Types of hearing loss:
Hearing loss can be divided into 3 types according to the site of lesion in the hearing system:
1. Conductive hearing loss (CHL): Where the lesion is in the outer or middle ear and can often be treated with medication or surgery.
2. Sensorineural hearing loss (SNHL): Where there is a lesion in the inner ear or auditory nerve. In most cases, it is incurable and hearing aids or cochlear implants are used to compensate for the lesion.
3. Mixed hearing loss: Where part of the lesion is in the outer or middle ear and part in the inner ear or auditory nerve. This type of hearing loss is actually a combination of CHL and SNHL hearing loss, and the CHL section is usually treatable.
What is cochlear implantation?
Cochlear implants are part of standard medical treatment to rehabilitation of hearing loss in children and adults. Determining the candidacy of cochlear implants is more complicated than other ear surgeries and is based on a complete medical history, clinical examination, and laboratory evaluations. Cochlear implant surgery has evolved over the past three decades. During this time, technologies have expanded significantly to reduce complications, especially in children less than 12 months old.
The cochlear implant has the following parts:
= Microphone, which receives sound from the environment.
= Speech processor, which performs complicated processing on sounds received by the microphone.
= Transmitter and receiver, which receives signals from the speech processor and converts them into electrical impulses.
= The electrode array which is a set of electrodes placed in to the cochlea, which deliver impulses of electrical stimulation to different areas of the cochlea and auditory nerve.
The selection criteria for children according to FDA clinical studies or measures proposed for cochlear implantation is a bit different. In general, patient selection criteria include:
1. Age 12months or older.
2. Profound hearing loss (pure-tone hearing thresholds in the better ear of 90 dB and more).
3. Little or no benefit with using a hearing aid.
4. No evidence of central auditory or auditory nerve damage.
5. No evidence of contraindication for surgery in general and cochlear implantation in particular.
In general, cochlear implant centers suggest that:
1. The child should use hearing aids for at least 3-6 months, unless cochlear ossification is detected or predicted.
2. Family members should have realistic expectations.
3. Participate in postoperative rehabilitation programs.
4. The family's willingness to complete the post-operation follow-up that determined by the center.
Evaluations include the following:
= Hearing, speech and sometimes balance tests
= Medical examinations to assess the health of the inner ear
= Occasionally, psychological tests are used to determine the learning ability or disability of the child.
The surgeon incise the back of the ear and creates a small hole in the skull bone (mastoid). The surgeon then makes a small hole in the cochlea and inserts the electrode array into the cochlea through it. The opening behind the ear is closed with sutures so that the inner part is placed under the skin. Most people are in good general condition on the day of surgery or the day after surgery to be discharged from the hospital. The person underwent surgery needs to see a doctor about a week later to have the stitches removed.
To activate the device, the responsible audiologist in rehabilitation team needs to do the following:
= Adjusts the sound processor.
= Inspects different parts of the device to ensure their performance.
= Determine what sounds to hear.
= Provide information on proper maintenance and use of the device.
Rehabilitation involves training the brain to understand the sounds heard through the cochlear implant. Everyday speech and environmental sounds are heard differently from what one remembers. The brain needs time to recognize the meaning of sounds. This process, improves with continuous use of the speech processor during waking hours.
The result of cochlear implantation is different from person to person two of the most important. Factors that may affect the results of cochlear implantation include:
= The age of the person when he/she loses his/her hearing
= The time interval between hearing loss and cochlear implantation
The best results with cochlear implants for children occur when the child is young. Also, Research has shown that young children who receive cochlear implants have better hearing and speech abilities in comparison with the hearing aid user peers. In post- lingual deafened adults, the best results are seen when the time interval between profound hearing loss and cochlear implantation is short. For pre-lingual deafened adults with little or no hearing experience, CI benefits are usually limited.
The latest cochlear implant devices:
One of the newest devices in the field of cochlear implants are the hybrid cochlear implants. In this type of cochlear implant, a shorter electrode array is used only for basal part of the cochlea. This new cochlear implant can be helpful for people who only have high-frequency hearing loss (while maintaining their normal hearing at lower frequencies).
How does a hybrid cochlear implant work?
The acoustic part of the device, like a hearing aid, amplifies and transmits the sounds through the natural way of hearing (outer ear, middle ear and inner ear). At the same time, The device processor converts high frequency sounds into digital (electrical) information and sends to the electrodes inside the cochlea and causes stimulation of the nerve fibers.