How Does Dementia Affect Hearing – Hearing loss is a major risk factor for the development of dementia, particularly Alzheimer’s disease (AD). Hearing loss in midlife increases the risk of dementia twice as much as any other single factor. However, considering this strong relationship between hearing loss and AD, the mechanism responsible for this link is still unknown. Data from studies on hearing and intellectual disability, measured by standardized questionnaires, have shown a strong relationship between them. Similar findings have emerged from animal studies, which show that exposure to loud noises can affect cognitive abilities. or blockage of the ear canal. Interestingly, patients with age-related hearing loss show increased tau phosphorylation in the cerebrospinal fluid, but no such relationship was found for amyloid-β. In addition, early hearing loss in social isolation promotes the development of dementia through perceived reduced cognitive load and processing demands. Given this relationship between hearing loss and dementia, the question arises whether hearing rehabilitation can mitigate the onset or progression of AD. Indeed, there is a growing body of research that suggests that people who wear hearing aids for age-related disorders maintain better cognitive function over time than those who do not. These are encouraging findings, as they suggest that the use of hearing aids can be an effective treatment for people with hearing loss both before (for those at risk of developing AD) and after development of symptoms. This review aims to summarize the current theories related to hearing loss in cognitive decline, presenting the main findings from animal studies, observational studies and summarizing gaps and limitations to be considered in this topic. Hereby, we propose directions for future studies to address the lack of randomized controlled trials in the field. This refusal is responsible for the inability to provide a definitive judgment on the use or not of hearing interventions aimed at cognitive decline related to hearing loss.
Dementia represents a global burden—a 2022 report estimated that by 2019, 55 million people will be living with dementia (Gauthier et al., 2022). Future projections suggest that dementia will continue to increase in prevalence, approaching 140 million people by 2050 (Gauthier et al., 2022). In order to slow down this excessive growth, new initiatives are needed, either to improve the management of those already diagnosed, or by preventing those at risk of developing it.
How Does Dementia Affect Hearing
Logically, the risk factor for disability precedes dementia, due to clinical progression. It should be noted that age is a significant risk factor – the natural aging process leads to a natural risk of cognitive decline, independent of dementia. In fact, aging is associated with a 70% increase in the incidence of dementia (Juan and Adlard, 2019). In addition, it is well known that loneliness and social isolation are factors that cause cognitive dysfunction, and are associated with increased decline (Cacioppo and Hawkley, 2009; Dominguez et al., 2021). In addition, research shows that people classified as socially isolated have a 50% increased risk of developing dementia (Evans et al., 2018; Dominguez et al., 2021). Interestingly, a recent Lancet study (Livingston et al., 2020) focused on various risk factors for the development of dementia, showed the importance of reducing the risk , which suggested that modifying these risk factors could reduce the risk of dementia by 40 .% (LaPlume et al., 2022). Among various risk factors, including brain injury, hypertension, depression, and diabetes mellitus, the review highlights hearing loss (HL) as one of the risk factors that, when excluded, reduces the risk of dementia by 8%. Mild, moderate or severe HL in middle age (defined as between the ages of 45 and 65) was associated with a 2, 3 and 5 fold increase in the risk of cognitive decline and dementia, respectively. (Lin et al., 2011). Livingston et al., 2020). Additionally, observational studies suggest that the severity of hearing loss (HI) is associated with an accelerated risk of cognitive decline (LaPlume et al., 2022), and one report even showed that the risk of dementia increases linearly with baseline severity. HL (1.27 per 10 dB loss) (Ford et al., 2018). However, limitations remain in confounding factors, which may affect the generalizability of these results (Ford et al., 2018).
Hearing Loss & Dementia
In this review, we examine HL as a risk factor for the development of cognitive impairment and dementia, focusing on molecular mechanisms. In doing so, we summarize the research surrounding the clinical manifestations of HL in dementia, the mechanisms proposed for this relationship, and the role of interventions, including the use of hearing aids. (HA) to reduce the impact of HL on the development of cognitive impairment. . dementia. in the development of dementia.
Hearing loss affects 466 million people worldwide (World Health Organization, 2021). People with HL disability represent more than 5% of the world’s population (World Health Organization, 2021) and it is estimated that by 2050, almost 1 in 10 people will need rehabilitation. ear. Causes of hearing loss include genetics, ear infections, exposure to serum (ear fluid), trauma to the ear or head, loud noise exposure (NHL), medications ototoxic and others.
The mechanism of HL involved in dementia revolves around sensorineural hearing changes, which involve dysfunction of the cochlea. It is generally considered age-related hearing loss (ARHL) or presbycusis, which affects about 40% of people over 65 years old (Gates and Mills, 2005). Causes of HL including presbycusis, noise-induced hearing loss (NIHL) and ototoxicity, cause chronic HL, and subsequently, result in limited management options (Lee and Bance, 2019).
ARHL is highly polygenic, with more than 100 genes identified in asymptomatic hearing loss (Lewis et al., 2018; Van Camp and Smith, 2023), and of these many genes, it is possible each brings its own contribution. legacy by 36-70% (Nagtegaal, 2019). Most cases of genetic deafness involve changes in the cochlea, the hearing organ; for example, the OTOF gene encodes the protein autoferlin, which allows synaptic vesicles to fuse with the plasma membrane at the ribbon synapse. Therefore, mutations in this gene can lead to impaired exocytosis of inner hair cells, which interrupts the transmission of hearing signals and can cause early deafness (Vona et al., 2020). ). However, some forms of genetic hearing loss can be caused by a malfunction of the central auditory system. Genome-wide association studies (GWAS) focusing on ARHL-related genes, although showing conflict, do not show strong agreement (Nagtegaal, 2019; Wells et al., 2019; Liu et al., 2021; Lewis ). et al., 2022). When examining the genetic link between hearing loss and AD, no GWAS has found a direct link between ARHL and AD. However, a recent study using UK biobank data (Brenowitz et al., 2020) has shown that genetic risk for AD is influenced by hearing speech in voice. Additionally, in another report (Mitchell et al., 2020), individuals with a high polygenic risk score (PRS) for AD were more likely to have hearing impairment than individuals with Lower PRS.
Hearing Difficulty Is Linked To Alzheimer’s Disease By Common Genetic Vulnerability, Not Shared Genetic Architecture
Sudden sensorineural hearing loss (SSHL) is also known to be a risk factor for the development of dementia. This form of HL is characterized as an otological emergency, defined by HL of at least 30 dB for 72 hours, which affects at least three hearing loss, usually with a viral, vascular, or autoimmune etiology ( Lee and Bance, 2019). In a retrospective cohort study comparing the incidence of dementia in people with and without a previous diagnosis of SSHL, it was found that the occurrence of SSHL was associated with a 1.39 higher risk in the case of all causes of dementia (Tai et al. , 2021).
However, especially in presbycusis, other types of mechanisms are also implicated, including metabolic (for example, mitochondrial dysfunction), oxidative stress (including changes in reactive oxygen species and deficiency ‘superoxide dismutase), other neurotransmitter deficiencies (GABA deficiency) (Jafari et al., 2021) (Figure 1). A better understanding of the underlying mechanisms and how the different classes of ARHL can reveal dementia is not yet fully understood (Bowl and Dawson, 2019).
Figure 1. Types of age-related atrophy, including stria vascularis atrophy, hair cell loss and primary cochlear neuron degeneration (Quaranta et al., 2015; Fortunato et al., 2016) ). Specific changes observed in the stria vascularis include age-related morphological changes such as numerous cytoplasmic vacuoles, enlargement of the intracellular space and abnormalities of mitochondria and their components, especially the disorganization of cristae (Lyu et al., 2020). These changes were shown by oxidative damage and reduction of TMEM16A, an active calcium chloride channel (Spicer and Schulte, 2005; Zhou et al., 2019). Cochlear disease or trauma has been shown to cause hair cell loss, with potential factors including exposure to industrial noise, reactive oxygen species and superoxide dismutase deficiency. (SOD1) (McFadden et al., 1999a, b, 2001; Emmerich et al. ., 2000;
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