Cochlear Implants

Clin Geriatr Med 22 (2006) 677–686

Cochlear Implants Sarah S. Connell, MD, Thomas J. Balkany, MD* Department of Otolaryngology, University of Miami Ear Institute, Miller School of Medicine, PO Box 016960, Miami, FL 33101, USA

Based on data from the National Center for Health Statistics, more than 2.2 million adults in the United States older than 70 have significant hearing impairment, making hearing loss the third most common chronic health condition affecting older adults [1]. An estimated 10% of individuals who have sensorineural hearing loss experience impairment so advanced that conventional amplification provides little benefit [2]. During the past 2 decades, improvements in cochlear implant (CI) devices and surgical techniques have made this form of aural rehabilitation a viable option for many geriatric patients. This article reviews current applications for multichannel CIs in elderly patients, the impact on quality of life, the evidence for safety and effectiveness, and promising new approaches for future technologic directions in bilateral cochlear implantation and electric acoustic hearing.

Epidemiology and clinical features In the Epidemiology of Hearing Loss Study (Beaver Dam, Wisconsin, 1993–1995 and 1998–2000), Cruickshanks and colleagues describe the risk for incident hearing loss as greater for men than women. The average age of onset was 65.9 years for men and 72.9 years for women [1]. This young age of onset suggests that older adults face many years of life with hearing impairment. The specific causes of presbycusis represent a combination of the effects of years of use, exposure to noise, chemical exposure, and genetically programmed biologic degeneration. Morphologic changes in humans demonstrate age-related loss of inner and outer hair cells and supporting cells, primarily from the basal turn of the cochlea.

* Corresponding author. Department of Otolaryngology, Miami Ear Institute, Miller School of Medicine, PO Box 016960, Miami, FL 33101, USA. E-mail address: [email protected] (T.J. Balkany). 0749-0690/06/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.cger.2006.04.003 geriatric.theclinics.com

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Geriatric patients who have hearing loss often are burdened further with the inability to communicate effectively because of cognitive impairment or development of depression, independent of socioeconomic status. Patients also experience tinnitus, poor voice monitoring, and difficulty using the telephone. There are additional reports correlating hearing loss with social isolation, poor self-esteem, and dementia. Despite the high numbers of people afflicted with this disorder, only 25% of patients who could benefit from hearing aids use them [3,4]. Unique to the older population is the effect of hearing loss compounded by other perceptual deficits, including dementia, visual impairment, or peripheral neuropathy. Overcoming age-related changes in the auditory system, diminished communication abilities, and coexisting medical and psychosocial problems, elderly adults with CIs achieve excellent audiologic and quality-of-life measures.

History Even though scientists in the 1700s were aware that electrical stimulation could produce auditory sensation, it was not appreciated until 1957 that some speech understanding was possible with electrical stimulation of the cochlear nerve. In the 1980s, single channel CIs were accepted as safe and efficacious therapy for individuals who had profound hearing loss. Since then, improvements in microelectronics, battery technology, packaging, and signal processing have broadened the application of this tool, such that there are more than 80,000 (as of September 2005) CI users worldwide, two thirds of whom are adults [5] (Steve Staller, personal communication, 2005).

Mechanism of cochlear implants In humans, sounds are converted from mechanical to electrical energy by hair cells within the cochlea. The CI serves to bypass this apparatus and provide direct electrical stimulation to auditory neurons. A CI is a surgically placed device that provides speech perception to individuals for whom hearing aids are not useful. Environmental sounds are transformed by a microprocessor into electrical signals, which are broadcast over multiple electrode channels. These electrodes are placed into the cochlea in a way that takes advantage of its tonotopic arrangement (ie, high frequencies are present at the basal turn and low frequencies represented in the apex). CIs work differently from hearing aids: hearing aids amplify sound. A CI, alternatively, transforms speech and other sounds into electrical energy that is used to stimulate surviving auditory nerve fibers in the inner ear. Unlike most hearing aids, CIs have internal and external components. A surgical procedure is needed to place the internal processor component of the implant.

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Hearing aids send amplified sound through the outer and middle ear and finally to the sensory receptor cells (commonly called hair cells) in the inner ear. The function of the hair cells is to change (transduce) the sound energy into electrochemical signals that are recognized by the hearing nerve. When hair cells are damaged or dead, parts of the signal may be distorted, or may not be sent to the hearing nerve at all. Because hair cell damage is by far the most common cause of hearing loss, CIs bypass the damaged hair cells and replace their function by converting sound energy into electrical energy that can stimulate the auditory nerve directly. The nerve recognizes this stimulation in much the same way normal sound is recognized, and the information is sent along the nerve to the brain where meaning is attached. Cochlear implant components All CI devices consist of internal and external hardware. External components are worn either behind the ear or on the body. They consist of a microphone, a sound processor, batteries, and a transmitter that sends coded electrical information and power to the internal parts. The internal components, which are placed surgically underneath the skin, include a receiver and an electrode array within the cochlea. Currently there are three CI devices available: HiResolution Bionic Ear (Advanced Bionics Corp, Valencia, California), Nucleus (Cochlear Americas, Englewood, Colorado), and Combi 40/40 þ (Med-el, Innsbruck, Austria). Similarities include type of components, such as body-worn and behind-the-ear sound processors.

Fig. 1. Mechanisms of CIs. (1) Sounds are picked up by the small, directional microphone located in the ear-level processor. (2) The speech processor filters, analyzes, and digitizes the sound into coded signals. (3) The coded signals are sent from the speech processor to the transmitting coil. (4) The transmitting coil sends the coded signals as FM radio signals to the CI under the skin. (5) The CI delivers the appropriate electrical energy to the array of electrodes, which has been inserted into the cochlea. (6) The electrodes along the array stimulate the remaining auditory nerve fibers in the cochlea. (7) The resulting electrical sound information is sent through the auditory system to the brain for interpretation.

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Differences include size, weight, color, batteries, and number of intracochlear stimulus contacts. The mechanisms of CIs are shown in Fig. 1.

Indications The first step in the work-up of patients who have hearing loss is an audiogram. This battery of tests includes pure tone thresholds from 250 to 8000 Hz, word recognition tests, speech reception thresholds, acoustic reflexes, and tympanometry. The majority of hearing loss in elderly patients is sensorineural. In mild to severe loss, hearing aids can be an effective therapy for social function, communication, and depression [6]. For those patients who cannot benefit from hearing aids because of the configuration or magnitude of their hearing loss or discrimination abilities, however, CIs are a good option. Audiometric criteria for cochlear implantation include bilateral severe to profound sensorineural hearing loss and pure tone average threshold (500, 1000, and 2000 Hz) worse than 70 dB hearing level. Patients undergo rigorous testing with appropriately fitted hearing aids and those who have less than 50% open-set recognition on taped sentence material also are candidates. High-resolution CT or MRI of temporal bones is performed to evaluate for cochlear ossification, the presence of a cochlear nerve, evidence of prior ear surgery, and active mastoiditis. Patients should have no medical contraindications to general anesthesia or any contraindications to surgical intervention or postoperative follow-up. Absolute contraindications include agenesis of the inner ear (Michel deformity), absence of the cochlear nerve, and systemic illness precluding anesthesia or surgery (Box 1). Patients must demonstrate psychologic stability and suitable motivation with realistic expectations for outcomes [7]. Selection criteria for choosing the ear are shown in Fig. 2 [8].

Box 1. Current selection criteria for adult cochlear implantation Moderate to profound bilateral sensorineural hearing loss Less than 40% correct in best-aided condition on tape-recorded test of open-set sentence cognition No contraindications to surgical placement of internal device and electrode array Benefit from hearing aids less than expected from CIs No medical contraindications to undergoing general anesthesia Family support, motivation, and appropriate expectations Absence of external or middle ear pathology or contraindication to participation in postoperative follow-up or evaluation

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Fig. 2. Considerations for selecting the ear to be implanted. aAfter candidate meets implantation criteria, demonstrates appropriate expectations and habilitation options. These are general guidelines. Each patient must be evaluated individually. bAbsolute contraindications.

Surgery Surgery to insert a CI usually requires 1.5 to 3 hours and can be performed on an outpatient basis. With patients under general anesthesia, a postauricular skin flap is elevated, a mastoidectomy is performed, and the middle ear entered. The cochlea is opened and the electrode inserted. The device is attached to the mastoid bone by suture. The wearable external microphone, microprocessor, and battery pack are coupled to the implanted device by radiofrequency transmission [7].

Rehabilitation The initial fitting process requires that the number of functioning channels be determined and made operative and that the dynamic listening range between threshold and comfort levels (upper level of the dynamic range) be established. Further, each electrode must be balanced with the others for pitch and loudness. This process is a substantial task for cooperative adults. Stapedial reflexes are used successfully to determine comfort levels in

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children as young as 2. Although this level may not be identical to the eventual comfort levels achieved, it does predict the general vicinity [7]. Complications Device failures are reported to occur in less than 1% of cases and failed devices can be replaced within the same ear without significant loss of benefit [9]. Complications are rare and can include dehiscence of skin incision, flap necrosis, improper electrode placement, dizziness, infection, facial nerve stimulation or injury, or cerebrospinal fluid leak. No deaths from CI are reported [10,11]. As humans age, general cognitive deficits and possible increased risk for surgical complications are concerns. A retrospective review, however, reveals no increased surgical risk. In comparison with younger patients, those over age 50 had no increased incidence of device extrusion, device malfunction, wound infection or dehiscence, flap-related problems, or anesthetic complications. They performed slightly worse in postoperative speech recognition scores but this could be explained by decreased spiral ganglion cell count or central presbyacusis with cognitive deficit [12]. Special consideration in elderly patients Central auditory pathways The elderly pose special considerations because of age-related degeneration of the spiral ganglion cells and deficits in central auditory pathways. Most often, the inner ear structures, the organ of Corti, and the stria vascularis show the greatest changes. Schuknecht postulated four basic types of presbyacusis: (1) sensory: organ of Corti hair cell degeneration; (2) neural: cochlear nerve degeneration; (3) metabolic: atrophy of the stria vascularis; and (4) mechanical: thickening of the basilar membrane. In addition to inner ear changes, evaluation of brainstem auditory centers in elderly subjects who have varying degrees of presbycusis reveals markedly smaller neural cell size in deaf versus normal hearing controls [13]. Although there is considerable variation, in general, advancing age is associated with an extensive loss of sensory cells and support structures. These ears reveal complete degeneration of the organ of Corti, particularly in the lower basal turn, with associated mild degeneration of some auditory nerve fibers. In contrast, the degeneration of the stria vascularis occurs primarily in the upper turns. In spite of these changes, as a group, individuals older than 65 have CI use characteristics similar to younger adult population [14]. Compromised healing Patients who have impaired wound healing capabilities from immunosuppressive medications or underlying medical conditions are at an increased risk

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for postoperative infection. In a study of patients who had liver and renal transplants or sickle cell disease, however, cochlear implantation is shown safe and effective. In a series of 13 CI patients who had chronic medical conditions or prolonged used of immunosuppressive medications, only one developed a mild infection, which resolved with antibiotics. When considering patients for cochlear implantation, the specific nature of their health problems and the overall medical and social situation of the patients should be considered to determine if the routine travel, postoperative visits, and longterm rehabilitation required are possible [15]. Acute otitis media Adults receiving CIs once were considered to be at risk for meningitis resulting from acute otitis media (AOM). CIs theoretically could create a potential spread of infection into the cochlea along the electrode array. Clinical studies demonstrate that the prevalence and severity of AOM are not increased by cochlear implantation. Oral antibiotics are effective in the treatment of postimplantation AOM, and there is no report of increased incidence of labyrinthitis or meningitis. All CI patients are given pneumococcal vaccination and preoperative intravenous antibiotics before implantation to decrease the risk for meningitis [8]. Impact of hearing on the quality of life of older patients The variability in outcomes with CIs is believed to be primarily the result of patient factors. A shorter duration of deafness, longer duration of implant use, and preimplantation hearing ability are predictors of postimplantation speech perception. A direct link between spiral ganglion cell survival and performance level is established. The etiology of deafness has been studied carefully and, in general, etiology does not seem to have an impact on the auditory performance of adults. Age at implantation for adults is not a major factor. The benefit experienced by older adults is not significantly different from that of younger adults, provided there are no other significant health issues. The age of onset of deafness does have important implications, as people who learn speech and language before becoming deaf adapt to CIs more quickly and achieve open-set speech discrimination earlier than those who have not learned speech and language. When subjects have measurable residual hearing before implantation, they perform better with CIs than those who do not. The amount of residual hearing, however, does not predict postimplantation performance [16]. Significant numbers of patients receive the secondary benefit of subjective tinnitus suppression after cochlear implantation [17]. The psychologic and social impact of CIs in adults is positive. Psychologic studies show decline in loneliness, reduction of depression, increase in self-esteem and independence, reduced isolation, and improved job opportunities [18].

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All adults who have functioning CIs are able to detect sound at lower intensities than who have hearing aids alone and almost all are able to lip-read better with auditory information provided by the implant. Many postlingually deafened adults develop significant open-set word recognition ability and more than half can converse to some degree on the telephone. The cost of the device ranges from $24,000 to $37,572, and mean total charge for unilateral implantation from time of implantation to 12-month follow-up is $36,837 [19]. Preliminary studies of the cost-effectiveness of CIs reflect a high value of this technology and procedure. Use of a cost per quality-adjusted life-year (QALY) method enables the cost effectiveness of selected medical technologies to be compared. CIs ($14,898/QALY) rank at the most cost-effective levels along with three-vessel coronary artery bypass grafts ($10,431/QALY) and a day of neonatal intensive care ($7,755/QALY) [7].

Advances in surgical therapy CI technology has undergone a rapid and constant evolution since the development of the devices in the early 1980s. Single channel devices gave way to multichannel devices. External equipment has become smaller and more versatile. Since the 1990s, users have been given multiple memory slots to choose the best speech processing strategy. At this time, the majority of adult implantees are able to conduct interactive telephone conversations. Significant advances in surgical techniques in the past 5 years include reduction in surgical complications, improvements in skin flap design, electrode fixation, and the ability to implant partially and fully ossified cochlea. The overall surgical complication rate for cochlear implantation has been reduced from 11% to 5% [7].

Future directions Several developments currently are being tested clinically and may become used widely in the next few years. The usefulness of bilateral cochlear implantation has been investigated in adults. It is anticipated that in the future the two implants will be integrated and will share a stimulation program to minimize channel interaction and improve hearing in noisy environments and localization of sounds [8,20,21]. Development of perimodiolar electrodes, implantable microphones, and rechargeable batteries promises fully implanted devices in the future [11]. Implantation of adults with residual hearing requires preservation of existing neural elements. Patients who have residual low-frequency hearing in conjunction with severe high-frequency hearing loss often do not hear well with conventional hearing aids. These patients are unable to distinguish high-frequency sounds, such as consonants, and have difficulty with speech

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perception in noisy listening conditions. One solution is a hybrid electro/ acoustic stimulator (a short atraumatic electrode coupled to an implantable hearing aid). In these experimental devices, severe to profound high-frequency losses are treated electrically and mild to moderate low-frequency losses are treated acousticallydthe ideal configuration for patients who have presbyacusis [8,22,23]. Poor speech perception in noise and music appreciation in CI users mainly is the result of their inability to encode pitch. The limited spectral resolution and inaccurate encoding of low-frequency information are believed the main reasons for poor pitch perception performance. Shallow insertion depth of present electrode arrays severely limits the transfer of low-frequency spectral information via CI. In addition, current speech processing strategies are unable to encode low-frequency temporal information. Multiple studies suggest that additional fine structure information at low frequencies allows for better encoding of pitch, which can improve music appreciation and enhance speech recognition in competing backgrounds [24]. Hence, those patients who are able to use both acoustic cues and electrical stimulation reap the benefits of speech understanding with highfrequency information from their CIs and speech perception in noise and music appreciation from the low-frequency hearing in the more apical (nonstimulated) portions of the cochlea. Another area of current development is neural preservation in association with implantation. Neurotrophins, molecular genetic techniques, and apoptotic pathway blockers, delivered either preoperatively or through CI electrodes, all are under study. If effective, more adults will be candidates to benefit from CIs [8]. Summary CIs are cost-effective auditory prostheses that safely provide a high-quality sensation of hearing to adults who are severely or profoundly deaf. In the past 5 years, progress has been made in hardware and software design, candidate selection, surgical techniques, device programming, education and rehabilitation, and, most importantly, outcomes. Cochlear implantation in the elderly is well tolerated and provides marked improvement in auditory performance and psychosocial functioning [25].

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[4] Dalton DS, Cruickshanks KJ, Klein BEK, et al. The impact of hearing loss on quality of life in older adults. Gerontologist 2003;43:661–8. [5] Koch DB, Staller S, Jaax K, et al. Bioengineering solutions for hearing loss and related disorders. Otolarygol Clin N Am 2005;38:255–72. [6] Yueh B, Souza PE, McDowell JA, et al. Randomized trial of amplification strategies. Arch Otolaryngol Head Neck Surg 2001;127:1197–204. [7] Balkany T, Hodges AV, Luntz MD. Update on cochlear implantation. Otolaryngol Clin North Am 1996;29:277–87. [8] Balkany TJ, Hodges AV, Eshraghi AA, et al. Cochlear implants in childrenda review. Acta Otolaryngol 2002;122:356–62. [9] Balkany TJ, Hodges AV, Gomez-Marin O, et al. Cochlear reimplantation. Laryngoscope 1999;106:351–5. [10] Cohen NL, Hoffman RA. Complications of cochlear implant surgery in adults and children. Ann Otol Rhinol Laryngol 1991;100:131–6. [11] Cohen N, Waltzman S, Fisher SG. Department of Veterans Affairs Cochlear Implant Study Group. A prospective, randomized study of cochlear implants. N Engl J Med 1993;328: 233–7. [12] Chatelin V, Kim EJ, Driscoll C, et al. Cochlear implant outcomes in the elderly. Otol Neurotol 2004;25:298–301. [13] Schuknecht HF, Gacek MR. Cochlear pathology in presbycusis. Ann Otol Rhinol Laryngol 1993;102:1–16. [14] Niparko JK, Kirk KI, Mellon NK, et al. Cochlear implant principles and practice. Philadelphia: Lippincott Williams and Wilkins; 2000. [15] Odabasi O, Mobley SR, Bolanos RA, et al. Cochlear implantation in patients with compromised healing. Otolaryngol Head Neck Surg 2000;123:738–41. [16] Gates GA, Daly K, Dichtal WJ, et al. National Institutes of Health: Consensus Development Conference statement: cochlear implants in adults and children. Bethesda (MD), May 15–17, 1995. [17] Miyamoto RT, Bichey BG. Cochlear implantation for tinnitus suppression. Otolaryngol Clin North Am 2003;36:345–52. [18] Knutson JF, Schwartz HA, Bantz BJ, et al. Psychological change following 18 months of cochlear implant use. Ann Otol Rhinol Laryngol 1991;100:877–82. [19] Palmer CS, Niparko JK, Wyatt R, et al. A prospective study of the cost-utility of the multichannel cochlear implant. Arch Otolaryngol Head Neck Surg 1999;125:1221–8. [20] Litovsky RY, Parkinson A, Arcaroli J, et al. Bilateral cochlear implants in adults and children. Arch Otolaryngol Head Neck Surg 2004;130:648–55. [21] Summerfield AQ, Marshall DH, Barton GR, et al. A cost-utility scenario analysis of bilateral cochlear implantation. Arch Otolaryngol Head Neck Surg 2002;128:1255–62. [22] Gstoettner W, Kiefer J, Baumgartner WD, et al. Hearing preservation in cochlear implantation for electric acoustic stimulation. Acta Otolaryngol 2004;124:348–52. [23] Adunka O, Unkelbach MH, Mack MG, et al. Predicting basal cochlear length for electricacoustic stimulation. Arch Otolaryngol Head Neck Surg 2005;131:488–92. [24] Kong Y, Stickney GS, Zeng F. Speech and melody recognition in binaurally combined acoustic and electric hearing. J Acoust Soc Am 2005;117(3) Pt 1:1351–61. [25] Hinderink JB, Krabbe PFM, van den Broek P. Development and application of a healthrelated quality of life instrument for adults with cochlear implants: the Nijmegen Cochlear Implant Questionnaire. Otolaryngol Head Neck Surg 2000;123:756–65.