Hearing Aids & Cochlear Implants PDF

# Other Types of Implantable Hearing Devices These implantable hearing devices are typically for individuals who cannot benefit from typical traditional hearing aids. ## Bone Anchored Hearing Aid (BAHA) The bone anchored hearing aid, or BAHA, is an implantable hearing aid designed to address cond...

# Other Types of Implantable Hearing Devices These implantable hearing devices are typically for individuals who cannot benefit from typical traditional hearing aids. ## Bone Anchored Hearing Aid (BAHA) The bone anchored hearing aid, or BAHA, is an implantable hearing aid designed to address conductive hearing loss, mixed hearing loss, or single-sided deafness (SSD). Some individuals with conductive hearing losses may have constant drainage from the ear that prohibits healthy or comfortable placement of a hearing aid or earmold in the ear canal. In a few cases, individuals have an absent ear canal, also known as aural atresia. The BAHA uses the concept of bone conduction, or how sound is delivered to the inner ear by vibration of the bones in the skull. The BAHA can be safely implanted in both children and adults. BAHA users have been found to have significantly better speech understanding ability than with traditional bone conduction hearing aids and also improved ability to hear in the presence of background noise. The BAHA system bypasses the middle ear system and delivers the sound signal directly to the inner ear. A titanium abutment, or screw, is surgically implanted in the bone of the skull behind the ear. Similar to a cochlear implant, an external sound processor is used. # Benefits and Limitations of Cochlear Implants Again, the cochlear implant is not a miracle cure. Research indicates that some children who have cochlear implants continue to demonstrate lags in a few areas of language development compared to their hearing peers. Research suggests that outcomes are significantly better for children who receive a cochlear implant at an earlier age and that the results help to direct the future of aural habilitation programs. Years of research indicates that cochlear implants have had a significant positive impact on the quality of life for children with profound hearing loss. The woman also stated that learning to listen with a cochlear implant, even for someone who had years of experience as a hearing person, was a challenge. It required dedication and work, but it was worth it. # Cochlear Implant Candidacy and Considerations A significant number of considerations impact potential success with a cochlear implant, including the following issues that should be taken into account for an individual patient or wearer: - The length of time that the patient has been deaf; individuals who have been deaf for a short time do better than those who have been deaf a long time. - The age at onset of the deafness; that is, whether patients were deaf before they could speak. - The rate at which individuals can learn; the quicker the better. - The quality and dedication of the learning support structure. - The health and structure of the individual's cochlea; in particular, the number of nerve (spiral ganglion) cells that exist in the cochlea. - Implating variables, such as the depth and type of implanted electrode and the signal-processing technique. - Intelligence and communicativeness of the patient These considerations again highlight the significant number of factors, many of which require patient commitment, for the implant to be successful. When cochlear implants were first approved by the FDA, the criteria were strict, including limiting the surgery to adults only. The cochlea is adult-like at birth, so implantation is feasible from a physical perspective. However, a number of other considerations must be addressed. Pediatric implant candidates, those aged 12 months to 17 years, must have a bilateral, profound sensorineural hearing loss. In addition, these children must demonstrate minimal or no benefit from appropriately fitted hearing aids. Hearing with a cochlear implant, I realized... was going to be like a stone skipping across the surface of a lake. I would have to learn to glide over the soundstream, not always fully in contact with it but getting the general meaning. I would have to learn to backfill the important information in my mind. I would have to give up the expectation that it would truly feel like hearing, and learn to use the implant as a tool that would enable me to do something that resembled hearing. It would not be hearing. It would just be equivalent to hearing. -Chorost (2005, p. 79) More than 120,000 people worldwide have received cochlear implants, and the devices are regarded as the most successful neural prosthesis available Although cochlear implants were initially designed to address issues faced by individuals with severe-to-profound hearing loss, the lessons learned from their use have advanced understanding in all areas of hearing loss, from speech and language development in children to understanding the development of the central auditory nervous system. Research continued in this area in the intervening years; however, it was not until the 1950s when electrical stimulation of the auditory nerve resulted in the perception of hearing in two deaf patients in France (Djourno & Eyries, 1957). Although the research of the 1950s was promising, researchers were discouraged when listeners reported that speech delivered by this electrical stimulation to the auditory nerve was unintelligible. The first commercially available cochlear implant was the 3M House implant, introduced in 1972. This implant used a single-channel electrode and was approved for use in the United States by the Food and Drug Administration (FDA) in 1984. As you will note from information provided in this chapter, the single-channel electrode was a beginning; albeit, it did a poor job of replicating the speech signal to the inner ear. At the time, the main benefit of this single-channel cochlear implant was as an aid to lipreading. Current electrodes have 22 to 24 channels available and are generally capable of delivering information to significantly fewer channels than those available. Research suggests that high levels of speech understanding can be obtained for adults with five to eight independent channels of input, whereas children may benefit from input from additional channels. ## The Nature of Cochlear Implants Zeng (2004) described cochlear implants as "... the only medical intervention that can restore partial hearing to a totally deafened person via electrical stimulation of the residual auditory nerve" This description provides a good foundation for understanding the cochlear implant. It is a medical intervention relying on electrical stimulation that is designed for individuals with severe-to-profound degrees of hearing loss. Cochlear implants were designed with the knowledge and understanding that not all individuals with a significant hearing loss can benefit from hearing aids. To understand the basics of how a cochlear implant works, it is important to consider how the cochlea codes sound. The basilar membrane, a flexible membrane within the cochlea in the inner ear, is displaced, or moved, by cochlear fluid. This process is associated with mechanical stimulation of the inner ear from the sound reaching the outer ear. These displacements contain information about the frequency, which is perceived by the listener as pitch of the signal. The displacements of the basilar membrane bend the hair cells attached to the membrane. This bending of hair cells results in the electrochemical substances being released that cause neurons to fire and transmit information about the auditory signal to the brain. In the case of a severe-to-profound sensorineural hearing loss, the hair cells attached to the basilar membrane are damaged and cannot translate sound to neural impulses, and this is considered a hallmark of hearing loss. Loizou (1998) describes this simply by explaining that sound information is able to travel through the outer, middle, and inner ear, but never makes it to the brain due to the broken link of the damaged hair cells. The sound reaches a dead end. One of the concerns is that the auditory neurons close to the damaged hair cells also deteriorate due to a lack of stimulation. In the case of an individual with a profound hearing loss, a significant number of hair cells and auditory neurons are damaged. The fact that most profound sensorineural hearing losses are related to damage to the hair cells is one of the foundations of the development of the cochlear implant. Another consideration in this hearing process is the tonotopic organization of the basilar membrane of the cochlea. Tonotopic organization refers to the representation of frequencies on specific areas of the basilar membrane. You could think of this as similar to the organization of a piano keyboard. High-frequency information is coded at the basal end of the cochlea where low-frequency information is coded at the apex. With this background on the cochlea, we can proceed to the operation of the cochlear implant. The electrode of the cochlear implant also has a tonotopic organization that simulates that of the basilar membrane. In contrast to hearing aids, which are designed to amplify sounds that are detected through a damaged portion of the ear (see Chapter 4), cochlear implants bypass the damaged hair cells and stimulate the auditory nerve directly. Auditory signals are generated from the auditory nerve to the brain, which recognizes the signal as sound. Tye-Murray (2009) describes the cochlear implant as the device that "replaces the hair-cell transducer system by stimulating the auditory nerve directly. bypassing the damaged or missing hair cells" However, hearing through a cochlear implant is perceived differently from typical hearing, and it takes time and practice to learn/relearn the interpretations of sounds. The cochlear implant itself has both internal and external components. The internal components include an internal receiver and an electrode array. An internal receiver (Figure 5-1) is surgically placed in the mastoid bone of the skull, and an electrode array (Figure 5-2) is surgically inserted into the cochlea. The electrode array has the same tonotopic organization as the cochlea. This electrode divides sounds into frequency bands and then stimulates the area of the basilar membrane of the cochlea that corresponds to the sounds being received. The only visible evidence of the internal placement of a cochlear implant is a small scar behind the ear from the surgical incision. External components of the cochlear implant include the processor, microphone, and transmitter. The most critical components of the cochlear implant are the microphone and the speech processor. The role of the microphone is to pick up sounds in the environment. The role of the speech processor is to transform the information from the microphone input to a set of stimuli that can be transmitted to the electrode array in the cochlea. The external components of a cochlear implant are visible, similar in size to a hearing aid, which is a significant improvement over the early devices that were worn on the body. ## Hearing Aid Components All hearing aids, regardless of the style or processing, are made up of three basic components: microphone, amplifier, and receiver. The two general types of microphones are directional, picking up sounds coming from the front of the hearing aid, and omnidirectional, picking up sounds from all directions. Directional microphones help the listener hear an individual speaker in a noisy environment. In contrast, omnidirectional microphones help the listener to be connected in that environment. Many digital hearing aids incorporate automatic directional microphones, which help the listener to understand speech in the presence of background noise while adapting to a number of listening situations. The microphone picks up acoustic energy in the environment, such as speech or environmental noise, and converts it to an electrical signal. The electrical signal generated by the microphone is then routed through an amplifier. The amplifier provides relative amplification based on the hearing loss and a prescription for gain, which is discussed later in this chapter. Once the signal is amplified, it is directed to the receiver of the hearing aid, where it is converted back to an acoustic signal and delivered to the listener's ear canal. ## Styles of Hearing Aids Hearing aids come in a variety of styles but can be placed into two general categories: behind-the-ear styles and custom products that fit into the ear canal. Behind-the-ear hearing aids, also referred to as BTEs, are standard in size. The components fit into a small case worn behind the pinna. The customized portion of the BTE-the earmold-is the part that attaches it into the ear canal. Custom products come in a variety of sizes-in-the-ear (ITE), in-the-canal (ITC), or completely-in-the-canal (CIC)- BTEs are often perceived as less cosmetically appealing than custom hearing aids. BTE hearing aids provide significant benefits in terms of power and features. Although BTEs are appropriate for anyone with a hearing loss, they are also the most appropriate choice for more severe degrees of hearing loss or for listeners with more complex hearing needs. For example, BTEs are the most appropriate choice for children, because these aids can be adjusted to address changing listening demands and can accommodate growing ears by remaking the earmold. Directional microphones, one method for improving signal-to-noise ratio, are easily built into these hearing aids, addressing the problem of noise so common for individuals with sensorineural hearing loss. BTEs have options, such as direct auditory input (DAI), that allow them to be connected to external audio sources, such as MP3 players, and provide a high-quality audio signal. ## Earmolds for BTES As noted previously, the BTE style of hearing aid requires some type of method to hold the hearing aid in the ear. The two options at this point are an earmold (see Figure 4-2), which is custom made and fabricated from an impression made of the wearer's ear. A well-fit earmold continues to be a critical factor in the success of a BTE hearing aid. Earmolds are designed to seal the ear canal. ## IN-THE-EAR and In-The-CANAL HEARING AIDS In-the-ear hearing aids, or ITES are both custom-made hearing aids in which all of the components are housed in a case or shell made from an impression of the wearer's ear. These hearing aids are considered to be self-contained. The case fits into the ear canal, with the primary difference being that the ITE fills more of the concha, or the C-shaped area, of the ear canal than the ITC. A number of wearers view these hearing aids as more convenient than having a hearing aid with an earmold. In addition, the sound quality for both the ITC and ITE is reported to be more natural because the microphone of the hearing aid is in the ear canal rather than behind the pinna, as is the case with BTEs. Although current ITEs can accommodate nearly any degree of hearing loss, ITCs tend to be more appropriate for no greater than moderate-to-severe hearing losses, due to their limited ability to provide sufficient gain and the possibility of acoustic feedback. These styles of hearing aids tend to be somewhat less durable than BTEs in terms of moisture and wax entering the receiver, which is housed directly in the hearing aid and close to the ear canal. ITEs are also more limited with respect to their use in conjunction with assistive technology. ## Are Two Ears Better Than One? ## Binaural Hearing Aids A question frequently asked by individuals who are considering hearing aids is if they really need two hearing aids. For some, this appears to be a perception that they are being oversold; that is, they only really "need" one hearing aid but are sold two by an unethical audiologist only wanting to make money. Others state that wearing one hearing aid is "bad enough," but that wearing two would be unacceptable from a cosmetic perspective or with regard to their self-perception. It is generally accepted that, when appropriate, two hearing aids are better than one, assuming that both ears have a hearing loss. Localizing the sound source, which is important for listening in noise and in providing direction of the sound (which is important for safety), is one of the most basic benefits of binaural hearing. Another benefit is binaural summation: that is, having the sum of information received at the two ears is greater than its parts.