Scientists have suggested that hearing loss causes structural changes in parts of the brain related to symptoms of dementia – regions associated with attention and executive function in the frontal cortex, as well as the auditory regions of the temporal lobe.
However, on the horizon is the possibility of reversing such changes with the use of hearing aids, reports Medical News Today.
Globally, more than 432m adults and 34m children have disabling hearing loss.
Due to the world’s ageing population, the WHO estimates that by 2050, almost 2.5bn people will experience hearing loss, with it being severe enough in about 700m people that they will require hearing aids.
A landmark 2020 study found that people with hearing loss are more likely to develop dementia, but that although an association between the two conditions is clear, their exact relationship is less so.
Now, a recent study has found that people with hearing loss exhibit microstructure differences in areas of the frontal cortex linked to executive function and speech and language processing. They also exhibit changes in the auditory regions of the brain’s temporal lobe.
Teams from the University of California-San Diego and Kaiser Permanente Washington Health Research Institute identified these areas through hearing tests and magnetic resonance imaging (MRI) scans.
The study involved 130 participants in the Rancho Bernardo Study of Healthy Ageing. Individuals had a mean age of 76.4 years, with 65% of them being women.
Researchers screened participants’ hearing between 2003 and 2005, at which time their better-hearing ear was tested for its average ability to hear a suite of oscillator-produced pitches at 500, 1 000, 2 000, and 4 000 Herz (Hz). These tones are roughly at the middle of the human hearing range.
MRI scans took place between 2014 and 2016, at which time, structural differences in participants’ brains were noted.
The study appears in the Journal of Alzheimer’s Disease.
Hearing loss impact on the brain
The study’s first author, Kaiser Permanente’s Dr Linda McEvoy, said: “The brain areas that the study found to differ between people with and without hearing loss are not the areas most implicated early in Alzheimer’s disease, e.g, medial temporal lobe structures such as the hippocampus, which are critical for memory.
“Instead, we observed differences in areas related to hearing, speech and attention. We need further research to really understand how hearing impairment increases the risk of dementia.”
Professor Jason Warren of University College London, in the United Kingdom, who was not involved in the study, pointed out that “the links between hearing, brain changes and dementia remain very complex, for a couple of major reasons”.
First, he cautioned that while this loss can be associated with brain changes and even with cognitive deterioration, “that is not the same as saying hearing loss causes Alzheimer’s disease”.
Second, he noted, the mere association between two things – in this case, hearing loss and dementia – does not explain which causes which, or if there is a causal relationship at all.
Describing the connection as a chicken-and-egg problem, Warren said: “Dementia may be exposed by hearing loss, but may also cause hearing loss in its own right. Eggs are just as necessary for chickens as chickens are for eggs, and one determines the other. It’s quite possible both are in play.”
Role of hearing aids in brain health?
Both McEvoy and Warren agree we do not know if brain structural changes might be slowed or reversed when a person with hearing loss uses hearing aids.
“We do not know whether these changes are reversible,” said McEvoy. “But we are beginning to obtain evidence that hearing aids may slow down these types of changes.”
She highlighted a soon-to-be-published study – with which she is not involved – called the ACHIEVE trial. It may offer some clues, since the trial describes the result of hearing aid effects on the brain.
Protecting hearing and the brain
Warren said: “Standard clinical hearing tests assess peripheral or ‘ear’-based (direct) hearing. At the moment, we are not very good at measuring ‘brain hearing’ – the sort of hearing we use, for instance, in following a conversation over background noise.”
This type of hearing is peripheral hearing.
“The key advice is that people should get their hearing checked out if they are worried about it, in case they have a measurable kind of peripheral hearing loss, and they should wear hearing aids if prescribed because that may help to relieve cognitive stress on their brains.”
While we may think of hearing as being primarily an issue for older people, it also often affects younger people, making structural changes in the brain especially concerning.
In the academic years of 2021–2022, in the United States alone, 71 000 people aged three to 21 had sufficient hearing issues that they qualified for coverage under the Individuals with Disabilities Education Act.
Study details
Elevated Pure Tone Thresholds Are Associated with Altered Microstructure in Cortical Areas Related to Auditory Processing and Attentional Allocation
McEvoy, Linda; Bergstrom, Jaclyn; Hagler Jr, Donald; Wing, David; Reas, Emilie.
Published in Journal of Alzheimer’s Disease on 21 November 2023
Abstract
Background
Hearing loss is associated with cognitive decline and increased risk for Alzheimer’s disease, but the basis of this association is not understood.
Objective
To determine whether hearing impairment is associated with advanced brain aging or altered microstructure in areas involved with auditory and cognitive processing.
Methods
130 participants, (mean 76.4±7.3 years; 65% women) of the Rancho Bernardo Study of Healthy Aging had a screening audiogram in 2003–2005 and brain magnetic resonance imaging in 2014–2016. Hearing ability was defined as the average pure tone threshold (PTA) at 500, 1000, 2000, and 4000 Hz in the better-hearing ear. Brain-predicted age difference (Brain-pad) was calculated as the difference between brain-predicted age based on a validated structural imaging biomarker of brain age, and chronological age. Regional diffusion metrics in temporal and frontal cortex regions were obtained from diffusion-weighted MRIs. Linear regression analyses adjusted for age, gender, education, and health-related measures.
Results
PTAs were not associated with brain-PAD (β= 0.09; 95% CI: –0.084 to 0.243; p = 0.34). PTAs were associated with reduced restricted diffusion and increased free water diffusion primarily in right hemisphere temporal and frontal areas (restricted diffusion: βs = –0.21 to –0.30; 95% CIs from –0.48 to –0.02; ps < 0.03; free water: βs = 0.18 to 0.26; 95% CIs 0.01 to 0.438; ps < 0.04).
Conclusions
Hearing impairment is not associated with advanced brain aging but is associated with differences in brain regions involved with auditory processing and attentional control. It is thus possible that increased dementia risk associated with hearing impairment arises, in part, from compensatory brain changes that may decrease resilience.
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