Cochlear Implants

What Is a Cochlear Implant?

A cochlear implant is a surgically implanted device that helps overcome problems in the inner ear, or cochlea. The cochlea is a snail-shaped, curled tube located in the area of the ear where nerves are contained. Its function is to gather electrical signals from sound vibrations and transmit them to your auditory nerve (or hearing nerve). The hearing nerve then sends these signals to the brain, where they're translated into recognizable sounds.

If important parts of the cochlea aren't working properly and the hearing nerve isn't being stimulated, there's no way for the electrical signals to get to the brain. Therefore, hearing doesn't occur. (Sometimes referred to as nerve deafness, 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.

How Normal Hearing Occurs

The ear is made up of three parts, and sound for a person who has normal hearing passes through all three on the way to the brain. The outer ear is made up of the outer, visible part of the ear and the ear canal.

When a person is exposed to a sound, the outer ear captures the sound vibration and sends it through the ear canal to the middle ear, which consists of the eardrum and three tiny bones. The sound vibration then causes motion in the three tiny bones, which makes the fluid in the cochlea move. The motion of the fluid stimulates the hair cells, which are thousands of tiny hearing receptors inside the cochlea. The hair cells bend back and forth and send electrical signals to the hearing nerve, and the hearing nerve then carries these signals to the brain, where they're interpreted.

Through aging, heredity, disease, infection, or repeated or severe exposure to loud noise, hair cells can be damaged or destroyed. If the hair cells don't work, the hearing nerve can't be stimulated and therefore can't send information to the brain. Thus, the person is unable 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, which simply make sounds louder, help. Those with profound or severe hearing loss might even have trouble understanding loud sounds. A hearing aid won't help in these cases, and a doctor might recommend a cochlear implant.

What a Cochlear Implant Does

The cochlear implant artificially stimulates the inner ear area with electrical signals, sends those signals to the hearing nerve, and allows the user to hear. Although sound quality is sometimes described as "mechanical" and not completely like that experienced by a person with normal hearing, the cochlear implant provides users with the ability to sense sound that they couldn't hear otherwise. Improvements in the way the implant processes sound information are continuously being made to make the sound seem more natural.

The actual 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 hearing nerve, and send it to the brain.

The 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 hearing 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. The sound and speech processor 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 components of the sound and speech processor include:

• the actual sound and speech processing device (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 implant package. The transmitter also includes a magnet that helps the user align the processor with the implant package.

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 as one device.

When the implant package and the sound and speech processor aren't completely aligned, the device doesn't work and the person can't hear. Because both components need to be aligned for the user to hear, some people take the sound and speech processor off at night to sleep soundly. Others leave it on all the time.

How a Cochlear Implant Works

Knowing what, exactly, the cochlear implant does 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 sound and speech 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 electrical current will determine loudness, and the position of the electrodes will determine the sound's pitch.
• The nerve endings in the cochlea (the area where the hair cells are located) are stimulated and the message is sent to the brain along the hearing nerve.
• The brain interprets the sound and the person hears.

Cochlear Implant Surgery

The actual surgical procedure, which takes 2 to 4 hours and uses general anesthesia, involves securing the implant package under the skin and inside the skull, and then threading 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). Next the surgeon opens up the mastoid bone behind the ear to allow access to the middle ear. Then, 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.

After having 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
• may be off-balance or dizzy for a few days
• may experience mild to moderate pain (the doctor may recommend giving pain medications)
• won't have to have the stitches removed — they're absorbable and dissolve on their own
• can lie on the side with the cochlear implant in a few days

Two to four 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.

Learning to Use a Cochlear Implant

Because the extent and type of hair cell damage, electrical signal patterns, and sensitivity of the hearing 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."

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. 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. In addition, an important part of the therapy includes 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 and attend mainstream schools.

Some families choose to have implants in both ears. This can help with speech detection when there is background noise and in localizing the source of sounds.

Can a Cochlear Implant Restore Hearing for Everyone?

Cochlear implants are very successful for some people, but not everyone is a candidate to receive one. Ideally, children 12 months of age or older with profound hearing loss in both ears are excellent candidates, but not every child is eligible.

Some common reasons that a child may not be eligible for a cochlear implant:

• the child's hearing is "too good" (meaning the child can hear some sound and speech with hearing aids)
• the reason for hearing loss isn't a problem with the cochlea
• the child has experienced profound deafness for a long period of time
• the hearing nerve itself is damaged or absent

Each potential candidate must be evaluated by a cochlear implant team to determine whether a cochlear implant is the best option.

For those who do receive a cochlear implant, benefits can vary. The length of rehabilitation varies from person to person, and many factors (such as the condition of the hearing nerve or the presence of scar tissue in the cochlea) can hinder the success of the implant.

Expectations should be realistic, and the doctor or surgeon will help you understand the level of success the implant can reasonably achieve for your child.

Reviewed by: Robert C. O'Reilly, MD
Date reviewed: November 2010