HEARING AIDS - BROADER PERSPECTIVES

Authors:

Dr.G.K.Hebbar, M.S(ENT),DLO                                  

Mr.T.Dattatreya, M.Sc (Sp & Hg)

Even during earlier times man has always tried to overcome his hearing loss by artificial means.  The first hearing aid was probably the hand cupped behind the ear.  This provides about 14 dB of cosmetically unappealing amplification.  Acoustic amplifiers such as horns and speaking tubes were used from the seventeenth to the nineteenth century.  Carbon hearing aids were developed at the beginning of the twentieth century using telephone technology.  In 1938, the vacuum type hearing aid was introduced.  Bell Telephone Laboratories invented the transistor in the 1950’s, which was the basis of the  hearing aids used then.  It was only when the chip was incorporated into hearing aids did the quality of hearing aids improve dramatically.  In the last two years hearing aid technology has boomed, with new digital models that can put the processing power of a laptop computer deep into the ear canal. There is a sea of difference between the grandfather hearing aids of yesteryears and the ones that are used today.

A hearing aid is a device consisting of a microphone, an electronic amplifier, and a receiver.  The microphone receives environmental sounds, the amplifier enhances a few or several frequencies depending on the needs of the user, and the receiver transmits the modified sounds to the middle ear.

There are typically three contributions that hearing aids can provide the user.  The first is to amplify normal conversational speech to levels that are maximally understandable, which leads to improved communication.  The second is to allow the patient to hear other environmental sounds, such as warning signals and music.  The third contribution is to promote education and development.  Hearing aid use in the hearing impaired child may allow the development of normal language and speech skills.

In order to understand how hearing aids function a brief introduction to the functioning of the ear and types of hearing impairment is in order.

Functions of the ear:  The ear is divided into three parts: an external ear, a middle ear and an inner ear. Each part performs an important function in the process of hearing.

 The external ear consists of the auricle and ear canal.  These structures gather the sound and direct it towards the ear drum.

The middle ear chamber lies between the external and inner ear and consists of an ear drum and the three small ear bones (ossicles): malleus (hammer), incus (anvil) and stapes (stirrup).  These structures transmit sound vibrations to the inner ear.  In so doing they act as a transformer, converting sound vibrations in the external ear canal into fluid waves in the inner ear.

The inner ear chamber contains the microscopic hearing and balance nerve endings bathed in fluid.  Fluid waves initiated by movement of the stapes bone stimulate the delicate hearing nerve endings, which in turn transmit an electric impulse to the brain where it is interpreted as sound.

Types of hearing impairment: The external ear and the middle ear conduct and transform sound; the inner ear receives it.  When there is some difficulty in the external or middle ear, a conductive hearing impairment occurs.  When the trouble lies in the inner ear, a sensori-neural or nerve hearing impairment is the result.  Difficulty in both the middle and inner ear results in a mixed impairment.  The type of hearing impairment is determined by means of a pure tone audiometer.  The ensuing document wherein the hearing levels are charted is called a pure tone audiogram.

Who will benefit with a hearing aid?  If you frequently ask people to repeat themselves, have trouble following conversations at party or get complaints about your loud TV, you might be benefited with a hearing aid.

The characteristics that determine hearing aid candidacy can be divided into audiologic factors and motivational factors.  The audiologic factors include the type of hearing loss and the degree of hearing loss.  Motivational factors relate to the patient’s lifestyle and acknowledgement of a hearing problem.

Those patients with intractable conductive hearing loss typically retain good inner ear function.  Most of such conductive hearing losses can be corrected by surgical intervention.  However in cases where surgery is contraindicated or wherein the patient does not desire such surgical intervention, a person with such a hearing loss will also benefit by using a hearing aid.

In sensorineural hearing loss, the condition of the inner ear is highly variable. Therefore, sound-processing capacity is not predictable from a pure tone audiogram.  Two individuals with sensorineural hearing loss and identical pure tone audiograms may have quite different sound processing capabilities, such that one relies heavily on amplification and the other does well without it.  The person with mild to moderate sensorineural hearing loss typically hears the louder portions of speech, such as vowels, but not the voiceless consonants like t, p, k, f, s and ch.  The result is a patient who can hear speech, but not understand it.  The likelihood of benefit from hearing aids is increased if the degree of hearing loss is moderate to severe.  Those with moderate to moderately severe hearing loss need amplification in almost all social skills.  Word recognition is usually lacking in those with profound hearing loss, which substantially reduces the derived benefits of amplification.  Patients with mild hearing loss are more difficult to assess.

Motivation for hearing aid use is influenced by degree to which the individual’s quality of life has been affected by hearing loss. This is referred to as the individual’s hearing handicap.  Denial of a hearing problem reduces motivation, which diminishes chances of a patient using the hearing device.  In children, motivational factors usually play a lesser role in determining candidacy.  With few exceptions, any child with significant long-term hearing loss is a candidate for hearing aids.

Why are people reluctant to use hearing aids?

 Different persons react differently to the use of a hearing aid. If the hearing impairment is of the sensorineural or nerve type, the person can hear speech sounds if they are loud enough but cannot understand what is being said.  It is true that speech must be loud enough to permit the listener to understand to his full capability.  But making speech increasingly louder will not necessarily lead to a corresponding improvement in discrimination because the hearing nerve has become less sensitive to the acoustic differences of speech sounds.  Because the prime function of an aid is to amplify sounds, some users of these instruments continue to experience difficulty in understanding in spite of increasing the loudness levels.  Another problem for a new hearing aid user is to adjust to the surrounding noise. People have very high expectations and expect to hear like a normal individual while using these devices.  When the quality of amplified sound does not match their expectations they discard these devices.

One or two hearing aids?  In most cases, binaural (both ears) fitting is indicated, because patients typically have improved speech understanding in noisy conditions, and are better at localizing sounds.  Studies have shown that auditory deprivation in the un-aided poorly hearing ear may result in loss of word-recognition which would not occur if  a hearing aid were present.  If the patient decides on a monaural (one ear) fitting for financial or other reasons, it is usually more beneficial to amplify the ear with better word recognition ability.

Type of hearing aids:   There are four common types of hearing aids, which differ based on size, and location on the patient.  In general, the smaller the hearing aid, the less electronic flexebility, the smaller and fewer the user controls, the fewer the possible acoustical modifications, and smaller the battery.  However, smaller aids do offer a more natural sound because the acoustic properties of the pinna and in some cases the canal is preserved.  In addition, smaller aids have higher cosmetic appeal.

Body level hearing aids(BLHA): Is the largest of the hearing aids.  This device is housed in a case, which is accommodated in one’s shirt pocket or a specially stitched pouch, and contains a microphone, an amplifier, and a battery.  This is connected to a receiver through a cord.  The receiver is fitted snugly into one’s ear canal using either custom built or standard ear molds.  The advantages of BLHA is that it is cheap, rugged and easily serviced.  The maintenance is also cheap since it uses pen torch batteries that is easily available.  It is powerful and can therefore be prescribed for persons with profound hearing impairment.  A single device with a ‘V’ cord can be used for a binaural fitting.  Hence these type of aids are prescribed for the economically backward and in case of children.  The disadvantages are that it is cosmetically unappealing and is not preferred by the more sophisticated persons.

Behind the ear (BTE) hearing aids:  This device has a module that fits behind the ear, which contains a microphone, an amplifier, a battery, and a receiver.  The microphone connects connects to an earmold in the external auditory meatus via a piece of clear tubing that travels over the top of the post auricular crease.  This device is more cosmetically appealing and is the most popular.  These devices are rugged and easily serviced.

In-the-ear(ITE) hearing aids: This device sits in the concha or the the shallow depression at the entrance of the External auditory canal(EAC) and extends into the EAC.  Most ITE aids are custom devices, with the components built into a shell made from an impression of the user’s ear.  They are more expensive than the BTE aids but having the same cosmetic value as that of BTE. 

In-the-canal(ITC) hearing aids: It is similar to the ITE device, except the components are small enough to fit entirely within the EAC. ITC aids are custom molded and have high cosmetic appeal.  They also have acoustic advantages such as reduced feedback when using the telephone.  The microphone location within the ear reduces wind noise interference and enhances high frequency sounds in the 2-4-kHz region.

As technology advances, hearing aids can be packaged into increasingly smaller housings.  Devices that fit entirely within the bony canal (the deeper portion of the EAC) have been developed, called completely-in-the-canal (CIC).  These aids have high cosmetic appeal because they are virtually undetectable. They also have several acoustic advantages.  These include reduced occlusion effect, reduced gain requirements, and preservation of the natural acoustic properties of the pinna and external ear.  Because of occlusion of the lateral portion of the EAC, amplification of low frequency vibrations in the patient’s voice produces an echoing, hollow sound. This is called occlusion effect and occurs with traditional hearing aids.  Because the volume of air between the hearing  aid and the ear drum  is reduced CIC devices have reduced gain requirements.  This allows thehearing aid to operate with lowerr acoustic power, resulting in reduced distortion and improved sound quality.  The CIC wearer has better understanding of speech in noisy conditions and localization of sounds is enhanced.

More specialized types of hearing aids also exist.  The CROS (contralateral routing of signal) devices uses a microphone on the worse hearing side to transmit sounds to a receiver at the better hearing ear.  Transmission occurs via a wire that runs around the back of the neck, or in a wireless mode by radio frequency.  In this way, the wearer can use the good ear to hear sounds from the impaired ear.  The CROS unit may benefit those with no usable hearing  in one ear and minimal hearing loss or normal hearing in the other ear.  For example, a taxi driver  with an impaired left ear may benefit from the CROS unit, since the left ear is closer to the passengers.

Until recently all contemporary hearing aids were utilizing analog circuitry.  The disadvantage with these types of analog devices is that the electroacoustic characteristics of the aids could not be modified satisfactorily to suit individual patient requirements.  This resulted in a lot of patient dissatisfaction and poor usage of these devices.  The revolutionary breakthrough in hearing aid technology occurred when programmable or digital circuitry was used in hearing devices.

Programmable hearing aids have analog type amplifiers and filters, which are controlled, by an external digital source.  These hearing aids contain a memory module such as a CMOS (complimentary metal oxide semiconductor), RAM (random access memory), or EEPROM (electrically erasable programmable read only memory).  An external microprocessor (computer) accesses the memory locations within the chip to modify the hearing aid’s electroacoustical performances.  The memory module provides more precise control of the acoustic characteristics.

In entirely digital hearing aids, the input signal is digitized, then processed with digital signal processing circuitry.  These hearing aids are fitted using software packages, and can be programmed to make changes in electroacoustic performance based on the input signal.  Digital and programmable hearing aids offer the potential for better fitting to the individual hearing loss.  Adjustments to suit individual requirements are to be made by the dispenser using a programming unit provided by the manufacturer.

Experts say that millions of urban Indians between the ages of 45 and 60 have some degree of hearing impairment.  Not only does this hearing loss comes with aging but today’s urban youth are suffering such impairment much earlier than their parents did thanks to noisier surroundings.  Too many people do not realize that hearing loss has serious consequences. A recent survey by the National Council of Aging, U.S.A., of hearing impaired individuals found those without hearing aids were more likely to be depressed, anxious and isolated from society.  Today’s fast paced lifestyle demands a keen perception of hearing. Thanks to the booming hearing aid technology the user has a wide variety of hearing aid models to suit most patient requirements.  The key to a good hearing aid is testing by a professional to uncover the type of hearing loss, and then getting an appropriate hearing aid fitted properly. If it’s not fit properly, the best hearing aid will not function.  It is therefore not advisable to get your relatives to get you a hearing aid from abroad or to mail order a hearing aid responding to an advertisement appearing either on the TV or a magazine.