While most people with hearing loss are quite successful with hearing aids, there are some individuals with severe to profound hearing loss for whom hearing aids do not provide enough benefit. For these people with severe damage to the sensory cells in the inner ear, a cochlear implant is often the best option for better hearing.
These tiny, complex medical devices work differently than hearing aids. Rather than amplifying sound, which helps an individual with residual hearing ability, a cochlear implant provides the sense of sound by stimulating the auditory nerve directly. Cochlear implants do not cure hearing loss or restore hearing, but they do provide an opportunity for the severely hard of hearing or deaf to perceive the sensation of sound by bypassing the damaged inner ear.
How do cochlear implants work?
A cochlear implant operates using two main components: an external part that hooks over the ear and a surgically implanted internal part. The two components are coupled using a powerful magnet.
The external component of a cochlear implant contains a microphone, a speech processor and a transmitter. The microphone and speech processor are housed in a small unit that looks like a behind-the-ear hearing aid. A small wire usually links them to the transmitter, which is positioned over the internal part of the device. The microphone picks up acoustic sounds and sends it to the speech processor. The processor analyzes and digitizes the signal before sending it to the transmitter. The transmitter then codes the signals and sends them to the implanted receiver via the magnetic coupling.
The internal part of a cochlear implant includes a receiver, which is located under the skin on the temporal bone, and one or more electrode arrays. The receiver collects the signals from the transmitter and converts them to electrical pulses. It then dispatches the pulses to the electrodes that have been inserted deeply into the inner ear. These electrodes directly stimulate the auditory nerve throughout a portion of the cochlea and the brain then interprets these signals as sound.
History of cochlear implants
Scientists have long pondered and experimented with the use of electrical stimulation for aiding hearing, rather than acoustic. In fact, Alessandro Volta experimented on himself in the year 1800, inserting metal rods in his ear canal and stimulating them with electricity. He reported that this created an auditory sensation. In the mid-1950s, experimentation with electrical stimulation to the auditory nerve really ramped up as researchers performed several successful studies on deaf individuals. By 1972, researchers developed the first implantable single-channel cochlear implant. More than a thousand people, mostly military veterans and children, were surgically fitted with the single-channel implant from 1972 through the mid-1980s.
Though the single-channel device was able to provide more auditory stimulation than hearing aids could for this population, the success was generally limited to awareness of environmental sounds and enhanced speech reading ability. Then, in 1984, Cochlear Corporation developed the first multi-channel cochlear implant for commercial use, which was initially approved by the FDA for adults, age 18 and older. Just 5 years later, FDA extended the age range down to 2 years and older. By the year 2000, certain devices were approved for use in children as young as 12 months of age. This approval is significant because it allows children to be fitted with a cochlear implant at a young enough age that they can develop speech and language rapidly, often catching up to their same-age peers by the time they start school.
The NIDCD reported more than 324,000 people worldwide - including about 58,000 adults and 38,000 children in the U.S. - have received a cochlear implant as of December 2012.
Who is a candidate for a cochlear implant?
Both children and adults can make exceptional candidates for cochlear implants depending on the hearing loss. Eligibility Candidacy requirements have expanded exponentially thanks to ongoing research. The FDA approves candidacy criteria when new devices are brought to market but insurance companies also set criteria of their own. As a result, cochlear implant candidacy is usually determined on a case-by-case basis. The general candidacy guidelines include the following, from the American Speech-Language-Hearing Association:
Children with hearing loss as young as 12 months old can be eligible for a cochlear implant. Experts recommend implantation as early as possible to expose children to sounds during the critical period of language acquisition. After implantation, they must undergo intense speech and language therapy in order to achieve the best possible outcome from the device.
Children are considered viable candidates when they:
- Have profound hearing loss in both ears.
- Get little or no benefit through the use of hearing aids.
- Are healthy and any medical conditions would not compromise surgery.
- Understand (when able), along with their parents, their role in the successful use of cochlear implants.
- Have support from an educational program that will emphasize the development of auditory skills.
Adults may qualify for cochlear implantation regardless of whether they lost their hearing before or after learning language. Those adults who developed language before losing their hearing (postlingually deafened) typically have greater success with cochlear implants than those who had not developed language before losing their hearing (prelingually deafened). Adult candidates are generally eligible for an implant if they:
- Have severe or profound hearing loss in both ears.
- Get little or no benefit from hearing aids.
- Have no medical problems that could put them at risk during surgery.
- Have a strong desire to be part of the hearing world and communicate through listening, speaking, and speechreading.
What is the process for getting an implant?
A potential cochlear implant candidate should visit an ENT physician or an audiologist for a referral to a cochlear implant center. At the center, the candidate will undergo audiologic and psychological testing, a medical exam and imaging studies to determine if they will benefit from a cochlear implant. There will also be counseling for the candidate, to make sure he or she - or parents, for a child - understand the large follow-up commitment required after the implant surgery, as well as what to expect regarding device performance and limitations.
After a child or adult is considered a viable candidate, he or she will undergo the implantation surgery, which is done under general anesthesia. It typically takes between two and four hours and most people spend one night in the hospital afterward. When they leave the hospital, the person will not be able to hear yet. Although the internal components have been placed, the surgical site must heal before the external device is placed.
About four to six weeks after the surgery, the patient will return to the cochlear implantation center to be fitted with the external device. At this appointment, the audiologist will activate the cochlear implant and begin the process of mapping the processor for the individual's specific needs. When the cochlear implant is "turned on," this is the first time many children and adults are experiencing sound. Whether first hearing a spouse's voice, their own voice, a parent's voice or the audiologist, it's quite an emotional and memorable milestone for the patient or parent.
This first appointment will be followed by other fine-tuning and adjustments to the cochlear implant map as the patient begins his or her new hearing journey. Many individuals need several follow-up visits over a few months to adjust the mapping of signal to the electrodes, as well as to help the person become accustomed to his or her device. Especially for those who've never heard sound before, an auditory training program is necessary to help the brain learn how to process the new auditory stimulation. Much like hearing aids, individuals will want to schedule regular visits with their audiologist for occasional adjustments and hearing tests.
What does the future hold for cochlear implants?
- There are large-scale studies about children who were implanted very early with cochlear devices. Researchers are interested in which variables account for individual differences in speech and language abilities, academic performance and reading ability among children who use cochlear implants.
- More and more individuals are being implanted with bilateral cochlear implants. Researchers are investigating whether two implants make a significant difference in ability to localize sound (determine where it's coming from) and hear speech in the presence of background noise.
- Engineers are always investigating new signal processing technologies and improved microphones, as well as new battery technologies for more convenient use.
- Researchers are also expanding candidacy criteria with a hybrid design making implants more available to adults, particularly those with severe to profound hearing loss of high frequencies and some residual, lower-frequency hearing abilities. The first hybrid cochlear implant was approved by the FDA in March 2014.
If you are interested in learning more about whether you might be a good candidate for cochlear implantation, talk to an ENT physician or audiologist for a referral to a cochlear implant center.
- Cochlear Implants, U.S. Food & Drug Administration, http://www.fda.gov/MedicalDevices/ProductsandMedicalProcedures/ImplantsandProsthetics/CochlearImplants/default.htm
- Cochlear Implants, National Institute on Deafness and Other Communication Disorders, http://www.nidcd.nih.gov/health/hearing/pages/coch.aspx