What is a cochlear implant?
A cochlear implant is a device that helps to improve hearing in children and adults with severe to profound hearing impairments. To better understand how a cochlear implant works, it is helpful to understand the nature of a hearing loss.
What is sound?
Sound is vibration that travels through the air and is sensed by the ear. Sound is carried by air particles. We are surrounded by air particles.
When a vibration occurs, it moves the air particles around it. These particles vibrate neighbouring particles and the vibration is carried outward like ripples on a pond. When these air particles reach your ear, your brain can interpret their movement as sound.
There is a chain of events that occurs from when air particles move the ear drum to when the brain recognizes that movement as sound. This chain of events is divided into 3 stages:
- the outer ear
- the middle ear
- the inner ear
Most people explain the parts of the auditory (hearing) system as the outer ear, the middle ear and the inner ear.
The Outer Ear
The outer ear consists of the pinna (the part you hang your glasses on) and the ear canal.
The pinna is shaped like a hand cupped around the entrance to your ear. It is designed like this to improve how you collect sound coming from in front of you.
The ear canal also helps you to hear certain sounds better. Large elephant ears are designed to hear low sounds like the thunder of far away elephants. Mice have small ears designed to hear the high sounds of skittering bugs. Our ears are best for hearing human speech. When there is a sound, air particles vibrate from the source all the way to the ear canal. When these air particles vibrate the ear drum, sound is sent to the middle ear.
The Middle Ear
The ear drum separates the ear canal from the middle ear space. The ear drum moves best if the air pressure inside the middle ear space is the same as in the ear canal.
When there is less air pressure inside the middle ear than in the ear canal, the ear drum is pulled tight. This makes it had for the ear drum to move. A tube, called the Eustachian tube, runs from the middle ear to the throat. The Eustachian tube helps keep the pressure even between the ear canal and the middle ear. Chewing or swallowing can help to open this tube and make the pressure in these spaces equal again.
Have your ears ever "popped" in an airplane? That is because the air pressure in these 2 spaces was not equal. That is why it is good to chew gum on an airplane......to open up the Eustachian tube and make them equal again.
There are times when the Eustachian tube does not work well. In children, this tube lies more horizontally and is more fleshy. This makes it more likely to close or become clogged. Problems with fluid or negative pressure in the middle ear can affect the movement of the ear drum. This makes it hard to hear. The moving eardrum then moves a chain of 3 small bones, called the ossicles. All 3 of these bones can easily fit on one side of a penny. The third bone connects with the inner ear and the cochlea.
|Enlarged view of the 3 bones (ossicles) of the middle ear.|
Reproduced from the HSC Auditory Science Laboratory with permission.
The Inner Ear
After sound vibration passes through the middle ear, it is "conducted" to a small window (the oval window) on a structure called the cochlea. The cochlea is the size of the end of your little finger. It is a curved space inside the bone of your skull that looks like the inside of a snail shell!
Inside this tiny space are very delicate structures. These structures take information from the 3 middle ear bones and sort that information into something that the brain can make sense of. Here is how it works...
The cochlea is filled with fluid. Vibrations at the oval window make waves in this fluid. High sounds affect the cochlea nearest the window and low sounds affect it further away. Imagine unrolling the cochlea into a straight tube. There are more than 20,000 tiny hairs all along the cochlea. These hairs move back and forth like seaweed with the motion of the fluid in the cochlea.
This is a picture of a cochlea. Notice how it spirals like a snail's shell.
This is an enlarged view of the part of the cochlea that holds the hair cells.
This is an enlarged view of the hair cells.
Here is the important part. When these hair cells bend they send a small electrical signal to the auditory (hearing) nerve. The auditory nerve is like a cable that sends this information up to the brain to be sorted out.
The hair cells are like piano keys. As different parts of the cochlea vibrate, different "notes" are played and sent up to the brain.
When there is a problem in the cochlea that affects hearing, it is called "sensorineural hearing loss". For most children with profound sensorineural hearing loss, the problem is with the hair cells. The outer hair cells, (which amplify sound), are usually the first to be damaged. If you think of the hair cells as piano keys, most of the keys are damaged or missing.
Hearing aids make sound louder so that the remaining hair cells will respond. However, they often do not provide clear sound, which makes listening, speech and language learning difficult.
How can a cochlear implant help?
A cochlear implant bypasses the damaged or missing hair cells. The inner part of a cochlear implant is a small wire with very small electrodes along it. The doctor puts this wire down inside the cochlea.
Imagine putting this string of electrodes on a broken keyboard. When electricity goes through an electrode, it goes past the damaged hair cells and directly stimulates the auditory nerve.
This assumes, however, that the inside of the piano is intact. If there is a problem with the inside of the piano (auditory nerve), then the cochlear implant cannot help.
The parts of a cochlear implant
Although different companies make different implants, the parts are basically the same. There is an internal coil which the doctor puts into the cochlea. There is also an external part that includes the microphone, the transmitting coil, the speech processor, and the cords that connect them.
At The Hospital for Sick Children we use the Cochlear Corporation Nucleus 24 Speech Processor.
The internal wire (coil) with the electrodes is placed through the round window of the cochlea. The main body of the internal device is put under the skin behind the ear. It has a receiver coil and a magnet. The doctor attaches the receiver to the bone so it will not move. After the surgery, it is not usually noticeable.
Four weeks after surgery, a child is fitted with the external device and he or she hears through the cochlear implant for the first time. The microphone rests behind the ear and looks like a hearing aid.
Here is how sound travels with a cochlear implant:
Sound goes into the microphone (1) and down a long cord (2) to a speech processor (3). This is a tiny computer that will decide which electrodes to use and how much to use each one. It is a little bigger than a deck of playing cards. The information is then sent back up the long cord and then across a short cord to a transmitting coil (4).
The transmitting coil has a magnet in its centre. This holds the coil in place directly over the internal device that lies under the skin. The transmitting coil sends the information across the skin to the internal device (receiver (5)). The electrodes (6) in the cochlea are then stimulated at thousands of times per second and the child hears this as sound.