You are in: eMedicine Specialties > Otolaryngology and Facial Plastic Surgery > MIDDLE EAR AND MASTOID Implantable Hearing DevicesArticle Last Updated: Feb 14, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 11
  Author: Jack A Shohet, MD, Chairman of Otolaryngology, Hoag Hospital Jack A Shohet is a member of the following medical societies: American Academy of Otolaryngology-Head and Neck Surgery, American Medical Association, American Neurotology Society, American Tinnitus Association, and California Medical Association Editors: B Viswanatha, MBBS, MS, DLO, Professor of ENT, Sri Venkateshwara ENT Institute, Victoria Hospital, Bangalore Medical College and Research Institute, India; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Gerard J Gianoli, MD, Clinical Associate Professor, Department of Otolaryngology-Head and Neck Surgery, Tulane University School of Medicine; Vice President, The Ear and Balance Institute; Chief Executive Officer, Ponchartrain Surgery Center; Christopher L Slack, MD, Otolaryngology-Facial Plastic Surgery, Private Practice, Associated Coastal ENT; Medical Director, Treasure Coast Sleep Disorders; Arlen D Meyers, MD, MBA, Professor, Department of Otolaryngology-Head and Neck Surgery, University of Colorado School of Medicine Author and Editor Disclosure Synonyms and related keywords: implantable hearing devices, implantable hearing aids, implantable middle ear devices, implantable middle ear hearing devices, cochlear implants, auditory brainstem implants, ABIs, bone-anchored hearing aids, piezoelectric devices, electromagnetic hearing devices INTRODUCTIONSection 2 of 11
  Hearing loss affects up to 10% of the population in the United States. The prevalence increases with age and over one third of people older than 65 years have a significant hearing loss. Only approximately 20% of people with hearing loss seek assistance from hearing aids. Of these, as many as 16.2% do not wear their devices. Although technical improvements and modifications have improved the fidelity of conventional aids, hearing aids still have many limitations. The aids may be difficult to maintain, requiring frequent cleaning, dehumidification, and battery changes. Some patients may perceive them as being uncomfortable because they simply cannot tolerate an object in the ear canal. Patients often complain of the occlusion effect of an object occupying the entire ear canal. Chronic otitis externa, canal exostoses, or frequent cerumen impactions make it difficult for some to wear hearing aids. Poorly fitting ear molds, faulty circuitry, or canal issues can lead to annoying feedback. Poor sound quality and problems hearing background noise are frequent complaints of those who wear hearing aids. Finally, some patients may perceive a social stigma associated with hearing-aid use. For excellent patient education resources, visit eMedicine's Ear, Nose, and Throat Center. Also, see eMedicine's patient education article Hearing Loss. CATEGORIES OF IMPLANTABLE HEARING DEVICESSection 3 of 11
Several types of devices can be considered implantable hearing devices. These include cochlear implants, auditory brainstem implants, and bone-anchored hearing devices. Cochlear implants A cochlear implant provides sound perception by means of an electrode surgically implanted into the cochlea. Candidates must have bilateral, profound hearing loss and meet strict audiologic criteria. These devices are covered in other articles of this journal (see Cochlear Implants, Indications and Cochlear Implants, Surgical Technique). Auditory brainstem implants Auditory brainstem implants (ABIs) were designed to be used in neurofibromatosis type 2 (NF-2) in which tumors involving complexes of both cranial nerve VII and VIII render the patient anacusic. These devices, which bypass the cochlea and cochlear nerve, are implanted into the lateral recess of the fourth ventricle adjacent to the cochlear nucleus to provide the patient with auditory perception. These devices are usually implanted after the tumor is resected, during the same operation. House and Hitselberger implanted the first ABIs in 1979. The device was a single-channel percutaneous implant with a ball electrode, which they implanted in approximately 13 patients. The second-generation device incorporated the cochlear implant speech processor (3M/House) and was implanted in 25 patients with NF-2 starting in 1986. In 1992-1993, the first multichannel device (which had an 8-channel array) was developed. In 1994, a multicenter investigation was conducted to evaluate a multichannel ABI powered by a stimulator (Cochlear Corporation Nucleus 22) with transcutaneous signal transmission. The latest ABI incorporates a digital speech processor (Nucleus 24). It offers 4 user-selectable programs, as well as programmable volume and sensitivity controls. The processor uses the SPEAK speech coding strategy and has the flexibility to add future strategies as they are developed. The device is indicated in patients with NF-2 aged 12 years or older. Implantation may occur during tumor removal on the first or second side or as a separate procedure. The patient should be medically and psychologically suitable, and no audiologic criteria are applied. Because of possible injury to the cochlear nucleus due to radiotherapy, prospective recipients who have undergone gamma-knife irradiation should be considered with extreme caution. A study on 61 patients receiving the multichannel ABI showed that the device is effective and safe in providing useful auditory sensations in most patients with NF-2.1 The ABI provides enough auditory information to improve lip-reading abilities in most, and a few are able to achieve open-set (no lip-reading cues) speech understanding. Performance may improve for up to 8 years after implantation. ABIs have been used with varying success in patients born with cochlear nerve aplasia, those with traumatic cochlear-nerve avulsion, and those in whom cochlear implantation is unsuccessful (for salvage treatment). Bone-anchored hearing devices The Baha device (Entific Medical Systems) is a percutaneous implantable device primarily used for conductive hearing loss, or more recently, for single-sided sensorineural hearing loss. Developed in An osseointegrated titanium fixture with a percutaneous abutment is implanted in the postauricular area. An external sound processor is attached to the abutment at will. A microphone in the processor, which vibrates the bone in the skull by means of the fixture, picks up sound. The sound is transmitted directly to the inner ear on the side with conductive hearing loss or better sensorineural hearing than the other. The external processor is available in 3 sizes and is chosen depending on the bone thresholds of the ear to be aided. A body-worn external processor allows for the implantation for pure-tone-average bone thresholds as low as 70 dB HL. The 2 small processors are for pure-tone-average bone thresholds better than 45 dB HL. The Baha device can be placed bilaterally in patients with bilateral disorders to allow for sound localization and to improve speech recognition in noise. The surgery is typically performed in a single stage in adults. About 3-4 months is required to allow for osseointegration before the external processor can be attached and its benefits realized. A 2-stage procedure is recommended in children in whom the fixture is placed into the bone at the first stage. After 3-6 months to allow for osseointegration, a second-stage operation is done to connect the abutment through the skin to the fixture. Complications are few and mostly limited to local infection and inflammation at the implant site and a failure of osseointegration. The US Food and Drug Administration (FDA) approved the Baha device for use in children aged 5 years or older, but has been used successfully in Before the FDA approved this device for single-sided deafness, the contralateral routing of signal (CROS) hearing aid was the only option available for rehabilitation. Poor performance and aesthetic considerations limited the use of CROS aids. The Baha device can be implanted on the side of the deaf ear, and it transmits the sound by means of bone conduction to the contralateral cochlea. This process eliminates the head-shadow effect and allows for hearing from both sides of the head. The Baha aid substantially improves speech recognition in quiet and in composite noise compared with the CROS aid. IMPLANTABLE MIDDLE-EAR DEVICESSection 4 of 11
The implantable middle-ear hearing devices discussed in this article were developed to treat conductive and sensorineural hearing loss. Extensive trials are in progress to determine their efficacy. The rationale for development of implantable middle-ear devices is multifaceted. These devices improve fidelity by directly stimulating the ossicles, and they improve comfort by allowing the ear canal to remain open. In addition, most implantable middle-ear devices almost completely eliminate feedback, one of the most annoying adverse effects of conventional aids. Improved cosmesis by means of miniaturization and concealment of the components is another benefit. Finally, some devices may allow the patient to continue receiving amplification while swimming or bathing. TYPES OF IMPLANTABLE MIDDLE-EAR DEVICESSection 5 of 11
Implantable middle-ear devices are generally available in 2 types: piezoelectric and electromagnetic. Piezoelectric DevicesPiezoelectric devices operate by passing an electric current into a piezoceramic crystal, which changes its volume and thereby produce a vibratory signal. The major disadvantage is that power output is directly related to the size of the crystal. Studies of early designs indicate that such an approach benefits only people with up to moderate (about 60 dB) hearing loss. These are the same individuals who benefit from the small completely-in-the-canal (CIC) type of hearing aid. A CIC hearing aid may be most useful if cosmetics are the primary concern. Piezoelectric transducers have the advantage of being inert in a magnetic field and therefore compatible with MRI. Rion device, E-type One of the earliest piezoelectric devices, the Rion Device E-type (RDE), has been used for both conductive and sensorineural losses. It is a partially implanted device composed of an external, ear-level microphone and amplifier, and an internal electromagnetic coil and vibrator element. The piezoelectric vibrator element is anchored to the squamous portion of the temporal bone with a titanium screw. It is attached to the stapes with a hydroxyapatite tube, which is interposed between the tip of the vibrator and the head of the stapes. In 39 patients in Japan, an initial hearing improvement of 36 dB at 3 months after surgery eventually decreased to 21 dB with long-term follow-up.2 The reason for diminished performance was thought to be to a decrease in the sensitivity of the ossicular vibrator caused by aging and tissue reaction around the vibrator element. The authors reported that sensorineural hearing was not affected and that all patients preferred the device to a conventional aid. Totally integrated cochlear amplifier The Totally Integrated Cochlear Amplifier (TICA; Implex American Hearing Systems, now owned by Cochlear Corporation), is totally implantable. The microphone is implanted subcutaneously in the external ear adjacent to the tympanic membrane. A digitally programmable processor located subcutaneously on the mastoid bone processes the signal. A piezoelectric transducer is coupled to the body of the incus and drives the ossicular chain by vibratory actions. Many European and some North American research programs have used this approach. The TICA received the C.E. mark in Europe in the late 1990s but have not undergone studies in the Middle ear transducer The Middle Ear Transducer (MET; Otologics LLC, Boulder, CO) was first introduced as a semi-implantable device but was recently made totally implantable. The original device was composed of an implanted transducer mounted in a laser-drilled hole in the body of the incus. The transducer translates the electrical signals into a mechanical motion that directly stimulates the ossicles and enables the wearer to perceive sound. The transducer is coupled with an externally worn audio processor (Button processor), which contains the microphone, battery, and signal processor. The device has a CE mark for pan-European approval to market the device. To date, more than 300 patients have received the implant in Europe and in the The MET Fully-Implantable Ossicular Stimulator (Otologics) consists of 4 primary components: the implant, the programming system, the charger, and the remote control. The implant component consists of 2 main parts: the electronics capsule and the MET. The electronics capsule contains the microphone, battery, magnet, digital signal processor, and connector. A sensitive microphone located under the skin picks up sounds, which are amplified according to the wearer's needs, and converted into an electrical signal. The signal is sent down the lead and into the transducer, which is the same as that used in the semi-implantable version. The semi-implanted version can be upgraded to the fully implanted version in a single surgical procedure by using local anesthesia. The programming system coil is placed over the implant site and held in place magnetically. The coil couples with the implant by means of a radiofrequency signal that is used to program the device in the same manner as a traditional digital hearing aid. The programming system also allows for extensive testing and diagnostics of the stimulator. The charger system consists of the base station, charging coil, and charger body. To charge the implant, the wearer removes the charger body from the base station and places the coil on the skin, over the implant site. The charger body contains a clip that allows the charger to be attached to the belt of the wearer during charging. Typically, charging time will be about one hour if performed daily. While recharging the implant, the wearer can perform normal daily activities, turn the implant on and off, and adjust the volume. A remote is used to control the stimulator when the device is not being charged. The remote allows the wearer to turn the implant on and off, and to adjust the volume. To use the remote control, the wearer holds the remote against the skin over the implant. Envoy system Another totally implantable piezoelectric device is the Esteem by Envoy Medical (originally St Croix Medical). This device uses the eardrum as the microphone, taking advantage of the natural acoustics of the ear canal without obstruction, interference, or any external devices. Therefore, the input signals are identical to those received by a person with normal hearing. This mechanical signal is detected from a piezoelectric transducer at the head of the incus (the sensor) and converted to an electrical signal by using existing transducer technology. The electrical signal is amplified, filtered, and converted back to a vibratory signal. The processed vibratory signal is then delivered by means of a piezoelectric transducer (the driver) attached to the stapes capitulum. The incus lenticular process is removed to prevent feedback to the sensor. The newly designed piezoelectric transducer can provide an output close to 110 dB sound pressure level (SPL). An audiologist programs the implant using a device called the commander. After the device is programmed, patients are given a personal programmer that allows them to turn the device on or off, to adjust the volume, and to remotely modify background noise filters. The advantages of such a device are notable. Without any appliance in the external auditory canal, the occlusion effect is eliminated. Uncoupling of the sensor and driver eliminate most feedback. Even more important for some is the fact that the device is completely concealed in the body. The Envoy device faces some hurdles as it is further developed. Its battery life is an issue, as with any totally implantable device. The battery has an estimated life of 3-5 years depending on use and can be replaced by using local anesthetic. In addition, removal of a portion of the incus permanently alters the ossicular mechanism and prohibits full recovery of hearing to preimplantation baseline levels if the device fails or is in the off position. Modern ossicular reconstruction techniques may effectively restore hearing to within 10 dB. Finally, functional gain decreased by more than 3000 Hz in the phase I study of the Envoy device. This appears to have been substantially improved in the phase II studies. The phase I clinical trial included 7 patients, 5 of whom had working implants in the 2-month postactivation period. The remaining 2 had no benefit from the device as a result in a breach of the hermetic sealing of the device, which allowed moisture to pass into the circuitry. This breach was corrected in the current version of the device. Overall, the patients with functioning devices perceived a benefit with the Envoy device compared with their hearing aid in terms of ease of communication in favorable conditions, background-noise reverberation, and aversiveness of sound. Speech discrimination was markedly improved over hearing-aid conditions by 17%. Functional gain and speech reception thresholds were similar for the Envoy device and for the hearing aids. Electromagnetic Hearing DevicesElectromagnetic hearing devices function by passing an electric current into a coil, which creates a magnetic flux that drives an adjacent magnet. The small, 50-mg magnet is attached to one of the vibratory structures of the middle ear (eg, tympanic membrane, ossicles). To date, all of the research programs using this method use devices that are only partially implantable and that still require an external hearing-aid shell to house the electric coil. In some cases, the external coil can be housed in a CIC type of hearing-aid shell. The major disadvantage is that power is decreased by the square of the distance between the coil and the magnet; therefore, the coil and magnet must be close. A slight shift of coil position in the external ear results in unpredictable or insufficient power output. Furthermore, the anatomy of the middle ear space restricts the size of the magnet or coil. Vibrant Soundbridge device One example of an electromagnetic device is the Vibrant Soundbridge device. Originally developed by Symphonix Devices, Inc., the Soundbridge was the first FDA-approved implantable middle-ear hearing device to treat sensorineural hearing loss. It was marketed and implanted in the The Soundbridge device is a semi-implantable device composed of an external sound processor and amplifier, an audio processor, and an internal vibrating ossicular prosthesis (VORP). Sound passes into a microphone on the postauricular audio processor and is transmitted through the skin to an implanted receiver on the VORP. The VORP, which is implanted postauricularly (similar to a cochlear implant) conducts the sound to a magnet surrounded by a coil called the floating mass transducer (FMT). The transducer is attached to the long process of the incus and the magnet hugs the long axis of the stapes, which causes it to vibrate. One of the disadvantages of this device is that the confined space of the middle ear restricts the dimensions and crucial mass of the transducer, limiting power output. The phase III FDA trial was completed in 2000, and results from the 53 patients submitted to the FDA were published in 2002.4 The device was safe, with no notable change in preoperative and postoperative bone thresholds. Furthermore, functional gain and word-recognition scores were improved with the Vibrant device compared with the patients' conventional hearing aids. Self-assessment inventories indicated that 94% of the patients believed that the overall sound quality of the Vibrant Soundbridge device was better than that with their conventional hearing aid. Soundtec direct system The Soundtec implant was introduced to the Such a design offers several possible advantages. Because it works by electromagnetic energy through the ear canal, the Soundtec device does not require an acoustic seal, which may lead to the occlusion effect or alter the resonance qualities of the ear canal. Also, functional gain can be improved without necessarily precipitating feedback, a common problem with traditional aids that occurs when sound pressure escapes the ear canal and cycles back through the microphone. A relative disadvantage is that the procedure requires separation and then reconstitution of the incudostapedial joint, a maneuver that may be responsible for as much as a 4.2-dB increase in air-conduction thresholds after surgery. Subsequent studies further demonstrated a loss of average bone conduction of pure tone after implantation. This effect may be a result of movement of the mobile stapes into the vestibule during disarticulation of the incudostapedial joint, which may cause sensorineural hearing loss. Early reports indicated that, though the Soundtec did not provide a significant difference from optimal traditional amplification, it provided statistically significant high-frequency functional gain. Furthermore, patients' subjective reports indicated a cleaner, more natural sound without feedback than that achieved with traditional amplification. The phase II FDA clinical trial included 103 patients with moderate to moderately severe sensorineural hearing loss who had previously worn hearing aids for at least 45 days.10 An average 7.9-dB increase in functional gain in the speech frequencies and a 9.6-dB increase in the high frequencies was reported, as was a statistically significant increase of 5.3% in speech discrimination. Furthermore, subjective measures of feedback, occlusive effect, perceived aided benefit, patient satisfaction, and device preference over the patient's optimally fit hearing aid were demonstrated. A long-term follow-up retrospective review of 64 patients receiving the Soundtec device revealed a significant average functional gain of 26 dB.11 About 55% of patients complained of hearing the magnet move when the processor was not being worn. This effect was improved, but not completely eliminated, by further stabilizing the implant by placing adipose tissue between the implant collar and the neck of the stapes. A patient questionnaire to assess sound quality, speech in noise, and satisfaction with the Soundtec compared with conventional hearing aids failed to show a significant difference. The authors concluded that the ideal patient is one younger than 70 years with a moderate sensorineural hearing loss, speech discrimination scores equal or better than 60%, appropriately sized ear canals, and sufficient manual dexterity to insert the processor. To date, around 600 Soundtec devices have been implanted, with the most users in the Additional studies of the Soundtec in a magnetic field indicate that it should be mechanically stable and nondestructive during 0.3-T open MRI with a modified MRI protocol.12, 13 Another electromagnetic device is the semi-implantable middle-ear electromagnetic hearing device (SIMEHD). This device consists of an external unit, which is similar in appearance to a behind-the-ear hearing aid, and an internal device, which is anchored to the mastoid. The internal device contains a driving coil and a magnet cemented to the incus. IMPLANTABLE MIDDLE EAR HEARING DEVICE, BIOMECHANICAL ISSUESSection 6 of 11
A few major biomechanical issues must be addressed in developing implantable middle-ear devices. First, the device should not affect normal function of the middle ear. In the optimal situation, the device should not alter air-conduction thresholds. If the device is unsuccessful, the added mass of the unit attached to the vibratory structure of the middle ear should not affect that structure's ability to vibrate. Another important issue is the anchoring of the device to the ossicular chain. Even a little laxity at the interface between the prosthesis and bone could diminish the transmitted power enough to render the device ineffective. Long-term stability of the fixation must be considered as well. The mechanical forces acting at the interface could affect the life expectancy of the device. Finally, the direction of the transducer's vibration must be coincident with the axis of normal sound transmission through the ossicular chain. In effect, the device must be attached to the tympanic membrane, the long process of the incus, or the head of the stapes. Otherwise, the transmitted force is reduced. INDICATIONSSection 7 of 11
In general, candidates for implantable middle-ear devices should have tried conventional aids with no success. Some devices were designed for patients with mixed hearing loss. Candidates may also have a conductive hearing loss with an irreversibly disabled middle ear, as with severe tympanosclerosis or chronic eustachian tube dysfunction. The Japanese Rion device was designed for this group of patients. Most current devices are designed for patients with mild-to-severe sensorineural hearing loss. Mild-to-moderate loss is usually adequately improved with conventional aids. Severe cases are limited by feedback. Given adequate output production, an implantable aid can amplify a severe loss and avoid feedback. CHALLENGESSection 8 of 11
Many challenges remain in developing the ideal implantable hearing device. Limitations in the capacity and recharging cycles of available batteries necessitate transducer energy efficiency. Restrictions due to the size of the middle ear challenge the device to deliver enough gain to aid a severe hearing loss. Costs associated with the development of these devices, as well as surgical implantation, make these devices more expensive than conventional hearing aids. This cost may limit their widespread acceptance. Another hurdle is in measuring the devices' perceived benefit. All implantable middle ear devices have shown benefit over hearing aids in the patient's perception of the sound fidelity, a very important quality. These implants rely on the Abbreviated Profile of Hearing Aid Benefit (APHAB), a subjective questionnaire of sound qualities that patients complete preoperatively with their hearing aid and postoperatively with the implant. However, quantitatively measuring the psychoacoustic qualities of sound that are so important to the patient is difficult. CURRENT STATUSSection 9 of 11
Some devices are currently being studied in FDA trials, but the FDA has formally approved only the Vibrant Soundbridge and Soundtec direct systems so far. Many believe that these devices may represent a new era in hearing augmentation akin to the largely successful, but more narrowly indicated, cochlear implants. Although several hurdles still remain, the potential advantages and increasing number of people who may benefit from such devices continue to support their development. MULTIMEDIASection 10 of 11
REFERENCESSection 11 of 11
Implantable Hearing Devices excerpt Article Last Updated: Feb 14, 2008 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
