Scientists have developed a new theory as to how hearing loss may cause dementia and believe that tackling this sensory impairment early may help to prevent the disease.
Hearing loss has been shown to be linked to dementia in epidemiological studies and may be responsible for a tenth of the 47 million cases worldwide.
Now, published in the journal Neuron, a team at Newcastle University, UK, provide a new theory to explain how a disorder of the ear can lead to Alzheimer’s disease – a concept never looked at before.
It is hoped that this new understanding may be a significant step towards advancing research into Alzheimer’s disease and how to prevent the illness for future generations.
Newcastle experts considered three key aspects; a common underlying cause for hearing loss and dementia; lack of sound-related input leading to brain shrinking; and cognitive impairment resulting in people having to engage more brain resources to compensate for hearing loss, which then become unavailable for other tasks.
The team propose a new angle which focuses on the memory centers deep in the temporal lobe. Their recent work indicates that this part of the brain, typically associated with long-term memory for places and events, is also involved in short-term storage and manipulation of auditory information.
They consider explanations for how changes in brain activity due to hearing loss might directly promote the presence of abnormal proteins that cause Alzheimer’s disease, therefore triggering the disease.
Professor Tim Griffiths, from Newcastle University’s Faculty of Medical Sciences, said: “The challenge has been to explain how a disorder of the ear can lead to a degenerative problem in the brain.
“We suggest a new theory based on how we use what is generally considered to be the memory system in the brain when we have difficulty listening in real-world environments.”
Work on mechanisms for difficult listening is a central theme for the research group, including members in Newcastle, UCL and Iowa University, that has been supported by a Medical Research Council program grant.
Dr. Will Sedley, from Newcastle University’s Faculty of Medical Sciences, said: “This memory system engaged in difficult listening is the most common site for the onset of Alzheimer’s disease.
“We propose that altered activity in the memory system caused by hearing loss and the Alzheimer’s disease process trigger each other.
“Researchers now need to examine this mechanism in models of the pathological process to test if this new theory is right.”
The experts developed the theory of this important link with hearing loss by bringing together findings from a variety of human studies and animal models. Future work will continue to look at this area.
Hearing Loss and Age-Related Cognitive Decline: Two Highly Frequent Conditions in Aging
The global increase in life expectancy is obviously one of the major health achievements of the past 150 years, since, for the first time in history, most people worldwide can expect to live into their sixties and beyond.
The now well-known counterpart of this is that the number of age-related chronic conditions that negatively affect independence and life participation has also substantially increased, contributing to a pessimistic picture of old age. The challenge is not only to live longer, but to live longer with no or little incapacities.
Among the numerous conditions which affect the independence and life participation of older adults, hearing loss is one the most common. Age-related hearing loss, also called presbycusis, is a progressive, bilateral and symmetrical multifactorial disorder of hearing sensitivity due the consequence of the aging auditory system, including peripheral and central hearing .
It typically results from loss of hair cells at the basilar membrane, primarily affecting high-frequency hearing. As many as one-third of adults aged over 65 report hearing loss, making hearing loss the third most prevalent chronic health condition in older age [2,3].
In recent years, scientific research consistently highlighted the association between hearing loss in elders and numerous mental health outcomes, among which depressive symptoms have been frequently reported, as concluded by a recent meta-analysis of 35 studies, where hearing loss in older adults was associated with an increased risk of experiencing depression .
Moreover, hearing loss has been associated with self-perceived older age , social isolation , reduced daily living activity participation [7,8], and lower quality of life [9,10].
Beyond well-being and psychosocial outcomes, another major domain of mental health suspected to be impacted by hearing loss is cognitive functioning. There is now strong and consistent evidence that older adults experiencing hearing loss present a poorer cognitive performance.
Not only have cross-sectional associations between hearing loss and low cognitive scores been reported [11–13] but also higher rates of cognitive decline. Indeed, numerous studies involving longitudinal follow-up of participants reported an accelerated cognitive decline with advancing age in older adults suffering from hearing impairment compared to those with no hearing loss [14–23], results that have been updated and synthesized in several review papers [24–26].
With such a body of evidence on the link between hearing loss and accelerated age-related cognitive decline, the question of whether hearing loss increases the risk of developing neurocognitive disorders arises. First, as is well known, in the coming years, along with a life expectancy increase, the number of people suffering from neurocognitive disorders is expected to increase.
Secondly, although the causes of most dementing illnesses remain to be clarified, the physio-pathological pathways (accumulation and deposition of beta-amyloid for Alzheimer’s disease or vascular neurodegeneration for vascular dementia) are assumed to be modulated by a wide range of factors—including psycho-social, life-style, nutritional factors—that contribute to moderate, either positively or negatively, the incidence and clinical expression of symptoms.
Therefore, the question of whether hearing loss increases the risk of developing dementia when one ages appears to be particularly relevant. These last 5 years, a number of studies involving a longitudinal design provided valuable input. Although still limited in number, these studies report consistent results, supporting the hypothesis of an increased risk of dementia which remains significant even after statistically accounting for potential confounders such as demographic and/or cardiovascular factors.
Such results have been gathered from studies considering hearing loss either with audiometric measures [15,27,28], speech recognition tests , or indirect/self-reported measures [30–32]. Although most of the studies considered peripheral hearing loss, the link between age-related central auditory processing disorder  and cognitive decline and/or dementia has also been reported by two longitudinal studies [34,35].
Possible Mechanisms Explaining the Relationship Between Hearing Loss and Cognitive Decline in Older Adults
To explain the relationship between hearing loss and cognitive impairment, the literature has proposed several potential mechanisms. Two recent articles [36,37] offer a quite thorough review of it, so these hypotheses will be only briefly mentioned in the present paper.
A first, relatively practical, explanation is that hearing impairment may bias the cognitive testing procedure, often involving verbal tests or orally provided test instructions. For instance, if a list of to-be-remembered words is orally given to a participant with hearing loss, the participant may require additional effort to accurately perceive the words, with less cognitive resources being available for encoding/learning processes.
Future studies should make use of assessment tools suitable for people with a hearing impairment. It is noteworthy that two such tools have been recently developed: the Repeatable Battery for the Assessment of Neuropsychological Status for Hearing-Impaired individuals (RBANS-H)  providing audiovisual presentation of the instructions and the test items and the Hearing-Impaired Montreal Cognitive Assessment (HI-MoCA) , an adaptation of the MoCA.
Nonetheless, if the explanation of possible testing biases can partly account for cross-sectional associations between hearing impairment and low cognitive performances in the studies consisting of comparing the cognitive scores of hearing-impaired participants to non-impaired ones, such potential biases alone can hardly explain the now numerous studies reporting accelerated decline, since in longitudinal studies the participant’s performance is compared to his/her own over time and what is compared to a control group is the rate of decline, not inter-individual performance.
Thus, although this hypothesis cannot be fully discarded, and possibly, for some of the results reported in the literature, may explain part of the association, other mechanisms probably come into play.
A major potential explanation relies on what is known as the “deprivation hypothesis” [19,40]. According to this hypothesis, hearing loss may lead to cognitive decline, and eventually dementia, because hearing impairment is suspected to affect brain integrity, as suggested by several MRI studies.
For instance, a study conducted with 126 participants aged 56 to 86 years showed that the participants with hearing impairment exhibited accelerated brain atrophy (whole brain atrophy but more particularly in the right temporal lobe) compared to participants with normal hearing .
Consistently, another study conducted within the Rotterdam study on 2908 participants (mean age 65) reported that hearing impairment was associated with smaller total brain volume . Moreover, a study showed that experimentally induced hearing loss in young mice caused a decreased performance in spatial working and recognition memory tasks at 6 months, providing evidence for a causal relationship between hearing loss and cognitive impairment .
An alternate view postulates that hearing loss may have a link with cognition because it leads to reduced physical activity, lower mood and poorer engagement in social and leisure activities. As shown by two studies [14,44], the strength of the association between hearing impairment and cognitive decline is reduced after statistically controlling for social participation variables. Moreover, low physical activity, depressive symptoms, as well as social isolation, have been reported as risk factors of cognitive decline and dementia in numerous studies [45–50].
Therefore, the link between hearing loss and cognition could be an indirect link mediated by reduced social and physical activity. One mechanism that has been hypothesized to underlie the link between hearing loss and cognitive decline is related to the “cognitive load hypothesis” .
In the presence of hearing difficulty, the neural activity required to understand spoken language becomes more demanding if the brain is compelled to recruit additional neural populations to achieve accurate performance. The brain is assumed to constantly re-allocate attentional resources to help with deciphering and decoding the garbled auditory signal, which, in turn, may have a cognitive cost.
In sum, cognitive impairment associated with hearing loss could be the result of devoting high cognitive resources to effortful sensory perception, to the detriment of other cognitive processes/brain areas .
Finally, the “common cause hypothesis” assumes the existence of a common factor that is responsible for the progressive degradation in physiological systems with aging. Cognitive as well as non-cognitive changes with advancing age are the result of multiple dysfunctions which are partly distinct and partly entangled.
According to this hypothesis, both hearing loss and cognitive impairment are caused by common underlying processes such as generalized effects of the aging brain or age-related cerebrovascular disease. In its original view, the hypothesis stated that sensory acuity (including hearing and vision acuity) could be markers of global physiological integrity of the brain .
Although presented as alternatives, the different hypotheses proposed to explain the hearing–cognition relationship may not be mutually exclusive. On the contrary, it is more than likely that several mechanisms, or all of them, act in conjunction.
reference link : doi:10.3390/jcm9030805
More information: Timothy D. Griffiths et al, How Can Hearing Loss Cause Dementia?, Neuron (2020). DOI: 10.1016/j.neuron.2020.08.003