Cochlear Implantation

INTRODUCTION

llowing the deaf to hear has been a goal of humanity since antiquity. The Bible reports Jesus healing a deaf man by placing his fingers in the man’s ear canals (1). Some 2000 years later modern otolaryngologists have

been able to reproduce this miracle via the development of cochlear implants. These surgically-implanted devices, with externally worn hardware, convert sound energy to electrical energy which is used to stimulate the brain allowing sound perception. Since Food and Drug Admin-istration (FDA) approval in the 1970’s and subsequent approval for children in the 1980’s, cochlear implants have been used to restore hearing in children born deaf as well as adults who lose their hearing. Approximately 40,000 individuals world wide have cochlear implant, and approximately 8,000 implants per year are performed in the United States. It is predicted that upwards of 750,000 Americans could benefit from this technology (2, 3). This disparity between use and need is due to a number of factors including lack of awareness of health care providers to this exciting technology, cost concerns related to technology that has a large front end cost, and moral debate between the hearing and deaf cultures. In this paper, we will provide an update as to the current standard of care regarding cochlear implants. In addi-tion, we will discuss the issues stalling their universal acceptance.

DISCUSSION

Hearing Loss

Typically, human beings perceive sound via vibra-tion of the eardrum which initiates a chain reacvibra-tion ending with higher cortical sound perception. The chain reaction is perpetuated as the vibrations are transmitted and amplified by the bones of the middle ear-the malleus, incus, and stapes (Figure 1). The last bone in the chain causes vibrations in the oval window

MAKING THE DEAF HEAR

Cochlear Implantation

A

Goal: Objectives:

David S. Haynes, M.D, is a member of the Otolaryngo-logy Head and Neck Surgery Associates at Saint Thomas Hospital. He received his M.D. at the University of Tennessee College of Medi-cine, and completed his train-ing in Otolaryngology, Head and Neck Surgery at Van-derbilt University Medical Center. He completed a fel-lowship in Neurotology at the Otology Group/EAR Founda-tion in Nashville. Dr. Haynes is currently an associate pro-fessor, Department of Oto-laryngology at Vanderbilt University, where he is the director of the Division of Otology and Neurotology. He is also an associate pro-fessor in the Department of Hearing and Speech Sciences at The Vanderbilt Bill Wilkerson Center for Com-municative Disorders. Dr. Haynes is currently a board member of both the Saint Thomas Neurosciences Insti-tute and the EAR Foundation at Baptist Hospital. His inter-ests include all aspects of ear surgery including cochlear implant surgery and acoustic neuroma surgery.

Robert F. Labadie, MD, PhD is board certified in Otolaryn-gology — Head and Neck Surgery and practices as a member of The Otology Group with offices at St. Thomas Hospital, Baptist Hospital, and Vanderbilt University Medical Center. He received his B.S. in Mechanical Engineering from the University of Notre Dame, his M.D. from the University of Pittsburgh, and his Ph.D. in Bioengineering from the University of Pittsburgh. His residency training was com-pleted at the University of North Carolina at Chapel Hill. He is a member o the American Academy of Otolaryngology-Head and Neck Surgery, The Nashville Academy of Otolaryngology, the Tennessee Medical Asso-ciation, and the American Medical Association. He is the Principal Investigator of an NIH funded grant investi-gating the use of image-guid-ance systems in otologic sur-gery. He has been an invited speaker nationally and inter-nationally presenting his research and lecturing on cochlear implantation. External Ear Middle Ear _Inner Ear Eardrum Incus Stapes Cochlea Eustachian Tube FIGURE 1 Malleus

of the cochlea leading to fluid waves within the inner ear. These fluid waves distort cilia laden cells, hair cells, which launches a molecular cascade leading to depolar-ization of the auditory nerve and stimulation of Brocha’s area within the cerebral hemisphere. (Figure 1)

There are many reasons why people lose this ability to hear. Otolaryngologists categorize these into two broad categories—conductive and sensorineural hearing loss. Conductive hearing loss occurs when a mechanical problems impedes the progression of the sound wave from the outer ear to the inner ear. Plugging one’s ears with earplugs produces a temporary conductive hearing loss. Other causes of conductive hearing loss are wax impaction, middle ear infections, ear drum perforations, and diseases affecting the middle ear bones (i.e. otoscle-rosis). Conductive hearing loss is treated by removing or correcting the obstruction. Sensorineural hearing loss occurs when damage to the hair cells prevents transmission of the signal to the auditory nerve and brain. The most common cause of sensorineural hearing loss is presbycusis, or age-related hair cell death. This multifactorial process is most-likely genetically mediated and exacerbated by repeat exposure to loud noises. Other causes of sensorineural haring loss are drug toxicity (i.e. gentamycin), infections (i.e. meningitis), and autoim-mune processes. At early stages of sensorineural hearing loss, amplifying sound with a hearing aid may overcome the depleted hair cell population and allow functional hearing. Hearing aids are typically used to treat mild to moderate levels sensorineural hearing loss. (Table I) For those individuals with advanced sensorineural hearing loss (severe to profound), cochlear implants are the treat-ment of choice.

Cochlear Implants

A cochlear implant overcomes sensorineural hear-ing loss by bypasshear-ing damaged hair cells and directly stimulating the auditory nerve. To accomplish this, an electrode array is surgically threaded into the cochlea placing electrical contacts in close proximity to the

auditory nerve (Figure 2). This electrode array is directly coupled to a subcutaneously placed internal receiver which is magnetically linked to an external receiver/ processor. The external receiver processor converts sound energy to electrical energy and sends the appro-priate signal across the skin and via the electrode array to the inner ear (Figure 3). A cochlear implant differs from a hearing aid in that it is not dependent upon the survival of inner hair cells within the cochlea.

Candidacy

At present, individuals with profound to severe sensorineural hearing loss are candidates for cochlear implantation (Table I). As mentioned earlier, the two largest groups of candidates are children born deaf and adults who lose their hearing.

For children born deaf, evaluation begins shortly after birth during newborn hearing screening. While not mandated in the state of Tennessee, 90% of Tennessean newborns have hearing screens before they leave the hospital (4) Such screening helps in identify-ing the approximately 5 per 1000 children born deaf each year. These children then undergo thorough med-ical evaluation in attempts to identify the etiology of hearing loss. Increasingly, genetic screening plays a role as mutations in the gene encoding a potassium channel protein, Connexin 26, may be responsible for up to 50% of autosomal recessive, non-syndromic, congenital hear-ing impairment (5). Followhear-ing medical clearance, a 6 month trial of hearing aid use is undertaken. During TABLE 1

LEVELS OF HEARING LOSS

The externally worn, behind-the-ear processor transmits data to the surgi-cally implanted electrode array via an indcuctance coil. A magnet couples the coil to the subcutaneous, surgically-implanted, internal receiver.

FIGURE 3 FIGURE 2

Cable Connecting Electrode Array to Subcutaneously

Implanted Receiver

Cochlear Implant Electrode Array

the hearing aid trial, continual assessment of hearing acuity occurs via a number of age-dependent tests. If best aided thresholds remain in the severe-to-profound range, cochlear implantation is recommended. For chil-dren who qualify, FDA-guidelines allow implantation for children as young as one year of age.

Severe-to-profoundly hearing impaired adults are further stratified to either pre-lingual deafness, e.g. an individual who has never heard, and post lingual deaf-ness, e.g. an individual who once heard but has lost that ability. Pre-lingually-deafened adults, i.e. former Miss American Heather Whitsone, McCullum are a distinct minority. While they can undergo cochlear implantation, expected outcome is limited to acquisition of environ-mental sounds (i.e. hearing alarms). Ms. Whitestone’s continuing story is detailed on-line (6). More typically, adult candidates are post-lingually deafened. An example of a post-lingually deafened adult who underwent coch-lear implantation is the conservative talk-show host Rush Limbaugh. Such candidates undergo a thorough assessment to identify possible causes of hearing loss. Age-related hearing loss, presbycusis, remains the lead-ing diagnosis. While much effort is currently belead-ing expended in attempts to slow or even reverse the hair cell death characteristic of presbycusis, clinical applica-tion is years away. Adults undergo intensive hearing tests and, under best-aided conditions, must have severe-to-profound hearing loss and/or sentence identification below 40% to qualify for a cochlear implant. As a rule-of-thumb, patients who can no longer communicate over the telephone are most likely candidates for coch-lear implantation. There are few contraindications for cochlear implantation in adults. Included would be severe comorbidity preventing any surgical intervention. Advan-ced age is not a contraindication for implantation (7).

Surgery

Cochelar implantation is usually performed under general anesthesia in an outpatient surgical facility. The operation lasts 3-4 hours in duration. The technique involves a post-auricular incision (Figure 4) following which a mastoidectomy is performed. Within the mas-toid, vital landmarks are identified, most notably the facial nerve which supplies innervation to the muscles of facial expression. Using the nerve as a landmark, a recess is opened anterior to the facial nerve and the middle ear is entered from behind (Figure 5). The cochlea is identified and a small hole made into it following which the electrode array is inserted. The associated internal receiver is secured in the temporo-parietal skull and the incision closed. Complications are extremely rare and include facial nerve injury, pain, ver-tigo, CSF leak, flap necrosis, device failure, device migration out of the cochlea, cholesteatoma formation,

and eardrum perforation. Because of recent concerns regarding recurrent meningitis in cochlear implant patients, Streptococcus pneumoniae vaccination to prior to surgery is recommended as per CDC guidelines (8). After implantation, care must be taken when performing magnetic resonance imaging (MRI) studies. Of the three FDA approved manufacturers, one (MED-EL Corpora-tion; Innsbruch, Austria) is FDA approved for use in 0.2-Telsa MRI units with ongoing studies aimed at high-er strength MRI units. Anothhigh-er (Cochlear Coporation; Melborne, Australia) recommends an in-office surgical procedure to remove the coupling magnet prior to MRI. The coupling magnet is then reimplanted after scanning is completed.

Device activation and outcome

Approximately 3 weeks following surgery, allowing time for post-operative edema to subside, the externally-worn receiver is magnetically coupled to the surgically-implanted receiver. Data is transmitted from the external receiver to the internal receiver via a coil of wire and electrical principles of inductance. At initial hook-up, adjustments are made to each electrode but stimulating each electrode individually and determining the most comfortable level of sound intensity for the patient. FIGURE 5

Surgical Incision. Patient is shown in surgical postion with skin inci-sion shown as dotted line behind ear and magnified mastoidectomy shown as inset.

DRILL SITE FOR ENTRY INTO COCHLEA FIGURE 4

This mapping process is analogous to tuning each indi-vidual instrument of an orchestra. Following this, all electrodes are activated. Analogously, the orchestra begins to play.

For children, this may be the first time recognizable sound is presented to their brain and a range of emotions is common. This represents a new birthday—a hearing birthday—following which intensive tutoring is neces-sary to teach children to hear. With such tutoring, speech recognition without visual clues approaches 90%

and children enter mainstream education (9, 10). For adults, speech understanding exceeds 85% and many adults quickly reacclimatize to the hearing world, some-times communicating via telephone immediately after device activation (11).

Cost-benefit analysis

With the continued escalation of health care costs much has been written about the costs of cochlear implants. The cost (including surgery, hardware, and rehabilitation) can be upwards of $50,000per patient. Despite this large up-front investment, cost-benefit analysis has shown that the devices are extremely eco-nomical. Using validated techniques analyzing cost per year of use, a study out of Johns Hopkins University reported that cochlear implants are much more cost-effective than other standard surgical implant proce-dures including implantable defibrillator and knee

replacement (12). Another approach to appreciating the cost-savings is comparison of educating a deaf child versus educating a main-streamed, cochlear-implanted child. For a deaf child, the cost of primary education (K-12) approaches one million dollars—roughly 50 times the cost of educating a normal hearing child. Despite this markedly increased cost, 44% of deaf chil-dren fail to graduate from high school (compared to 19% of normal hearing individuals) and only 5% of deaf individuals graduate from college (compared to 13% of normal hearing individuals) (13).

The above cited and extensive other studies sub-stantiate that both for children and adults, cochlear implantation is economically justifiable. Debate contin-ues, however, as the upfront cost is being born by the health care community and the long-term savings are benefiting society at large. There are models of govern-ment support of cochlear implantation which have been successful in reducing the high cost of deaf education (14). Such programs, however, have met with resistance from the deaf community.

Deaf Culture

As cochlear implants threaten the very existence of their community, many deaf individuals and parents of deaf children oppose the use of cochlear implants. Some, most notably the National Association of the Deaf feel that hearing impairment is not a disability which needs to be fixed (15). Some also believe that their unique language—American Sign Language— assures status as a separate culture with appropriate respect for autonomy. For the interested reader, this ongoing debate between the hearing and non-hearing worlds is the subject of an Academy Award Nominated Public Broadcasting Service documentary entitled Sound and Fury (16). While the documentary does not offer any resolution to the debate, it does provide for under-standing of each point-of-view.

CONCLUSION

Cochlear implants are standard of care for hearing impaired individuals who met FDA approved criteria based upon results of formal hearing tests. They are not experimental. They have a high incidence of effective-ness in restoring speech understanding. They have an extremely low incidence of complications. Most insur-ance carriers cover their costs. This technology, while expensive, is cost-effective. The impact of cochlear implants on deaf culture remains to be seen and is the topic of ongoing debate.

“

”

For adults,

speech understanding

exceeds

85%

and

many adults quickly

reacclimatize to the

hearing world,

sometimes

communi-cating via telephone

immediately after

CME credit is available for this activity until the expiration date of Jan. 31, 2005. DISCLOSURE STATEMENT:

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15. http://www.nad.org/infocenter/newsroom/positions/ CochlearImplants.html