Hearing impairment is equal top of the list of modifiable risk factors associated with the greatest proportion of people developing dementia globally, a new report from the 2024 Lancet Commission shows.
Experts also said evidence that treating hearing loss decreases the risk of dementia was now stronger than their previous report four years ago.
The new report estimates the risk factors associated with the greatest proportion of people developing dementia are hearing impairment and high LDL (low-density lipoprotein) or “bad” cholesterol in midlife, each responsible for 7% of dementia cases globally.
The third Lancet Commission on dementia prevention, intervention, and care was presented at the Alzheimer’s Association International Conference in Philadelphia, America on 31 July 2024 ET (today 1 August, 2024 AEST) and published simultaneously in The Lancet.
“The evidence that treating hearing loss decreases the risk of dementia is now stronger than when our previous Commission report was published,” the researchers said. “Use of hearing aids appears to be particularly effective in people with hearing loss and additional risk factors for dementia.”
They said they previously discussed the evidence that hearing aid use was protective against dementia and reduced cognitive deterioration rates after beginning hearing aid use.
“Since then, a systematic review and meta-analysis of eight cohort studies with 126,903 participants, followed up for two–25 years, reported that people with hearing loss who used hearing aids had a significantly lower risk of cognitive decline and dementia than those who did not use assistive devices,” the experts reported.
In another cohort of 2,114 people older than 50 with self-reported hearing loss, 1,154 people had mild cognitive impairment and those who used hearing aids were at significantly lower risk of developing all-cause dementia during the follow-up than those not using hearing aids. The median time to incident dementia was two years for non-hearing-aid users and four years for hearing aid users.
Based on the latest available evidence, the Commission found nearly half of dementia cases worldwide could be prevented or delayed by tackling 14 risk factors including two new risks – high cholesterol and vision loss. They found addressing these, starting in childhood and continuing throughout life, could prevent or delay 49% of dementia cases, even as people around the world live longer and cases are set to rise dramatically.
The report outlined recommendations for governments and individuals to adopt to reduce risk including preventing and treating hearing loss, making hearing aids available for all those with hearing loss and reducing harmful noise exposure.
The Commission was authored by 27 world-leading dementia experts including Australian Professor David Ames from the National Ageing Research Institute and The University of Melbourne Academic Unit for Psychiatry of Old Age.
Globally, an estimated 20% of people have hearing loss, sometimes related to occupational or environmental exposures to noise or untreated infections, they said. About 62% of those with hearing loss are over 50 years of age and hearing loss is often untreated.
“This paper is the most comprehensive analysis to date of the population attributable fraction for potentially modifiable risk factors for dementia and updates previous calculations with newly incorporated risk factors with convincing evidence and updated worldwide estimates of relative risks and prevalence for the risk factors,” they said.
They also completed new meta-analyses of the risk of hearing loss and depression for future dementia.
Significant associations
In their previous Lancet Commissions, they performed a meta-analysis of high-quality cohort studies with participants who were free of dementia but had objectively measured peripheral hearing loss at baseline.
There were five further meta-analyses on the association between hearing loss and subsequent dementia including one which focused on Sinitic tonal languages.
“All of these analyses reported significant associations between hearing loss and subsequent dementia,” they said. “In the most recent study, incident hearing loss was associated with incident dementia risk and was related to dose, as each 10-dB worsening of hearing was associated with a 16% increase in dementia risk.”
However, none of these analyses included all of the criteria that they judged to ensure high-quality data in their previous meta-analysis so they conducted a further meta-analysis of six high quality studies. In this meta-regression studies with a higher proportion of people who wore hearing aids reported a lower likelihood of dementia than those with a lower proportion of people who used hearing aids.
“As severity of hearing loss increases, dementia risk increases: all four studies that investigated dose–response between hearing and dementia risk reported that every 10 dB decrease in hearing ability increased dementia risk,” they said. “The magnitude of this risk increase varied between studies, from a 4% increase to a 24% increase in dementia risk per 10 dB decrease in hearing ability.”
Another study found that compared with people with typical speech-in-noise hearing, there was an increased risk of dementia in people with insufficient and poor speech-in-noise hearing.
Researchers said a causal link between hearing loss and dementia was supported by longer exposure to hearing loss being associated with higher dementia risk, with maximum risk in people who were diagnosed with hearing loss for more than 25 years.
“The evidence described here raises the question of whether the use of hearing aids in people with hearing loss can eliminate or mitigate the increased dementia risk,” they added.
The ACHIEVE study, the first randomised controlled trial of hearing aids and cognition, recruited 739 healthy people aged 70–84 years with hearing loss and 238 people from the ARIC study. It found no overall effect of the use of hearing aids on the primary outcome of cognition at three years follow-up.
But in the ARIC study population who had more dementia risk factors, there was a large protective effect of hearing aids on cognition – a 48% reduction in three-year global cognitive decline compared with the control population.
“The explanation of the large effect in the ARIC cohort might be that hearing aids in groups at high risk of dementia also change social contact, low mood, cognitive stimulation, and improve motivation and communication about medical treatment, but this evidence does not exist yet,” they added.
“The observational evidence of the benefits of hearing aids for dementia risk is increasing. “Even if only the studies with long follow-up are considered, to reduce the chance of reverse causality, the evidence on hearing aids reducing dementia risk is consistent and supportive. Implementing the use of hearing aids, if effective in preventing dementia, would likely be cost saving.”
They said that although association was not causation, the effect on cognition seen in randomised controlled trials assessing multicomponent interventions including hearing aid provision, hearing and vision treatments, and increases in cognitive stimulation continue to suggest a causal association with the clinical expression of dementia.
Recommendations for people with dementia
The researchers recommended that if a person with dementia was not distressed by psychosis, management should continue to start with non-pharmacological interventions to maximise stimulation, such as improving hearing and sight and increasing social and other stimulation.
“It is important to energetically treat delirium, both treating the underlying illness and using non-pharmacological means of increasing orientation. Vision and hearing maximisation, management of pain and hypoxia, fluid support, and ensuring food intake are also crucial,” they added.
“Our new report reveals that there is much more that can and should be done to reduce the risk of dementia. It’s never too early or too late to take action, with opportunities to make an impact at any stage of life,” said lead author Professor Gill Livingston from University College London, UK. “We now have stronger evidence that longer exposure to risk has a greater effect and that risks act more strongly in people who are vulnerable.
“These actions are especially important given new evidence which shows that reducing the risks of dementia not only increases years of healthy life but also reduces the time people who develop dementia spend in ill health.”
The researchers said the prevention estimates assumed there was a causal relationship between risk factors and dementia. While they were careful to only include risk factors with convincing evidence, some associations may only be partly causal as while the strength of the evidence was good, causality could not be proven.
They added that risk modification affected the population and did not guarantee that any individual would avoid dementia. However, “evidence is increasing and is now stronger than before that tackling the many risk factors for dementia that we modelled previously (ie, less education, hearing loss, hypertension, smoking, obesity, depression, physical inactivity, diabetes, excessive alcohol consumption, traumatic brain injury, air pollution, and social isolation) reduces the risk of developing dementia,” they said.
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