Published on 13/06/2015 by admin
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Last modified 13/06/2015
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Chapter 32 Implantable Hearing Devices
William H. Slattery, III
It is estimated that 32 million Americans have a hearing loss severe enough to cause problems with communication. The severity of this loss ranges from mild, in which the individual may have difficulty only when significant background noise is present, to profound, in which the patient is unable to understand and communicate even in the quietest situation. Most hearing loss is sensorineural in nature. Less than 10% of hearing-impaired individuals have losses correctable by medical or surgical means. Although hearing loss may affect all frequencies, it is most common for individuals to have some component of high-frequency sensorineural hearing loss. Patients with a mild loss may receive no treatment other than instructions on modifying their acoustic environment to diminish background noise when selecting seating arrangements to allow improved listening conditions. Individuals with conductive losses usually have the opportunity to undergo surgical therapy to have the loss corrected. In some cases, owing to congenital abnormalities or infection, surgical correction is not an option. Individuals with profound sensorineural loss may receive cochlear implants, which provide electric stimulation directly to the cochlear nerve.
Most individuals with sensorineural hearing loss must rely on amplification to provide a better means of improving communication. This amplification is most commonly accomplished with conventional air conduction hearing aids. In recent years, hearing aids have decreased in size, and improved microchips have been developed, resulting in improved signal processing capabilities. Hearing aids have become easier to program with digital processors; they are able to provide much better individualization of amplification according to each patient’s needs. Despite the improvement in conventional hearing aid technology, only approximately 20% of hearing-impaired individuals who could receive benefit from amplification actually use these devices.
There are many reasons why patients do not wear a hearing aid. One of the most common reasons is that conventional air conduction hearing aids do not provide enough amplification of the sounds the individual actually wishes to hear. In addition, they produce troubling amplification of unwanted sounds, especially when background noise is present. Another major problem with conventional hearing aids is the limited high-frequency response. High-frequency output is limited by the high-frequency feedback that occurs when the microphone and receiver of conventional air conduction hearing aids are in close proximity. Although new circuitry (feedback cancellation) has diminished some of these complaints, some users of conventional air conduction hearing aids still complain of feedback problems. Poor fit can result when the ear canal enlarges because of long-term use of the device, causing feedback problems. As microchip technology has allowed hearing aids to become smaller to improve their cosmetic acceptance, it can result in greater problems with feedback as the microphone and speaker are placed closer together. Limited frequency output may produce problems with distortion, and, in particular, limited high-frequency amplification restricts sound localization abilities.
Cosmesis is still a major complaint of conventional air conduction hearing aids. Many patients believe that wearing such a device indicates a disability or carries a stigma of old age. Ear molds may cause an occlusive effect in blocking residual hearing, and produce discomfort, skin irritation, and the potential for increased infections of the ear canal (external auditory canal [EAC]). Discomfort is especially noted with smaller devices that fit more medially in the EAC. The occlusion effect, in addition to being uncomfortable, can result in loss of low-frequency information. There is a significant breakdown rate in conventional hearing aids because of wax in the receiver. Cerumen not only can occlude the receiver of the hearing aid, but also may cause wax impaction by the medial displacement of wax in the EAC.
Implantable hearing devices have become a viable alternative to conventional hearing aids. There are two main types of implantable devices. The more commonly used device is the bone-anchored cochlea stimulator (Baha). Used in Europe since 1977, the Baha received FDA clearance in 1996 as a treatment for conductive and mixed hearing losses. In 2002, the Baha was also approved for treatment of unilateral sensorineural hearing loss or single-sided deafness. The other class of implantable devices is the middle ear implant, in which stimulation of the ossicular chain or direct stimulation to the cochlea is performed. This chapter reviews both of these devices.
The goal of the middle ear implant is to improve the efficiency of the device by improving gain, sound quality, hearing, and noise, and eliminate acoustic feedback. The implant should improve the quality of life. The ideal implantable hearing device should be easy to implant. It should cause no trauma or damage to the normal auditory system. It is crucial that the auditory system remain intact in case of device failure. Experience with cochlear implants has shown the reliability of devices implanted in the postauricular and mastoid area, and revealed potential complications. Potential risks of implantation include further sensorineural hearing loss, damage to the dura, cerebrospinal fluid leak, and the possibility of cholesteatoma or skin implantation into the middle ear or mastoid area. Skin complications can occur with any type of implant, and infection of the device is always a risk. Additionally, difficulties arising from coupling of the device to the ossicular chain or inner ear may directly damage these structures. The facial nerve and chorda tympani may also be at risk with surgical approaches for implantation, resulting in facial paralysis or weakness, or taste disturbance. Increased stiffness of the ossicular chain resulting from device fixation to the ossicles may impede low-frequency response, whereas implants that increase the mass effect on the ossicular chain reduce the high-frequency response.
The ideal device must prove to be safe over the long-term. Batteries are required for these fully implantable devices; changes should be able to be easily performed in the office or on an outpatient basis. It is to be expected that these devices, worn for years, will require future upgrades. The devices should allow technology that is easily upgraded allowing better speech processing strategies to be programmed when made available.
Theoretically, a device that worked all the time—24 hours a day, 7 days a week—would offer a significant advantage over conventional air conduction hearing aids. The ability to use a device in all normal daily activities such as water exposure during bathing or swimming is also a significant benefit. Patients who wear conventional air conduction hearing aids are unable to use them while sleeping or during water exposure. The ideal implant offers significant cosmetic benefit by being as invisible as possible. The lack of maintenance or need to clean the EAC is a significant advantage to hearing impaired patients. A long battery life is essential to reduce the need for additional surgical procedures to change the battery.
Gain is the amount of acoustic energy a device is able to deliver above the incoming signal. An implantable hearing aid must provide better gain than conventional air conduction devices, or some other real benefit to justify its use in an individual patient. Amplification of high frequencies is expected, and gain must be significant enough to provide real benefit to the patient. Most patients with significant hearing loss require significant gain levels in the high frequencies to receive any benefit from a device. If the device does not provide enough amplification in the high frequencies, the amount of benefit the patient receives would be reduced. The maximum level of output in decibels SPL (Sound Pressure Level) required to accommodate various levels of hearing loss is approximately 50 dB above the hearing threshold. To provide benefit to patients with moderate hearing loss to moderate to severe hearing loss, a hearing aid must provide a maximum output level equivalent to 90 to 115 dB SPL, whereas a flat frequency response of 8 kHz is desirable for maximum speech comprehension.
Conventional air conduction hearing aids amplify sound before it reaches the middle ear. A microphone converts the incoming acoustic signal into an electric signal, and the amplifier and signal processor modify the electric signal to increase its strength. The receiver converts the amplified electric signal into an acoustic signal for presentation to the tympanic membrane for transmission via the middle ear to the inner ear in the normal physiologic manner.
In contrast, implantable devices provide acoustic energy to the middle ear or inner ear, bypassing the external ear canal, and in some cases the middle ear space. The microphone converts the incoming acoustic signal into an electric signal. The amplifier and signal processor modify the electric signal to increase its strength. The receiver converts the amplified electric signal into a vibratory signal for presentation to the ossicular chain or to the cochlea directly, bypassing the tympanic membrane. Middle ear implants take advantage of the direct vibration of the middle ear ossicles to drive the ossicular chain.
Middle ear implants may be totally or partially implantable. The partially implantable device consists of a microphone and a speech processor connected to a transmitter fitted with an external coil that transmits electric energy transcutaneously to the internal device. An internal receiving coil connected to a receiver provides electric energy to a transducer connected to the ossicular chain. The external device also has a battery to power the system.
A fully implantable device contains essentially all of the elements of a partially implantable device with the exception of a transducer coil and receiver. A microphone is placed under the skin, or is attached to the middle ear space and is connected to the internal speech processor. The entire system is powered by a rechargeable battery.
All middle ear implantable devices stimulate the ossicular chain or cochlear fluids; however, they differ primarily by the type of transducer used to connect to the ossicular chain or cochlea. These devices provide a broad frequency response with low linear and nonlinear distortion. They are able to amplify high frequencies without the problem of acoustic feedback seen in conventional hearing aids, allowing the potential for better hearing in background noise with a more natural sound quality. A fully implantable device can also eliminate the perceived social stigma of visible conventional hearing aids.
An implantable hearing device is any surgically implanted device that converts acoustic energy to mechanical energy, which delivers vibratory stimulation to the inner ear. A transducer is a device that converts one form of energy to another. Essentially two types of transducers currently are used in middle ear implants: piezoelectric and electromagnetic. Each type has advantages and disadvantages related to power, efficiency, frequency response, and reliability.
Piezoelectric devices make use of ceramic crystals that change shape when voltage is applied. This change in shape can provide mechanical energy to stimulate the ossicular chain or inner ear. The change in the shape of the ceramic is temporary; the ceramic reverts to its original shape when the electric current is no longer applied. There are two types of piezoelectric ceramic crystals: monomorph and bimorph. The monomorph piezoelectric crystal consists of a single layer, which expands and contracts to create the vibrations directly. The bimorph consists of two bonded ceramic layers, which are arranged in opposing electric polarities. When the current is passed through the bonded layers, the entire structure bends, creating the vibration. Anatomic size restrictions limit use of piezoelectric ceramic materials because the amount of bending is proportional to the length of the crystal, reducing the amount of transductive power available.
The electromagnetic transduction of sound involves the creation of mechanical vibration by passing a current through a coil proximal to a magnet. As the electricity passes through the coil, an electromagnetic field is created, vibrating the magnet, which by direct or indirect contact causes movement of the middle ear structures or cochlear fluids or both. The magnet may be attached directly to the middle ear vibratory pathway—the tympanic membrane, incus, or stapes. A fluctuating magnetic field is generated when the coil is energized by electric signals that correspond to the acoustic input. The magnetic field causes the magnet to vibrate, inducing vibration of the ossicular chain or the cochlear fluids directly. The force generated by this system is directly proportional to the proximity of the magnet with the induction coil.
One method to produce electromagnetic stimulation is to separate the magnet from the induction coil. The coil is housed in a separate device, usually within the external ear canal while the magnet is attached to the ossicular chain. It can sometimes be challenging, however, to control the spatial relationship of the magnet and the coil when the magnet is attached to one part of the ear, and the coil is located within the ear canal, which may result in a wide variation in device performance. This can manifest as varying frequency responses, or fluctuation of output levels, if the distance between the alignment of the coil and magnet changes.
Another method of electromechanical stimulation is to house the coil and magnet together, often in a single assembly. If the magnet and coil are housed together, a probe extending from this assembly must be in contact with the ossicular chain. As current is passed through the assembly, vibrations from the probe are sent directly to the ossicular chain. This form of electromechanical stimulation optimizes the spatial and geometric relationships to avoid the problem of changing alignment that may occur between the coil and magnet. The major limitation of housing the magnet and coil together is the attachment of the stimulating device, or coupler, to the ossicle or inner ear. If the device shifts relative to the position of the ossicles or inner ear, there may be a reduction in the optimal transmission of auditory stimulus.
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