Cochlear Implantation

TITLE: Cochlear Implantation
SOURCE: Grand Rounds Presentation, Department of Otolaryngology
The University of Texas Medical Branch (UTMB Health) <= /span>
DATE: October 29, 2012
RESIDENT PHYSICIAN: Joseph L. Russell , MD
FACULTY PHYSICIAN ADVISOR: Dayton Young, MD
FACULTY PHYSICIAN ADVISOR: Tomoko Makishima, MD, PhD
DISCUSSANT: Dayton Young , MD
SERIES EDITOR: Francis B. Quinn, Jr., MD
ARCHIVIST: Melinda Stoner Quinn, MSICS

"This material was prepared by resident physicians in partial fulfillment of educational requirements established f= or the Postgraduate Training Program of the UTMB Department of Otolaryngology/= Head and Neck Surgery and was not intended for clinical use in its present form.= It was prepared for the purpose of stimulating group discussion in a conference setting. No warranties, either express or implied, are made with respect to= its accuracy, completeness, or timeliness. The material does not necessarily reflect the current or past opinions of members of the UTMB faculty and sho= uld not be used for purposes of diagnosis or treatment without consulting appropriate literature sources and informed professional opinion." =

Introduction

The cochlear implant stands unparalleled in its a= bility to restore one of the human senses. Since the first patients were implanted just over 50 years ago, the cochlear implant has rapidly evolved from single-channel electrodes that provided little more than the awareness of sound in the environment to comp= lex multi-channel electrode arrays coupled with innovative speech processing technology to provide many implanted patients with near normal understandin= g of speech and, in some cases, enjoyment of music.

History of Cochlear Implantation

Although the development of cochlear implants has occurred only over the last 50 years, Alessandro Volta of Italy in 1800 described an experiment on h= imself that gave evidence that electrical stimulation of the ear could produce a s= ense of noise:

“I (Volta) introduced= right into both ears two probes or rods of metal with rounded ends; I linked them= up immediately to the two extremities of the apparatus. At the moment when the circuit was completed in this way, I received a jolt in the head; and a few moments lat= er (the circuit operating continuously without any interruption), I began to f= eel a sound, or rather a noise, in my ears which I cannot define clearly; it wa= s a kind of jerky crackling or bubbling, as though some paste or tenacious matt= er was boiling. This noise conti= nued without stopping and without increasing all the time the circuit was complete… The disagreeable sensation of the jolt in the brain, which I feared might be dangerous, was such that I did not repeat the experiment several times.^1”

Over one hundred and fifty years later in 1957, André Djourno, an otolaryngologist, and Charles Eyriès, an electrophysiologist, both of France, collaborated to implant a coil electro= de into the shredded stump of the cochlear nerve of a patient with bilateral deafness and facial paralysis following multiple cholesteatoma surgeries. <= span style=3D'mso-spacerun:yes'> The patient was able to discriminat= e high versus low frequency sounds, could detect environmental sounds and even a f= ew words, but could not understand speech.

In 1961, inspired by the work of Djourno and Eyriès, William House in Los Angeles= implanted single electrodes into the scala tympani of three profoundly deaf patients. The patients’ hearing results were similar to those of Djourno and Eyriès’ patient; unfortunately all electrodes had to be removed within weeks of implantation due to wound infections. Over the next few years both F. Blair Simmons of Stanford U= niversity and Robin Michelson of the University of California-San Francisco also began implanting patients with single electrode devices.^1,2

During the 1960s there were several objections to= human cochlear implantation by many in the basic science community. Essentially, it was believed to be physically impossible for any meaningful hearing to be achieved based on wh= at was known at the time of the physiology and physics of the ear. Objections cited previous studies = that had shown that there was only a 10 decibel dynamic range for electrical stimulation of the ear, which, when compared to the 120 decibel dynamic ran= ge of the normal ear was felt to be too small to be usable. Additionally, it was thought that = trauma from the electrode insertion would ultimately lead to spiral ganglion cell deterioration; furthermore, if those cells did survive the electrode placem= ent, long term electrical stimulation was posited to ultimately lead to their de= mise. In 1967, Simmons published results= of his studies in cats that showed the electrodes could be inserted into the cochlea without subsequent widespread cochlear degeneration and that long t= erm electrical stimulation of spiral ganglion cells did not lead to any signifi= cant deterioration.¹

In 1972 House developed, in cooperation with the = 3M Corporation, what would become the first FDA-approved single-channel cochle= ar implant. Between 1972 and the mid-1980s, over 1000 patients received this implant. Patients were found post-implantat= ion to have some improvement in speech discrimination, improved voice modulation, = and the ability to hear environmental sounds; however, open set speech discrimination was not obtained.² In 1978, Graeme Clark of Austr= alia implanted the first multi-channel electrode array in a human subject; this patient was able obtain some meaningful open set speech discrimination. Results from subsequent implantees= would eventually lead to the consensus that multi-channel electrode arrays were s= uperior to single-channel electrodes.¹

Over the next two decades cochlear implant develo= pment proceeded steadily with ever-improving results. In 1985, the FDA approved the first multi-channel implant for adults. In 1990, after several successful trials in children, FDA approval w= as given for multi-channel implantation in children as young as 2 years old.2 Ten years later (2000), FDA approv= al was given for cochlear implantation in children as young as 12 months of age, w= here the lower age limit currently stands. Nevertheless, several institutions have been implanting children as young as 6 months of age in clinical trials, with good results.

Current Implant Technology

There are currently three companies that manufact= ure FDA-approved cochlear implants: Advanced Bionics Corporation, Cochlear Corporation, and Med-El Incorporated. The current implant systems produced by these companies at the time = of this writing are the HR90 K, Nucleus 5, and Sonata ti100, respectively. The general design of these systems share common functional components (see slides for figures). Sound is recei= ved by a microphone located on the behind-the-ear (BTE) sound processor; it is processed and coded, then sent via the transcutaneous radiofrequency link to the implanted receiver-stimulator; data are decoded and sent to the multi-electrode array, stimulating spiral ganglion neurons, which then tran= smit the signal via the auditory nerve toward higher processing centers.³

In addition to standard electrode arrays with 12 = to 22 single or paired electrodes, there are several special electrode arrays designed for implantation of partially ossified cochleas, for implantation = of common cavity cochleas, and for electric and acoustic simulation (EAS).

Candidate Selection and Preoperative Evaluation

Adult selection criteria^4,5

Best-aided scores on open-set sentence tests of <50% in the ear to be implanted and <60% in contralateral ear

For Medicare patients, <30% in the ear to be implanted and <40% in = the contralateral ear

Failure with conventional hearing aids
No evidence of central auditory lesions or lack of auditory nerve
No evidence of contraindications to surgery in general

Pediatric selection criteria^4,5

In children <2 year old, determined by lack of auditory milestones
In children ≥2 years old, scores of <30% on single-syllable word tests

Otologic assessment

The otologic assessment begins with a complete hi= story and physical examination. The history should include the onset, etiology (if known), and progression of hearing loss, the patient’s experience with amplification, history of meningitis (if applicable), the number of past and recent ear infections, a= nd previous otologic surgeries. = The examination should include a diligent search for signs of active otitis med= ia or externa, perforations, and the presence of tympanostomy tubes, as these conditions must be remedied prior to implantation.^4,5=

Audiologic assessment^4-7

For adults, the audiologic assessment includes un= aided and aided thresholds for pure tones and the Minimum Speech Test Battery (MSTB). The MSTB is used at m= any cochlear implant centers to assess preoperative and postoperative hearing performance. It consists of s= ets of compact disc recordings and includes the Consonant-Nucleus-Consonant (CNC) Monosyllable Word Test, the Arizona Biomedical (AzBio) Sentences (in quiet = and in noise), and the Bamford-Kowal-Bench Sentences in Noise (BKB-SIN) Test. Hearing-in-noise (HINT) sentences = were previously part of the MSTB but have fallen out of favor due to a ceiling effect for post-implant performance.

For children, the audiologic assessment is freque= ntly more difficult due the fact that the patients are frequently too young to participate in speech discrimination tests and benefit most from being implanted before the age of normal speech development. Therefore, assessments rely heavil= y on electrophysiologic testing and behavioral observations. Both auditory brainstem response (= ABR) and otoacoustic emission (OAE) testing should be performed. Implant candidates typically have = no response at the limits of the testing equipment. If a patient is found to have audi= tory neuropathy as indicated by absent ABR response but present OAEs, it is imperative that these patients undergo MRI prior to implantation, as 16% of these patients will have an absent or hypoplastic auditory nerve. Speech perception tests include the Meaningful Auditory Integration Scale (MAIS), Early Speech Perception (ESP) Test, and the Lexical Neighborhood Test (LNT). The MAIS is a questionnaire that is completed by family members of children too young to participate in other speech perception tests. In t= he ESP Test a word is spoken without visual cues and the patient selects the corre= ct object or picture that relates to the stimulus. The LNT consists of 50 monosyllabic words ranging from “easy” (high frequency, few lexical neighbor= s) to “hard” (low frequency, many lexical neighbors).

Imaging: CT versus MRI^8-10

High-resolution computed tomography (HRCT) has traditionally been the gold-standard imaging modality for preoperative plan= ning for cochlear implantation. It provides superior visualization of the bony structure of the otic capsule a= nd the course of the facial nerve; however, its weaknesses are that it can miss cochlear fibrosis, retrocochlear pathology, central nervous system abnormalities, and cochlear nerve hypoplasia/aplasia. Magnetic resonance imaging (MRI) i= s more effective at identifying cochlear fibrosis and is able to identify the pres= ence or absence of cochlear fibrosis. The weaknesses of MRI include inferior visualization of bony anatomy, particularly of the fallopian canal; inability to detect the presence of the round window, oval window, or an enlarged vestibular aqueduct; and it often requires anesthesia for young patients.&nb= sp; In recent retrospective studies, MRI has been shown to be both more sensitive and specific than CT in identifying inner ear abnormalities that affect surgical planning; in fact, MRI is now the preferred imaging modalit= y in some centers. However, it sho= uld be noted that HRCT is still advocated at these centers in cases of malformed external canals, semicircular canals, or vestibule due to the high incidenc= e of an anomalous facial nerve in these patients.

Vaccination¹⁴

Children with cochlear implants are at higher ris= k for meningitis, although the overall rate is low (<0.6%). Streptococcus pneumoniae has been the most common organism isolated in children with cochlear implants who developed meningitis. Therefore, the current vaccine recommendations are as follows:

· Patients <2 years old

Prevnar (7-valent) only

· Patients 2-5 years old

Prevnar and Pneumovax (23= -valent)

· Patients >5 years old

Pneumovax only

· Additionally, all patients <5 year old sh= ould receive the Hib vaccine

· Vaccination should be completed at least 2 w= eeks prior to surgery

Surgical Approach^11-13 (see slides for images)<= /p>

The standard procedure for cochlear implantation = is a transmastoid facial recess approach to the round window and basal turn of t= he cochlea. Continuous facial ne= rve monitoring is used. The skin = is marked with a dummy sound processor and transmitter. Methylene blue can be injected thr= ough the skin to periosteum with an 18 gauge needle to mark position of the receiver-stimulator package. = The incision is a modification of the standard post-auricular incision with a posterosuperior extension to provide exposure to seat the receiver-stimulat= or. When planning the incision and pla= cement of the receiver-stimulator package, there should be 1 cm between the skin incision and the periosteal incision and 1 cm between the periosteal incisi= on and the edge of the receiver-stimulator.&n= bsp; The patient is prepped and draped in standard fashion for a mastoidectomy. When making the incision, it is carried to the level of the temporalis fascia superiorly an= d to the level of the mastoid periosteum inferiorly. Skin flaps are developed anteriorl= y to the external auditory canal and posteriorly to allow for placement of receiver-stimulator. Next, a musculoperiosteal flap is created= by incising the temporalis fascia, muscle, and periosteum vertically, then rai= sing this flap anteriorly to the bony EAC, revealing the spine of Henle. A similar flap is raised posterior= ly to create a pocket for the receiver-stimulator. A cortical mastoidectomy is then performed; however, the superior and posterior margins are not saucerized to aid in containment of the excess electrode within the mastoid cavity.

The facial recess is then developed. The short process of the incus is = used as a pointer to define the level at which to open the facial recess; drilling = too medial will damage the facial nerve, while drilling too lateral will lead to perforation of the canal wall. The incus buttress is kept thin to optimize exposure. The round window ni= che usually is visible just inferior to the stapedius tendon; a small diamond b= urr is used to remove the lip of the niche to expose the round window membrane.=

The next step is either a cochleostomy followed by electrode insertion or direct round window insertion (RWI) of the electrode. RWI was the origin= al means of electrode insertion; however, as large arrays with more electrodes= were developed, the RWI technique was abandoned due to buckling of the electrodes upon insertion which led to severe cochlear trauma. However, modern electrodes are much thinner and less prone to buckling, leading to a recent reemergence of RWI.= A recent study showed no differenc= e in hearing outcomes or complications when comparing RWI to cochleostomy.

Regardless of which technique is used to place the electrode, the electrode is placed in the scala tympani with every effort m= ade to minimize the trauma of insertion. Once the electrode array is in place, monopolar cautery should no lo= nger be used due to the theoretical risk of injury to the cochlea, the electrode array, or the receiver-stimulator. The cochleostomy or round window is sealed with muscle or fascia.

Next, the receiver-stimulator is implanted. A well is drilled in the calvarium= to accommodate the receiver-stimulator. It is important during this step to avoid dural compromise, especial= ly in children, who have much thinner bone.&n= bsp; Many surgeons advocate securing the implant with sutures to the calvarium, while others do not. The excess electrode is left coiled in the mastoid to allow for the 1.7 cm incr= ease in the distance between the electrode array and receiver stimulator that oc= curs with growth between infancy and adulthood.= The musculoperiosteal flap is closed, followed by deep dermal suture= s to close the skin flaps; finally the skin is closed. A standard mastoid dressing is pla= ced and removed on the first post-operative day.

Complications¹⁴

The most common complications of cochlear implant= ation are related to the surgical wound—occurring in about 4% of cases. This includes infection, flap necr= osis, and extrusion of the receiver-stimulator.&= nbsp; These complications can be avoided primarily by careful placement of= the incisions, as described above, and by keeping the skin flap over the receiver-stimulator 6-7 mm thick.

Other complications include acute otitis media (2= %), damaged or misplaced electrodes (1%), persistent cerebrospinal fluid leak (= 1%), and facial nerve paresis (0.5%).

Perhaps the most feared complication, meningitis, occurs rarely (<0.6%). In = 2003, a study in the New England Journal of Medicine showed that the incidence of streptococcal meningitis in children with cochlear implants was >30 times the incidence in age-matched controls.&nbs= p; However, the study had several limitations—11.5% of children w= ith implants in the study had a prior history of meningitis, which predisposes a patient to future bouts of meningitis, and 8.5% of children with implants in the study had labyrinthine dysplasia, which also predisposes a patient to meningitis. Nevertheless, lat= er studies showed that cochlear implants do increase the risk of meningitis in rats and this risk was mitigated by the Pneumovax vaccine. This has led to the implementation= of Streptococcal and Haemophilus vaccination as a requirement prior to cochlear implantation. Of note, the ma= jority of the cases of meningitis prior to 2002 were linked to a positioner device manufactured by Advanced Bionics that has since been discontinued.

Revision Cochlear Implantation

The rates of revision cochlear implantation are 5= .4 to 7% in adults and 8 to 12% in children.&nbs= p; The primary reasons for revision are hard failure of the device (46%= ), medical-surgical related complications (wound complication, malposition, cholesteatoma formation, 37%), and soft failure (device begins to not funct= ion well for the user even though diagnostic tests on the device are normal, 15%). In general, patients pe= rform as well after reimplantation as their best performance prior to revision.

Outcomes^15-17

Measuring outcomes of cochlear implantation is challenging for several reasons. First and foremost, the benefits of cochlear implantation vary widely across individuals, making the average result not highly representative of = what an individual achieves. Study methodology and outcome metrics vary considerably, and most studies are relatively small due to rapid changes in implant technology.

Speech Perception<= /p>

For adults, after 6 months of implantation open-s= et word test scores typically range from 30 to 60%, though scores as high as 7= 5% are being achieved with the most recent speech processing strategies. Words-in-sentence testing scores a= re typically >75%.

For children, >75% achieve substantial open-set speech recognition after 3 years of implant use. Implanted patients have, on average, language learning rates that match normal-hearing peers. Greater than 50% who use early edu= cation intervention exhibit age appropriate vocabulary scores by kindergarten. Five years post-implantation, impl= ant users have a 75% rate of assignment to mainstream classrooms, compared to 1= 2% of similar-hearing peers with hearing aids.

Cost Outcomes

The cost utility of cochlear implantation is high= ly favorable in adults, even more so than for a knee replacement or heart tran= splant. The cost benefit is highly favorab= le in children, with estimated net savings of $30,000 to $200,000 per child if implanted at age 3 years.

Factors affecting implant performance

The age at implantation is a crucial factor impac= ting implant performance—the earlier, the better. Children should be implanted by ag= e 3 years, preferably by age 2 years, to maximize the benefits of implantation. In cases of acq= uired profound hearing loss, the shorter the duration of the loss, the better the implant will perform. The dur= ation of implant use affects performance—in fact, maximum benefit is not se= en until at least 3-5 years post-implantation. For patients who derived some bene= fit from amplification and attempts were made to facilitate communication with = the aids, these patients will perform better with implants than their peers who= did not have the early amplification/linguistic experience. If other disabilities are present = in a patient, especially cognitive disabilities, implant performance will be adversely affected; however, these patients can still derive benefit from implantation. Of course, stro= ng family support results in improved implant performance while lack of such support has the opposite effect.

Recent Advances¹⁵

Bilateral cochlear implantation

A majority of cochlear implant centers are curren= tly implanting the majority of children bilaterally. Large multi-center long-term investigations are pending, but small studies have shown improved sound loc= alization and understanding of speech in noise with bilateral implantation. Other potential advantages include= more natural hearing, reduced listening effort, and improved quality of life. Disadvantages include the cost of = an additional implant per patient and the potential exclusion of the patients = from future innovations, such as hair cell regeneration.

Electric and Acoustic Stimulation

This is an option for patients with residual low frequency hearing. A shortened electrode is inserted as atraumatically as possible into the cochlea to preserve this residual low frequency hearing; a cochlear implant and hearing aid are then used on the same side. A subgroup of 11 patients at the University of Iowa improved their average CNC word scores from 32% correct with binaural hearing aids to 75% correct with one implant and binaural hearing aids at 9 months post-implant= . Other benefits include improved he= aring in noise and better appreciation of music compared to a standard cochlear implant.

Future Directions

Totally implantable cochlear implants¹⁸

In 2008, Briggs reported the results of three adu= lt subjects implanted with a modified Cochlear Corporation receiver-stimulator that contained an internal microphone and rechargeable battery. All three had improved hearing res= ults at 12 months; however, implantees performed twice as well on CNC word scores when using an external (regular) processor compared to the fully implanted = mode. Additionally, swallowing and breat= hing were also noted to interfere with hearing when using the fully implanted mode.

Robot-assisted/image-guided cochlear implantation^{19, 20}

This innovative research is being conducted by gr= oups at Hanover Medical School in Germany and at Vanderbilt University in the USA. Percutaneous postauricul= ar transmastoid access to the basal turn of the cochlea is made with either an image-guided frame through which a powered drill is guided (USA), or with an image-guided robot-controlled drill (Germany). Cadaver studies with 6 to 10 speci= mens have been promising, showing no facial nerve injuries with a total of two planned stapes injuries and three planned chorda tympani sacrifices. While this research is in its infa= ncy, it stands to become the foundation for minimally-invasive cochlear implants= in the future.

Discussant’s Remarks

Dr. Dayton Young’s remarks on Dr. Russell’s presentation on Cochlear Implantation, October 29, 2012

There’s a lot of confusion in what people th= ink about cochlear implantation, in particular regarding what the hearing level should be marking the patient as an implant candidate. The bottom line is that’s not really the way it’s done. It’s really a clinical evaluation of the patient and what he c= an hear and what he can’t hear, usually done by the audiologist. The driver for this measurement is= the speech discrimination score, which is tough in children. You can’t figure out what a child’s speech discrimination is before they learn how to speak. The old FTA criteria for adults wa= s 70 db, but when the patient drops below a certain speech discrimination score, they essentially can’t communicate anymore and that’s when they become an implant candidate. = The standard we use is when it’s less than 50% word discrimination or sentence discrimination determined by a battery of tests. You basically stick a hearing aid = on a patient, you optimize that hearing aid to get the best possible hearing and then you run word discrimination or speech discrimination testing on them.<= span style=3D'mso-spacerun:yes'> If they do less than 50% with the hearing aid, that’s basically the criterion for implantation.

In children and in infants that’s harder, be= cause how can you assess speech discrimination in the very young? So, what we do is use surrogate markers. With a newborn with profound hearing loss, you measure that with an ABR. When they get to be about two year= s old, we look for a severe hearing loss, about 70 db. All the while that you’re te= sting them, you’re putting hearing aids on these kids. And you watch how they progress. This is done by the parents, the audiologist, and the speech pathologist as well. Sometimes it boils down to a paren= tal questionnaire, and what the audiologist and the speech pathologist think ab= out this child and how it’s progressing.=

A big thing on the horizon for cochlear implants i= s that for a long time it’s been an adage that you don’t implant a pat= ient with unilateral deafness. That’s actually changing and they’re doing it now in Europe. Whereas we always tho= ught that it would be like hearing normal English in one ear and Chinese in the other, but that’s really not so, and the patients actually do better = with an implant in the deaf ear. <= /p>

To buy the implant from the manufacturer is $30,000. The surgery itself m= ay run another $100,000. The post-implantation therapy is also expensive, but when you compare these cos= ts to the amount of money actually saved in the education of what would otherw= ise be a deaf child, as well as their employability and life-long productivity = as a hearing adult, ultimate cost of implantation tends to level out and becomes less daunting. Commercial insurance companies will cover these costs, and although Medicare is our worst payer, and believe it or not, Medicaid is our best.

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