12 Cochlear Implants
Cochlear implantation has evolved to become a safe and reliable means of providing auditory rehabilitation in both adults and children with severe or profound hearing loss.
12.1 Indications and Goals of Cochlear Implantation Programmes
The device aims to provide perception of sound by attempting to emulate the transducer function of the cochlea, thereby stimulating residual auditory neural tissue. In appropriately selected individuals, the original premise that cochlear implantation would allow recognition of environmental sound and serve as an adjunct to lip reading has been realised. In reality, many individuals have derived greater benefit through cochlear implantation including the ability to understand speech with little or no lip reading, ability to use the phone, and enjoyment of music.
12.2 History
The first report of cochlear implantation was from Djourno and colleagues in 1957 who described the insertion of a device into two totally deaf individuals. Paralleled by developments in pacemaker technology, this stimulated considerable interest in the 1960s and 1970s amongst several investigators: the House group in Los Angeles (United States), Michelson and colleagues in San Francisco (United States), Clark and colleagues in Melbourne, (Australia), Hochmair–Desoyer’s team in Austria, and Chouard and colleagues in France. Not surprisingly, many of the original and current commercially available devices bear their origins in these pioneering research programmes. 3M and Clarion devices from California, United States (Advanced Bionics); Nucleus (Cochlear) devices from Australia (Cochlear), and Med-El devices from Austria.
The success of these early implant programmes generated considerable interest in the United Kingdom; the first single channel device was implanted in London by Fraser in 1984 with the first multichannel device being inserted by Ramsden in Manchester in 1988. Initially, implant programmes in the United Kingdom were funded through research and charitable sources, but the MRC report by Summerfield and Marshall (1995) based on a multicentre pilot study, in effect procured central funding for cochlear implantation.
12.3 Implant Design
Current cochlear implants consist of two parts: an external component and a surgically implanted internal component. The external part comprises a microphone, speech processor, and transmitter coil. The microphone unit is hooked behind the ear in a manner not dissimilar to a conventional behind-the-ear hearing aid. Sound received by the microphone is converted into electrical energy, which is conveyed to the speech processor. This body-worn component utilises various speech strategies (see later) and sends the processed signal to the transmitting coil which is held on the scalp behind the ear by a magnet in the coil and the implanted part of the device. The transmitter coil transfers the processed information to the internal implanted receiver–stimulator package by transcutaneous induction. From the receiver–stimulator package, information is conveyed to the electrode array which is implanted within the cochlea; the current hypothesis is that the implanted electrodes stimulate the spiral ganglion cells of the auditory nerve directly. Depending on the manufacturer, the number of active electrodes varies from 12 to 24 (multichannel devices). Modified electrode arrays are available for use in the partially ossified cochlea; compressed arrays carrying a smaller number of electrodes or double electrode arrays for insertion into the basal and middle cochlear turns independently. The implanted component of the device needs to be constructed from biocompatible materials with high tensile strength and resistance to corrosion. The electrodes themselves are made of a platinum–iridium alloy housed in silicone with either a ceramic or silicone casing for the receiver package. At the time of writing, the cost of the implant hardware in the United Kingdom is around £20,000.
12.4 Speech Coding Strategies
A speech signal has two main components: spectral (pitch) information and temporal (loudness and change in loudness) information. Various speech coding strategies have evolved over the last 20 years in an attempt to emulate and present this information to the auditory nerve. The original strategies based on extracting vowel and fundamental formant information are now obsolete. Current devices digitise the input and utilise bandpass filters to divide the signal into frequency-specific components. This information is presented in a pulsatile waveform to the individual channels of the electrode array. This reduces cross-interaction between channels thereby enhancing spectral information. By stimulating a smaller number of the available electrodes with the signal components that have the highest amplitude, the overall rate of stimulation increases, enhancing temporal information. The implementation of the principles of the speech strategies differs between manufacturers and continues to evolve. In terms of outcomes, the current strategies for multichannel implantation from the different manufacturers are all capable of providing comparable results.
12.5 Neural Plasticity
This is the ability of the central nervous system to be programmed to learn a task. In cochlear implantation, two components are of paramount importance: changes in the brainstem auditory nuclei and the auditory cortex in response to sound and also the neural plasticity of speech articulation. Auditory plasticity, and with it the ability to listen, is lost by the age of 8 years while the ability to develop good speech articulation only occurs if speech sounds are heard by the age of 3 years. On this basis, hearing-impaired individuals are classified as post-lingual (speech acquired before becoming deaf) or pre-lingual (become deaf before acquiring speech). In addition, a third group comprises those children who lose their hearing around the time of speech development (peri-lingual). Post-lingual adults and children may be considered for implantation. In pre- and peri-lingual children, the timing of cochlear implantation is critical taking into account the issues of neural plasticity.
12.6 Candidacy
The process of selecting appropriate individuals for implantation is of critical importance for a successful outcome and is a task that involves all the members of the multidisciplinary implant team. The selection criteria continue to evolve and may be considered under the following headings:
1. Age:
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