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The risk of
noise-induced hearing loss from amplified music was
predicted by Carter et al in 1982, who noted that, in a study of 994
subjects aged 16 to 20 years, ". . . the accumulated exposure of some of
them 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
20s. Further empirical studies are necessary to determine whether
these hearing losses will eventuate".1 In this issue of the Journal
(page 588), LePage and Murray use the relatively new
technique of otoacoustic emission (OAE) analysis to investigate
noise-induced hearing loss resulting from the use of personal stereo
headsets.2 Their results suggest that
personal stereo use results in a decline in cochlear function
analogous to rapid ageing of the cochlea, and comparable to hearing
loss from industrial noise trauma. They also emphasise that, as OAE
analysis can detect decline in cochlear function long before there is
any clinically detectable hearing loss, this technique can
potentially provide early warning of noise-induced hearing loss.
Otoacoustic emissions were first described by Kemp3 in 1978. They are sounds thought to be generated by the cochlear outer hair cells in response to an external sound stimulus. Normal hearing threshold is achieved by a cochlear mechanism, thought to reside in the healthy outer hair cell, which magnifies the stimulus internally. When this mechanism loses the peak of its performance, OAEs diminish and hearing threshold is raised. As OAEs can be recorded in the outer ear, they may provide an objective, non-invasive and quantitative measure of hair-cell function.4,5 High test-retest reliability has been demonstrated for individuals, although there is variability between subjects.6 Four types of OAEs have been described.7,8 Spontaneous OAEs occur in 68% of infants younger than 18 months, but the incidence falls to 35% in adults under 50, and to 20% of adults over 50 years.9 Transient-evoked OAEs (TEOAEs), a response to acoustic clicks delivered to the outer ear, are currently thought to be the most clinically useful OAEs, as they are detectable in 98% of people with normal hearing, regardless of age or sex, and the two ears of any individual produce similar TEOAEs. Stimulus-frequency OAEs occur in 88%-100% of people with normal hearing, and resemble TEOAEs in behaviour. They represent fixed-place emissions corresponding to specific frequency sites along the organ of Corti, but their usefulness as a clinical test is limited by technical factors. Distortion-product OAEs (DPOAEs) also occur in 100% of people with normal hearing, and, while small in amplitude, can be used to intentionally test a specific frequency region of the cochlea. DPOAEs are technically difficult to measure, but will become an essential tool in the investigation of tonotopic outer hair cell function. Probst et al have provided an extensive review of the technical details, experimental and clinical findings of otoacoustic emission analysis.10 Measurement of OAEs has become a useful audiologic and otoneurologic diagnostic test for neonatal screening,4,9 otosclerosis,11 sensorineural hearing loss,4,5,12 Meniere's disease,5 acoustic neuroma,5 tinnitus,13 ototoxicity,7 and noise-induced hearing loss.5,12,14 A limiting factor in this kind of ear testing is that eustachian tube dysfunction will reduce otoacoustic emission energy.4 Thus, tympanometry is essential if no otoacoustic emission can be measured. The reduction in outer hair cell activity in patients with noise-induced hearing loss, measured by DPOAEs, is directly related to frequencies of the audiometric loss.5 In one study, emissions were abnormal in 93.2% of ears with noise-induced hearing loss and in teenagers exposed to noise, and were found to be useful in the prediction of noise susceptibility.12 In another, ears with a noise-induced impairment showed a significant reduction in the incidence of both spontaneous emissions and spectral peaks in evoked emissions that was not evident in ears with similar patterns of hearing loss caused by other factors.14 Are the listening habits of the younger generation potentially dangerous to hearing? Ising et al studied 681 students aged 10 to 19 years.15 Although 50% of students listened to music for less than one hour per day, 10% listened for four or more hours. Among those aged 12 to 16 years, 10% chose to set the listening level at 110 dB(A). It was estimated that 7% were exposed to noise levels likely to damage the cochlea. They recommend that the sound levels for portable music players be limited to 90 dB(A).15 Hearing loss has been documented in people who attended rock music concerts,16 in employees of urban music clubs,17 and one report indicates that exercise combined with exposure to music presents a greater risk to hearing than the music alone.18 These authors conclude that "the results have implications related to contemporary lifestyle issues such as aerobics and the utilisation of personal music systems during physical exertion". The risk of recreational noise-induced hearing loss is real, and our patients must be advised of this risk. LePage and Murray have demonstrated that early warning is now available in the form of the transient-evoked otoacoustic emission test.
John T Redhead
©MJA 1998 | |||
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