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Cochlear
implants represent one of the greatest advances in medicine
in the management of children with bilateral severe to profound
hearing loss. For children not benefiting from hearing aides
or other more conservative therapies, and whose families are
committed to an oral mode of communication, cochlear implants
offer a singular opportunity to restore functional hearing levels
to children.
Who
is a candidate for a cochlear implant? Among children, the
FDA has released the following guidelines for cochlear implant
candidacy:
- Children
12 months of age through 18 years of age
- Children
demonstrating severe to profound sensorineural hearing loss
bilaterally
- Children
not benefiting from hearing aide amplification
- Children
with little to no speech recognition when testable.
In general, a child is a candidate for cochlear implantation
if he or she has no useful hearing on audiometric testing in
both ears and/or is not obtaining adequate auditory stimulation
with maximal hearing aide use. If a child has severe hearing
loss but can still obtain benefit from hearing aides, then such
a mode of management is preferable to cochlear implantation.
Also, if a child has one profoundly deaf ear, but has a normal
hearing ear on the opposite side, then that child is not a candidate
for a cochlear implant.
A fairly common scenario is to have a very young child who may
have sound detection in a useful range, however, the quality
of such a child's hearing may be insufficient for that child
to discriminate complex sounds such as speech and in turn, develop
oral communication skills. This highlights two levels of hearing
in hearing impaired children. First, sound detection is a much
lower level of auditory function. The ability to detect sounds
(a bell, whistle, car horn, etc.) requires a certain level of
cochlear and cochlear nerve function. However, speech recognition
requires a much higher level of cochlear and cochlear nerve
function. It is fairly common for children to be able to detect
sound but not obtain the quality sound and speech input that
is so critical for them to develop proper communication skills.
In the case of children being considered for a cochlear implant,
while the sound detection level is taken into consideration,
a greater weight is placed on the speech recognition ability
of the child. So even though a child may be capable of detecting
sound at useful hearing levels she may not be able to discriminate
speech and therefore would be considered seriously for a cochlear
implant. Often the cochlear implant team's decision whether
to recommend an implant (or not) for a child will depend upon
whether the child is showing signs of adequate hearing and speech
development.
Other specific criteria required for children to receive a cochlear
implant are medically determined. For example, some children
are born deaf due to a lack of an inner ear and cochlea. Such
a child would physically have no structure within which a cochlear
implant could be inserted and therefore would not be an appropriate
candidate. Similarly, one of the most common causes of deafness
in children in the United States is still bacterial meningitis.
And although vaccines have reduced the incidence of some forms
of meningitis, Pneumococcal meningitis is still the most common
cause of meningitis-related hearing loss. And an unfortunate
sequelae of Pneumococcal meningitis can be scarring and obliteration
of the cochlea. On imaging studies such as computed tomography
(CT scan), such cochleae show obliteration of the cavity into
which an implant is usually inserted. While implantation is
still feasible, outcomes are typically poorer compared to that
of children with normal cochleae. Factors such as these help
determine candidacy for cochlear implantation in children.
The
Evaluation Process Prior to Cochlear Implantation
All children (and their families) being considered for a cochlear
implant undergo a substantial preoperative evaluation by the
CHDR Cochlear Implant Team. Initially, serial audiologic testing
(hearing evaluations) need to be performed to determine if the
child is audiologically a candidate for cochlear implantation.
Particularly in young children, hearing tests may need to be
performed several times in order to obtain an accurate measure
of their hearing levels. Children will also occasionally show
a recovery of some hearing depending upon the cause of their
hearing loss, and therefore, serial testing is one way of determining
if the child should be implanted or given time to recover whatever
hearing they may without more aggressive intervention.
Using noninvasive, modern hearing testing techniques, an accurate
determination of a child's hearing can be obtained for any age
child - including newborns. Most people are familiar with the
traditional hearing test in which the patient is placed in a
sound-treated booth and asked to respond when tones are heard
through the headset. These tests remain a standard that can
be modified by the incorporation of play/games into the testing
process. For infants or young children who may not be capable
or interested in performing such tests, Auditory Brainstem Response
(ABR) allows the audiologist to measure the electrical activity
a child's hearing system in response to a sound stimulus (usually
a "click" type sound). For ABR, electrodes are stuck
to the child's skin behind the ear and at the forehead, and
a sound delivered to the child's ear. The electrodes detect
the electrical impulses triggered by the sound as that electrical
signal is sent through the hearing pathways. By measuring this
electrical activity, it is possible to determine if the child's
ear has responded to the sound at normal or abnormal levels.
Occasionally, children will require sedation (e.g. chloral hydrate
syrup) in order to allow the test to be performed. But commonly,
this noninvasive and non-painful test can be performed with
the child falling asleep spontaneously. An additional audiologic
test being performed on all hearing impaired children at CHDR
is Otoacoustic Emission (OAE) testing. This relatively new technique
allows the audiologist to evaluate the function of hair cells
located within the cochlea itself. By placing a small ear plug
(that contains an ultra sensitive microphone as well as two
miniaturized speakers) in a child's ear, it is possible to deliver
sounds to the ear, then measure an actual sound (the otoacoustic
emission) that is generated by the cochlea's outer hair cells
in response to the sound stimuli. These OAEs indicate whether
the hair cells are functioning normally or not. A very common
cause of hearing loss in children is loss or damage of these
hair cells, and accordingly, OAEs would be absent in such children.
Very recently, children have been identified with varying degrees
of hearing loss yet normal OAEs. These data would suggest that
something other than hair cell loss/damage is the cause of the
hearing loss (e.g. Auditory Neuropathy).
Once a child is determined to be a cochlear implant candidate
by audiologic criteria, evaluations are set up with the CHDR
team members. This includes evaluation by the aural rehab, speech-language,
child development and pediatric neuropsychiatry staff, as well
as a cochlear implant surgeon. A primary reason for this multidisciplinary
evaluation is to determine if there are any factors relevant
to rehabilitation after surgery or if there are any other needs
that require resolution or addressing prior to considering implantation.
Medically, all candidates routinely undergo a comprehensive
evaluation to determine if there are any readily treatable causes
for the hearing loss - e.g. occult infection, hypothyroidism,
metabolic abnormalities, etc. Also included are imaging test(s),
typically a CT or MR scan of the inner ear region to study the
cochlea and related structures.
Once the evaluations are complete, all candidates are discussed
at weekly CHDR Cochlear Implant Team meetings and a consensus
decision reached as to the candidacy of individual children.
Once decided, the family is informed of the decision and invited
in to discuss either the surgery and scheduling, or alternative
recommendations and plans.
The
Technology
Cochlear implants are completely different from a hearing aide.
Hearing aides generally amplify sound and speech and allow whatever
residual hearing a child has to process and perceive that auditory
input. A cochlear implant, in contrast, does not amplify or
generate any sounds. Instead, the external speech processor
component of a cochlear implant takes in sound and speech, and
converts it into an electrical (digital) signal that is then
transmitted to an implanted internal component. This surgically
implanted device includes a micro-electrode array (a fine wire)
that is inserted directly into the cochlea (the hearing organ
of the inner ear). Upon receiving the encoded signal from the
external speech processor, the electrodes in the internal device
are then activated in a precise pattern/sequence to stimulate
the hearing nerve directly. In this way, sound and speech is
encoded by the cochlear implant and in turn, stimulate the hearing
nerve to allow the child to hear again.
At present, three cochlear implants are FDA-approved in the
United States: the Nucleus cochlear implant, Cochlear Corporation
www.cochlear.com,
the Combi40+, MED-EL corporation www.medel.com,
and the Clarion cochlear implant, Advanced Bionics Inc www.advancedbionics.com.
All devices have shown their successful use in the pediatric
population with grossly equivalent results in terms of sound
detection and speech recognition. Components similar to each
device include an external speech processor and an implanted
internal component that includes the electrode array. The three
devices differ in terms of their absolute electrode designs
and speech processing strategies. The Nucleus 24 implant utilizes
24 electrodes that are now arranged along either a straight
or precoiled array in the latest generation devices. The Clarion
implant uses a 16 electrode design that has always been based
on a precoiled array. The Clarion latest generation implant
now includes a recently FDA-approved positioning device that
is felt to position the electrode array as close to the hearing
nerve fibers as possible in order to make electrical stimulation
more efficient and effective. The MED-EL is the newest device
and has been used successfully in Europe for many years
All three devices have been used at the CHDR with excellent
hearing and speech results among both patient groups. Further
information can be obtained by visiting the individual corporate
websites.
Surgical
Techniques
Cochlear implantation is performed under general anesthesia
and requires an incision that is typically S-shaped behind the
ear. After initial exposure through this incision, the mastoid
bone is removed using a drill to allow identification of the
facial nerve and the cochlea. The facial nerve is a critical
in this and most ear surgeries since it runs directly through
the middle ear and mastoid region where surgery is being performed.
At this point, an opening is created in the cochlea just small
enough to allow insertion of the electrode array. Depending
upon the device being inserted, testing of the implant can usually
be performed before insertion, immediately after insertion,
or both, to confirm integrity of the device prior to permanently
implanting the device into a child. Also depending upon which
device is being implanted, an insertion tool or stylet may or
may not be used to facilitate placement of the electrode array
inside the cochlea. An atraumatic insertion of the device is
critical at this stage of the procedure as this can affect how
hair cells and nerve fibers survive after implantation and ultimately
the performance of the implant in any given child.
In order to secure the implanted processor and reduce the prominence
of the processor on the side of the child's head, a bony depression
is drilled in the designated position of the internal processor
(typically, above and behind the outer ear). A variety of techniques
have been described to permanently tie the device in position
(permanent suture, Gore-Tex sheeting, etc.). The skin incision
is then closed using absorbable sutures and a compression/protective
head wrap is then applied for the next several days. In order
to allow the skin incision and operative site to heal adequately,
initial activation of the implant is delayed anywhere from 4-6
weeks after surgery.
Complications from cochlear implant surgery include skin incision/flap
problems (most common complication), facial nerve injury, bleeding
and infection. To address the skin flap problems, the S-shaped
incision has been uniformly used to allow better preservation
of the bloodflow to the area, reduced tension on the skin after
placement of the implant, as well as better venous and lymphatic
drainage. In contrast to earlier C-shaped flaps behind the ear,
no significant problems have been observed with this type incision.
The facial nerve is responsible for moving the facial muscles
on the same side of the face (closure of the eye, moving the
facial muscles during speech, eating, expression). Injury to
the facial nerve results in weakness or paralysis of that side
of the face and is a dreaded complication. In order to minimize
this risk, continuous intraoperative facial nerve monitoring
is used to further reduce the chance of facial nerve injury.
The risk of injury to the facial nerve is generally cited at
1% or less during a procedure such a cochlear implant. Facial
nerve injury remains a small but obviously significant risk
during cochlear implant surgery. The nerve must first be identified
then drilling performed fractions of a millimeter in front of
the nerve in order to expose the cochlea adequately for implantation.
Therefore, surgical expertise with the microscopic anatomy along
with routine use of facial nerve monitoring are essential to
maintaining a risk of injury rate in the 1% or less range. Bleeding
and infection are possible complications from any surgery and
remain minor risks during cochlear implantation. The use of
prophylactic antibiotics is a routine at our institution particularly
when implanting a permanent device into a child. Infection of
the device could possibly require surgical removal and therefore
all precautions are taken to avoid such an outcome.
What
happens after surgery?
A cochlear implant is typically an overnight stay in the hospital
following a same-day admission. However, some institutions are
routinely performing cochlear implantation as an outpatient
procedure for many of their patients. Approximately 4-6 weeks
after surgery, the patient is scheduled for their first implant
activation and initial mapping. The exact timing will depend
upon the healing progress in each individual case. All the electrodes
are checked electrically for their integrity and each electrode
needs to be programmed for threshold and comfort levels. For
young children, it may take several sessions to program all
the electrodes in the array. Much depends upon the cooperation
and tolerance of the child to the mapping process.
By far, the most difficult and demanding part of the cochlear
implant process is the rehabilitation and training following
the surgery. The younger the child, the longer the rehab period
typically runs. For some children, this may entail weekly, bimonthly,
monthly or periodic rehab sessions for several years. Specific
therapy is needed to train children in hearing skills, speech
and language skills as well as periodic mapping and fine-tuning
of the implant to individualize the settings to that child's
optimal benefit.
From the first mapping session, we uniformly expect children
to obtain sound awareness with activation of the device. After
several weeks to months and as the child gets used to hearing
and stimulation by the implant, the output of the device can
be increased to provide greater dynamic range (wider hearing
thresholds). Most children end up with pure tone thresholds
in the mild-moderate hearing loss range. However, speech recognition
abilities typically take longer to develop and again, the younger
the child, the longer the time before substantial advances in
speech recognition and generation are noticed.
What
determines success with a cochlear implant?
Research over the past several years has focused on determining
which factors/characteristics of children predict who will benefit
the most from a cochlear implant. This has been prompted by
the wide range of performance noted amongst seemingly similar
children undergoing cochlear implantation. Although this area
remains extremely controversial, several factors are now fairly
established as being good or poor indicators for post-implant
performance in children.
Original cause of the deafness is an important factor. Some
diseases such as meningitis cause hearing loss that can be associated
with cochlear scarring or new bone formation (ossification).
Such patients can undergo cochlear implantation but may not
benefit as much as a child without a history of meningitis and
cochlear ossification.
Age at onset of deafness is another important consideration
in terms of cochlear implant performance in children. Children
born severely to profoundly deaf are clearly considered pre-lingually
deaf (i.e. deaf prior to the acquisition of oral speech and
communication skills). In contrast, a 7 year old child that
lost his hearing at that age, has already established the hearing,
speech and communication skills in his auditory and neural pathways,
and therefore, is considered post-lingually deaf. Children who
were post-lingually deafened and receive a cochlear implant
fairly soon after the loss of their hearing, generally do extremely
well in a fairly short timeframe. Pre-lingually deafened children,
however, are essentially starting at "time 0" in terms
of their oral communication skills development after cochlear
implant activation. Therefore, a lengthy rehab course (commonly
~1 year) may be necessary in pre-lingually deafened children
before they may demonstrate major strides in speech and communication.
Duration of deafness is also a key factor. In general, the longer
the duration of deafness, the poorer the performance after implantation.
This is not to say that children with long-standing deafness
are not cochlear implant candidates, however, the progress and
overall benefit in such children may not be optimal. As an example,
a girl born deaf and initially evaluated for a cochlear implant
at 16 years of age, is not an ideal candidate for implantation.
The reasonable explanation for this phenomenon is that the auditory
system is in many ways a "use it or lose it" system.
Lack of auditory stimulation will eventually result in degeneration
of the cells, nerves and neural pathways responsible for relaying
that information. Therefore, in some children with a long duration
of deafness (e.g. >10 years), there may be atrophy of the
hearing system such that it cannot be used or stimulated by
an implant. In children such as the case example discussed here,
there is a 50% chance that the child will not be using the device
at the end of 5 years. Obviously, it would be preferable to
select candidates better preoperatively such that children would
not have to go through the procedure only to find that the implant
is not beneficial to them. (For research in this area, see the
section on Functional Magnetic Resonance Imaging and Cochlear
Implant Candidates). CHDR is actively involved in using novel
functional imaging techniques to address this problem and therefore
provide families with better prognostic information before surgery.
Social/Family Factors are clearly the most important determinants
of a child's post-implant performance. Despite the incredible
technology and exponentially expanding experience in using cochlear
implants in children, how well the child performs is largely
determined by the family and school environments. Children who
are not brought for postop mapping, aural and speech rehab sessions
inevitably do worse than children that are brought consistently
for rehab sessions. As an analogy, giving a child a cochlear
implant and not bringing them to rehab sessions is like buying
a child a $40,000 sports car and not teaching them how to drive.
This frustrating fact is the rationale for our CHDR Cochlear
Implant Team stressing to families at every opportunity before
implantation, that their postoperative commitment to the process
is the greatest contribution they can make to their child's
progress. The school environment is also vitally important to
optimal oral communication skill development and maximum benefit
from a cochlear implant. A learning environment that encourages
and provides the special educational needs of hearing impaired
children greatly enhances the child's rehab after implantation.
And while Total Communication (TC) educational programs are
certainly well-established for hearing impaired children, clinical
studies have shown that children with cochlear implants obtain
better oral communication skills if immersed in a school environment
in which aural (hearing) and oral communication are emphasized
over manual communication (e.g. sign language, ASL). Similarly,
a home environment in which parent(s), siblings, and other family
members encourage and stimulate aural-oral communication also
predicts favorably for good performance with a cochlear implant.
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