Download How the Ear Works

How the Ear Works
The ear consists of the following three parts: the outer ear, the middle ear,
and the inner ear. The outer ear consists of the “ear” as we see it as well as
the ear canal. A person hears speech and other sounds as a result of sound
waves entering the ear through the ear canal.
The middle ear consists of the eardrum as well as three bones that are
attached to the eardrum. These bones are called the malleus, incus, and stapes
and are referred to as the ossicles. When sound travels through the ear canal
it vibrates the eardrum. Because the ossicles are attached to the eardrum,
when the eardrum vibrates they vibrate as well. The ossicles vibrate against a
membrane, called the oval window that leads to the inner ear.
The inner ear consists of two parts – the cochlea, which is the organ of hearing,
and the semicircular canals, which are the organs of balance. The cochlea looks
like a snail and is filled with fluid. When the ossicles vibrate against the oval
window the vibration is transferred to the fluid. The cochlea also contains
thousands of hair cells that sway back and forth as the fluid moves. The
movement of the hair cells stimulates the nerve fibers of the auditory nerve
and sends a signal to the brain, which allows us to hear the sound.
Hearing loss can originate in any of the three main parts of the ear. Depending
on which part of the ear is involved, the characteristics of the hearing loss as
well as the way we treat that hearing loss is different.
Types of Hearing Loss
Sensorineural hearing loss
Sensorineural hearing loss is caused by damage to the cochlea and/or auditory
nerve. The hair cells in the cochlea, which are necessary to stimulate the
auditory nerve, may be damaged or absent in a sensorineural hearing
impairment. The hair cells are “tuned” to respond to specific pitches, so
depending on which hair cells are damaged, a child may be able to hear certain
pitches better than others. When sensorineural hearing loss occurs sounds
need to be louder, especially in the pitch range where the hair cells are
damaged, for a child to hear them. Because of the nature of inner ear/auditory
nerve damage, people with sensorineural hearing loss often have difficulty with
clarity of sound, not just loudness. Sensorineural hearing loss can not be
medically repaired, so people with this type of hearing loss are good candidates
for hearing aids or other hearing devices.
Conductive hearing loss
Conductive hearing loss is caused by a problem in the outer or middle ear which
inhibits the transmission of sound to the inner ear. The hair cells and nerve
fibers in the inner ear, in this case, are not damaged. A conductive hearing loss
effectively acts as an earplug by diminishing the loudness of the sounds, while
having little effect on clarity since the inner ear is not damaged. Middle ear
infections, fluid behind the eardrums, ear wax, and certain conditions involving
the ossicles can cause conductive hearing loss. Conductive hearing loss is
usually medically treatable; however, in some cases of conductive hearing loss,
hearing aids may be recommended.
Mixed hearing loss
Mixed hearing loss involves both the outer/middle ear and the inner ear.
Middle ear infections are common in young children; if a child has a
sensorineural hearing loss and then develops a middle ear infection which causes
a conductive component to the loss, the loss is called a mixed hearing loss. A
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conductive impairment combined with a sensorineural impairment can further
diminish a child’s hearing, leaving him or her with decreased access to sounds.
Regular visits to the audiologist, pediatrician, or ENT (ear, nose and throat)
doctor are advisable to monitor your child’s hearing health.
How we Assess Hearing in Children
It is important to understand the necessity of a battery of tests for
determining your child’s hearing status. The results of any one test alone are
not as strong as the culmination of results of two or more tests. Appropriate
measurement of the type and degree of your child’s hearing loss is paramount
for providing appropriate intervention and amplification if indicated.
Behavioral hearing tests
When a child is willing and able, the preferred method of hearing testing is
using behavioral testing. This requires the child to respond to sounds at
different loudness levels and different pitches to determine the minimum levels
to which he/she can attend. This testing is performed in a sound-treated
booth. Your child may wear headphones, earphones, or listen to sounds through
speakers placed in the testing booth. The audiologist looks for consistent
responses to the sound, such as head turns from younger children or more
complex play responses as the child gets older, such as dropping a block in a
bucket when he/she hears a sound. By using different procedures and
equipment, behavioral testing can provide information about the type
(sensorineural, conductive, or mixed) and degree of the hearing loss for each
ear. Results are recorded on a graph called an audiogram.
Objective hearing tests
For children younger than about 6 months of age, or for those who are unable
to respond behaviorally for audiological evaluation, objective tests that do not
require behavioral responses are used. A common test used is called
otoacoustic emissions. This tests the integrity of the hair cells in the cochlea
by measuring tiny echoes that the inner ear sends out in response to sounds
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presented to the ears. This test requires the child to be quiet but not
necessarily asleep and generally takes less than 5 minutes per ear. Generally, if
otoacoustic emissions are present they indicate that the cochlea is functioning
properly. There are rare occasions, however, when the cochlea is functioning
properly but a child still does not respond to sound behaviorally.
Another objective test often used to estimate hearing sensitivity is called an
auditory brainstem response (ABR) test. This test requires that your child be
asleep and takes 30-60 minutes to complete. Electrodes are placed on your
child’s skin, which pick up the electrical activity generated from his/her
auditory nerves and part of the brainstem that is involved in hearing. The
audiologist examines the recorded waveforms and interprets them to determine
an estimate of the degree and type of hearing loss that is present.
Tympanometry
Tympanometry is a test that evaluates middle ear and eardrum function.
Because middle ear problems are common in young children, tympanometry
should be performed routinely on your child. A rubber tip, connected to a
machine called a tympanometer is held up to your child’s ear canal. The machine
measures the eardrum movement in response to different tones and air
pressure changes and records the results on a graph called a tympanogram.
This provides information about your child’s middle ear and eardrum.
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