Joseph Attias DSc
Institute for Audiology & Clinical Neurophysiology
Schneider Children’s Medical Center
European Federation of Audiology Societies
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).
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.
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.
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.