Alex D. Sweeney, M.D.

Disclaimer: The information contained within the Grand Rounds Archive is intended for use by physicians and other health care professionals. These documents were prepared by resident physicians for presentation and discussion at a conference held at Baylor College of Medicine in Houston, Texas. No warranties, either express or implied, are made with respect to accuracy, completeness or timeliness of this material. This material does not necessarily reflect the current or past opinions of the faculty of Baylor College of Medicine and should not be used as a basis for diagnosis or treatment, and is not a substitute for professional consultation and/or peer-reviewed medical literature. 

The whole of the ear is a very elegant structure in a number of ways. One in particular is how structure reinforces function. Take the pinna – angles and curves play an integral function in sound localization. In order to abstract sounds from the outside world into the external auditory canal and then from the external ear into the middle ear and then to the inner ear, through which we hear and understand speech and noise.

Similar to how there’s more than meets the eye when looking at the auricle, looked at a picture of the developing embryo doesn’t do justice to the intricacies of the development of a single structure. Even looking at the early ear doesn’t show how the inner, middle and outer divisions, each with their own unique contribution to the function of the ear, are developing simultaneously and separately. Focusing on the inner ear and cochlea, from the otic placode, we see the formation of two compartments – the vestibular and cochlear division at about week 3. Then you have progressive elongation of the cochlea and thickening around the endolymphatic duct, which leads to the formation of hair cells (inner and outer). Ultimately a division occurs, which results in two separate fluid filled structures whose integrity is important for cochlear physiology and the integrity of the Organ of Corti.

The fluid compartments of the cochlea differ based on their composition. Their content is actively maintained in a way that helps maintain potential energy and allows for the creation of an action potential in hair cells, which occurs when sound vibrates the tympanic membrane and is amplified and conducted into the scala vestibuli via the oval window. With the help of outer hair cell mechanics, the frequency of vibration is optimized for a particular location along the basilar membrane (tonotopic map) in the cochlea.

Within the cochlea, there are 3 principal fluid filled spaces (vestibuli, tympani and media). The first two are filled with perilymph (high Na, low K) and are contiguous. The latter is filled with endolymph (intracellular electrolytes – high K, low Na). This gradient is proposed to be maintained by an active passage of electrolytes and is important for the endocochlear potential which helps to propagate action potential (signal transduction). With the movement in the basilar membrane, depolarization occurs (Ca++ in and K+ out), which promotes the release of glutamate into the synapse between the hair cell and a nerve fiber, which leads to an action potential that is then propagated.

Over time, there is some degree of wear and tear on the system, which is represented by the trends of decreased hearing sensitivity) increased thresholds over time with a predisposition for the basilar cochlea), and specificity (decreasing speech discrimination) shown in this graph of the Framingham cohort of patients.

A system was developed by Schuknecht et al in 1993 to explain age-related changes in hearing. Four types of loss were described, and, in some cases, thresholds at which hearing loss were experienced. Pathologic analysis done since that time has shown that things are not so clear cut: most people demonstrated changes from all these types of hearing loss.

But where does the damage come from? Genetics aside, there are a variety of environmental sources that may contribute: microvascular disease causing perfusion issues; noise exposure; drug effects; cigarette smoking; and even diet, which was studied in a recent article from Nigeria, which found significantly different levels of folate in patients that correlated to their degree of age?related hearing loss.

The geriatric population is the fastest growing segment of the population of all industrialized nations, including the United States. As life expectancy increases and healthcare costs rise, the medical profession will be challenged to provide cost-effective quality care. Aging affects every system and organ in the body; it does not spare the auditory or vestibular systems but instead causes presbycusis and presbyastasis, respectively. Compounding these problems, the elderly suffer concomitant age-related declines in the immune, musculoskeletal, visual, proprioceptive, cardiovascular, and central nervous systems. As otolaryngologists, our care can impact our patients' longevity and quality of life. Between 25 and 50% of the elderly suffer sufficient hearing loss to degrade their quality of life. Loss of hearing negatively affects social interaction, leading to progressive isolation and withdrawal. Although hearing loss in the geriatric population is often due to presbycusis, other causes should always be sought.

Age may be nothing but a number, but it seems to have significance in terms physiologic function. Time leads to “wear and tear” and hearing loss is frequently seen. The over-80s are the fastest growing demographic in this country. Cochlear dysfunction is common in this age group. There are psychosocial ramifications for hearing loss that seem to be increasingly experienced by the elderly.

Our patient was an 88-year-old man who presented to VA Clinic with bilateral hearing loss. He worked as a cook in the Navy and reported loud noise exposure and stated that, since that time, he had not been able to hear as well, and that, over the past six or so years, he began to feel as though his hearing loss was advancing. He did not report any acute changes or any associated syndromes with that hearing loss. His past medical history was significant for coronary artery disease, for which he had a bypass graft in 2005; hypertension; and prostate hypertrophy. Social history was significant for alcohol and tobacco use, which he quit in the 1960s and no illicit drug use was reported. His otologic exam showed no external ear deformity. Bilateral external ear canals appeared to be clear and the tympanic membranes showed no evidence of effusion, perforation, or retraction. He had significantly increased pure tone threshold in both ears that were fairly symmetric and additionally had extremely poor speech discrimination, as you can see from his audiogram. In the course of his workup, he received a CT of the temporal bone, which was not significant for any otosclerotic pathology, but did show that there was a well formed cochlea and internal auditory canal. 

So after leaving his room, I remember asking myself if the 80-year-old patient would benefit from a cochlear implant, as he is certainly not benefiting from his hearing aids. Additionally, would the benefit the patient gained from the implant be worth the cost of the cochlear implant. Also, if we did an implant, which side would it placed on? We also needed to consider if the elderly patient would tolerate such a surgery.

There is an abundance of key sources of information concerning cochlear implants. Sometimes referred to as the “bionic ear,” there were 60,000 implants done in the 20 years preceding the publication of the paper by Gates et al in 2003. Since that time, well over 100,000 have been done, with a fairly even distribution between the number of adults and children implanted.

A history of cochlear implants is very interesting. One might argue that their history begins with Count Alessandro Volta, an Italian physicist, who was the first to stimulate the auditory system electrically by connecting a battery of 30 to 40 couples (approximately 50V) to two metal rods that were inserted into his ears. When the circuits were completed, he received the sensation of “une secousse dans la tete” (a boom within the head), followed by a sound similar to that of boiling of thick soup. Incidentally, Volta was the inventor of the battery. 

The contemporary history of the cochlear implant really begins with Djourno and Eyries in 1967, who were both otolaryngologists. A patient had had multiple bilateral cholesteatoma surgeries, in which the 7th and 8th cranial nerves were sacrificed. Djourno designed the implant that was placed directly onto the auditory nerve and Eyries performed a facial nerve graft. With the device in place, the patient actually did have some response, and it was reported that the patient could hear noises. Two months later, the device stopped working. They decided to re-implant the device, which they did, but again, about two months later, it ceased to function. At that point, Eyries decided to abandon the project. Djourno continued the work, rejecting any corporate sponsorship and eventually went bankrupt.

In the United States, there were a number of people who were simultaneously working on the idea. One of note was William House, who through his efforts came up with a device that was the first commercially marketed device – through 3M. It was a single channel implant and included a speech processor, first marketed in 1972. Additionally, Robin Michelson started to work on a multichannel implant. Through their efforts, and the NIH support of a multi-institutional study on cochlear implantation, more research money become available to support the development of a viable cochlear implant.

So when we think about cochlear implant, we want something that can capture sound from the outside world, convert it into a signal that can be understood by the inner ear, transmitting it across bone into the inner ear - essentially replicating the function of the ear. Essentially an external microphone picks up speech signals that are transformed into digital impulses that a carrier transmits percutaneously into the internal receiver-stimulator and electrode array. Ultimately, the auditory cortex is stimulated.

In United States, currently there are three different brands that have FDA approval. There is the cochlear implant of the Cochlear Brand from Australia, the MED-EL from Austria, and the Advanced Bionics/Clarion from the US. All of these brands have extensive websites for their products that discuss the pros and cons of their products versus the other products. Each has certain features that are important in determining which implant to choose. Some important considerations include: size and appearance; power of the computer; speed of the computer; reliability; battery life; ease of use; etc. It is important to become familiar with all implants in order to select the one that best fits the patient’s needs. Truly, the most important thing is to know the types and features of each, and like so many things in medicine, providing the facts so that the patient can make the choice.

Brands are marketed on their differences. And there are differences in what is offered. However, at this point, there does not seem to be a good, reliable method for predicting success or failure among brands. Considerations taken into account should be the preferences of the patient, the surgeon and the team.

As far as who is eligible for cochlear implant, there are three groups that are listed, but these can be broken down into two groups, children and adults, further defined by when the deafness occurred, prelingual or postlingual. Selection criteria seem to vary between different centers. The adult population, for these selection criterion, is defined as older than 17. Patients must have profound sensorineural hearing loss and minimal benefit from hearing aid. They do have a present cochlea in internal auditory canal. There should be no medical contraindications and there should be an appropriate expectation of outcomes. One interesting thing, in terms of our patient, is that very few of the centers impose an upper age limit for the placement of a cochlear implant. As long as the patient is healthy, age is not a factor.

As far as the surgical principles of placing a cochlear implant, in general, there are different ways of doing it, but the incision provides exposure to the temporal bone permitting placement of a bone well for the anchoring of the implant. The drilling exposes the facial recess and goes through the recess to the cochlea. The implant goes in the cochlea.

As far the complications from this surgery, there are some and they can be diverse. This study by Cote et al in 2007 looked at about 700 consecutive cochlear implants and some of the complications that arose. It appears that the most common complications from cochlear implants are flap complications relating to the actual incision and the skin flap. These complications can be hematoma, infections, or just vascular compromise of the flap.

As far as costs for cochlear implant, this was a very tricky subject. It is very tough to get a reliable estimate on how much the cochlear implant cost and how much reimbursement there is. These are some of the numbers that I came across. The device itself, in a 2002 study by Garber et al at Johns Hopkins, was said to cost in the neighborhood of $20,000. In 1999, Cheng et al published their experience with the cost of the device estimated at between $14,000 and $29,000. From the audiologic literature, in 2010 there was a discussion of the CPT codes and reimbursement, and it suggested that the CPT code for the cochlear implant (69930) reimbursed $28,906, which was for the device and the non?physician surgical cost. This did not include the surgeon’s fee. In actuality, the total cost of the cochlear implant will be far more than this since it should include not only the cost of the device itself and its implantation, but also the extensive rehabilitation afterwards. The Gates et al article of 2003 suggested that the total cost in children could be approximately $60,000, and in adults, about $40,000. The website of the American Academy of Otolaryngology-Head and Neck Surgery estimates total costs of upwards of $100,000. Suffice it to say that not all costs are met by a third party.

In terms of current trends in cochlear implantation in the adult population, there appears to be, as I mentioned before, no reliable method for predicting success or failure amongst the different brands of cochlear implants. As far as the benefit of the implant as related to the patient’s age, which was the primary issue with our patient, a couple of studies by Budenz et al and Vermeire et al provide some information. There are a lot of different ways to determine speech perception in patients and a lot of different tests that have been devised for this purpose. Unfortunately there are no widely accepted standards for speech perception that are widely used. Budenz et al compared patients by age and their preoperative hearing. It seemed as though there was a pretty significant difference between patients preoperatively and postoperatively, and this was true of those in different age groups. Meanwhile, comparing the two groups that were younger than 70 and older than 70, the only real statistical difference was in the perception of phonemes. Vermeire et al looked at the quality of life using that HHIA measure, and found no statistical difference in post-implant quality of life between the different age groups (grouped as young, middle-aged, and geriatric). So, in these two studies, it was suggested that there were comparable results between the age group of less than and greater than 70 years old. Both suggested that there is benefit to be obtained from a cochlear implant and that the subjective benefit that was reported seemed to be equivalent between patients who are less than or greater than 50 and 70 years old.

For further discussion on this particular topic, we do not really look for other beyond our department where Dr. Williamson et al in a 2009 study looked at patients who were more than 65 years old and patients who were more than 75 years old. These patients demonstrated a significant benefit from their preoperative levels to their postoperative levels, measured at six months and one year postoperatively. This was true of both age groups, demonstrating that age alone does not limit the benefits that can be achieved with implantation.

More recently a paper from the Mayo Clinic, by Carlson et al, looked at patients under 79 years and those over 80 years. There was very little statistical significant different when comparing their pre- and post-operative speech perception between the two groups.

One issue that cannot be discounted is the fact that patients in the elderly population have more surgical risk factors on average than younger patients. This study from the Journal of the American College of Surgeons in 2006 (Turrentine et al) looked at the risk factors over time with increasing age and found that they increased in a linear fashion for postsurgical morbidity and the postsurgical mortality. I should caution you that this paper was written by a vascular surgeon who also included general surgical procedures that may or may not have been elective, as well as thoracic surgical procedures, which one might imagine have more risk factors, might be sicker patients and have a higher chance of causing morbidity and mortality following the surgery.

Looking at the paper from the Mayo Clinic, again a risk of complication does exist. In the group that was 80 years or older, they had a statistically significant higher incidence of TIAs or cardiac arrhythmias and also what was considered to be more minor complications, such as postoperative delirium and urinary retention. Additionally, it seemed that these patients would require hospitalization more often and then, when hospitalized, required admission to more intensive care type setting. But, in general, it appeared that cochlear implantation was well tolerated even in patients whose age exceeded 80 years. The authors suggested that this may be the result of the fact that since cochlear implantation is an elective procedure, more patients who were in good health may have qualified for the surgery. But, the low blood loss during the procedure as well as the relatively short operative time may also be factors. So, age is not a surgical contraindication, even in this elective procedure, but we must stress the importance of an informed consent, pre-operative workup and optimization of comorbidities.

In regards to the timing of hearing loss, the first thing that will discuss is the issue of prelingual versus postlingual deafness. The findings on this subject are fairly consistent across all of the literature. The patients who have deafness that is postlingual in nature seem to do significantly better in tests of speech perception than patients who have prelingual deafness. Also, patients who have prelingual deafness that have received the cochlear implantation as an adult seem to plateau some time around six months to a year, after which they stopped receiving significant benefit from their implants. Patients with postlingual deafness were more likely to continue to receive increased benefit over time. Additionally, for patients with postlingual deafness, implantation is more successful for those who have been deaf for a shorter period of time.

As far as laterality is concerned, this is an interesting and burgeoning area of research. Two studies (Gifford et al in 2007 and Morera et al in 2007) In the Gifford study it appeared that patients with this bimodal implant plus the hearing aid seemed to perform better in speech perception and this also carried over to sound localization. In the Morera study, bimodal hearers did better in tests of comprehension in different noise situations. So, there were advantages to using the contralateral ear even in patients who qualify for the implant and have poor hearing in the bilateral ear with the hearing aid that extended over just implantation alone.

Another interesting topic is the concept of bilateral implantation or binaural implantation. The benefits noted in the previous studies carried over into this. A study by Grantham et al in 2008 looked at 6 recipients of unilateral implants and tested their ability to localize when hearing sounds from different speakers in a horizontal plane. This graph shows the responses of normally hearing individuals, indicated by “normal,” and that which would be expected by random chance is indicated by “chance.” On the right of the graph, two different studies are compared: B = localization in patients with bilateral implants and U = patients with bilateral implants in which one was turned off (studies done under similar conditions as this one). The interesting discussion concerns patients who seem to be able to localize with just one implanted ear vs. why those with both implanted ears and one turned off can’t do this. It appears that those with one implant are used to localizing and use different cues (perhaps they can pick up on very subtle differences caused by head shadowing) to help, while those with two implants are not used to doing this. Also, the ability of the unilateral implanted subjects to localize is only present in the familiar category (speech) implying that there is a baseline familiarity/reference with a voice that can give an idea of where it is coming from.

There is not a lot of research comparing the two modes of having a hearing aid plus a cochlear implant versus two cochlear implants. This study by Cullington et al in 2011 made comparisons and showed that there really was not that much of a difference. This lack of research makes it difficult to state which is better.

This brings us to the question of cost vs. benefit. The concept of QALY (Quality Adjusted Life Year) is as disconcerting as it is interesting. It appears that quality of life may be too subjective to be objectively measurable, although we use it as a consideration in end of life care (stenting in carotid blowout),so although I cringe whenever we use QOL scores to determine whether or not an elective should be done, it doesn’t seem logically inappropriate, and, in some places, these types of score are the guardians of reality. Ultimately, one is seeking procedures that yield a higher quality of life at a lower cost.

It’s tough to judge what the acceptable $/QALY ratio should be, but in the NHS, the maximum amount that seems to be acceptable for spending is about 50,000 euro. Below this number, interventions seem to be favored by the NHS. There is also a lower benchmark below which things are considered to be “good money.” 

An earlier study shows a pretty low $/QALY in cochlear implants in patients older than 17 years of age. A larger, more recent study from the UK stratified $/QALY versus age and duration of profound deafness. As you know, QALY is “quality adjusted life year”: 1 QALY = 1 year of perfect health; 0 = death. If (hypothetically), 0.3 = life with deafness x 1 year, and a cochlear implant costs $10,000 and would increase the QALY to 1, you get the formula: 0.7 QALY = $10,000 or 1 QALY = $14,285. This seems to mean that for $14,285, you can eliminate deafness as a detriment to quality of life. These studies do not appear to take into account the subsequent costs after implantation. This study by the UK cochlear Implant Study Group in 2004 did suggest that with increasing age and decreasing life expectancy, the cost benefit analysis may increase.

So, our patient JH underwent a right-sided cochlear implant with a cochlear device. His postoperative course was complicated by epistaxis, which self-resolved. At his last follow-up with us, his surgical site was healing very well and at his last follow-up with Audiology, he was noted to be pleased with his hearing, mentioned that he was able for the first time in a long while to hear his pastor at church. However, during his last time at church, he did not hear anything. He had forgotten how to turn the processor on and off, which we helped him with, and he was happy once again.

So, in conclusion, the first thing is that the older cohort of the US population is growing rapidly and with that, some degree of higher incidence of vestibular cochlear dysfunction, which makes this subject all the more relevant. As far as the type of cochlear implant that should be used, there is currently no reliable method of predicting success or failure amongst the different brands. Which is ultimately implanted seems to have a lot more to do with patient preference, surgeon preference, and institutional preference. As far as the age is concerned, there does seem to be benefit across all ages with unilateral cochlear implantation. Surgical risk is certainly present, but with proper informed consent, a proper preoperative workup and optimization of medical comorbidities, the surgery is certainly doable. As far as the timing of hearing loss is concerned and how to place in the cochlear implantation, postlingual patients seem to have better speech perception over time and more recently deaf patients or more recently deaf ears seemed to achieve more benefit over the long run. As far as the laterality is concerned, it seems as though the general principle is that having some sort of bilateral stimulation is beneficial over having just unilateral stimulation from an implant. That can be achieved with a hearing aid in the contralateral ear or having a bilateral implantation. In some of the earlier studies comparing the two modes of bilateral stimulation, it does not seem at this point that there is a statistically significant different. As far as the cost benefit analysis is concerned, it is certainly favorable, but the benefit may fall with decreasing life expectancy over time. 

Case Presentation

An 88-year-old man with a history of coronary artery disease, hypertension and benign prostate hypertrophy who presented to the Otolaryngology-Head and Neck surgery clinic at the MED VAMC for evaluation of hearing loss. He stated that his hearing loss began during his time in the military, when he worked as a cook. Since that time, he has reported symmetric hearing loss. Over the past 6 years, he believes that the hearing loss has become significantly worse. Currently, he denies otalgia, otorrhea, vertigo, visual changes, headache, rhinorrhea, epistaxis, facial pain, and paresthesia.

His past medical history was significant for coronary artery disease, hypertension and an unspecified prostate procedure that he believes was related to gland hypertrophy. He denies prior otologic surgery.

Physical exam revealed a man in no acute distress. There were no signs of external trauma. His facial strength and sensation were intact, bilaterally. His extraocular movements were intact. His pupils were equal, round and reactive to light, bilaterally. There were no external ear deformities. His external auditory canals were clear bilaterally, without evidence of a mass or lesion. The tympanic membranes were intact, without evidence of effusion, perforation or retraction. The middle ear appeared well aerated, bilaterally, with no visible mass or lesion. There were no sinonasal masses or lesions visible with anterior rhinoscopy. The nasal mucosa appeared pink and healthy. He was edentulous. There were no masses or lesions visualized or palpated in the oral cavity. The floor of his mouth and the base of his tongue were soft to palpation. There was no palpable lymphadenopathy in his neck.

An audiogram was available for review, which revealed a severe-to-profound, sloping sensorineural hearing loss. Speech discrimination was noted to be 0% and 8% on the right and left, respectively. A computed tomography scan of the temporal bones had no findings suggestive of a causative pathology, and it revealed a well-formed cochlea and internal auditory canal.

A cochlear implant was performed on the patient without complication. His post-operative course was complicated by epistaxis, which was self-resolved. At his most recent Otolaryngology-Head and Neck surgery appointment, his surgical site was noted to be healing well. At his most recent Audiology appointment, he was noted to be pleased with the function of the implant.

Bibliography

American Speech-Language-Hearing Association. Technical report: cochlear implants. ASHA 2003;24:1-35.

ASHA 2010 Medicare Fee Schedule and Hospital Outpatient Prospective Payment for Audiologists. Available at: http://www.asha.org/uploadedFiles/10AudMPFS.pdf. Accessed March 2011.

Basura GJ, Eapen R, Buchman CA. Bilateral cochlear implantation: current concepts, indications, and results. Laryngoscope 2009;119:2395-401.

Bond M, Elston J, Mealing S, Anderson R, Weiner G, Taylor R, Stein K. Systematic reviews of the effectiveness and cost-effectiveness of multi-channel unilateral cochlear implants for adults. Clin Otolaryngol 2010;35;87-96.

Cheng AK, Niparko JK. Cost-utility of the cochlear implant in adults: a meta-analysis. Arch Otolaryngol Head Neck Surg 1999;125:1214-8.

Ching TY, Incerti P, Hill M. Binaural benefits for adults who use hearing aids and cochlear implants in opposite ears. Ear Hear 2004;25:9-21.

Cote M, Ferron P, Bergeron F, Bussieres R. Cochlear reimplantation: causes of failure, outcomes, and audiologic performance. Laryngoscope 2007;117:1225-35.

Cullington HE, Zeng FG. Comparison of bimodal and bilateral cochlear implant users on speech recognition with competing talker music perception, affective prosody discrimination, and talker identification. Ear Hear 2011;32:16-30.

Eisen MD. Djourno, Eyries, and the first implanted electrical neural stimulator to restore hearing. Otol Neurotol 2003;24:500-6.

Firszt JB, Reeder RM, Skinner MW. Restoring hearing symmetry with two cochlear implants or one cochlear implant and a contralateral hearing aid. J Rehabil Res Dev 2008;45:749-67.

Gantz BJ, Turner C, Gfeller KE, Lowder MW. Preservation of hearing in cochlear implant surgery: advantages of combined electrical and acoustical speech processing. Laryngoscope 2005;115:796-802.

Garber S, Ridgely MS, Bradley M, Chin KW. Payment under public and private insurance and access to cochlear implants. Arch Otolaryngol Head Neck Surg 2002;128:1145-52.

Gates GA, Miyamoto RT. Cochlear implants. N Engl J Med 2003;349:435-45.

Gates GA, Cooper JC Jr, Kannel WB, Miller NJ. Hearing in the elderly: the Framingham cohort, 1983-1985. Part I. Basic audiometric test results. Ear Hear 1990;11:247-56.

Gifford RH, Dorman MF, McKarns SA, Spahr AJ. Combined electric and contralateral acoustic hearing: word and sentence recognition with bimodal hearing. J Speech Lang Hear Res 2007;50:835-43.

Grantham DW, Ricketts TA, Ashmead DH, Labadie RF, Haynes DS. Localization by postlingually deafened adults fitted with a single cochlear implant. Laryngoscope 2008;118:145-51.

Harris MS, Hay-McCutcheon M. An analysis of hearing aid fittings in adults using cochlear implants and contralateral hearing aids. Laryngoscope 2010;120:2484-8.

Lasisi AO, Fehintola FA, Yusef OB. Age-related hearing loss, vitamin B12, and folate in the elderly. Otolaryngol Head Neck Surg 2010;143:826-30.

Miyamoto RT, Kirk KI. Cochlear implants and other implantable auditory prostheses. In: Bailey BJ, Johnson JT, et al, eds. Head and Neck Surgery – Otolaryngology, 4th ed. Philadelphia: Lippincott, Williams and Wilkins; 2006.

Morera C, Manrique M, Ramos A, Garcia-Ibanez L, Cavalle L, Huarte A, Castillo C, Estrada E. Advantages of binaural hearing provided through bimodal stimulation via a cochlear implant and a conventional hearing. Acta Otolaryngol 2005;125:596-606.

Nelson EG, Hinojosa R. Presbycusis: a human temporal bone study of individuals with downward sloping audiometric patterns of hearing loss and review of the literature. Laryngoscope 2006;116(Suppl 112):1-12.

Palmer CS, Niparko JK, Wyatt R, Rothman M, de Lissovoy G. A prospective study of the cost-utility of the multichannel cochlear implant. Arch Otolaryngol Head Neck Surg 1999;125:1221-8.

Pensak ML. Healthcare transformation: implications for implantation. Curr Opin Otolaryngol Head Neck Surg 2009;17:333.

Peters BR, Wyss J, Manrique M. Worldwide trends in bilateral cochlear implantation. Laryngoscope 2010;120 (Suppl 2):S17-44.

Roland PS, Samy RN. Aging and the auditory and vestibular system. In: Bailey BJ, Johnson JT, et al, eds. Head and Neck Surgery – Otolaryngology, 4th ed. Philadelphia: Lippincott, Williams and Wilkins; 2006.

Shah SB, Chung JH, Jackler RK. Lodestones, quackery, and science: electrical stimulation of the ear before cochlear implants. Am J Otol 1997;18:665-70.

Summerfield AQ, Marshall DH, Barton GR, Bloor KE. A cost-utility scenario analysis of bilateral cochlear implantation. Arch Otolaryngol Head Neck Surg 2002;128:1255-62.

Teoh SW, Pisoni DB, Miyamoto RT. Cochlear implantation in adults with prelingual deafness. Part I. Clinical results. Laryngoscope 2004;114:1536-40.

Turrentine FE, Wang H, Simpson VB, Jones RS. Surgical risk factors, morbidity, and mortality in elderly patients. J Am Coll Surg 2006;203:865-77.

van den Broek E, Dunnebier EA. Cochlear implantation in postlingually hearing-impaired adults: choosing the most appropriate ear. Int J Audiol 2009;48:618-24.

Venail F, Sicard M, Piron JP, Levi A, Artieres F, Uziel A, Mondain M. Reliability and complications of 500 consecutive cochlear implantations. Arch Otolaryngol Head Neck Surg 2008;134:1276-81.

Wareing MJ, Lalwani AK, Jackler RK. Development of the ear. In: Bailey BJ, Johnson JT, et al, eds. Head and Neck Surgery – Otolaryngology, 4th ed. Philadelphia: Lippincott, Williams and Wilkins; 2006.