Home » 3 -Assistive Hearing Devices

3 -Assistive Hearing Devices

Early provision of assistive
hearing devices

Andrea Bohnert

displays the possibilities of providing assistive devices to young children and what has to be taken into consideration when choosing the kind of device and the handling of technology.

Introduction

The potential and limitations of technical intervention to combat hearing impairment in early childhood are determined not only by how early the diagnosis is made, but also by the quality of this intervention. For children with congenital or early acquired hearing defects, the experience of hearing in the first years of life is of crucial importance. The neurological maturation processes depend upon it, as do the development of hearing ability and the acquisition of language. Within this sensitive phase, as hearing matures postnatally, this sensory ability can develop normally only when sufficient acoustic stimulation is provided. If this phase elapses ‘unused’, so to speak, then this will have serious consequences for hearing and language development in these children. A large number of studies (Robinshaw, 1995; Apuzzo and Yoshinaga-Itano, 1995) have shown that children with severe hearing loss who are fitted with hearing aids before the age of 6 months and receive early intervention as appropriate can undergo speech development that that is in the ‘low normal’ range for their age. Their language acquisition is superior to that of children who are diagnosed and treated later in life (Yoshinaga-Itano, 2001).

Richard Seewald once said: “Children are not small adults.” By this he means that audiological methods developed for intervention in adults cannot be used in children without suitable modification. The process of fitting with hearing aids in infants and children is, in many respects, considerably different from that in adults, and these child-specific aspects must always be taken into account. Methods are now available that have been specially developed for intervention in children and thus enable children to be fitted (appropriately for their age) with assistive hearing devices within the first half-year of life.

The following chapters describe these child-specific aspects as well as the use and handling of these technical devices (hearing aids, cochlear implants, and FM systems that provide additional benefit) in children between the ages of 0 and 3. The suggestions and information given here are designed to help you relate with greater confidence to a child with hearing impairment.

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Chapter I – Specific aspects of hearing-aid fitting in infancy

Learning goals

Newborn hearing screening (NHS), improved electrophysiological diagnostic techniques and tremendous advances in hearing-aid technology have, in recent years, considerably enhanced the prospects for children with hearing loss. The following chapter describes those aspects where hearing-aid intervention in infants and children differs from that in adults, and why it is so important to use procedures specially developed for children.

The ‘hearing situation’

The ‘hearing situation’ in infanthood is different from that in adulthood. When spoken to, babies and toddlers are usually on their mother’s arm or lap or at her breast. They spend most of the day in a recumbent position. Infants have no control at all over their heads, which can make it very much harder to avoid the occurrence of feedback when they are wearing their hearing aids. As a child ages, however, things change; their growing mobility means they become exposed to different hearing situations that shift and alternate ever more frequently. During the early years of life, the focus is on the transmission of a broad spectrum of frequencies from the child’s immediate ‘auditory vicinity’; as the child learns to walk, however, the distance between them and the sound source rapidly increases. As the child’s ‘action radius’ continues to grow, the range of demands made on their hearing aids therefore changes.

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Anatomical conditions / earmould

The anatomical conditions present in infants and toddlers are also different from those in adults. The auricle and ear canal are made of highly elastic tissue, and the ear canal itself is shorter and straighter. In these young children, therefore, sound is picked up by an ear canal that is smaller and (in contrast to the situation in adults) generates sound pressure transformations that are displaced to higher frequencies (FIGURE 1). These special anatomical conditions and the child’s lack of control over their own head often make it more difficult to avoid feedback when the child is wearing hearing aids, especially in the first months of life. These devices are frequently prone to whistling, which is unpleasant for the caregiver and, depending on the level of hearing loss, may also cause great discomfort to the child themselves. The greater the degree of hearing loss, the sooner this feedback squeal occurs. It is therefore important to have a cast of the ear taken, and an earmould created, by an experienced professional. While this mould (FIGURE 2) serves to securely hold the hearing aid in place at the outer ear, its main function is the acoustic transmission of the amplified signal from the hearing-aid receiver to the eardrum. It is important that the earmould fits as snugly as possible in order to ensure a high level of acoustic sealing and thus prevent feedback. Another function of the earmould may be to enhance the acoustic transmission characteristics. This can be achieved by creating additional openings.

Making and inserting what is known as a ‘Müller claw’ (German: ‘Müller-Kralle’; FIGURES 3 and 4) may help to keep the feedback problem better in check. If feedback persists, however, it is a good idea to get the hearing aid tested as to its ‘leak tightness’ and to have a new earmould made. On no account should the amplification be lessened in an attempt to reduce feedback, as this would lead to recipients being insufficiently aided. It is recommended that soft materials be used to make the earmould, in order to keep the risk of injury as low as possible.

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The function of sound transmission through the outer ear

As long ago as the early 1970s, Keller and Biesalski drew attention to the fact that the ear canal in infants and toddlers has a smaller volume than that of adults, and that it is essential that this be taken into account in calculating the amplification output of hearing aids when children are fitted with these devices. Kruger and Ruben, too, reported in the late 1980s that the ear canal volume in newborns may be two to three times smaller than the average volume in adults. Feigin and coworkers concluded that the difference between the sound pressure levels found in children under the age of 5 in a sealed ear canal and those measured using a coupler is significantly higher than the equivalent values in adults. The coupler (a device for coupling the hearing aid to the test microphone) represents the standard ear canal volume in an adult and is used to provide reference values for hearing-aid intervention. A number of studies have been conducted since (Kruger and Ruben, 1987; Bohnert et al., 2001; Bohnert und Brantzen 2004; Bagatto et al., 2002) which show that the readings obtained using a coupler in adults do not provide sufficient information about age- and frequency-specific properties of the ear canal in infants and toddlers. These data are, however, necessary in order to calculate a hearing aid’s required amplification output. A large ear canal volume means that higher amplification must be factored in, since the sound pressure level at the eardrum declines; if the ear canal volume is small, however, less amplification needs to be provided, as the sound pressure level is increased at the eardrum. If, therefore, the ear canal volume of a child is not determined exactly but only estimated on the basis of an average adult volume (coupler measurement), then the hearing aid may be set at too loud or quiet a level. If it is too loud, the child may refuse to wear it and their residual hearing would not be sufficiently protected. If it is too quiet, speech signals cannot be adequately transmitted. It is, therefore, not acceptable for infants and children to be fitted with hearing aids solely on the basis of coupler response in adults. Instead, therefore, the real-ear-to-coupler (RECD) value should be used, which takes account of the specific conditions relating to each particular child’s ear canal and each individual earmould during the fitting process (Moodie et al. 1994). This procedure involves a small probe-tube being inserted into the child’s ear canal. The customized earmould is also placed into the ear, coupled with an insert earphone. Readings can then be taken; measurement itself takes only a few seconds. The results are stored in a database and can then, without the child having to undergo any further discomfort, be used as the basis for setting the hearing-aid levels (FIGURE 5). Studies have shown that measurements of this kind can be reliably made from the age of one month onwards. If, in an individual case, it does not prove possible to take readings based on the child’s actual ear canal, the system provides average values for each month of life. These averages are based on extensive measurements of the ear canal in children (infants and above).

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Fitting algorithms

Hearing-aid fitting requires frequency-specific data to be provided. These are used in setting the device parameters on the basis of ‘fitting rules’ that have been devised especially for children. These rules are also known as fitting algorithms. Of the available algorithms, two methods – DSL[i/o] v 4.1, DSL m[i/o] v 5.0 (Seewald et al., 2005) and NAL-NL1, NL2 – fulfil these criteria. In Germany, the DSL technique is the one most commonly used with children. The DSL[i/o] method takes into account the child’s actual age, the type of transducer system used in audiometric testing (free field, insert earphones, headphones), the sound threshold, the uncomfortable loudness level, and the long-term average speech spectrum (LTASS). The upgraded DSL m[i/o] v 5.0 version also incorporates data from electric response audiometry (ERA), revised normal values for RECD, sound conduction components, multi-channel compression, target curves for both quiet and loud situations, and binaural fitting (i.e. for both ears).

In accordance with audiometric data, especially the frequency-specific hearing threshold, the hearing aid is thus pre-programmed on the basis of the fitting algorithm.

The outcome of calculations based on the fitting algorithm and the setting of the hearing-aid parameters should be checked using what is known as an ‘SPLogram’ (sound pressure level output diagram) (FIGURE 6). This diagram is used to demonstrate whether the LTASS is being optimally transmitted, i.e. both quiet and loud components of speech within the residual dynamic range of the hearing-impaired child are depicted. Inadequate transmission of (for example) quiet or high-frequency components of speech, in particular, requires investigation to find the causes (FIGURE 7).

After initial fitting with hearing aids, the process of hearing development begins. This entails dynamic progression and depends on a wide range of factors. For this reason, hearing-aid fitting must also be a dynamic process, involving gradual adjustment as the child develops. However, this does not refer to the ‘fine-tuning’ stage. Rather, the aim here is to ensure that the hearing aid is – while the child’s hearing and language are developing – optimally adjusted throughout. Fine-tuning refers to optimization following the first fit. This process of optimization is a general requirement and does not depend on the way in which the child’s development progresses.

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Choice of hearing aids

Various requirements must be considered when selecting the right hearing aid. In general, digital behind-the-ear devices, preferably with highly flexible setting options, should be used in order that even children with complicated or progressive threshold curves benefit sufficiently. The hearing aids selected should be small and robust, with a childproof battery compartment and an FM connection (wireless radio system; see Chapter V). When checking for possible feedback, only feedback managers that operate out of phase should be used. Please note that after the feedback manager has been activated, checks should also be made to ensure that the device still provides sufficient amplification. Multi-microphone technology, noise suppression managers, multi-programme managers and volume controls should be activated in the first year of life. To date, no studies have been published that allow meaningful conclusions regarding the minimum age at which adaptive directional microphones can be beneficially used. There are, however, various research teams that are addressing this aspect.

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Monitoring the outcome

Once the hearing-aid fitting process has been completed, a SPLogram needs to have been created demonstrating optimized transmission of LTASS in the child’s residual dynamic range. It must be ensured that quiet components of speech or high-frequency regions can also be adequately transmitted. The aided threshold of hearing is determined during visually based tests that have been described as ‘observational and distraction audiometry’. These procedures can be used as a tolerance test and serve as a plausibility check. They entail testing changes in the child’s behaviour – such as reactions that involve the eyes or head turning towards the sound source, changes in facial expression, gestures or breathing, as well as changed activity in response to acoustic stimuli – both in free field and via bone conduction receivers. It is essential to have well-structured questionnaires, and that both the parents and early-years practitioner make use of monitoring sheets.

Regular ongoing checks and monitoring are required in a number of areas during the first year of life, encompassing:

all diagnostic data (growth; status of the middle ear);
RECD data (growth; status of the middle ear);
hearing-aid settings;
regular intervention to help auditory, speech and language development;
regular checks on language status.

As soon as the suspected diagnosis has been confirmed, this must be reported to the relevant early-intervention provider in order that regular intervention to help auditory, speech and language development can take place. During the period of hearing-aid fitting, the parents must be kept informed about the use and maintenance of the equipment, as well as about accessories and additional technical devices. When a prescription for a hearing aid has been issued, if not before, the parents should be presented with a ‘passport’-style record for the device.

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Review questions

  1. What makes it particularly hard to prevent feedback in hearing aids worn by infants?
    1. The fact that they lack control over their heads
    2. The fact that they lack control over their heads and / or have poorly-fitting earmoulds
    3. Poorly fitting earmoulds
  2. Why is it important to measure the volume of a child’s ear canal?
    1. So that the hearing aid provides neither insufficient benefit nor too much benefit
    2. So that the hearing aid does not provide too much benefit
    3. So that the hearing aid does not provide insufficient benefit
  3. What should a hearing aid for infants and toddlers be like?
    1. As large as possible, so as to optimize transmission of the signal
    2. As large as possible, with a childproof battery compartment and flexible setting options
    3. Small and robust, with a childproof battery compartment and flexible setting options
    4. Small and robust, with flexible setting options

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Chapter II – Use and handling of hearing aids

Learning goals

In the previous chapter you will have got an idea of the particular challenges that surround hearing-aid fitting in infants and toddlers. This chapter will explain how you yourself can check that a hearing aid is in proper working order, how to reinsert it if it has fallen out, and what you can do if the device whistles, its LED is flashing or the child suddenly no longer responds when spoken to. In other words, you will receive practical tips for dealing, on a day-to-day basis, with a child who wears hearing aids.

Checking that hearing aids are working properly
All hearing aids require batteries as their power supply. Hearing aids tend to use zinc-air batteries. The mercury batteries formerly utilized are no longer in use as they contain heavy metals that are environmental contaminants. Zinc-air batteries also contain toxic substances. While they are not as hazardous as mercury batteries, it is still essential to make sure that the child does not accidentally swallow one. To check whether the hearing aid is working properly, place it in your cupped hand and switch it on: it should whistle, since the sound amplified by the hearing aid re-enters the device’s microphone and is re-amplified. If it does not whistle, the fault must be traced. Both the tubes and the earmould need to be regularly checked, as condensation can form in the tube and earwax may block the earmould.

The hearing aids and earmould should be checked daily to ensure they are working properly.

When putting on the hearing aids, make sure you do not get the sides mixed up, as these devices are usually specially adjusted to suit either the left or right ear. This is especially important if the hearing impairment is more pronounced on one side than the other. The hearing aids should therefore be marked appropriately; a red dot is commonly used for the right-ear device and a blue dot for the left-ear device. As this is not necessarily the case, however, you should ask the parents how their child’s hearing aids are marked. Do not switch the devices on until you have inserted them, and do not remove them until you have switched them off, in order to avoid feedback whistle. This will be more comfortable for the child.

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Troubleshooting

The hearing aid does not whistle when you hold it in your hand:

Check that it is switched on.
Check battery function.
Check whether the earmould, sound tube or ear hook is blocked.
The hearing aid whistles when the child is wearing it:

If the device has a volume control, check it is set properly.
Check the fit of the earmould. The mould may not be fitting properly within the ear canal or there may be hairs between the earmould and the ear canal.
Check whether the sound tube fits properly or has a hairline crack.
Check to see if the housing is broken.
The earmould may be too small as the child’s ear has grown. Try applying a little lubricant (vaseline or baby oil); this will, however, only help temporarily. A new earmould must be made as soon as possible.
The child refuses to wear the hearing aid or does not respond as normal when spoken to:

Check the power supply.
Check to make sure the left- and right-ear hearing aids have not got mixed up.
Check the switch settings.
Review questions

Why do the tubes and earmoulds need to be checked on a regular basis?
To prevent feedback or blockage
To ensure they are always clean
To prevent blockage
To prevent feedback
Why is it important not to mix up the left- and right-ear hearing aids?
Because they will then consistently be too loud
Because they will then continually whistle
Because they will then consistently be too quiet
Because the child will not hear as well, as one device will be too loud and the other too quiet
3. What power source is used for hearing aids?

Mercury batteries
Rechargeable batteries
Zinc-air batteries
Either zinc-air or mercury batteries

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Chapter III – Specific aspects of cochlear implantation in infancy

Learning goals

Even if a hearing-impaired child is diagnosed and fitted with suitable hearing aids at an early stage, this is not always enough to enable them to develop language successfully. In some children, the function of the sensory hair cells in the cochlea is so badly impaired that the child cannot adequately understand speech even with the aid of very powerful hearing aids; these individuals can be fitted with a cochlear implant. The following chapter describes how these implants work and the difference between unilateral, bilateral and bimodal cochlear implantation.

How a cochlear implant works

The cochlear implant (also referred to as a CI) converts sound waves into electrical impulses and thus functionally replaces the eardrum, ossicular chain and sensory hair cells. The CI basically consists of two parts: the implantable portion, and the speech processor that is visible from outside. The implanted part (i.e. the receiver coil with electrode) is surgically inserted with the recipient under general anaesthetic. The implant, which is inserted following an incision in the skin behind the ear, is positioned and secured in place beneath the skin on the bone of the skull. The implant electrode is inserted into the cochlea; in the process, allowance is made for changes that may occur during growth. The skin incision behind the ear is closed, usually leaving a small scar. The outer part of the CI consists of the speech processor with the microphone, a battery pack, the transmitter coil and a lead. The speech processor is worn behind the ear rather like a hearing aid. Depending on the make of device and the recipient’s age, different versions are available. With the aid of a magnet, the outer transmitter coil is held in place on the skin behind the ear, exactly above the implanted receiver coil. The signals picked up via the microphone are passed to the speech processor where they are converted by a computer chip such that they can, in the form of electrical impulses, be transmitted wirelessly through the skin from the transmitter coil to the receiver coil. From the receiver coil, these electrical signals are passed to the electrode in the cochlea, from where they reach the auditory nerve. This electrical stimulation of the auditory nerve generates a hearing sensation in the brain.

The outer part of the CI must be removed before showering or swimming. If a technical fault occurs, the speech processor can be replaced at any time. If the implant malfunctions, however, it can be replaced only in a surgical procedure. The implant’s power supply is provided externally via the speech processor.

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Indication

A patient becomes a candidate for a cochlear implant if the function of the sensory hair cells in the cochlea is so badly impaired that speech cannot be understood, or language learned, even with the assistance of very powerful hearing aids. Once diagnosed with hearing loss, infants and toddlers tend to initially undergo a trial phase using hearing aids for several months. If this still does not sufficiently benefit their hearing, so that they are also falling behind in their language development, a CI can (once the diagnosis has been completed) be used. The only exception is where hearing loss was caused by meningitis; in these cases prompt action is required as, in some individuals, the cochlear may be obliterated – which can, in subsequent implant surgery, make it impossible to place the electrode in the cochlea. The prerequisites for implantation are that the cochlea is structurally normal and the auditory nerve functionally intact. This means that, prior to implantation, full diagnostic procedures must be performed, involving measurements at the ear. Imaging techniques such as CT and MRI scanning are also required. As the child needs to lie completely still for these examinations, it is usually necessary to give them a sedative. Not until all of these tests have been carried out, there has been consultation with the paediatric audiology centre providing early intervention (and, if necessary, with the nursery school), and the parents have been fully advised and informed, should a joint decision be taken about CI surgery. When weighing up the pros and cons, the overall development of the hearing-impaired child and their family situation should always be considered. The decision-making process and subsequent rehabilitation should therefore be regarded as a multidisciplinary undertaking and requires close cooperation by all the different specialist professionals involved.

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Unilateral / bilateral implantation

Thanks to newborn hearing screening and the excellent scope for differential diagnostics, today children can be bilaterally fitted with hearing aids within the first year of life. If language development does not progress as hoped for despite early fitting with hearing aids, the child will generally receive a cochlear implant. As a rule, the child is initially fitted with only one implant (i.e. implantation is unilateral) – whereas hearing aids are almost always fitted bilaterally (i.e. on both sides). There are various reasons for this. Parents often initially opt for unilateral cochlear implantation as they want to wait and see whether the procedure is successful and, with appropriate intervention, whether implantation proves beneficial in terms of language development. Many parents may wish to retain any residual hearing in the other ear so that future technological advances can be taken advantage of as and when they occur. Often, however, it is only our capacity to hear with both our ears (binaural hearing) that enables us to cope with the diverse hearing situations we face in everyday life. This is because directional hearing, spatial hearing and signal-source separation are possible only when both ears act in combination. For people with two fully functional ears, rapidly finding one’s bearings in everyday life is a straightforward matter, as is listening for specific noises or speech; even under difficult acoustic conditions – as, for example, in echoey spaces with high reverberation levels, or where there is a babble of voices in the background, or when sounds and speech are very quiet – these people still take in the auditory information they need. In order to hear directionally, individuals with normal hearing exploit interaural differences in terms of time and sound level, as well as diffraction phenomena and the way the sound reflects back from the auricle. This means that a sound signal is received differently by the left and right ears (owing to their being laterally arranged on the head), with the slight differences between the two ears being analysed in the central auditory processing system (consisting of the auditory nerve and auditory pathway, and the auditory centre in the brain). On the basis of this subtle and complex comparison of differences between the way the two ears register the sound, it is possible to locate sound sources and to separate speech from background noise. Children with only a unilateral fitting (i.e. with a single CI or hearing aid) are unable to perceive these differences and so cannot exploit them to achieve directional hearing and speech understanding in a noise-filled environment such as at school or nursery. For children with hearing loss, the lack of this capability considerably affects their ability to concentrate – and hence their performance and achievement potential – as well as, for example, their safety as road users. A further argument for bilateral cochlear implantation is that it enables hearing ability in one ear to be preserved in order to cover the eventuality that function is temporarily lost on the other side (as when the speech processor is being repaired, for example).

Bilateral cochlear implantation can involve the two implants being received either at the same time or sequentially. The relative merits of these two alternatives are the subject of some controversy among professionals. In general, it has emerged that the earlier a patient is fitted with an assistive hearing device, the more effectively this sensitive phase – when central processes are maturing – is exploited. This also applies, of course, to bilateral cochlear implantation. The greater the time interval between successive implantations, the more intensive the intervention that is required. Research has shown that children who receive implants sequentially are also able to learn to hear binaurally. Sequential implantation does not, therefore, based on current experience, appear to be necessarily disadvantageous. A prognosis as to how well the hearing-impaired child will be able to hear and understand at a later stage is not possible at the time of implantation. The CI settings must be determined separately for each ear. On no account can the speech processor settings for one side be adopted unchanged for the other side.

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Bimodal implantation

Children who have been unilaterally implanted and still have sufficient residual hearing in the other ear can continue wearing a hearing aid in the non-implanted ear. The previous section described in detail the benefits of binaural over monaural hearing. Bimodal fitting (which involves the recipient using a cochlear implant in one ear and a hearing aid in the other at the same time) thus also makes binaural hearing possible. Despite obtaining different hearing sensations from the CI and hearing aid, children may still benefit from being fitted in this way. The residual hearing of the non-implanted ear can continue to be stimulated so that, if a decision has been made to implant the second ear later, the hearing ability of this second ear can be more effectively activated and exploited by the implant.

Where children have bimodal-binaural fittings, the process of determining the devices must be accorded the same importance for the hearing aid as for the CI. Both devices must be mutually adjusted to each other as well as possible, in order that the child’s hearing or understanding is not poorer with two devices than with only one. This, of course, proves more difficult in infants and toddlers than in adults. The benefit gained from each device must first be gauged individually and then using both devices in combination, making use of the standard test procedures for the age concerned (see Chapter I). This means that, although children of course have shorter attention spans and lesser powers of concentration than adults, they must actually undergo testing procedures that can take three times as long overall as for adults – which calls for a great deal of experience on the part of the tester.

The same applies, of course, to bilateral cochlear implantation. However, this solution is in many cases indisputably superior to bimodal fitting.

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Review questions

What does bimodal fitting involve?

Having a cochlear implant in both ears
Having a cochlear implant in one ear only
Having a cochlear implant in one ear and a hearing aid in the
other at the same time

Having hearing aids in both ears
What are the components of the implanted part of a cochlear implant system?

Receiver coil with electrode
Receiver coil and speech processor
Speech processor and microphone
What are the external components of a cochlear implant?

Receiver coil
Speech processor with microphone, battery pack, transmitter coil and lead
Receiver coil with electrode
Transmitter coil with microphone, battery pack and lead
What is the difference between bimodal and bilateral cochlear implant fitting?

Bimodal fitting involves the child wearing a hearing aid on one side and a cochlear implant on the other, whereas bilateral fitting involves the child wearing two cochlear implants
Bilateral fitting involves the child wearing a hearing aid on one side and a cochlear implant on the other, whereas bimodal fitting involves the child wearing two cochlear implants
Bimodal fitting involves the child wearing only one cochlear implant, whereas bilateral fitting involves the child wearing two cochlear implants

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Chapter IV – Use and handling of a cochlear implant

Learning goals

Chapter III will have familiarized you with distinctive aspects of cochlear implantation in infants and toddlers. Here in Chapter IV we will look at everyday considerations (as Chapter II did for hearing aids, only this time for cochlear implants): how you yourself can check that a CI system is in proper working order, how to put on a child’s speech processor, and what you can do if the device’s LED flashes or the child suddenly no longer responds when spoken to.

Checking that the cochlear implant system is working properly
Just like a hearing aid, the cochlear implant system requires a power supply. Unlike a hearing aid, however, standard zinc-air batteries should not be used; instead, use batteries specifically recommended for the system by the manufacturer. The battery life specified varies according to make and depending on how many hours a day the speech processor is used; it tends, however, to be around three to five days.

Here, too, as with hearing aids, it is essential to make sure that the child does not accidentally swallow a battery.

An earmould can be used to hold the speech processor more securely in place at the child’s ear, just as with a hearing aid. The earmould should, for reasons of hygiene, be cleaned just as regularly as that used with a hearing aid. Unlike a hearing aid, however, feedback whistle cannot occur with a CI. And here, if the earmould or tube is blocked with earwax or condensation, this has no impact on hearing ability.

If a child is to be fitted bilaterally with CIs, it must be ensured that the left- and right-ear implants are not reversed by mistake as these implants are always specially programmed for the respective ear. CI systems are available that have automatic recognition and thus do not emit a stimulation pulse if the left-ear and right-ear devices are mixed up. As not all systems have this feature, however, the parents should (as with a hearing aid) make sure the devices are marked appropriately. A CI speech processor must be protected from humidity and soiling; sharp knocks and impact should also be avoided. As electrostatic charge can affect implant function, the speech processor should be checked after the child has, for example, been playing in the ‘children’s ballroom’ at IKEA, on plastic slides, or in a plastic crawling tunnel – or even removed beforehand as a precaution.

Checking that speech processors are in proper working order involves different procedures for the various CI systems. Please follow the manufacturer’s recommendations and, if necessary, have the parents explain the switch settings to you.

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Troubleshooting

First check the power supply. Make sure the batteries used are working properly or, if they are rechargeable, that they are kept fully charged. Further steps involved in checking system function can vary widely according to make. Specific information on any given make of cochlear implant system can be found on information sheets that can be requested from the manufacturer in question (if the parents themselves do not already have them).

The child does not respond when spoken to:

Check the power supply.
Check the switch settings.
Check the lead connections.
If the child has a bilateral fitting, make sure the left-ear and right-ear devices haven’t been switched by mistake.
The coil keeps falling off:

The parents should be told about this.
It may be possible to use a stronger magnet (following consultation with the implanting centre).
The speech processor’s light diode (LED) is flashing:

Check the power supply.
The flashing patterns and their meaning vary widely according to make, so it is essential to consult the manufacturer’s information sheet and/or to ask the parents!
Review questions

1. How can the function of the speech processor be checked?

By using a test tool specially designed for cochlear implant systems
By checking the power supply
By checking the lead connections
By checking the power supply and lead connections and following the manufacturer’s guidance notes
What kind of power supply is used in cochlear implant systems?
Zinc-air batteries suitable for CI systems
Batteries suitable for CI systems and recommended by the manufacturer
All zinc-air batteries are suitable
Mercury batteries suitable for CI systems
What is the purpose of the earmould in a cochlear implant system?
To prevent feedback during signal transmission
To enhance signal transmission
To hold the speech processor more securely in place at the ear

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Chapter V – FM systems

Learning goals

For a child with hearing loss, the path towards good hearing and language development begins with timely diagnosis, early fitting with hearing aids and/or cochlear implants, and early intervention to help foster both hearing and speech. And yet even when these devices are correctly adjusted to suit a child’s individual needs, they are unable to ensure good hearing or good understanding in all situations. It is here that the use of wireless radio systems (also known as FM systems) can be helpful. The following chapter describes what is meant by FM systems, the different types available, when and in which situations they should be employed, and how they can be used.

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What is an FM system?

An FM system is a wireless transmission system that works on the basis of frequency modulation. These systems consist of a microphone with a transmitter worn by the teacher and receivers worn by the child. These receivers are either coupled to the child’s hearing aids or cochlear implants, or integrated within them. The FM transmitter amplifies and modulates the acoustic signal and transmits it to the FM receiver by means of radio waves. The FM receiver demodulates the signal and transmits it to the hearing aid or CI. Speech signals are thus picked up directly at the source and conveyed, clearly and without any interference, directly into the child’s ear. FM systems are available from various manufacturers, just as hearing aids or CIs are.

There are different types of transmitters and receivers. Some transmitters have an external microphone. The receivers can easily be connected to a hearing aid. Depending on which type of hearing aid the child uses, the receiver is either integrated into the hearing aid or a device called an audio shoe is plugged into the hearing aid which is, in turn, coupled to the receiver. Alternatively, in a small number of hearing aids, the battery compartment needs to be opened in order for the audio shoe to be plugged in.

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Indication

An FM system provides a valuable basis for improved speech understanding irrespective of the acoustic environment. In certain situations – at playgroup, day nursery or nursery school, for example – a child’s ability to understand speech can be greatly affected by background noise or the acoustics of a space. If many children are all playing or speaking at once in a room, it may sometimes be impossible for a child with hearing loss to filter out the background noise. Both distance and reverberation also reduce speech understanding. The best way to overcome these difficult situations is to use an FM system. This equipment should not, however, be used on a permanent basis. It is worthwhile using during activities such as ‘circletime’, when a child with hearing loss is not sitting right next to the teacher, and for other group events where only one person is speaking at a time. It is not, however, necessary to use it when the child and the speaker are only a short distance apart or in quiet environments. There is some controversy surrounding the minimum age at which an FM system should be used. It is, however, always advisable that both the child and their family first become familiar with the hearing aid and/or CI. An interdisciplinary approach is also called for here. Specific early intervention for the hearing impaired (also referred to in Germany by a term that translates as ‘paediatric audiology-based early intervention’) will involve the relevant staff regularly seeing the child and their caregivers in the home environment. This is why it is recommended that an FM system be used only after consultation with the early-years practitioner.

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Checking that an FM system is working properly

Unlike the receiver, the FM system’s transmitter requires its own (either standard or rechargeable) battery. The receiver gets its power from the battery in either the hearing aid or the speech processor of the cochlear implant. Before using the device for the first time, make sure the batteries have sufficient capacity (if non-rechargeable) or are charged (if rechargeable). Before each use, the correct switch position at the receiver (i.e. the child’s hearing aid) and at the transmitter must be checked. If, for example, the receivers are switched on but the transmitter is not, then only a hissing noise will be transmitted. The child will then, far from being in an acoustically enhanced situation, actually be acoustically worse off. It is important that the transmitter microphone is positioned about 20 cm in front of the teacher’s mouth. It must be ensured that no rubbing noises (as caused, for example, by clothing or jewellery) are transmitted. Have the parents or the paediatric audiological/early-intervention professional explain how to correctly use and handle these devices. Let the parents or practitioner involved know straight away if anything is not right with the hearing aid, CI or FM system.

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Troubleshooting

The power supply should always be checked before using an FM system. So make sure you check the transmitter’s charge status and recharge it on a regular basis. There should always be enough charge for, say, an entire morning’s session at school or nursery.

Functional problems with the hearing aid and FM system:

  • Check the power supply.
  • Check all settings on both the transmitter and receivers.
  • Check the adapter connections.
  • Check to make sure there is no interference from other electronic equipment.
  • Functional problems with the cochlear implant and FM system:
  • Check the power supply.
  • Check all settings on the transmitter and receivers.
  • Check the adapter connections.
  • Check to make sure there is no interference from other electronic equipment.

While checking function, it is recommended that the child not be in the same room when signal transmission is being verified. In other words: ask the child out to leave the room, have them stand just in front of the door and briefly try out the system. If the child can hear and understand you even when they are outside the door, then the system is working fine. If more than one child in your group has their own FM system, there is a risk of frequency overlap. If this is the case, then what is known as a ‘frequency plan’ should be drawn up to ensure that each child uses a different frequency. (You will also need to consult the manufacturer’s instructions in this regard). If the child complains of a crackling or rasping sound, this may indicate a cable break affecting the microphone of the transmitter.

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Review questions

When is it a good idea to use an FM system?
In ‘quiet’ play situations
In ‘quiet’ therapy situations
During ‘circletime’ and in group situations where only one person is talking
From what age should an FM system be used?
An FM system should be provided at the same time as the cochlear implant or hearing aid
When the child starts primary school
When the child starts nursery school
Following consultation with the provider of early intervention for the hearing impaired
If a child complains of a crackling or rasping noise in the system, what might be the reason for this?
A cable break affecting the transmitter microphone
The hearing aid or cochlear implant being switched to the wrong position
A flat battery

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Figures

Figure 1: The ear canal in adults and children compared

Figure 2: Earmould for a two-month-old baby

Figure 3: ‘Mueller claw’

Figure 4: ‘Mueller claw’ in a child’s ear

Figure 5: Measuring RECD in a three-month-old child with Down’s Syndrome

Figure 6: SPLogram

Figure 7: SPLogram with hearing-aid measurement

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