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A cochlear
implant is an electronic device that can restore useful hearing
and provide improved communication abilities for persons who have
a bilateral (both ears) severe to profound sensorineural hearing
loss. Persons who receive little to no benefit from hearing
aids are considered for cochlear implant candidacy.
The field of
cochlear implants is well-established. The earliest research
on cochlear implantation was conducted over 30 years ago in France.
Since that time, the technology of cochlear implants has evolved
rapidly from a primitive device using a single electrode to stimulate
the ear to devices that transmit sound information via multiple
channels or electrodes. The latest advance in cochlear implant
technology involves entirely behind-the-ear speech processors.
Currently there
are three different cochlear implant systems available at the University
of Michigan Cochlear Implant Program. These are:
All three of
these devices are multi-channel cochlear implant systems.
All provide multiple speech perception strategies and features.
The Food and Drug Administration (FDA) has approved all of them
for clinical use. To learn more about each system, and to explore
current technology updates and trials, click on the implant name
above for a link to that company’s website.
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How
is an implant different from a hearing aid?
Cochlear implants
differ from hearing aids in two important ways:
1.
Hearing aids and other assistive listening devices simply amplify
sounds. A cochlear implant, on the other hand, transforms
speech and other sounds into electrical energy that is used to stimulate
surviving auditory nerve fibers in the inner ear.
2.
Unlike most hearing aids, cochlear implants have both internal (inside
the body) and external (worn outside the body) components.
A surgical procedure is needed to place the internal processor component
of the implant.
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How does the cochlear
implant work?
Understanding
normal auditory function is essential to understanding how the cochlear
implant can restore auditory perception to people with severe to
profound hearing loss.
The ear is
comprised of three parts:
  
The
outer ear
Consists of the pinna (visible outer portion) and ear canal |
The
middle ear
Consists of the eardrum, ossicles (three small bones) and
middle ear space |
The
inner ear
Consists of the snail-shaped cochlea and organs of balance |
Sound waves
in the environment are collected by the pinna and funneled down
the ear canal to the eardrum. These waves strike the eardrum,
causing it to vibrate. There are three small bones attached
to the eardrum, called the ossicles. As the ear drum vibrates,
so do the ossicles. The smallest of these bones is the stapes,
which is attached to the snail-shaped cochlea. The motion
of the stapes causes the fluid inside the cochlea to move.
The movement of this fluid activates thousands of sensory receptors,
called hair cells, which line the cochlea. The hair cells
are arranged tonotopically, or in pitch order, with high pitch
sounds coded in the base of the cochlea and low pitch sounds in
the apex. As the hair cells are stimulated, they convert
the vibrations into electrical pulses, which are sent along nerve
fibers to the brain. The brain interprets these pulses as
sounds.
For people
who are cochlear implant candidates, the outer ear and the middle
ear function normally. However, as the fluid travels in the
cochlea, the hair cells are not stimulated and do not generate electrical
pulses to be sent to the brain. Therefore, the brain does
not perceive the sound. The hair cells may be absent or damaged
although typically there are some residual nerve fibers. The
cochlear implant attempts to utilize these residual fibers by replacing
the function of the hair cells with electrical stimulation.
The cochlear
implant is an electronic device that stimulates residual nerve fibers
in the inner ear. These electrical pulses are sent to the
brain and interpreted as sound. An implant system consists
of an external speech processor and headset and an internal, surgically
implanted electrode array. All three manufacturers currently
have ear level processors available for their devices that are much
more convenient to the user than the traditional body processor.
However, the body processor is useful to illustrate the implant's
function and is described below:
1. Sounds in the
environment are picked up by a microphone (1) on the headset.
2. The long
cable (2) carries the sound signal from the microphone (1) to the
speech processor, a powerful miniaturized computer (3).
3. The speech
processor (3) filters, analyzes and digitizes the sound signal into
coded electrical signals.
4. These coded
signals are carried from the speech processor (3) to the transmitting
coil (4) via the long (2) and short (5) cables.
5. The transmitting
coil (4) sends the signals across the skin to the implanted receiver/stimulator
(6) via an FM radio signal.
 6.
The receiver/stimulator delivers the correct amount of electrical
stimulation to the appropriate electrodes (7) on the array to represent
the sound signal that was detected.
7. The electrodes
(7) along the array stimulate the remaining auditory nerve fibers
in the cochlea, which carry the signal on to the brain, where it
is interpreted.
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Cochlear
Implant Surgery
Cochlear implant
surgery lasts about three hours and is performed while the patient
is under general anesthesia. Some conditions that may affect
the cochlea, such as cochlear ossification (bony growth) or cochlear
abnormalities may lengthen the time of surgery. Prior to surgery,
a small portion of hair is shaved around the ear to be implanted.
The surgeon makes a postauricular (behind the ear) incision.
A small depression is created in the mastoid bone to hold the receiver/stimulator
so that it is flush with the skull. The surgeon drills through
the mastoid bone to the inner ear. The electrode array is
then inserted into the cochlea. The receiver/stimulator is
secured to the skull, and the incision is closed with stitches.
Typically, patients remain in the hospital for one night and return
home the day after surgery.
Stitches are
removed approximately 10 days after surgery. Patients return
to school or work as soon as they feel well enough to do so, usually
within a week of surgery. Activation of the implant takes
place four to six weeks after implantation, allowing enough time
for the incision to heal properly.
Cochlear implantation
has the same surgical risks as other procedures conducted under
general anesthesia and other routine surgeries of the middle or
inner ear. Other risks include:
- During
surgery any remaining hearing in the implanted ear will be permanently
lost.
- Because
the surgery is done in the vicinity of the nerve that moves the
face, there is a remote possibility that temporary or permanent
facial paralysis may occur after surgery.
- There is
a slight risk that the patient may experience taste disturbances,
such as a metallic taste, following implant surgery.
- There is
the risk that the surgical site may become infected, which might
require removal of the device.
- The patient
may experience pain at the wound following surgery; this is typically
temporary.
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