By Dr. Sandra Roglić

Phonac amplifier

Unfortunately, not everyone can enjoy the benefits that the senses, particularly hearing, offer. Some people cannot hear music or speech. Others can hear music, but they cannot fully experience its beauty, because they cannot hear all the sonic frequencies and cannot distinguish between a drum and a violin. Until recently, these people used various hearing aids, like amplifiers which make it possible for a person with a hearing defect to better hear the sounds that their ears can detect. In some cases, this is not enough, because if there is serious damage in the area of some frequencies, they cannot be heard, no matter how much the sound is amplified.

Ear Structure

Ear section

The ear consists of three basic parts: the outer, middle and inner ear. The external auditory canal which begins at the auricle ends with the tympanic membrane (membrana tympani) which delimits the middle ear. The tympanic membrane transfers sound waves onto the system of auditory ossicles (malleus - hammer, incus - anvil, stapes - stirrup) which are a part of the middle ear. These ossicles further transfer the sound waves to the inner ear, which consists of two, functionally different, parts. These are three semicircular canals (canales semicirculares) which are essential to the sense of balance, and the cochlea with sensory cells which can "translate" the sound waves into electric impulses. The auditory nerve (nervus statoacusticus) transfers these impulses to the brain and we ultimately experience them as sound.

Sound and Hearing Defects

The sounds that our ear can hear are of different pitches or frequencies. Certain frequencies stimulate only the sensory cells located at specific sites within the cochlea. In the case of a hearing defect, not all sensory cells are equally affected. Most commonly, the cells enabling the perception of high frequency sounds (e.g. the sound of a violin or the phoneme s) are damaged, followed by other cells which are affected depending on the type and extensiveness of the defect. The defects involving the cochlear cells are called perceptive hearing defect or deafness.

The remaining hearing ability in cochlear implant candidates is merely a remnant – it is not enough to enable a child to hear someone behind his back calling his name or the horn of an approaching car.

Cochlear Implant

Cochlear's Nucleus 24 speech processor, microphone and trasmitter
Cochlear's Nucleus 24 receiver and electrode

Modern technology has made great progress in the treatment of perceptive deafness. The cochlear implant system, namely the artificial cochlea, which facilitates the perception of sound, including the frequencies which the ear cannot hear due to damage to certain sensory cells, has been created. This device is significantly more sophisticated compared with the earlier hearing aids which merely amplified the sound.

The cochlear implant system consists of two parts: external and internal. The internal part comprises the electrode and the receiver. The external part consists of the speech processor, microphone and coil (transmitter) connected by wires. The cigarette pack-sized speech processor is generally power-supplied by standard AA or accumulator 1.5 V batteries.

The receiver is implanted by surgery, subcutaneously, behind the auricle. The electrode connected to the receiver is surgically implanted into the cochlea and is in contact with the auditory nerve, thus taking over the role of sensory cells.


The wound on the morning after surgery

The surgical procedure during which the internal part of the artificial cochlea is fitted is usually performed on one ear only and lasts a few hours. The procedure is performed under general anaesthetic but is not too stressful to the child's body. The procedure takes such a long time because it involves auricular microsurgery and an electrode performance checkup. In rare cases, the surgery is performed on both ears, with two cochlear implant systems.

The post-operative recovery is quick. The wound is checked daily until the external stitches are removed, which is usually done seven to ten days after surgery. The external part of the system, namely the processor, the microphone and the transmitter, are first placed and adjusted approximately six weeks after the surgery. The child is then stimulated by the sound via the new hearing device for the first time, but he/she takes a while to recognize and distinguish sounds.

Use and Performance of the Cochlear Implant and the Speech Processor

Med-El's processor, microphone and coil

There are also smaller versions of speech processors, the same size as the standard ear amplifiers, which are worn behind the ear. However, they are not suitable for children, since their programming possibilities are modest and they can easily be damaged or lost.

The speech processor can be worn in different ways. It is usually worn on a belt around the waist. Children carry it in a backpack on their backs, which minimizes the risk of damage. The other option is to sew a special pocket onto a vest, on the back or at the side, above the waist.

Speech processor backpack

The microphone is worn behind the ear. The external transmitter coil is fixed to the receiver beneath the skin by a magnet, thus maintaining close contact with it. The coil and the microphone, as well as the adjoining cables, can be covered with hair to improve aesthetic appearance.

The microphone behind the ear registers the sound, which is then transmitted to the speech processor. Inside the processor, the sound is analyzed by different strategies (programs) and transformed into electric codes. Coded signals are transmitted to the receiver beneath the skin through the coil. The receiver forwards the information to the electrode inserted into the cochlea. The electrode is in close contact with the auditory nerve and stimulates it by electric impulses. The partially processed sonic information is transmitted via the auditory nerve to the central auditory system and the area of the brain which is responsible for hearing. By processing the received sonic information, the central auditory system facilitates its comprehension and appropriate use. Simply put, the cochlear implant enables the child to hear and the brain gives him the possibility to understand what he hears.

How it works (image courtesy of Cochlear)

Adjustment (Adaptation) of the Processor and Rehabilitation

After the surgical procedure and the first speech processor adjustment, the child starts the rehabilitation process. It is a time-consuming task which requires team efforts of both professionals and parents, but it yields excellent results. If the rehabilitation is done properly, the child can hear and understand speech and starts to use it as primary means of communication.

During the first year, the speech processor is adjusted every six to eight weeks, but later, the intervals between adjustments are longer. The aim is to find the best strategy of sound processing in the speech processor, in order to ensure comfortable and high-quality listening, which facilitates comprehension.

The Parents' Decision

There are different (and contradictory) views regarding cochlear implant – the artificial cochlea. Some people emphasize the surgical procedure and the fact that the implantation of the electrode destroys the remaining hearing ability in the ear. As I stressed earlier, the surgery is almost painless, the patients tolerate it well, and it is performed routinely today (the first procedure was done about twenty years ago).

Certainly, the artificial cochlea is not the right solution for everybody. An ORL specialist will suggest the cochlear implant on the basis of numerous tests (from tonal audiometry to the auditory evoked potentials test to CAT scan of the inner ear). It is the parent who must make the effort to gather as much information as possible and decide what is best for the child. The road to surgery, for which expensive devices need to be obtained (costing 30,000 – 40,000 DEM), is not an easy one.

Once again, I would like to stress that it is essential to get reliable information from people who have both theoretical knowledge and experience to give such advice. It is also important to discuss the matter with the parents of children with the same problem, i.e. parents of children with cochlear implants.

Today, the Association offers moral support to parents, as well as easier access to information based on personal experience, without seeking to impose its views on anyone.