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12/6/00 Meeting Highlights
by Bob Zurek
Otoacoustic Emissions (OAEs)
On Wednesday December 6th, Dr. Laurel Christensen of Etymotic Research gave a
presentation entitled �Otoacoustic Emissions (OAEs).� Laurel began her
presentation with discussion of anatomy and physiology of the ear. Dr.
Christensen began by explaining that the ear is divided into three regions,
the outer ear, the middle ear, and the inner ear. She explained that the
outer ear serves as an amplifier in the 2kHz to 5kHz region. She went on to
explain that the middle ear�s purpose is that of an impedance matching device
between the air in the outer ear and the fluid in the inner ear. Dr.
Christensen stated that damage done to the middle ear is usually repairable.
This damage can result from ear infections, head trauma, or the stapes
ossifying into the window. She also mentioned that the middle ear serves
another purpose, which is to attenuate high intensity sound over about 85dB.
She stated that there is controversy as to whether this action is a hearing
protection apparatus in the ear or not. Dr. Christensen then talked about the
inner ear eventually leading to discussion of the Inner and Outer Hair Cells
(IHC and OHC). The inner hair cells send the information to the brain whereas
the outer hair cells amplify low level sounds. She then explained that the
outer hair cells are typically the first to go, accounting for a large
percentage of hearing loss cases. Dr. Christensen then showed the group
audio-visual demonstrations of the workings of the ear using software
developed by the University of Wisconsin�s Auditory Physiology Department.
These demonstrations included pictures of cross sections of normal cochlea
and damaged ones showing the damage done to hair cells at different levels of
hearing loss. She explained that the high frequency OHCs of the cochlea are
the first to experience damage due to their location. She also explained that
human speech extends out to approximately 4kHz and that 90% of the speech
signal is carried by the high frequency consonants. Thus this type of damage
would lead to reduction in speech intelligibility. Dr. Christensen then
presented the differences in losses due to different types of exposure. She
specifically showed the distinction between asymmetric losses often found in musicians
and the symmetric loss found in noise induced hearing loss. She also
explained that while music may not be a steady state noise, long exposure to
loud levels in music results in the same hearing loss. She also mentioned
that impulse noise as seen in percussion may cause more damage than noise
exposure, and that in all cases, the greatest damage is done in the first
years of exposure.
Dr. Christensen then discussed the topic of Otoacoustic Emissions or OAEs.
She described how the OAEs are produced by the contraction of the outer hair
cells in the cochlea in response to sound applied to the ear and can be
measured in the ear canal at levels between 5dB and 15dB. She explained the
two methods used to measure the tones, the transient and distortion methods.
She explained that the transient or �click� method allows for a measurement
in the 500 Hz to 4kHz range, where the two-tone distortion method is useful
in the 1.5kHz to 16kHz region. In the two-tone method, two pure tones are
applied simultaneously to the sealed ear, and the Tartini tone generated by
the hair cells is measured. Dr. Christensen next discussed some of the
advantages to the measurement of OAEs. First, the test provides an objective
clinical test of outer hair cell function, and second it often detects damage
before the results of an audiogram does. Some of the other characteristics of
OAE testing are that it is particularly good for newborns, since no response
is required from the subject being tested, it is frequency specific, and it is
a reliable form of measurement. One of the drawbacks of the method is that it
is also sensitive to middle ear pathology as well as inner ear. In other
words, an ear infection or middle ear problem will show a reduction in the
OAEs. At this point Dr. Christensen discussed a few clinical examples of
outer hair loss and the role OAE measurements played in diagnosis and
treatment.
After the presentation, Dr. Christensen was kind enough to test the OAEs of
attendees who were interested using Etymotic Research�s ERO*SCANtm OAE test
instrument. The Chicago Section would like to thank Dr. Christensen and
Etymotic Research for presenting and hosting the meeting.
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