Various implantable hearing aids are now approved for the rehabilitation of conductive, mixed, and sensorineurial hearing loss. (click here for more information on Hearing Loss).
Conductive hearing loss occurs when sound transmission through the external or middle ear is impeded due to mechanical problems (but the hearing nerve is intact). Common causes of conductive hearing loss include congenital aural atresia, persistent ear disease, persistent draining mastoid cavities (as in recurrent cholesteatoma), or surgical defects. Because of the abnormalities in the ear caused by these conditons, many patients can not use conventional hearing aids without feedback.
Bone Anchored Hearing Aids (BAHAs) use a titanium screw implanted into the mastoid process. After this screw becomes integrated (3 months for adults, 6 months for children), a pedestal and vibrating hearing aid are attached to the screw, usually in an office procedure. The vibrating hearing aid stimulates the cochlea, essentially providing a bridge across the defective area, and sound is once again processed by the inner ear.
BAHAs improve hearing sensitivity, permitting speech recognition and detection of quiet sounds. BAHAs do not carry the risk of infection that is associated with the closed earmolds of traditional hearing aids.
Mixed hearing loss occurs as a result of some sensorineural deficit combined with a conductive impairment. High power BAHAs (the Cordel) can achieve bone conduction and hearing improvement comparable to that achieved by BAHAs for individuals with conductive hearing loss.
Sensorineural hearing loss occurs when the cochlea, eighth cranial nerve, or auditory pathways are damaged. Sensorineural hearing loss is most commonly caused by ototoxicity, noise trauma, infection, head injury, tumors, labyrinthine disorders such as Meniere's Disease, genetic causes, aging, and other factors. When an individual experiences sensorineural hearing loss but still maintains a substantial level of hearing, a number of devices can be effective (including conventional and digital hearing aids and assistive listening devices).
The first implantable hearing aid for sensorineural hearing loss, the Vibrant Soundbridge, received FDA approval in 2000. The Soundbridge combines implanted and external components; an external audio processor, held by a magnet to a screw behind the ear, processes acoustic signals. The internal receiver sends the processed signal via a wire to a Floating Mass Transducer (FMT) attached to the incus. The Vibrant Soundbridge is appropriate for patients with substantial remaining hearing, in particular those who recognize at least 50% of speech under headphones. Research is currently underway to develop fully implantable devices, to miniaturize further, and to increase battery life.
When the cochlear structures are damaged but the eighth cranial nerve remains intact, cochlear implants can be very effective. Cochlear Implants are especially effective for patients with poorer than 50% speech recognition. A cochlear implant involves the implantation of an internal receiver behind the ear and a multichannel electrode into the cochlea. An external speech processor then passes electrical stimulation via the electrode to stimulate the auditory nerve. Children are now routinely implanted at the age of 12 months due to the demonstrated advantages in speech and language development. In children, growth of the skull is anticipated by leaving extra lead wires in the mastoid. (see entcolumbia.org/cochkid.htm)
Cochlear implants are undergoing rapid development, with new devices available in body-borne and ear-borne versions. Signal processing mechanisms are becoming increasingly sophistocated as well, permitting vast hearing improvements.
People affected by Neurofibromatosis II (NF2), also called Von Recklinghausen's Disease, develop tumors (vestibular schwannomas) on both auditory nerves. Their removal often results in total deafness in both ears. Hearing aids and cochlear implants are ineffective for NF2 patients. Auditory Brainstem Implants (ABIs) are specifically designed for neural deafness, and they received FDA approval in 2000. ABIs involve the placement of a multichannel electrode over the cochlear nucleus area after tumor removal. An external speech processor is individually programmed and provides electrical stimulation that allows perception of sound. Implantation may be performed during the removal of an acoustic tumor or in a separate procedure. ABIs provide limited speech perception.