Left-right asymmetries in hearing threshold levels in three age groups of a random population

Revisiting Age-Related Normative Hearing Levels in Korea

BACKGROUND

Hearing level reference values based on the results of recent audiometry have not been established for the general population of South Korea. This study aimed to evaluate the mean hearing levels of each age group and to measure the annual progression of hearing loss.

METHODS

We used the database of the eighth and ninth Korea National Health and Nutrition Examination Survey from 2020 to 2022, and included participants with normal tympanic membranes and without occupational noise exposure. Mean hearing levels were calculated by averaging hearing levels at 0.5 kHz, 1 kHz, 2 kHz, and 4 kHz. Hearing loss was defined as a hearing level exceeding 25 dB in the better ear, and severity of hearing loss was classified as moderate, moderately severe, severe, and profound, following the World Health Organization classification.

RESULTS

Hearing levels and the proportions of patients with hearing loss increased with age. Hearing levels were 9.11 ± 6.28 dB, 13.33 ± 7.85 dB, 19.90 ± 10.30 dB, and 31.95 ± 13.37 dB in those in their 40s, 50s, 60s, and 70s, respectively, and the proportions of patients with hearing loss were 1.81%, 7.51%, 25.32%, and 63.94%, respectively. The annual increase in hearing level was 0.71 dB, and increases were greater at higher frequencies. Hearing loss was also worse in men.

CONCLUSION

The present study provides reference values for normal hearing levels by age group, and indirectly reveals the progression of age-related hearing loss. Based on the results, physicians will be able to provide appropriate advice regarding hearing levels and the need for monitoring in the elderly.

Perceptual and Cognitive Effects of Focal Transcranial Direct Current Stimulation of Auditory Cortex in Tinnitus

OBJECTIVES

Transcranial direct current stimulation (tDCS) has been studied as a potential treatment for many brain conditions. Although tDCS is well tolerated, continued study of perceptual and cognitive side effects is warranted, given the complexity of functional brain organization. This study tests the feasibility of brief tablet-based tasks to assess auditory and cognitive side effects in a recently reported pilot study of auditory-cortex tDCS in chronic tinnitus and attempts to confirm that this untested multisession tDCS protocol does not worsen hearing.

MATERIALS AND METHODS

Participants with chronic tinnitus completed two hearing tasks (pure-tone thresholds, Words In Noise [WIN]) and two cognitive tasks (Flanker, Dimension Change Card Sort) from the NIH Toolbox (2024 Toolbox Assessments, Inc, Lincolnwood, IL). Participants were randomized to active or sham 4×1 silver/silver-chloride tDCS of left auditory cortex (n = 10/group). Tasks were completed immediately before and after the first tDCS session, and after the fifth/final tDCS session. Statistics included linear mixed-effects models for change in task performance over time.

RESULTS

Before tDCS, performance on both auditory tasks was highly correlated with clinical audiometry, supporting the external validity of these measures (r2 > 0.89 for all). Although overall auditory task performance did not change after active or sham tDCS, detection of right-ear WIN stimuli modestly improved after five active tDCS sessions (t34 = -2.07, p = 0.05). On cognitive tasks, reaction times (RTs) were quicker after sham tDCS, reflecting expected practice effects (eg, t88 = 3.22, p = 0.002 after five sessions on the Flanker task). However, RTs did not improve over repeated sessions in the active group, suggesting that tDCS interfered with learning these practice effects.

CONCLUSIONS

Repeated sessions of auditory-cortex tDCS do not seem to adversely affect hearing or cognition but may modestly improve hearing in noise and interfere with some types of motor learning. Low-burden cognitive/perceptual test batteries could be a powerful way to identify adverse effects and new treatment targets in brain stimulation research.

Perceptual and cognitive effects of focal tDCS of auditory cortex in tinnitus

OBJECTIVES

Noninvasive brain stimulation continues to grow as an effective, low-risk way of improving the symptoms of brain conditions. Transcranial direct current stimulation (tDCS) is particularly well-tolerated, with benefits including low cost and potential portability. Nevertheless, continued study of perceptual and cognitive side effects is warranted, given the complexity of functional brain organization. This paper describes the results of a brief battery of tablet-based tasks used in a recent pilot study of auditory-cortex tDCS in people with chronic tinnitus.

METHODS

Volunteers with chronic tinnitus (n=20) completed two hearing tasks (pure-tone thresholds, Words In Noise) and two cognitive tasks (Flanker, Dimension Change Card Sort) from the NIH Toolbox. Volunteers were randomized to active or sham 4×1 Ag/AgCl tDCS of auditory cortex, and tasks were completed immediately before and after the first tDCS session, and after the fifth/final tDCS session. Statistics included linear mixed-effects models for change in task performance over time.

RESULTS

Before tDCS, performance on both auditory tasks was highly correlated with clinical audiometry, supporting the external validity of these measures (r2>0.89 for all). Although overall auditory task performance did not change after active or sham tDCS, detection of right-ear Words in Noise stimuli modestly improved after five active tDCS sessions (t(34)=-2.07, p=0.05). On cognitive tasks, reaction times were quicker after sham tDCS, reflecting expected practice effects (e.g., t(88)=3.22, p=0.002 after 5 sessions on Flanker task). However, reaction times did not improve over repeated sessions in the active group, suggesting that tDCS interfered with learning these practice effects.

CONCLUSIONS

Repeated sessions of auditory-cortex tDCS does not appear to adversely affect hearing or cognition, but may modestly improve hearing in noisy environments and interfere with some types of motor learning. Low-burden cognitive/perceptual test batteries could be a powerful way to identify adverse effects and new treatment targets in brain stimulation research.

Interaural and sex differences in the natural evolution of hearing levels in pre-symptomatic and symptomatic carriers of the p.Pro51Ser variant in the COCH gene

Hearing impairment constitutes a significant health problem in developed countries. If hearing loss is slowly progressive, the first signs may not be noticed in time, or remain untreated until the moment the auditory dysfunction becomes more apparent. The present study will focus on DFNA9, an autosomal dominant disorder caused by pathogenic variants in the COCH gene. Although several cross-sectional studies on this topic have been conducted, a crucial need for longitudinal research has been reported by many authors. Longitudinal trajectories of individual hearing thresholds were established as function of age and superimposed lowess curves were generated for 101 female and male carriers of the p.Pro51Ser variant. The average number of times patients have been tested was 2.49 years with a minimum of 1 year and a maximum of 4 years. In addition, interaural and sex differences were studied, as they could modify the natural evolution of the hearing function. The current study demonstrates that, both in female carriers and male carriers, the first signs of hearing decline, i.e. hearing thresholds of 20 dB HL, become apparent as early as the 3rd decade in the highest frequencies. In addition, a rapid progression of SNHL occurs between 40 and 50 years of age. Differences between male and female carriers in the progression of hearing loss are most obvious between the age of 50 and 65 years. Furthermore, interaural discrepancies also manifest from the age of 50 years onwards. High-quality prospective data on the long-term natural evolution of hearing levels offer the opportunity to identify different disease stages in each cochlea and different types of evolution. This will provide more insights in the window of opportunity for future therapeutic intervention trials.

Obstructive sleep apnea risk and hearing impairment among occupational noise-exposed male workers

This study aimed to investigate the association between obstructive sleep apnea (OSA) risk and hearing impairment among workers exposed to occupational noise. A cross-sectional study was conducted among 607 healthy male workers at a tire-manufacturing factory. The subjects underwent audiometric testing, and their OSA risk was examined based on the STOP-Bang questionnaire. Hearing impairment was defined as a hearing threshold >25 dB hearing level (HL) in any frequency of 1, 2, 3 and 4 kHz in either ear. High OSA risk was defined as a STOP-bang score of ≥3. Hearing thresholds at 1, 2, 3 and 4 kHz in both ears were significantly higher among workers with high OSA risk than among those with low OSA risk after adjusting for confounders. Multiple logistic regression analysis examining the association of OSA risk and STOP-Bang score with hearing impairment revealed an odds ratio of 1.738 (95% confidence interval [CI] 1.113-2.713, p = 0.015) and 1.256 (95% CI 1.031-1.529, p = 0.023), respectively, after adjusting for confounders. In addition, when the hearing impairment was reclassified into high- and low-frequency hearing impairment, a statistically significant OR was seen for high-frequency hearing impairment. In conclusion, high OSA risk was associated with hearing impairment in occupational noise-exposed workers, especially in the high-frequency range of 3 and 4 kHz. More efforts are required to improve the management of OSA and its risk factors to preserve hearing in occupational noise-exposed workers.

Genome-wide association study and polygenic risk score analysis for hearing measures in children

An efficient auditory system contributes to cognitive and psychosocial development. A right ear advantage in hearing thresholds (HTs) has been described in adults and atypical patterns of left/right hearing threshold asymmetry (HTA) have been described for psychiatric and neurodevelopmental conditions. Previous genome-wide association studies (GWASs) on HT have mainly been conducted in elderly participants whose hearing is more likely to be affected by external environmental factors. Here, we investigated HT and HTA in a children population cohort (ALSPAC, n = 6,743). Better hearing was associated with better cognitive performance and higher socioeconomic status. At the group level, HTA suggested a left ear advantage (mean = -0.28 dB) that was mainly driven by females. SNP heritability for HT and HTA was 0.13 and 0.02, respectively (n = 4,989). We found a modest negative genetic correlation between HT and reading ability. GWAS for HT (n = 5,344) did not yield significant hits but polygenic risk scores for higher educational attainment (EA, ß = -1,564.72, p = .008) and schizophrenia (ß = -241.14, p = .004) were associated with lower HT, that is, better hearing. In summary, we report new data supporting associations between hearing measures and cognitive abilities at the behavioral level. Genetic analysis suggests shared biological pathways between cognitive and sensory systems and provides evidence for a positive outcome of genetic risk for schizophrenia.

Prevalence and clinical aspects of hearing loss among the South Korean adolescent: Data from a population-based study

OBJECTIVES

Slight/mild hearing loss is detrimental to communication and academic achievements. This study aimed to describe the prevalence of hearing loss, and to investigate the factors related to noise-induced hearing loss among South Korean adolescent.

METHODS

As a population-based retrospective study, 1845 South Korean adolescents aged from 12 to 19 years were analyzed using the data from Korea National Health and Nutrition Examination Survey V (KNHANES V, 2010-2012). The prevalence of hearing loss according to the side, severity, and frequency was calculated. For assessing the noise-induced hearing loss in adolescent, the prevalence of hearing loss only in high-frequency (onlyHFHL, defined as 1) thresholds at 0.5 and 1 kHz of ≤15 dB HL, and 2) maximal thresholds at 3, 4, or 6 kHz ≥ 15 dB HL higher than the highest threshold for 0.5 and 1 kHz) was analyzed. Moreover, the relevance of onlyHFHL in context of sociodemographic factors and noise exposure history was evaluated.

RESULTS

The prevalence of unilateral and bilateral hearing loss based on the average of six frequencies (0.5, 1, 2, 3, 4, and 6 kHz) in South Korean adolescents were 8.56% and 1.03%, respectively, and most cases were hearing loss with slight/mild degree. The prevalence of unilateral and bilateral onlyHFHL were 32.74% and 5.53%, respectively. Factors found to be associated with onlyHFHL were sex (female) and household income (high).

CONCLUSIONS

According to this population-based study, the prevalence of slight/mild hearing loss and onlyHFHL in the South Korean adolescents were considerably high. With knowledge of the factors related with onlyHFHL, paying more attention to slight/mild hearing loss will be helpful in preventing hearing loss in adolescents.

Assessment of occupational noise-related hearing impairment among dental health personnel

Objectives

The purpose of the study was to examine hearing thresholds among dental personnel. The secondary aim was to evaluate sound levels among dental equipment that dental personnel are exposed to.

Methods

Two hundred forty‐four dentists, dental technicians, dental assistants, and dental students participated. Sixty‐two participated as a control group. Audiological thresholds for the test groups were compared to the control group. All participants were from Jordan University Hospital. Participants completed a questionnaire in addition to their audiometric testing. Otoscopy, tympanometry, and pure tone audiometry were included in their assessment. Three‐factor ANOVA and t tests were utilized to assess the statistical differences of hearing thresholds among the groups and between the two ears. Pearson correlation test was used to assess the effect of age, experience, and duration of exposure on the degree of hearing loss in the test groups for both ears.

Results

The authors reported statistically significant differences among hearing thresholds between the control group and others. Left hearing thresholds were noted to be significantly poorer in the left versus right ear at 1000, 2000, 4000, and 8000 Hz in dental assistants. The authors also reported a significant relationship between the degree of hearing impairment among dental assistants and the daily duration of exposure to dental occupational noise, followed by age.

Conclusion

Hearing impairment was higher among dental professionals than the control group and especially among dental assistants and technicians. The authors recommended screening guidelines and adapting hearing protection methods for dental professionals and particularly for dental assistants and technicians.

Hearing threshold levels among steel industry workers in Samut Prakan, Thailand

Background

Industrial noise is generally considered from the perspective of environmental health and safety, with requirements for reduced occupational noise exposure, strict standards for hearing protection, and public health awareness campaigns.

Materials and methods

This cross-sectional study aimed to identify the hearing threshold levels (HTLs) among steel industry workers exposed to hazardous noises in Samut Prakan Province, Thailand, and investigate the associations between hearing loss and age and length of employment in the industry. Audiometric data for 93 participants were collected at the Samut Prakan Provincial Hospital. Qualified technicians performed audiometric screening using an audiometer in a quiet environment. Tests were conducted after participants had completely rested for ≥14 hours after the last exposure to workplace noise.

Results

The results showed that all participants had been employed in a work zone with a noise hazard for >1 year. Participants’ age ranged from 19 to 59 years and their work experience from 1 to 39 years. Average mean hearing thresholds in the right ear at 4, 6, and 8 kHz were 31.34, 29.62, and 25.64 dB, respectively. Mean hearing thresholds in the left ear at 4, 6, and 8 kHz were 40.15, 32.20, and 25.48 dB, respectively. Hearing loss related to work experience was greater than that attributable to age and work station. Duration of noise exposure ranged from 1 to 39 years, and more than half of the participants (60.2%) were exposed to hazardous noises from 6 to 25 years.

Conclusion

These findings show that participants’ age and experience were significantly associated with hearing loss at all levels. HTLs among industrial workers should be regularly assessed. At the health policy level, these workers need to start being protected when they begin working.

The Effect of Short-Term Noise Exposure on Audiometric Thresholds, Distortion Product Otoacoustic Emissions, and Electrocochleography

Purpose The purpose of the study was to examine the effect of short-term noise exposure on audiometric thresholds, distortion product otoacoustic emissions (DPOAEs), and electrocochleography (ECochG) as a function of ear and sex. Method Preexposure and postexposure measures of audiometric thresholds, DPOAEs, and ECochG indices were examined. Sixteen male and 16 female adults participated. Participants were exposed to a 2000-Hz narrowband noise presented at 105 dBA for 10 min. Results Following noise exposure, significantly ( p < .0001) larger auditory threshold differences were observed for left ears. Larger auditory threshold differences were also observed for 3000 and 4000 Hz versus 2000 and 6000 Hz. DPOAE absolute amplitude differences increased with decreasing L ₁, L ₂ level ( p < .0001). DPOAE absolute amplitude differences also significantly rose with increasing f₂ frequencies ( p < .0001). Females generally had larger DPOAE absolute amplitude differences than males ( p < .05). Summating potential amplitudes were significantly larger for female left ears following noise exposure ( p = .03). Left-ear summating potential/action potential amplitude ratios and summating potential/action potential area ratios were increased following noise exposure ( p < .05). Conclusions Utilizing a test battery to examine the effects of short-term noise exposure can reveal the functional status of different structures in the cochlea. There appears to be a susceptibility of the left ear to short-term loud noise exposure. Findings with respect to a sex susceptibility to short-term loud noise exposure were not strong, as sex differences were only observed for a subset of the conditions.

Noise exposure and auditory thresholds of military musicians: a follow up study

Background

Military musicians are working in a noisy environment with high sound exposure levels above the international standards. Aim of the current study is to find out, whether they develop the expected hearing impairments. Adherence to the regulations for prevention in musicians is more difficult than in other occupational fields.

Methods

In an interval of 13.3 years, 36 out of 58 male military musicians of a German army music corps were subjected twice to an audiometric audit. There were no exclusion criteria apart from acute ENT infections (three musicians). These results were compared with one another and evaluated by means of statistical methods for relationships with several factors.

Results

At frequencies below 3 kHz, the follow-up audiograms were up to 5 dB better than the preliminary examination. From 4 kHz up to 8 kHz the preliminary investigations showed less hearing impairment. Averaging all frequencies the improvement of hearing ability was around 1 dB. Above 1 kHz the average hearing of the right ear was up to 7 dB better than that of the left ear. Age-induced hearing loss was 3 to 8 dB lower than predicted by ISO standards over the entire frequency range. The side of the ear (right/left) and the frequency (3, 4, and 6 kHz) were significant (p < 0.05) in hearing loss, whereas the influence of the instrument and the acoustic traumata were not.

Conclusion

Despite the high noise levels, the average hearing ability of the 36 military musicians during the investigation period only slightly deteriorated in the noise-sensitive frequencies (3, 5 and 6 kHz). Music may be less harmful than industrial noise, or the long-term auditory training of the musicians leads to a delayed presbycusis.

Noise exposure and auditory thresholds of German airline pilots: a cross-sectional study

OBJECTIVE

The cockpit workplace of airline pilots is a noisy environment. This study examines the hearing thresholds of pilots with respect to ambient noise and communication sound.

METHODS

The hearing of 487 German pilots was analysed by audiometry in the frequency range of 125 Hz-16 kHz in varying age groups. Cockpit noise (free-field) data and communication sound (acoustic manikin) measurements were evaluated.

RESULTS

The ambient noise levels in cockpits were found to be between 74 and 80 dB(A), and the sound pressure levels under the headset were found to be between 84 and 88 dB(A).The left-right threshold differences at 3, 4 and 6 kHz show evidence of impaired hearing at the left ear, which worsens by age.In the age groups <40/≥40 years the mean differences at 3 kHz are 2/3 dB, at 4 kHz 2/4 dB and at 6 kHz 1/6 dB.In the pilot group which used mostly the left ear for communication tasks (43 of 45 are in the older age group) the mean difference at 3 kHz is 6 dB, at 4 kHz 7 dB and at 6 kHz 10 dB. The pilots who used the headset only at the right ear also show worse hearing at the left ear of 2 dB at 3 kHz, 3 dB at 4 kHz and at 6 kHz. The frequency-corrected exposure levels under the headset are 7-11 dB(A) higher than the ambient noise with an averaged signal-to-noise ratio for communication of about 10 dB(A).

CONCLUSIONS

The left ear seems to be more susceptible to hearing loss than the right ear. Active noise reduction systems allow for a reduced sound level for the communication signal below the upper exposure action value of 85 dB(A) and allow for a more relaxed working environment for pilots.

Current insights in noise-induced hearing loss: a literature review of the underlying mechanism, pathophysiology, asymmetry, and management options

BACKGROUND

Noise-induced hearing loss is one of the most common forms of sensorineural hearing loss, is a major health problem, is largely preventable and is probably more widespread than revealed by conventional pure tone threshold testing. Noise-induced damage to the cochlea is traditionally considered to be associated with symmetrical mild to moderate hearing loss with associated tinnitus; however, there is a significant number of patients with asymmetrical thresholds and, depending on the exposure, severe to profound hearing loss as well.

MAIN BODY

Recent epidemiology and animal studies have provided further insight into the pathophysiology, clinical findings, social and economic impacts of noise-induced hearing loss. Furthermore, it is recently shown that acoustic trauma is associated with vestibular dysfunction, with associated dizziness that is not always measurable with current techniques. Deliberation of the prevalence, treatment and prevention of noise-induced hearing loss is important and timely. Currently, prevention and protection are the first lines of defence, although promising protective effects are emerging from multiple different pharmaceutical agents, such as steroids, antioxidants and neurotrophins.

CONCLUSION

This review provides a comprehensive update on the pathophysiology, investigations, prevalence of asymmetry, associated symptoms, and current strategies on the prevention and treatment of noise-induced hearing loss.

Tinnitus as an Early Indicator of Permanent Hearing Loss

A retrospective study was designed to evaluate tinnitus (ringing or other sounds in the ears or head) as a potential early indicator of permanent hearing loss in a population of noise exposed workers. Data were examined from 91 male employees working in environments with noise levels ranging from 8 hour time weighted averages of 85 to 101 dBA over a period of 15 years. Results of annual audiometric testing were obtained as part of an ongoing hearing conservation program conducted since 1971 by ESCO Corporation, a steel foundry located in the Portland, Oregon metropolitan area. Results indicate the prevalence of tinnitus increases more than two and one half times for workers experiencing maximum threshold shifts ≥15 decibels in hearing level (dBHL). Results also provide evidence that reports of tinnitus at the time of annual audiometric testing may be useful in identifying workers at greater risk for developing significant shifts in hearing thresholds.

Hearing loss in the Royal Norwegian Navy: A longitudinal study

The aims of this longitudinal study were to investigate a significant threshold shift (STS) among personnel working on board the Royal Norwegian Navy's (RNoN) vessels between 2012 and 2014 and to identify possible determinants of STS. Hearing thresholds were measured by pure tone audiometry in two consecutive examinations (n = 226). STS was defined as an average change in hearing thresholds ≥ + 10 dB at 2,000 Hz, 3,000 Hz, and 4,000 Hz in either ear. Determinants of STS were assessed through a questionnaire. The incidence of STS was 23.0%. Significant determinants of STS were the number of episodes of temporary threshold shifts (TTS) in the Navy, exposure to continuous loud noise during work on board, and the number of gun shots (in the Navy, hunting, and sports). This study indicated a significant association between noise exposure on board Navy vessels and development of STS.

Dietary vitamin intake correlates with hearing thresholds in the older population: the Korean National Health and Nutrition Examination Survey

BACKGROUND

Previous animal studies have shown that vitamins may prevent age-related hearing loss. However, no concrete conclusions have been reached about the association between vitamin intake and presbycusis in humans.

OBJECTIVE

We investigated the association between dietary vitamin intake and hearing thresholds in adults between 50 and 80 y of age by using data from a large population-based survey.

DESIGN

We used data from the 2011 Korea National Health and Nutrition Examination Survey. A pure-tone audiogram and physical examination of the ear were performed, and vitamin intake was calculated by using the 24-h recall method. Data from 1910 participants were analyzed through univariate and multivariate linear regression analyses.

RESULTS

After adjustment for age, sex, smoking, and exposure to occupational and explosive noise, intake of vitamin C correlated with better hearing at midfrequency (2000 and 3000 Hz) (coefficient: -0.012; 95% CI: -0.022, -0.002). Dietary supplement use was positively associated with better hearing at all frequencies. The univariate analysis indicated that dietary intakes of retinol, riboflavin, niacin, and vitamin C were positively correlated with better hearing at most frequencies. In contrast, serum concentrations of vitamin D were associated with worse hearing at mid and high (4000 and 6000 Hz) frequencies.

CONCLUSIONS

Dietary intake of vitamin C was associated with better hearing in the older population. Because less than one-half of elderly participants in this study consumed a sufficient amount of vitamins, and vitamin intake decreased with age, we should consider proper diet counseling to prevent hearing decline.

Effect of workplace noise on hearing ability in tile and ceramic industry workers in Iran: a 2-year follow-up study

INTRODUCTION

Noise as a common physical hazard may lead to noise-induced hearing loss, an irreversible but preventable disorder. Annual audiometric evaluations help detect changes in hearing status before clinically significant hearing loss develops. This study was designed to track hearing threshold changes during 2-year follow-up among tile and ceramic workers.

METHODS

This follow-up study was conducted on 555 workers (totally 1110 ears). Subjects were divided into four groups according to the level of noise exposure. Hearing threshold in conventional audiometric frequencies was measured and standard threshold shift was calculated for each ear.

RESULTS

Hearing threshold was increased during 2 years of follow-up. Increased hearing threshold was most frequently observed at 4000, 6000, and 3000 Hz. Standard threshold shift was observed in 13 (2.34%), 49 (8.83%), 22 (3.96%), and 63 (11.35%) subjects in the first and second years of follow-up in the right and left ears, respectively.

CONCLUSIONS

This study has documented a high incidence of noise-induced hearing loss in tile and ceramic workers that would put stress on the importance of using hearing protection devices.

Laterality of basic auditory perception

Laterality (left-right ear differences) of auditory processing was assessed using basic auditory skills: (1) gap detection, (2) frequency discrimination, and (3) intensity discrimination. Stimuli included tones (500, 1000, and 4000 Hz) and wide-band noise presented monaurally to each ear of typical adult listeners. The hypothesis tested was that processing of tonal stimuli would be enhanced by left ear (LE) stimulation and noise by right ear (RE) presentations. To investigate the limits of laterality by (1) spectral width, a narrow-band noise (NBN) of 450-Hz bandwidth was evaluated using intensity discrimination, and (2) stimulus duration, 200, 500, and 1000 ms duration tones were evaluated using frequency discrimination. A left ear advantage (LEA) was demonstrated with tonal stimuli in all experiments, but an expected REA for noise stimuli was not found. The NBN stimulus demonstrated no LEA and was characterised as a noise. No change in laterality was found with changes in stimulus durations. The LEA for tonal stimuli is felt to be due to more direct connections between the left ear and the right auditory cortex, which has been shown to be primary for spectral analysis and tonal processing. The lack of a REA for noise stimuli is unexplained. Sex differences in laterality for noise stimuli were noted but were not statistically significant. This study did establish a subtle but clear pattern of LEA for processing of tonal stimuli.

Mean hearing thresholds among school children in Taiwan

OBJECTIVES

The primary aim of this study was to obtain the mean hearing thresholds among school-age children residing in the Datong and Jhongshan Districts of Taipei City, Taiwan.

DESIGN

Hearing thresholds were obtained from a stratified sample of 1411 students in grades 1, 4, 7, and 10 based on their participation in the annual health examination of school-age children carried out by the Taipei City Government. After otoscopic examination, audiometric testing was undertaken at 0.5 to 4 kHz along with tympanometric screening. The means and SDs obtained from these examinations are reported by grade, gender, ear, and frequency.

RESULTS

The mean hearing thresholds ranged between 6.8 and 16.7 dB HL, with the highest (poorest) hearing threshold being obtained at a test frequency of 0.5 kHz. At all the tested frequencies, the hearing thresholds of boys were found to be higher than those of girls. Logistic regressions demonstrating that those students with the highest odds ratios of hearing loss were to be found in the sample of children in grade 1. The odds ratios of hearing loss were also found to be higher among boys than girls and for the left ear rather than the right.

CONCLUSIONS

These results indicate that the mean thresholds among the study sample were well above (meaning poorer) the pure-tone average of 15 dB HL, as recommended in a previous study. Therefore, the results of this study suggest the need to further determine the etiology of hearing loss among children in grades 1, 4, 7, and 10 in the Datong and Jhongshan Districts of Taipei City and perhaps Taiwan.

Abnormal auditory forward masking pattern in the brainstem response of individuals with Asperger syndrome

Abnormal auditory information processing has been reported in individuals with autism spectrum disorders (ASD). In the present study auditory processing was investigated by recording auditory brainstem responses (ABRs) elicited by forward masking in adults diagnosed with Asperger syndrome (AS). Sixteen AS subjects were included in the forward masking experiment and compared to three control groups consisting of healthy individuals (n = 16), schizophrenic patients (n = 16) and attention deficit hyperactivity disorder patients (n = 16), respectively, of matching age and gender. The results showed that the AS subjects exhibited abnormally low activity in the early part of their ABRs that distinctly separated them from the three control groups. Specifically, wave III amplitudes were significantly lower in the AS group than for all the control groups in the forward masking condition (P < 0.005), which was not the case in the baseline condition. Thus, electrophysiological measurements of ABRs to complex sound stimuli (eg, forward masking) may lead to a better understanding of the underlying neurophysiology of AS. Future studies may further point to specific ABR characteristics in AS individuals that separate them from individuals diagnosed with other neurodevelopmental diseases.

Asymmetry in Noise-Induced Hearing Loss

OBJECTIVE

Noise-induced hearing loss (NIHL) is more severe in the left ear than the right ear. The aim of this study was to examine the possible association of handedness and acoustic reflex parameters on right or left NIHL predominance.

STUDY DESIGN

Retrospective case review and prospective series.

SETTING

Tertiary center.

PATIENTS

Three samples were studied: 1) the files of 4,277 army personnel with NIHL were reviewed to analyze the relation between the side affected and age, sex, duration of noise exposure, and acoustic damage; 2) an additional 119 army personnel with NIHL were evaluated for the possible association of handedness and NIHL asymmetry; and 3) fifty-one normal-hearing subjects were tested for threshold and latency of acoustic reflex by handedness.

INTERVENTION

None.

MAIN OUTCOME MEASURE

Differences in NIHL asymmetry by background and noise-related variables and handedness.

RESULTS

NIHL was more pronounced in the left ear, regardless of demographic characteristics, noise exposure parameters, acoustic reflex measures, or handedness.

CONCLUSION

The asymmetry in hearing loss severity may be attributed to the cortical pathways, specifically to the more pronounced efferent auditory system on the right side, which reduces the susceptibility of the right ear to cochlear insult.

Hearing loss among operating engineers in American construction industry

OBJECTIVE

Occupational noise exposure and noise-induced hearing loss (NIHL) among construction workers has long been recognized as a problem in the United States, yet little is known about the prevalence of NIHL among American construction workers. The purpose of this study was to determine the prevalence and characteristics of hearing loss among operating engineers (OEs) who operate heavy construction machinery.

METHOD

As a part of hearing protection intervention, an audiometric test was conducted for both ears at frequencies 0.5 through 8 kHz in the soundproof booth. Prior to the audiometric test, a paper-pencil pre-hearing test questionnaire was administered and an otoscopic examination was completed. Prevalence of hearing loss was determined based on hearing threshold levels (HTLs) in the worst ear with a low fence of 25 dB.

RESULT

A total of 623 workers were included in the analysis and they were predominantly middle-aged Caucasian males (mean age = 43 years, Caucasian = 90%, male = 92%). Over 60% of OEs showed hearing loss in the noise-sensitive higher frequencies of 4 and 6 kHz. The rate of hearing loss was particularly higher among workers who reported longer years of working in the construction industry. Workers showed significantly poorer hearing in the left ear, and a typical characteristic of NIHL, a V-notch at 4 or 6 kHz, was not shown in this population. Thirty-eight percent reported ringing/buzzing in the ear and 62% indicated having problems in understanding what people say in loud noise. Average reported use of hearing protection devices (HPDs) was 48% of the time they were required to be used. Significant inverse relationship was found between higher frequency (4-6 kHz) hearing loss and use of HPDs (r =-0.134, p < 0.001). Workers using HPDs more had significantly better hearing than those who did not.

CONCLUSION

The study demonstrated a significant NIHL problem and low use of HPDs in OEs. An effective hearing conservation program, including a periodic audiometric testing and hearing protection intervention, for this study population should be in place.

Development of the human fetal cochlear nerve: a morphometric study

Ontogenesis of the human peripheral auditory pathway is relatively less explored. While the distal part of the auditory perception apparatus (i.e. the cochlea) received attention, studies on the neural element carrying information to the brainstem (i.e. the cochlear nerve) are scarce. In the present study, axonal differentiation, maturation and myelination of the distal end of the human cochlear nerve (CN) were assessed using light and electron microscopy. Seven human fetuses of 12, 15, 18, 20, 22, 28 and 38 weeks' gestation (WG) were analyzed. Light microscopy revealed nerve fascicles as early as 12 WG, initially arranged loosely but later compacted by 18 WG. Myelinated fibers were clearly detected at 28 WG. Ultrastructurally, at 12 WG developing Schwann cells were present between the thin unmyelinated axons. At 15 WG, the fascicular arrangement was distinct with blood vessels in the perineurium. The maximum number of axons was found at 20 WG, which subsequently reduced to reach the adult level at 22 WG. The myelinated axons in the CN were first observed on the left side at 20 WG, following which the number and proportion of myelinated axons increased until term, incorporating both small and large axons. The right CN lagged behind in maturation. Axon size also increased with age. Thus, the maturation of the human CN commences during the mid-gestation period and produces exuberant axons that are eventually pruned at a time when axons start to myelinate. During this developmental period the human CN maintains maturational asymmetry, the functional consequences of which remain to be elucidated.

Hearing loss among workers at an oil refinery in Taiwan

In this study, the authors determined the characteristics of noise-induced hearing loss (NIHL) and its association with hearing-loss-related symptoms (HLRSs) among workers at an oil refinery in Taiwan. Workers from the refinery (N = 384) who had chronic noise exposure (range = 73 to 89 dBa; mean = 81.2, standard deviation = 5.4 dBa) were enrolled in the study. Workers chronically exposed to noise levels that were less than 85 dBa experienced slight hearing loss at high frequencies [(3k + 4k + 6k)/3]. Workers with > or = 1 HLRSs had increased hearing loss at low frequencies [(0.5k + 1k + 2k)/3] and high frequencies (odds ratios = 7.2 and 3.7, respectively). An increased hearing threshold shift for high frequencies (3k, 4k, and 6k Hz) was found in workers who had chronic noise exposure for more than 15 yr. These findings indicate that self-reported HLRSs provide valuable early warning signs of NIHL.

Gender distinctions and lateral asymmetry in the low-level auditory brainstem response of the human neonate

Threshold measures of auditory brainstem response (ABR) were generated in 72 full-term newborn infants in response to clicks and tone burst stimuli between 500 and 8000 Hz as detailed in a previous study. These results were further analyzed for differences in response related to ear (lateral asymmetry) and subject gender. Thresholds obtained in male infants were significantly lower than those of females (P = 0.0485). The greatest differences in threshold between male and female infants occurs in the right ear (7.45 dB) as opposed to the left ear (1.56 dB). Both male and female infants have significantly larger wave V amplitude elicited from the right ear than the left (P = 0.0002) using low-level stimuli. Also, as has been noted in adults, female infants have larger amplitude ABRs than males (P = 0.0018), but amplitude differences across gender are significant only in the right ear (ear by gender interaction P = 0.0278). Results of this study indicate that gender differences and lateral asymmetry in auditory function are not a result of gender bias for or unbalanced auditory trauma, but a biologically significant phenomenon that is present at birth. The argument is made that superior right ear performance may be part of cerebral laterality in auditory function.

Intrinsic differences in hearing performances between ears revealed by the asymmetrical shooting posture in the army

Left ear noise-induced hearing losses are dominant in the army. The common explanation is the asymmetrical effect of the shooting posture. However, firm evidence to support this possibility is still lacking. In the French army shooters, eye preference rather than hand preference for shooting determines the side of shooting and hence the ear more exposed to noise. To test whether left-right asymmetry of hearing thresholds really relates to the shooting posture, we analyzed audiograms from 644 officers of the infantry and artillery branches. The interaural differences reached 7 dB for right-eyed subjects, and less, about 5 dB, for left-eyed subjects at 6-7 kHz, both with disadvantage for the left ear. In contrast, hearing thresholds of both groups in the low frequency range were significantly better for left ears. Our results suggest each ear has different intrinsic characteristics. The right cochlea might be a less sensitive but more robust sensor than the left cochlea, which might be a finer sensor but more sensitive to noise.

The hearing threshold levels of children at school age

OBJECTIVE

To measure the hearing threshold levels by air conduction in a group of unselected and a group of selected (otologically normal) school-aged children.

DESIGN

Six hundred eighty-seven unselected school children representing three age groups and 471 selected, otologically normal children were examined by pure-tone audiometry.

RESULTS

The obtained results showed that hearing levels improved with age from 7 to 10 yr in children in both groups. The pure-tone averages (PTAs) of the right and the left ear in otologically normal children were, respectively, 1.8 and 1.2 dB HL in 7-yr-old children, -0.9 and -1.7 dB HL in 10-yr-old children, and -1.3 and -2.0 dB HL in 14-yr-old children. The PTAs were 0.2 to 1.9 dB poorer in the groups of unselected children.

CONCLUSIONS

The present study provides new data on the hearing sensitivity among unselected 7-, 10-, and 14-yr-old school children. Furthermore, it provides normative values on the hearing thresholds of otologically normal children in these three age groups.

An investigation of the association between exposure to styrene and hearing loss

The importance of the association between advancing age and hearing loss is well recognized. Further, prolonged significant noise exposures are also known to result in permanent hearing loss. However, little is known of the contribution of industrial chemical exposures to hearing loss. Information available, from both animal and human studies, raises the possibility that certain aromatic hydrocarbons are ototoxic. The purpose of this study was to assess whether occupational styrene exposure causes hearing loss in a group of workers in the fiber-reinforced plastics manufacturing industry. The hearing acuity of 299 subjects was determined, using pure-tone screening audiometry, at the beginning of a single workshift and again at the end of the shift. On the same day, the personal, time-weighted average exposures of each subject to both styrene and noise were measured. In addition, information on the following factors was obtained from each participant: previous work history, including exposures to noise and chemicals; use of personal protective equipment for noise or solvents; personal and family history of hearing problems; and smoking history. Current exposures together with work histories were used to construct lifetime noise and styrene exposure indices. No conclusive evidence was found for a chronic styrene-induced effect on hearing acuity, when both noise and styrene lifetime exposures were taken into account. As expected, age and noise exposures were highly important variables, both positively associated with hearing loss. In addition, the detrimental effect of noise exposure on hearing acuity was found to be strengthened with increased age. Noise and styrene exposures were highly correlated, clearly illustrating the importance of considering all associated variables in analysis of such data. No evidence was found for a relationship between smoking, recreational noise, solvent exposures, and hearing loss.

A speculation about the parallel ear asymmetries and sex differences in hearing sensitivity and otoacoustic emissions

Hearing sensitivity and the prevalence of spontaneous otoacoustic emissions show parallel ear and sex differences in humans. Here it is proposed that these lateral asymmetries and sex differences may all result from differences in the 'strength' of the efferent inhibition delivered to individual cochleas. Specifically, it is proposed that the amount of efferent inhibition is relatively less in right ears and in females than in left ears and males. While it is unclear why or how ear asymmetries or sex differences would develop in the efferent system, by assuming that they do, one can begin to explain a number of basic facts of hearing. It is even possible that the lateral asymmetry in the efferent system may be related to the well-known cortical asymmetries that are believed to underlie speech perception, speech production, and other human abilities.