ELECTRONIC
IMPLANTABLE HEARING DEVICES
COCHLEAR IMPLANTS
Introduction:
Over the past 30
years, otology has irrevocably crossed the threshold of the inner ear.
The electronic cochlear prosthesis is the chief example of this marriage of technology and
medicine in the inner ear.
The principle objective in the development of cochlear implants is to restore useful hearing
to profoundly deaf subjects who have requisite surviving neuronal populations, partially
replacing the functions of the cochlea.
To accomplish this, basic discoveries have been and continue to be necessary.
Unlocking these discoveries and translating them into cochlear implant devices has
required close interaction of numerous technical disciplines , including surgery,
diagnosis, audiology, electrophysiology, histopathology, psychoacoustics,
speech and language rehabilitation, engineering (electronic, mechanical, biomaterials),
and manufacturing technologies.
Only in the past
37 years or so have physicians been able to adequately treat profound
sensorineural hearing loss.
Interest in electrical
stimulation of the auditory system dates back to the eighteenth century,
Volta, in1800, tried it on
himself.
Djourno and
Eyries (1953), in France, provided the first detailed account of the effects of
direct stimulation of
the auditory nerve in deafness. In a patient undergoing surgery for
facial nerve paralysis prior to cholesteatoma
surgery they placed a monopole on the
eighth nerve. The patient
described hearing high frequency sounds. By using a 1000 Hz
signal generator, the
patient could recognize common words and improved their speech
reading capabilities.
This provided the early beginnings for cochlear implant development.
The sixties saw a number of developments which had a significant impact on the development
The operating microscope, for example, opened up vast new possibilities.
In 1961, House and Doyle and
others separately described approaching the auditory nerve
via the scala tympani. Simmons, three years later, placed an electrode directly into the
modiolar segment of the
auditory nerve through the promontory and vestibule and
demonstrated that some
tonality could be achieved. House and Michelson refined the clinical
applications of
electrical stimulation of the auditory nerve via
the scala tympani implantation
of electrodes.
In 1972, with the help of
Jack Urban (an innovative engineer) the first commercially available
device was developed.
It consisted of a wearable speech processor that interfaced with the
House 3M
single-electrode implant.
In 1984, multiple channel
devices were introduced and became the approach of choice based
on enhanced spectral
perception and open-set speech understanding.
In the 1990s, improvements
in both the technical and clinical approaches have developed
from further basic
science and clinical investigation. Now there is a trend for earlier
implantation of
cochlear implants based on the findings from research. As of early 1997
almost 20,000 people
had cochlear implants placed.
Basic Science – Technology
Cochlear
implants consist of implantable circuitry and information processing systems.
Because much of the central
auditory pathway remains vital in deafness, and processing
capabilities are retained,
cochlear implants are capable of restoring physiologically meaningful
activity in that
pathway.
Rather than
introducing a processed acoustic signal, implants receive,
process, and transmit
acoustic information via
electrical stimulation.
Electrode
contacts implanted within the cochlea serve to bypass nonfunctional
cochlear
transducers and directly depolarize auditory nerve fibers.
Implant systems employ an
electrical code that
is based in those features of speech that are critical to word understanding
in normal listeners.
The cochlear implant is
fundamentally an electrical device. Most systems consist of internal
and external
components.
The internal component acts as the control tower that directs the
signal from the external
component to the central nervous system. This
receiver-stimulator accepts, decodes, and
transmits signals.
The signals are transmitted by way of a connecting lead down to an array of
electrodes
implanted within the scala tympani.
The target of
the electrical stimulus is the cell bodies of the auditory nerve.
The external component is a speech processor that converts
microphonic input into a
distinct code of electric stimuli for each electrode. The processed signal is
amplified and
compressed to match the narrow electrical
dynamic range of the ear. An external antenna
enables radiofrequency
transmission transcutaneously to the internal system. Batteries
housed within the processor drive the system.
The cochlear implant is a transducer, which changes acoustic signals into
electrical signals
used to stimulate the
auditory nerve. The electric signals are always processed to amplify
the signal level, compress
the signal to limit stimulation levels, filter the signal to shape or
divide the acoustic
frequency spectrum to match neural requirements, and encode the
information in the
signal for transmission to the implanted receiver. Each of
these processes will be
identified separately for clarity of discussion:
1.
Amplification of the signal occurs within the processor.
The signal from
the microphone is usually several millivolts.
This is generally too
small to be directly used in the electrical
circuits.
Amplifiers are used to
increase signal levels.
2.
Compression
is the second basic processing step
performed by the
cochlear implant electronics. The normal ear
responds over a
pressure range of nearly 120dB SPL. In general,
as hearing
becomes more impaired, the dynamic range from
threshold to
maximum loudness decreases. For the totally deaf
subject
with a cochlear implant, the
acoustical
dynamic range is
generally
limited to 10 to 25dB and it is not unusual to see
acoustical dynamic
ranges of 5dB at frequencies above
3000Hz.
3.
The
third basic signal processing operation is filtering the
input signal on the
basis of frequency. The acoustic frequency
spectrum of interest
ranges from 100Hz to possibly over
4000hz. Three basic
types of filters are used to alter the
frequency
characteristics of the incoming signals: low pass, high
pass, and band pass
filters. The low pass filter passes a
frequency below a
specified cutoff frequency. For example, a
100Hz low pass filter
passes frequencies below 100Hz and
stops frequencies
above 100hz. A 100Hz high pass filter would
have the opposite action. A bandpass
filter passes frequencies
within a band
specified by two cutoff frequencies. There are two
principle reasons for
filtering.
The frequencies that provide
no useful information can be
removed and the
frequency spectrum can be divided into
separate bands so that these
bands of information can be
processed
independently.
The signals that have been
processed so far must be encoded in some manner for transmission
to the implant receiver.
Encoding
preserves the information that has been processed to this point and provides a
means of getting this
information to the electrodes for stimulation of the auditory nerve.
The objective of the
above steps is to condition the input signal so that the maximal amount
of information can be
transferred to the acoustic nerve.
Charge is simply
the flow of current for a period of time.
Cochlear
implants typically transfer less than one microcoulomb of charge during each
phase
of stimulation current.
Charge density
is defined as microcoulombs per cm2 of electrode surface area. Charge density
is thought to be related to
the safety of the electrode. Initiating an action potential in a neuron
requires that the
normally charged cell membrane be depolarized. Stimulation current
supplied by two
electrodes establishes an electric field between the electrodes, which induces
a current flow in
nearby nerve fibers and initiates an action potential.
There are two
general configurations of electrodes; monopolar and bipolar. Monopolar
electrode designs place one
or more active electrodes adjacent to the neural target and a ground
electrode external to
the cochlea.
Conversely, bipolar
electrode designs place both active and ground electrodes within the
cochlea. They provide a more
restrictive field of current and the potential for more discrete
patterns of neural
stimulation with less channel interaction.
Multichannel Cochlear Implants vs. Single-channel Cochlear Implants
Controversy
still exists in the selection of single-channel or multichannel devices for
patients
needing cochlear implants.
A channel is a pathway through which information is transmitted
from the implant to the auditory nerve (Cohen et
al., 1993).
The design of
electrodes for the cochlear prosthesis in single-channel and multichannel
devices
is what distinguishes the two.
The issues associated with
electrode design consist of the number of electrodes, electrode
placement, and electrode
configuration. These two devices vary in many different aspects,
and each has its own advantages and disadvantages.
In
multichannel devices, the signals are transmitted through several
independent channels.
An electrode array
(consisting of multiple electrodes) is inserted in the cochlea so that
different auditory nerve fibers
can be stimulated at different places in the cochlea. Electrodes
near the base of the
cochlea are stimulated with high frequencies, while electrodes near the
apex are stimulated
with low frequencies.
In principle, the larger the
number of electrodes, the finer the place resolution for coding
frequencies. However,
frequency coding is constrained to the number of remaining auditory
neurons that can be stimulated at a particular site in the
cochlea.
Ideally, it would be best to
have the surviving auditory neurons lying along the length of the
cochlea, so that the use of multiple
electrodes could stimulate the neurons at different sites.
On the other hand, if the
surviving auditory neurons are restricted to a small area, a few
electrodes implanted near
the area would be as good as eighty electrodes distributed all along
the cochlea.
So, using a large number of
electrodes will not necessarily create better results, because the
number of surviving auditory
neurons that can be stimulated by the electrodes limits the
frequency coding.
Advantages
include potential for lower current density, wider dynamic range, and more
convenient tonotopic stimulation.
Although
single-channel cochlear implants preceded multichannel devices, they are
still
used today.
In a single-channel cochlear
implant, the signals are transmitted through one channel or only
one electrode is used.
These types of devices are
of interest because of their simple design and low cost compared
to the multi-channel implants.
They are also appealing
because they do not require much hardware, allowing almost all the
electronics to be stored within a behind-the-ear
device.
During the single-channel
operation, a short, single-channel electrode that does not extend
beyond the first bend in the cochlea is used.
The disadvantages to using
single-channel devices are that they produce a narrower dynamic
range, a higher current
density, and a greater potential for stimulating other neural tissue,
possibly resulting in
facial nerve stimulation.
This small dynamic range may
be due to an abnormal response of the nerve activity in electric
stimulation (Zeng and Galvin, 1999).
A study was
conducted on six postlingually deaf adults with at least 6 months of experience
with a single-channel cochlear implant. After replacing their single-channel
implant with a
multichannel device in the same ear, all six patients had
substantial improvement in speech
recognition (Rubinstein et al., 1998).
There are
different speech processing strategies for the different types of cochlear
implants.
The
Nucleus 22-channel implant utilizes the Spectral Peak (SPEAK) strategy
implemented
in the Spectra 22 processor.
This uses a
vocoder in which a filterband consisting of 20 filters cover the center
frequencies
from 200 to 10,000Hz. Each
filter is allocated to an active electrode in the array. The filter
outputs are scanned and the
electrodes that are stimulated represent filters that contain
speech components with
the highest amplitude.
The
Clarion multichannel cochlear implant offers two types of speech
processing
strategies: compressed
analog and continuous interleaved sampling. The compressed analog
strategy first compresses the analog
signal into the restricted range for electrically evoked
hearing and then
filters the signal into a maximum of eight channels for presentation to the
corresponding
electrodes.
The continuous interleaved
sampling strategy filters the incoming speech into eight bands and
then obtains the speech
envelope and compresses the signal for each channel. More than 90%
of Clarion multichannel implant
recipients use the continuous interleaved sampling strategy
for speech processing.
The
MED-EL Combi 40-Cochlear Implant system utilizes the continuous
interleaved
sampling strategy for its speech processor.
Patient Selection
Original FDA
guidelines for cochlear implantation contained narrow specifications of
audiologic, medical, radiologic, psychologic, and cognitive criteria for implant
candidacy.
Indications have
now been broadened.
Cochlear
implants were originally limited to postlingually deafened adults who received
no
benefit from hearing aids and had no
possibility of worsening hearing.
This population has been the
most readily identifiable beneficiary of cochlear implants.
No upper age limit is used
in the selection process. Cochlear implantation is appropriate if
other selection criteria are met and
the patient’s general health status will allow a general
anesthetic.
The entry criteria have been
expanded to include some patients with residual hearing.
Adult selection criteria include :
postlingual,
profound bilateral sensorineural hearing loss in excess of 95dB pure tone
average
(90dB for the Nucleus
implant), little or no benefit from conventional hearing aids, and
psychological, and
motivational suitability.
In the best-aided condition,
the candidate should not have word discrimination scores
better than 30% or speech detection
threshold of 70dB sound pressure level.
FDA guidelines hold that
candidates should generally have at least 6 months experience
with high-powered binaural amplification and should undergo aided speech audiometry.
Each
implant has its own FDA approval
criteria typically specified in the product labeling or in
the training manuals.
The audiologic evaluation is
the primary means of determining suitability for cochlear
implantation.
Thresholds for both aided
and unaided using conventional amplification are determined
for the candidate.
Hearing aid performance is
compared with normative cochlear implant performance.
Not all patients with profound sensorineural
hearing loss are implant candidates. Many with
pure tone thresholds between 90dB
and 100dB HL with residual hearing through 2000 Hz
demonstrate speech
recognition skills with conventional hearing aids that are
superior to
multichannel implant users.
In the
physical examination it is important to identify preoperatively any external
or
middle ear disease that must be treated prior to
cochlear implantation.
The ear proposed for
cochlear implantation must be free of infection and the tympanic
membrane must be intact.
If these conditions are not
met then medical or surgical treatment to correct them should
be employed prior to cochlear implant placement.
Radiologic
evaluation of the cochlea is performed to determine whether the cochlea is
present and patent and to identify congenital deformities of the cochlea. This
is accomplished
through high resolution computed tomography of the
temporal bone. Many kinds of
congenital deafness
are caused by osseous deformities of the inner ear, which can be
visualized by CT.
Abnormal anatomy of the cochlea may result in surgical problems or
complications.
Congenital
malformations of the cochlea are not contraindications to cochlear implantation.
Two exceptions
are the Michel deformity, in
which there is a congenital agenesis of the cochlea, and the
small internal auditory
canal syndrome, in which the cochlear nerve may be congenitally
absent.
Psychological
testing is performed for exclusionary reasons to identify patients who have
organic brain dysfunction, mental
retardation, undetected psychosis, or unrealistic
expectations.
A cochlear implant will
provide the most benefit to individuals who possess sufficient
motivation and support to complete a program of postimplantation
device activation and
rehabilitation.
Psychological testing
screens for any conditions that can severely complicate the
implantation process.
Counseling is often provided to families who have misconceptions and
unrealistic
expectations regarding the benefits and limitations of cochlear implants.
Patient selection : Expanding indications in children:
Criteria for cochlear implantation in children have evolved substantially in the past 5 years.
Experience has shown that children with more residual hearing often perform better with
implants, and those with some measurable open-set speech recognition ability before
implantation perform better with cochlear implants than do children without residual open-set
It is perhaps more important than ever that each case be considered individually by an
experienced cochlear implant team.
Medical and radiological criteria have expanded to include children with significant cochlear
abnormalities in addition to other substantial medical conditions.
Current selection criteria include:
1. Physiologic age of 12 months or older
2. Severe to profound bilateral sensorineural hearing loss
3. Benefit from hearing aids less than that expected from cochlear implants
4. Medical clearance to undergo general anesthesia
5. Family support, motivation, and appropriate expectations
6. Rehabilitation and educational support for development of oral language and hearing
Surgical Implantation
Once a candidate
has been selected, the next question is which side to place the implant.
If there is no difference
acoustically, then the implant goes on the side of the better surgical
ear as determined by CT evaluation.
If the patient has had
different durations of profound hearing impairment in each ear, better
results are achieved by placing the implant in
the ear that has the shortest duration of deafness.
In the candidate who has the
same etiology for the deafness and a similar duration of deafness
in both ears but has used a
hearing aid in only one ear, placing the implant in the ear that used
the hearing aid should be discussed with the patient.
Potential candidates who
have residual hearing in the ear to receive the implant must be told
that following implantation they will
likely lose the residual hearing in that ear.
The details of
implantation differ from prosthesis to prosthesis.
The patient is placed in the
supine position with the surgeon and surgical nurse at the head of
the bed.
The electrodes for
monitoring the facial nerve are placed prior to prepping the patient. The
hair is shaved based on the
design of the incision to be used; most frequently shaving four
fingerbreadths above and four fingerbreadths behind the
ear is sufficient.
The patient is then prepped and draped similar to other ear surgeries.
The position of the
internal component of the implant about 1 cm behind the auricle is then
determined and marked on the external
surface using a dummy coil.
Once the location of the
implant is determined, a skin flap is designed. A variety of skin flaps
have been used in the past,
but it is important to maintain a 1 to 2 cm distance from the edge of
the implant and the incision.
The flap must have a good
blood supply. An alternative incision to the classic wide c-shaped
incision is an extension of a postauricular incision near the postauricular
crease, extending
superiorly over the temporal squama and middle fossa, curving slightly
posteriorly at the most
superior aspect of the incision. The skin is incised
with a knife and the subcutaneous tissue
is incised with electrocautery.
The temporalis
muscle and fascia is preserved and the mastoid musculoperiosteal tissue
is divided to expose the
mastoid cortex. The site for the internal receiver in the skull is created
at the position previously determined.
A circular depression is
created using drilling burrs for the internal device. Again a dummy
device is used to correctly
mark out the dimensions of the receiver so that an accurate sink may
be created.
Suture tunnel holes created
on either side of the seat with a burr will be used to help hold
the receiver-stimulator in place.
A complete
mastoidectomy is performed creating a mastoid cavity with a minimal amount
of saucerization of its edges. A shelf of mastoid cortex is created
posteroinferiorly for securing the
electrode array.
The facial
recess is opened in the usual fashion by first identifying the horizontal semicircular
canal and incus.
A thin layer of one can be
preserve over the facial nerve to minimize surgical trauma.
Once exposure through the
facial recess is completed, the round window membrane
is inspected. The basal turn of the cochlea is opened near the anteroinferior
aspect of the round
window membrane. A small fenestra
slightly larger than the electrode to be implanted is
developed. A small diamond
burr is used to blue-line the endosteum
of the scala tympani, and
the endosteal membrane is removed by small picks.
At this point
the surgical site has been prepared for electrode insertion. The monopolar
electrocautery devices are disconnected and not used once the implant is in
place. Bipolar
cautery should be used for hemostasis as necessary.
The implant may be inserted
one of two ways. The electrode array may be inserted
first, followed by securing
of the internal coil. Alternatively, the internal coil is secured, after
which the electrode array is
inserted into the cochlea. The internal coil is secured by placing it
within its bony well and secured with
sutures placed across the internal receiver. With micro
claws, the electrode array
is inserted into the cochlea.
It is important to prevent
kinking of the electrode, which may cause breakage of the wires
leading to the
electrode.
Force must not be used in
inserting the electrode as this may cause buckling of the electrode
within the cochlea.
This buckling of the
electrode may disrupt the basilar membrane and result in additional
degeneration of ganglion
cells and degradation of the electrical signal.
Deep insertion of the
electrode array is desirable in order to reach the area of high density
of ganglion cells near the upper basal turn. The cochlear multichannel
device has 32 metal bands
along the stimulating electrode, of
which the distal 22 are active electrodes and the proximal 10
are inactive
stiffening rings.
Once inserted, one can count
the remaining rings to determine depth of insertion of the
electrode array.
Once the electrode array is in place within the cochlea,
connective tissue is used to obliterate the
cochleostomy site and fill the facial
recess.
The surgical site is closed
in multiple layers without a drain and a large bulky mastoid dressing is
applied.
In approximately 50 % of
cases, the round window niche and membrane are replaced with
new bone growth.
This condition is more
common in patients who have had meningitis. In these cases, the surgeon
must drill forward along the basal coil
for as much as 4 to 5mm. Usually the new bone is white
and looks different than the surrounding otic capsule. If new bone growth completely obliterates
the scala tympani, the
electrode may be placed into a patent scala vestibuli. With complete
ossification of the cochlea, a channel is drilled
following the anatomic
location of the scala tympani.
Care must be taken to
not injure the internal carotid artery in its location anterior to the cochlea.
In cases of cochlear dysplasia,
there is significant risk of
encountering a gusher on fenestrating the
cochlea while creating
a cochleostomy.
A gusher is managed by allowing the CSF reservoir
to drain off and inserting the electrode
array into the
cochlea.
A connective tissue plug is
placed around the active electrode array at the cochleostomy
site, sealing the leak.
The external processor and
antennae are fit 3 weeks after implantation of the internal
device. This allows time for
the incision to heal. The internal device and external processor
contact one another
via a small antenna that is retained magnetically behind the ear.
The overall surgical
complication rate for cochlear implantation has been reduced in the
last 5 years from 11% to 5%.
Among the most commonly encountered problems are those
associated with the
incision and postauricular flap. Problems involving the skin flap include
wound infection,
wound breakdown with possible extrusion,
and poor percutaneous
transmission.
The
complications related to mastoidectomy and cochleostomy include facial nerve
paralysis, bleeding from a dural sinus, CSF leak, and meningitis.
Device related
complications include breakage of electrode array or wires during insertion,
excessive intracochlear
damage from traumatic insertion, slippage of electrode out of the
cochlea, and improper
electrode placement.
Eliminating specific
electrodes when the array is mapped during postoperative
programming of the device
can usually control undesirable vestibular or facial nerve
stimulation.
Rehabilitation
Rehabilitative needs differ
in implant recipients based on their auditory experience
prior to onset of deafness.
For the prelinguistically
deafened implant recipient, auditory and speech training are
critical in facilitating communication change.
For the postlinguistically deafened implant
recipient, auditory
training often focuses on the more complex listening skills.
Success with cochlear
implants in children requires a rehabilitation team’s significant
skill and a family’s
commitment to optimizing the use of the device.
Cochlear implant
rehabilitation must address the development of receptive language
skills and expressive
language skills. The rehabilitation of a cochlear implant user
depends on a team approach.
The goal of pediatric
cochlear implant rehabilitation is to enable the early deafened child
to learn incidentally from
their environment at home and at school. In order to accomplish
this there must be
structured intervention.
In order to maximize the
effectiveness of the implant in children, the rehabilitative program
must be committed to
the development of auditory and speech training.
RESULTS
The definition
of success is different from patient to patient and family to family.
Several factors, including
age at time of deafness, age at implant surgery, duration of
deafness, status of
remaining auditory nerve fibers, training, educational setting, and
type of implant, affect the
benefit a patient receives from an implant.
The variability in outcomes
with cochlear implants is thought to be due primarily
to patient factors.
Postlinguistically deafened
patients do better than prelinguistically deafened patient
with cochlear implants.
Children who have not
previously learned language, however, continue to improve
over a period of 2 to 5
years, and during that time, their scores become closer to those
achieved by postlinguistically deafened children.
There have been several well
documented studies that clearly demonstrate that
prelinguistically deafened
children who receive cochlear implants at an early age and are
educated in aural-oral
settings achieve open-set word recognition.
Gantz and colleagues
at Iowa University, in a study of 54 implanted children, reported
that after 4 years of use, 82% of prelinguistically
deafened children achieved open-set word
understanding.
At Washington University,
Lusk and associates reported on 25 congenitally deaf
children, all of who showed
improvement with cochlear implants. Within 36 months of
surgery, all of the
children implanted under the age of 5 showed open-set speech
understanding.
This reveals that the
earlier a child is identified with profound hearing loss and given
an implant, the better the
results.
A retrospective study done
at Atlanta Ear clinic, Georgia, about Multichannel cochlear
implantation in children ,
concluded:
-
Patients with ossification of the
cochlea do not perform as well with cochlear
-
implants as do patients
-
with no ossification,
-
An increase in ossification may
result in a decrease in cochlear implant
-
performance,
-
Even patients with complete ossification receive benefit
from multichannel
-
cochlear implants.
Ethical
consideration:
Cochlear implants have the potential to change the way deaf people live.
Although cochlear implants are s safe and effective treatment for the disability of deafness,
some deaf people view cochlear implants as unnecessary and demanding to their way of life.
As a result of the perceived threat from cochlear implants, organized attempts to
suppress their use, particularly in children, occurred throughout the 1990s. It is important
to recognize that 90% of deaf children have two hearing parents, and 97% have at least one
If significant numbers of these deaf children receive implants, the numbers of deaf persons
could be substantially diminished.
The principal ethical issues associated with cochlear implantation are:
Who should decide whether a deaf child receives a cochlear implant, and
What should be the basis of the decision?
It is generally believed that parents have the right and responsibility to exercise free,
informed consent on behalf of their child and that their decisions should be based on
the best interests of the child.
As experience continues to demonstrate both the safety and effectiveness of cochlear
implants, organizations for the deaf are moderating their opposition to cochlear implantation
Specific challenges for the
future cochlear implantation:
To provide better
reproduction of the coding of sound,
To further improve the
perception of speech and other sounds in noise,
To develop a totally
implantable cochlear implant, ( started…)
To use nerve growth factors to
protect the hearing nerve from die back after deafness,
To reestablish auditory
plasticity to help implanted children achieve optimal
speech perception.
A lab representation of the implant electrode
Graphic representation of the coil inside the cochlea
The External Component in place
Internal Component [Cochlear]
External component with Body processor [Cochlear]
Internal + external Components with Processor [Clarion]
Internal + external Components with Processor [Medel]
Nucleus [Cochlear]
Children are being implanted at a younger age,
Help improve speech development
Easily hidden under long hair
Manufacturers of
cochlear implants:
1: External Component--Magnet
2:Processor
3:Microphone
4:Internal Component
5:Electrode
