Figure 8.1. External and Internal (Implanted) Components of a Cochlear Implant. (Artwork courtesy of National Institutes of Health [NIH] Medical Arts.)

HOW A COCHLEAR IMPLANT WORKS

A cochlear implant is an electronic device made of several different parts. External components are worn outside the body; internal components are surgically implanted under the skin and within the cochlea. Figure 8.1 shows where the external and internal components are placed. Figure 8.2 shows the components themselves; the numbers in the figure correspond to the numbers in the text.

Similar to a hearing aid, an ear-level microphone (1) picks up sounds from the environment. Sound information is sent to a speech processor (2), which may be worn on the ear like a behind-the-ear (BTE) hearing aid or on the body like a pager (processor controls [3]). The processor contains a tiny computer that digitizes sound information and organizes it into patterns according to a code. The patterned information travels through a cable (4) to a transmitter (5), which is worn on the head behind the ear. The transmitter sends the information to a receiver/stimulator

(6) implanted under the skin. The transmitter and receiver, each about the size of a quarter, are held together by magnets. No wires or cables pass through the skin, reducing the chance of infection. The receiver converts

Figure 8.2. External and Internal Components of a Cochlear Implant. (Courtesy of Cochlear Americas.)

digitized information into patterned electrical pulses and sends them through tiny wires to the electrode array (7) that winds through the cochlea. The array has tiny electrodes attached to it (at present, a maximum of 24 electrodes substitute for more than 16,000 hair cells). The electrodes deliver the coded electrical pulses at very high rates to auditory nerve fibers located in different frequency regions of the cochlea (see Figure 3.7 in Chapter 3). The auditory nerve fibers come together as CN VIII and carry the bioelectrical (neural) information to auditory centers in the brain. With practice, the brain learns to interpret the coded electrical pulses as meaningful sound.

COCHLEAR IMPLANT CANDIDACY

Food and Drug Administration (FDA) regulations allow infants 12 months of age and older to receive cochlear implants; however, infants younger than 12 months have been implanted under special conditions. There is no upper age limit, and people in their 90s have been implanted very successfully.

Generally speaking, adult candidates must have a severe-to-profound sensorineural hearing loss in both ears (thresholds of 70 dB or poorer) and receive limited benefit from hearing aids. Limited hearing aid benefit is a criterion because conventional implantation procedures sacrifice residual

Figure 8.3. Ear-level Speech Processor and Transmitter. (Courtesy of Cochlear Americas.)

(remaining) hearing in the implanted ear. Hearing loss criteria continue to change (always becoming less stringent); therefore if you were told in the past that you had too much hearing, it might be worth checking again. If your audiologist thinks you may derive more benefit from an implant than from hearing aids, he will refer you to a cochlear implant center for extensive evaluation by members of a cochlear implant team. The evaluation might require more than one visit, and it will likely include sessions with the implant team coordinator, a physician, an audiologist, and a psychologist or social worker.

The physician/surgeon will evaluate your overall health and the condition of your ears to be sure that it’s physically possible to perform the surgery and implant the device. She will examine images of your cochlea (CT scan or MRI) in search of anatomic variations that could affect insertion of the electrode array into the cochlea. The audiologist will do a comprehensive hearing evaluation, with and without hearing aids. If you don’t currently wear state-of-the-art hearing aids, you may be required to try them for 3 to 6 months to evaluate their benefit. The implant audiologist must be convinced that you’re likely to receive greater benefit from an implant than from hearing aids or no device at all. Members of the implant team will also want to know about your expectations, motivation, and family support. Team members must determine that you have the personal resources to handle a cochlear implant and keep it working over a lifetime.

Members of the team will discuss all aspects of the process: the hearing evaluation, the medical evaluation, the surgical procedure, surgical risks, postsurgical recovery, activation and programming of the device, and

long-term follow-up care. Team members will provide information about the level of success that you can reasonably expect and the limitations that you might face. They will also discuss the relative advantages and disadvantages of the implant devices available to you. They will discuss costs, warranties, and insurance coverage. Armed with this information, you and your family can decide if you want an implant and, if so, which device is best for you.

EXPECTED OUTCOMES FOR COCHLEAR IMPLANT USERS

Chapter 2 suggested that hearing occurs on three levels: primitive, warning, and symbolic. At the primitive level, hearing provides a constant auditory background that makes us feel connected to the world around us. The auditory background includes everything from the drone of traffic noise to more enjoyable sounds like birds chirping overhead, leaves crunching underfoot, and children laughing in the distance. In nearly all cases, a cochlear implant can restore the auditory background and reduce feelings of isolation.

An implant also restores hearing at the warning level. At this level, sounds provide information about what’s happening around us (even in places we can’t see, like behind us, through walls, and in the dark). A smoke alarm signals danger. An unexplained noise could signal the presence of an intruder. A scream could signal a need for help. An inability to hear warning sounds creates feelings of anxiety and insecurity. A cochlear implant almost always makes the user feel safer and more independent.

The third level of hearing, the symbolic level, is the most important. At this level, hearing is used to understand speech. Success with implants at this level is more variable. With therapy and practice, most users learn to understand speech with considerably less need for speechreading. Many are able to communicate over the telephone, and newer technologies are making it possible for more users to appreciate music. Not all implant users achieve the same level of success, however.

Success depends on many factors. Those that are especially critical include the age at which hearing loss began, the duration of deafness, past use of hearing aids, and age at the time of implantation. Generally speaking, two groups of patients experience remarkable success: children who are implanted at a very young age and adults who lose their hearing later in life (after developing speech and language). In contrast, expectations are somewhat lower for adults who were born deaf (or became deaf before speech and language developed) and have not relied on hearing for communication in the past. In this case, CN VIII and hearing areas in the brain haven’t been adequately stimulated, and benefit from an implant might be more limited. Even in this case, however, it’s likely that an implant will improve speechreading ability by allowing the user to combine visual cues with the rhythm of speech, which can be heard with an implant.