Sept 2019 The Hearing Journal

Breakthrough Research Seeks Causal Link

Emerging research on the relationship between hearing loss and dementia reveals a possible causal link that could impact audiology and memory care drastically in coming years. Though the exact association between the two conditions is yet to be identified, recent studies have determined four potential theories for the biological and cognitive processes underlying the link. With further research on the horizon, researchers predict breakthroughs in the near future.

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Approximately 30 percent of adults aged 65 and older and 55 percent older than 80 years show some degree of hearing loss. The number of people affected by age-related hearing loss is estimated to grow to 580 million worldwide by 2050. In 2015, 47 million people worldwide were living with dementia, amassing a global cost of $818 billion, according to a study published in The Lancet Commissions. By 2050, when the proportion of people older than 60 years will double, accounting for 21 percent of the projected global population, the number of those living with dementia is expected to triple, costing nearly $2 trillion. The risk of developing dementia doubles for older adults with mild hearing loss, and triples for those with moderate hearing loss. And for those with severe hearing loss, the risk is five times that of someone who does not suffer hearing loss. Notably, the Lancet study also found that of nine potential risk factors for developing dementia, hearing loss was the highest at nine percent.

In addition, recent research has found that shrinkage of brain tissue is fast-tracked for those with hearing loss, with accelerated rates of brain atrophy compared to those with normal hearing. Overall, those with hearing loss were found to have lost more than an additional cubic centimeter of brain tissue each year compared with those without impaired hearing. Shrinkage of brain tissue for those with hearing loss was most prevalent in regions of the brain responsible for processing sound and speech, including the superior, middle, and inferior temporal gyri.

Common symptoms of both dementia and hearing loss align, including confusion during conversation, changes in methods of communicating, difficulty completing everyday tasks, and feelings of fatigue or stress. As a result, hearing loss can be misdiagnosed as dementia or make the symptoms of dementia appear worse, and research has found that hearing aid use was positively associated with episodic memory scores, with a potential mitigating effect on the trajectories of cognitive decline later in life. Despite correlational evidence between the two conditions, work remains to identify causal factors to link hearing loss and dementia.

The true link between hearing loss and dementia is a mystery. “We don't have enough information to say if hearing loss causes dementia, as we don't yet know the exact nature of the relationship between the two,” said Danielle S. Powell, AuD, a PhD student in epidemiology at Johns Hopkins Bloomberg School of Public Health and a fellow at Johns Hopkins Cochlear Centre for Hearing and Public Health. “There's a lot of research showing an association, but to be able to say hearing causes dementia—we aren't there yet.” There are, however, four predominant theories to explain the link, the first of which contends that hearing loss and cognitive decline occur in parallel as the result of a common underlying pathology. “It's certainly possible that hearing loss and dementia share a common cause,” Karl Doerfer, MD, of Medical College of Wisconsin, said. “There's clearly a strong association between the two, though we haven't been able to determine causality,” he said. For instance, the Lancet study found that central hearing loss could be a prodromal symptom of Alzheimer's disease, causing impaired speech perception, especially in the presence of competing sounds; however, it likely does not contribute to the high prevalence of dementia, as it occurs in only two percent of patients with Alzheimer's disease. “There are people who have dementia without hearing loss and people with hearing loss who don't have dementia, so there is not necessarily a correlation or connection between the two,” Doerfer said. “However, studies have shown a strong association, and, in addition, we have seen that the severity of cognitive decline is associated with the severity of hearing loss, which shows a compelling connection.”

“Statistically, there is a possibility that hearing loss and dementia share a common cause, but that chance is very small and not very likely,” added Tracey Pollard, the senior research program manager at the UK-based non-profit, Action on Hearing Loss. “Current research evidence is strong that there is a greater connection between the two.” Such a connection could be the result of a cognitive load on perception, in which a cognitive decline reduces resources, leading to impaired auditory perception. “In the case of cognitive reserve, we have to ask how resilient patients’ brains are,” Pollard said. “With dementia, there is actual brain damage; some people cope with it better than others. Different people have different ways of compensating for that damage, and some may use a different area of the brain as a coping strategy,” she explained. This theory, however, has gained minimal support, partially due to a lack of evidence that cognitive decline precedes hearing loss in behavioural or longitudinal studies.

A third theory proposes that hearing loss increases demands on cognitive resources, causing information degradation as resources are taken away from other cognitive tasks to support hearing. “Individuals who suffer from untreated hearing loss, even mild untreated hearing loss, often struggle to follow conversations in noisier settings,” said Leslie P. Soiles, AuD, the chief audiologist at HearingLife and Campaign for Better Hearing. “Their participation requires more listening effort, which can result in more cognitive effort. That struggle to grasp the conversation leaves less cognitive bandwidth to recall everything that was said, which may impact memory.” The decreased cognitive reserve due to hearing loss could make the brain more likely to develop dementia, explained Pollard. “If patients are getting degraded intonation in the brain because of hearing loss, that puts a cognitive load on the brain and calls on a greater cognitive reserve,” she added.

A final hypothesis poses that hearing loss causes brain reorganisation and/or social deprivation, resulting in cognitive decline. “Dementia may be impacted by the social isolation that could be a result of hearing loss,” Soiles said. “People with hearing loss may not be confident in responding or re-acting appropriately and may choose to disengage from conversations. As a person with hearing impairment withdraws from social engagement, there is also a reduction in the amount of cognitive function necessary to assess and interact with his or her environment. This may impact executive function ability.”

Despite the compelling early research on the connection between hearing loss and dementia, there remains a wide knowledge gap to determine the actual causal or correlational link between the two conditions. “This area has a lot of opportunity for research to better understand the specific neurocognitive processes that might be involved,” said Michael S. Harris, MD, of the Medical College of Wisconsin. “Most of the existing testing and studies have used rather blunt tools and reading measures, which results in a big knowledge gap on the mechanisms that link hearing loss and dementia.” Pollard added that there is a need for evidence that further supports existing theories. “We need an actual demonstration that shows how hearing loss causes dementia or how a third factor causes both,” she noted.

IMPLICATIONS FOR CLINICAL PRACTICE

Using the knowledge currently available, clinical audiologists can advise patients of the potential link and make informed treatment decisions. “Though all four hypotheses are potentially viable and deserving of further research and resources, for hearing health professionals, the theories that are most intuitive are the information degradation explanation and the sensory deprivation hypothesis,” Doerfer said. “If we can improve some of the information degradation or sensory deprivation through hearing restoration, that's compelling.”

Soiles noted that audiologists have the opportunity to get ahead of hearing loss and the potentially associated cognitive decline. “It all starts with assessing the degree of hearing loss and its impact on functional communication,” she said. “In addition to performing pure tone and speech audiometry, at HearingLife we include a protocol where our professionals measure speech understanding ability in everyday situations.” Soiles said that patients’ caretakers’ or family members’ voices are presented in quiet and background noise environments without the use of visual cues to assess the person's performance in everyday conditions. “Such information helps in understanding the patient's potential for improved hearing with amplification, then appropriate recommendations are made and expectations are discussed based on this result.”

By understanding the mechanisms of hearing loss, hearing care professionals can make recommendations that could prevent or reduce cognitive decline. “Current research affirms the connection between hearing aid use and reduction in cognitive decline, particularly in the area of working memory,” Soiles said. “In some instances, hearing aid use is the one controllable and effective action a person can take to help reduce cognitive decline. Today's hearing aid technology allows for easier speech understanding in the presence of background noise, due to improved signal processing. If we are saying that the cognitive load associated with not hearing well triggers a cascade effect on memory and executive function, then improving hearing reduces that cognitive load.”

When assessing patients, it is important to determine if there is already cognitive decline present, which can be accomplished even prior to formal testing, Soiles noted. “As I input the patient demographic information, I will always ask the patient their date of birth. A person who is experiencing cognitive decline will typically stumble on their answer,” she said. “The second thing I notice is if there are frequently repeated questions, even though answers have already been provided. If I am sensing an issue, I will note my observations in the visit report that I share with the patient's primary care physician.”

CONSIDERATIONS FOR FUTURE RESEARCH

As research on the link between hearing loss and dementia continues, sights are set on the lasting impact that the results could have on global resources for each condition. “There's been a lot more attention today on the impact and the strain seen on the health care system as the population ages,” Doerfer said. “Since hearing loss and dementia affect older individuals disproportionately, treating the two conditions will become an increasing burden on the system.” Harris added that the existing research provides support for the notion that hearing loss is a public health issue. “This research should serve as motivation for advocacy efforts directed at policy change to address hearing loss as a public health matter and broaden access to hearing health technology, increasing the penetration of hearing aids and making them more accessible for the people who need them.”

Institutions such as the Johns Hopkins Bloomberg School of Public Health are attempting to do just that with a dedicated research centre to study the effects of hearing loss with a goal of preventing and mitigating the consequences of hearing impairment, including dementia and cognitive health. With more than $20 million in NIH funding committed to the study, the centre aims to develop and test interventions for hearing loss, while helping to craft policy and strategies to ensure successful implementation at local and national levels. “The focus of the centre is to bring hearing into the public health conversation and the bigger picture,” Powell said. “It aims to identify options, solutions, and strategies for mitigating the negative health impacts of hearing loss in adults while training individuals and expanding the public health world to audiologists.” Powell noted that from an economic standpoint, the research regarding the link between hearing loss and dementia could potentially do a lot for society. “The end goal is to inform clinical recommendations and policy recommendations,” she said. “But the hope is that some of the research will show how much hearing impacts adults and provide intervention options and clinical recommendations. We hope to bridge hearing into the overall health conversations among physicians, policy makers, and families.”

Hearing care professionals are in a unique position to contribute to this breakthrough research and impact patients’ lives on a global level, noted Soiles. “As audiologists, we are truly the experts in the assessment and treatment of hearing and balance disorders,” she said. “Our abilities to verify the hearing ability of older adults, understand the role hearing loss has in their overall health and well-being, and offer appropriate solutions play a critical part in collaborative health care. We must be prepared to engage our patients’ other health care providers with our insights and recommendations.”

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