Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Implantable Hearing Devices
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 even in the quietest situation the patient is unable to understand and communicate. Most hearing loss is sensorineural in nature, and less than 10% of the hearing impaired population has a hearing loss that is potentially correctable by medical or surgical means. Although sensorineural hearing loss may affect all frequencies, this type of hearing loss most commonly affects the high frequencies initially. Patients with a mild loss may receive no treatment other than instructions on modifying their acoustic environment to diminish background noise, such as creating preferential seating arrangements to allow improved listening conditions. Individuals with conductive losses may have the opportunity to undergo surgical treatment to have the loss corrected. However, in some patients, because of congenital abnormalities or infection, surgical correction is not an option. Individuals with profound sensorineural loss may receive cochlear implants, which provide electrical stimulation directly to the cochlear nerve.
Most patients with sensorineural hearing loss must rely on amplification to provide a better means of improving communication. Amplification is most commonly accomplished with conventional air-conduction hearing aids. Continuous improvements in technology and signal processing have led to decreased size and increased fidelity for hearing aids. Still, despite the improvement in conventional hearing aid technology, only approximately 20% of the hearing impaired population who may receive benefit from amplification actually uses these devices.
There are many reasons why patients decline to wear hearing aids. A common reason is that conventional air-conduction hearing aids may not provide enough amplification of the sounds that the individual actually wants to hear. In addition, they may produce troublesome 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 partially limited by the feedback that occurs when the microphone and receiver of conventional air-conduction hearing aids are in close proximity. Although the human ear has a range of hearing from 100 to 18,000 Hz, conventional air-conduction hearing aids often have difficulty amplifying sounds above 3000 to 4000 Hz. Feedback cancelation is often incorporated in modern hearing aids, but many conventional air-conduction hearing aids still present problems with feedback. Ear canal stenosis and meatoplasty can both result in a poor hearing aid fit, exacerbating feedback issues. Limited frequency output may produce problems with distortion, and furthermore, limited high-frequency amplification restricts sound localization abilities.
Cosmesis is still a major complaint regarding conventional air-conduction hearing aids. Many patients believe that wearing a hearing aid indicates a disability or carries the stigma of old age. The other disadvantages of conventional air-conduction hearing aids, particularly those with molded earpieces, include the occlusion effect; blocking of residual hearing; and discomfort, skin irritation, and the potential for increased infections of the external auditory canal (EAC). Occlusion resulting from blockage of the EAC results in decreased comfort in addition to the attenuation of low-frequency sounds. Cerumen can occlude the speaker of the hearing aid, and patients who wear conventional hearing devices are at increased risk of EAC blockage by wax impaction caused by the medial displacement of wax in the EAC.
Over the past four decades, implantable hearing devices have become a viable alternative to conventional hearing aids. Implantable devices overcome many of the deficiencies found with conventional air-conduction hearing aids. Because surgery is required, some degree of risk is inherent. For patients with nonserviceable hearing secondary to profound sensorineural loss, cochlear implants are the mainstay of hearing rehabilitation. For patients with serviceable hearing remaining, two main classes of implantable devices exist. The more commonly used type harnesses calvarial bone-conduction for sound transmission and includes the Cochlear Baha, the Oticon Ponto, and newer devices, such as the Med-EL Bonebridge and the Cochlear Osia. Used in Europe since 1977, the Cochlear Baha received Food and Drug Administration (FDA) clearance in 1996 as a treatment for conductive and mixed hearing losses. In 2002, it was also approved for treatment of unilateral sensorineural hearing loss or single-sided deafness.
The other class of implantable devices in patients with serviceable hearing remaining is the middle ear implant, in which stimulation of the ossicular chain or direct vibratory stimulation to the cochlea via the round or oval window is performed. In 2000, the Vibrant Soundbridge became the first semi-implantable middle ear implant approved by the FDA; it was later acquired by Med-EL. In 2010, the Envoy Esteem became the first fully implantable middle ear implant approved by the FDA. As cochlear implants and bone-conduction devices are covered in separate chapters, this chapter is focused on the history, technology, and current status of middle ear implantable devices.
Requirements of Implantable Devices
The goal of the middle ear implant is to improve on gain, sound quality, and hearing in noise and acoustic feedback issues relative to conventional hearing aids. The ideal implantable hearing device should be easy to implant and increase a patient’s quality of life. In addition, this implant should cause no trauma or damage to the normal auditory system, including in the case of device failure. Experience with cochlear implants has demonstrated that these devices can be implanted reliably in the postauricular and mastoid areas but that potential complications exist. The potential risks of implantation include further sensorineural hearing loss, damage to the dura, cerebrospinal fluid (CSF) leak, and the possibility of cholesteatoma or skin implantation into the middle ear or mastoid. Skin complications can occur with any type of implant, and infection of a surgically implanted device is always a risk. In addition, the act of 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 the low-frequency response, whereas implants that increase the mass effect on the ossicular chain reduce the high-frequency response. An increase in stiffness or mass effect on the ossicular chain may also cause a disruption in the normal relationship between the eustachian tube and tympanic membrane.
The ideal device must prove to be safe over a long period of time. Fully implantable devices require batteries; battery changes, if necessary, should be performed easily in the office or on an outpatient basis. Devices designed with an external processor and battery, such as the Soundbridge, are immune to this consideration. It is to be expected that these devices, worn for years, will require future upgrades. The devices should be easily upgradable, allowing better speech processing strategies to be programmed when available.
Theoretically, a device that worked continuously would offer a significant advantage over conventional air-conduction hearing aids. The ability to use a device in all normal daily activities, such as during water exposure (showering, bathing, or swimming) and even during sleep, is also a significant benefit. The ideal implant offers cosmetic benefit by being as invisible as possible. The lack of maintenance or need to clean the EAC provides another significant advantage.
Conventional Versus Implantable Hearing Aids
Gain reflects the amount of acoustic energy a device is able to deliver above the incoming signal. An implantable hearing aid must provide better gain than a conventional air-conduction device to justify its use in an individual patient. Amplification of high frequencies is crucial for speech understanding, and most patients with significant hearing loss require significant gain in the high frequencies. The level of output in decibels sound pressure level (dB SPL) required to accommodate various levels of hearing loss is approximately 50 dB above the hearing threshold. To provide benefit to patients with moderate to moderate-to-severe hearing loss, a hearing aid must provide a maximum output level in the approximate range of 90 to 115 dB SPL, and a relatively flat frequency response up to 8 kHz is desirable for maximum speech comprehension.
Conventional air-conduction hearing aids amplify sounds before they reach the middle ear. A microphone converts the incoming acoustic signal into an electrical signal, and the amplifier and signal processor modify the electrical signal to increase its strength. The receiver then converts the amplified electrical signal into an acoustic signal for presentation to the tympanic membrane and transmission via the middle ear to the inner ear in the normal physiological manner.
In contrast, implantable devices provide acoustic energy to the middle or inner ear, bypassing the EAC and, in some procedures, the ossicles themselves. The microphone converts the incoming acoustic signal into an electrical signal. The amplifier and signal processor modify the electrical signal. The receiver converts the amplified electrical signal into a vibratory signal for presentation to the ossicular chain or to the cochlea directly via the round or oval windows, bypassing the tympanic membrane.
These implantable devices may be categorized as partially or totally implantable. Partially implantable devices, such as the Soundbridge, consist of a microphone and processor and a battery that couples transcutaneously to the implanted internal device. An internal receiving coil connected to a receiver provides electrical energy to a transducer connected to the ossicular chain. The battery to power the device is located within the external device.
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 in the middle ear space and connected to the internal speech processor. The entire system is powered by an internal battery.
Middle ear implants all 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 distortion. They are able to amplify high frequencies without the problem of acoustic feedback experienced with conventional hearing aids, allowing the potential for better hearing in the presence of background noise with a more natural sound quality. A fully implantable device can also eliminate the perceived social stigma of visible conventional hearing aids.
Types of MIddle Ear Implants
The definition of an implantable hearing device is any surgically implanted device that receives external acoustic energy and delivers vibratory stimulation to the inner ear. With most implants, this means that acoustic energy must be transformed into an electrical signal, which is then transformed into a mechanical vibration applied to the inner ear. A transducer is a device that converts one form of energy to another. There are essentially two types of transducers currently used in middle ear implants: piezoelectric and electromagnetic. Each type has its advantages and disadvantages related to power, efficiency, frequency response, and reliability.
Piezoelectric devices make use of materials, most often ceramics, that change shape when voltage is applied. This change in shape then provides mechanical energy to stimulate the ossicular chain or inner ear. The change in the shape of the ceramic is not permanent; the ceramic reverts to its original shape when the electric current is no longer applied. Broadly speaking, there are two types of piezoelectric elements: monomorph and bimorph. The monomorph piezoelectric element consists of a single layer that expands and contracts to create the vibrations directly. The bimorph consists of two bonded ceramic layers, typically arranged in opposing electrical polarities. When current passes through the bonded layers, the entire structure bends, creating a vibration. Anatomical size restrictions limit the use of piezoelectric ceramic materials, because the amount of bending is proportional to the length of the crystal, thus limiting the amount of transductive power available.
In contrast to piezoelectric transduction, electromagnetic transduction involves the creation of a 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. A fluctuating magnetic field is generated when the coil is energized by electrical signals that correspond to the acoustic input. The magnetic field causes the magnet to vibrate. The magnet may be attached to the tympanic membrane or ossicles or directly to the inner ear fluids via the round window. 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 EAC, and the magnet is attached to the ossicular chain. However, it can sometimes be challenging 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. In this situation, 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 this type of device is the attachment of the stimulating device, or coupler, to the ossicles or inner ear. If the device shifts relative to the position of the ossicles or inner ear, there may be a reduction in optimal transmission of auditory stimuli.
History of Middle Ear Implants
The use of a magnetic field to induce mechanical vibrations to stimulate the ossicles is not new. This concept can be traced back to 1935 when Wilska placed iron particles directly on the tympanic membrane. A magnetic field was generated by an electromagnetic coil inside an earphone, which caused the iron fillings to vibrate in synchrony with the magnetic field, producing vibration of the tympanic membrane, simulating hearing. Later, Rutschmann glued 10-mg magnets onto the umbo, causing it to vibrate via the application of a modified magnetic field with an electromagnetic coil. The resulting vibration of the ossicles produced hearing sensation. The implantation of devices within the middle ear space did not occur until the 1970s with the Rion device, to be discussed later in this chapter. Frederickson et al. developed the first mechanical device at Washington University in St. Louis, Missouri in 1973. This device, which used a multichannel digital signal processor that transmitted power to the implanted coil via a transcutaneous link, was implanted in 12 rhesus monkeys. After 2 years of implantation, there was no damage to the cochlea or peripheral auditory system. Frederickson found that the results from mechanical stimulation were similar to those produced by acoustic stimulation and that high-intensity signals could be delivered to the middle ear effectively.
Maniglia et al. at Case Western Reserve University in Cleveland, Ohio demonstrated the use of an electromagnetic device in cats. This group used the malleus as the microphone for a totally implantable device, which was implanted in cats for approximately 9 months. As with Frederickson’s experiments, the results demonstrated thresholds from mechanical stimulation comparable with acoustic stimulation with no adverse effects on the middle or inner ear. Dumon et al. in Bordeaux, France worked on piezoelectric devices placed in contact with the round window membrane. Twelve guinea pigs were stimulated over a 7-month period. This group attached the piezoelectric devices to the round window without removing any other component of the ossicular chain.
You're Reading a Preview
Become a Clinical Tree membership for Full access and enjoy Unlimited articles
If you are a member. Log in here