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COCHLEAR IMPLANTS
What is a cochlear implant?
The cochlear implant is a prosthetic device, a part of which is surgically implanted
inside the cochlea. Cochlear implants have been found to be beneficial for children
and adults with severe to profound hearing loss and steeply sloping hearing loss who
do not benefit adequately with hearing aids but have an intact auditory nerve. While a
hearing aid provides amplified sound energy to the ear, the cochlear implant directly
provides electrical stimulation to the nerve endings in the cochlea.
How does a cochlear implant work?
A Cochlear implant has an externally worn device which includes the microphone,
speech processor and transmitting coil and an internal device which is surgically
implanted in the skull and cochlea. The internal device consists of the receiver
stimulator package and the electrodes.
Given below is a picture of a Nucleus cochlear implant device.
In the picture above, the labelling is as follows:
a: the microphone
b: the headset
c: the speech processor
d: the transmitting coil
e: receiver stimulator package
f: the electrode array in the cochlea
g: the cochlea
h: auditory nerve
Working of a cochlear implant:
 Sound is picked up by a microphone placed on the ear.
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The microphone converts the sound into electrical energy. This electrical
signal is transmitted through a cable to the speech processor.
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The speech processor may be a body worn device or a ear level device. It
analyses and digitises the sound into coded signals. This coding is done by the
processor depending on how it is programmed. The speech processor decides
how much current is to be sent into the cochlea so that the wearer will hear the
sound. Different types of speech coding strategies are available on speech
processors.
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The coded signal from the speech processor is sent to the transmitting coil
worn on the head. This coil is held in place with a magnet. The transmitting
coil sends the coded signal across the skin via radio frequency link, to the
receiver stimulator package.
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The receiver- stimulator package is surgically fitted in the mastoid bone of the
skull just behind the ear. The receiver stimulator package contains a magnet so
that the transmitting coil and receiver are aligned with each other without any
direct contact through the skin.
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The signal from the receiver stimulator package is sent to the electrode array
which has been surgically put into the scala tympani of the cochlea. (Given
below is a picture of an electrode array. The black bands are the electrodes)
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Stimulation of the electrode array leads to stimulation of the nerve endings in
the cochlea. This results in a sensation of sound.
How does one evaluate the need for an implant?
Before deciding whether a child or adult is a suitable candidate for a cochlear implant,
detailed assessments have to be done. These include:
 Audiological assessment including assessment of benefit from hearing aids
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Speech and language evaluation
Psychological assessment
Evaluation of the expectations of the client and the family
Medical investigations
Radiological investigations (CT scan and MRI)
Neurological evaluation
It is only after all these detailed assessments that candidacy can be determined. If the
child or adult is deriving adequate benefit through hearing aids, a cochlear implant
would not be necessary. It is obvious that for a cochlear implant program to be
successful a team of professionals is required. The team includes audiologists and
speech language pathologists, the ENT surgeon, the paediatrician in case of children,
the neurologist, the special educator, the psychologist and the social worker. Other
professionals may be called to give their inputs if required for a particular patient.
What does the surgery involve?
The surgery for the cochlear implant may take about 2 and a half hours. The surgeon
makes an incision behind the pinna and then surgically implants the electrodes inside
the cochlea and the receiver-stimulator package in the mastoid bone behind the ear.
The patient may have to remain in hospital for a day or two.
What happens after surgery?
After the surgery, one has to wait for the scar to heal. This period is approximately 2
to 3 weeks. During this time, the implantee cannot hear through the implant because
the external part is not coupled to it yet. After this healing period is over, the implant
and processor are programmed or mapped for the first time. This is called the ‘switch
on’. During the switch on, the speech processor of the patient is connected to a
computer which has the mapping software. The processor is worn by the patient and
the transmitting coil is place on the head so that it can communicate with the internal
device. Mapping is done by an audiologist to decide how much current is required for
the person to hear sounds well without causing any discomfort. After the mapping is
complete, the device is switched on. The maps can be stored in locations on the
speech processor and upgraded at each subsequent mapping session.
Initially in the first few months, the person will need frequent sessions of mapping to
improve the signal which is being sent to the implant. It is also important to continue
with training to improve listening skills. The roles of the special educator and the
speech – language pathologist are very important in rehabilitation.
While the cochlear implant helps persons with profound loss to hear soft sounds, the
user still needs to be trained to understand the auditory signal. In case of children, the
focus is on developing listening skills as a means to developing age appropriate
speech and language skills. Cochlear implants do not make the hearing normal. Hence
post-implant rehabilitation is very important for successful outcomes. Outcomes also
depend on many other factors such as the age at implantation, pre-implant hearing and
speech- language status and motivation of the implantee and family.