Hearing Tests

Hearing test is the hearing capacity of a person who has some hearing issues. Hearing capacity is a term of an audiology of ENT. It doesn’t require special preparation and doesn’t hurt. People who practice audiology are called audiologists. Audiologists are accountable for managing and recuperating disorders to love hearing and balance. Audiologists provide comprehensive services for people with hearing disorders. There are different kinds of hearing tests that use different techniques to identify hearing loss. Hearing tests may be screenings to see if you can hear or evaluations to find out if you have hearing issues. Audiologists undertakes hearing tests like newborn hearing screening; Pure Tone Audiometry (PTA), Impedance Audiometry (TYMPANOMETRY), OtoAcoustic Emission (OAE), Brain stem  Evoked Response Audiometry (BERA), Auditory Steady – State Response (ASSR). Sound is presented through earphones recorded with recording pads placed on the head.


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DIFFERENT KINDS OF HEARING TESTS

1.PURE TONE AUDIOMETRY (PTA):

PTA is called an audiological investigation of people having hearing difficulties. We test the hearing sensitivity of a subject only for pure tone sounds. But the sounds that we hear  in nature are complex sounds and not pure tone sounds. Pure tone audiometry is as much a science as it is an art of ascertaining the hearing acuity (hearing threshold level) of a subject for pure tone sounds of various frequencies. The result, when plotted graphically, is called a pure tone audiogram. 

The instrument used for this is an electronic device called a pure tone audiometer. It consists of an audio-oscillator which generates pure tone sounds of various frequencies usually at regular steps of 125, 250, 500, 1000, 1500, 2000, 3000, 4000, 6000, 8000 Hz. (Though the  normal range of human hearing is nearly up to 20,000 Hz, yet for routine clinical purposes it is sufficient to test up to 8000 Hz only). Each tone can be separately amplified to a maximum of 100 or 120 dB in most frequencies except the very low and very high frequencies where the range is slightly lesser. The tones are attenuated (control of intensity) by an attenuator dial which is marked in decibels (dB) and graduated in 5 decibel steps from -10 to 120 dB..



2.IMPEDANCE AUDIOMETRY (Tympanometry and Acoustic Reflex Tests):

Impedance Audiometry has been one of the major advancements in the fields of otology and otoneurology in recent times. It is becoming increasingly popular because of the very wide range of otological and neurological information it provides about the nature and anatomical site of a lesion. The uses of Impedance tests can be briefly summarized as:- 

  1. a) Objective differentiation between conductive and sensorineural hearing loss;
  2. b) Differential diagnosis in cases of conductive deafness;
  3. c) Measurement of middle ear pressure and evaluation of eustachian tube function:
  4. d) Differential diagnosis of a sensorineural deafness whether a lesion is cochlear or retro-

retro-cochlear;

  1. e) Identification of the site of lesion in facial paralysis and certain brainstem pathologies.       

     

The impedance audiometry tests also provide very useful information for objective           assessment of the approximate hearing threshold in patients where subjective  audiometry is difficult, for ruling out cases of malingering (i.e. feigned deafness) etc.The  Inclusion of impedance tests into the routine diagnostic test battery has now become mandatory not only for oto-neurologists, neurologists, pediatricians, etc. who may have  reason to examine the functional or structural integrity of the 7th and 8th cranial nerves, other pathologies related to auditory dysfunction, etc.

Impedance audiometry comprises two separate but related tests namely tympanometry and acoustic/ stapedial reflex tests. These two parts of the impedance audiometry tests are discussed separatly The procedure of tympanometry test is more or less similar with all impedance 

Audiometers made by different manufacturers. The tympanometry test is best done by 

an otologist with sufficient clinical experience. Interpreting the test results requires skill of 

a clinician who understands and appreciates the diagnostic patterns and can correlate it 

with the clinical findings and other audiological tests. Before starting the test the 

following points should be explained to the patients :-

  1. i) The test is virtually painless and objective but it does require at least a minimum 

of passive cooperation from the patient.

  1. ii)         The test will take not more than 2-3 minutes in each ear but during the tenure of 

the test the patient should not move the head and should not swallow or breathe 

            very hard. There is to be no talking or opening of the mouth during the test.

iii) A rubber or plastic ear tip will be inserted into the ear in an airtight fashion which 

may cause a slight discomfort.

In some difficult to test and restless patients, especially children, some sedation may be 

used. This will in no way affect or invalidate the test results. 

3.OTOACOUSTIC EMISSION (OAE):

It has been found by scientists that the normal cochlear generates a sound-believed to be due to the biological activities of the outer hair cells. This sound emitted by the normal cochlea which can be picked-up, recorded and measured by placing a microphone-receiver in the deep external meatus is called otoacoustic emission (OAEs). There are two types of OAEs viz. spontaneous otoacoustic emission (SOAEs) and evoked otoacoustic emission (EOAEs). The SOAEs are generated automatically i.e. spontaneously and they do not require any external stimulation for being generated. The SOAEs are found in about 50% of normal hearing subjects. Initially they were thought to be the objective cause of tinnitus  but in spite of a lot of research, it could not be established that SOAEs are not of much diagnostic importance at the present state of our knowledge.

The other variety of OAE occurs only when a sound stimulus is presented to the ear. Since it is not spontaneous and has to be evoked by presenting a sound stimulus to the ear, it is termed as Evoked Otoacoustic Emissions (EOAEs). The EOAEs are sometimes also referred to as Transient Evoked Otoacoustic Emissions (TEOAEs) .The character i.e.the frequency, latency, duration, etc. of the evoked EOAE is dependent upon the character of the evoking stimulus i.e. the sound presented the cochlea. If the frequency of the EOAE is analyzed it will be found to consist mainly of low frequency sounds. Similarly, if high frequency sound is presented to the cochlea the EOAE will originate from the basal turn of the cochlea and will consist of mainly high frequency sounds. Since the OAEs in response to high frequency sounds originate from the basal turn of the cochlea they have a shorter latency period compared to the low frequency sounds which have longer latency period since they originate in the apex of the cochlea.

4.BRAINSTEM EVOKED RESPONSE AUDIOMETRY (BERA):

The brainstem evoked response audiometry test is useful to the otoneurologist in the following conditions namely :

 

(a) detection of deafness in the difficult-to-test patients like infants and mentally retarded or malingering subjects from whom requisite cooperation will not be available for foolproof subjective pure tone audiometry test. This test can be carried out correctly even in deeply sedated and anesthetized patients. 

 

(b) assessment of the nature of deafness (e.g.whether conductive or sensory or neural) in difficult-to-test patients, especially in circumstances when the patient can not follow or respond adequately in tests like SISI, ABLB, tone decay, etc.

 

(c) identification of the site of lesion in retrocochlear pathologies. The retrocochlear pathway is a fairly big area in the cerebral cortex. Whereas, the other tests allow the oto neurologist to merely suspect whether any retrocochlear disease is present or not, the BERA test helps us to delineate the approximate area in the retrocochlear pathway where the lesion is present. This helps in diagnosing conditions like acoustic neuroma with utmost precision and accuracy.

 

(d) study of central auditory disorders, Evoked response audiometry has been found to be of use in separating diseases of the auditory cortex from diseases of the more peripheral organs.

 

(e) study of maturity of the central nervous system in newborns, objective identification of brain-death, assessing prognosis in a comatose patient etc.

5. AUDITORY STEADY-STATE RESPONSE (ASSR) :

Auditory Steady-State Response (ASSR) Procedure is almost the same as BERA, But ASSR Takes more time to perform as it is frequency specific, meaning it is measured in 1000Hz, 2000Hz and seen in different intensities.



6. SPEECH AUDIOMETRY :

In speech audiometry the hearing sensitivity of a subject for speech is assessed. The test is of limited diagnostic value to the otoneurologist, but is very valuable to persons who are concerned with therapeutic and rehabilitative audiology. The speech audiometry tests have however been found to be very useful in the assessment of central auditory function. Speech Audiometry comprises of the following tests-

Speech Reception Threshold (SRT) :-

  1. The SRT is the lowest hearing level in dBHL at which 50% of a list of spondee words are correctly identified by a subject. A spondee is a two-syllable word with equal stress on each syllable e.g. eardrum, toothbrush, armchair, workshop, baseball, horseshoe, whitewash, headlight, etc. “CID Auditory Test List W-1” is a list of 36 such spondee words in  the English language prepared by the central Institute for the Deaf. Words from this list are usually used for ascertaining the SRT. The simplest way of estimating the SRT is to present groups of 6 spondee words first at a step 25dB above the pure tone hearing threshold level, and  then at successively lower intensities till a level is reached at which the subject correctly identifies just 3 of 6 spondee words presented. In such cases the higher value (at which all the words or more than 50% of the words could be correctly identified) is accepted as the SRT. The SRT of a normal subject is very closely related to his pure tone hearing threshold and the SRT is usually about 2dB lesser than the average of the pure tone hearing thresholds at 500 and 1000Hz.In neural lesions, the SRT is much poorer than this pure tone average, However, exceptions are not uncommon e.g. in a patient with steeply stoping or steeply rising audiometric contour, SRT may be much poorer than expected even if the lesion is cochlear in type.

Speech Discrimination Score (SDS) :-

  1. The SDS is the percentage of correctly identified words, when words from a specially prepared list called “Phonetically balanced word list” is presented to the subject. This Phonetically balanced word list consists of monosyllabic words, kike “as, can, age, your,” so selected that, most, if not all, of the phonetic elements used in the language are covered in the word list,and that to they are present in such frequency as they  occur in normal speech in that language. The percentage of the total number of such presented words that the subject correctly identifies gives the speech discrimination score. The SDS is normally between 90 to 100%, but in neural lesions the SDS is considerably low. However, this is not a very reliable parameter to differentiate between cochlear and  neural types of sensorineural deafness and in quite a few patients of neural deafness.

A somewhat better way of differentiating between neural and other types of hearing loss is by graphically plotting the performance—intensity function. This is done by ascertaining the speech discrimination score at different sensation (intensity) levels and plotting the percentage of correctly identified words as a function of the intensity of presentation of the words from the PB-word-lists.




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