Deafness and dementia
Hearing correction – prevention of Alzheimer's disease and senile dementia?
Senile dementia affects tens of millions of people today. The main cause of age–related dementia – Alzheimer's disease - is still incurable, although scientists are working to study the mechanisms, as well as ways to correct and prevent this pathology. In this sense, the role can be played by a far from obvious connection between a decrease in mental abilities and deafness, found in elderly people.
Recently, scientists from the USA and the UK have proposed several hypotheses regarding how hearing loss can contribute to the development of Alzheimer's disease – a chronic neurodegenerative pathology in which incorrectly folded beta-amyloid and tau protein molecules accumulate in brain tissues.
Article by Griffiths et al. How can hearing loss cause dementia? published in the journal Neuron.
The first of these hypotheses assigns the main role to the "general" pathological process (for example, in blood vessels), which simultaneously covers the organs and nerve structures of the auditory pathway and the cortical structures of the brain. However, this mechanism is considered unlikely by the researchers themselves.
Secondly, hearing loss can affect the functions of certain areas of the brain directly - by stopping their stimulation by sound impulses. This assumption is supported by the fact that people who actively use their hearing (for example, musicians or piano tuners) change brain structures that play a key role in memory formation and analysis of auditory information. On the other hand, people with impaired hearing perceive speech worse, and this reduces the quality of social interactions, which is considered a risk factor for dementia.
There is one caveat here: usually the quality of hearing is assessed by the minimum volume level at which a person can hear sounds at different frequencies. But in the case of the threat of dementia, the ability to hear sounds against the background of noise is more significant, since it requires the use of many cognitive processes related to focusing attention, word recognition, etc.
The third hypothesis is based on the idea that people with hearing impairment spend too much mental resources just to hear something. And there are simply not enough of them to solve other cognitive tasks. We encounter a similar phenomenon in everyday life: everyone knows how difficult it is to focus on something else while listening to someone's speech, especially in the noise.
The fourth hypothesis focuses on the medial temporal lobe of the brain, which does not belong to the auditory system, but is involved in the processing of sound information. As is known, in the typical course of Alzheimer's disease, pathological tau proteins accumulate early in the tissue of this particular structure. It is likely that the reason may be a decrease in the activity of this part of the brain due to hearing loss. Researchers even propose a specific molecular mechanism associated with the NMDA cellular receptor mediating the mutual enhancement of deafness and protein pathology inherent in Alzheimer's disease.
By the way, the scientists themselves believe that all the hypotheses put forward do not exclude each other at all. And it will be possible to understand which of these mechanisms or their combinations actually works when they are tested on the corresponding animal models of pathologies.
A practical conclusion is important to us: timely hearing correction can prevent cognitive impairment – with the help of a hearing aid or the installation of a cochlear implant. This will restore the volume of social interactions and/or normal activity of brain structures, which in any case will have a positive effect on mental health.
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