Dr. Fasih Ullah

 Mechanism of Hearing

The sound waves are received by the external ear pinna. These sounds waves then move in the ear and vibrate the tympanic membrane  ( also called ear drum ). See illustration
The tympanic membrane is attached to a bone called malleus , which in turn is joined to a bone called incus and which is again joined to abone called stapes. When the tympanic membrane vibrates it also vibrates the chain of bones with it accordinglyand with the vibration of the last bone the stapes, the fluid within the inner ear cochlea is moved. Thus the cells generate an impulse in the system of inner ear(contained in the bony snail shaped portion of the ear called coclea ). From cochlea the nerve impulses are then carried to the brain where the details of the sound are analysed. It is at the  brain level the sound is differentiated and the speech is understood in minute details.

The deafness or the impairment of hearing occurs as a result of the defect in the different parts of the ear mentioned above. It may be due to the defect in the tympanicmembrane or in the chain of bones responsible for the conduction of sound, or maybe due to the stiffness of the chain of bones or infection. In all types of deafness the inner ear the cochlea is functioning normally. The deafness may also be due to a defect or weakness in the cochlea or otherwise the nerve may be weaker. All types can be corrected by surgical means which may include medical or surgical correction. For example if the deafness is due to the stiffness in the chain of bones ,the bone can be removed and an artificial bonemadeup of plastic like material is inserted and  deafness is thus improved. Incase of hearing loss due to infection and perforation of the ear drum( tympanic membrane), the operation is performed and the tympanic membrane is repaired by plastic surgery known as Tympanoplasty. Likewise many different procedures are available for the improvement of hearing. All these depend on one factor that the inner ear in these circumstances is normal. In these cases the hearing loss is caused  due to defects other than those of the inner ear.

When a person suffers from hearing problems as a consequence of old age, is due to the weakness of the nerve cells responsible for generation of impulse of the cochlea. In such cases the hearing impairment is due to the defects in the inner ear.In this situation the tympanic membrane and the chain of bones responsible for the conduction of the sound waves are normal and there is no defect in the middle or the external ear.
The hearing level to determine the hearing loss is measured by means of a machine called audiometer. Different tests are available to determine the cause of hearing loss and are dealt with accordingly. Most patients who suffer from such snags in the hearing mechanism are the old people and they are prescribed a hearing aid.
The function of a hearing aid is to pick up sound (by its reliever) and then it ampliphy it and conveys it to the cochlea.

Depending on the type and depth of deafness, different types of hearing aids are available in the market. Some people have such low levels of hearing impairment that it is not possible to correct their hearing impairment even by means of stronger hearing aid. In this group are children born deaf and adults who have lost their hearing as a result of injuryor due to side effects of some medicines ( ototoxic medicines).The hearing loss is classified as profoundloss of hearingand the strongest possible hearing aids are in effective. Such patients need a stronger device. Devices overcome such a situation, which take over the job of the cochlea. Another big group is thedeaf and mute childrenwho are born deaf, I n this group the hearing impairment leads to a delayed speech or no speech at all. Efforts aremade to detect the level of hearing loss and to provide some aid so that patient hears and learns the speech. When it is established that the hearing level isprofound, a strong hearing aid is provided and prescribed. Such children now days are provided with cochlear implant.

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