April 25, 2023, by brzam5

The humble hearing aid versus giant Dementia

The Lancet Commission reports published in 2017 and 2021 have highlighted hearing loss as a major risk factor for dementia. Tom Dening, Clinical Professor in Dementia Research, has enthusiastically worn hearing aids since September 2022. Encouraged by the reports, here he discusses the limitations and the positive messages within them and what this means for hearing aid users. 

The Lancet Commission reports published in 2017 and 2021 have highlighted hearing loss as a major risk factor for dementia, suggesting that hearing loss from midlife onwards is responsible for about 8% of the potentially modifiable risk of dementia. Therefore, potentially, eliminating the problem could prevent 8% of future cases of dementia. This would be a huge effect and would dwarf the effects of any current or near-future pharmacological treatments.

The question then arises as to whether treating hearing loss does reduce dementia risk. This is attracting a lot of research attention at present. What evidence do we have, and how good is its quality? Papers published in recent years have tended to support the idea that hearing aids are beneficial, as they usually report that people wearing hearing aids have lower risks of developing dementia over time compared to those with untreated hearing loss. However, studies are often limited by relatively small numbers and the fact that hearing aid users and non-users may differ in important ways. For example, the latter group may be developing early features of dementia, may live in more socially deprived settings or may have worse physical health, any of which could explain lower hearing aid use.

Two papers published this year* have taken matters further. The first (Yeo et al., 2023) is a systematic review including 31 studies of hearing interventions and cognitive decline (so, not just dementia). A meta-analysis of eight of these studies (with nearly 127,000 participants in total) found a reduction of 19% in hazard of developing cognitive decline in hearing aid users compared to those with uncorrected hearing loss. There was also a 3% increase in cognitive test scores in the short-term. In the second paper (Jiang et al., 2023), the authors used data from the UK Biobank from nearly half a million participants, comparing groups with and without self-reported hearing problems in relation to future diagnosis of dementia. Compared to people with normal hearing, those with hearing loss and without hearing aids had an over 40% increased risk of dementia (hazard ratio = 1.42). However, people with hearing loss and using hearing aids had the same risk of dementia as those with normal hearing.

These papers are very encouraging reading as they strengthen the impression that hearing aid use may well be beneficial for brain health and therefore that clinicians should encourage patients to seek hearing assessment and treatment. However, what we still don’t know is whether, if everyone with hearing loss used hearing aids, that would produce similar large reductions in dementia risk. That would require a randomised controlled trial, which would be difficult to design because of the ethical problem of having a control group who were denied hearing aids.

Another important issue concerns how hearing conditions may contribute to cognitive impairment. There are several hypotheses, which include (1) increased cognitive load (if you can’t hear, your brain has to work harder all the time), (2) sensory deprivation (if you can’t hear, you miss out on much of the information in your environment), (3) common cause of neurodegeneration in brain and auditory pathways (e.g. vascular disease), (4) central auditory dysfunction (which can be an early manifestation of dementia), and (5) psychosocial isolation and withdrawal (which can result from either hearing loss or cognitive changes, but is exacerbated by the two occurring together). These hypotheses are also relevant to how hearing aids might work. For example, you would expect that hearing aids would be effective against mechanisms (1), (2) and (5), but less so against (3) and (4). Thus, if people who don’t use hearing aids have a preponderance of vascular pathology or central auditory dysfunction, this may explain why they don’t use aids. And making them do so would probably not affect their dementia risk.

One final question that occurs to me is that maybe eventually we will find that hearing impairment is only a proxy from the real risk factor. By this I mean that we should be looking carefully at the characteristics of the hearing non-user group. Perhaps at least some of them are non-users for very good reasons, and their increased dementia risk may be due to factors that also underlie their hearing loss (vascular, neurodegenerative, adverse social circumstances, etc).

In the meantime though, recent research is good news as it suggests that hearing aids can be useful little blighters when it comes to reducing your risk of future dementia. Audiologists will of course welcome the news and hopefully the public too. However, do we have a system that can cope with the potential demand? One of the most potent ways of promoting hearing aid acceptance is through the relationship of the patient with the audiologist, though I suspect that we are rarely able to offer this within the NHS.

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