Pure Tone Audiometry Threshold Changes for 10 Years in the Same Individuals of General Population: A Retrospective Cohort Study

Abstract

Objectives:

The objectives of this study were to measure the changes in hearing thresholds in the same individuals during a period of 10 years and suggest a clinical reference for the threshold changes by aging.

Methods:

In this retrospective cohort study, we used regular health checkup data including 2 pure tone audiometry results with a 10-year interval in the same individuals from 1288 subjects. The subjects’ data including demographics, smoking habits, and the diagnosis of chronic diseases were used.

Results:

Age, male gender, smoking, and osteoporosis were identified as factors affecting age-related hearing loss (ARHL). The sole effect of aging on ARHL for 10 years according to age groups and genders was as follows: a loss of 1.4 dB in 20s, 4.0 dB in 30s, 5.0 dB in 40s, 8.2 dB in 50s, and 11.2 dB in 60s of males compared to a loss of 2.3 dB in 20s, 2.9 dB in 30s, 5.1 dB in 40s, 6.5 dB in 50s, and 9.4 dB in 60s of females.

Conclusions:

We could demonstrate the actual effect of aging on ARHL, and it can be used as a clinical reference. Hearing ability decreases more in males than females but seems to decrease exponentially with age in both males and females.

Keywords

Introduction

Aging is the most influential factor in the aggravation of hearing ability, aside from several ear diseases. It has been recognized that sensorineural hearing loss is profound in the elderly and their hearing thresholds of pure tone audiometry (PTA) are significantly higher than those of the young.^1-4 Hearing impairment caused by aging is called age-related hearing loss (ARHL) or presbycusis; however, the impairment, in fact, is caused not only by aging. Some chronic diseases such as hypertension (HTN), diabetes mellitus (DM), and dyslipidemia have been suggested as illnesses affecting ARHL.^5-8 Furthermore, smoking, socioeconomic status, and noise exposure have also been recognized as the risk factors for ARHL.^8-11 For these reasons, it was difficult to estimate the sole effect of aging on ARHL, although it is regarded as an age-related disorder. As we cannot determine the actual effect of aging on hearing, it sometimes disturbs a clinical diagnosis of a patient’s hearing impairment. There has been difficulty in assessing legal compensation for hearing loss caused by noise exposure, especially in modern industrial society. Since ARHL almost always occurs with comorbidity, compensation is controversial due to the ambiguity of whether hearing loss was caused by noise, aging, or both.

Cross-sectional studies on ARHL have been conducted in Korea; however, these studies had a tangible limitation in that it was infeasible to identify changes in hearing ability in a single individual over a specific period.^3,4 To overcome this limitation, qualified longitudinal studies were conducted. Nevertheless, those investigations also had shortcomings; (1) some were performed only on subjects in particular occupations^9-12; (2) subjects’ hearing levels were assessed by self-reporting, not PTA¹²; (3) some focused on risk factors other than aging, such as smoking, central adiposity, and poorly controlled DM in the Beaver Dam Eye Study⁸; and (4) most studies were published quite a long time ago and did not reflect changes in people’s environment and lifestyle, for instance, the decrease of occupational noise exposure and the increase of vulnerability to other sound sources including music concerts, film theaters, and portable audio devices.^13,14

To determine the effect of aging on ARHL and overcome the limitations of previous investigations, the authors designed a study fulfilling the following requirements. First, it should be a longitudinal study performed on the same individuals over a specific period, estimating the difference in hearing ability for the duration. Second, factors other than aging potentially affecting hearing must be controlled. Lastly, hearing ability should be assessed by audiometric methods such as PTA rather than questionnaires. Considering these aspects, data from medical health checkups were chosen. As people who underwent medical health checkups typically tended to maintain better medical health,^15,16 their illnesses were thought to be relatively well managed. We can therefore assume that other conceivable influences on hearing could be controlled in this population. In addition, since many subjects underwent sequential medical examinations sequentially, a retrospective cohort study could be designed.

In this study, we compiled 2 PTAs separated by a 10-year interval, together with individuals’ demographics. The objectives of this study were to measure the changes in pure tone thresholds over 10 years and suggest a clinical reference for age-related hearing threshold changes according to demographics, especially age and gender.

Materials and methods

Study Population

Medical health checkup data during 1999 to 2013 were initially obtained from the Health Promotion Center of Ajou University Hospital that is a tertiary referral center in Republic of Korea. The medical health examination results of 1326 subjects were compiled from the data, who underwent PTAs twice with a 10-year interval. This medical health checkup was a typical examination program for the general people and did not include special programs, such as a regular special examination for people persistently exposed to loud industrial noise. In fact, most of them performed PTAs more than twice, as a part of their regular checkups, during the period, but we only took the 2 PTAs with 10-year interval. No one was repeatedly involved in this study population. If some performed PTAs almost every year, they could be overlapped—for example, if one underwent PTAs 6 times in 2001, 2003, 2006, 2009, 2011, and 2013, he or she could be enrolled as 2011-2013 and 2003-2013 group. In this case, we selected only one—the most recent result.

Five subjects reported in questionnaire that they had ear diseases. However, we thought that the questionnaire alone was not enough to determine the presence of otologic diseases. Therefore, if a subject showed an average threshold difference more than 10 dB between right and left ears, suggesting ear diseases on the side presenting higher thresholds, he or she was not included. Consequently, we excluded 22 showing the threshold differences, including the 5 reporting their ear diseases. Then, 13 outliers were also excluded; their hearing threshold changes were more than 30 dB for 10 years; it might be because their results were misconducted or recorded in errors or other unidentifiable causes could affect their hearing. Finally, 1288 subjects were enrolled in this study: 128, the year of 1999 as a baseline and 2009 as a follow-up; 165, years 2000 and 2010; 334, years 2001 and 2011; 481, years 2002 and 2012; and 180, years 2003 and 2013. The subjects were categorized into 5 age groups based on the age at the baseline: age 20 to 29, 30 to 39, 40 to 49, 50 to 59, and 60 to 69.

This study was performed with the approval of the Institutional Review Board of Ajou University School of Medicine, Republic of Korea (AJIRB-MED-MDB-17-413). The informed consent was waived.

Audiometric Measures

Air conductive hearing thresholds by PTA were first measured in each ear of the subjects at frequencies of 0.5, 1, 2, 4, and 8 kHz. Then, mean thresholds at each frequency was calculated as an average of the right and left ear. Next, the average hearing value of a subject was reckoned by using the weighted 4-frequency method (threshold at 0.5 kHz + 1 kHz + 1 kHz + 2 kHz + 2 kHz + 4 kHz)/6. This method of calculation is the standard when assessing hearing in various areas of Korea. This average value was used in most analyses in the study. Since 8 kHz was not included in the calculation formula, the results are shown separately. Hearing loss was defined as the average in either ear, not in both ears, was greater than 25 dB. The degree of pure tone threshold changes in 10 years was calculated by subtracting the baseline thresholds from the follow-up thresholds.

Demographics and Other Variables

The subjects’ age indicates the age at their baseline year. Information of cigarette smoking was obtained by the subjects’ self-reporting. If an individual reported that he or she had never smoked or been cessation status at the baseline year and has kept continuing the status at the follow-up year, the person was categorized to a nonsmoker. The others were regarded as smokers. In other words, the nonsmoker means that he or she has never smoked or stopped for more than 10 years, and the smoker indicates that the subject was a current smoker at the follow-up year or has quitted smoking but for less than 10 years. The existence of a chronic disease in a subject was determined on the diagnosis at his or her baseline year. The diagnosis was based on the electrically registered diagnosis code by our institution’s electric medical record system, not on the subjects’ self-reporting. Nine hundred ninety-four subjects who were diagnosed with neither with HTN, DM, dyslipidemia, nor osteoporosis were categorized as no disease group.

Statistical Analyses

Two-tailed independent t test was used for analyzing the mean difference of the thresholds according to gender, smoking, and chronic diseases. Two-tailed paired t test was applied for the significance of the threshold changes for 10 years. To identify other potential risk factors for the hearing impairment, multivariate linear regression was used. Gender, age, smoking habit, and chronic diseases were included for the multivariate analysis. The P value <.05 was considered statistically significant. All statistical analyses were performed using IBM SPSS Statistics for Windows (Version 21.0; Armonk, New York, USA).

Results

Of the 1288 subjects, 818 (64%) were males, and 470 (36%) were females. Mean age was 44.1 ± 9.0 years old in males and 44.1 ± 9.5 in females. The degree of the threshold changes was larger in males then females both in the weighted 4-frequency average and at 8 kHz. The amount of the threshold changes increased along with their initial age (2.5 dB in 20s, 3.6 dB in 30s, 5.3 dB in 40s, 7.5 dB in 50s, and 10.6 dB in 60s in the weighted 4-frequency average and 4.8 dB, 6.5 dB, 10.9 dB, 15.9 dB, and 17.8 dB at 8 kHz). Smoking and DM did not present statistical significance, while HTN, dyslipidemia, and osteoporosis showed statistical significance either in the weighted 4-frequency average or at 8 kHz (Table 1). However, the mean ages were different among each group. Therefore, all the variables were included in the following multivariate linear regression analysis.

Table 1.

Differences of Pure Tone Threshold Changes for 10 Years Between Gender, Smoking, and Chronic Diseases (N = 1288).

				Pure Tone Threshold Changes
				4-Frequency Average (dB)		At 8 kHz (dB)
	No	Age^a	P	Δ^a	P	Δ^a	P
Gender
Male	818	44.1 ± 9.0	Referent	5.7 ± 5.1	Referent	11.5 ± 11.2	Referent
Female	470	44.1 ± 9.5	.833^b	4.7 ± 5.4	.001^b	8.6 ± 11.6	<.001^b
Age groups
20s	74	27.7 ± 2.3		2.5 ± 3.8	<.001^c	4.8 ± 8.7	<.001^c
30s	431	36.7 ± 2.6		3.6 ± 4.6	<.001^c	6.5 ± 10.1	<.001^c
40s	468	44.9 ± 2.8		5.3 ± 4.8	<.001^c	10.9 ± 11.2	<.001^c
50s	245	54.9 ± 2.8		7.5 ± 5.5	<.001^c	15.9 ± 11.5	<.001^c
60s	70	63.4 ± 3.0		10.6 ± 6.3	<.001^c	17.8 ± 10.5	<.001^c
Smoking
Nonsmoker	897	45.0 ± 9.3	Referent	5.2 ± 5.4	Referent	10.2 ± 11.5	Referent
Smoker	391	41.9 ± 8.5	<.001^b	5.6 ± 4.8	.260^b	10.9 ± 11.2	.303^b
Chronic diseases
No disease	994	42.5 ± 8.6	Referent	5.1 ± 5.2	Referent	9.6 ± 11.0	Referent
HTN	120	49.9 ± 9.0	<.001^b	6.2 ± 5.4	.032^b	12.6 ± 12.3	.012^b
DM	54	50.2 ± 9.5	<.001^b	4.9 ± 4.8	.793^b	11.2 ± 12.2	.304^b
Dyslipidemia	124	47.6 ± 8.0	<.001^b	5.5 ± 5.6	.476^b	11.8 ± 11.8	.036^b
Osteoporosis	43	55.9 ± 6.9	<.001^b	7.8 ± 5.8	.001^b	17.9 ± 13.4	<.001^b

Abbreviations: DM, diabetes mellitus; HTN, hypertension; Δ, difference of pure tone thresholds between the 10 years, thresholds at the 10 year – thresholds at the initial year.

Values are expressed as mean ± standard deviation.

P value from independent t test comparing to each referent.

P value from paired t test between the baseline and the follow-up hearing threshold.

In the weighted 4-frequency average, age, male gender, and smoking were suggested as the risk factors for the threshold changes. At 8 kHz, age, male gender, and osteoporosis showed negative effect on hearing changes (Table 2). To demonstrate the actual effect of aging on ARHL, the individuals, 433 subjects, having the suggested factors, smoking or osteoporosis, were excluded. Of the other 855 subjects, Figure 1 showed the changes in PTA in 10 years according to gender and age groups. We could assume that the changes in the thresholds meant the sole effect of aging on ARHL.

Table 2.

Factors Affecting the Degree of Pure Tone Threshold Changes by Multivariate Linear Regression.

	Estimate^a	95% Confidence Interval
4-frequency average^b
Age	0.211 (0.015)	0.181-0.240
Male	0.637 (0.316)	0.016-1.258
Smoking	0.722 (0.335)	0.064-1.380
8000 Hz^c
Age	0.405 (0.033)	0.340-0.471
Male	3.162 (0.630)	0.340-0.471
Osteoporosis	4.579 (1.738)	1.170 -7.989

Values are expressed as estimate ± standard error.

All variables in Table 1 were initially included as variables in the multiple linear regression. Nonsignificant variables were excluded by stepwise method.

In Figure 2, each dot was an encountering point between mean ages and means of the threshold changes in the weighted 4-frequency average in each age group. The velocity of hearing aggravation increased exponentially along with age in both male and female. Regression equation in males was y = 0.0024x² + 0.0426x − 1.4498, and its coefficient of determination was 0.99. The equation in females was y = 0.0037x² – 0.1297x + 3.0239, and the coefficient was 0.99.

The prevalence of hearing loss between the 2 periods, 1999-2003 and 2009-2013, was described in Table 3.

Table 3.

Change of the Proportion of Hearing Loss in Age Groups Between the 2 Points of Time (1999-2003 and 2009-2013).

	Baseline (1999-2003)^a	Follow-Up (2009-2013)^a	P ^b
Hearing loss in 20s	0/740.0% (0.0%-4.9%)	—
Hearing loss in 30s	18/4314.2% (2.5%-6.5%)	1/741.4% (0.0%-7.3%)	.238
Hearing loss in 40s	25/4686.2% (3.5%-7.8%)	40/4319.3% (6.7%-12.4%)	.023
Hearing loss in 50s	44/24518.0% (13.4%-23.3%)	94/46820.1% (16.5%-24.0%)	.495
Hearing loss in 60s	21/7030.0% (19.6%-42.1%)	106/24543.3% (37.0%-49.7%)	.046
Hearing loss in 70s	—	35/7050.0% (37.8%-62.2%)

Values are expressed as number of hearing loss out of number of the subjects in each age group and the proportion of hearing loss (95% confidence interval for the percentage).

P value from chi-square analysis.

Discussion

Hearing loss is one of the most common reasons for people to see otolaryngologists. Otolaryngologists, then, attempted to find a cause of the hearing impairment by PTA, speech audiometry, otoscopy, and physical examinations. The physicians, however, often encountered somewhat ambiguous clinical circumstances with the diagnosis—especially in the elderly, it was sometimes unclear whether the hearing loss was by ARHL or other causes such as noise-induced hearing loss (NIHL), sudden sensorineural hearing loss, or drug-induced hearing loss. If we could estimate the actual effect of aging on hearing, it would be helpful to make a diagnosis when confronting these uncertain circumstances. Moreover, there is a growing concern about fair compensation for people complaining occupational hearing loss. Because ARHL also affects occupational hearing problems, there are growing controversies whether the impairment was caused by their noise exposure or aging. For these reasons, we have been interested in sole effect of aging on ARHL. And this study can contribute as a clinical reference toward clarifying the influence of aging on the various kinds of hearing loss.

As previous studies already suggested that male was a risk factor on ARHL,^8-17 male gender showed bigger changes both in the weighted 4-frequency average of the threshold and at 8 kHz for the 10-year period than female (5.7 dB and 4.7 dB in males and females, respectively, in the weighted 4-frequency, P = .001, and 11.5 dB in males and 8.6 dB in females at 8 kHz, P < .001) (Table 1). One of the possible explanations is that the gender difference in smoking habits could affect this result. According to a study on general population of Korean, 83.3% of males had been or were current smokers, whereas it was only 9.3% in females.¹⁸ Of the 391 smokers in this study, 376 (96.2%) were males. However, after multivariate analysis, both male gender and smoking were revealed as the risk factors, respectively, on ARHL. Other than smoking behavior, what did make these different results between male and female? Here, the impact of noise on hearing should be considered. NIHL is one of the most common occupational and environmental problems, resulting in irreversible hearing loss.¹⁹ Male workers consist of more proportion in construction, manufacturing, and transportation industries generating continuous loud noise. In addition, military service for almost 2 years in men has been still mandatory in South Korea, and it has long-term negative effect on hearing.²⁰ Of course, the enrolled individuals in this study were not categorized as a high-risk group for noise but general population; however, for those reasons, male individuals could be estimated more vulnerable to sources of noise than females. Decreases in high frequencies on PTA, over 3 kHz, have been frequently observed in NIHL. The typical audiometric pattern of NIHL is a sharp fall around 4 kHz, called a V-shape dip or notch.²¹ And Kim et al²⁰ demonstrated that declining at 8 kHz on PTA was notable by long-term effect of noise exposure during military service in South Korea. In our study, it was evident that the thresholds at 4 and 8 kHz were more depressed in males (Figure 1). One possible explanation on these results is that the depression at 4 kHz could arise from the occupational noise exposure and that the long-term effect from their military service period could affect decline at 8 kHz. This gender difference and the association with noise were suggested in a former study. Hoffman et al¹³ reported that sex-specific prevalence of hearing loss declined with the decrease of industrial noise exposure. Consequently, we could postulate that the gender difference of hearing aggravation in our study also resulted from vulnerability to noise in men.

HTN, DM, and dyslipidemia did not show any effects on the change of the hearing ability (Tables 1 and 2). However, this outcome could be explained with a consideration of the enrolled subjects’ characteristics. The referred chronic diseases in them may be better managed than other populations. Low density lipoprotein cholesterol levels in the subjects at the follow-up, for example, could be obtained in 120 of 124 dyslipidemia patients in our study population. Their mean low density lipoprotein cholesterol level was 114.8 ± 34.1 mg/dL. It was under the level of treatment target, 130 mg/dL.²² Interestingly, it was even lower than the mean level of the “no disease” group subjects, 116.6 ± 30.5 mg/dL, though there was no statistical significance between the two values (P = .580). Therefore, although the negative effect of chronic diseases was not revealed in this study, we should not neglect the influence of the illness on hearing. Rather, this result in this study could mean that management of the diseases is more important than the diagnosis itself. On the other hand, osteoporosis showed a negative effect on ARHL at 8 kHz. Kahveci et al²³ reported that osteoporosis patients had worse hearing levels at high frequencies. Demineralization of cochlear capsule in osteoporosis was proposed as the hypothesis.²⁴ However, underlying mechanisms causing this high-frequency hearing loss in osteoporosis are still uncertain.

Does the individual’s hearing ability aggravate constantly or acceleratively in ARHL? As described in Figure 2, the degree of the threshold changes increased along with age. Considering the extension of one’s life span, ARHL may be an increasingly prevalent illness.²⁵ Furthermore, this disability can induce other psychological and cognitive problems; severe hearing loss increased the risk of depressive disorder and was associated with cognitive decline.^26,27 Unfortunately, recovery of hearing ability is not currently possible in most patients.²⁸ Hearing aids amplifications are available for ARHL, but it is not used for restoring normal hearing. Nevertheless, hearing aids can improve speech recognition and hearing-related quality of life; also, it can prevent other disabilities provoked by hearing loss.²⁹ In 2012, only 14.2% of hearing loss patients, however, were using the aids regularly in the United States, and it was even lower in Korea, 12.6%.^30,31 Promoting the use of the devices and vigorous researches to solve the hearing problems would be necessary.

Of subjects aged 30 to 69, number of hearing loss was 108 (8.9%) of 1214 in baseline years, and it was 241 (19.8%) of 1218 in follow-up years (Table 3). Due to the difference of age proportion between the baseline and follow-up, indirect Standardized Incidence Ratio was applied to calculate the prevalence again. By the indirect Standardized Incidence Ratio, the prevalence of hearing loss was 12.3% and 15.5%, respectively. Unlike the report in the United States that the prevalence of hearing loss declined due to the decreased noise exposure,¹³ our data showed the increase of the prevalence in more recent period. In fact, it is also true in Korea that industrial and occupational noise exposures have been managed by the Occupational Safety and Health Act.³² Nonetheless, other potential factors, such as the increase of vulnerability to loud sound from various portable devices, may still affect hearing continuously, so investigating the effect of those could be maintained.^14,33

We assumed that males were more vulnerable to occupational noise, though, if the degree of exposed noise levels in each subject could be assessed, the sole effect of aging could be estimated more accurately; unfortunately, it was impossible in this study design, therefore this is a limitation of this retrospective cohort study. However, despite considering the limitation, the result of this study is still valuable; if a patient who does not show any abnormalities except persistent loud noise exposure in his or her workplace visits an otolaryngologist for his or her hearing impairment and the patient demonstrated more aggravation than the reference that we suggested in this study, we will at least be able to suggest an expert opinion, with the clinical evidence, that the patient deserves to be compensated for the loss.

In conclusion, we demonstrated the sole effect of aging on ARHL for 10 years, and it might be used as a clinical reference. Hearing ability decreases more in males than in females but seems to decrease exponentially with age in both males and females.

Footnotes

Acknowledgements

The subjects’ data we reviewed in this study were provided from the Health Promotion Center of Ajou University Hospital.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Hantai Kim

Hun Yi Park

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