The links between hearing impairment, cognitive decline and dementia are receiving much attention as studies point to an association. Other research is investigating whether hearing intervention such as hearing aids and cochlear implants can lower dementia risk or slow progression. What is the evidence and what does it mean for audiologists and patients? We asked three experts.
The 2024 Lancet Commission on dementia ranked hearing loss as the equal highest modifiable risk factor for dementia at a population level, claiming 7% of dementia cases may be due to hearing loss.
While this latest summary adds to the evidence, Australian audiologists who are experts in the field say that although there is a link between hearing impairment and cognitive decline, it is not yet proven that hearing loss causes dementia – or that hearing intervention reduces risk or progression.
Audiologist Professor Julia Sarant is head of The University of Melbourne’s Hearing Loss and Cognition Program which she established in 2016 after becoming aware of ‘this significant health crisis, probably the biggest we’re going to have due to global population aging.’ “A couple of decades ago, only a few countries were aging globally and had significant aging populations. Now it’s the whole world,” she says.
“With aging comes hearing loss, so we have greater prevalence of older adults, hearing loss, and dementia and cognitive decline.”
She says there have been many studies in recent decades and growing awareness and evidence around an association between hearing loss and cognitive decline/dementia.
“When I started looking at it, there were a couple of good reviews which seemed to synthesise the evidence to suggest quite strongly that there was an association between hearing loss and cognitive decline,” Professor Sarant explains.
“I would describe it as an independent association between hearing loss and cognitive decline, and hearing loss and dementia, but not a causal link as there’s no evidence of a causal relationship – that hearing loss causes cognitive decline or dementia.”
The 2024 Lancet Commission states there are 14 modifiable risk factors for dementia and the more a person has, the higher their risk of dementia.
“Risk does not equal cause and just because you have a risk of something doesn’t mean you’re going to develop the condition,” Prof Sarant says. “It depends on genes too which can interact with exposure to risk factors.”
She says the commission has delivered more evidence as it is now based on six meta-analyses compared with three in 2020.
“They’ve doubled the evidence, so I guess that’s stronger evidence but is it definitive evidence, is it strong evidence? I don’t think it is, it doesn’t prove causality,” Prof Sarant says.
Randomised controlled trials (RCTs) cannot follow people for long enough to see if they develop significant cognitive decline and dementia, she says, adding the ACHIEVE study was only three years due to it being ethically unacceptable to deprive people with hearing disability of an intervention.
But she rejects that observational evidence, used in the commission, is not good enough. An RCT, for example, was not done to prove smoking causes lung cancer; instead, enough evidence was accumulated in high quality observational studies to show that smoking causes lung cancer.
“We need high quality observational studies because they’re the only ones that can be long term, following people for at least a decade,” she says.
It also depends on populations studied as ACHIEVE found hearing aids did not reduce dementia risk but its ARIC sub-group of sicker, older people with many dementia risk factors showed hearing-aids slowed loss of thinking and memory abilities by 48%, she says.
‘Amazing results’ by treating hearing loss
Prof Sarant’s own studies to see if hearing intervention impacts cognitive outcomes in people wearing hearing aids or cochlear implants have had impressive results, finding no or reduced cognitive decline, and even significant improvement in those treated with cochlear implants.
However, she says this does not prove hearing intervention prevents or slows dementia, and as sample sizes were small, her team is recruiting larger samples with longer follow-up.
“We know there’s an association, with pretty strong evidence to suggest people with hearing loss decline faster, and that’s a dose response effect. “Four studies show that for every 10dB of hearing loss, there’s a faster rate of cognitive decline,” she says.
“Our ENHANCE study showed a greater effect than the ARIC subgroup. “Our group with hearing loss who had hearing aids were at much lesser risk of decline in terms of physical health and had fewer risk factors (than the ARIC group) but still we found a bigger impact of hearing intervention and saw no cognitive decline.
“We saw stability over three years. In our comparative group, a representative sample of the normal community of adults – about 50% who had no hearing loss and the rest with mainly very mild hearing loss – we saw a greater rate of decline per year on three out of four tests compared to our intervention group with hearing loss. Results of our studies indicate that hearing intervention can stabilise cognitive function and perhaps delay the onset of cognitive impairment.”
Results she is submitting for journal publication show the cochlear implant study had “amazing results”. “We saw stability in three of five tests over four and a half years but significant improvement in working memory and executive function. It’s a much stronger result and fits nicely with theories of how hearing intervention could help.”
The comparative group did worse overall than the implanted group even though cochlear implantees were more poorly educated, started off with lower cognitive scores, and had more mood disorders and cardiovascular disease.
Could hearing intervention help?
Prof Sarant says three hypotheses could explain how hearing aids and cochlear implants may assist cognitive health. The first is that reduced auditory stimulation causes degeneration in brain structure and function, for which there’s physiological evidence.
“The second is that reduced communication ability leads to reduced social interaction, which can lead to loneliness, isolation and mood conditions such as depression and anxiety, which are known risk factors for dementia,” she explains.
“The third is that processing a degraded auditory signal requires more cognitive resources and these are diverted from other cognitive functions which impairs those functions. A classic is working memory. Speech, perception and understanding is a cognitive task, so we need skills like working memory, inhibition and attention to take in information, make sense of it, and look for what words match with the stimulus.”
Supportive evidence includes imaging studies where areas of the brain light up when a person is processing speech, and studies which show other areas appear to be recruited for listening tasks. However, these theories need more investigation, she adds, because like cancer, there are multiple risk factors, different people have different combinations and factors can interact and be additive.
Head of Brain and Hearing at the Ear Science Institute Australia, Perth audiologist Associate Professor Dona Jayakody, is also studying the links and interventions, and was in the US for the 2024 Lancet Commission launch. She says commission experts advised more evidence was needed to confirm links as “we have evidence but it needs strengthening.”
“There is strong evidence of an association between hearing loss and dementia but more evidence is needed to confirm if using hearing aids reduces dementia risk or slows progression,” she adds. “While a causal link has not been established between hearing loss and dementia, increasing severity of hearing loss has been associated with poorer cognitive function, which could indicate greater dementia risk.”
Contributing factors include the aging auditory system and the way it processes information, she adds. “In untreated hearing loss, people can feel lonely and isolated which can boost depression, anxiety and stress which are dementia risk factors,” she says..
A/Prof Jayakody says the Lancet Commission reduced the percentage of dementia cases that could be due to hearing loss from 8% in 2020 to 7% in 2024 but it could only draw conclusions on published work so more research was needed to help them make better conclusions.
Around 2015 her team began investigating impacts of untreated hearing loss on older adults. The study in more than 400 people found untreated hearing loss increases risks of cognitive impairment, depression, anxiety, stress, social isolation, loneliness and physical frailty. The researchers then studied outcomes in those treated with hearing aids, cochlear implants and bone conduction implants.
“In cochlear implant recipients, we saw a significant improvement in executive functions and reaction time which are cognitive functions. We assumed this was because they had good speech scores after a year (of being implanted) and improved quality of life,” A/Prof Jayakody says. “The rate of cognitive decline in the hearing aid cohort was a bit less than those who didn’t receive a hearing aid.”
To further investigate, the team expanded to include geriatrician, psychiatry and imaging specialists. Their HearCog randomised controlled trial which began in 2020 includes 180 people aged over 70 years with hearing loss at risk of dementia who have never used hearing aids. One group received delayed hearing aid treatment and both groups were followed for two years. The study has finished with results due soon.
“Regardless of the evidence related to hearing loss and dementia, treating hearing loss has positive health benefits including better quality of life, improved communication and improved mental distress and psychosocial well-being,” A/Prof Jayakody adds.
if there are benefits, they are not clinically relevant at an individual level. Image: Piers Dawes.
Marker of dementia risk
More sceptical is director of the Centre for Hearing Research at the University of Queensland, audiologist Professor Piers Dawes, who believes hearing loss is a marker of dementia risk as it’s a marker of general frailty.
He says the 2024 Lancet Commission states treating hearing loss may reduce dementia cases by 7% at a population level but he stresses this does not apply to each individual; for example, hearing loss does not raise a person’s chance of dementia by 7%.
“The commission reported a relative risk (RR) of 1.4 at an individual level, meaning a 40% increased risk of dementia associated with hearing loss. “But the risk of dementia at an individual level is still small overall,” Prof Dawes says.
“The top personal risk factors for dementia in the report were depression (a 220% increased risk), traumatic brain injury, diabetes, less education, social isolation and untreated vision loss; all were more of a risk individually for dementia than hearing impairment.
“The new Lancet Commission hasn’t really changed anything; we still haven’t got solid evidence that hearing loss causes dementia. There is evidence that hearing loss is a marker of risk for dementia but that is not to say that hearing loss causes dementia. Even the ACHIEVE study did not deliver definitive proof of benefit of hearing aids everyone was hoping for.
“The latest Commission report represents an enormous amount of work and deservedly receives widespread attention but it is not perfect when discussing hearing loss and hearing aids. Like Alice in Wonderland, we are in danger of going down a professional rabbit hole: the attention devoted to hearing loss and dementia has become a distraction from the evidence-based importance of healthy hearing for healthy ageing, an important social responsibility.”
He adds that all the evidence linking hearing loss to dementia risk is observational, saying we know hearing loss is associated with dementia risk but because two things are associated does not mean one causes the other.
“There are numerous reasons why hearing loss could be associated with dementia as almost all modifiable risk factors for dementia identified in the report are also risk factors for hearing loss. These include high cholesterol, traumatic brain injury, inactivity, diabetes, smoking, hypertension, obesity and excessive alcohol,” Prof Dawes says.
“It’s the same with studies looking at benefits of hearing aids for cognitive outcomes. Most are observational – comparing hearing aid users with non-users. When you see a difference it’s hard to say if it’s because of the hearing aids or some other difference between hearing aid wearers and non-users. Hearing aid users tend to be more well educated and affluent, and rates of use are much lower in ethnic minority groups.
“These social demographic factors are associated with health outcomes including dementia risk. Researchers try to measure and control for them statistically but there’s not a perfect control for these potential confounds so you can never be sure any apparent benefit of the hearing aid in these observational studies is due to hearing aids and not some confound.”
Prof Dawes says one RCT looked at whether hearing aids could reduce cognitive decline. “To avoid limitations of an observational study, you need a trial to randomly allocate people with hearing loss to a hearing aid intervention or a control condition to see if there is difference in cognitive decline over time,” he says. “Such trials are rare but an impressive recent trial was ACHIEVE.”
The main finding of ACHIEVE was negative, he adds, as there was no effect of hearing aids in reducing cognitive decline. But the Lancet report does not discuss this main finding, he adds.
“It focuses instead on a post-hoc secondary analysis that shows a benefit of hearing aids in reducing cognitive decline in a subgroup of people described by ACHIEVE authors as a ‘high risk’ group. The Lancet report would have benefitted from more scrutiny of this secondary result because there are several reasons we suspect this result may be spurious (e.g. a small effect, unclear mechanism of effect and no dose-response effect of hearing loss).”
‘Inappropriate and unethical’ without convincing evidence
Prof Dawes says many in the audiology community ‘share our concerns that we may be going down a professional rabbit hole.’ “It’s an appealing idea that treating hearing loss would prevent dementia but one concern is that we’ve lost proper scientific objectivity about whether this is true or not.
“It is inappropriate and unethical to advocate addressing hearing loss in terms of dementia prevention on the grounds of relevance to the individual and lack of convincing evidence of benefit,”he says. “If there are benefits, they are not clinically relevant at an individual level.”
Another concern is the dementia focus may turn people off acting on hearing impairment due to stigma, he says. Linking hearing loss to dementia may reinforce the stigma, denial and dis-engagement with hearing care that many people experience at hearing loss diagnosis.
Rather than ‘scare tactics’ around dementia risk, he says positive messaging by audiologists is preferable about well-proven benefits of hearing aids in improving communication and maintaining an active, socially engaged lifestyle.
“Hearing loss is a major public health problem and it’s important to address hearing loss in its own right but if people say hearing loss is important because it causes dementia, we risk agreeing with a common perception among policy makers, clinicians and the public that hearing loss is not that important,” Prof Dawes says.
Prof Sarant says audiologists can advise patients that some studies suggest hearing intervention may improve cognitive health but there are limitations to most and evidence is not strong of a causal link.
“Audiologists could discuss the topic in the context of healthy aging and use positive messaging. Fear-mongering will only make people anxious and increase stigma so hearing intervention should be supported and promoted for its multiple well- established benefits, and cognitive health may be one,” she says.
“There’s also a huge role for audiologists in working with people with cognitive impairment; they can refer people early to get the support they need and modify their clinical practice.” She is concerned some audiologists are doing cognitive screens and urges them to instead refer to a geriatrician, memory clinic, or psychologist trained in these procedures.
