Cochlear implants in children: safety as well as speech and language

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Abstract

The development of cochlear implants for children at the University of Melbourne and the Bionic Ear Institute, has consisted of a routine of biological and engineering safety followed by evaluation of speech processing strategies on adults before they are undertaken on children. The initial safety studies were to ensure that insertion was atraumatic, the electrical stimulus parameters did not lead to loss of ganglion cells and that the electrode could be inserted without the risk of middle ear infection leading to meningitis.

The initial second formant extraction scheme was shown to produce significant open-set speech understanding in adults and was approved by the US Food and Drug Administration (FDA) in 1985. Following this, an international study was undertaken for the FDA on children using a strategy that also included the first formant, and was approved in 1990.

Additional advances in speech processing have been evaluated on adults. However, before using one with high rates of stimulation, it was tested for safety on experimental animals. Further advances have been anticipated in particular through the development of a peri-modiolar array, the Nucleus Contour. Prior to its use on adults, it was tested in the human temporal bone and found to lead to minimal trauma. It was evaluated in adults and found to lead to better current localization and lower thresholds. A study was undertaken in children using a spectral maxima scheme at high rates (advanced combination encoder (ACE)) and the Contour array as it had given best results in adults. It was approved as safe and effective for use in children in 2000.

Studies were also undertaken to look at plasticity and visual dominance particularly through cognitive studies and the use of the McGurk effect. This demonstrated that deaf children with implants rely heavily on visual information and there is a great need to have unambiguous auditory stimuli to get best results.

Introduction

Speech perception and production as well as language must be considered in evaluating the benefits of cochlear implants for children. They are inter-related and their development is important for the child’s education, social development and career opportunities. Language is the medium for communicating a message, and can be broadly classified as expressive and receptive. The message has the following components: phonology, morphology, syntax, and pragmatics. Phonology refers to the basic perceptual and acoustic units in a word; morphology or semantics refer to the meanings of words and sentences; syntax or grammar refer to the rules governing the ordering of the words; and pragmatics is the skills to carry on a two-way conversation and is assessed by analysing interactions between child and family or therapist.

Children with a hearing loss and a hearing aid have language that falls predominantly outside the normal distribution of equivalent language age for children with good hearing, and it is worse for those who are severely-to-profoundly deaf and need a cochlear implant.

Cochlear implants not only provide information in the low frequencies, but also in the mid to high speech frequency ranges not available to the hearing-impaired children. Thus, implants should improve the speech and language in severely-to-profoundly deaf children.

Before implanting children in 1985 at the University of Melbourne’s Cochlear Implant Clinic at the Royal Victorian Eye and Ear Hospital, biological safety was studied on experimental animals and human temporal bones. The efficacy of the speech processing strategies was also established on post-linguistically deaf adults before trialing in children.

Section snippets

Safety

The areas of biological safety examined were trauma, biotoxicity, electrical stimuli, and the risk of middle ear infection leading to labyrinthitis and meningitis. The most serious risk is middle ear infection spreading to the inner ear and leading to meningitis. This was a reasonable concern as in the 1960s and 1970s, cases of fatal meningitis were reported in patients who had otitis media some time after a stapedectomy [1], [2], [3], [4], [5]. These reports showed the stapes prosthesis could

The efficacy of speech processing for children

The first and second formant/voicing (F0/F1/F2) and first and second formant, and high spectral frequency/voicing (“Multipeak”) speech processing strategies were first shown to be effective on post-linguistically deaf adults before they were used on children from 1985 to 1994. With the F0/F1/F2 strategy, the first (F1) and second (F2) formant frequencies were extracted with filters with wide bandwidths and presented as place of stimulation to the appropriate frequency site in the cochlea, the

Safety of implanting infants and young children

Special biological safety risks in infants and young children had to be addressed. These were the risk of middle ear infection extending to the inner ear and leading to meningitis, the effect of the implant on head growth, and the effect of electrical stimulus parameters on a maturing nervous system.

Firstly, the effects of Streptococcus pneumoniae rather than S. pyogenes were studied, as it is a very common pathogen causing otitis media and meningitis in children. S. pneumoniae also has

Efficacy for adults and infants and young children

The efficacy of the most recent strategies had to be assessed in adults before implanting young children. There were three speech-processing strategies to be evaluated in adults before use in young children. They were the continuous interleaved sampler (CIS), spectral maxima sound processor (SPEAK), and advanced combination encoder (ACE). CIS samples the outputs of six or more filters to stimulate six to eight electrodes on a place-coding basis non-simultaneously at a constant high rate of 800

Advances in speech processing

Future advances in speech processing are likely to require a better interface with the auditory nervous system so that more electrodes can be stimulated with a finer temporal and spatial pattern of responses. This could require placing the electrodes closer to the nerve fibers. Consequently if children are to be able to take advantage of these advances a peri-modiolar electrode was designed to lie close to the modiolus, and this also allows the creation of a sheath that can be used by newer

Safety and efficacy of peri-modiolar electrode array

To produce a peri-modiolar electrode that would be safe and not cause trauma a single-component array was developed and evaluated in the human temporal bone in the ARC Human Communication Research Centre. This array had a single component with an in-built curl, and was held straight until it was inserted into the cochlea before being released. The single-component array lay close to the modiolus within the scala tympani and caused minimal trauma.

The single-component pre-curved array was further

Efficacy of peri-modiolar electrode array

The advantage of the Contour electrode is its potential to provide new strategies for children and adults in the future, and also improve results with existing strategies. In order to evaluate advanced strategies on adults, the Nucleus 24 speech processor and 24R receiver–stimulator and Contour array have been developed (Fig. 11). The Contour provides lower thresholds and comfortable listening levels. It also produced more localized stimulation as shown by the narrower masking curve for the

Factors leading to improved speech and language

Not only is it necessary to develop new speech processing strategies, but it is also essential to know the other factors of importance for improving speech perception as they will not only show how to achieve the best performance with present strategies, but also how to develop improved systems. The general factors leading to good speech perception were analysed from the results on 102 children at the University of Melbourne and Bionic Ear Institute in 2000. The factors contributing to the

Psychophysical factors related to improved speech and language

The variations in performance may also be due to neural connectivity in early development and are reflected in the psychophysical responses in children. This was studied firstly by seeing whether the children’s ability to discriminate electrode place of stimulation correlated with age at implantation. The results showed that the discrimination of electrodes was better the younger the child at operation [35], [36]. This suggests there is a limited time over which the neural connectivity for

Cognitive factors for improved speech and language

Language and other higher cognitive functions have been assessed to determine their role in speech perception. The close relation of language with speech perception is illustrated in Fig. 14 for data from both implant and hearing aid children over time. The relation is steep up to an equivalent language age of 6 years. The importance of language for speech perception is illustrated in this figure. In addition, three cochlear implant children were educated in the meaning of words they did not

The McGurk effect and habilitation

The results have important implications for habilitation. They show firstly there is a visual bias in integrating information, and that a higher level function for fusion of auditory and visual information is present in deaf children. These findings suggest that either the visual signal be used to help children with ambiguities or that speech processing strategies should be used to minimize key ambiguities.

The studies of the McGurk effect indicate the strong bias of the visual signal for

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