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Author 's Accepted Manuscript
Techniques in Cochlear Implantation
Heather M. Weinreich MD,MPH, Howard W. FrancisMD, John K. Niparko MD, Wade W. Chien MD
PII: S1043-1810(14)00068-2
DOI: http://dx.doi.org/10.1016/j.otot.2014.09.002
Reference: YOTOT645
To appear in: Operative Techniques in Otolaryngology
Cite this article as: Heather M. Weinreich MD,MPH, Howard W. Francis MD, John K.
Niparko MD, Wade W. Chien MD, Techniques in Cochlear Implantation, Operative
Techniques in Otolaryngology, http://dx.doi.org/10.1016/j.otot.2014.09.002
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The
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Techniques in Cochlear Implantation
Heather M. Weinreich, MD MPH1, Howard W. Francis, MD1, John K. Niparko, MD2, Wade W. Chien,
MD1,3
1Division of Otology, Neurotology & Skull Base Surgery. Department of Otolaryngology-Head & Neck
Surgery. Johns Hopkins School of Medicine, Baltimore, MD
2Department of Otolaryngology-Head and Neck Surgery. Keck School of Medicine, University of
Southern California, Los Angeles, CA
3 National Institute on Deafness and Other Communication Disorders, National Institutes of Health,
Bethesda, MD
Abstract
Cochlear implantation surgery has evolved over the past 30 years. Although the standard
mastoidectomy with posterior tympanotomy has not change, recent techniques have focused on minimal
incisions and hearing preservation. Evidence shows that smaller incisions with modest manipulation of
soft tissue do not affect post-operative healing outcomes. There has been a trend advocating “soft-
surgical” techniques in an attempt to preserve the cochlear sensory epithelium. However, the literature is
inconclusive as to the outcomes of “soft-surgical” over standard techniques for hearing preservation
including round window insertion and usage of adjuvants during insertion. Regardless of techniques,
cochlear implantation remains an important therapeutic option for hearing loss.
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Introduction
Cochlear implants convey sound information by stimulating the auditory nerve, bypassing a
dysfunctional cochlea. A typical cochlear implant consists of an external component (microphone and
speech processor) and an internal component (receiver-stimulator and the stimulating electrodes) (Figure
1). As of 2008, over 172,000 cochlear implants have been implanted worldwide1. For over 30 years,
cochlear implants have restored auditory sensitivity in hearing impaired patients and provided access to
speech and environmental sounds. During this time period, various surgical techniques have been
employed and evolved. There are three FDA approved device manufacturers in the United States
(Cochlear™ Americas, Med-El Corp, and Advanced Bionics Corp). Here we provide a discussion of the
most relevant techniques for general implantation. Specific techniques for each device will not be
addressed.
Indications/Contraindications
Candidates for cochlear implants include those with significant hearing loss that receive little to
no benefit with hearing aids. Severe to profound impairment of cochlear function in both ears and
anatomic preservation of the auditory nerve in the implanted ear are requirements for implantation.
Cochlear implants have been used for the management of cochlear dysfunction due to congenital or
acquired causes secondary to genetic, ototoxic, infectious or autoimmune etiologies. Cochlear implants
are also used for cases in which radiation therapy or the surgical removal of a posterior fossa tumor
threatens function in the only-hearing ear. Treatment that involves the better hearing but diseased ear may
result in complete hearing loss secondary to damage to the cochlea or cochlear nerve. However,
implantation of the contralateral, un-operated hearing ear can provide auditory function for the patient.
Candidacy
Comprehensive candidate assessment is essential to minimize risks. Audiologic, medical,
surgical, developmental, cognitive and psychosocial factors need to be addressed. Candidates should
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understand that cochlear implant is a communication tool and not a cure for deafness, since expectations
largely shape postoperative satisfaction with any form of auditory rehabilitation2.
Age, hearing loss etiology, unaided and aided hearing, deafness duration, social support and
environments that promote spoken language carry predictive value. Regarding duration of hearing loss,
speech perception and duration of profound deafness before implantation are significantly correlated with
post-implantation hearing outcome3. Although there are negative factors, a prolonged period of deafness
does not rule out prospects for speech understanding with a cochlear implant if basic foundations of
communicating through audition (e.g. prior hearing aid use, use of lip-reading) are in place.
A preoperative medical examination should be performed to determine suitability for general
anesthesia. An otologic evaluation is performed to identify structural changes in the temporal bone that
may affect surgical approach and feasibility. High-resolution computed tomography (CT) scans of the
temporal bone can aid in surgical planning. Temporal bone surgical anatomy including mastoid
pneumatization, ossicular anatomy, position of the facial nerve, caliber of the internal auditory canal,
cochlear malformation/ossifications, enlarged vestibular aquaduct presence, and labyrinthine anatomy can
be assessed4. CT findings of cochlear patency generally correlate with surgical findings5, but
discrepancies can occur as a result of volume averaging6, 7
.
Magnetic resonance imaging (MRI) may be a useful adjunct to CT for assessment of implant
candidacy by imaging soft tissues such as the membranous labyrinth, nerves in the internal auditory canal,
and soft tissue within the cochlea8-10. High-resolution T2-weighted MRI images are helpful for assessing
cochlear patency by revealing the presence or absence of fluid within the scalae. Special consideration
should be given to patients who need future assessment with MRI as the implanted magnet in the internal
device may be contraindicated. Baumgartener et al.11
showed that in patients with cochlear implants of
different devices, undergoing MRI with 1 Tesla (T) did not cause implant malfunction or patient injury.
Our own experience suggest that MRI imaging with 1.5 T magnet poses no significant threats if the
device is immobilized using externally applied molding material and is firmly bound12.
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Adult candidates
Adult candidates typically have unaided thresholds in the severe to profound hearing loss range
defined as a pure-tone average (PTA) hearing loss of 70 dB or greater in both ears. Current U.S. Food and
Drug Administration (FDA) guidelines require speech recognition on sentence test material at normal
conversational levels (50 or 60 dB SPL) in best aided condition of less than 60% in the better hearing ear
and less than 50% in the ear to be implanted13. Medicare (CMS) and Medicaid guidelines are stricter than
the FDA guidelines. As of 2005, CMS requires a score of less than 40% on sentence test material. If the
patient is enrolled in an FDA-approved category B investigational device exemption clinical trial,
implantation may be covered with a hearing test scores of greater than 40% and less than or equal to
60%14.
Child candidates
Children candidates are those with bilateral severe-to-profound sensorineural hearing loss
(assessed by PTA) who have demonstrated limited or no functional benefit from hearing amplification,
and lack of progress in auditory skills. The FDA has approved implantation down to 12 months of age.
Hearing loss should be confirmed by acoustic reflex data and auditory brainstem responses to both clicks
and tonal stimuli when appropriate. Many children with congenital deafness exhibit cochlear
malformation such as cochlear hypoplasia or a common cavity15. A hypoplastic cochlea is associated with
poor definition of cochlear turns and partitions between the modiolus and internal auditory canal (IAC)
and relatively low spiral ganglion cell populations16. However, if an auditory nerve is present,
implantation can be attempted, even though the hearing outcome may not be as good as patients without
cochlear malformation.
Bilateral Cochlear Implants
Bilateral cochlear implantation, either sequential or simultaneous, has been described for
individuals with bilateral profound hearing loss who do not receive adequate benefit with a hearing aid in
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the better hearing ear. The potential benefit of bilateral implantation relies on increased auditory
sensitivity due to summation effects, improved sound source localization, and improved speech
recognition in noise18. In adult populations, bilateral implantation is less common due to lack of insurance
coverage, inability to tolerate usage of a hearing aid with a contralateral cochlear implant, and a lack of
motivation to proceed with another surgery and rehabilitation efforts.
Neurofibromatosis 2 (NF2)
Preservation of auditory nerve integrity should be strongly considered in all NF2 cases whether or
not the contralateral ear has already demonstrated hearing loss. If the contralateral non-operated tumor ear
continues to benefit from a hearing aid but is undergoing measurable decline in function, early
implantation provides an opportunity for continued nerve stimulation as hearing diminishes. Lustig et al.17
examined seven NF2 patients who were implanted following surgical resection with nerve preservation or
stereotactic radiation of vestibular schwannomas. Hearing acuity and awareness of environmental sound
was achieved in all cases; however, there was variability in speech understanding.
Surgical Technique
Cochlear implant surgery is performed in a supine position with the use of an operating
microscope and a rotating drill. Prophylactic antibiotics are administered to provide coverage for the
placement of a prosthetic device. Implantation is usually performed under general anesthesia. In our
center, facial nerve monitoring is routinely used during cochlear implantation.
Design of the flap/soft tissue work
The design of the incision and device placement is critical to preventing complications. Wound
complications have been associated with a delay in activation19. There are three main principles for the
placement of the incision:1) it should not be near the internal receiver, 2) it should not compromise the
blood supply, and 3) it should provide exposure for a standard mastoidectomy20. Additionally, the
location of the incision should be posterior enough to enable placement of the internal device sufficiently
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away from the pinna compatible for an ear-level external processor. Marking of the drapes and skin with
mock devices can assist in placement.
The typical approach incorporates a standard post-auricular incision used in mastoidectomy
surgery. In the past, an inverted J-shaped incision was utilized as it was based on a posterior-inferior
arterial supply and allowed incorporation of prior post-auricular scars if the patient had undergone
previous otologic surgery. Over time this incision has been shortened, with some institutions using a
minimal access design with a 2-4 cm straight incision20. In a comparative study of a standard cochlear
implant incision versus a minimal access technique, an overall complication rate was noted 18.4% vs.
11.0%, respectively21. This difference was not statistically significant. Minor complications of the
minimal access technique included poor positioning and migration of the device. Of the two reported
devices that migrated in the study, one occurred after trauma and the other was attributed to the technique.
Advocacy for a minimal access incision includes decreased operative time leading to decreased time
under anesthesia and decreased costs. In the Prager et al. study21, mean operative time was reduced from
45 minutes to 28 minutes (p
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enter the facial recess using a small diamond burr (Figure 2). Adequate thinning of the posterior canal
bone and thinning of the bone anterior to the vertical segment of the facial nerve can improve and
maximize visualization of the round window niche via the facial recess.
Cochleostomy
A cochleostomy is created in the scala tympani, either indirectly through the promontory, antero-
inferior to the center of the round window niche, or directly through the round window membrane. The
preferred approach is debatable, but the priority should be to access the scala tympani without inducing
direct injury to the basilar membrane, while providing adequate access for unimpeded insertion of the
electrode array22. The round window niche should be exposed, and the round window membrane should
be identified23. If the round window niche is obliterated with bone and difficult to identify, a round
window drill out can be done by drilling approximately two millimeters (mm) from the inferior margin of
the oval window or approximately 1-1.5 mm inferior to the stapes tendon to enter scala tympani24.
When drilling the cochleostomy, a technique of “soft-surgery” implantation has been proposed.
Principles include minimizing trauma upon opening the cochlea, avoiding entrance of foreign material
(e.g. blood, bone dust) into the cochlea and limiting injury upon insertion of the electrode. Prior to
entering the cochlea, a topical hemostatic agent such as a cotton ball soaked in 1:1000 epinephrine can be
placed on the promontory to minimize bleeding while entering the cochlea. A cotton pledget placed in the
antrum can minimize blood from entering the middle ear 25.
Traditionally, the cochleostomy is placed through the promontory anterio-inferior to the round
window membrane utilizing a small diamond burr. Placement of the cochleostomy is critical to avoiding
inadvertent insertion into scala vestibule or the osseous spiral lamina. The endosteum should be exposed
but not penetrated as this exposes inner ear to acoustic trauma. Care should be taken to prevent bone dust
from entering the inner ear. At our institution, sodium hyaluronate (Healon®) is applied during the
cochleostomy. This helps to remove and prevent bone dust and blood from entering the cochlear as well
as lubricate the electrode upon insertion. Evidence supports that bone dust can create a source for intra-
cochlear bone formation while the implication of blood within the middle ear is limited to animal studies
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suggesting an increased low frequency hearing loss25. A straight pick is used to open the endosteum and
care should be to avoid suctioning the perilymph.
Some otologic surgeons favor cochlear implant insertion directly through the round window
membrane. They claim that this technique reduces damage to the cochlea by providing an ideal insertion
angle and permits correct electrode position26. Round window insertion has been shown to provide
preservation of residual low-frequency hearing26 and produce no negative effects on utricular, saccular
and semicircular channel function. In this technique, the round window membrane is visualized by
drilling away the niche as well as bone anterior to the descending segment of the facial nerve. If the
membrane is visualized with complete view, Healon® can be placed over the window. The window is
then opened with a straight pick and the electrode inserted.
The debate between the two approaches centers on hearing preservation strategies especially in
the light of the development of softer, shorter or hybrid arrays for patients with residual low frequency
hearing. Friedland and Runge-Samuelson25 provides a review of the “soft surgical” technique and
implications. New bone formation at the cochleostomy site is more pronounced in the traditional
cochleostomy compared to round window insertion; however, implications for hearing preservation are
unknown. Havenith et al.28
performed a systematic review to determine if there was a difference in post-
operative residual hearing. There was no clear benefit to one approach over the other. Literature is limited
to case series and cohort designs with no published randomized controlled studies. One of the most recent
published case series showed comparable post-operative speech perception scores at up to 48 months
follow up; however, the study did not examine hearing preservation rates29.
Insertion
The electrode array is advanced under direct visualization along a trajectory tangential to the
basal turn of the scala tympani (Figure 3). For a round window insertion, the electrode is inserted at an
oblique/anterior angle. Resistance to array insertion can produce buckling of the carrier-array leading to
spiral ligament and basilar membrane injury, and neural loss. Aggressive insertion should be avoided as
damage occurs when the electrode is inserted past the point where resistance is first detected. Cochlear
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trauma with array placement is associated with poorer outcomes in clinical trials of adults and children30,
31. Current electrode carriers are typically inserted over a distance of up to 25-30 mm which places array
adjacent to fibers of the auditory nerve that normally subserve the entire range of speech frequencies.
“Soft surgical” techniques when utilized with shorter electrodes focus on a depth of 17-20 mm to preserve
low frequency thresholds within 35 dB of pre-operative hearing25. These techniques can be employed for
full length electrodes as well. However, deeper insertions may result in mechanical damage leading to
further deterioration of residual hearing25.
After insertion of the array, the cochleostomy should be sealed gently with a small piece of fascia.
In a round window insertion, the electrode itself can seal the incision or a small amount of
muscle/periosteum can be used to further seal the opening. The connecting lead should be stabilized
within the facial recess to reduce the likelihood of the array extruding from the cochlea. Balkany and
Telishi32 describe a technique which includes drilling a notch in the incus buttress that provides a slot in
which the array lead can be stabilized. The electrode should be coiled into the mastoid cavity.
Special situations
Diseases such as otosclerosis or meningitis can lead to labyrinthitis ossificans. Labyrinthitis
ossificans results in the formation of fibrous tissue and possible calcifications deposited in the scalae
leading to obstruction of the scalae. The scala tympani, especially in the basal turn, is the most common
site of fibrous tissue and new bone growth, regardless of the etiology. Since many patients with cochlear
ossification receive only partially inserted electrode arrays, performance may suffer either because of
smaller numbers of available channels or spiral ganglion cell depletion. A study performed by Francis et
al.19 showed a significantly smaller proportion of electrodes were initially activated in children with
history meningitis. This proportion remained stable at 12 and 24 months post-implant. The need for
additional cochleostomy drilling was also noted to have lower proportion of activated electrodes.
Securing the device
Prior to placement of the electrode array, a subperiosteal flap is elevated deep to the temporalis.
The pocket should be elevated to fit the device and minimize movement. A well is drilled for the internal
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receiver. In general, the receiver should be placed posterior to the incision and at a 45 degree angle,
avoiding overlap with the external processor. The well should be drilled to accommodate the device and
allow the device to lay flush against the skull. In the past, it has been advocated that for children or adults
with thinner bone, the well be drilled down to dura with or with a floating island of bone to decrease the
profile of the internal receiver. Complications to this technique can occur. In one retrospective study
involving both children and adults, a well drilling complication rate of < 1% was published with injuries
to dura resulting in CSF leaks and in one case, a subdural hematoma33. The authors concluded by
advocating for no or minimal dural exposure33. Indeed the trend amongst surgeons is to avoid dural
exposure. In a survey of otolaryngologists, only 35.1% usually or always expose dura while even less
surgeons create a bony island (25%)34.
In light of concerns about the depth of the well, device migration is considered a potential
development of infection or extrusion. Thus methods for securing the internal receiver have been utilized.
Titanium screws with nylon suture, mesh or Gortex, and isomeric bone cement have been instituted20.
More recent research has supported that the receiver-stimulator is stabilized to the bony cortex by virtue
of a tight-fit to the pericranium and deep fascia of the temporalis muscle also known as the temporalis
pocket technique35
. In a study performed by Prager et al21
, comparison of a minimal access technique that
included creation of a tight fitting pocket without drilling a well resulted in migration of one device after
head trauma and a second incidence of an electrode migrating into the vestibule for a major complication
rate of 2.7%. This number was not statistically different when compared with standard techniques. For
children, a minimal access technique should still include a method of fixation given a thinner soft tissue
envelope and frequency of head trauma in this population36.
Closing
As the incision is closed, the implanted device is covered completely. Care should be taken to
ensure the periosteum is closed over the electrode. Closure includes approximation of the periosteum,
dermal layer and epidermis with suture. Our institution utilizes 3-0 chromic gut suture for the periosteum
and dermis while 5-0 fast absorbing plain gut suture is used for the skin. Antibiotic ointment is applied to
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the incision, followed by antibiotic impregnated petroleum dressing (Xeroform™) and a mastoid
dressing.
Outcomes
Results in Adults
Implanted adults with six months of experience have been found to receive an average score of
40% correct on word testing with a range of 0% to 100%37. Sentence testing on patients results in an
average of 75% correct, with a wide range of scores from 0% to 100%. In adults, especially the elderly, it
has been suggested that cochlear implantation slows age-related cognitive decline and can improve
quality of life among the elderly38.
Results in Children
There is a wide variability in the speech perception outcomes in children due to differences in
residual hearing, age of implantation, mode of communication, family support, and length of deafness.
Miyamoto et al.39 found in children between 1-4 years after implantation, roughly half achieved at least
some open-set speech recognition. Furthermore, earlier implantation in children results in better outcomes
and is important in predicting language abilities. A study by Niparko et al.40 found that children who were
implanted before 18 month of age showed significantly higher rates of language comprehension and
expression than those implanted at a later age, and the trajectory of language acquisition was on par with
that of the normal hearing controls.
Complications
Cochlear implantation entails risks inherent with mastoid surgery and those associated with the
implanted device. Cohen et al.41 characterized implant-related complications as major if they required
revision surgery, and minor if they resolved with minimal or no treatment. Major complications include
facial nerve paralysis and implant exposure due to flap loss. In several surveys of implant operations,
major complication rates ranged from 8-13.7% while minor ranges from 4.3-13.7%41-43. Facial nerve
injury is uncommon and, when recognized promptly, is unlikely to produce permanent paralysis. Otologic
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surgeons should not solely rely on the facial nerve monitor for locating the facial nerve and should
maintain a keen understanding of the anatomic course of facial nerve.
Device failure is attributed to either flaws in manufacturing or trauma. Device failure as a result
of loss of electrical function in the external processor commonly produces a sudden loss of function and,
therefore, hearing. External processor function may be lost with direct trauma, exposure to water, and,
most frequently, normal wear and tear. An internal device failure typically presents as either an
immediate cessation of hearing or intermittent hearing loss associated with reduced quality of sound and a
period of diminishing function over days to weeks.
Revision implant surgery is indicated with device malfunction, infection, electrode migration,
facial stimulation, and upgrades from single-channel to multiple-channel cochlear implants. At our
institution, there was a 1.3% annual risk of revision surgery44. Device failure (65%) was the most
common indication of re-implantation. A third of those presented with complete cessation of device
function, and two-thirds experienced less dramatic declines in function. Other indications included
infection (12%), electrode extrusion (15%) and facial nerve stimulation (8%).
Risk of meningitis remains low in cochlear implantation. In animal studies, inoculation of
streptococci resulted in inflammation of the round window niche; however, cochlear inflammation was
absent possibly due to the barrier the electrode created45, 46. Greater than 10 years ago, an increased risk of
meningitis was associated with a particular device design (positioner) of which the manufacturer
ultimately recalled. Children appeared to be particularly affected: of the 52 cases originally reported by
the FDA, 33 (63%) were under the age of 7. Reefhuis et al.47 conducted a study of 4,264 children
implanted between 1997 and 2002 and found 29 cases of bacterial meningitis in 26 children. The rate of
meningitis amongst children with the positioner was 30 times the incidence in the general population
while children implanted without the positioner were 16 times higher than the general population. Risk
factors included a history of placement of a ventriculoperitoneal cerebrospinal fluid (CSF) shunt, a history
of otitis media prior to implantation, the presence of CSF leaks alone or inner-ear malformations with
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CSF leak, signs of middle-ear inflammation at the time of implantation, and exposure to smoking in the
household.
Since this study, the incidence of meningitis has decreased but likely depends on assurance of
vaccination against Streptococcus pneumonia, evaluation of pre-operative imaging for cochlear anatomy
that may place patient at risk for CSF leak, aggressive treatment of otitis media and surgical techniques
used to seal cochleostomy48. Pneumococcal vaccination schedules now include both pneumococcal
conjugate vaccine (PCV) 13 (e.g. Prevnar 13®) and pneumococcal polysaccharides vaccine (PPSV) 23
(e.g. Pneumovax®23). Specific immunization schedules are published by the Centers for Disease Control
and Prevention49. Although published, many children are still not vaccinated. In one study of a state
maintained registry, only 56% of pediatric cochlear implant recipients were immunized50. Determination
of vaccination status is paramount for the cochlear implant surgeon.
Conclusion
Cochlear implantation surgery has evolved over the past 30 years. Implantation criteria have
expanded where younger candidates have shown improved outcomes. Although the approach has not
change, recent techniques have focused on smaller incisions and hearing preservation methods that
minimize trauma to the cochlear. Complications continue to remain low secondary to careful patient
selection, meticulous surgical technique and diligence in ensuring vaccination status.
Disclosures
The authors report no proprietary or commercial interest in any product or concept discussed in this
article.
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Figure 1: A cochlear implant consists of an external component (microphone and speech processor) and
an internal component (receiver-stimulator and the stimulating electrodes). Reprinted with permission51
Figure 2: Location of the facial recess. Landmarks include the chorda tympani, facial nerve and buttress.
A small diamond burr is utilized to enter the facial recess. Reprinted with permission51
Figure 3: Insertion of the electrode. The electrode array is advanced under direct visualization along a
trajectory tangential to the basal turn of the scala tympani.Reprinted with permission 51
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Fig 1
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Fig 2
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Fig 3