Better Hearing Australia Conference
Adelaide 7-11 August 1994

1st presentation

Ageing characteristics of the Australian population in terms of otoacoustic emission strengths: global and individual picture

N. M. Murray, E. L. LePage and K. Tran
Hearing Loss Prevention Research Unit
National Acoustic Laboratories,
126 Greville Street, Chatswood, NSW 2067.

ABSTRACT

Using an Otodynamics ILO88/92 analyser in standard screening mode with stimulus peak clicks pre-adjusted to 80 ± 1.5 dB SPL, nearly 6000 click-evoked otoacoustic emission records from normal and pathological ears of all ages in the Australian population have been examined. The strength of the emission for any ear is reduced to a single number defined as Coherent Emission Strength (CES dB SPL) which varies from 38 dB in neonates to the -12 dB noise level for individuals with non-measureable emissions, assumed to represent almost total loss of outer hair cell function.

CES values are plotted versus age, both for a subgroup of 1450 ears with no know ear pathology other than noise-induced hearing loss and for a small group of individuals followed for periods of up to five years. A general deline of CES values with age (greater for males than females) exists in the greater subgroup. For the first two decades the mean rate of decline is 8.6 dB/decade, leaving an apparent plateau in the values between ages 15 and 45. A further linear decine of the mean values is not shared by that of the maximum CES values in each age range. A “minimal” linear rate of decline in emission strength of ❤ dB/decade equates to 7 decades with vaery little hearing loss, but 8 dB/decade affords no more than 3 decades without hearing loss. These global rates of ageing with be compared with apparent rates of ageing in individuals in the various age ranges studied. It is envisaged that to be able to denote an accelerated rate of cochlear ageing will be of value in any hearing loss prevention program.

INTRODUCTION

Over the last thirty years the most commonly accepted ageing picture for the ear has been based on the pure tone audiometric threshold, particularly those published in the U.S. National Survey of Health Statistics for 1960-1962. These show that no until age 55 for men and 65 for women is there any audiometric hearing loss; over these ages hearing loss occurs only at high frequencies, 2000 Hz and above. As well, there is no hearing loss evident in the audiometric pattern of the speech range (taken as the values for teh 3-frequency average 500, 1000 and 2000 Hz) until age 78 for both men and women (Figure 1).

Evidence from otoacoustic emission data gathered in our Hearing Loss Prevention Research Unit at the National Acoustic Laboratories over the pass five years now appears to demonstrate that the ageing processes in the cochlea pre-empt those indicated by the earlier audiometric evidence.

Fig. 1. A. Median pure tone audiometric hearing levels for both males and females in age groups ranging from 18 to 79 years. B. Median hearing levels for speech (3- frequency average: 500, 1000 and 2000 Hz) for both males and females in age groups ranging from 18 to 79 years. (U.S. National entre for Health Statistics, Series 11, No. 11, 1965).

The area of our research stems from basic hearing research and has led to a clinical test which is not a hearing test, does not rely on patient subjective assessment of their state of hearing or whether they can hear either a sound or an unwanted osunt. It is an objective assessment of inner ear damage.

Apart from the middle ear muscles, inside the inner ear, or cochlea, are a large group of cells which have been shown to have activity of their own. Normally when a sound comes into the ear it triggers a tiny motor or muscular response. By placing a small microphone in the ear canal we can pick up the sound of their response. This sound (rather like an echo) which comes from the inner ear is called an “otoacoustic emission”. It appears that the health of the ear can be directly assessed by the strength of the ear emission.

METHOD

Using an Otodynamics Analyser, nearly 6000 click-evoked otoacoustic emission recordings have been collected from almost 600 males and 500 females from birth to 83 years og age from all states in Australia. From the overall group, all repeat records, and all records from ears where there has been any suspicion of ear pathology, have been removed. However, cases of noise-induced hearing loss have not been eliminated, because in such a group as this, there is no strong basis for separating the known noise-exposed from the remainder of the ageing population. There was no prerequisite for subjects to have pure tone audiometric hearing thresholds within normal limits. Standard screening mode of 260 repetitions of the nonlinear stimulus sequence of clicks at approximately 80 dB SPL was used. From the two paramters output by the Otodynamics Analyser (reproducibility and emission sound level) we ahve empirically derived a single-number parameter measure, Coherent Emission Strength (CES dB SPL). This, we believe, provides a better estimate of the status of permanent cochlear damage by weighting the size of the emission by its reproducibility. The values of the the CES have been computed and plotted versus age for males and females.

RESULTS

From Figure 2 it can be seen that there is a dependence of emission strength upon age; that is, emission strength, in general, declines with age. Whether it is reasonable to assume that this decline should be linear is discussed below. This characteristic has potential value for the preventionof hearing loss when it is realised that hearing loss is not generally registerd until the CES value has decreased to what we regard as a critically low value of emission strength of approximately -3 dB SPL. This we associate with the onset of symptoms of hearing loss (LePage, Murray & Tran, 1994; next article). In fact, the curve representing the mean minus one standard deviation tends to mimic the shape of the well- documented presbycusis curve as shown in Figure 1.

Fig. 2. Age dependence of otoacoustic emissions in normal Australian ears (600 males and 500 females) with no known ear pathology.

Click-evoked otoacoustic emissions thus allows us to assess and monitor the accumulation of outer hair cell damage prior to the onset of hearing loss. As will be discussed later, CES scores for any individual’s ear can be placed on this characteristics and afford a crude measure of susceptibility; i.e. the hight the value, the less susceptibility to further damage; the lower the value the more susceptible.

However, in these population trends, different rates of ageing are apparent in different age ranges: firstly, there is a very slow rate of decline which applies to those individuals with maximum values of CES in each age range; secondl, a rate which applies to those up to the age of approximately 25; thirdly, a plateau between 25 and 45 and fourthly, another, slower decline after the age of 45.

Fig. 3. A. Minimal rate of ageing of -2 dB/decade. B. Decline of -3 dB/decade after 45 years of age. C. High rate of ageing of the cochlea at -8 dB/decade.

Firstly, from Figure 3A, it can be seen that those who obtained the maximum CES values in the range for each age group appear to be declining at the slowest rate, estimated to be -2 dB per decade and would thus be considered to be among the least susceptible for hearing loss, as the values for individuals in these groups never approach that critical value of -3 dB.

Secondly, in Figure 3B there appears to be a general steady decline of -3 dB per decade after 45 years og age. It is estimated that if the population as a whole aged at this rate then there would be little evidence of even a mild hearing loss until about age 65 or 70 years of age. These 45 to 80 year olds may be equated with the group in the U.S. Statistics of 30 years ago.

However, as can be seen in Figure 3C, there is a “dip” which suggests that there appears to be much faster rate of decline of -8.6 dB per decade which is showing up in the first two decades of life. An individual in this group could reach a critical level for the onset of hearing loss by age 25.

In these terms therefore, of Figs. 3A, B and C it would seem that the mean CES values for the Australian population up to age 15 may be declining at more than four times the rate of the few individuals whose emission strengths remain close to values typical for neonates. In our population we have observed that these begin with a mean emission strength of 22 dB SPL and an observed maximum of 38 dB SPL. The “minimal” linear rate of cochlear ageing of 2 dB per decade showing in Fig. 3A, equates to 6 decades with a ver low incidence of hearing loss whie CES values remain positive. On the other hand, a rate of decline of 8 dB per decade (Fig. 3C) affords no more than two and a half decades without hearing loss.

Figure 4 suggests that a new phenomenon may be occurring and that Australian young people may have substantially more “aged” ears in comparison with those of the previous generation when they were teenages and who are now aged 45 to 60 years. Indeed the apparent rate of ageing is highest in young people and, on the basis of our present calculation of Coherent Emission Strength, may be indicateive of early latent damage in the cochlea.

Fig. 4. The new phenomenon of an 8 dB before decade decline in emission strength in the first two decades of life which may be indicative of early latent damage in the cochlea.

The above describes apparent trends in the Australian population. These can only give an indication of what is happening to any individual. On further looking at the possibility of a new phenomenon occurring in the young people, and whether the dip inthe population statistics is real, we have observe a large number of records from young people in the age range 14 to 25 years with low emissions, where the depth and width of spectral notches indicates the extent of damage.

Fig. 5. The “Picket fence effect”: A selection of click-evoked emission spectra showing a notched feature characteristic of individuals exposed to noise yet who exhibit no hearing loss.

It can be seen that the correlated part of the spectrum (black area) has not only decreased considerably, but is rippled to various extents and periodicity. The notches may grow deeper and wider with more noise-exposure. In the top left example, he narrower, more frequent “pickets” are being found to be typically associated with heavy music exposure in younger people, particularly musicians, who as yet have no audiometric hearing loss.

If, as suggested previously individual Emission Strengths can be plotted against normative values established for chronological ages, we can see in the next figure (Figure 6) what essentially could amount to a crude meaure of susceptibility. Those having a higher CES value and/or, over time, a minimal rate of ageing are the less susceptibility to further damage; a lower CES value and/or, over time, a maximal rate of ageing, are the more susceptible.

Fig. 6. CES results for four individuals place on the results (Mean and Mean ± 1 S.D.) of a normative Australian population.

To further examine the ageing process on an individual basis we are tracking the emission strength of a large number of normal-hearing individuals. From our complete database were selected those individuals for whom a minimum of four repeat records had been obtained over at least four years. For individual ears inthis subgroup of 13 males and 6 females the gradients of the decline of emission strength after the period of repeat recordings was calculated by linear regression and are plotted in Figure 7.

Fig. 7. Tracking 19 normal hearers over 4 to 5 year period.

Figure 7 shows the gradient of CES change in dB per decade and the regression coefficients for each of the 19 individuals tracked. From this can be seen that a rate of ageing per decade may be estimated for each person. If, as we have shown, for example, a decline in Coherent Emission Strength of 8 dB per decade in the population of young people affords no more than 3 decades without hearing loss. The four individuals plotted on Fig. 6 are seen to be highly variable. The two individuals on the lower portion of figure 5 are declining at rates of 30 and 22 dB per decade in the case of the 10-15 year old, and 18 to 25 dB per decade in the case of the 45 to 49 year old. In the cases plotted in the upper portion of Fig. 7, there is no apparent ageing for the 34 to 38 year old, while only 4 and 1 dB per decade is seen in the example of the 52 to 56 year old.

As some individuals appear to be ageing at a much faster rate, it is anticipated that cochlear ageing described in terms of rates of decline of Coherent Emission Strength or some equivalent parameter, may be used as a potent tool in education about potential hearing loss, particularly for young people, e.g. a simple notion is that of the inner ear performance as a standard deposit at birth with a “withdrawal-only” bank account and hearing loss presenting when the account is empty.

It would appear, therefore, that the evoked otoacoustic emission test holds promise as a fast, objective and non-invasive procedure to study the ageing processes in the cochlea and at the same time would be a useful adjunct in hearing loss prevention programs.

REFERENCES

National Center for Health Statistics, 1965. Vital and Health Statistics: data from the National Health Survey. Hearing Levels of Adults by age and sex, United States, 1960-1962.

LePage, E. L., & Murray, N. M. Click-evoked otoacoustic emission strength: its reproducibility and variation with age in the Australia population. 10th National Conference of the Audiological Society of Australia .1992.

LePage, E. L., & Murray, N. M. (1993). Click-evoked otoacoustic emissions: comparing emission strengths with pure tone audiometric thresholds. Aust.J.Audiol. 15 , 9-22.

LePage, E. L., & Murray, N. M. Otoacoustic emission measures of susceptibility to hearing loss. 18th Annual NHCA Conference .1993.

LePage, E. L., Murray, N. M., Tran, K., & Harrap, M. J. (1993). The ear as an acoustical generator: otoacoustic emissions and their diagnostic potential. Acoustics Australia, 21(3), 86-90.

Murray, N. M., & LePage, E. L. (1993). Age dependence of otoacoustic emission strengths and apparent rates of aging of the inner ear in the Australian population. International conference on hearing rehabilitation, 74. Sydney Australia: SSSH Australia.

Murray, N. M., & LePage, E. L. (1993). Age dependence of otoacoustic emissions and apparent rates of ageing of the inner in an Australian population. Aust.J.Audiol., 15(2), 59-70.

LePage, E. L., & Murray, N. M. Click-evoked otoacoustic emission derivation of one measure of susceptibility to sensorineural hearing loss in terms of rates of decline of net motor activity. 17th mid-winter meeting of the Association for Research in Otolaryngology .1994.

LePage, E. L., Murray, N. M., & Tran, K. Comparison of otoacoustic emission measures of cochlear damage in the Australian population with hearing loss in the Australian and British populations. In: Better Hearing Australia conference. Better Hearing Australia.1994.

---

Last Modified: Tuesday, 30 November 2004