Sometimes called a "bionic ear," the cochlear implant offers the hope of gaining or restoring the ability to sense sound for some people with significant hearing loss.
What Is a Cochlear Implant?
A cochlear implant is a surgically implanted device that helps overcome problems in the cochlea (KO-klee-uh). The cochlea are snail-shaped, curled tubes near nerves in the ears. They transform sound vibrations reaching the ear into signals that can be sent to the auditory nerve (or hearing nerve). The auditory nerve then sends these signals to the brain, where they're translated into recognizable sounds.
If important parts of the cochlea aren't working as they should and the auditory nerve isn't stimulated, there's no way for the sounds to get to the brain. As a result, hearing doesn't happen. This is called sensorineural hearing loss.
By completely bypassing the damaged part of the cochlea, the cochlear implant uses its own electrical signals to stimulate the auditory nerve, allowing the person to hear.
About Normal Hearing
The ear is made up of three parts, and in normal hearing, sound passes through all three on the way to the brain:
- The outer ear: This is the outer, visible part of the ear and the ear canal. When a person hears a sound, the outer ear captures the sound vibration and sends it through the ear canal to the middle ear.
- The middle ear: This consists of the eardrum and three tiny bones (the malleus, incus, and stapes). The sound vibration from the outer ear causes motion in these bones.
- The inner ear: This is where the cochlea are. The motion of the bones in the middle ear makes the fluid in the cochlea move, which stimulates the hair cells (the thousands of tiny hearing receptors inside the cochlea). The hair cells bend back and forth and send chemical signals to the auditory nerve, which carries these signals to the brain, where they're interpreted.
Hair cells can be damaged or destroyed through aging, heredity, disease, infection, or repeated or severe exposure to loud noise. If the hair cells don't work, the auditory nerve can't be stimulated and can't send information to the brain. So, the person won't be able to hear.
Hearing loss can be mild, moderate, or severe, depending on the number of hair cells that are defective, damaged, or destroyed. People with mild or moderate hearing loss may find that hearing aids help (hearing aids make sounds louder). Those with profound or severe hearing loss might even have trouble understanding loud sounds. A hearing aid won't help in these cases, so a doctor might recommend a cochlear implant.
What a Cochlear Implant Does
The cochlear implant artificially stimulates the inner ear area with electrical signals, which sends those signals to the auditory nerve, letting a person hear.
Although sound quality from a cochlear implant is different from that in normal hearing, the cochlear implant lets someone sense sound that he or she couldn't hear otherwise. And regular improvements to the way the implants work are helping to make the sound even more natural. Most infants, even if they never heard before, will be able to make sense of these sounds and develop speech and language.
A cochlear implant consists of an implant package, which is secured inside the skull, and a sound and speech processor, which is worn externally (outside the body). Several components of the cochlear implant work together to receive sound, transfer it to the auditory nerve, and send it to the brain.
The cochlear implant package is made up of:
- a receiver-stimulator that contains all of the electronic circuits that control the flow of electrical pulses into the ear
- an antenna that receives the signals from the external sound and speech processor
- a magnet that holds the external sound and speech processor in place
- one wire containing electrodes that are inserted into the cochlea (the number of electrodes can vary depending on the cochlear implant model type used). The electrodes act much like normal functioning hair cells and provide electrical charges to stimulate the auditory nerve.
The sound and speech processor is a minicomputer that processes sound into digital information, and then sends that information to the implant package in the form of electrical signals. This is worn externally and looks a lot like a normal hearing aid. Depending on the type of sound and speech processor used, it can either be worn as a headset behind the ear or in a belt, harness, or pocket.
The sound and speech processor is made up of:
- a sound and speech processor (which can either be a body-level model that can be clipped onto clothing like a portable radio, or an ear-level model that's hooked over the ear)
- a microphone
- a transmitter that sends the signals to the cochlear implant package. The transmitter also includes a magnet.
For the cochlear implant to work, the implant package and the sound and speech processor must be aligned — that's what the magnets are for. By lining up the magnets, both the implant package and sound and speech processor are secured and work together. If they aren't completely aligned, the device doesn't work and the person can't hear.
How a Cochlear Implant Works
Knowing how a cochlear implant works may help kids better understand their new bionic ear and the cool technology behind it that allows them to hear better.
Here's how the implant works:
- The microphone picks up sound.
- Sound is sent to the sound and speech processor.
- The processor analyzes the sound and converts it into an electrical signal. (The signal contains information that determines how much electrical current will be sent to the electrodes.)
- The transmitter sends the signal to the implant package, where it's decoded.
- The implant package determines how much electric current should pass to the electrodes and sends the signal. The amount of electric current determines loudness, and the position of the electrodes determines the sound's pitch.
- The nerve endings in the cochlea are stimulated and the message is sent to the brain along the auditory nerve.
- The brain interprets the sound and the person hears.
Cochlear Implant Surgery
Surgery for a cochlear implant takes 2–4 hours and uses general anesthesia (which keeps a patient completely unconscious). The surgeon will place and secure the implant package under the skin and inside the skull, then thread the wires containing the electrodes into the spirals of the cochlea.
To secure the implant, the surgeon first drills a 3- to 4-millimeter bed in the temporal bone (the skull bone that contains part of the ear canal, the middle ear, and the inner ear). Then, the surgeon opens up the bone behind the ear to allow access to the middle ear. A small hole is drilled in the cochlea and the wires containing the electrodes are inserted. The implant package is then secured and the incision is closed with stitches.
After cochlear implant surgery, a child:
- will probably be able to go home the next day
- will have to wear a dressing over the implant area for 24 hours
- might be off-balance or dizzy for a few days
- may have mild to moderate pain (the doctor may recommend giving pain medicines)
- won't have to have the stitches removed (they're absorbable and dissolve on their own)
- can lie on the side of the cochlear implant in a few days
About 2 to 4 weeks after surgery, the sound and speech processor is matched with the implant package and is programmed and fine-tuned to meet the child's individual hearing needs.
Nowadays, children who are born deaf or with a profound hearing loss in both ears can even receive two cochlear implants, one for each ear, at the same time. This is a great advance as it requires only one surgery.
Learning to Use a Cochlear Implant
Because the hair cell damage, electrical signal patterns, and sensitivity of the auditory nerve are different for each person, a specialist must fine-tune the sound and speech processor for every patient.
By measuring the lowest and highest current for each electrode, the clinician finds the softest and loudest sounds that will be heard (each electrode produces a different sound with different pitch). The sound and speech processor matches sounds on different electrodes with different volumes and attempts to create an accurate version of the original sound. However, because a limited number of electrodes are taking over the function of the thousands of hair cells in a normal ear, sounds won't be totally "natural." However, infants who never heard before will be able to make great sense of these sounds and will quickly learn language.
After the first few programming sessions, the user begins to pick up sounds with the implant, but giving the implant full power is a gradual process that takes several months. In children who are born deaf, the stimulation from the implant will allow them to develop the brain pathways necessary to hear sounds and develop speech and language. This is an extended process with programming and intensive therapy that often lasts for several years.
During the programming process, the user attends speech and language therapy sessions to help identify and interpret the new sounds he or she is hearing. An important part of the therapy is parent education and training.
Therapy will help a child develop and understand spoken language through detecting, imitating, and associating meanings of sounds. These sessions last at least a year, along with parent education and training programs. In many cases, therapy has helped kids with cochlear implants develop speech and language on par with their peers.
Some families choose to have implants in both ears. This can help with speech detection when there is background noise.
Can an Implant Restore Hearing for Everyone?
Cochlear implants are very successful for some people, but not everyone is a candidate to receive one. Children 12 months of age or older with profound hearing loss in both ears are usually excellent candidates, but not every child is eligible.
Common reasons that a child might not be eligible for a cochlear implant include:
- the child's hearing is "too good" (meaning the child can hear some sound and speech with hearing aids)
- the hearing loss isn't due to a problem with the cochlea
- the child has had profound deafness for a long time
- the auditory nerve itself is damaged or absent
Each potential candidate must be evaluated by a cochlear implant team to see whether a cochlear implant is the best option.
Benefits of an implant can vary. The rehabilitation period can be long, and many factors (such as the condition of the auditory nerve or the presence of scar tissue in the cochlea) can affect the success of the implant.
The doctor or surgeon will help parents understand what an implant can reasonably achieve for their child.