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Definition

Audiology is a science that deals with the study of the hearing and hearing disorders. This includes the anatomy, physiology and pathology of the auditory system. The auditory system consists of the external ear, middle ear, inner ear and the auditory nerve. Sound waves pass through the auditory tract where they are amplified, then converted from acoustical to mechanical to electrical impulses, and finally received by the brain as a discernible signal.

 

Early detection and intervention of hearing loss is imperative as impaired hearing can isolate an individual from his/her environment and impede normal social and intellectual growth in children. When an individual repeatedly asks us to repeat what was said, or adjusts the television volume louder than usual, or uncharacteristically watches facial and lip movements while someone is speaking, hearing difficulties should be considered. Hearing problems may also be observed in classrooms, as the teacher may be able to pick up on some symptoms. An audiologist should be consulted to perform hearing tests and interpret their results. Global Communication Services can identify existing hearing problems in virtually anyone. While we work with the general population, we specialize in services to individuals with mental retardation and/or developmental delays.

 

Anatomy of the Ear ( )
The ear is divided into three parts
1. External Ear : The external ear is comprised of the pinna and the external auditory meatus (ear canal). The pinna helps in collecting the sound waves, which then pass through the ear canal to the tympanic membrane. The tympanic membrane acts as a wall separating the external ear and the middle ear.

2. Middle Ear: Middle ear is comprised of an air filled cavity with a chain of three ossicles namely, malleus, incus and stapes. These bones connect the external ear to the inner ear. The malleus connects one end of ossicular chain to the tympanic membrane and the stapes connects the other end of the chain to the inner ear. The incus connects the malleus and the stapes. The middle ear pressure is regulated by the Eustachian tube. When the sound waves touch the tympanic membrane, they cause the tympanic membrane to vibrate. Here the acoustic energy is converted into mechanical energy. These vibrations are transferred from the tympanic membrane to the ossicular chain. From the ossicular chain these vibrations are transferred into the inner ear by movement of the stapes into the oval widow of the cochlea.

3. Inner Ear : Inner ear is comprised of the cochlea and the vestibular system. The vestibular system is responsible for balance. The cochlea is responsible for hearing. The cochlea has hair cells, which play a major role in the perception of sound. When the mechanical from the ossicular chain reaches the cochlea, it is transmitted to the auditory nerve in the form of electrical impulses through a series of movements of membranes and hair cells present in the cochlea. The auditory nerve then transmits the sound from the cochlea to the brain by means of chemical and electrical impulses.

 

Modes of Hearing (Illustration)
There are two sound pathways for human hearing, air conduction and bone conduction. When any sound courses through the outer ear, middle ear, inner ear and beyond, this sound is heard by air conduction. It is possible to bypass the outer and middle ears by vibrating the skull mechanically and introducing sound waves directly into the inner ear. In this way the sound is heard by bone conduction. Therefore hearing by air conduction depends upon the function of the outer, middle and inner ear and the neural pathways beyond; bone conduction depends upon the function of the inner ear and beyond.

 

Measurement of Hearing
Audiometry is the measurement of hearing. In order to obtain an objective measure of hearing, various instruments are used such as audiometers, impedance meters and instruments for electrophysiological measurements. These instruments incorporate various kinds of stimuli such as puretone signals, noise, pulse tones and speech. Hearing is evaluated in terms of intensity (i.e. loudness of the signal measured in units called deciBels) and frequency (i.e. pitch of the signal measured in units called Hertz). The responses of an individual whose hearing is evaluated are traced on a graph of frequency versus intensity called an audiogram. The minimum level at which an individual is able to respond to a signal at least fifty percent of the time is called the threshold of hearing. The procedure by which threshold are measured at different frequencies by varying the levels of intensity of the signal (usually puretone or single tone) using an audiometer is called puretone audiometry. In speech audiometry, the signal used is live speech or recorded speech instead of puretones.

 

Severity of Hearing Loss
Hearing loss can be divided into various categories based on the severity of hearing loss. This is important because the type of hearing loss determines the nature of the treatment. The severity of hearing loss is measured by calculating the puretone average of the hearing thresholds from the audiogram. It is divided into following categories:

25 dB and below------ Hearing within normal limits

26 dB - 40 dB -------- Mild hearing loss

41 dB - 55 dB -------- Moderate hearing lossv

56 dB - 70 dB -------- Moderately severe hearing loss

71 dB - 90 dB -------- Severe hearing loss

90 dB - above-------- Profound hearing loss


Note: The above classification is for adult population. For infants and children, 15dB and below is considered as hearing within normal limits, 16dB to 25dB is minimal hearing loss.

 

Types of Hearing Loss
There are three types of hearing loss. Depending on the site of lesion, hearing loss can be classified into various types as follows:
1. Conductive Hearing Loss A pathology that occurs anywhere in the external and/or the middle ear may lead to a conductive hearing loss (i.e. infection of the middle ear). In this kind of loss the bone conduction is at or near normal and the air conduction is affected. This kind of loss could be rectified by medication or surgery.

2. Sensorineural Hearing Loss A pathology affecting the cochlea or the auditory nerve could lead to this kind of loss (i.e. congenital hearing loss and hearing loss due to old age). Some pathologies that lead to this type of hearing loss may be rectified by surgery or medication. In cases where the loss is not rectifiable, devices such as hearing aids and cochlear implants may be used to improve hearing.

3. Mixed Hearing Loss Mixed loss is the combination of conductive and sensorineural hearing loss. The conductive component of the hearing loss may be rectified medically (or surgically). The sensorineural component may not be rectifiable medically (or surgically). In such cases depending on the severity of the hearing loss, hearing aids are recommended to improve hearing sensitivity.

 

Audiological Tests
Various kinds of audiological tests are used to detect and differentiate between types of hearing loss. There are tests available to suit almost all age groups. These tests are designed using puretones, noise and speech as signals. Besides pure tone audiometry and speech audiometry, there are different kinds of procedures used in the field of audiology. Impedance audiometry is one of the special procedures used to detect and differentiate middle ear and inner ear pathologies. It is an objective and automated test conducted with an instrument called as an Immittance meter. Another special procedure is Brain Stem Evoked Response Audiometry (BSERA). This is also an objective and automated test conducted to differentiate between types of hearing loss and to locate the site of the lesion. BSERA is also widely used for research purposes. These tests are administered and interpreted by an audiologist.

 

Rehabilitation
Besides administration and interpretation of audiological tests, an audiologist also plays a major role in rehabilitation of patients with hearing problems. Hearing aids, sign language, speech reading, total communication methods are used in rehabilitation. The audiologist also plays an important role in auditory training for the post-surgical cases of cochlear implants. Auditory training helps the patient obtain the best possible results from their aids.

 

Hearing Aids
There are three kinds of hearing aids.

1. Body level hearing aids

2. Behind the ear hearing aids (BTE)

3. In the ear hearing aids (ITE)

Body level aids are becoming less frequent. BTEs and the ITEs are commonly used. Hearing aids are prescribed based on the degree and type of hearing loss. Communication significantly improves as the hearing of an individual is enhanced by the use of hearing aids. In some cases, strategies like speech reading are also combined with the use of hearing aids to facilitate better communication. Hearing loss caused due to aging is called presbycusis. Hearing aids play a major role in improving hearing sensitivity for individuals with presbycusis. Depending on the degree and type of hearing loss and the speech audiometry test results, hearing aids are recommended. Cochlear implants are also successfully used in some cases. /font>

 

Early Intervention
An audiologist also plays an important role in early intervention of hearing loss. Research has shown that early detection and rehabilitation of hearing loss leads to better development of speech and language abilities of the child. In children with hearing loss, where hearing aids are not beneficial, methods like sign language, speech reading, total communication methods etc. are employed to develop communicative abilities of the children. Recent developments like cochlear implants are also used and seem to be effective.

 

Contributed by Nishant Bagul and Sangita Krishnamurthi

 

 
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