Cochlear Implants

I feel extremely privileged to be working in the field of Cochlear Implants.

Cochlear Implants are one of the most amazing medical technologies of the last 25 years.

A cochlear implant can allow a child born with no hearing to develop good hearing and normal sounding speech.

A cochlear implant can allow an adult who loses their hearing to hear again.

The effects of a complete loss of hearing can be devastating to a patients quality of life(see Effects of Hearing Loss). Communication is a fundamental part of the human condition and it becomes very difficult when a hearing person loses their hearing. People can become very isolated. A cochlear implant can ‘return someone to the world’.

A cochlear implant does not always work in everyone and can work to varying degrees. Patients may still struggle in certain situations.

However, fortunately, the vast majority of patients gain significant benefit from a cochlear implant and their quality of life is greatly improved.

In the South Of England Cochlear Implant Centre in Southampton , where I am one of the surgeons we have just performed our 1000th cochlear implant operation.

WHAT IS A COCHLEAR IMPLANT?

A cochlear implant is an electronic device which is implanted under the skin and takes over the function of the cochlea.

The cochlea is that part of the inner ear which converts sound energy into nerve impulse.

Sound vibrations stimulate the nerve endings (called hair cells) in the cochlea and these nerve endings trigger electrical impulses which travel along the hearing nerve into the brain where we hear them as sound.

One of the commoner causes of deafness is where the nerve endings die off and no longer work. The nerves (or some of them) remain in place but without any nerve endings. This means that when sound comes into the cochlea there is no mechanism by which the sound can stimulate those hearing nerves – so the person will not hear.

Loss of hair cells happens to most people as they get older but fortunately the majority of the time enough hair cells remain to allow the person to hear with a hearing aid. A hearing aid puts more energy into the inner ear and the remaining hair cells can detect the sound.

A cochlear implant can be considered when there are so few functioning hair cells left that a hearing aid is no longer of sufficient benefit.

A cochlear implant has 2 parts. An internal part and an external part. Both parts have a coil of wire with a magnet in the middle. This magnet holds the outside part in place holding the outside coil directly over the inside coil. This is how the outside device communicates with the inside device.

HOW DOES A COCHLEAR IMPLANT WORK?

THIS EXPLANATION IS VERY SIMPLIFIED BUT SHOULD HELP YOU UNDERSTAND THE PRINCIPLES.

A cochlear implant is primarily designed to help people understand speech.

When we say a word it is made up of a combination of sounds.

The sounds in a word will be made up of different frequencies at different volumes.

In the normal ear the cochlear will detect all the different frequencies in the word and the volume of those frequencies. A louder volume will stimulate more nerve fibres. The cochlear is like a tube divided in two by a membrane. The nerve fibres are spread along this membrane from one end to the other.

A particular frequency will always bend the membrane in the same place and stimulate the nerve fibres at that point. High frequencies stimulate the nerve fibres at one end of the tube and low frequencies stimulate the nerve fibres at the other end. The membrane inside a cochlear implant is a bit like a piano keyboard. Press a key at one end and you hear a high note, in the middle you hear a a mid frequency note and at the other end you hear a low frequency note.

This arrangement of the nerve endings is called ‘Tonotopic’.

When the nerve endings die off part of the nerve fibre (called the spiral ganglion) often remains but, because there are no hair cells, bending the membrane no longer stimulates these spiral ganglia.

The cochlea is coiled like a snail shell around a central shaft. The spiral ganglia spiral around in this central shaft. The spiral ganglia are also arranged in a tonotopic fashion.

In a cochlear implant a wire called an electrode array is passed into the cochlea and coils around this central shaft. The electrode array has a number of electrodes on it. These electrodes send a small electric current to the spiral ganglia which then triggers a nerve impulse up to the brain where it is perceived as a sound corresponding to the frequency detected by that spiral ganglia.

In a cochlear implant sound comes into the microphone of the external part. It travels to the speech processor. Here that split second of sound is analysed as to what and how much of each frequency it contains. Then, depending on the processing strategy, a variable number of the strongest frequencies are selected. Lets say 6. This information passes through the skin to the internal part. The internal part then activates the 6 electrodes which are closest to the spiral ganglia which detect those frequencies. Nerve impulses pass up along the appropriate nerves to the brain and the patient ‘hears’ that sound.

This process repeats itself many times a second.

Below is a video which shows how a cochlear implant works:

WHO IS SUITABLE FOR A COCHLEAR IMPLANT?

A cochlear implant could be considered for anyone who does not gain sufficient benefit from a hearing aid.

This could be a child who is born profoundly deaf or a child who becomes profoundly deaf or an adult who becomes profoundly deaf.

In the UK the National Institute for Clinical Excellence (NICE) will pay for a cochlear implant for:

• a child with hearing thresholds in both ears of 90dB or worse.

• an adult with hearing thresholds at 2 and 4kHZ of 90db or worse and a speech discrimination score of less than 50%

It is felt that people with these levels of hearing loss have a good chance of doing better with a cochlear implant than with a hearing aid.

More information coming soon…

• WHAT DOES THE ASSESSMENT INVOLVE?
• WHAT DOES THE SURGERY INVOLVE??
• WHAT ARE THE RISKS OF THE SURGERY?
• WHAT HAPPENS AFTER THE SURGERY?