Increased Risk of Dementia Following a Diagnosis of Hearing Impairment: A South Korean Nationwide Cohort Study

Abstract

Background:

Among the older adults, hearing impairment is a common problem and may contribute to dementia.

Objective:

Therefore, we aimed to examine the association between hearing impairment and the risk of dementia among older adults in South Korea.

Methods:

Using the Korean National Health Insurance Service-Senior Cohort from 2005 to 2019, we collected data of 44,728 patients. Hearing impairment was determined using the national disability registry. Propensity score matching (1:1) was performed to match patients with and without hearing impairment (case: 22,364, control: 22,364). A Cox proportional hazards regression model was built to analyze the association between hearing impairment and dementia.

Results:

Patients with hearing impairment had a higher risk of dementia than those without hearing impairment (hazard ratio [HR] = 1.28, 95% confidence interval [CI] = 1.23–1.34). Assessing the degree of disability, both severe (HR = 1.25, 95% CI = 1.16–1.35) and mild conditions (HR = 1.29, 95% CI = 1.23–1.35) had an increased risk of dementia, respectively.

Conclusions:

Older patients with hearing impairment exhibit an increased risk of dementia, thereby warranting a new approach to dementia care among these patients regardless of the degree of hearing impairment.

Keywords

Alzheimer’s disease dementia hearing impairment hearing loss

INTRODUCTION

Dementia is characterized by global cognitive impairment, with a decline in functioning when compared with the previous level and disturbances in both psychiatric and behavioral aspects [1]. The global incidence of dementia is growing. Currently, 50 million individuals live with dementia, and this number is estimated to increase to 152 million by 2050. As dementia affects both individuals and their families, the economy can also be impacted, given that the global costs associated with dementia are approximately 1 trillion dollars annually [2]. It is anticipated that Korea will become an aged society by 2026 [3], with the number of individuals with dementia set to increase. According to the Korean Ministry of Health and Welfare, the prevalence of dementia is estimated to increase from 10.28% in 2019 to 16.09% in 2050, with the total number of patients expected to reach 3 million [4].

With a growing older population, there is an increase in hearing impairment. According to the World Health Organization (WHO), 278 million individuals worldwide suffer from moderate to profound hearing loss (HL) [5]. An estimated one-third of older adults have disabling HL, the third most prevalent health condition among older individuals [6]. A similar pattern has been detected in South Korea. After physical disability, hearing impairment was found to be the second most prevalent type of disability in Korea, accounting for 10.4% in 2011 and increasing to 16% in 2022 [7]. Considering only registered disabled individuals aged ≥65 years, the rate of hearing impairment increased to 24.5% [7]. Hearing impairment markedly reduces the ability to perform day-to-day activities and seriously alters interpersonal relationships, as communication with other individuals can be challenging [8].

The importance of understanding and addressing dementia risk factors cannot be overstated. Furthermore, with a progressively aging society, hearing impairment will inevitably increase, accompanied by an increase in social costs. However, HL is potentially a modifiable risk factor that can be treated with early intervention and rehabilitation. Therefore, it serves as a potential target for public health policies aimed at reducing the burden of dementia. Accordingly, understanding the risk of functional limitations on hearing among older adults is crucial for healthcare planning to achieve favorable aging and provide necessary aid to those with hearing impairment.

The relationship between hearing impairment and dementia has been previously documented. For example, Kim et al. [9] reported the risk of hearing impairment and neurogenerative dementia in 4,432 Korean individuals aged 40 years or older. Deal et al. [10] reported hearing impairment and incidence of dementia and cognitive decline among 3,075 well-functioning, biracial adults aged 70–79 years. However, the former study reported findings without particularly addressing the older population, and the latter missed examining a large population and including Asian adults. Hence, our study aimed to investigate the relationship between hearing impairment and dementia in Korean adults over 60 years of age using data from the Korean Health Insurance Service.

METHODS

Study population

Herein, we used data from the Health Insurance Service National Sample Cohort-Senior Database (NHIS-SC), which comprises a 10% sample of randomly selected older individuals aged over 60 years who were living in Korea at the end of 2002. The database contains demographic and household income data (including socioeconomic factors), medical records, health screening check-up results, and questionnaires [11]. The NHIS-SC data also includes general demographics and clinical information regarding diagnoses, treatments, and prescribed medications for all visits to medical institutions [12]. Given that nearly all Koreans are covered by the National Health Insurance Service, the NHIS Claim Database can be considered representative of the Korean population [13].

To select individuals who were diagnosed with hearing impairment during the study period, we excluded those diagnosed with hearing impairment or dementia prior to 2005. Participants with hearing impairment were matched 1:1 to those (control group) who had never been diagnosed with a hearing impairment between 2005–2019. The matches were processed for age, sex, income group, region of residence, and Charlson Comorbidity Index (CCI).

Ethical consideration

This study was reviewed and approved by the Eulji University Institutional Review Board (IRB number: EUIRB2023-024) and adhered to the tenets of the Declaration of Helsinki. The NHIS-SC data do not contain any identifying information necessitating additional approval.

Variables

The variable of interest in the present study was hearing impairment. We included participants who were registered as hearing-impaired in the Ministry of Health and Welfare. To be registered as hearing-impaired in Korea, individuals undergo a single auditory brainstem response test, along with the pure tone audiometry test performed three times. The average hearing threshold was calculated as follows: (500 Hz + 2 * 1000 Hz + 2 * 2000 Hz + 4000 Hz)/6 [10]. Following the abolition of the disability rating scheme in 2019 [14], Korea has two levels of hearing impairment based on the calculated average pure tone threshold: severe (better ear ≥80 dB HL) and mild (better ear ≥60 dB HL or worse ear ≥80 dB HL and better ear HL ≥40 dB HL) [15].

Herein, the dependent variable was dementia. We extracted individuals who were diagnosed with Alzheimer’s disease (International Classification of Diseases, 10th revision code: G30), dementia in Alzheimer’s disease (ICD-10: F00), vascular dementia (ICD-10: F01), or other dementia (ICD-10: F02, F03).

We included independent variables such as sex, household income level, insurance type, residential area, CCI, hypertension, diabetes mellitus, and stroke. CCI was assessed with diagnostic codes using the Quan ICD-10 coding algorithm of the CCI score 26.

Statistical analysis

Baseline characteristics were compared using standardized differences to assess the covariate balance. Differences of <0.01 (<10%) were deemed negligible. Follow-up periods (24, 25–60, and ≥61 months) were considered for survival analyses. The incidence of dementia (and 95% confidence interval [CI]) was calculated using the product-limit method of survival probability. The incidence rate of dementia (95% CI) was calculated based on a generalized linear model with a Poisson distribution. A Cox proportional-hazard model with a robust variance estimator was used to present the effect size and to account for clustering within matched pairs. Considering both patients and matched controls, the date of hearing impairment was established as “time-zero”. Survival time was defined by the number of days from the incidence date (time-zero) to December 31, 2019, or the date of death, whichever came first. Data analyses were conducted using SAS Enterprise Guide (version 7.1; SAS Institute). A p-value of <0.05 was deemed statistically significant.

RESULTS

Table 1 summarizes the baseline characteristics of matched cohorts. The present study included 22,364 patients with hearing impairment and 22,364 matched controls. In both cohorts, 55.9% of participants were male.

Table 1

Baseline characteristics of Hearing Impairment patients and their risk set-matched cohort

Variables	Hearing impairment cohort ^†	Matched cohort	Standardized difference
Total	22,364 (100.0)	44,728 (100.0)
Sex			0
Male	12,499 (55.9)	24,998 (55.9)
Female	9,865 (44.1)	19,730 (44.1)
Age			0
60–69	8,751 (39.1)	17,502 (39.1)
70–79	9,017 (40.3)	18,034 (40.3)
80–	4,596 (20.6)	9,192 (20.6)
Household income level			0.0196
Low	8,751 (39.1)	13,403 (30.0)
Middle	9,017 (40.3)	10,651 (23.8)
High	4,596 (20.6)	20,674 (46.2)
Insurance Type			0.0264
Health Insurance	20,973 (93.8)	41,800 (93.5)
Medical Aid	1,391 (6.2)	2,928 (6.5)
Region			0.0121
Urban	9,422 (42.1)	18,711 (41.8)
Rural	12,942 (57.9)	26,017 (58.2)
Charlson Comorbidity Index (CCI)			0
0	17,002 (76.0)	34,004 (76.0)
1-2	4,425 (19.8)	8,850 (19.8)
3–	937 (4.2)	1,874 (4.2)
Diabetes mellitus			0
Yes	1,668 (7.5)	3,336 (7.5)
No	20,696 (92.5)	41,392 (92.5)
Hypertension			0
Yes	6,403 (28.6)	12,806 (28.6)
No	15,961 (71.4)	31,922 (71.4)
Stroke			0
Yes	537 (2.4)	1,074 (2.4)
No	21,827 (97.6)	43,654 (97.6)

^*Values are presented as mean±standard deviation or number (%). ^†Hearing impairment is defined as those registered as hearing impaired in the Ministry of Health and Welfare.

Table 2 presents the results of the association between hearing impairment and the risk of dementia. Individuals with hearing impairment had a higher risk of dementia than those without hearing impairment (hazard ratio [HR] = 1.28, 95% CI = 1.23–1.34). According to the grade of hearing impairment, both severe and mild hearing impairment were associated with an increased risk of dementia (HR = 1.25, 95% CI = 1.16–1.35; HR = 1.29, 95% CI = 1.23–1.35).

Table 2

Results of the association between Hearing Impairment and the Risk of dementia

	Risk of Dementia
Variables	HR	95% CI
Hearing Impairment^*
Yes	1.28	(1.23–1.34)
- Severe Hearing Impairment†	1.25	(1.16–1.35)
- Mild Hearing Impairment‡	1.29	(1.23–1.35)
No	1.00
Sex
Male	1.00
Female	1.37	(1.23–1.53)
Age
60–69	1.00
70–79	3.08	(2.91–3.26)
80–	7.30	(6.80–7.83)
Household income level
Low	1.01	(0.95–1.07)
Middle	1.02	(0.96–1.08)
High	1.00
Insurance Type
Health Insurance	1.00
Medical Aid	1.63	(1.50–1.78)
Region
Urban	1.00
Rural	1.23	(1.17–1.29)
Charlson Comorbidity Index (CCI)
0	1.00
1-2	1.10	(1.03–1.16)
3–	1.36	(1.23–1.51)
Diabetes mellitus
Yes	1.17	(1.07–1.27)
No	1.00
Hypertension
Yes	1.09	(1.03–1.15)
No	1.00
Stroke
Yes	1.18	(1.04–1.34)
No	1.00

^*Hearing impairment is defined as those registered as hearing impaired in the Ministry of Health and Welfare. †Severe Hearing Impairment is defined as those average pure tone threshold of more than 80 dB HL in the better ear. ‡Mild Hearing Impairment is defined as those average pure tone threshold of more than 60 dB HL in the better ear or more than 80 dB HL in the worse ear and more than 40 dB HL in the better ear.

Table 3 presents the results of the association between hearing impairment and the risk of dementia by type. Patients with hearing impairment had a high risk of dementia regardless of the type of dementia. Table 4 presents the incidence rates of dementia in patients with hearing impairment and matched cohorts. During the first 24 months of the follow-up period, 999 cases of dementia were identified for 40368.2 person-years (IR, 2474.7 per 100,000 person-years) among the 22364 patients with hearing impairment. Patients with hearing impairment were 1.28 times more likely to have dementia than the matched controls during the same period.

Table 3

Result of the subgroup analysis according to the types of dementia

	Risk of Dementia
	Hearing Impairment^*			Severe Hearing Impairment†			Mild Hearing Impairment‡
	No	Yes		No	Yes		No	Yes
	HR	Adjusted HR	95% CI	HR	Adjusted HR	95% CI	HR	Adjusted HR	95% CI
Types of Dementia
Alzheimer’s disease	1.00	1.29	(1.09–1.52)	1.00	1.38	(1.04–1.84)	1.00	1.26	(1.06–1.51)
Dementia in Alzheimer’s disease	1.00	1.29	(1.22–1.37)	1.00	1.22	(1.11–1.35)	1.00	1.31	(1.23–1.39)
Vascular dementia	1.00	1.25	(1.08–1.44)	1.00	1.32	(1.03–1.68)	1.00	1.23	(1.06–1.43)
Other dementia	1.00	1.36	(1.23–1.50)	1.00	1.45	(1.23–1.71)	1.00	1.33	(1.20–1.48)

^*Hearing impairment is defined as those registered as hearing impaired in the Ministry of Health and Welfare. †Severe Hearing Impairment is defined as those average pure tone threshold of more than 80dB HL in the better ear. ‡Mild Hearing Impairment is defined as those average pure tone threshold of more than 60dB HL in the better ear or more than 80dB HL in the worse ear and more than 40dB HL in the better ear.

Table 4

Incidences rates of dementia in the hearing impairment cohort and matched controls

Exposure	Number of	Number of	Person	Incidence rate (95% CI) per	Hazard Ratio
	Subjects	Dementia	Years	100,000 person years
0–24 Months
Matched Controls	22364	999	40368.2	2474.7 (2326.1–2632.8)	1.00
Hearing Impaired Patients^*	22364	1281	40331.5	3176.1 (3007.1–3354.6)	1.28 (1.18–1.39)
25–60 Months
Matched Controls	15408	916	37984.8	2411.4 (2260.4–2572.6)	1.00
Hearing Impaired Patients^*	15244	1165	37052	3144.2 (2968.7–3330.1)	1.30 (1.19–1.41)
0–60 Months
Matched Controls	22364	1915	78353.0	2444 (2337.1–2555.9)	1.00
Hearing Impaired Patients^*	22364	2446	77383.0	3160.8 (3038.1–3288.5)	1.29 (1.22–1.36)
>60 Months
Matched Controls	9511	1639	36691.0	4467.0 (4265.5–4678.0)	1.00
Hearing Impaired Patients^*	9207	1875	34831.1	5383.1 (5156.3–5619.8)	1.20 (1.13–1.27)

^*Hearing impairment is defined as those registered as hearing impaired in the Ministry of Health and Welfare.

DISCUSSION

Herein, we aimed to determine the possible relationship between sensory impairment and dementia. Our results revealed a significant association between hearing impairment and dementia. Moreover, the risk of dementia was elevated regardless of the type of hearing impairment.

Our results are consistent with those previously reported. According to a German study [16], hearing impairment increases the risk of dementia when accounting for diagnosis by a specialist, depression, and other related factors. Furthermore, an Australian study [17] analyzed 37,898 adult males and found that individuals with HL had an increased risk of dementia than those without HL.

Although there is no definite pathology regarding the relationship between hearing impairment and dementia, several mechanisms have been suggested. HL can lead to decreased cognitive stimulation. According to one theory [18], auditory deprivation creates an impoverished environment, especially when communication input is diminished, which negatively affects brain development. The subsequent development of dementia is reportedly influenced by this altered brain structure and function. Another theory is the effortfulness hypothesis, or cognitive burden [19], which suggests that sound signals, especially in the high-frequency range, are more distorted in individuals with HL, resulting in considerable effort in perceiving sound. Individuals with HL may experience challenges encoding and processing speech into memory owing to additional cognitive load involved.

Considering the degree of hearing impairment, the results revealed minimal differences, with both degrees associated with an increased risk of developing dementia. In addition to low accessibility to hearing aids, those with hearing aids fail to wear the devices as required. Before 2015, only 340,000 KRW (290 in US dollars) were provided as hearing aid benefits to individuals diagnosed with hearing disability [20]. Considering that the price of hearing aids ranges between 1,000,000 to 1,500,000 KRW (approximately 900 to 1300 US dollars) [21], it is likely that the appropriate hearing aid was not provided based on individual needs. In addition, until the policy change in 2015, the costs of hearing aid fitting and aural rehabilitation were not insured. The proper regular use of hearing aid requires regular fitting, fine tuning [22], and aural rehabilitation [23]. In the absence of appropriate hearing aid fitting and inadequate use, there is an increased possibility of patients not wearing hearing aids, which could, in the long term, impact the development of dementia.

For both dementia as well as HL, identifying and treating exacerbating factors is crucial [24]. As a preventive measure, hearing screening is recommended by several groups. The American Geriatrics Society recommends annual hearing screening for those above 65 years of age [25], whereas the World Health Organization has recommended hearing screening every 5 years for those aged 50–64 years and every 1–3 years for those above 65 years of age [26]. In South Korea, hearing examinations are included in the biennial health checkups conducted in the country for all adults over the age of 20. However, the participation in health checkups is lower among those with disability than among those with no disability, owing to accessibility issues. Furthermore, formal audiometric testing at various frequencies is not possible, as hearing assessments are conducted using pure tone audiometry at a single frequency of 1,000 Hz, with the result recorded as ‘pass’ (less than 40 dB hearing level) or ‘fail’ (criterion for referral for further workup) [27]. Additionally, cognitive function is only tested biennially in individuals over the age of 66 years [27]. In order to develop more effective preventive measures, there should be greater accessibility for those with disability, a wider range of audiometry hearing tests, and more frequent testing for those who are potentially more vulnerable to cognitive decline.

The current study was divided according to the number of days of diagnosis of dementia after hearing impairment. Similar levels of risk were observed on analyzing between 24 months ago, 25–60 months, and 60 months later. These results were similar to those of a study conducted in the UK [28], where the risk of dementia was robust within three years, with a similar risk to other years. However, the time interval between the occurrence of a disability and registration as a legally disabled person needs to be considered. To receive disability support services in South Korea, there is a waiting period of 6–12 months for registration, depending on the disability type [29]. In addition, there is a considerable time interval between the initial decrease in hearing and diagnosis of HL. Meeting the necessary threshold for disability may be delayed, especially in individuals with mild HL; hence, the diagnosis of dementia after HL may be delayed.

Our study has certain important limitations. First, our study was limited to patients registered as hearing impaired, excluding those with a moderate degree of HL. Future studies should examine a wider degree of hearing impairment. Second, this study was based on a South Korean database; hence, the introduction of potential biases should be considered. However, given that this study compares the relative risk of dementia between individuals with hearing-impaired and a matched control group, we believe the results could be applied to other populations. Despite these limitations, the strengths of our study need to be highlighted. Given that the NHIS-SC represents a population over 60 years of age, it offers a large sample size. Additionally, owing to the nature of the data, there is a relatively low rate of loss in the follow-up process.

CONCLUSIONS

Collectively, our findings suggest an association between hearing impairment and dementia. We observed that the risk of dementia increased regardless of the degree of hearing impairment. In addition, there was no significant difference in the period from HL to dementia diagnosis. Therefore, it is necessary to establish thorough policies and manage individuals with hearing impairment.

Footnotes

ACKNOWLEDGMENTS

We like to thank the NHIS for providing us with the data.

FUNDING

This research was supported by a fund from the research program of the Korea Medical Institute (Funding Code: 23-042).

CONFLICT OF INTEREST

The authors have no conflicts of interest to declare.

DATA AVAILABILITY

The data supporting the findings of this study available in National Health Insurance Data Sharing Service at .

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