Aids to hearing

From the previous chapters, it has become clear that most causes of deafness are sensori­neural (involving the inner ear, auditory pathway and brain) in origin and only relatively few are conductive. At present there is no specific cure or indeed any medical treatment for the vast majority of sensorineural losses.

It is the conductive losses that can often be treated with surgery to repair and reconstruct damaged or diseased parts of the outer and middle ears. Some of these conditions and their treatments have been described in preceding chapters. Ironically, it is the conductive losses that are also most easily managed by hearing aids because the inner ear is usually intact. All the hearing aid has to do is increase the volume of sound to overcome the conductive ‘block’. In sensorineural losses, the loss of hair cells and auditory nerve fibres reduces the quality of the inform­ation going to the brain, so that, however good the aid, it is limited by the quality of the information that can be passed on to the brain.

Aids to hearing come in a wide variety of forms and this chapter tries to outline what is available. Hearing therapists within the NHS are trained to give appropriate advice and provide help wherever possible and within the constraints of a rather small budget. A special service called LINK is provided for those with severe problems, but people must be referred to it by an NHS consultant. LINK has been of enormous assistance to many, but its funding is not assured, although in the author’s personal view, and with respect to the many grateful patients, it should be.

SIGN LANGUAGE

For those born deaf, especially into families where the parents are deaf, ‘signing’ is a truly marvellous form of communication and there are virtually no barriers to what can be achieved. The major obstacle to its use is that hearing people tend to treat this part of the deaf com­munity as if they have a disease – which of course they do not. The deaf community generally distrust doctors and the like who try to ‘cure’ them of their hearing loss when they, the deaf community, rightly, do not accept that hearing loss, in itself, is a handicap. It is society in general that is intolerant and society should be more accepting.

The other barrier is that there is not yet an internationally accepted sign language, which is a problem. There is a fairly uniform British sign language that takes time to learn but, with effort, fluency can be achieved, and the more ‘hearers’ who can learn sign language, the quicker the communication barriers between people with and without hearing can be broken down. The information in Useful addresses includes details of how to find out more about signing.

LIP-READING

For people who acquire deafness, even quite profound deafness, lip­reading skills may be the only means they have available to overcome their difficulties in face­to-face communication. Under­standing the spoken language is a very high-level skill. It requires the ears to hear the various major elements of speech, the eyes to read the lips and assess facial expression and body posture, and the brain to analyse the context of what is being said and to make sense of all the incoming inform­ation in the light of experience. The ability to hear speech is only one part, albeit a large part, of com­munication.

We must all have played the game of Chinese whispers where a secret message is whispered from one person to the next. Being a whisper, this will often be misheard; you cannot lip-read and you don’t know the context. Normally, you would ask for the words to be repeated if you could not quite make sense of what was being said. However, the rules of the game insist that you make a decision as to what you think you heard and the message that comes out at the end of the chain is usually quite wrong and often very amusing.
The most common reason for poor communication is reduced hearing and enhancing the lip­reading abilities of those who have developed deafness may be very useful. There are definite strategies than can enable you to become a better lip-reader and most local education authorities run classes to teach them.

ENVIRONMENTAL AIDS

As electronic technology becomes increasingly sophisticated, a wide range of devices is being devised to help individuals cope with day-to-day life. In the early days, the options were simple: to increase the volume or to alter the tone of the sound in question so that it stood out from the background sounds. The doorbell could be made louder, extension bells might be placed around the house or the tone of the bell could be made low pitched – as most forms of hearing loss have relatively well-preserved low tones.

The same modifications can be made to baby alarms and fire alarms, and all can have flashing lights incorporated somewhere sensible. For all alarm systems, even for alarm clocks, vibrators buried within pillows for use at night can be incorporated into the system to make it almost foolproof. Tele­phones can be amplified and televisions can have suitable personal amplifiers either directly wired or radio-transmitted to the individual listener.

Most hearing aids have facilities for detecting the electromagnetic waves that are produced from loudspeakers or from ‘loop’ systems. In these, a wire is run around the outside of the room or hall and attached to an amplifier, which is in turn connected to an appropriately placed microphone or to the sound output of a television, radio or other device. The hearing aid has a switch (usually marked ‘T’) and, when this is on, the aid detects the signal from the loop without the background noise. This can be an absolute boon in meetings, at theatres and in church, although there are occasional interesting moments when the vicar inadvert­ently forgets to turn off his microphone and those using the loop system can hear his comments although the rest of the congre­gation cannot.

Until recently, it was not possible to provide a text or signed translation of what was being said on the telephone, television or radio, but there has been much progress in this area of late. There is Teletext on the television and Minicom telephones. Some tele­vision programmes can be broad­cast with split screens, with a signer providing simultaneous translation. With the advent of digital radio, it should be possible to add a visual text bar to the set much as Teletext is added to the television.

All NHS hearing aid centres should be able to provide inform­ation about how to obtain and install the relevant environmental aids and the RNID also offers comprehensive guidance.

CONVENTIONAL HEARING AIDS

Conventional hearing aids are available either through the NHS or privately. At present the provision of an NHS aid generally requires an initial referral by the general practitioner to the local ENT department, where a doctor first excludes treatable diseases that could be causing the hearing loss. The individual is then transferred to the hearing aid centre. In some regions, direct referral to the hearing aid centre attached to the ENT unit is possible. Here the audiologists and hearing therapists undertake the screening and, if they find problems, send the individual on to the ENT surgeon.
Hearing aids can be bought privately and are generally quite expensive, although the aids are now much more robust than they used to be and should last for eight to ten years. They can be provided only by people who have passed the Hearing Aid Council examin­ations so that quality control should be in place. There should be a trial period of four to six weeks, after which the aid can be returned with only administrative costs. If this option is not offered then go elsewhere.

Acoustic aids

A conventional hearing aid consists of a microphone, an amplifier requiring a source of power and a loudspeaker. Since the introduction, in the 1950s, of the rather large and cumbersome Medical Research Council aids for the NHS, there have been major developments in battery technology and electronics, which have allowed the aids to become smaller and more effective.

At present mild-to-moderate hearing losses can be helped with small aids that sit within the ear canal and in the shell-like portion of the pinna – the concha. There are smaller aids called IC (in-the-canal) aids, which are more or less hidden within the ear canal, but there is a limit to their power at present because of their size. Aids that are totally within the canal are being developed.

Very severe losses need more power and in general need a larger aid than can be fitted in the canal and concha. This type of aid (called a BE aid) therefore sits behind the ear and a small tube carries the sound into a mould or shell that fits into the ear canal. A very few people will need an aid with still more power than a BE aid can produce and this can be provided with a body-worn (BW) aid, which can be carried, for example, in a top pocket. The aid is connected by a wire to the ear mould, which houses the small loudspeaker.

The processing of the incoming sound by the amplifier has also undergone enormous changes. Many hearing losses such as those that arise from ageing or noise damage are restricted to the higher frequencies. This sort of loss with relatively normal mid- and low-tone hearing would be difficult to overcome if the amplifier simply magnified all sound frequencies. The result would be that the low tones, which are often general environmental noises such as traffic, air conditioning, etc., would become unbearable.

One way out of this is to make a ventilation hole in the ear mould itself, which reduces the volume of the low tones at the eardrum.
However, this can introduce ‘feedback’ which is a high-pitched whine as amplified sound from the loudspeaker escapes from the ear canal via the vent, is detected by the microphone of the hearing aid and amplified, and again escapes only to enter an endless loop. This ‘feedback’ sound also occurs when hearing aid moulds do not fit properly and is often a source of severe irritation to the family and friends of the person wearing the hearing aid. This can easily be remedied by having a well-fitting mould made.

An alternative is to use elect­ronics so that the high frequencies are let through and amplified by high-pass filters (filters that let through the high frequencies). Recently, the introduction of digital signal processing – the basis of CD technology – has made it possible to produce digital hearing aids. In digital signal processing, signals are broken down into very short seg­ments, each being given a numerical value that can then be processed. Many people who have moved to digital aids say that, although these aids are not perfect, they are a lot better than the earlier analogue aids. There are no scientific studies to confirm this yet. At present digital aids are more expensive than conventional, analogue aids.

An amplifier may also make sounds too loud for comfort. This is especially a problem for people with a cochlear type of sensori­neural hearing loss in which recruitment occurs. For these people there is only a small volume change between the level of sound that they can just hear and that which is uncomfortable. Before a hearing aid can be fitted, the loudness discomfort levels at each pitch should be measured and the output of the aid adjusted accordingly. This is possible with conventional aids but much easier for a digital aid, which can usually be pre-set or programmed with the individual user’s audiogram and discomfort levels to ensure comfortable listening levels.

Amplifiers are now available that reproduce the incoming signal perfectly. They can be tuned or programmed to give an output that matches the pattern of the audio-gram, so that only the involved frequencies are made louder. Yet many people who wear hearing aids still complain that they cannot hear speech in background noise or that picking out one voice in a crowd is impossible. One part of the problem is that the performance of all hearing aids falls away in the frequencies above 4 kHz, which are those needed for performing difficult auditory tasks such as hearing individual voices in background noise. However, even if perfect aids could be made, a greater problem still exists: as the outer and then the inner hair cells are lost, there is an eventual loss of auditory nerve fibres. When this happens, there is simply not enough information reaching the person’s brain to enable them to hear speech in difficult background noise. The hearing is still good enough to hear warning sounds, but it cannot provide the very high-level information that is needed for understanding speech.

Many people are disappointed when they first wear a hearing aid because the sounds are ‘tinny’ and not what they expect. They want an aid to restore perfect hearing in the same way that a good pair of glasses restores near-perfect vision for people with many types of visual loss. However, hearing is a complex skill and, despite the fact that an aid suddenly restores sounds that have not been heard for many years, it still takes time for the brain to acclimatise. Once it has done so, the sounds become clearer. It is generally accepted that this acclimatisation can take up to three months; when you first start to use a hearing aid, you should increase the length of time you wear it a little each day until, eventually, you are wearing it for most of the day. Once you have acclimatised then you can wear the aid when needed.

Bone-conducting aids

Sound energy can be transferred by means of a vibrator to the inner ear by direct passage through the skull. This is called bone conduction and can form the basis of a very useful form of hearing aid, although it relies on having good inner-ear function on at least one side. There is, however, a limit to the usefulness of bone-conducting aids because the power output and energy transfer of the vibrator are not as great as with a conventional acoustic aid. A bone-conducting aid will not help someone with significant sensorineural hearing loss, but can be extremely useful for people with a significant conductive deafness in both ears. This might include those who have a discharging wet ear, perhaps as a result of a perforated eardrum, or some other problem such as a sensitivity to the materials of a conventional hearing aid mould or inflammation of the skin of the ear canal caused by eczema.

The simplest version of the bone conductor comprises a sprung metallic band that goes across the head with a small microphone, amplifier and vibrator contained in a unit at one end of the band, which presses against the mastoid bone. Some degree of pressure is needed to achieve good sound conduction and some people find this uncom­fortable at first. Such an arrangement is a good temporary measure after middle-ear and mastoid surgery, when there is a conductive loss in both ears. Some hearing has to be provided as the operated ear is usually full of antiseptic packing material and there is still the conductive loss on the unoperated side. (It is usual only to perform major surgery on one ear at a time.)

Wearing a hair band all the time may just be acceptable for a woman, but for a man who cannot conceal it under his hair, this type of aid is not usually a reasonable solution. Fortunately, the bone-conducting aids can be fitted into the frames of spectacles with the curved portion that goes behind the ear incorporating the electronics and the battery, making the aid relatively invisible. The microphones can be hidden in the surrounds of the lenses.

Bone-anchored hearing aids

The bone-conducting aids described above work well but, if it is likely that they will be needed on a long-term basis, an alternative is a bone-anchored aid. A titanium screw is fitted into a carefully threaded hole that has been drilled in the skull behind the ear, usually at the hairline. The skin surrounding the screw is thinned right down or else removed and grafted with a very thin skin graft so that there is not a big bulge of skin around the device. The titanium is then allowed to integrate with the bone over a six-week period, after which the vibrator can be attached to the screw with a specially designed intermediary piece.

Before such an aid can be fitted, it is necessary to ascertain that the person has good inner-ear function. This means that trials with conven­tional bone-conduction devices must have shown good results so that a successful outcome from surgery can be predicted. The skin around the titanium screw needs continued care, because there is a ‘foreign body’ in the scalp and this has to be kept clean to avoid infection. The aid may be rather conspicuous if the wearer has short hair.

Despite all these constraints, bone-anchored aids have provided a remarkable improvement in the quality of life for many people because they have both restored hearing and often allowed wet, unpleasant ears to dry out.
Bone-anchored hearing aids are available through the NHS, although not all ENT units are able to provide the service because a skilled team is necessary for the assessment, surgery and follow-up.

IMPLANTABLE AIDS

Recently, there has been a fashion for designing very small vibrators, which can be fixed by special glues or by fine wire clips on to the malleus or anvil in the middle ear. The vibrators are small crystals that change shape when electricity is applied to them. (This so-called piezo-electric effect is used in reverse in some gas lighters where pressure on the crystal generates a spark.) In the ear the electricity to vibrate the crystal comes from the amplifier of a hearing aid, so that the crystal vibrates ‘in tune’ with the sound; this vibration is passed on to the inner ear by the ossicles. Some research groups feel that this would give better sound quality than is possible by conventional aids.

There still has to be an external microphone and the resulting electrical signal still has to be transferred across the skin to the underlying amplifier and electrical output device. The long-term effects of having a vibrating weight glued or clipped to the ossicles has not been evaluated and, once again, there has to be relatively good inner-ear function with losses no greater than 50 or 60 dB and a normal middle ear. A recent development has been to have the microphone implanted into the walls of the ear canal so that the whole device is buried. The batteries, which are buried under the skin in either of these two types of device, are rechargeable using a radiofrequency energy transmitter. The applications of technology are amazing. Although surgeons are very keen on these devices because they test their surgical skills, it is not at all clear that these aids offer any hearing benefit over a modern digital acoustic aid and certainly there is a surgical risk. At present implantable aids are experimental and are not generally available

CLEVER TRICKS

People who have complete hearing loss in one ear can experience a major problem in terms of hearing in groups, especially at meetings or around a meal table where speakers can be sitting on the individual’s deaf side. The devices that help this are CROS aids. This stands for contralateral routing of signal and means that the sounds from the deaf side are picked up by a microphone and the signal is taken round to the better hearing ear, amplified if necessary and then presented to that ear.

This can be achieved by having the transfer wires hidden in spectacle frames, by thin wires running in the hair behind the head or even by radio-transfer from small receivers/ transmitters in the deaf ear, with a receiver/loudspeaker in the good ear. With time, the brain starts to recognise the difference between the direct input and the re-routed signal, so that the ability to localise sound can also return. Many people find that this has revolutionised their lives as far as meetings and group occasions are concerned. Some of these CROS systems are available through the NHS although the patient might have to be very persistent to gain access to the service. In some parts of the country, however, there is not the funding available locally to provide the service.

ELECTRONIC AIDS: COCHLEAR IMPLANTS

When the inner ear is severely damaged, conventional aids cannot help because there are simply not enough hair cells remaining to generate sufficient signal to make sense of language. In the past, individuals in this situation were consigned to a life of near silence, relying on lip-reading or signing for communication. For those who acquired a profound loss in adult life, the transition to a silent world was usually extremely depressing and some became suicidal.

For children born profoundly deaf, their silent world was not such a problem, but even with the best will in the world those children rarely, if ever, acquired speech that could be understood by anyone but their parents.

Each year in the UK, out of the 800,000 births, there are about 150 children born profoundly deaf and about another 80 who become so after having meningitis. Fortunately, this second figure should decline as more vaccines are developed against bacterial meningitis and parents become more aware of the dangers.

Hair cells are needed to convert sound to electrical impulses and to start the process of hearing. It is not yet possible to replace missing or dead cochlear hair cells, but if there are still acoustic nerve fibres left these can be used to carry any electrical impulses that can be delivered to them. This job can be carried out by electronic rather than acoustic aids.

The system still comprises a microphone and amplifier linked to a signal processor, but instead of a sound output it produces an electrical output that is related to the pattern (waveform) of the original sound. Early attempts to do this involved the use of single electrodes placed on the round window membrane. This showed that the idea could work and subsequent developments resulted in multiple electrodes embedded in a fine flexible strip which could be threaded into the cochlea by way of a small drill hole (cochleostomy) made near the round window.
This electrode coils around the central core of the cochlea and lies very close to the remaining acoustic nerve fibres. A signal processor with a magnet in it is buried in a recess drilled in the skull under the scalp behind and above the external ear. A radiofrequency transmitter with a magnet in it is on the outside and is held to the skin overlying the processor by magnetic attraction. A microphone collects sound and converts it to an electrical signal, which is then transferred across the skin along with a power supply by the transmitter to a receiver in the signal processor. This then generates the electrical impulse that travels to the electrode buried in the cochlea.

There are many different makes of electrode and several different speech-processing strategies in use today. These are all trying to get the best ‘hearing’ response for the individual user. For those in whom the device works well, the effects are miraculous and some patients can even use a conventional telephone.

Adults can have such a device implanted at any time, but babies born deaf and those becoming deaf before they can talk (prelingually deafened) should be implanted as soon as possible and certainly before the age of three or four years. If this is not done their speech will never develop. If implantation is delayed past the age of four, the results are very disappointing because the auditory pathways in the brain fail to develop in the absence of stimu­lation and, after a while, shut down, so that the child never acquires speech of any quality.
This makes it very important to screen all babies using OAE (otoacoustic emission testing – see page 36). Any babies with profound losses can be detected early, and the difficult decisions about whether to implant can be made without undue hurry. Unfortunately, this screening is not yet done in the UK because of the high cost implications of detecting deafness.

Children who are born hearing and become deaf (usually through meningitis) after they have learnt to talk can also be helped by a cochlear implant. The sooner it can be performed the less time is lost from education and general social development.

Cochlear implants and bone-anchored hearing aids are provided by the NHS, provided local funding can be obtained. The aids them­selves are expensive, for example, the cochlear implant device itself costs about £12,000 and requires a skilled team to ensure that the best implant type is chosen, fitted and tuned. Most health authorities will support the cost of cochlear implants in children and babies because of the adverse publicity and public outcry if they refuse funding. This is not the same for adults and several health authorities refuse to fund implants for those profoundly deaf people who would be likely to benefit!