Introduction:
Hearing loss affects almost two-thirds of adults over 70 and is significantly undertreated. Investigating whether hearing loss independently contributes to cognitive decline is a crucial initial step in determining the potential effectiveness of hearing rehabilitative interventions in mitigating cognitive decline. With the prevalence of dementia expected to double every 20 years due to global population aging, it is essential to identify factors and understand mechanistic pathways leading to cognitive decline and dementia in older adults. Hearing loss is a significant risk factor for dementia, particularly Alzheimer’s disease (AD). mid-life hearing loss doubles the risk of developing dementia more than any other factor. Despite the strong association between hearing loss and AD, the mechanisms underlying this connection are unknown.
What Is Meant by Hearing Loss and Cognitive Decline in the Elderly?
Hearing loss in the elderly can be sensorineural hearing loss (SNHL), an age-related issue arising from various mechanisms like auditory deterioration, manifesting in different forms based on the affected portions of the auditory pathways. Sensorineural hearing loss can be peripheral or central. Peripheral SNHL occurs when damage is limited to the cochlea or cochlear nerve, while central SNHL involves damage to the superior auditory pathways, including the cochlear nuclei and auditory cortex.
Cognitive decline refers to reduced cognitive function associated with age, and when it exceeds the expected level for a person’s age, mild cognitive impairment (MCI) is diagnosed. AD is a progressive neurodegenerative disease characterized by deficits in episodic memory, working memory, and executive function, with specific biomolecular markers. MCI is considered an early sign.
Central SNHL arising from superior auditory pathways, like vascular damage affecting signal transmission and brain atrophy, as a consequence of vascular damage or aging, is a link between cognitive decline, MCI, AD, and hearing loss. In these conditions, due to brain atrophy, memory progressively diminishes, and psychological diseases can further impact memory function, particularly affecting the elderly.
How Do Hearing Loss and Cognitive Decline Occur in the Elderly?
The relationship between hearing loss and cognitive decline involves various mechanisms:
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Increased Cognitive Load: Cognitive load theory suggests that cognitive effort or the amount of information processing required for a specific task can affect learning. According to this theory, hearing loss likely increases the cognitive effort needed to process and understand speech, as reduced or distorted sensory input demands greater brain effort. Excessive cognitive load in auditory perceptual processing may lead to significant brain structural changes and neurodegeneration, potentially creating a cycle where available cognitive resources for auditory perception are reduced, hypothetically contributing to cognitive decline.
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Changes in Brain Structure and Function: Contemporary evidence shows that hearing impairment is associated with cerebral alterations. Magnetic resonance imaging (MRI) reveals a correlation between hearing loss and reduced volume of the whole brain and the primary auditory cerebral cortex in the temporal lobe. Chronic hearing impairment also leads to less activation of central auditory pathways, dysfunction of the auditory-limbic pathway, and atrophy of the frontal lobe and hippocampus. In parallel, hearing impairment is associated with increased stimuli from other sensory organs, such as eyes, prompting a compensatory increase in volume through cross-modal cortical reorganization. While this reflects the brain’s adaptability to loss through neuroplasticity mechanisms, it can negatively impact cognition. In individuals with hearing loss, compensatory adaptation significantly reduces the brain’s ability to process sounds, affecting speech understanding, and even mild hearing loss weakens hearing areas of the brain. Consequently, areas necessary for higher-level thinking compensate for weaker areas, diminishing cognitive performance, particularly in executive functions, during neuropsychological assessments.
- Common Pathological Conditions: Some researchers propose that a common pathological condition may trigger hearing loss and cognitive impairment. According to this hypothesis, both conditions result from a common neurodegenerative process in the aging brain involving the degeneration of the stria vascularis, loss of hair cells and primary afferent neurons, and changes in neurotransmitter release. Both age-related hearing loss (ARHL) and cognitive impairment share common risk factors, mainly vascular, such as atherosclerosis, smoking, and diabetes, increasing the risk of cardiovascular disease and stroke. The role of the apolipoprotein E (APOE) gene, strongly associated with neurodegeneration, has also been suggested, with some studies indicating an association between the APOE e4 allele (linked to AD) and hearing loss. However, the specific molecular link between hearing loss and dementia remains unknown. Syndromic associations of dementia with cochlear or ascending auditory pathways dysfunction are uncommon and typically occur in the context of neurological impairment, often in younger individuals.
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Social Disengagement: Social isolation is another mechanism underlying the connection between hearing loss and cerebral alterations. individuals with hearing impairment often experience less satisfactory social interactions due to difficulties filtering out conversions from background noise. Communication challenges associated with hearing loss may encourage solitude, considered a risk factor for cognitive disorders, reduced cognitive stimulation, apathy, and potentially depression. Greater hearing loss correlates with increased odds of social isolation. Depression associated with hearing loss and cognitive impairment may manifest episodically, presenting more in somatic symptoms and lethargy (depression without sadness), making diagnosis challenging.
Social isolation also promotes negative biological mechanisms, such as increased transcription of pro-inflammatory status, a major risk factor for cerebral function damage.
How to Diagnose Hearing Loss in Cognitive Impairment?
Assessing hearing loss in patients with cognitive impairment:
The National Institute on Deafness and Other Communication Disorder (NIDCD) reports that individuals with hearing loss typically wait an average of seven years before seeking treatment. Hearing loss may impact cognitive function during this delay, potentially leading to irreversible decline. Additionally, in people with cognitive impairment, hearing loss is often overlooked or untreated.
Cognitive test performance, crucial for dementia assessments, is influenced by various factors, including sensory impairment. If someone cannot hear the questions asked during assessments, they are less likely to provide accurate answers. This becomes particularly problematic in noisy environments like hospitals, where misdiagnosing normal cognitions as cognitive impairment is possible.
The NICE guidelines on hearing loss recommend conducting hearing evaluations for individuals with suspected cognitive impairment, repeating them every two years for those diagnosed with dementia. Screening for central auditory dysfunctions is advised as a low-cost and effective method to identify precursors to AD pathology, especially in at-risk populations.
Central hearing loss appears more common in mild cognitive impairment and AD than in aging individuals. Deficits in central auditory processing may serve as potential biomarkers for neurodegenerations.
What Is the Treatment for Hearing Loss and Cognitive Decline in the Elderly?
Addressing hearing loss in those with cognitive impairment can enhance communication, improve quality of life, and reduce behavioral symptoms associated with dementia, thereby alleviating stress for families and caregivers. Currently, treatment options for hearing loss in patients with cognitive decline include hearing aids, amplification devices, and cochlear implants. The impact of these interventions remains unclear, while preventive measures involve local or systemic antioxidant therapies that demonstrate protective properties against oxidative stress mechanisms leading to hearing loss.
Conclusion:
The relationship between hearing loss and cognitive decline in the elderly underscores the importance of timely intervention. Evidence suggests that treating hearing loss through aids and devices may enhance communication and potentially mitigate cognitive decline. The mechanisms linking these two conditions involve increased cognitive load, changes in brain structure, pathological conditions, and social disengagement. Therefore, addressing hearing loss is valuable for promoting cognitive well-being in aging populations.
