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Sunday, August 12, 2001

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Choice of hearing aids

NOT only is hearing important for speech development, the quality of developed speech is constantly monitored by proper hearing. The human ear can hear sounds of 20 Hz to 20,000 Hz. However, the frequency of speech is only from 500 to 4,000 Hz. Sound travels in the air and reaches the tympanic membrane through the ear canal. When the membrane vibrates, the three ossicles - malleus, incus and stapes - also vibrate.

When the stapes bone vibrates, it sets up corresponding vibrations of fluid in the inner ear. The peripheral end organs of hearing are situated in the inner ear and are stimulated by the vibrations of the fluid. These signals are picked up by the eighth cranial nerve or the acoustic nerve which transmits the impulses to the hearing centre in the brain where it is decoded so that the person is able to hear.

Thus, the transmission of sound in the external and middle ear is mechanical, whereas in the inner ear and beyond it is through the nerve of hearing. Pathologic conditions of the inner ear and the eighth cranial nerve lead to nerve deafness or sensorineural deafness.

Types of hearing aids:

Aid conduction hearing aids: Almost 98 per cent of hearing aids are air conduction hearing aids, where the amplified sound is transmitted via the ear canal to the tympanic membrane.

Bone conduction hearing aids: Instead of the receiver there is a bone vibrator which fits snugly on the mastoid bone and directly stimulates the cochlea, the organ of hearing. These are specially useful in persons with actively discharging ears, inflammed external ears, stenosed or absent ear canals where ear inserts cannot be fitted. This is suitable for mild to moderate conductive deafness only.

Air conduction hearing aids are of different types. (1) A hearing aid worn on the body where a microphone, an amplifier and a battery are in a single case worn at chest level. The receiver is at the ear level. This allows a high degree of amplification with minimal feedback.

It is indicated in severely deaf persons or children with severe or profound congenital deafness. This hearing aid uses ordinary pen torch batteries which last for 60 to 80 hours. A pocket model may cost about Rs. 2,000 to Rs. 5,000. It is to be used in the following cases:

1. Congenital ear defects where there is no ear canal.

2. Patients who have discharging mastoid cavities and hearing aid cannot be fixed to the ear canal.

3. Chronic adhesive otitis media.

Contra lateral routing of signals: In this type, a microphone is fitted on the side of the defective ear and the sound thus picked up is passed to the receiver placed in the better ear. This is useful for a person with one ear totally impaired and helps in sound localisation coming from the side of the affected ear.

Programmable hearing aids: Here the hearing health care professional programmes or adjusts certain sound features in the aid using a programmer or computer. The sound features are designed to accurately fit individual needs and give the patient distinct, high quality amplification. It can be readjusted or reprogrammed.

Digital hearing aids: It offers all the advantages of digital technology. You hear loud and soft sounds, high and low tones practically at the level of a person with normal hearing. Added to this comes excellent speech intelligibility in noise. This means that low frequency noise is reduced and the clarity of speech enhanced. You hear your environment again just as you would like to.

BTE, ITE, ITC, CIC - These are conventional, otherwise called analogue or programmable or digital hearing aids. The programmable ones will cost about 50 per cent more than the conventional ones and digital ones may be much more expensive than the programmable ones.

Cochlear implants are still in the developmental phase. They are electronic devices which convert sound signals into electrical impulses which then directly stimulate the cochlear nerve. Even if the cochlear nerve is non-functional, the electrode can be placed on the brain stem. Thus cochlear implants replace the non functional transducive system of hair cells of the cochlea.

It is a misconception that a cochlear implant is a new type of hearing aid implanted inside. However, the patient who undergoes cochlear implant surgery has to wear the speech processor instead of a hearing aid. Post lingual adults benefit by cochlear implants. Careful selection of cases and extensive rehabilitation are necessary.

Vibratory ossicular replacement prosthesis (VORP): The Vibrant Soundbridge System is the latest development. The hearing aid is fixed on the incus bone of the middle ear. This is suitable for mild to moderate hearing loss.

Total implantable hearing aids are (1) a microphone implanted deep inside the ear canal, under the skin near the ear drum. (2) An electro-mechanical transducer directly moves the ossicles in the middler ear. (3) The main module for electronic amplification is placed in a recess behind the ear.

There is no need for a ear mould, there is no distortion and the person can take a shower or have a swim with the hearing aid since all the parts are under the skin. The tiny battery which forms the power source is contained with the main module. This battery is recharged from outside by wireless power transmission.

The patient has to wear a device like a headphone for two hours every two or three days. During charging, the device offers different settings to suit different listening situations. This type of hearing aid system is also in the developmental stage and holds promise.

Hearing aids are designed to give the best reception for speech frequencies and, therefore, the range of amplification is usually from 250 to 4,000 Hz. The frequency response of the hearing aid can be adjusted to the patient's requirements. In many types of hearing aids there is a tone control which can be used to increase gain in the higher frequencies which can help in understanding speech.

Persons with conductive type of deafness get the best results by wearing hearing aids. Of course, surgery like removal of fluid from the ear and various types of tympanoplasties in discharging ears will restore the hearing and eliminate the necessity of a hearing aid. Tympanoplasty operation consists of removal of the disease from the ear and mastoid and reconstruction of the ossicular chain which may be damaged by the chronic discharge.

Fitting: It is not advisable for the patient to buy a hearing aid across the counter. The patient may have a condition which can be cured so an aid may not be necessary. There may be wax or fungus blocking the ear which has to be cleaned before fixing the hearing aid.

Otherwise the sound transmission will not be good. The ear canal skin may have tendency for inflammation and this must be treated before fixing the hearing aid.




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