What are Cochlear Implants?Cochlear Implants are electrical devices which process audiosignals and then transmit them across the skin to the cochlear implant which in turn then transmits the decoded auditory signals to the electrodes placed in the cochlea which then stimulate the nerve cells allowing the person to hear.
How are Cochlear Implants able to help a person to hear even though they have nerve deafness?The hair cells in the scala media are responsible for stimulating the nerve cells which are responsible for hearing. In most persons who suffer from nerve deafness it is because the hair cells are affected. The cochlear implant bypass the hair cells and electrically stimulate the nerve cells also known as the spiral ganglion cells. This then allows a person to hear.
Who will benefit from cochlear implants?There are 2 sets of patients, those who are Prelingually deaf and Postlingually deaf.
Prelingually deaf persons are children who are deaf without ever having heard sound. They are deaf since birth. They are children who have not acquired speech.
Postlingually deaf persons: are those who have gone deaf after having heard sound and have acquired speech.Prelingually deaf children should have a cochlear implant inserted at the very earliest. By the age of 2 to 3 years the center in the brain responsible for acquiring speech starts to shrink. Most surgeons think that the cut off age limit for inserting cochlear implants is up to the age of 4 years. Beyond the age of 4 the benefits of cochlear implantation falls dramatically.
Who are candidates ( suitable persons ) for cochlear implants?Candidates for cochlear implants are persons who have
- Have profound sensorineural ( nerve) hearing losses in both ears.
- The cochlear should not be damaged or diseased.
- The person should be motivated.
- The brain should be in good condition.
- The person should not have a contraindication for insertion of a cochlear implant.
How are Prelingually deafened children tested for hearing disabilities?These children need to undergo the following tests
- Brainstem evoked response audiometry also known as BERA
- ASSRT is a variation of BERA to determine residual hearing.
- Otoacoustic emission audiometry also known as OAE
- Impedance audiometry
- High resolution CT scanning of the temporal bones in which the cochlear and hearing apparatus is situated.
- Magnetic resonance imaging of the brain and the auditory nerve.
- Psychological evaluation.
- The parents also need to be counselled about the implications of cochlear implantation.
- The child must be fitted with hearing aids as soon as hearing losses are detected. It has been found that such children do better when fitted with cochlear implants
Where is the cochlear implant placed?
The cochlear implant is placed in the scala tympani through the round window of the cochlea. The Cochlear implant goes all the way to the apex of the cochlea in order to stimulate all the nerve cells of the spiral ganglion. In this way all the frequencies of hearing are stimulated by the cochlear implant.
The mastoid is usually opened and through an operation termed the posterior tympanotomy the round window membrane of the cochlea is approached.
What are the parts of a cochlear implant?
The cochlear implant is divided into two parts. The cochlear implant is the device inserted into the round window and is placed under the skin just behind the ear.
The speech processor is that part of the cochlear implant that takes surrounding sound converts it into an electrical signal, codes it and then transmits it across the skin to the cochlear implant where the electrical signal is decoded and transmitted to the nerve cells in the cochlea.
- Ideally must be inserted by the age of 2 for maximum benefit.
- Cochlear implants should be inserted in both ears to get the maximum benefit
- Both implants can be inserted at the same time through two different operations carried out on both ears.
- Two weeks after the implant has been inserted the device (cochlear implant) is switched on.
- The child hears sound for the very first time and may get startled and cry. The child soon gets used to it and then wants the device on as often as possible. This is a very good sign.
- The loudness of the various frequencies are then adjusted and the child is encouraged to wear it for as long as possible and as often as possible.
- The child and its parents are called regularly for ‘mapping’. Mapping is nothing but tuning of the device to make sure that each frequency of hearing is adjusted to the child’s comfort levels.
- Cochlear implants allow the child to develop normally provided the parents follow the guidelines for rehabilitation.
Cochlear implants in Prelingually deaf children
Postlingually deaf candidatesThese patients suffer from profound sensorineural hearing loss in both ears.
- This is confirmed on hearing tests like pure tone audiometry, BERA, OAE and Impedance audiometry
- They will need high resolution CT scans of the temporal bones and MRI scans of the brain.
- Psychological evaluation of these patients needs to be done.
- There is no age limit. Patients of very advanced ages , even in their 90’s are found to benefit tremendously from cochlear implants.
Postlingually deaf patients usually benefit most from these devices. They are able to hear even on the telephone.
Single sided deafnessThis type of deafness in which there is profound hearing loss only in one ear can be treated through a variety of devices.
The devices offered for the treatment of this problem are the following:
- CROS (contralateral routing of signal) hearing aid. This device does not need surgery for insertion.
- Bone bridge: This works like the CROS hearing Aid but need surgery for insertion.
- Cochlear implantation.
Some things you need to know about Cochlear Implants
- They are implants not transplants.
- A person is likely to need two cochlear implants in their life time as the first implant may need to be removed and a second new one inserted as technology improves and changes.
- Technology is changing rapidly. Now auditory stimulation is also included. A hearing aid like device is also inserted in the ear canal to achieve better hearing results.
- Complications like facial paralysis either permanent or temporary can occur. Leakage of brain fluid from the round window can occur. On rare occasions removal of the cochlear implant may be needed. This is termed explantation.
- Regular and frequent mapping is mandatory.
- Cochlear implants need to be inserted in both ears to achieve the best possible result.
- Most cochlear implant patients can undergo an MRI scan with a magnet strength of 1.5 tesla.
- Some companies are manufacturing implants that are water proof and can be worn while bathing.
- There is no age limit for cochlear implantation in persons who are Postlingually deafened.