A
GUIDE TO OTOACOUSTIC EMISSIONS (OAEs) FOR SCHOOL NURSES
STEVEN
D. SMITH, Au.D., F-AAA NEURO-AUDIOLOGIST
Introduction
In 1978,
Dr. David Kemp first showed and described that the cochlea (inner ear
hearing organ) was capable of producing, as well as receiving, sounds.
These sounds produced by the cochlea are now known as "evoked otoacoustic
emissions."
Since 1978, a tremendous amount of energy has gone into investigating
otoacoustic emissions (OAEs). There is numerous research articles published
on OAEs and related clinical topics. In 1995 a variety of FDA-approved
otoacoustic emission devices were available for clinical applications.
Since 1995 the OAEs devices have undergone a tremendous transformation.
We have available a wide variety of screening and diagnostic OAE devices
for every type of clinical application and facility.
What are otoacoustic emissions (OAEs)?
Otoacoustic emissions are sounds produced either spontaneously
or evoked by the cochlea, specifically the outer hair cells, and measured
in the outer ear canal or external ear canal. Due to the outer hair
cells unique property of motility, which either spontaneously or in
response to acoustic stimulation (sound), produces mechanical energy
within the cochlea. This energy is transmitted back through the middle
ear mechanism and the tympanic membrane and converted into an acoustic
signal in the ear canal. These emissions are then measured or detected
in the ear canal by utilizing a very small microphone contained within
a probe assembly.
Simply stated, "Our perception of sounds or hearing relies on a
specific chain of events to occur." First sound is passed through
the ear canal and reaches the eardrum where through the middle ear and
vibratory motion it is transmitted to the cochlea or inner ear. Within
the cochlea or inner ear this vibration is transmitted throughout the
entire hearing organ stimulating thousands of tiny nerve hair cells
(outer and inner). The neural signal from these tiny hair cells is then
send to the hearing nerve (eighth nerve) and forwarded from the lower
to upper auditory areas of the brain where the sound is perceived.
A byproduct of this outer hair cell stimulation is otoacoustic emissions.
OAEs only occur in a normal cochlea (inner ear) with normal hearing
sensitivity. If there is damage to the outer hair cells, which produce
hearing loss, then the OAEs will not be present. Generally it is a good
rule of thumb to remember OAEs will be present if hearing is at least
30 dB or better.
There are essentially three types of otoacoustic emissions. These are:
1. Spontaneous (SOAEs):
These are recorded without any presentation of a stimulus and
are not typically of any clinically use. They occur in about 35 to 50%
of normal hearing ears.
2. Transient (TEOAEs):
These are evoked responses from stimulating the cochlea with
a transient signal such as a click or tone burst acoustic signal. TEOAEs
are a wide frequency response in the 500 to 5,000 Hz range. They typically
do not occur in hearing loss of about 30 dB or greater.
3. Distortion Product (DPOAEs): These
are evoked response OAEs from stimulating the cochlea with two simultaneously
presented pure tones of different frequency. This type of OAE can be
recorded in individuals with greater degree of hearing loss, at higher
frequencies, with more frequency specificity. DPOAEs are obtainable
in the frequency range of 500 to 8000 Hz. They typically do not occur
in hearing losses greater than 30 dB.
How is OAEs Measured?
The test procedure typically takes less than 2 minutes for
both ears. It is noninvasive and does not require sedation for the patient.
The OAEs, whether TEOAEs or DPOAEs are measured by presenting a series
of very brief acoustic stimuli, usually clicks, to the ear through a
probe that is insert within the outer ear canal. Within this probe assembly
there is a loudspeaker that generates the acoustic stimulus and a microphone
that measures the resulting OAEs that are produced within the cochlear
and then transmitted back through the middle ear into the outer ear
canal. The resulting emission is picked up by the microphone, analyzed,
digitized and processed by the specially designed OAE hardware and software.
The recorded OAEs, which are very low-level, are differentiated from
the ambient background noise by the software provided within the equipment.
What does the OAE test
results mean?
The production of OAEs by the cochlea, specifically by the
outer hair cells of the cochlea, is thought to be the by-product of
the nonlinear and active processes of the cochlear mechanisms. The ongoing
clinical significance of OAEs is that they are reliable, consistent,
valid evidence of the vital sensory process arising within the cochlea.
OAEs only occur in a normal cochlea with normal sensory function. If
there is damage to the outer hair cells producing a mild hearing loss,
then OAEs are not present.
Typically if OAEs are present then one can assume the individual's hearing
is at least 30 dB or better. Due to the fact OAEs are evoked by a transient
signals that process a wide frequency response, a broad region of the
cochlea responds, providing information across a wide spectrum of the
cochlea.
The utilization of OAEs has many clinical applications,
but typically within a school setting the following uses are recommended:
1. Hearing screening for all school ages.
2. Hearing screening for developmentally delayed or physically impaired
children.
3. Hearing screening in non-cooperative or difficult to test children.
4. Comparison between OAEs and Pure tone audiometric hearing screening
tests.
OAE testing is recommended by the Joint Committee on Infant Hearing
as one of two methods of evaluating newborn infant hearing. Because
of OAEs quick noninvasive nature, it is promoted as an excellent hearing
screening method of use in any setting. Many of the new OAE devices
are handheld and user friendly. This allows the school nurse or assistant
ease of use in performing the test. A simple PASS or REFER screening
criteria is utilized. The test time is less than 2 minutes and can provide
the school nurse with excellent information regarding the integrity
of the individual's cochlear (inner ear) sensory function. It is not
a test of hearing, but rather a test of cochlear (sensory) function.
This screening method allows for the school nurse to appropriately refer
only those individuals who have failed their hearing screening for more
comprehensive, diagnostic audiometric testing.
Due to the nature of measuring OAEs middle ear function must be considered.
Middle ear fluid or dysfunction can reduce or eliminate the response.
This does not mean the OAE test should not be performed, this only means
that in the presence of middle ear dysfunction (otitis media, Eustachian
tube dysfunction, etc.) the OAE response may be absent or reduced. This
will further provide the school nurse with information regarding the
possible status of the patients hearing in the presence of middle ear
dysfunction. It is well document that middle ear dysfunction (otitis
media or Eustachian tube dysfunction) can have an adverse effect on
an individual's hearing sensitivity.
It is recommended that OAE testing be utilized as a method of screening
individuals for possible hearing loss. It is a quick and reliable method,
which does not require the active participation of the individual. It
will, however, require that the patient and test environment be relatively
quiet.
What does OAE test results mean?
As stated previously, an OAE test is not a test of hearing,
but essentially a test of cochlear function or sensitivity for the frequency
range tested. The following information can be obtained from the OAE
test:
1. PASS - A PASS means that the
OAE response was obtained for the frequency region tested and the response,
based on the criteria set by the manufacturer, was met. This PASS suggests
normal cochlear function or sensitivity for that frequency range.
2. REFER - A REFER means either
there were no OAEs present or the OAEs did not pass the required number
of frequencies. This REFER is suggestive of a possible hearing loss
greater than 30 dB. It should be noted that debris in the ear canal
such as cerumen or drainage can contribute to a REFER finding.
The Pass/Refer criteria I would personally recommended and most research
studies would support, is a 3 frequency out of 3 frequency (2, 3, and
4K) PASS, with the otoacoustic emissions being 5 dB above the noise
floor. This is recommended to ensure that you indeed have a low false
positive rate. I would not recommend the utilization of a 2 out 3 frequency
PASS criteria.
How much does the equipment
cost?
Since 1995 the cost of the OAEs has decreased dramatically.
The cost of OAE equipment varies from manufacturer to manufacturer,
but typically can run from $3,000.00 to $8,000.00 depending on if the
equipment is a handheld screening device or a full diagnostic device.
Most all facilities will find the handheld devices are appropriate and
cost effective for their environment and patient population.
Conclusion
Since 1995, OAE testing has become a vital and important
test procedure. Across a wide variety of health care specialties and
facilities (audiology, otology, pediatrics, speech pathology, educational)
this test procedure has added a tremendous amount of diagnostic information
to our battery of audiological tests. The information derived from OAE
testing provides site-specific information regarding the cochlear or
inner ear integrity and to some degree middle ear status.
The importance of assessing hearing in a timely, noninvasive, cost effective
manner has provided all health professionals with a method of appropriately
assessing hearing in their own clinical setting. We are now able to
screen and assess more individuals due to the availability of this technology,
which in return, has allowed for the identification of more individuals
with hearing loss and for better diagnosis.
If utilized appropriately this equipment will become a vital test modality,
which will provide you with information typically not accessible within
your facility. OAE testing has become a recognized, standardized, reliable,
and valid test option especially for a school based population in the
area of hearing screening and sensitivity.
It is my experience in using OAEs, both as a newborn infant hearing
screening and diagnostic method, in a busy audiology and otology setting,
that OAEs are a tremendous asset. Furthermore, I have used OAE is a
large school based hearing screening program and have found it to be
a fast, reliable, accurate and cost effective method of testing. This
single method of testing has truly revolutionized audiology and our
ability to assess and screen cochlear function. The ability for the
educational (school) community to utilize this testing modality I feel
is important and a very necessary component in the overall ability for
the early identification of hearing loss. Furthermore, it ensures those
individuals that need further diagnostic testing are identified and
subsequently referred to their appropriate family physician or audiologist.
Additionally, I would like to state the decision of which type of OAEs
(Transient or Distortion Product) is very important. The difference
between the two is that Distortion Product OAEs are frequency specific
and will closely approximate pure tone audiometric test results. In
theory OAEs will provide earlier information regarding cochlear function
than behavioral test results. The preferred OAEs modality used in most
clinical facilities is currently Distortion Product Otoacoustic Emissions
(DPOAEs) due to this frequency specificity.
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