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Cochlear Implants: An Overview

INTRODUCTION TO COCHLEAR IMPLANTS

A cochlear implant is a surgically implantable device that provides hearing sensation to individuals with severe-to-profound hearing loss who do not benefit from hearing aids. Typically, people with hearing losses in the severe-to-profound range have absent or malfunctioning sensory cells in the cochlea. In a normal ear, sound energy is converted to mechanical energy by the middle ear, which is then converted to mechanical fluid motion in the cochlea. Within the cochlea, the sensory cells-the inner and outer hair cells-are sensitive transducers that convert that mechanical fluid motion into electrical impulses in the auditory nerve.


Cochlear implants are designed to substitute for the function of the middle ear, cochlear mechanical motion, and sensory cells, transforming sound energy into electrical energy that will initiate impulses in the auditory nerve.

Although it has been known since the late 1700s that electrical stimulation can produce hearing sensations, it was not until the 1950s that the potential for true speech understanding was demonstrated. In 1957, two French surgeons placed an electrode on the auditory nerve of a deaf man during an operation for facial nerve repair. When current was passed through electrode, the patient was able to discriminate some sounds and understand a few simple words.

Based on that observation, several research groups around the world began exploring the feasibility of implantable electrical stimulators that could be used on a long-term basis by hearing-impaired individuals. Through their efforts, cochlear implants have evolved from single-channel devices, introduced in the late 1970s, into technologically advanced microprocessor systems that deliver a higher level and greater range of hearing benefits than ever would have been predicted or expected. (For a history of cochlear implant development, see Schindler, 1999, Beiter & Shallop, 1998; Loizou, 1998; Shannon, 1996). Currently, four cochlear implant systems are available or under investigation in the United States. They are:

1. The Clarion Multi-Strategy Cochlear Implant System (Advanced Bionics Corporation, Sylmar, CA).
2. The Nucleus Multichannel Cochlear Implant System (Cochlear Corporation, Englewood, CO)
3. The Med-El Cochlear Implant (Med-El Corporation, Research Triangle Park, NC)
4. The AllHear Cochlear Implant (AllHear, Inc., Aurora, OR).




REGULATION OF COCHLEAR IMPLANTS


Cochlear implants are regulated by the U.S. Food and Drug Administration (FDA). Prior to commercial distribution, they must be evaluated for safety and efficacy in clinical trials monitored by the FDA under what is termed an 'Investigational Device Exemption' (IDE). During an IDE study, patients who meet specified criteria can be implanted at a limited number of investigational clinics. These clinics, under supervision from local Institutional Review Boards (IRB), implant patients and collect safety and efficacy data, which is sent to the implant manufacturer. The manufacturer compiles the data and submits an application for Pre-Market Approval to the FDA. After review, the FDA may grant the manufacturer a release to distribute the cochlear implant in the U.S. Typically, the device is released for a defined population (e.g., profoundly deaf adults), so additional clinical trials must be conducted if other populations (e.g., children) are to receive the device. Moreover, additional clinical trials are required if changes are made to the device itself. Currently, the FDA status of the implants available in the United States is as follows:

Clarion:
* Released for adults in 1995
* Released for children in 1997
Nucleus:
* Released for adults in 1985
* Released for children in 1990
Med-El:
* Under clinical investigation in adults and children
AllHear:
* Under clinical investigation in adults and children



COMPONENTS OF A COCHLEAR IMPLANT


All cochlear implant systems consist of both internal and external components. The external components, which are worn on the head, over or next to the ear, include (1) a microphone, which converts sound into an electrical signal, (2) a speech processor, which manipulates and converts the signal into a special code (i.e., speech processing strategy), and (3) a transmitter, which sends the coded electrical signal to the internal components. The surgically implanted components include (1) a receiver, which decodes the signal from the speech processor, and (2) an electrode array, which stimulates the cochlea with electrical current. The systems are powered by batteries located in the speech processor.

Although all cochlear implant systems have basic features in common, they differ in how those features are implemented. Important differences exist in the (1) implanted electronics packaging, (2) electrode design, (3) stimulation waveform and temporal pattern of stimulation (4) speech coding strategy, and (5) telemetry. Detailed descriptions of these features for each of the U.S. implants can be found at the following websites.

Clarion: www.cochlearimplant.com
Nucleus: www.cochlear.com
Med-El: www.cochlearimplants.com
AllHear www.allhear.com

Current Issues in Implant Design

Most recently, new electrode designs and options in speech-processing algorithms have been introduced.

Electrode Design and Coupling

Typically, cochlear-implant electrode arrays are inserted into the scala tympani of the cochlea longitudinally to take advantage of the place-to-frequency coding mechanism used by the normal cochlea. Information regarding low-frequency sound is sent to electrodes at the apical end of the array, whereas information regarding high-frequency sound is sent to electrodes nearer the base of the cochlea. The exception is the AllHear single-electrode implant, which is inserted a short distance into scala tympani. It delivers an amplitude-modulated signal to one location at the base of the cochlea.

The ability to take advantage of the place-frequency code is limited by the number and pattern of surviving auditory neurons in an impaired ear. Recently, there has been an effort to design electrodes that will lie as close as possible to the surviving spiral ganglion cells. Modiolar-hugging designs are now available in the Clarion (HiFocus electrode plus Positioner), the Nucleus (Contour electrode), and the Med-El (C40+ electrode plus positioning system) cochlear implant systems. Presumably, these electrodes should reduce the amount of current required for hearing, increase the electrical dynamic range, and reduce interactions between electrical fields resulting from stimulation of adjacent electrodes.

For all systems, electrical current is passed between an active electrode and an indifferent electrode. If the active and indifferent electrodes are remote, the stimulation is termed monopolar. When the active and indifferent electrodes are close to each other, the stimulation is referred to as bipolar. Bipolar stimulation focuses the current within a restricted area and presumably stimulates a small localized population of auditory nerve fibers. Monopolar stimulation, on the other hand, spreads current over a wider area and a larger population of neurons. Less current is required to achieve adequate loudness levels with monopolar stimulation, whereas more current is required for bipolar stimulation. The use of monopolar or bipolar stimulation is determined by the speech processing strategy and each individual's response to electrical stimulation.

The Nucleus, Clarion, and Med-El, and AllHear cochlear implants all have extracochlear electrodes so that monopolar stimulation can be implemented. The Nucleus and the Clarion also can implement various bipolar stimulation modes.

Stimulation Waveform

There are two types of stimulation currently used in the multi-electrode cochlear implants, analog and pulsatile. Analog stimulation consists of electrical current that varies continuously in time. Pulsatile stimulation consists of trains of square-wave biphasic pulses. The pattern of stimulation can be either simultaneous or non-simultaneous (sequential). With simultaneous stimulation, more than one electrode is stimulated at the same time. With non-simultaneous stimulation, electrodes are stimulated in a specified sequence, one at a time. Typically, analog stimulation is simultaneous and pulsatile stimulation is sequential.

The Nucleus and Med-El implant offer only non-simultaneous pulsatile stimulation. The Clarion system can provide both non-simultaneous and simultaneous stimulation. The Clarion also has the capability of implementing hybrid strategies that combine analog and pulsatile stimulation, and simultaneous and non-simultaneous stimulation.

Speech Coding Strategy

Coding strategy defines the way the implant system transforms sound into electrical stimulation of the auditory nerve. In particular, coding strategies differ in the way they transform acoustic speech signals into an electrode stimulation pattern. In order to represent speech accurately, the coding strategy must reflect three parameters in its electrical stimulation code-frequency, amplitude, and time. Frequency information is conveyed by the site of stimulation, amplitude is encoded by amplitude of the stimulus current and temporal cues are conveyed by the rate and pattern of stimulation. The speech processing strategies currently available in the multi-electrode implant systems are described below.

Non-simultaneous strategies. These strategies fall into two general categories--continuous interleaved sampling (CIS) and 'n of m' strategies. In a CIS strategy, trains of biphasic pulses are delivered to the electrodes in an interleaved or nonoverlapping fashion to minimize electrical field interactions between stimulated electrodes. The amplitudes of the pulses delivered to each electrode are derived by modulating them with the envelopes of the corresponding bandpassed waveforms. A CIS strategy uses the full spectrum of the incoming acoustic waveform without compromising temporal information. The rate at which the pulses are delivered to the electrodes is an important variable in the implementation of CIS strategies. High-rate stimulation typically results in better speech understanding than low-rate stimulation (Wilson 1993, Loizou et al. 2000). Because only one channel is stimulated at a time, monopolar electrode coupling is typically employed with this type of strategy. The Clarion, Nucleus, and Med-El implant systems can implement a CIS strategy although the number of analysis channels, number of electrodes, and stimulation rates differ among them.

In an 'n of m' strategy, a specified number of electrodes out of the maximum number available are stimulated. The implementation of this type of processing in the Nucleus implant is referred to as the spectral peak extraction (SPEAK) strategy. SPEAK analyzes the incoming sound to identify the filters that have the greatest amount of energy, selects a subset of filters, and then stimulates corresponding electrodes (always less than the total number of electrodes in the cochlea). The stimuli are pulsatile and non-simultaneous. With SPEAK, 6-10 electrodes are activated sequentially at a rate that averages approximately 250 pulses per second on each activated electrode. The Advanced Combination Encoder (ACE), offered in the Nucleus system, combines the spectral maxima detection of SPEAK with a higher stimulation rate. With ACE, the number of maxima and electrodes used, which will influence the stimulation rate, can be specified for each patient. The Med-El system also offers an 'n of m' strategy with a fixed number of channels stimulated at a high rate.

Simultaneous strategies. The Clarion implant is the only device that supports simultaneous stimulation. The simultaneous strategy currently available is the Simultaneous Analog Stimulation (SAS) strategy. The SAS strategy evolved from compressed analog (CA) schemes which used a vocoder approach. In that approach, a bank of filters separated the incoming sound into different frequency bands, and the resulting analog waveforms were compressed and delivered to appropriate electrodes. In the Clarion, a digitized version of the incoming acoustic signal is filtered, compressed and processed, and is then transmitted to the implanted electronics. Following digital-to-analog conversion, the analog waveforms are sent simultaneously to all electrodes. One problem encountered with simultaneous stimulation is channel interaction, which is reduced through using a bipolar electrode coupling mode.

Additional simultaneous strategies are under development or evaluation for the Clarion. The Multiple Pulsatile Sample (MPS) is a partially simultaneous strategy that stimulates two or more non-adjacent electrodes at the same time with biphasic pulses. The Hybrid Analog Pulsatile (HAP) strategy combines simultaneous analog stimulation of the apical electrodes with non-simultaneous, pulsatile stimulation of the basal electrodes.



COCHLEAR IMPLANT CANDIDACY


Since the first cochlear implants became commercially available in the 1970s, implant systems have evolved to provide ever-increasing benefit to people with severe-to-profound hearing impairment. Originally, cochlear implants were available only for adults with profound hearing loss (> 90 dB HL) who lost their hearing after acquiring oral speech and language. Because the average speech-perception abilities of these individuals increased as implant technology improved, it became clear that people with lesser degree of hearing impairment might derive as much or more benefit from implants than from conventional hearing aids. Thus, over time, the criteria for postlinguistically deafened adults has changed to include individuals with severe hearing losses (average hearing loss for 500 Hz, 1000 Hz, and 2000 Hz > 70 dB HL) and a greater degree of speech recognition with hearing aids. The current general candidacy criteria for adults are:

18 years old or greater
Severe-to-profound bilateral sensorineural hearing loss
Limited speech understanding with hearing aids
Some oral speech and language experience
No radiologic or medical contraindications

In addition, the number of prelingually deafened adults seeking cochlear implants is increasing as the candidacy criteria are expanded. Research shows these individuals can derive substantial benefit from cochlear implants, although their performance on speech recognition tests is poorer than adults with postlingual onset of severe or profound hearing loss. The ideal candidates with prelingual severe or profound hearing loss are adults with a consistent history of hearing aid use, preferably with some residual hearing, who use oral communication. Counseling regarding appropriate expectations post-implantation is extremely important for these individuals

In children, the situation is somewhat more complex. Upon initial approval by the FDA, only children between ages 2 and 17 years with profound hearing impairment were permitted to have cochlear implants. As implants improved and as developmental data became available from implanted children, the benefits and limitations of pediatric implantation became evident. For example, the language-development benefit provided by the auditory information delivered by the implant exceeded the benefit derived from conventional amplification (e.g., Svirsky and Meyer 1999). Moreover, children who were implanted earlier in life showed greater improvement in speech perception than children implanted later (e.g., Miyamoto et al. 1999).

Based upon those observations, it was clear that earlier implantation was desirable in order to optimize the acquisition of oral speech and language. Thus the candidacy criteria were modified to allow implantation of younger children. The current general candidacy criteria for children are:

12 months to 17 years old
Profound bilateral sensorineural hearing loss
Limited benefit from hearing aids
Failure to progress in auditory skill development
No radiologic or medical contraindications




COCHLEAR IMPLANT BENEFITS


Speech Perception Results in Adults

Most adults with postlingual onset of severe or profound hearing loss (i.e., onset > 6 years of age) demonstrate dramatic improvements in speech recognition abilities after relatively limited implant experience. For example, Figure 1 shows mean preoperative speech-perception scores obtained with hearing aids compared to one-, three-, and six-month postoperative performance for 57 adults implanted with the Clarion HiFocus Electrode plus Positioner.

Figure 1. Word and sentence recognition scores over time for 57 postlinguistically deafened adults who use the Clarion HiFocus Electrode plus Positioner (CNC = consonant-nucleus-consonont monosyllabic words, CID = CID Everyday Sentences, HINT = Hearing in Noise Sentences).

Notably, Rubinstein et al. (1999) has found that preoperative sentence recognition performance has a significant impact on post-implant word recognition scores. Patients who have better preoperative scores on the CID sentence test achieve higher word recognition scores post-implant. Specifically, Rubinstein and colleagues reported that each additional 2% on the CID test preoperatively results in a 1% increase in the post-implant CNC word score. These researchers caution that individuals who show no preoperative speech recognition scores (i.e., score of 0% on sentence tests with hearing aids) also do extremely well post-implant, although even small amounts of residual hearing are associated with a higher average performance and less variance. The findings of this study support the current trend to implant adults with greater residual hearing (i.e., less hearing loss).

Speech Perception Results in Children

Children benefit from cochlear implantation. The outcomes of recent studies of cochlear implantation in children are important for determining implant candidacy in the future and for counseling parents and guardians. Clinical research results indicate that implanting children as young as possible will give them the most advantageous auditory environment for speech and language learning. Furthermore, providing auditory input early will increase a child's ability to learn in a normal classroom setting and reduce the need for special education services. Older children also benefit significantly from a cochlear implant, especially if they use oral communication, but their benefit may be limited if they have been deaf for a long time or if they use total communication (for reviews, see Osberger & Koch 2000, Waltzman & Cohen 2000).

Most studies to date have reported results from children 2 years of age and older. However, because the lowest age for implantation is now 12 months, some data are now available for children younger than 2 years. For example, on the IT-MAIS, a test that evaluates milestones in speech and auditory perception skills, very young children showed significant improvement in vocalization, spontaneous alerting to sound, and meaningful responses to sound in the first 3 months after implantation (Zimmerman-Phillips et al. 2000) (Figure 2).

Figure 2. Composite means of scores on the Infant-Toddler Meaningful Auditory Integration Scale for 35 children under the age of 2 years who received a Clarion implant. These children had no consistent responses to sound with hearing aids before implantation. More dramatic improvement in deriving meaning from sound occurs after 12 months of implant use.



Other factors besides age influence cochlear implant benefit in children. For example, educational method impacts the performance of older children. Children who use primarily oral communication have higher speech perception scores than children who are educated using a total communication approach. Moreover, children who have some preoperative speech perception skills using hearing aids do better than children who showed no hearing aid benefit. Nonetheless, children with no preoperative auditory perceptual skills still show significant speech-perception benefit after implantation.




FUTURE DIRECTIONS


The efficacy of cochlear implants for the remediation of severe-to-profound hearing impairment is now well accepted. Research and development of cochlear implant technology will continue to focus on modifying electrode/neural interfaces to reduce power consumption and to limit undesirable channel interaction. Speech processing strategies that use a variety of stimulation waveforms and patterns will allow implants to be better tailored to the needs of each individual user. New miniaturization processes will result in smaller behind-the-ear processors and, eventually, completely implantable systems. In addition, technological advances should result in improved implant reliability and enhanced speech perception benefit. As the perceptual benefits improve, the candidacy criteria will likely expand to include other individuals with hearing impairment who do not benefit satisfactorily from traditional acoustic amplification.



ACKNOWLEDGEMENT


Some text in this article has been published previously in:

Osberger, M.J., & Koch, D.B. Cochlear implants. In R.E. Sandlin (Ed.), Textbook of Hearing Aid Amplification, 2nd ed. (pp. 673-703). San Diego: Singular Publishing Group, Inc. 2000. Reprinted with permission of Delmar, a division of Thomson Learning.



REFERENCES


Beiter, A. L. & Shallop, J. K. (1998). Cochlear implants: past, present, and future. In W. Estabrooks (Ed.), Cochlear Implants in Kids (pp. 3-29). Washington, DC: Alexander Graham Bell Association.

Loizou, P. C. (1998). Mimicking the human ear. IEEE Signal Processing Magazine, September, 101-130.

Loizou, P.C., Poroy, O., & Dorman, M. (2000). The effect of parametric variations of cochlear implant processor on speech understanding. Journal of the Acoustical Society of America, 108(2), 790-802.

Miyamoto, R.T., Kirk, K.I., Svirsky, M.A., & Sehgal, S.T. (1999). Communication skills in pediatric cochlear implant recipients. Acta Oto-Laryngologica, 119(2), 219-224.

Osberger, M.J., & Koch, D.B. (2000). Cochlear implants. In R.E. Sandlin (Ed.), Textbook of Hearing Aid Amplification, 2nd ed. (pp. 673-703). San Diego: Singular Publishing Group, Inc.

Rubinstein, J. T., Parkinson, W. S., Tyler, R. S., & Gantz, B. J. (1999). Residual speech recognition and cochlear implant performance: Effects of implantation criteria. American Journal of Otology, 20, 445-452.

Schindler, R.A. (1999). Personal reflections on cochlear implants. Annals of Otology, Rhinology, & Laryngology, 108 (supplement 177), 4-7.

Shannon, R.V. (1996). Cochlear implants: what have we learned and where are we going? Seminars in Hearing, 17, 403-415.

Svirsky, M., & Meyer, T. A. (1999). Comparison of speech perception in Clarion cochlear implant and hearing aid users. Annals of Otology, Rhinology, & Laryngology, 108 (supplement 177), 104-109.

Waltzman, S.B., & Cohen, N.L. (2000). Cochlear Implants. New York: Thieme.

Wilson, B. E. (1993). Signal processing. In. R. S. Tyler (Ed.), Cochlear Implants: Audiological Foundations (pp. 35-85). San Diego: Singular Publishing Group, Inc.

Zimmerman-Phillips, S., Robbins, A., & Osberger, M. J. Assessment of functional device benefit in infants and toddlers. Paper presented at the 5th European Symposium on Pediatric Cochlear Implantation. Antwerp, Belgium, June 2000.


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