The Accuracy of 0 db HL as an Assumption of Normal Hearing

Transcription

1 The Accuracy of 0 db HL as an Assumption of Normal Hearing

2 Introduction An operating assumption of diagnostic audiology is that hearing level of a young adult with no known hearing loss or history of noise exposure should be 0dB HL. Hearing thresholds of 0 25 db HL are generally considered a loose clinical standard of normal hearing, yet hearing loss is consistently measured as elevation from 0 db. Recent studies have suggested that thresholds of 0 db may be an optimistic assumption for this population due to increased recreational noise in our society.

3 Previous Research Mostafapour et al. (1998) tested young adults ages for the presence of noise induced hearing loss. They were particularly interested in the role of personal listening devices and other sources of leisure noise on young peoples hearing thresholds. Results: All hearing thresholds were found to be within normal limits (less than 25 db), and although notches were present in a percentage of the sample, the notches did not correlate with any single or cumulative noise source.

4 Previous Research The authors of the study concluded that the majority of young users of personal listening devices are at low risk for substantive NIHL. However they warn that NIHL is an additive process, and even subtle deficits may contribute to unequivocal hearing loss with continued exposure.

5 Previous Research Carter at al. (1982) studied a group of year olds to explore the occurrence of hearing loss caused by exposure to loud music. The study found no evidence of widespread hearing loss, but it was noted that the accumulated noise exposure of some of [the subjects] to noise is such that, if their recreational patterns remain the same, they are at risk of some noise-induced hearing loss by their mid twenties.

6 An Additive Process The controversy in the literature raises an important question about the accuracy of our standards for normal hearing in young adults. Determining a correct baseline is important because the concept of 0 db HL as a baseline of normal hearing is critical for determining the degree of hearing injury in cases of occupational noise exposure. Noise induced hearing loss is an additive process, so even a small trend toward NIHL in the early adult years may impact the patient s hearing loss in the future.

7 The purpose of this study This study was instituted to determine the audiometric thresholds of a screened sample of graduate students with normal hearing.

8 The Sample All subjects were between 20 and 29 years of age and identified themselves as having normal hearing. Exclusion criteria included diagnosed hearing loss, history of chronic middle ear dysfunction, history of occupational noise exposure, occluded ear canals, and abnormal immitance on the day of testing. A case history was taken verbally for each subject to determine eligibility in the study.

9 The Sample 21 graduate students volunteered to participate; one student was excluded due to abnormal tympanograms on the day of testing, leaving 20 people in the sample. 6 men and 14 women

10 Materials and Methods Audiological examination was performed by the author, an AuD student in her fourth year of study. Otoscopy Bilateral screening tympanometry Pure tone air conduction audiometry with ER-3 insert ear phones using a GSI 61 audiometer 250, 500, 1000, 2000, 4000 and 8000 Hz in 2 db steps. In order to insure accurate thresholds, each subject was carefully instructed about response criteria, and thresholds were rechecked for consistency.

11 Results To explore audiogram configuration, the thresholds of both ears of each participant were averaged to create a single average threshold for each frequency. Examined under a series of comparisons to determine how close thresholds were to 0 db HL and if significant differences were present based on age, sex or ear. Results were compared to ISO 1999 Annexes A and B.

12 db HL Average Thresholds of All Participants Average Threshold of All Test Subjects Average Threshold Frequency (Hz) 8000 Average thresholds of 20 subjects, tested twice using rigorous audiometric testing methods, and SEM.

13 db HL Average Thresholds of Participants by AGE Average Threshold by Age years years (n=6) (n=14) Frequency (Hz) 8000 Average thresholds of subjects divided by age, and SEM.

14 Ear Effect? To determine the presence of a threshold difference between the ears, thresholds for all subjects were averaged by ear for each test frequency. No significant difference in thresholds was observed between ears at any frequency tested.

15 db HL Average Thresholds of all participants by SEX Average Threshold of Males vs. Females Males Females (n=8) (n-12) Frequency (Hz) 8000 Average thresholds of sample divided by sex, and SEM.

16 ISO 1999 Annexes A and B The mean thresholds of males and females were compared to ISO 1999(1990), a document produced by the International Organization of Standardization to explore the prevalence of occupational noise induced hearing loss by duration and intensity of noise exposure in the work week. ISO 1999 annexes A and B show hearing threshold levels associated with age of a typical screened (annex A) and unscreened (annex B) population of an industrialized country.

17 ISO 1999 Annexes A and B The ISO 1999 data is not representative of hearing threshold levels for young adults in their twenties, but to obtain a general comparison, data from the present study was compared to the 30 year age group. Results were compared at 500, 1000, 2000 and 4000 Hz.

18 Comparison to ISO 1999 Annexes A and B Both males and females showed elevated thresholds compared to Annexes A and B data for 30 year old subjects. This is an interesting finding, as we would expect the thresholds from test subjects to be lower than those from the ISO study because they are younger than thirty years old.

19 db HL Males compared to ISO 1999 Average Thresholds of Males in Sample vs ISO Annex-A Annex-B Males in Sample Frequency (Hz) 4000 Average thresholds of males in sample compared to ISO Annexes A and B.

20 db HL Females compared to ISO 1999 Average Thresholds of Females in Sample vs ISO Annex-A Annex-B Females in Sample Frequency (Hz) 4000 Average thresholds of females in sample compared to ISO Annexes A and B.

21 Discussion The results obtained in this study show that the average thresholds of a small carefully screened group of graduate students in their twenties are not 0 db. Although individual subjects presented with some thresholds equal to or better than 0 db across the frequency range, the mean thresholds ranged from.7 to 8.2 db HL from 250 to 8000 Hz, not 0 db HL.

22 Discussion Despite elevated thresholds in the sample, the audiometric configuration does not generally show the pattern of loss we would expect to see from the emergence of noise induced hearing loss. We would expect the hearing loss to manifest itself in a notch at 3000 to 4000 Hz with essentially unaltered low frequency thresholds.

23 Discussion Comparison of thresholds by age: As a whole, the group of subjects in their younger twenties presented with lower thresholds. The elevation of high frequency thresholds in the older group, although not large enough to be a true 4000 Hz notch, may be evidence of emerging noise and/or age related hearing loss.

24 Discussion It should be noted that the samples of each age group were not even. The year old group included 8 subjects while the year old group included 12 subjects. It would be interesting to further explore the differences between adults in their later or early twenties with even numbers in each age group.

25 Low Frequency Loss? Consistent elevation of low frequency thresholds for males and females of both age groups There is no audiological reason to suspect low frequency hearing loss, particularly since each subject was screened on the day of testing for middle ear involvement. Low frequency loss is not consistent with age or noise induced hearing loss. It is possible that there was some source of low frequency masking present in the booth. Another possible explanation is an error in calibration.

26 Conclusion The results of this study indicate that 0 db HL may be an optimistic assumption of normal hearing for young adults with no known otologic insult or history of noise exposure. The configurations of mean thresholds in the older vs younger samples imply the possible beginning of high frequency hearing loss. It is impossible to determine the cause of threshold elevation from this study, but it is important to note that even adults under age 30 may have emerging hearing loss and be susceptible to further noise induced and age related hearing loss.

27 References Carter, N.L., et al. (1982). Amplified music and young people s hearing. Medical Journal of Australia, 2(3), Dobie, R. A. (1993). Age-Related Hearing Loss. Medical-Legal Evaluation of Hearing Loss. Van Norstrand Reinhold, New York; chapter 6, Dobie, R.A. (2008). The burdens of age-related and occupational noise-induced hearing loss in the United States. Ear & Hearing, 29, International Organization for Standardization Acoustics: Determination of Occupational Noise Exposure and Estimation of Noise-Induced Hearing Impairment, ISO Geneva: International Organization for Standardization. Mostafapour, S.P., et al (1998). Noise-induced hearing loss in young adults: the role of personal listening devices and other sources of leisure noise. American Laryngological, Rhinological & Otological Society, Inc., 108, Sera, M.R., et al. (2005). Recreational noise exposure and its effects on the hearing of adolescents. Part I: An interdisciplinary long-term study. International Journal of Audiology, 44, Sera, M.R., et al. (2005). Recreational noise exposure and its effects on the hearing of adolescents. Part II: Development of hearing disorders. International Journal of Audiology, 44,