Deafness and the Bionic Ear

Deafness is a by no means negligible social problem. The subject suffering from profound deafness in Italy amount approximately 120,000 and, according to the ISTAT data, the proportion of deaf subjects aged over 50 is steadily increasing and will have reached as much as 10%% of the population by the year 2001.

It is estimated that 150 to 200 children a year are born with profound congenital bilateral neurosensory hearing loss; if we consider the pre and periverbal forms acquired in preschool infancy, the number rises to some 1000 cases a year and, if we go on to consider the forms acquired in children of school‐‐going age, the number of cases per year rises to as many as 2000–3000. Approximately 47%% of the population present hearing problems and 18,000 adult subjects suffer from profound bilateral deafness.

These figures necessarily bring us to the concept of the socio‐‐economic analysis of the problem of deafness.

The presence of severe or profound loss of hearing reduces the ability to perceive spoken language and this, in turn, leads, in the deaf child, to the development of models of articulation which differ significantly from those of his or her normally hearing counterparts of the same age ((Welsh et al., [1996], [1983])).

These two factors create enormous barriers and obstacles to the child's ability to interact with society. What is more, the impact of the inability to hear on the acquisition of oral and written language severely limits the child's range of choices in life and his education. Equally dramatic is the psychological condition of these children who inevitably develop feelings of inferiority towards others, of insecurity with regard to their abilities, and of diffidence towards everyone around them. These feelings will ultimately lead to conditions of dependence on a member of the family, or to isolation, aggressiveness and depression ((Gatty, [1998])).

In adults, too, the onset of progressive or sudden deafness leads to unmistakable transformations in conditions of life involving difficulties of communication with family and friends, inability to carry out one's professional activity adequately and social maladjustment, other inevitable consequences being isolation, diffidence and depression.

The only valid alternative to all this for subjects suffering from profound or severe bilateral neurosensory deafness who are unable to benefit from traditional prosthetic hearing aids is the bionic ear.

The bionic ear is an implantable artificial organ capable of restoring hearing. It consists of two parts, one of which internal and the other external. The internal part is inserted surgically in the retromastoid region or in the squamous portion of the temporal bone and consists of a receiver–stimulator, a magnet and an electrod array. The electrodes are placed in the cochlea or, in the absence of the acoustic nerve, in the brainstem and are capable of electrically stimulating the fibres of the acoustic nerve or the auditory areas of the brainstem. The external part consists of a microphone which is positioned behind the pinna and a small sound‐‐processing computer capable of converting the sounds into electronic impulses. It may take the form of a small walkman or it can be contained in the retroauricolar appliance. Through a magnet‐‐transmitter it adheres by electromagnetic attraction to the magnet of the internal part.

The functioning of the bionic ear can be summarized as follows: the sound arrives at the retroauricolar microphone and isrelayed to the small sound‐‐processing computaer which converts the most useful elements for the understanding of words into electronic codes. These codes then reach the magnet‐‐transmitter unit which transmits them, through the skin, to the subcutaneous receiver–stimulator. The latter contains an integrated circuit which converts the codes into special electronic signals that ultimately reach the electrodes inserted in the cochlea by direct stimulation of the cochlear nerve fibres and in the brainstem by stimulation of the cochlear nuclei.

The internal part is positioned by means of a surgical operation which, according to the classic technique, involves a retroauricolar approach. A milling cutter is used to produce a niche at the level of the squamosa to house the receiver–stimulator, and a mastoidectomy and posterior tympanotomy are performed to make it possible to reach the cochlea via a cochleostomy. This enables the surgeon to insert the electrode array in the cochlea. This surgical technique presents major advantages but also a nimber of limitations which can be overcome by means of an operation developed and perfected by Professor Vittorio Colletti involving gaining accessto the cochlea via a subtemporal route. With this approach, the middle ear is abandoned and the electrode is inserted from above, thus making it possible to reach the middle and apical areas of the cochlea.

As we have already mentioned, even if the auditory nerves are damaged, as is typical of Von Recklinghausen neurofibromatosis, hearing can be restored by means of the brainstem implant.

Brainstem implants have been fitted hitherto only in adult patients, including approximately 50 patients in Europe and some 200 in the USA. On February 15, 2000 in the ENT Department of Verona University, Professor Vittorio Colletti became the first ENT surgeon in the world to fit a brainstem implant in a child. The 4‐‐years‐‐old child was suffering from a severe cochlear malformation and aplasia of the cochlear nerve. Since the condition was bilateral, it was not possible to use the classic cochlear implant approach. The child responded brilliantly to the operation and the study of auditory evoked potentials and neurotelemetry enabled the surgeon to confirm correct anatomical position of the implant.

One month later, a 3‐‐years‐‐old child with a similar malformation underwent the same operation successfully.

The bionic ear then is capable of restoring hearing thanks to this series of surgical inventerventions.

It should be pointed out that, after the operation, when approximately one months has elapsed, which is necessary for scar formation, the implant has to be activated and adjusted ideally for each individual subject. This procedure is called mapping and entails the stabilization for each electrode of the minimum ((T level)) and the maximum energy capable of causing an auditory sensation without causing discomfort ((C level)). In this way a dynamic range of stimulation of the fibres of the acoustic nerve or cochlear nuclei is defined for each electrode for the two types of implants, respectively. Furthermore, as each electrode is activated for different frequencies, the brain, by virtue of neuroplasticity, is again capable of receiving all the frequencies and intensities and of distinguishing words from sounds and noises. In addition, there are other technical parameters that can be modified in order to achieve the optimal auditory outcome.

To be able to exploit the bionic ear to the full, in each case a period of logopedic rehabilitation will be necessary, since the subject must adapt to the new way of hearing, almost as though he or she had to learn a new language.

The rehabilitation therapy varies according to the subject's needs. For instance, for subjects suffering from post‐‐verbal deafness, i.e. deafness after they had learnt to speak, acoustic training will often be enough, since they have been able to develop an auditory memory to which they can refer. Subjects with pre‐‐verbal deafness, on the other hand, have to learn to listen to and inwardly assimilate sounds so as to be able to go on and develop verbal language.

From what we have said, it is clear that achieving optimal outcomes requires a substantial effort on the part both of the implant recipient and the team assisting the subject, but the advantages that can be obtained from using the implant are very considerable. The implanted child, in fact, quickly develops auditory perception and subsequently verbal production and the acquisition of language which will enable him or her to achieve social integration. The implanted adult, on the other hand, is capable of regaining hearing capacity and thus of resuming his or her habitual life style.

The basic factors affecting the final outcome are numerous. In the first place, there is the time of onset of the deafness: rehabilitation is simpler and faster in subjects with post‐‐verbal deafness. A second major factor is the duration of the deafness: the shorter the duration of hearing loss, the easier it will be to reactivate auditory memory. Anatomical conditions should also be considered and therefore also the quantity of residual fibres of the cochlear nerve or hearing centres. Finally, the subject's age at the time of implant is also important: in children with congenital deafness, the sooner implantation is performed, the better the functional results will be. Family support also plays a basic role in the rehabilitation process ((Brackett and Zara, [1998]; Fryauf‐‐Bertschy et al., [1997]; Nikolopoulos et al., [1999])).

The experience world‐‐wide with more than 20,000 patients treated by implanting a bionic ear has made it possible to overcome a serious clinical problem in a satisfactory manner, yet, as a result of the ongoing research in the biomedical field, increasingly advanced goals can be perceived and techniques adapted to individual needs.

Our personal case series consists of 124 patients, 50 of whom received implants by the retromastoid route and 71 by the subtemporal route, while 3 had bilateral implants in the same surgical session.

In addition, 9 brainstem implant have been applied, 7 in adult patients ((6 subjects suffering from type II neurofibromatosis and 1 with a solitary neurinoma in the only hearing ear)) and 2 in children, aged 3 and 4 years, respectively suffering from severe bilateral cochlear malformations and aplasia of the cochlear nerve.