Cochlear Implant for Sensorineural Deafness
How a 'Bionic Ear' Can Help Restore Hearing Loss
Sep 13, 2009
What might seem like a simple process – the hearing of a sound – is dependent on the proper functioning of a complex sequence of events. A problem with any step in this sequence can lead to hearing loss.
How Does the Ear Work?
- In the outer ear, sound is collected in the auricle (the external part of the ear) and funnelled down the ear canal to the eardrum.
- In the middle ear, a chain of tiny bones (or ossicles) transmits vibrations in the eardrum to the inner ear.
- In the inner ear, the vibrations transmitted by the ossicles cause movements in the fluid that fills a cavity called the cochlea. The cochlea is lined with ‘hair cells’ bearing flexible protein filaments, and movements in the fluid cause the filaments to bend. The bending stimulates the hair cells and this in turn stimulates the cochlear nerve, which carries information about the sound that was heard to the brain, for processing.
Sensorineural Hearing Loss and Cochlear Implants
Loss of hearing can stem from problems at any point in this sequence. Sensorineural hearing loss is caused by abnormalities in the hair cells that prevent them transmitting normally to the cochlear nerve. These abnormalities can be present at birth, or may be caused by drugs, infection or prolonged exposure to very loud noise.
Sensorineural hearing loss may be overcome with a hearing aid, which amplifies the sounds that enter the outer ear. However, people with hair cell damage often find it difficult to distinguish between different sounds – including speech – and simply increasing the volume of those sounds does not solve the problem. For these people, a cochlear implant (sometimes called a 'bionic ear') may help.
Cochlear implants are used both in children born with sensorineural deafness and in people who have developed the condition later in life.
How Does a Cochlear Implant Work?
Cochlear implants take the place of the damaged hair cells by directly stimulating the cochlear nerve.
- A microphone worn behind the ear picks up sounds, converts them to an electrical signal and transmits this to a processor. The processor may be also be positioned behind the ear, or may be carried in a pocket or on a belt.
- The processor organizes and filters the signal, then sends it to a transmitter in the form of radio waves. The transmitter is worn on the side of the head, held in place by a magnet.
- Under the skin, implanted into the bone directly beneath the transmitter, is a second magnet together with a receiver/stimulator. The receiver/stimulator picks up the radio signal from the transmitter and passes this down a thin wire that has been threaded into the cochlea.
- Electrodes on the wire stimulate the cochlear nerve as would healthy hair cells.
Cochlear Implant Procedure
Cochlear implant surgery is performed with a general anaesthetic and normally takes around two hours. To fit the internal parts of the device, the surgeon makes a small incision in the skin behind the ear for the receiver/stimulator, then cuts through the skull and into the inner ear to position the electrode wire.
The patient can usually return home the same day, but the implant is not switched on for up to a month after the surgery. An extended period of training follows, during which the patient learns how to interpret the sounds that he or she hears through the implant. With the aid of a speech therapist and the family, children born with sensorineural deafness can soon acquire age-appropriate language skills.
In 2003, a study suggested that children with a cochlear implant were at increased risk of bacterial meningitis compared with children of similar age in the general population (Reefhuis J et al. N Engl J Med 2003; 349: 435–45). The risk was greatest in those with a particular type of implant. This implant was quickly withdrawn from the market, but the Centers for Disease Control and Prevention have issued recommendations for children with any implant type.
Benefits of a Cochlear Implant
A cochlear implant does not restore normal hearing, but it allows users to recognize important sounds such as alarms and warnings, and to understand speech in person and on the telephone. Anyone referred to a cochlear implant centre can expect a thorough assessment of their suitability for an implant, as well as advice on the risks and benefits, before any decisions are made.
Reference
British Cochlear Implant Group. Accessed 12-09-09
Disclaimer
The information contained in this article is for educational purposes only and should not be used for diagnosis or to guide treatment without the opinion of a health professional. Any reader who is concerned about his or her health should contact a doctor for advice.
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