Auditory prostheses fall into two general categories. In the first category are cochlear implants and auditory brainstem implants that electrically stimulate the auditory system. The second category consists of a wide range of devices that mechanically stimulate the auditory system. These devices are for individuals with sensorineural, conductive, or mixed hearing loss, or for those with single-sided deafness who are unable to use conventional air conduction hearing aids.
These implants can be further subdivided. In one subcategory are middle-ear implants for sensorineural hearing loss, which use different designs and differ primarily in the means by which they deliver vibratory mechanical energy to the ossicular chain. However, they all impart mechanical energy to the cochlea at the round window via motion of the stapes (e.g., Fredrickson et al., 1995). The external portion of these devices is similar to a cochlear implant (CI), consisting of a microphone, speech processor, and radio frequency (RF) transmitter. The implanted portion consists of an RF receiver, electronic components, and a mechanical vibrator.
In the second subcategory are a number of devices for conductive or mixed hearing losses. These devices impart mechanical energy directly to the cochlea, circumventing the ossicular chain, through direct bone conduction or mechanical energy. Direct bone conduction devices couple energy from a mechanical vibrator to the skull (e.g., Tjellstrom et al., 1995). Because the skull and otic capsule surrounding the cochlea are rigidly coupled, cochlear fluids are vibrated, causing bone conduction hearing. The external component of these devices, which includes an integrated microphone, speech processor, and vibrator, is clipped to an abutment on a pure titanium bone screw that protrudes through the skin. After surgery, the surrounding bone becomes osseointegrated with the screw.
Alternatively, mechanical energy can be imparted to the cochlea through a vibratory transducer placed on the round window or stapes footplate. The external and implanted portions of these devices are similar to middle-ear devices for sensorineural hearing loss, except that the transducer directly stimulates the cochlea (e.g., Colletti et al., 2006).
Middle-ear implants with ossicular placement are indicated for adults with sensorineural hearing loss and normal ossicular function. The implant provides frequency-specific amplification, similar to a hearing aid, to compensate for sensorineural hearing loss. There are some fully implantable middle-ear devices that provide an alternative for individuals with cosmetic or other concerns about hearing aid use. Implants with ossicular placement are not indicated for children.
Middle-ear implants with round window or stapes footplate placement are indicated for adults with conductive or mixed hearing loss caused by middle-ear disease or unsuccessful middle-ear reconstructive surgeries. In the case of conductive losses, the implant bypasses the conductive loss and provides unamplified output directly to the cochlea. For mixed losses, the conductive component is bypassed, and frequency-specific amplification compensates for the sensorineural hearing loss. Implants with round window or stapes placement are indicated for pediatric populations in Europe, but not in the United States.
Osseointegrated implants are indicated for adults and children older than 5 with conductive or mixed hearing losses. Children younger than age 5 with external or middle-ear malformation caused by atresia or microtia undergo a staged treatment. The child wears a headband holding the integrated microphone, speech processor, and vibratory transducer until age 5, at which time the skull is thick enough for the titanium bone screw to be implanted.
Osseointegrated implants also are indicated for adults with single-sided deafness due to sudden hearing loss or subsequent to acoustic neuroma surgery. The implant is placed adjacent to the deaf ear. Vibratory energy from the implant travels across the skull to stimulate the hearing ear. This means of stimulation overcomes the headshadow effect that attenuates sounds contralateral to the hearing ear.
Outcomes are generally successful for implant recipients with normal or near-normal cochlear hearing in the stimulated ear. Outcomes for recipients with sensorineural hearing loss depend on the severity of the loss and are similar to those for air conduction hearing aids.