Association between Subjective Hearing Impairment and Cognitive Status among Korean Elderly

Background: The goal of this study was to investigate the impact of subjectively assessed hearing impairment on cognitive function in elderly Koreans living in the community. Methods: In the 2020 Survey of Living Conditions and Welfare Needs of Korean Older Persons Survey, 9920 subjects (5949 females; 60%) aged 65 or more years were examined. Using the Korean version of the Mini-Mental Status Examination (MMSE-KC), cognitive function was evaluated. To investigate the relationship between hearing impairment and cognitive status, multiple logistic regression analysis was performed with adjustment for multiple confounding variables (socioeconomic, health behavior, psychological factors, and functional status). There were 2297 participants in the hearing impairment group (23.2%) and 7623 subjects in the no-hearing impairment group. Results: Cognitive impairment was significantly higher in the hearing impairment group (37.2%) compared to the no-hearing impairment group (27.5%). After adjusting for confounders, hearing impairment was significantly associated with an increased risk of cognitive decline (odds ratio [OR] 1.21; 95% confidence interval [CI] 1.08–1.35) compared to no-hearing impairment group. Conclusions: Although a cross-cectional design of this study does not allow a causal reasoning, our findings show a significant association between the hearing loss of older adults and their cognitive impairment. Hearing impairment should be regarded as a risk factor for cognitive disorders.

Keywords: cognition, elderly, hearing impairment

How to cite this article:
Lee S, Chung JH. Association between Subjective Hearing Impairment and Cognitive Status among Korean Elderly. Noise Health 2023;25:113-9
Introduction

According to the most recent Global Burden of Disease report, hearing loss has become the third leading cause of years lived with disability and a major concern for global health, particularly among the elderly.[1] According to National Health and Nutrition Eamination Survey (NHANES) audiometric data, 61 million U.S. adults have hearing loss in at least one ear.[2] Hearing loss becomes more common with age and is more common in men than in women; it is estimated that one-third of men aged 40 and up have hearing loss.[3] Hearing loss and cognitive decline both have multifactorial causes and are frequently progressive, owing to cumulative auditory and neurodegenerative damage that accumulates over time.[4] Understanding whether hearing loss influences early cognitive decline could thus reveal a valuable opportunity for identifying determinants of early cognitive decline and provide critical insights for earlier intervention and prevention.[5] Hearing loss associated with aging may be associated with neurodegenerative, functional, physical, and psychosocial impairment. For example, hearing loss is a cause of cognitive impairment.[6] A growing number of studies have found a link between hearing loss and cognitive decline, suggesting that hearing loss could be a possible early sign of dementia.[6],[7],[8] Hearing-related cognitive decline in mice suggested that early hearing impairment can be one of the determinants between physiological and pathological cognitive aging.[9]

Some studies appeared to provide some support for the link between hearing loss and cognitive impairment; however, it also identified some studies that argued against this link. Although there have been many studies on the relationship between hearing impairment and cognitive function, there have been few studies on the relationship between cognitive impairment and subjective hearing impairment in the elderly. Eliminating all modifiable risk factors for dementia could prevent or postpone up to 40% of dementia cases globally.[10] So, in our study, we investigate whether there is an association between subjective hearing impairment and cognitive decline using the data from a nationally representative Korean elderly population.

Methods

Study Participants

In this study, we used data from the Survey of Living Conditions and Welfare Needs of Korean Older Persons, which is a representative nationwide survey of noninstitutionalized elderly persons in Korea conducted by the Korea Institute for Health and Social Affairs in 2020. The participants in the 2020 National Survey of Older Koreans were aged 65 years or older. The sample was selected using a proportional two-stage stratified sampling method. Based on the Population and Housing Census, the sampling and weighting were applied on 17 cities and provinces.[11] This survey was conducted by well-trained interviewers in a face-to-face manner. No further ethical approval was required as informed consent was obtained from all participants and the data were publicly accessible.[11]

The participants in this study comprised 10,097 individuals aged 65 years or older. We excluded 177 people who had not completed the Korean version of the Mini-Mental Status Examination (MMSE-KC), short version of the Geriatric Depression Scale (GDS), limited activities of daily living (ADL), limited instrumental activities of daily living (IADL), and K-FRAIL scale. The final sample included 9920 participants (males, n = 3971; females, n = 5949; age range: 65–99 years). The subjective evaluation of hearing impairment was measured as (1) comfortable, (2) slightly uncomfortable, and (3) very uncomfortable, using a self-report method. We recategorized as subjective hearing impairment group (slightly uncomfortable or very uncomfortable, n = 2297) and subjective no-hearing impairment group (comfortable, n = 7625).

Assessment

Sociodemographic and health-related characteristics

Sociodemographic variables, such as sex (male or female), age (65–69, 70–74, 75–79, ≥ 80 years),[12] marital status (married or single, widowed, or divorced), educational status (0–3 years, 4–6 years, or ≥ 7 years), job status (employed or unemployed), economic level, and religion (yes or no) were investigated. As physical inactivity may result in a loss of muscular mass and strength, which could eventually lead to frailty,[13] we adjusted physical activities for frailty. The International Physical Activity Questionnaire-Short Form,[14] which has been validated in the Korean elderly population,[15] was used to define physical activity. Physical activity was defined as > 150 minutes/week of exercise.[16] Alcohol consumption was defined as drinking alcohol ≥ 2 days/week. Heavy alcohol consumption was defined as consuming ≥ 7 standard-sized drinks/day for males and ≥ 5 standard-sized drinks/day for females.[17] We used the Nutritional Screening Initiative criteria to assess the nutritional status of the elderly (good: 0–2 points, moderate: 3–5 points; high nutritional risk: ≥ 6 points).[18] Frailty in seniors is linked with the use of polypharmacy.[19],[20] Thus, we adjusted number of medications for frailty. The number of chronic diseases was categorized as follows: 0–1 and ≥ 2.

Cognitive function

Cognitive function was assessed using the MMSE-KC, which was scored as 0–30.[21] Cognitive impairment was determined when the MMSE-KC score was < 1.5 standard deviations from age-, sex-, and education-adjusted elderly Korean populations in a previous study.[22]

Depression

Depression was defined using the short Korean version of the GDS, which was validated in a previous study.[15] GDS scores ranged from 0 to 15. The optimal cut-off score was ≥ 8, which has been shown to have high sensitivity (0.9429) and specificity (0.7255) for depression.[23]

Functional capacity

Visual and hearing impairment, ADL, and IADL were investigated. The evaluation of ADL was based on the Korean ADL scale.[24] Participants with limitations in more than one of the following items were defined as having a limitation of ADL: (1) dressing, (2) face washing, tooth brushing, and hair washing, (3) bathing, (4) eating, (5) arising from a sitting position and walking across the room, (6) toilet use, and (7) bowel and bladder control. Limitations in IADL were defined using the Korean Instrumental ADL Scale.[24] We defined IADL as the inability to perform any one of the following 10 items: (1) personal grooming, (2) housework, (3) meal preparation, (4) clothes washing, (5) taking medicines as prescribed at the prescribed time, (6) money management, (7) traveling a short distance from home, (8) making purchasing decisions, paying for goods, and receiving change, (9) making and receiving phone calls, and (10) using transportation.

Evaluation of frailty

Frailty was defined using the K-FRAIL scale. The K-FRAIL scale is a 5-item questionnaire (fatigue, resistance, ambulation, illness, and loss of weight).[25] A score of ≥ 3 for three items on the K-FRAIL scale was accepted as frailty. Fatigue was determined using the following question: “Have you recently been much less active or felt less motivated to be active?” (yes = 1). Resistance was ascertained by asking the following question: “Do you have difficulty climbing 10 steps without a rest?” (not difficult at all or slightly difficult = 0, very difficult or cannot do at all = 1). Ambulation was determined using the following question: “Do you have difficulty walking 400 meters without an assistive device?” (not difficult at all or slightly difficult = 0, very difficult or cannot do at all = 1). Illness was established by asking the following question: “Have you been diagnosed with a chronic disease by your physician?” (0–4 comorbidities = 0, ≥ 5 comorbidities = 1). Finally, weight loss was determined by asking the following question: “Have you experienced weight loss of more than 5% in the last year?” (yes = 1). Based on the total scores, the participants were defined as nonfrail (0) prefrail (1–2), or frail (3–5).

Data Analysis

Descriptive statistical methods were used to describe the basic characteristics of the study population, with numbers and percentages reported for each variable. Multiple logistic regression analyses were performed with complex sampling adjusted for sociodemographic variables (i.e., age, sex, residence area, religion, spouse, living status, job, BMI, education level, economic status), health behavioral variables (i.e., smoking, alcohol, nutritional status, regular exercise, and drug), psychological variables (i.e., health status, stress, depression, abuse, and suicidal idea), comorbidities, and functional status (visual, hearing, chewing impairment, ADL, IADL, and leg muscle weakness). Data were analyzed using SPSS Statistics for Windows (version 21.0; SPSS Inc., Chicago, IL, USA). P < 0.005 was considered to indicate statistical significance.

Results

[Table 1] shows sociodemographic characteristics of participants. In the hearing impairment group compared to the no-hearing impairment group, there were more males (p < 0.001), higher average age (p < 0.001), more frequent: urban residence (p < 0.001), living alone (p < 0.001), unemployment (p < 0.001), lower education (p < 0.001), lower family income (p < 0.001), less regular exercise (p < 0.001), more chronic diseases (p < 0.001), more medication (p < 0.001), more depression (p < 0.001), less social membership participation (p < 0.001), and more suicidal ideas (p < 0.001).

Table 1 Differences of Sociodemographic and Clinical Characteristics between the Hearing Impairment Group and the No-hearing Impairment Group

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Functional status variables differences between the no-hearing impairment group and the hearing impairment group are shown in [Table 2]. The hearing impairment group compared to no-hearing impairment group had lower average MMSE-KC score (P < 0.001) and more frequent: cognitive impairment (p < 0.001), dependent ADL (p < 0.001), IADL (p < 0.001), frailty (p < 0.001), history of fall in the previous year (p < 0.0001), and hospitalization in the previous year (p < 0.001).

Table 2 Differences of Functional Status between the Hearing Impairment Group and the No-hearing Impairment Group

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After adjusting for sociodemographic variables (i.e., age, sex, residence area, religion, living status, job, BMI, education level, economic status), health behavioral variables (i.e., smoking, alcohol, regular exercise, drug and comorbidities), psychological variables (i.e., depression, social membership participation, abuse, and suicidal idea), comorbidities, and functional status (ADL, IADL, frailty, and falling history in the previous year), hearing impairment was significantly associated with an increased risk of cognitive decline (odds ratio [OR] 1.21; 95% confidence interval [CI] 1.08–1.35) compared to no-hearing impairment group.

Discussion

Our findings show that self-reported hearing loss is linked to cognitive decline, regardless of sociodemographic or other health-related factors. This finding is consistent with the findings of previous longitudinal studies that used objective measures of hearing impairment, confirming (by statistical analysis) the hearing function–cognition relationship in older people.[26],[27],[28] Thus, gathering evidence from observational studies could be an important initiative in determining whether age-related hearing loss is a modifiable factor among dementia prevention strategies. Randomized controlled trials cannot address the risk of hearing impairment on cognition due to ethical concerns about potential harm to participants’ wellbeing (studies could not be designed to deliberately assign “hearing impairment” to participants). Recent well-designed cohort studies[3],[27] have emerged as an alternative to thoroughly investigating the problem. Nonetheless, some studies came to negative conclusions about the relationship between hearing function and cognition.[29],[30]

The association between hearing function and cognition has been the subject of research in earlier meta-analyses.[31] Whether their hearing was being corrected or not, people with hearing loss had worse cognitive symptoms, according to the first meta-analysis.[31] However, this conclusion was drawn using data from individuals in a wide range of age groups, a variety of hearing and cognitive status tests, and a variety of statistical techniques. Using the UK Biobank data set, a large-scale cross-sectional study,[32] with information from 164,770 persons between the ages of 40 and 69, associations between hearing loss, cognitive function, social isolation, depression, and hearing aid use were examined. Despite social isolation and despair, using hearing aids was linked to improved cognition. Poorer cognition and hearing were both linked to social isolation. Hearing loss had a significant effect on cognition even when hearing aids were not used, implying that the effect of hearing loss on cognition is only partially mediated by hearing aid use. Social isolation was linked to both poorer cognition and hearing. Loughrey et al.[33] evaluated the relationship between hearing impairment and cognitive function, cognitive impairment, and dementia by analyzing the findings of 36 distinct studies with an estimated 20,264 distinct participants based on inclusion criteria. In the prospective cohort studies, there was a small but statistically significant correlation between hearing impairment and 7 cognitive domains, including episodic memory and processing speed. There was also a significant correlation between dementia and cognitive impairment (OR, 1.28; 95% CI, 1.02–1.59). Although there are some evidences that support the link between age-related hearing loss and cognitive impairment; there are, however, some studies that argued against this link.[34] Studies with negative results[35],[36] have prevented us from definitively concluding that hearing function is connected to the risk of cognitive impairment in elderlies.

The underlying pathophysiology of the link between hearing loss and cognitive decline is unknown; however, several potential processes could explain it. Hearing loss related degradation or depletion of sound cues may cause increased cognitive burden, more effortful listening, and changes in brain networks.[37] Poorer performance on cognitive tests could be the result of compensatory use of cognitive resources that would otherwise be available for task completion.[38]

There may be an increased reliance on additional cognitive processes to infer meaning from heard speech as the integrity of the auditory input signal deteriorates. As a result, insufficient auditory signal processing may negatively affect performance on cognitive tests that rely on auditory input by necessitating the activation of additional cognitive processes for speech and listening processing. Cognitive spare capacity may be affected if more cognitive resources are devoted to processing the auditory signal degradation. Persistent hearing loss compensation may lead to brain remodeling, change how cognitive resources are allocated, and diminish cognitive capacity.[39] Furthermore, there is evidence that reductions in other sensory modalities are also connected to declines in cognition, suggesting that the same mechanisms may underlie both the auditory and cognitive deficits that come with aging.[40],[41] Vascular, oxidative, and inflammatory mechanisms, genetics, and other factors are frequent causes of neurodegenerative diseases.[42] Cognitive decline is likely caused by a combination of shared causal mechanisms and sensory-related elements acting in combination.[43] Moreover, social interaction may be hindered by hearing loss, increasing social isolation and sadness, both of which raise the risk of cognitive decline.[44]

Our study also had some limitations. First, our cross-sectional study design does not allow causal reasoning. Second, although MMSE is the most commonly used screening method for assessing cognitive impairment, MMSE has limited value for diagnosis of mild cognitive impairment.[45] In spite of these limitations, the strengths of this study included the large representative sample, using household sampling procedures and the sizable sample of elderly, and adjusting multiple confounding variables including sociodemographic variables (i.e., age, sex, residence area, religion, spouse, living status, job, BMI, education level, and economic status), health behavioral variables (i.e., smoking, alcohol, nutritional status, regular exercise, and drug), psychological variables (i.e., health status, stress, depression, abuse, and suicidal idea), comorbidities, and functional status (visual, hearing, chewing impairment, ADL, IADL, and leg muscle weakness).

In conclusion, our present study showed that there was an association between self-reported hearing impairment and cognitive impairment. This result calls for further interventions to reduce the burden of cognitive impairment and hearing impairment among elderly. So far, there have been no prospective studies to delay or slowing down cognitive impairment through preventive strategy of hearing impairment. However, proper hearing impairment management might contribute favorably to the prevention of cognitive impairment. Future well-designed prospective study might provide further evidence for the associations between hearing impairment and cognitive impairments in elderly.

Author contributions

All authors contributed to the conception, analysis, interpretation, revision, and final preparation of the manuscript. SJ Lee served as the principal investigator and had full access to all study data. JH Chung takes responsibility for the integrity of the data and accuracy of the data analysis.

Financial support and sponsorship

Nil.

Conflict of interest

None of the authors has any conflicts of interest.

Funding

None of the authors has any funding.

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