May/June 2021

Hearing Loss and Dementia: Untangling the Connection Between Two Common Conditions
By Jamie Santa Cruz
Today’s Geriatric Medicine
Vol. 14 No. 3 P. 10

Hearing loss is highly prevalent among older adults: Almost three-quarters of individuals aged 70 and older have hearing loss in at least one ear, and two-thirds have hearing loss in both ears.1 Although hearing impairment is an age-old condition, it’s been receiving new attention in recent years due to its increasingly apparent link to dementia risk. Dementia is also a prevalent condition, affecting approximately 50 million adults around the world at any given time.2

So far, it’s unclear exactly how hearing loss is connected to dementia. However, researchers are working to untangle the link in hopes that it might open new possibilities for dementia prevention. Here’s what they know so far.

How Strong Is the Connection?
Three high-quality prospective studies in recent years have compellingly demonstrated the link between hearing impairment and dementia. The first, a 2011 study from Johns Hopkins University, followed 639 individuals aged 36 to 90 for a median of approximately 12 years. The researchers found that individuals with mild hearing loss had close to twice the risk of developing dementia than did those without hearing loss. Those with moderate hearing loss had three times the risk of dementia, and those with severe hearing loss had nearly five times the risk, compared with those with normal hearing.3

In 2017, another prospective study—this one following 3,075 participants aged 70 to 79 for nine years—found that those with moderate to severe hearing loss had 1.5 times the risk of incident dementia during the study period as those with normal hearing.4 A third study of 1,057 men with a mean age of 56 similarly found that each 10-decibel decrease in hearing loss as measured at baseline was associated with an increased likelihood of dementia at the end of the follow-up period 17 years later.5

“The evidence is pretty robust,” says Tim Griffiths, DM, a professor of cognitive neurology at Newcastle University. These studies “took into account other factors such as hypertension or smoking or diabetes [and overall provide] very high-quality evidence.”

In addition to being linked with a greater risk of incident dementia, hearing loss has also been associated in multiple studies with a faster rate of cognitive decline, especially in the domains of memory and executive function.5-9 For instance, a 2013 prospective study of 1,984 individuals enrolled in the Health Aging and Body Composition study found that the rate of cognitive decline was 32% to 41% faster in individuals with hearing loss compared with those without.9 A 2017 meta-analysis found that the link between hearing impairment and rate of cognitive decline was “small” overall across the 36 studies included, but that the effect size was nevertheless significant and was comparable to other factors that have received more study.10

What Explains the Connection?
Although the connection between hearing loss and cognitive decline is well established, it’s not yet clear how they are linked. “We’re still determining the true nature of that connection,” says Danielle Powell, AuD, an audiologist as well as a PhD candidate in epidemiology at the Johns Hopkins Bloomberg School of Public Health, where she’s studying the association of hearing loss with dementia. “The mechanism behind that association—and if hearing loss truly causes dementia—is still unknown.”

One possibility is that dementia causes hearing loss. Although dementia can affect hearing in the later stages of the disease, this is unlikely to be the major explanation for the link between hearing loss and cognition, simply because the prospective data indicate that hearing loss usually appears prior to dementia.

A second possibility is that hearing loss and dementia are caused by a common etiological factor (in other words, the relationship between the two is only associational, not causal). For instance, a common pathology, such as Alzheimer’s disease’s pathology or that of vascular disease, could explain the connection. However, although common pathology probably plays a role in some cases, Griffiths says it’s unlikely to explain the majority of the association. Alzheimer’s-related pathology can affect both the brainstem and the auditory cortex, but hearing loss due to brainstem or cortical pathology is uncommon and isn’t consistent with the type of hearing loss that’s typically been observed in the prospective studies connecting hearing impairment with cognitive decline. Similarly, vascular disease could only explain cases of vascular dementia but not other types of dementia. “If you look at the prospective studies [that have linked hearing loss to dementia], the commonest cause of dementia […] was Alzheimer’s disease, rather than vascular dementia,” Griffiths says.

Other common causes are also possible. “It may be that both are just a process of aging,” says Phillip Hwang, PhD, MPH, a postdoctoral fellow at Boston University School of Medicine. Genetic variants that contribute to both hearing loss and cognitive loss could also play a role. At this point, the possibility of a common etiological factor that causes both hearing loss and dementia can’t be ruled out.

However, a third possibility—one that many researchers in the field think is likely—is that hearing loss promotes cognitive decline and is a causal factor in dementia. There’s no proof of this theory, but the following possible mechanisms have been proposed:

• The social isolation model: One leading theory is that hearing loss leads to social isolation, which is a risk factor for dementia. “They are not able to engage with their surroundings,” Powell says. “They pull away from doing their general activities that they used to enjoy. It increases behaviors that then increase the risk for dementia.” It’s not clear exactly how social isolation would lead to dementia; however, there’s evidence that isolation increases inflammation and glucocorticoid levels, which could affect brain structure and has been linked to dementia and cognitive loss.11

• The impoverished input model: It’s also possible that hearing loss leads to dementia via reducing the amount of input individuals are receiving into their brains, especially with respect to speech and language. This effect could be direct (that is, the individual simply cannot hear speech and language input), or it could be mediated by social withdrawal: The individuals can’t hear well, so they retreat from interaction with others, resulting in an impoverished acoustic environment.12 Regardless of whether the effects are direct or indirect, the thinking is that hearing-impaired individuals experience less cognitive stimulation, which could make them more vulnerable to dementia. “If you have impoverished input to the brain, that might cause a loss of function in high-level brain areas,” Griffiths explains.

A significant number of animal studies support the theory that impoverished environments negatively influence brain structure. These studies show that the richness of an animal’s environment affects not only cortical thickness but also the function of synapses, dendrites, somata, axons, glia, and vasculature.13

• The increased cognitive load model: Yet another model holds that hearing loss leads to increased cognitive load on the brain, which in turn increases the risk of cognitive decline.14 According to this model, a hearing-impaired individual has to use more brain resources than does the average person in order to listen. “That may reduce the cognitive resources available to do other cognitive tasks, and that that may lead to cognitive impairment,” Hwang says.

• The dementia pathology add-on model: The previous models are based on the premise that hearing loss can alter brain function and structure. Recognizing this, some argue that the changes in brain structure that result from hearing loss interact with the changes in brain structure seen in dementia, and the combination of the two therefore exacerbates dementia. “Cortical volume [is] decreased in those with hearing loss,” Powell says. “Cortical connections are altered with hearing loss as well. And all of that can add to the brain pathology we see with different types of dementia. So you can think of it as a second hit that’s happening to the brain, which can increase someone’s presentation of dementia symptoms.”

As support for this theory, Griffiths notes that hearing loss alters cortical activity not only in the classical auditory system but also in the medial temporal lobe (MTL), which is where the early pathological processes of Alzheimer’s disease first appear. Although the MTL isn’t traditionally considered part of the auditory system, the structures of the MTL, including the hippocampus, are active in sound analysis, and animal models indicate that these regions play a role in auditory processing.12 Thus, according to Griffiths, the thought is that “the activity changes in [the] area [of the MTL], and that there is an interaction between increased activity in that area and the Alzheimer’s disease process, which generally starts in that area in typical cases.”

Importantly, none of these models are mutually exclusive. It’s entirely possible, for instance, that social isolation contributes to inflammation but simultaneously causes an impoverished acoustic environment, and that both of these factors independently influence brain function and promote cognitive decline. “It’s probably a combination of all of them in some capacity,” Powell says.

Can Use of Hearing Aids Prevent Cognitive Decline?
If hearing loss plays a causal role in dementia, that raises obvious questions about the possibility of prevention. “That’s the question that everyone wants to know in the end, because if there’s a causal relationship, then hopefully older adults with hearing loss can get some sort of treatment for their hearing loss to reduce the risk of dementia,” Hwang says.

Since the causal relationship isn’t established, it’s difficult to know whether hearing aids do in fact help prevent dementia. However, according to Hwang, there’s some preliminary evidence that they do. A 2018 longitudinal study from researchers at the University of Manchester measured the association between hearing aid use and cognition in a group of more than 2,000 American adults aged 50 and older. The data for this analysis came from the Health and Retirement Study, which measured cognitive performance every two years for a total of 18 years. The researchers found that individuals who used hearing aids had higher scores on tests of episodic memory than did those who didn’t use the aids. Among individuals who started using hearing aids partway through the study, they also found that declines in episodic memory were slower after the participants began using hearing aids than before. The researchers concluded that use of hearing aids may help alter trajectories of cognition later in life.14

A few other studies have produced similar findings. One analysis of 3,777 older adults prospectively followed the participants for 25 years and found that self-reported hearing loss was positively associated with incidence of dementia, except among those who used hearing aids.15 A cross-sectional analysis of adults in the United Kingdom aged 50 and older likewise found that hearing loss was linked to reduced cognition, but only in those who didn’t use hearing aids.16 Finally, a small 2020 study found that use of hearing aids might actually improve some aspects of cognition in hearing-impaired adults—namely, executive function for both genders and working memory, visual attention, and visual learning in women.17

The 2020 report of the Lancet Commission on dementia prevention, intervention, and care argued that the long follow-up times in these studies suggest that addressing hearing loss reduces dementia risk, rather than that people with dementia are less likely to use hearing aids. The commission therefore recommended use of hearing aids in individuals with hearing loss as a potential way of decreasing dementia risk.18

That said, all three of the above-mentioned studies on hearing aids were observational in design. “So, there is some suggestive evidence,” Hwang says. “Obviously, the gold standard is doing a randomized clinical trial.” One such trial is currently underway through Johns Hopkins University and is scheduled to be completed in 2022.19

Even if experimental studies demonstrate that hearing aids are protective against cognitive loss or dementia, it remains to be seen whether they could actually reverse cognitive decline that has already occurred. The answer to that question will depend on the specific mechanism by which hearing loss causes cognitive impairment, Griffiths says. “If you’re cognitively impaired because you’re using a lot more brain to hear things, then if you restore the hearing, it might even be possible for the cognition to be restored,” he explains. However, in the final model, which postulates an interaction between changes in brain structure resulting from hearing loss and changes in brain structure resulting from dementia, "then that [cognitive loss] would not be reversed by the restoration in hearing.”

Implications for Clinicians
Although some questions remain about the specific mechanisms linking hearing loss and cognition, the evidence is sufficient to show that hearing loss should be taken seriously. “It’s not just thinking about cognition,” Hwang says. “Hearing has so many other known impacts of quality of life and functional independence, so it’s important to evaluate hearing for patients in the clinic.”

Powell offers the following recommendations for clinicians:

• Be proactive. According to Powell, the average person waits seven years before seeking treatment for hearing loss. There’s still a lot of stigma around hearing loss for adults as they get older, which explains some of the delay. Many people assume loss of hearing ability is just a normal part of aging and think they should just tolerate it. Others don’t necessarily know where to go or what to do if they start noticing hearing loss.

The upshot is that patients may not seek treatment themselves. Thus, according to Powell, physicians must be proactive in order to catch patients who need treatment.

• Treat early. Waiting too long to start treatment can mean permanent losses in hearing ability. “Hearing aids are not like glasses,” Powell says. “They’re not something you can just put on and suddenly you can see better and everything is back to normal. Especially with prolonged hearing loss, the brain gets used to hearing in quiet with all of their auditory stimulus being reduced. So if you suddenly try to put hearing aids on somebody and they’ve had hearing loss for an extended period of time, it can be way too much, too stimulating, too distracting.” For that reason, she says, early identification and management is quite important.

• Take hearing loss into account when testing cognition. There’s some thought that hearing loss could potentially worsen an individual’s performance on cognitive tests, biasing the results and leading to a score showing worse cognition than the individual actually has. According to Powell, this biasing effect is probably minimal, if it plays a role at all. Still, she says, “a provider performing a cognitive screen test should at least be aware of that [possibility of bias] to try to ensure, can the patient actually hear the test well enough to be able to complete it appropriately?” To avoid unnecessary risk of bias, providers should try not to put all eggs in the basket of auditory tests. “Do both auditory and visual-based tests when you’re looking at cognitive impairment,” Powell says.

— Jamie Santa Cruz is a health and medical writer based in Parker, Colorado.

References
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2. Dementia. World Health Organization website. https://www.who.int/news-room/fact-sheets/detail/dementia. Updated September 21, 2020. Accessed December 9, 2020.

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