocognitive assessments, and laboratory studies or medical imaging if available. A dementia diagnosis may stem from multiple potential etiologies. While it seems pedantic, these distinctions are essential when we consider both the possibility of hearing loss causing dementia and/or cognitive decline (not just if each condition is associated with either) and subsequently advises the direction and potential fruitfulness of our intervention efforts. Interested readers are referred to the reference list for articles with more comprehensive discussion of the points presented here.

Peripheral hearing loss: the ability to detect a sound or speech

The peripheral portion of the auditory system is the part of the hearing process where incoming sound is encoded — an incoming sound wave is picked up by parts of the peripheral system (the external ear, ear canal, ear drum, middle ear bones, and cochlea) and is transferred into an electrical signal which will be sent up to the brain.⁵ You might consider this peripheral ability as that which allows a person to detect different sounds in the environment and is commonly measured through a clinical testing process known as audiometry.

Speech-in-noise performance: the ability to hear and understand in the presence of background noise

Speech-in-noise performance (SPIN) is of course dependent on peripheral hearing— you cannot understand speech if you are not able to hear it. SPIN also depends on higher-level cognitive processes involving what is considered the central auditory system which includes the brain stem and the brain. The central auditory system takes the electrical signal created from the peripheral system and then decodes this sound for the brain to interpret and provide meaning to what we hear.⁵ This system is what allows a person to separate a conversation with a specific speaker from others talking within a noisy environment, a listening environment that is commonly challenging for older adults.

While we cannot always replicate the conversation and the environments where an individual struggles to understand speech in our clinics, we commonly evaluate this ability through testing the presentation of words or sentences in the presence of background noise (i.e., SPIN).

Self-Reported Hearing Loss: a person’s perception of how they hear

Self-reported hearing is much more complex and integrates aspects of not only peripheral hearing, but also a person’s perceived hearing and communication ability, which can be influenced by additional aspects such as listening environment, fatigue, listening motivation, or hearing expectations. Self-report hearing might be akin to how someone feels they function with their current hearing, and that self-evaluation has value for clinical management. However, self-reported hearing also may be influenced by a person’s mental health and cognitive status, making the understanding of the association between self-report hearing status and cognitive impairment more challenging to distinguish.

Distinguishing between Dementia or Cognitive Decline

Dementia is an umbrella term, which is often utilized to represent a symptom of a variety of underlying etiologies, each of which has a separate pathology. While the presence of a combination of forms of dementia, called mixed dementia, is the most common, the most common unique form of dementia is Alzheimer’s disease.⁶ Alzheimer’s disease by itself may represent about 23% of dementia cases, followed by cerebrovascular related dementia representing about 6%.⁷

It is important to remember that dementia is not a normal process of aging. Normal aging might include a slight decrease in cognitive performance. Cognitive decline is indicated by a change in prior abilities.⁸ For those who will go on to develop dementia, we currently understand there is often a prolonged silent preclinical phase of something like 20 years. This preclinical phase is marked by changes in the brain that may be noticed by the individual and are not yet measurable or detectable on cognitive tests. For some, this phase will continue throughout the remainder of their life. However, others may experience further cognitive decline and progress towards mild cognitive impairment (MCI).⁹

With MCI, cognitive changes become evident to the individual and family. One or more of the cognitive domains by which we can measure cognition (memory, attention, executive function, visuospatial, language)¹⁰, show impairment on testing. Yet with MCI, the individual is overall able to function about their general daily activities (activities of daily living: eating, bathing, dressing, toileting, transferring).

An individual progresses to dementia at the point when their cognitive impairment begins to interfere with their ability to complete these everyday activities.¹¹ A diagnosis of dementia is made based off careful consideration of clinician opinion and expertise, detailed case history, family or patient report, neurocognitive testing, and lab work or medical imaging if available.^8-14

Prior research has primarily focused on the dementia phase of this process. What we are learning is that focusing on interventions at this phase is quite possibly too late, and frankly as of yet there are no interventions for dementia itself beyond symptom management. Even if treatment does exist the need for effective prevention strategies remains paramount due to the growing number of older adults likely to experience dementia in the future. Therefore, there is now focus on prevention and identification of potentially modifiable dementia risk factors which take place earlier in the disease process.

Is there a link between hearing and dementia — what do we know so far?

The majority of the research to date has centered on the association between peripheral hearing loss and dementia, or the ability of the individual to detect sounds or speech and dementia or cognitive change. Part of the reason for this is due to the complexity of speech-in-noise as described above and the challenges of isolating the central hearing aspects vs. the higher-level cognitive processing of SPIN. Additionally, peripheral hearing measures are often more readily and easily obtained either in the clinic or in community-based studies. Increasing research has begun to recognize the value of self-report hearing ability as it relates to cognitive ability, although less is understood about the complexity of this association.

Peripheral hearing loss

A handful of previous reviews have provided summaries of current evidence.^15-21 In perhaps the most rigorous review to date, The Lancet Commission² reported a pooled relative risk from three longitudinal studies^23-25 which indicated nearly 2 times greater risk of incident dementia for older adults with hearing loss compared to those without. Interestingly only three studies met the specified high research standards to be considered for The Commissions review including audiometrically measured hearing, study follow-up for at least 5 years, and accounting for other conditions which may affect hearing and cognition and therefore provide misleading results if not considered in an analysis. Peripheral hearing has been specifically associated with declines in global cognitive function, executive function, processing speed, and memory.^{17, 26-28}

Speech-in-noise performance

The relationship between cognitive decline/dementia and central auditory function as measured through speech-in-noise performance remains much more abstract as the boundaries between processes are blurred. Dryden et al. systematically reviewed evidence in 2017²⁹, suggesting a weak overall correlation between cognitive performance and speech perception.

Prior work has hypothesized that central auditory dysfunction (CAD) may be a prodromal symptom and therefore an early marker of cognitive decline in older adults³⁰, with subjects with severe central auditory dysfunction demonstrating over 9 times greater risk for incident dementia. We have much to learn on how to use the measure of speech-in-noise performance to understand and inform cognitive impairment in older adults, but an opportunity to identify those who may present greater risk for cognitive decline or early identification of cognitive change in close collaboration with other medical specialties may be possible with further research.

Why might we see this association between hearing and dementia — understanding mechanism

It is possible that a separate factor (i.e., genetics, vascular disease, neurodegeneration)^31-32 may lead to both hearing loss and dementia, which would mean intervening on hearing loss would likely have no direct impact on dementia risk. At this time, we can’t rule out this possibility. But given the strength of the relationship between hearing loss and dementia and the consistency of the findings from different studies, it seems unlikely that this separate factor would account for the entire association, meaning other drivers are likely also at play.

How might hearing loss cause dementia?

While a direct causal link remains to be determined, research supports a few key theories for a mechanistic link behind the hearing loss and dementia association. More detailed discussion of the potential path between the hearing-dementia association has been discussed across a variety of mediums elsewhere^15-21, however we will briefly summarize each here.

First, prolonged hearing loss appears to lead to structural and functional changes in the brain, particularly the temporal lobe. With hearing loss, a degraded auditory signal and reduced stimulation from the cochlea may lead to reorganization in the brain and, with prolonged auditory deprivation, ultimately, brain atrophy.

Second, the effort and processing ability required to overcome a degraded auditory signal from age-related hearing loss may pull vital cognitive resources away from general cognitive processing in older adults who demonstrate decline in cognitive ability, thereby exacerbating symptoms and limiting reserve to cope with existing brain pathology (e.g., amyloidosis, neurodegeneration) that leads to dementia .

Lastly, we know individuals with hearing loss frequently decrease their engagement in social activities or have an impaired social or emotional connection with others, which may lead to social isolation or feelings of loneliness – known risk factors for dementia.

Hearing Loss Management and Risk for Dementia

A key question that remains is – are these proposed mechanistic theories of what drives this association amenable to aural rehabilitation (i.e., can we modify the risk presented through management of hearing loss with hearing aids, cochlear implants, or communication-related strategies etc.), thereby decreasing risk for dementia? Answering this question is complicated.

Hearing Aid Use

A handful of studies have investigated this question and suggest that hearing aid use may in fact be protective against dementia. However, hearing aid users may have better cognitive performance due to extraneous factors that similarly are also drivers for the use of hearing aids. For example, those who pursue hearing aids are often more educated, have a higher income, or have more social contacts or engagements than those who do not.³³ Since these characteristics are also protective against dementia, isolating the effect that hearing aid use may have on cognitive performance becomes more difficult. Similarly, community-based studies which include appropriate identification of hearing aid use are rare.

So where does that leave us in determining if hearing aid use ameliorates dementia risk? For this answer, we look to a clinical trial of hearing aid use, of which large-scale investigations are underway.³⁴ In a clinical trial, these external influences are minimized. What these trials hope to ascertain is whether the use of hearing aids alters the course of someone’s expected cognitive trajectory compared to an individual with a similar pure tone average who does not use hearing aids? Hopefully, these studies will give us an answer, but that answer is likely still 2-3 years away.

Cochlear Implants

A much smaller proportion of those with hearing loss are currently considered eligible for a cochlear implant, although guidelines for who is deemed a candidate continue to change and expand. With expansion of candidacy, studies have been investigating how cochlear implantation may influence dementia or cognitive status.

Evaluations of cochlear implant recipients pre and post-surgery have overall focused change in performance on assessments of SPIN and perceived function or quality of life, rather than specific neurocognitive outcomes. Pre/post investigation has suggested improved cognitive performance on global cognitive performance and executive function.^35-36 At this current time, detailed investigation of the effect of cochlear implantation on cognitive status and in particular cognitive change over time is non-existent or extremely limited at best.

Management Summary and what this means for clinical care and our family members

What does this mean for hearing care and how we approach care for older adults with hearing loss or expressed hearing difficulty now? Understanding that hearing loss has a broader impact on quality of life for older adults beyond impaired communication is essential for general practitioners and for helping older adults feel informed about their hearing loss. Coincidentally, this may also encourage adherence to broader hearing and medical rehabilitation recommendations.

Early identification of hearing loss and aural rehabilitation can have long-term down-stream effects on quality of life and mental health. To this end, a growing body of epidemiologic evidence now supports what many Audiologists in the clinic have observed for decades. Hearing loss has a broad impact on many aspects of life^37-38 – increased social isolation, greater risk of depression, decreased physical activity or social engagement – each of which independently increases the risk of dementia. While this evidence does not mean hearing loss causes dementia – studies suggest that is a possibility, and management of hearing loss can have far reaching benefits.

Regardless of the results of clinical trials, treatment of hearing loss through hearing aids, cochlear implants where appropriate, communication strategies, social and behavioral changes, or other forms could indirectly reduce the risk of these other known risk factors of dementia, in turn decreasing the burden of dementia. For those already living with dementia, studies suggest many individuals still demonstrate positive outcomes with hearing aid use or hearing management, including reduced dementia-related behaviors and decreased reported hearing handicap.^39-43

What researchers and clinical providers alike still have to determine is how we best provide hearing health care to those with dementia and/or their caregivers who are essential to dementia care. What works for a middle-aged adult to meet their listening needs is likely not the same for persons living with dementia, yet our hearing care model and evidence-based recommendations at this point are the same for each. Person-centered care needs to be forefront in clinical work- something I am passionately pursuing.

Conclusion

The coming years will likely see formidable changes to hearing health care as research continues to advance and policy changes transform care models and access to hearing care. The approach of hearing care providers and family members or caregivers can have a lasting impact on adults with hearing loss and their health. Involving patients in a continued dialogue about their communication needs, as well as overall health and disease prevention, can markedly improve our collective efforts to help our old adults live longer and healthier lives. Understanding the effect of hearing loss on overall health and the role of aural rehabilitation in a comprehensive health strategy across medical specialties and with primary care providers can improve patient and family connection and support older adults in their years ahead.

Acknowledgements: A portion of this text was printed in the Minnesota Academy of Audiology Newsletter in 2020 titled: Hearing and Dementia- A Look Ahead and written in collaboration with Dr. Jennifer Deal. Dr. Powell is supported by NIH/NIA grant T32AG066576.

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