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Audiological disorders in children: current innovations

Joseph Attias
1 July, 2006  

Joseph Attias DSc
Head
Audiological  Department
Haifa University
Israel
Director
Institute for Audiology & Clinical Neurophysiology
Schneider Children’s Medical Center
Petach Tikvah
Israel
President
European Federation of Audiology Societies
W: www.efas.ws

Congenital hearing loss is one of the most common birth  defects, affecting approximately two to four out of every  1,000 newborns. The rate is particularly high among  premature infants and infants of consanguineous parents.  Hearing loss has a pervasive effect on quality of life,  interfering with speech, language, communication, learning  and social skills – especially in infants and young children  whose capacity to communicate is still undeveloped. The  longer the deafness lasts and the more profound it is, the  more severe the consequences. Therefore, early  identification and early treatment are crucial. Studies have  shown that rehabilitation started before the age of four can  close the gap between the hearing-impaired and hearing  child; any later and the success rate decreases  significantly. In developed countries, identification and  intervention no later than 12 months is a standard of  healthcare.

Hearing loss detection
Until 10–15 years ago, testing for hearing loss in infants  was difficult and costly, and it was applied mainly to those  at high risk. Screening large populations of newborns first  became possible with the discovery of otoacoustic emissions  (OAE).(1) Normally, sound is transmitted from the outer ear  to the inner ear or cochlea by the vibrating eardrum and the  middle ear bones. The cochlea contains a basilar membrane  covered by tiny hair cells that detect the vibrations and  convert them into signals to conduct to the brain (see  Figure 1).

[[HHE06_fig1_C62]]

Researchers found that by placing a sensitive microphone in  the external ear canal and connecting it to a computer, they  could measure the inner ear’s response to sound in the form  of otoacoustic signals or emissions. In this way, they could  determine if the function of the outer hair cells was intact  at the threshold level (sensory hearing).

This innovation was important, because most cases of  congenital hearing loss are caused by an impairment of  cochlear hair cells. The test has been found to have almost  100% sensitivity. It is also quick (lasting only a few  seconds), noninvasive and objective, making widespread  screening both feasible and cost-effective.

Although the detection of OAE indicates normal hearing, the  absence of emissions does not necessarily indicate cochlear  hearing loss – it could also be caused by a malfunction in  the middle or external ear. Therefore, infants who fail the  OAE screen are referred for testing of the auditory neural  system. Neural tests are also appropriate for the small  number of infants (2–10% of infants with hearing loss) with  an auditory nerve malfunction or auditory neuropathy. In  these cases, the inner ear may be completely intact, so the  diagnosis of hearing loss is missed on OAE testing at birth.

Auditory neuropathy is usually caused by lack of  synchronisation in the neural tract due to an impairment at  the junction of the cochlear sensors with the auditory nerve  fibres, or to a disease that affects the propagation of the  neural volley. Premature babies are particularly vulnerable.

To measure auditory brainstem-evoked responses (ABR) three  electrodes are attached to the scalp. The brain potentials  induced by decreasing levels of auditory stimuli are  recorded.(2) The lowest stimulus level at which potentials  can be recorded is considered the “cochlear hearing  threshold”. By using different stimulus paradigms,  audiologists can predict the hearing threshold at each  frequency. This test can also assess the speed at which  neural activity is propagated along the auditory pathway  from the cochlea to the brainstem. A speed significantly  lower than normal may indicate the presence of a lesion  beyond the cochlea or abnormal development of the neural  tracts.

Hearing loss management
A partial absence of cochlear hair cells causes a hearing  loss at the frequency corresponding to the location of the impairment. Activating the remaining cells by amplifying  external sounds could compensate for the loss. Therefore, children with partial  hearing loss may benefit from a hearing aid.

Today’s hearing aid models are much smaller than the  originals and hardly visible. They have multiple channels  for processing different frequency bands and temporal cues,  and multiple microphones for collecting speech signals from  a wide area around the head. Special attention has also been  addressed to enhancing the distinction of speech from  background noise (signal-to-noise ratio), attenuating the  background noise, and reducing the occlusion effect (the  distorted perception of one’s own voice caused by the  presence of the hearing aid in the ear canal). Programme  selectors with data logging features allow users to store  their personal listening settings for music or noisy  environments. This is especially relevant in small or  disabled children who cannot report their hearing problems  or cooperate with the hearing aid programmer.

However, in children with very few intact hair cells and the  consequent severe or profound hearing loss, the  amplification possible with even the best available hearing  aids may not be sufficient to make sounds audible. These  children would probably fare better with a cochlear implant,  with a mechanism based on electrical stimulation. This is  also true for children with auditory neuropathy.

Cochlear implants are designed to compensate for the lack of  activity (transduction) of the missing hair cells in the  inner ear (see Figure 2).(3) They consist of an external  directional microphone that picks up sounds from the  environment and transmits them to a speech processor. This  processor extracts the information necessary for speech  understanding, produces a code for the signal and transmits  the signal by inductive coupling to a receiver-stimulator  implanted in the mastoid bone. The receiver-stimulator then  decodes the signal and produces a pattern of electric  impulses in an array of electrodes inserted at the turns of  the cochlea for transmission to the brain.

[[HHE06_fig2_C62]]

Since its introduction, the cochlear implant has undergone  tremendous technological improvements of the electrode  array, speech coding strategies and transmission paradigm.  It has also been greatly miniaturised. Data collected on the  more than 100,000 cochlear implants in use indicate that  with complete insertion in the inner ear early in the life  (12–24 months) – together with intensive auditory training –  success rates can reach beyond 95%.

Nevertheless, several important challenges remain. First,  better reproduction of the sound coding is needed in both  the high and low frequencies. Secondly, researchers are  seeking ways to make the cochlear implant invisible to the  eye by also implanting a speech processor and microphone,  with power supplied by an implantable rechargeable battery  or a coil placed over the implant. Full implants would make  it possible for children to freely participate in all  activities without removing the device. Thirdly, combining  the electrical stimulation with delivery of nerve growth  factors might protect the cochlear nerve from further  deterioration and induce its regeneration by activating the  appropriate gene.

The progress in hearing aid and cochlear implant  technologies has led researchers to combine the two modes of  management in children with residual hair cells in the low  frequency range only. An electrode array is inserted into  the basal turns of the inner ear for high frequencies and an  acoustic hearing aid is added for the low frequencies.  Recent preliminary studies reported great promise for this  method for improving understanding of speech in noisy  environments and enhancing enjoyment of music.

Hearing aid and cochlear implant technology has also been  applied to the development of implantable – and recently to  fully implantable – hearing aids for patients with a  moderate impairment who cannot use a hearing aid due to the  absence of external ears, infections induced by hearing  aids, or poor sound quality with hearing aids. Implantable  hearing aids pick up sound through an external microphone  attached by a magnet to a motorised or piezoelectric device  implanted under the skin behind the ear. Sound is  transmitted to the middle ear bones by the tip of the device  vibrating in response to sound, causing the bones to move.  From this point, sound is processed as in healthy children.

The implantable bone-anchored hearing aid (BAHA) is a good  option in children with severe bilateral impairment of the  middle ears but normal inner ears. A vibrating device  attached to a screw inserted into the bone behind the ear  stimulates the cochlea in response to noise, thereby  bridging the affected area. However, the BAHA may be  associated with infection or discomfort around the abutment  site.   

Genetic treatment is a novel approach to irreversible  sensorineural deafness. Studies in animal models have shown  that the incorporation of the Atoh1 gene, which plays a key  role in regulating hair cell development, into inner ears  that contain nonsensory supporting cells leads to  regeneration of functional hair cells and subsequent  improvement in the hearing threshold. Such biological  technology may be the solution of the future.

Conclusion
In conclusion, recent years have witnessed tremendous  progress in the early detection and management of  significant hearing loss in infants. Latest developments  including elegant and efficient methods to restore or  preserve hearing. Many of the advanced technologies are  already available in a large number of Western countries.  Our mission is to introduce them to developing areas and to  encourage their acceptance and clinical application.