Chapter 16 - Cochlear implants
Introduction
Prior to the late 1970s there was no therapeutic intervention for patients with a severe-profound, bilateral, sensorineural hearing loss (SNHL). Because of the extensive loss of the sensory hair cells, these patients did not receive clinical benefit from conventional hearing aids. It is only over the last four decades through the development of cochlear implants (CIs) and auditory brainstem implants (ABIs) that patients with an extensive SNHL have been able to receive useful clinical benefit. CIs have an electrode array that is inserted into the scala tympani to electrically stimulate discrete sectors of auditory nerve thereby taking advantage of the tonotopic organization of the cochlea. CIs transmit important speech cues to the auditory brain, including loudness (stimulus intensity); pitch (site of cochlear stimulation); and temporal cues that convey information about the dynamics of the speech envelope.
Originally these devices were designed as an “aid to lip-reading”; however, with improved technology and habilitation techniques CIs have proven to be very successful, with some patients being able to communicate in an almost normal fashion. By 2011, a total of 200,000 CIs had been implanted worldwide. As clinical performance has improved, patient selection criteria have expanded to include children as young as 6 months of age, and severely deaf patients who receive only limited benefit from conventional amplification. This chapter will review the pathophysiology of a severe-profound SNHL, present an overview of contemporary CIs, briefly review central auditory prostheses and discuss new developments associated with CI technology.
Section snippets
Sensorineural hearing loss
Hearing loss is a major health and economic burden on society. For example, it is estimated that 22% of Europeans are affected by a hearing loss, costing the community €284 billion annually (http://www.hear-it.org/multimedia/Hear_It_Report_October_2006.pdf). Hearing impairment can result in significant communication disorders including poor development of spoken and written language in children, leading to important educational, social and vocational ramifications that can adversely affect
Cochlear implants
The development of CIs has dramatically changed the clinical management of patients with a profound, bilateral SNHL. CIs electrically stimulate discrete populations of residual SGNs via an electrode array located in the scala tympani, thereby taking advantage of the known tonotopic organization of the cochlea by dividing speech into discrete frequency bands mapped to specific electrodes.
Central auditory prostheses
Although cochlear implants have clearly demonstrated clinical benefit for severe–profoundly deaf patients (Bond et al., 2009), there have been numerous attempts to develop auditory prostheses based on electrical stimulation of the central auditory pathway. Studies of the mammalian auditory pathway have demonstrated well organized cochleotopic maps within the major auditory nuclei from the cochlear nucleus to the auditory cortex (Irvine, 1992), implying that appropriately positioned electrode
Future directions
There are a number of active areas of research directed towards improving the clinical performance of CI users. This research covers a wide spectrum of hearing ability ranging from patients with a profound SNHL to those with useful levels of acoustic hearing and includes: combined electric/acoustic stimulation (Gantz et al., 2000, Von Ilberg et al., 2011); bilateral cochlear implantation for improved sound localization and speech perception in noise (Van Hoesel and Tyler, 2003, Litovsky, 2011);
Conclusions
This chapter reviews the biological basis of CIs and provides an overview of both existing technologies and future research that will impact on the field. CIs have made a major impact on the quality of life of severe-profoundly deaf adults and children. With this clinical success come even broader patient selection criteria both in terms of age at implantation and hearing ability. The broadening of selection criteria will influence CI research in the future, including the development of
Acknowledgments
The authors would like to thank their colleagues within the Bionics Institute for useful discussions on aspects of this work.
We gratefully acknowledge the following financial contributions to this work: National Institutes of Health (HHS-N-263-2007-00053-C), the Garnett Passe and Rodney Williams Memorial Foundation, and the National Health and Medical Research Council of Australia. The Bionics Institute also acknowledges the support it receives from the Victorian Government through its
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