Could Dementia be Solved with Hearing Aids?
As populations age and healthcare improves, people live longer and are at greater risk of being affected by age-related diseases. Life expectancy is increasing every year and shows no sign of slowing down. By 2030, it is predicted that 20% of the population will be over 65 years of age. Consequently, diseases and disorders that affect elderly groups are of increasing and immediate interest (Jagger et al., 2007; Kirkwood, 2017; Martini et al., 2014). Although dementia is not part of normal ageing, ageing is one of the greatest risk factors for all neurodegenerative diseases. The cell degradation seen in neurodegeneration accelerates, and the body’s ability to repair reduces with age (Wahl et al., 2019). About 50 million people suffer from dementia worldwide. This is predicted to increase to 150 million by 2050 as populations continue to age. Dementia is not an inevitable side effect of ageing, however, and may be prevented. There is a growing body of evidence that age-related hearing loss may be a causal factor in dementia and that treating this hearing loss could slow or prevent dementia’s progression (Chern & Golub, 2019).
Dementia
The word ‘dementia’ is often used as if it were a disease. Dementia is sometimes used interchangeably with diseases like Alzheimer’s Disease, which causes a lot of confusion when talking about these conditions. ‘Dementia’ is the term used to describe a set of cognitive symptoms, including memory loss, that are caused by diseases that deteriorate the brain. As the brain controls cognition, disorders that affect the brain can affect a variety of cognitive functions. Although most associated with memory loss, dementia can also result in the worsening of other cognitive abilities such as mood, emotion, social behaviours, executive function and even language (Arvanitakis & Bennett, 2019; Dening, 2019). Dementia implies the decline of several cognitive and mental abilities. Dementia is specifically caused by an acquired decline in these abilities, distinguishing dementia from neurological disturbances that result in amnesia or brain lesions that cause aphasia (Gustafson, 1996).
Figure 1: Dementia is an Umbrella Term (Unknown, n.d).
There are several diseases that can result in dementia, each distinguished by its cause. Causes of dementia can range from brain injuries to a buildup of particular proteins in the brain. For example, the most common cause of dementia, Alzheimer’s Disease (AD), is caused by the accumulation of amyloid β (Aβ) and neurofibrillary tangles in the brain. AD makes up about 50% of dementia cases, while other dementia types, such as vascular dementia, make up 20-25% of cases, and mixed dementia makes up 5-10% (Chen et al., 2009). Vascular dementia has a worse prognosis than AD, with a life expectancy of only three to five years after diagnosis. The life expectancy for AD depends on the severity, or aggressiveness, of the disease in an individual. Life expectancy for mild dementia is six years, while for severe AD life expectancy is two to three years. There is also a difference in life expectancy in the age of onset of the disease. Those who develop AD below 75 years of age survive longer with the disease than those who develop AD after 75 (Kua et al., 2014).
There is a difference between a normally ageing brain and dementia. There are certain biological processes considered ‘hallmarks’ of normal brain ageing. As it ages, the brain goes through a decline in neuroplasticity and resilience, impaired molecular waste disposal and neuronal network activity, as well as oxidative damage and inflammation. The normal ‘Hallmarks of Ageing’ will occur in healthy brains but will also be present in those with dementia as they age. Conversely, people that don’t experience cognitive impairment can sometimes show features of dementia. Therefore, the degree of neuropathology, such as a build-up of Aβ in AD, differentiates normal ageing brains from those with dementia (Wahl et al., 2019). However, there are commonalities in brain characteristics between ageing and the onset of dementia. The hallmarks of ageing can even make the brain more vulnerable to neurodegenerative disease and stroke. There is evidence that a sedentary lifestyle will increase the rate of brain ageing and cognitive decline, while the brains of those whose lifestyles involve exercise, good nutrition and intellectual challenges will age healthily (Mattson & Arumugam, 2018).
Figure 2: Hallmarks of Ageing (Mattson & Arumugam, 2018).
Hearing Loss
The senses can be an important determinant of cognitive decline. The pathways and systems that hearing, sight, taste, touch and smell take in our brains can be used to diagnose and even treat neurological disorders. Hearing and sight have both been shown to be important diagnostic tools in dementia-related cognitive decline. The decline of a person’s sense of smell and touch is highly indicative of the progression of neurodegenerative conditions. As a result, it has been proposed that restoring these senses in patients with cognitive impairment could improve the prognosis of these neurological diseases. Studies have shown that not all sense restoration has the same benefit. However, sight and hearing can be reliably restored and have shown cognitive improvement following this restoration. In particular, hearing loss can be restored through well-developed electrical devices like hearing aids which have been proven to effectively treat hearing loss (Martini et al., 2014). Hearing loss is the most common sensory impairment and can occur in newborn babies, children, and adults, as well as the elderly (Martini et al., 2014). In the over-60 age group, hearing loss is usually due to the loss of inner and outer hair cells, the specialised sensory receptor cells on the basilar membrane of the Organ of Corti in the inner ear. These are called ‘hair cells’ due to the stereocilia on their surface, which detect movement in the perilymph of the ear caused by sound. The hair cells that detect higher frequencies are lost first, explaining the loss of high-frequency hearing experienced during ageing (Ashmore, 1989; Lawrence et al., 2019; Pickles & Corey, 1992).
Figure 3: Organ of Corti and Hair Cells of the Ear (Nankali et al., 2020).
Hearing loss causes an increase in ‘cognitive load’ on the rest of the brain. This occurs as other parts of the brain try to compensate for the lost function of the ears and neuronal cells specialised for hearing. The brains of individuals with hearing loss will use other neural populations and recruit them to understand and recognise speech, a function they are not best suited for (Martini et al., 2014). This increased cognitive load and redirection of resources towards auditory perceptual processing causes auditory signals to degrade further. When the brain diverts too many resources towards one task, resources for other cognitive tasks are depleted. The individual must also pay greater attention to trying to hear, causing further cognitive load. Developed in 1998, the cognitive load theory shows that each task carried out by the brain uses a certain amount of cognitive effort and information processing. If a task requires more resources than this, due to cognitive decline in other areas of the brain or information overload, cognition is hampered and less effective (Uchida et al., 2019). Restoring hearing by using hearing aids can reduce this cognitive load and demand on the brain. By restoring the function of hearing-specialised neurons, the excessive cognitive load on the rest of the brain is reduced, which can protect the brain from further deterioration (Martini et al., 2014).
Hearing Loss and Dementia
Hearing loss was first linked to dementia in 1989 by Uhlmann et al. This study demonstrated a strong association between dementia and age-related hearing loss and concluded that hearing loss contributes to cognitive decline in adults. Furthermore, it was found that the severity of hearing loss increased this risk: greater hearing loss was associated with more severe dementia. This strong association suggests that hearing loss may be a risk factor for developing dementia. Multiple clinical research studies have since supported these findings. Hearing loss has been shown to increase the rate of cognitive decline by 30-40%. Mild hearing loss can double the risk of developing dementia, while moderate hearing loss triples this risk, and severe hearing loss can cause five times the risk of developing dementia (Lin & Albert, 2014). Hearing loss is also linked to depression, which may be, in turn, linked to dementia. Changes in the white matter of the brain, vascular changes and loss of neurotransmitters seen in depression are similar to the effects of AD. As with hearing loss, depression could cause cognitive decline to such an extent that it results in dementia (Korczyn & Halperin, 2009; Lawrence et al., 2019).
Figure 4: Contributing Factors of Dementia (Livingston et al., 2020).
However, there is an argument against the hypothesis that hearing loss leads to dementia. As almost every person will experience hearing loss as they age, it can be argued that hearing loss, and cognitive decline, are normal parts of ageing. Although both hearing loss and dementia primarily affect older people of the population, they are distinct issues (Lin & Albert, 2014).
Restoring hearing loss
Hearing loss is a common target for improving the quality of life in the elderly, as this is usually a treatable condition. Hearing loss causes isolation, cognitive deterioration, and increased morbidity. Despite the prevalence of this condition, it is underdiagnosed and often undiagnosed. Up to 60% of elderly people suffer from hearing loss, a figure that rises to 97% of elderly people in institutions. It is clear that hearing loss detrimentally affects people and leads to further cognitive decline (Allen et al., 2003). As hearing loss has been shown to impair cognitive function and is linked to dementia, many studies have investigated the effects of improving hearing in those suffering from dementia. In one study, Allen et al. (2003) used hearing aids to improve auditory acuity in dementia subjects with hearing loss. 42% of these subjects showed improvement. However, this study did not show any clear improvement in cognitive function or ability to perform daily activities. Furthermore, the study showed that the removal of ear wax was sufficient to significantly improve hearing in 10% of patients, showing that dementia patients are receptive to simple auditory procedures.
Figure 5: Hearing aid (Garrison, 2022).
Another possibility for improving dementia prognosis is to reduce the rate of cognitive ageing. As brain deterioration leading to dementia is age-related, preventing or reducing brain ageing could delay the onset of dementia. Poor metabolic health, obesity, heart disease and smoking are all risk factors for dementia. Lifestyle changes, such as exercise, cognitive stimulation, increased cardiovascular health, metabolic health and improved nutrition, such as a Mediterranean diet, have been shown to directly reduce the rate of ageing and age-related diseases, including dementia (Wahl et al., 2019).
Which comes first?
The statistically significant correlation between hearing loss and dementia is well known, with about 9% of dementia cases associated with hearing loss (Griffiths et al., 2020). However, the exact role of hearing loss in dementia is less understood. Does hearing loss cause dementia, or is it a risk factor? Determining the cause-effect link between dementia and hearing loss could determine the approach new dementia treatments take. Hearing loss and cognitive decline can happen at similar times in an individual’s life, so it is difficult to determine which happens first or if one can cause the other (Martini et al., 2014).
Figure 6: Dementia is not a normal part of ageing (Unknown, n.d).
Conclusion
As the fastest growing age group is the over 60s, the need to understand disease that affects elderly populations is urgent. Understanding the contributing factors of dementia is key to developing preventions and treatments for the diseases that cause these debilitating effects. Hearing loss in adults can cause cognitive deterioration, social isolation and depression, which can all lead to dementia. The social isolation caused by hearing loss alone can increase disorientation and cognitive decline. Treating this hearing loss has been shown to reduce the risk of developing dementia and even slow the progression of this life-altering disease. One of the most significant factors is that hearing loss is highly treatable with hearing aids. Something as simple as wearing hearing aids when required can improve a patient's prognosis, while a true cure for dementia remains out of reach. By treating hearing loss, cognitive decline can be stopped in its tracks, and the quality of life of our ageing population can be drastically improved.
Bibliographical References
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