Detecting Hearing Loss in Children (cont.)
How can hearing be assessed in a child who is unable to cooperate?
Some children are unable to cooperate for hearing evaluations, either due to
their age or to a developmental delay (for example, a child with severe mental
retardation). Currently, there are two different types
of tests that are utilized for children who are unable to cooperate. The first
is the frequency-specific auditory brainstem response (ABR) evaluation. An ABR
is a physiological measure of the brainstem's response to sound. It tests the
integrity of the hearing system from the ear to the brainstem. The test is
performed by placing four to five electrodes on the child's head, after which
a variety of sounds are presented to the child through small earphones. As the
hearing nerve fires, the sound stimulus travels up to the brain. The electrical
activity generated by the nerve can be recorded by the electrodes and presented
as waveforms on a computer screen. The audiologist can then present different
loudness levels of each sound and determine the softest levels at which the
child can hear. The child can be tested using all the sounds of a conventional
hearing evaluation (adult hearing evaluation).
The limitation of the ABR is the need for the child to be quiet and still.
The electrical potential the computer is recording from the auditory nerve is
very small. Any muscle movement, including something as small as an eye blink,
can obliterate the hearing response; therefore, the infant or child must be
sleeping during the test. Infants less than 3 months old can be tested
during natural sleep. Children older than 3 months are typically sedated for
about one hour (under the supervision of a physician) during the test. The most
common oral sedative used is chloral hydrate.
The second type of test to objectively evaluate children is the otoacoustic emission (OAE) test. This test can be done as a supplement to
the ABR or as an initial screen of hearing. An otoacoustic emission test
measures an acoustic response produced by the inner ear (cochlea). The acoustic
response measured is in essence the response produced by the inner ear as it
bounces back out of the ear in response to a sound stimulus. The test is
performed by placing a small probe that contains a microphone and speaker into
the child's ear.
As the child sits or rests quietly, sounds are generated in the probe and
responses that come back from the cochlea are recorded. Once the cochlea
processes the sound, an electrical stimulus is sent to the brainstem, but in
addition, there is a second and separate sound that does not travel up the
nerve but comes back out into the child's ear canal. This
"byproduct" is the otoacoustic emission. The emission is then recorded
with the microphone probe and represented pictorially on a computer screen. The
audiologist can tell which sounds yielded a response/emission and the strength
of those responses. If there is an emission present for those sounds critical to
speech comprehension, then the child has "passed" the hearing screen.
As a supplement to the ABR, the OAE serves as a crosscheck to either confirm
normal hearing or verify the site-of-lesion for the hearing loss as the inner
ear. OAE tests the integrity of the hearing organ for sound (the cochlea),
but it does not evaluate the hearing beyond the cochlea. That is why the OAE is
often paired with the ABR or with a behavioral test that can evaluate a child's
responsiveness to sound.
The results from an ABR and an OAE evaluation can predict the child's
hearing, determine if there is a loss, determine the type of hearing loss, and
help with decisions regarding intervention. Intervention can include medical
treatment, surgery, or hearing aids and therapy.
Next: Are any additional tests done during a pediatric hearing evaluation? »
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