Hearing loss and dementia – correlation not cause
Last year saw increasing media interest in the links between hearing loss and dementia. In her blog, On “World Hearing Day” Dr. Jenna Littlejohn, NIHR Manchester Biomedical Research Centre (BRC), Hearing Health Researcher provides a timely reminder of why and how Manchester BRC is investigating these possible links further.
Does hearing loss cause dementia?
In the Hearing Health theme at Manchester BRC we are interested in understanding the links between hearing loss, cognitive decline (problems with memory, language, thinking and judgment) and dementia. Dementia is a world public health and Greater Manchester priority and knowing how hearing loss may play a role in this syndrome, could help to reduce the risk or the burden associated with dementia.
There is a large body of evidence linking hearing loss to cognitive decline and dementia.
People with hearing loss appear to:
(a) perform worse on cognitive tests
(b) decline more rapidly on these tests over time
(c) be more likely to develop dementia years later
Additionally, the severity of hearing loss seems to be associated with cognitive decline and the onset of dementia. Research has shown people with a mild hearing loss are twice as likely to develop dementia as someone with normal hearing, but people with severe hearing loss are almost five times as likely to develop dementia. Due to this, hearing loss has been reported as a ‘modifiable risk factor for dementia’. If hearing loss is a risk factor, then it would have to cause dementia. However, from current research, we can’t officially say this, as all of the studies have simply shown an association between hearing loss and dementia.
Delving deeper into the research
Evidence to support hearing loss as a risk factor has come from just three longitudinal studies (observational studies that follow the same subjects repeatedly over a period of time) based on data sets collected in the USA and the UK (see references below). Hearing loss was measured when participants enrolled onto the study and cognitive decline and dementia were the outcome measures, some 10-12 years later. All studies reported that peripheral hearing loss was a risk factor for dementia. However, these large data studies need to be interpreted with caution. Indeed, the same findings could also possibly be explained as ‘dementia increasing the risk of hearing loss,’ or ‘another shared factor increases the risk of both hearing and cognitive impairments.’
Could hearing loss cause dementia?
There are different theories as to how hearing loss could lead to dementia, none of which are widely accepted. One theory is hearing loss causes the brain to work harder to process sounds, at the detriment of other cognitive processes. Another is that over time, if the auditory parts of the brain are no longer being stimulated, they become inactive, leading to widespread neuronal changes. A third option is hearing loss may indirectly increase the risk for dementia. The best example of this indirect link is via social isolation. Loneliness and isolation have been shown to increase the risk of cognitive decline and dementia, due to physiological changes such as increasing blood pressure. Hearing loss is also associated with higher rates of loneliness and isolation.
Could dementia cause hearing loss?
Neuroscientists have shown that the abnormal proteins associated with Alzheimer’s disease, the most common cause of dementia, are present in the brain up to 20 years before the onset of clinical symptoms. This indicates, on study enrolment, participants who were destined to develop dementia would already have had the disease in their brains. It may have even been present before any hearing loss. Therefore, underlying dementia could lead to hearing loss as part of the neuro-degenerative process. Another option is, if someone has underlying cognitive impairment, they may struggle to undertake the hearing test, under-perform and appear to have worse hearing.
Could another factor cause both hearing loss and dementia?
It could of course be that another factor common to both hearing loss and dementia could explain this association. Due to the nature of the research it is impossible to test every single factor that could possibly lead to dementia – but is one of these responsible rather than the hearing loss? Hearing loss and dementia both have shared risk factors such as older age, hypertension, stress, alcohol abuse, poor sleep hygiene and lower education. In this sample, the people who have hearing loss may also have one or more of these other factors which could be increasing the risks of dementia.
What do we hope to achieve from our research?
At Manchester BRC, we hope to continue our work in this field, by investigating how hearing is specifically affected in the very early stages of dementia, and how this may change as dementia progresses. This would help to further understand a potential mechanistic link between hearing loss and dementia.
This is a really important issue; if people with hearing loss are at a greater risk for developing dementia, pro-active early management of hearing impairment with hearing aids could be protective. Since hearing loss is so prevalent in the ageing population; this could potentially reduce many cases of dementia.
References