Damage to the inner ear resulting in faulty transmission of auditory information to the brain is referred to as sensorineural hearing loss; damage which is often permanent. One of the more promising treatments is the cochlear implant, an electronic device that provides a sense of sound to people who would have little or no benefit from hearing aids.
A cochlear implant is surgically placed in the inner ear and activated by a device worn outside the ear. The implant functions similar to that of an artificial inner ear, taking over the job of the cochlea. The cochlea then translates sounds into electrical signals and sends them to the brain for interpretation. The implant directly stimulates the auditory nerve to send information to the brain.
The first research on cochlear implants began in the late 1950s when scientists began to experiment with ways to compensate for the damaged hair cells. Since then, cochlear implant technology has continually improved with approximately 70,000 people worldwide receiving implants, many of them children.
Although a cochlear implant does not restore normal hearing, it can dramatically improve the ability to hear and to understand speech. Benefits from the surgery vary from one individual to another, but most find that the implant allows them to handle such tasks as talking on the telephone. After a few months of wear, the user usually finds that other voices begin to sound more natural. For children, implants can help them acquire speech, language, and other essential developmental skills.
A cochlear implant is very different from a hearing aid, which amplifies sounds and delivers them to the ear canal. An implant does not make sounds louder. Instead, it compensates for damaged or nonworking parts of the inner ear, identifying useful sound information and translating this information into a form that the brain can understand.
Normally, the inner ear converts incoming vibrations from the middle ear into electrical impulses. The delicate hair cells stimulate the auditory nerve to send the electrical impulses to the brain. The brain recognizes the impulses as sound, unless the hair cells are damaged. In this case, they are unable to stimulate the auditory nerve. Although many nerve fibers may remain intact and can still transmit electrical impulses, these fibers are unresponsive because of the hair cell damage.
In people with mild or moderate hearing loss, sounds that are amplified by a hearing aid are converted into electrical impulses by the hair cells that are not damaged, in the same way that sounds are transmitted in a normal-hearing ear. But if there is profound sensorineural hearing loss, extensive hair cell damage prevents the ears from processing the auditory information, no matter how loud a hearing aid might amplify the sound.
Cochlear implants bypass the hair cells and stimulate the surviving nerve fibers in the cochlea. These fibers send electrical signals through the auditory nerve to the brain, allowing the perception of sound.
How Implants Function
Several different cochlear implant systems are available, but all work by identifying sounds in the environment electronically and sending the impulses to the brain. The implant is not a single unit but has both internal and external components. The external parts consist of a microphone, speech processor, transmitter, and connecting cords. The internal components are a receiver and electrodes.
These parts work together as follows:
- The microphone picks up sounds. It is located in a headset or case worn behind the ear, similar to a behind-the-ear hearing aid.
- A thin connecting cord carries sounds from the microphone to the speech processor, a small but powerful computer that digitally converts the sounds into coded electrical impulses. The coded impulses contain information about the frequency and loudness of sounds. Speech processors generally come in two styles. One is about the size of a pager and can be worn on a belt or in a pocket. The other is small enough to fit behind the ear and may be part of the same headset or case that contains the microphone.
- Coded impulses are sent to a transmitter, sometimes called a transmitting coil. A magnet holds the transmitter in place behind the ear, directly over the receiver that is implanted beneath the scalp.
- The transmitter relays the coded impulses as radio waves through the skin to the receiver. The receiver relays the signals to an array of electrodes threaded directly into the cochlea on a bundle of tiny wires.
- The electrodes stimulate nerve fibers in the cochlea that trigger the creation of electrical impulses. This information is sent to the auditory nerve and on to the brain for interpretation.
The process may sound complicated, but it all happens very quickly. The length of time between when the microphone picks up a sound to when the brain receives the information is just a few thousandths of a second. .
Cochlear implants are not alternatives to hearing aids and help only those who cannot receive any benefit from hearing aids. Adults and children who are candidates for cochlear implants typically have severe to profound sensorineural hearing loss in both ears or have great difficulty understanding speech.
The best age for children is still being debated, but most who receive implants are between the ages of one and six. The younger the child at the time of implantation, the less delay there will be in the speech and language development – as long as there is appropriate therapy and education after the implantation.
Among adults, there is no upper age limit. Several studies have shown that people over the age of 65 can experience excellent results. The duration of hearing loss is the foremost predictor of any success with the implants: the shorter the duration, the better the results. The decision to receive an implant should be made only after talking to a cochlear implant audiologist and an experienced cochlear implant surgeon.
As well as having some degree of severe hearing loss, the best candidates for implants must have the following:
- Realistic expectations, that is, a clear understanding of the benefits and limitations of a cochlear implant;
- Willingness and ability to make a time commitment for the pre-implant evaluations and post-surgical follow-up services;
- Motivation, along with the support of family and friends to be a part of the hearing world.
It may come as a surprise to know that many people in the deaf community strongly object to cochlear implants since many are often content in their unique culture. The deaf community usually includes a shared sign language, social customs and lifestyle, literature, art, and political, economic, and recreational organizations. However, not all people who are deaf participate in this culture and, for them, implants are a viable option.
For many in the deaf community, deafness is not regarded as a disorder to be altered. They have an especially negative reaction to implantation in children who are born deaf. Some parents have reported dealing with unfavourable comments and adverse reactions if they choose an implant for their child. However, some headway is being made in reconciling the two perspectives. Many are now recognizing the value of being fluent in both worlds that is, continuing to use sign language and remaining part of the deaf culture while also participating in the larger hearing world.
The condition of the auditory nerve fibers will play a factor in the success of a cochlear implant. People with a greater number of functioning nerve fibers in the cochlea may benefit more from an implant. Although no test can determine the number or location of surviving fibers, such tests as magnetic resonance imaging (MRI) can indicate whether the cochlea can accommodate implant electrodes.
An otolaryngologist (ENT doctor), performs cochlear implant surgeries, although not all perform the procedure. Before proceeding with the implantation, several tests will be performed by an implant team, which includes an otolaryngologist and an audiologist. Tests will include:
- Otologic examination: An ENT will perform a medical exam involving the outer, middle, and inner ear to ensure that no active infection or any type of abnormality exists that would void the use of a cochlear implant. This exam will also determine if the patient can safely undergo general anesthesia.
- Imagery examinations: includes X-rays, CT scans (computerized tomography), and/or MRIs (magnetic resonance imagery) to see if the cochlea is suitable for inserting implant electrodes.
- Audiologic evaluation: includes extensive hearing tests to determine how much can be heard without a hearing aid. At the same time, hearing, speech, and language tests are conducted to establish a baseline of information for comparison with tests following implantation.
- Psychological examinations: will determine if the patient can cope with the implant. They will also examine issues that could affect adjustment to and satisfaction with an implant.
Once all the tests prove satisfactory, the surgery will be scheduled.
Implant surgery is performed under general anesthetic and lasts from one to three hours. The procedure may also be done on an outpatient basis.
After anesthesia is administered, the surgeon will proceed as follows:
- An incision will be made behind the ear and a small depression in the skull behind the mastoid bone will be made. This will be where the receiver is placed.
- A second incision in the mastoid bone opens up the middle ear.
- A tiny hole is made in the cochlea and the electrodes are inserted.
- A few electronic tests are performed to make sure the device is functioning properly before the incisions are closed.
Bandages are usually removed a day or so after surgery. Complete healing takes about 4-6 weeks and, during this time, the implant will not be activated. Activation and programming will be done only after the surgical site heals completely.
Activating the Implant
When the healing process is complete, the patient returns to the cochlear specialist for the fitting of the external components and mapping the speech processor.
During the initial session, a headset or case containing the microphone is placed on the patient’s head and a transmitter is positioned on the side of the head. It is held in place by a magnet that couples with a magnet in the implanted receiver. The speech processor is connected to the microphone and to the audiologist’s computer.
One by one, implanted electrodes embedded in the cochlea are turned on. Each one carries a slightly different frequency and the patient will be asked to respond to the sound, indicating how loud it is. The audiologist uses these measurements to program the speech processor with special computer software. This processor is set to the appropriate levels of stimulation for each electrode.
After programming is complete, the speech processor is disconnected from the audiologist’s computer. Rechargeable or disposable batteries are inserted into the processor and the patient leaves with the whole system. It does take time to adjust, and each person has a different experience using the system.
Total costs for a cochlear implant, including evaluations, surgery, hospital fees, and all other fees and hardware, can range from $30,000 to $50,000. However, unlike hearing aids, cochlear implants are covered by most private insurance plans. In the U.S., Medicare, some state Medicaid programs, and Veterans Affairs provide partial coverage for cochlear implants. The implant specialist will be able to assist in determining the extent of a patient’s coverage.