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The waiting game

Your questions answered on hearing aid choices

Professor Tony Wright explains the recent advances in hearing aids and how suitability is determined

All hearing aids consist of a microphone, sending a signal to an amplifier and some form of processor, depending on the type of aid, delivering the output to the patient. The type of output differs between the three classes of aid: acoustic, bone conducting and electronic.

·With acoustic aids (the most common) the output is amplified airborne sound, which enters the ear along the ear canal and is heard through the usual pathways.

·With bone-conducting aids, mechanical vibrations are transmitted through the skull to enter the cochlea and thereby stimulate the auditory sensory cells ­ the hair cells.

·Electronic aids have a series of electrodes that are fed directly into the coils of the cochlea and which stimulate the remaining fibres of the acoustic nerve in the absence of any hair cells. In the absence of an acoustic nerve an array of electrodes can be placed on the auditory areas of the brainstem.

Who needs what sort of aid?

Hearing loss is the second commonest disability in the UK. Most people with a significant problem can be helped with a conventional acoustic aid, whether the problem lies in the middle ear (conductive loss) or in the inner ear and acoustic nerve (sensori-neural loss). If there are changes in both the middle and inner ear the loss is 'mixed'.

With some conductive losses that cannot be helped by a conventional aid, either because the ear is constantly discharging or there is no canal (as happens in some congenital conditions), bone conduction is the answer. Bone-conducting aids can be attached to glasses or headbands, both of which press against the skull.

A new development is the use of a titanium screw, which is threaded into the skull behind the ear and clipped to a detachable vibrator so that sounds are directly transmitted to the inner ear. This is a bone-anchored hearing aid (BAHA) and is very effective for some people with conductive losses.

With a severe profound bilateral, sensorineural loss, which might be caused by meningitis or be congenital in origin, an electronic aid in the form of a cochlear implant will often restore some form of hearing.

Types of acoustic aid

Not so long ago conventional aids were bulky devices that had to sit in a pocket with a wire running up to an ear mould. With the rapid advances in technology, especially the miniaturisation of the electronic components and reduction in the size of batteries, the aids could be shrunk to a more cosmetically acceptable size. The microphone amplifier, processor, battery and loudspeaker can now all be fitted into a capsule that is completely in the canal (CIC) and is barely visible.

But small size limits the power and the gain of the aid may not be enough for some people. A slightly larger aid that sits in the shell-like part of the ear ­ the concha ­ may be needed. This is a conchal or in-the-ear (ITE) aid. If still more power is needed or some sophisticated processing is required then the body of the aid will have to sit in the crease behind the external ear as a behind-the-ear (BTE) aid. This may also be necessary if there are cost implications as BTE aids are generally cheaper, or if the manual dexterity of the patient means that a CIC aid could not be handled and inserted properly.

How are conventional acoustic aids fitted?

Depending on the type and severity of the loss and the preferences of the patient, a trained hearing aid fitter will decide on what type of aid is appropriate. For all aids an impression of the ear canal has to be taken so an ear mould can be made. To do this a small sponge is placed in the deep ear canal to protect the eardrum and a soft self-setting material is gently squeezed into the canal and allowed to set. When it has, the cast is removed along with the sponge and a mould made from the cast.

The mould can be made of many materials, including acrylic or softer silicone materials, and could even be coated in gold in case of sensitivities and allergies. The mould can be used to house an ITE aid, or be used as the ear insert for a BTE aid. The BTE aid has soft plastic tubing running from the aid through a hole in the mould to deliver amplified sound to the ear. The tubing needs to be changed from time to time as it hardens; it also needs to be kept clean from wax and other debris that can block up the opening.

Are hearing aids an instant success?

Providing a hearing aid is much more complex than fitting glasses and the patient needs to acclimatise to the newly heard sounds. Hearing speech is not just a question of having two ears that work. It also relies on visual clues, including lip reading and assessing facial expression, and a cerebral 'context editor' sorting what is heard, seen and expected to give the best guess at what was said.

When sounds that may not have been heard for many years are introduced the patient needs time for adaptation. The usual advice is to wear the aid for an increasing time each day. Acclimatisation may take several months.

What if hearing aids whistle?

The usual cause of whistling is that the mould is not a good fit and there is 'feedback'. The microphone detects sound that leaks around the mould and this is amplified and sent back to the ear canal, where it leaks again, causing a continuous 'sound loop'. This may happen when patients chew, or when the mould or the ear canal changes shape with ageing. The remedy is a new, snug-fitting mould.

Occasionally the ear canal is blocked by wax and this also results in feedback, as the wax prevents sound from entering the ear canal.

What is the T-setting on the aid?

The T-setting on many aids can be used where electro-magnetic induction delivers sound directly to the aid from the source rather than airborne sound waves. The common use is with telephones and televisions or with 'loop systems' in halls, meeting rooms and churches.

This has led to the occasional light moment when the vicar has forgotten to turn off his clip-on microphone and muttered unholy words under his breath. T-settings are usually only found on BTE aids.

What do digital aids do that is better than conventional analogue aids?

A digital aid has a conventional microphone and converts the smooth analogue waveform into a digital signal. The quality of this varies according to the frequency and sensitivity of sampling. Once digitised, the sound is filtered through different frequency bands with each having its volume represented by a number.

Incoming signals are broken down into a numerically coded package and processed to enhance parts of the frequency range and compress others so that the output is not too loud to be uncomfortable. Overall, there are many different programs to achieve this and they vary between manufacturer. The output can be controlled, in part, by the hearing aid fitter.

The chips that do the processing have improved dramatically in both performance and size over the last few years. For many patients with sensori-neural losses they are an improvement on the older analogue aids and the earlier digital aids.

Simple conductive hearing losses with normal inner ear function often do not benefit from the digital processing of sophisticated aids.

How can my patients get a

hearing aid?

The NHS is making major improvements in the provision of audiology services. There is a massive drive to implement digital technology, but it is labour intensive and technologically demanding ­ progress is slow. Nevertheless you should start the process by referring the patient to an ENT consultant, an audiological physician or in some areas directly to an audiology department. The waiting time may be unduly long in some areas as technology is being introduced without the resources to implement it.

In the private sector, the whole range of digital and analogue aids should be available; in the UK dealers have to be members of the Hearing Aid Council and pass rigorous exams. Part of their charter is that aids can be returned after a trial period at no cost other than the administration charges for time and appropriate hearing tests.

Are hearing aids expensive to run?

Owners of NHS aids are given 'battery books' and can obtain batteries from various NHS hearing aid centres free of charge

Commercial aid users need to buy their batteries. A box of 60, which should last for a year, costs about £42. It is unlikely batteries will cost more than £1 per week and with improving technology the power consumption of newer aids are reduced, so battery life is longer.

CIC aids need small guards to prevent wax getting into the aid. These cost £6 for eight and each guard lasts about two weeks (a total of about £20 extra a year). The newer digital aids are very robust and reliable and will last for seven or more years if well maintained and serviced. They can also be reprogrammed as the owner's hearing level changes.

What about disposable aids?

An American company has developed small, sealed aids that fit deep in the ear canal close to the eardrum. As they are so close they use less power, so an integral battery will last quite a long time. The aid is called Songbird.

They come in a variety of sizes and are configured to match typical patterns of hearing loss; they are not custom-made for the individual. It is claimed that the batteries last for 10 weeks if used for 12 hours per day.

There is a £35 initial assessment charge and the aids cost £26 per month per ear. The downside is that the deep canal skin is very delicate and great care has to be taken when inserting the aid. Custom-built commercial aids may be cheaper in the long-term.

Tony Wright is professor of otolaryngology at the Institute of Laryngology and Otology, University College London

Further Reading

Graham J, Martin M. (eds) Ballantyne's Deafness. Sixth Edition. London: Whurr Publications, 2001

Wright T. Deafness and Tinnitus. London: BMJ Books, 2003

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