What is a cochlear implant?
A cochlear implant is an electronic device that is implanted into the inner ear and is used to restore hearing in patients who have severe to profound hearing loss. In patients with normal hearing, sound is collected by the external ear and funneled down to the eardrum.
The sound waves then are carried through the middle ear by the middle ear bones (the ossicles). The sound pressure waves are then converted to nerve signals within the inner ear (the cochlea). A cochlear implant bypasses the normal sound conduction mechanism through the external, middle, and inner ear, and directly stimulates the auditory nerve.
An implant does not restore or create normal hearing. Instead, under the appropriate conditions, it can give a deaf person a useful auditory understanding of the environment and help him or her to understand speech. Cochlear implants were approved by the Food and Drug Administration (FDA) in the mid-1980s and are covered by insurance policies, Medicare, Medicaid, and Vocational Rehabilitation. There are now more than 60,000 patients worldwide who have received cochlear implants.
Who is a candidate for a cochlear implant?
Patients have to be so hard of hearing that a hearing aid is of no benefit. For most patients, a cochlear implant will not work as well as a hearing aid. The only way to determine whether or not a patient would benefit from a cochlear implant is to be evaluated by the implant team of physicians and audiologists.
The most common groups of patients who benefit from a cochlear implant are:
- Children who are born deaf
- Young or middle-aged adults with hearing loss due to genetic causes, autoimmune disease, or unknown reasons
- Older adults with progressive hearing loss due to aging or noise exposure
A cochlear implant is made up of two parts. The external device consists of a microphone behind the ear that hears the sounds in the environment. These sounds are then digitized and processed by a small computer called a speech processor. Small speech processing units can be hidden behind the ear while larger ones have to be worn on your belt. Of course, the bigger the processor, the more capability it has to select speech signals out of the environment, ultimately improving hearing. However, computer technology is continuing to improve so that behind-the-ear speech processors now provide quite good results. The signals from the speech processor are sent to the implanted part of the device though the skin using a magnet.
The implanted part is an electronic device that is put under the skin behind the ear.
An electrode connected to the device is inserted into the inner ear. The electrode is simply a bundle of tiny wires that have open contacts spread out along the length of the cochlea. Thus, the electrical signals can be sent to different areas of the cochlea and represent different frequency sounds.
State-of-the art cochlear implant devices now have up to 24 electrodes that stimulate the auditory nerve. These multi-channel implants have the advantage of stimulating many different nerve fibers individually, thereby transmitting more detailed information to the brain. The more information that reaches the brain, the greater the patient's ability to understand what is happening in his/her environment.
A cochlear implant allows a deaf patient to hear. Hearing through an implant may sound different from normal hearing, but it allows many people to communicate fully with oral communication in person and over the phone. One in 1,000 children are born with a severe to profound sensorineural hearing loss. Hearing loss of that magnitude occurring so early in life has a significant deleterious effect on speech and language development. Because a child learns much about his/her world by listening, a cochlear implant can provide significant improvements in a child's ability to learn to communicate.
A cochlear implant is also very beneficial for adults who have already learned to communicate, but are now unable to hear. Hearing loss isolates one from society in a way that is much different than other disabilities. Because you can't hear what others are saying, you can't communicate back with them. Thus, interpersonal relationships with others who hear normally become more difficult to develop and sustain. If a hearing aid is not provide useful benefit in this situation, a cochlear implant often can, bringing a patient back into a world of communication.
For children, once the diagnosis of severe hearing loss is made, parents may chose to have their child taught in either oral language or a non-oral language such as American Sign Language (ASL). Sign language is a way to teach children how to communicate without having to hear. They can develop close relationships with other people in the deaf community. One disadvantage to this method however, is that it becomes difficult for the child to communicate with people who do not know sign language. Thus, special schooling in schools for the deaf is required.
In contrast, if oral language training is selected, hearing aids will be fitted to amplify speech and environmental sounds to help the child learn to communicate. If hearing aids do not provide sufficient benefit for oral language development, a cochlear implant will be recommended. Cochlear implants have also benefited school age and older children as well to help them develop and improve their communication skills and achieve their educational goals. Most children who receive a cochlear implant end up being mainstreamed in community schools and do quite well with their peers.
For adults, the decision to proceed with a cochlear implant is personal, just like that to get a hearing aid. Interestingly, many patients don't treat their senses equally. Although not a perfect analogy, hearing can be compared to vision. If you began having progressive loss of vision, you would immediately get your eyes checked and get glasses. If surgery was needed to preserve your vision, most would have the surgery.
Cochlear Implant Surgery
This is a fairly routine type of surgery for physicians who are trained in ear surgery (Otology, Neurotology, and Skull Base Surgery). It takes about 3 hours and involves making an incision behind the ear. The mastoid bone behind the ear is drilled to get to the inner ear and the electrode is placed. The incision is then closed and a dressing applied. Often the patient is kept in the hospital one night for observation, but in other cases the patient can be discharged home the same day as surgery. It is not particularly painful, and patients can resume their normal activities within 2-3 days typically. The implant is not turned on until the incision has fully healed, about 4 weeks after surgery.
Although surgical implantation is almost always safe, complications are a risk factor, just as with any kind of surgery. Inserting a cochlear implant destroys any residual hearing in the operated ear. Thus, one can not turn back. The standard surgical risks of a cochlear implant are all quite rare. These include: bleeding, infection, device malfunction, facial nerve weakness, ringing in the ear, dizziness, and poor hearing result.
One long-term risk of a cochlear implant is meningitis (infection of the fluid around the brain). This is very rare and there have only been 91 cases out of 60,000 patients with cochlear implants. However, 17 of these patients have died. Those patients who had meningitis tended to have several predisposing risk factors including congenital inner ear malformations, previous history of meningitis, immune system dysfunction, less than 5 years of age, and a history of recurrent ear infections. Because the cochlear implant runs between the middle and inner ear, bacteria within the middle ear may tract along the implant into the normally sterile inner ear. The inner ear has connections with the brain, through which the infection may spread. Currently, all patients who receive a cochlear implant need to be immunized against the common bacteria that cause meningitis.
An additional consideration is learning to interpret the sounds created by an implant. This process takes time and practice. Speech-language pathologists and audiologists are the professionals frequently involved in this learning process. Not everyone performs at the same level with a cochlear implant. Prior to implantation, all of these factors need to be discussed.
We have several research projects regarding cochlear implantation. Currently, we are studying methods to predict which patients will have the best hearing outcomes with a cochlear implant. Also, we are studying the best methods for coordinating educational programs for children with implants.