Clinical Applications of OAEs



Managing Musicians: The Use of Otoacoustic Emissions in Monitoring Acoustic Trauma and Counseling






By Shanda Brashears, M.C.D., C.C.C.-A.
Louisiana State University Health Sciences Center
Department of Otorhinolaryngology and Biocommunication and the Kresge Laboratory

sbrash@lsuhsc.edu

Phone Int + , Fax Int +



Music as Acoustic Trauma

     The incidence of noise induced hearing loss (NIHL) among musicians is exceptionally high. Considering the fact that an acute sense of hearing is necessary to the career of a musician, hearing conservation and audiological management must be addressed in this population. The incidence of NIHL among musicians varies across studies depending on the definition of hearing loss used and the specific subgroup studied. In any case, the numbers are astounding. Axelsson & Lindgren (1978) studied eighty-three pop musicians with an average of nine years of exposure and a mean age of thirty years and found the incidence of NIHL to be 30 %. Royster et al. (1991) found hearing impairment in 50% of the musicians in the Chicago Symphony.

     Despite these startling statistics, musicians are not required to participate in hearing conservation programs, few take precautionary measures, and some are not even aware of the dangers. Hearing health care professionals need to take a proactive roll in educating musicians about these dangers and in providing them with necessary services. Otoacoustic emissions (OAEs) are useful both in monitoring acoustic trauma and in providing objective data that's very effective in educating and counseling patients. OAEs are an invaluable tool to be used as part of a comprehensive strategy for managing musicians.

OAEs: A Powerful Monitoring Technique



     OAE testing is a quick and non-invasive method for evaluating outer hair cell (OHC) function. OHCs are important for hearing because they are the cochlear amplifier that enables sounds below 60 dB SPL to induce auditory nerve firing. Without OHCs, a 55 - 60 dB hearing loss will be present (Berlin et al., 1996). OAEs are quite vulnerable to the effects of noise and are more sensitive to these effects than conventional audiometry. It is not uncommon to see a reduction in emission amplitude long before a loss of hearing sensitivity can be observed on the audiogram. This property of OAEs makes them quite useful in monitoring the ototoxic effects of noise. In fact, many people with years of noise exposure show normal behavioral pure tone thresholds but absent or reduced OAE amplitude (Attias et al., 1995). Figures 1 A, B, C demonstrate such a case of a young woman with fifteen years of noise exposure to classical music. She is a french horn player.

Figure 1A: Audiogram of a young woman with fifteen years of noise exposure to classical music


Figure 1B: Left ear TEOAE response


Figure1C: Right Ear TEOAE response


     In contrast, Figures 2 A and B show a similar audiogram with a much more robust transient OAE tracing of a young woman of the same age with no history of noise exposure.


Figure 2A: Audiogram of a young woman with the same age as the subject of Figure 1



Figure2B: Left ear TEOAE response, of the un-exposed young woman



Figure2C: Right ear TEOAE response of the un-exposed young woman

     Similarly, across normal hearing musicians, it has been found that compromised OAE integrity is strongly associated with greater exposure to noise (Mansfield et al., 1999).

     OAEs are also an effective and reliable way to evaluate temporary threshold shift (TTS) (Drake-Lee, 1992). A reduction of OAE amplitude is seen in the same frequency bands in where pure-tone threshold shifts are seen following exposure to noise (2-5 KHz). Emission testing is not only a faster way to measure TTS than pure-tone audiometry, but it is also more sensitive in that a reduction in OAE amplitude is often seen in cases of sub-clinical threshold shifts. For example, when exposure levels are moderate and therefore not intense enough to elicit a shift in pure-tone thresholds, noise affects are often apparent in OAE tracings.

     Both distortion product (DPOAEs) and transient evoked emissions (TEOAEs) can be used for the purposes of monitoring and evaluating TTS. DPOAEs have the advantage of evaluating OHC function across a higher frequency range, whereas TEOAEs have the ability to tolerate higher ambient noise levels, which is useful if testing is to be done on site.

Educating and Counseling the Musician



     In addition to its powerful roll in monitoring the effects of noise over time, OAEs can also serve very well in counseling musicians. The goal of counseling and educating is to create musicians that will take an active roll in maintaining their audiological health by using appropriate protection and having regular hearing evaluations. OAEs provide an effective way to objectively and visually demonstrate the effect of noise on hearing. To provide a basic understanding of OHCs and OAE's, it can be simply stated that emissions are recorded by a tiny microphone in the ear, that their presence represents healthy cochlear function, and that they are sensitive to the effects of noise. The most effective demonstration is to personalize the effect of noise by testing a musician's emissions before and after a loud concert thus demonstrating to them their own temporary loss of outer hair cell function. In most cases this is not possible, so for counseling purposes, having one such case on hand to demonstrate the measurable effects of noise on the cochlea is sufficient. Figure 3 A is a left ear DP-gram of a young woman with normal hearing. In Figure 3 B we see her emissions the morning after a concert that was reported to be "very loud". Note that the reduction in DP amplitude is most different between 3 and 4 KHz where noise has its biggest effect.



Figure 3A: Left ear DP-gram of a young woman with normal hearing


Figure 3B: Same ear as in 3A, the morning after a concert that was reported to be "very loud"

     In cases in which hearing and middle ear status are normal, but OAE amplitude is significantly reduced (See Fig 1), it can be useful to demonstrate for the musician the characteristics of a healthy OAE response (See Fig 2). With the understand that this apparent difference is likely to be resulting from years of acoustic trauma, seeing an objective measure of this may prompt the musician to take measures in preventing any further effects. This technique can be viewed as a scare tactic, however, and should be used with caution. It is important to qualify this demonstration by explaining that emission tests are not tests of hearing and that reduced emission amplitude is in no way indicative of, or a precursor to hearing impairment. Compromised emission status in the presence of otherwise normal hearing is simply a message from the outer hair cells that they need protection from noise.



Prevention



     Regardless of the means, the goal is to convince a musician that protecting his hearing is medically and professionally necessary thus enabling a long and audiologically healthy career in music. However, doing so will be futile if that musician is not equipped with ear protectors and other devices that make protection esthetically tolerable. One of the big hurdles in convincing a musician to wear protection is that past experiences wearing earplugs have not allowed them to hear the music in a natural manner. This is because foam earplugs cut as much as 50 dB in the high frequencies and 35 dB in the low frequencies. This is more attenuation than necessary to bring concert level music into a safe listening range, and the difference between the high and low frequencies makes the music sound muffled. Additionally, this type of earplug creates an uncomfortable side effect of voice distortion called the occlusion effect, which is the sensation of feeling like ones own voice is much louder than outside sounds.

     Musicians' earplugs, on the other hand, have filters called ER 9, ER 15 and ER 25 that attenuate 9, 15, or 25 dB respectively. The filters can be changed depending on musical setting and each musician's preference, and are designed to attenuate all frequencies evenly preserving the integrity of the music. Additionally, these earplugs are custom molded to each ear by an audiologist insuring that the earplugs fit deeply into the bony portion of the ear canal. This cuts down dramatically on the undesirable occlusion effect. The appropriate filter is chosen based on the musician's average daily dose, which is a combination of the intensity of the signal and time of exposure. These Occupational Safety and Health Administration (OSHA) guidelines can be used for calculating what level of attenuation is needed to bring their exposure levels into a safe listening range.

      Table I:(OSHA, 1992) AVERAGE SAFE HOURS PER DAY

Noise Level (dB A)

No Protection

15 dB Protection

90

8 hours

No limit

95

4 hours

No limit

100

2 hours

No limit

105

1 hour

8

110

hour

4

115

hour

2

120

0

1

125

0

0



      Ideally, sound level measurements should be taken of the musician's typical playing environment. This information should be taken into account in conjunction with a client history revealing average daily and weekly time of exposure. In cases where this is not possible, the clinician can consult a chart to estimate the intensity level of the musician's instrument and musical situation.

Table II(Peck, 1997):Sound Levels of Various Instruments, Orchestra, and Rock Concert

Instrument

Noise Level (dB A)

Violin

84-103

Cello

84-92

Bass

75-83

Piccolo

95-112

Flute

85-111

Clarinet

92-103

French Horn

90-106

Oboe

80-94

Trombone

85-114

Club

110

Orchestra

87-98

Headphones

110

Car Stereo

120

Band

120

Rock Concert

130



     Different filters can be used by the same musician in various venues according to the loudness level of that venue. Because of their many advantages over conventional earplugs these musician plugs gaining in popularity among musicians and avid concert goers alike.

     For most musicians, it is not realistic to expect them to use ear protectors 100% of the time that they are exposed to music. For this reason it is important that they understand two very basic principles of outer hair cell mechanics: the cumulative affects of noise over time and hair cell recovery. The cumulative effects can be demonstrated by explaining the OSHA guideline chart (See Table I). Recovery period refers to the fact that when hair cells have been temporarily damaged by moderately loud noise as in the case of TTS, they will take 24-48 hours to fully recover their function. This recovery time is evident in the slow recovery of normal emissions following exposure to noise. If again bombarded by such levels of noise within that very sensitive recovery time, the hair cells are at risk of permanent damage or total destruction. This knowledge enables the musician to realize that the risk of permanent damage can be reduced by "giving the ears a break" during long musical sessions and nightly concert schedules.

     Another useful tool in hearing conservation for musicians which is gaining rapidly in popularity are "in the ear monitors". These are small, high fidelity speakers that are seated inside an ear piece which can be universal or custom fit to the ear canal and is coupled to an FM receiver accepting signals from the mixing board. This is an excellent solution for amplified bands and is preferable to many musicians who would rather have a state of the art monitoring system than wear earplugs. Many musicians have reported these monitors to be a drastic improvement over the conventional floor monitors that are typically used because they prevent feedback and create a "sweet spot" everywhere, regardless of position on stage. The issue to be aware of for any audiologist distributing such equipment is that dB sound pressure at the eardrum can exceed safe listening levels. Real ear measurements must be taken and the musician must be counseled as to what volume and gain settings are not to be exceeded.

     In summary, OAEs are of great value in evaluating cochlear function, making them a useful tool for monitoring the ototoxic effects of noise. They should be an integral part of managing, educating, and counseling musicians who are at high risk for NIHL. OAEs can also be used to verify the effectiveness of both musicians' earplugs and in the ear monitors. S She can be reached by e-mail at sbrash@lsuhsc.edu.

References


Attias, J., Furst, M., Furman, V., Reshef, I., Horowitz, G. Noise-induced otoacoustic emission loss with or without hearing loss. Ear & Hear. 1995; 16(6): 612-618.


Axelsson, A., & Lindgren, F. Hearing in pop musicians. Acta Otolaryngology. 1978; 85: 225-231.


Berlin, C.I., Hood, L.J., Hurley, A., & Wen, H. Hearing aids: only for hearing aid patients with abnormal emissions. In: Hair cells and hearing aids. C.I. Berlin (ed.) Singular Publishing Group. San Diego, 1996; 99-111.

Drake-Lee, A.B. Beyond music: auditory temporary threshold shift in rock musicians after a heavy metal concert. J R Soc Med. 1992 85(10): 617-619.


Mansfield, J.D., Baghurst, P.A., & Newton, V.E. Otoacoustic emissions in 28 young adults exposed to amplified music. Brit. J. Audiol. 1999; 33(4): 211-222.


Mansfield, J.D., Baghurst, P.A., & Newton, V.E. Otoacoustic emissions in 28 young adults exposed to amplified music. Brit. J. Audiol. 1999; 33(4): 211-222.


Occupational Safety and Health Administration. Occupational noise exposure 1926.52 standars. [on line]; 1992.


Peck, K. . Hearnet. [on line]; 1997.


Royster-Doswell, J., Royster, L.H., & Killion, M.C. Sound exposure and hearing thresholds of symphony orchestra musicians. J. Acoust. Soc. Am. 1991; 89(6): 2793-2803.