Assessment of hearing

Assessment of hearing – Hearing tests

CLINICAL HEARING TESTS

Hearing assessments are crucial for diagnosing auditory function and identifying potential hearing impairments. Several clinical tests are employed to evaluate hearing thresholds and sound localization. The Finger Friction Test, the Watch Test, and the Free Field Voice/Speech Test are among these tests.

(i) Finger Friction Test:

The Finger Friction Test is a basic screening tool for assessing hearing thresholds and sound localization capabilities. In this test, the examiner generates a clicking sound by snapping their thumb and middle finger together near the patient’s ear. The patient is instructed to close their eyes and listen for the sound, allowing the examiner to evaluate the patient’s auditory perception.

(ii) Watch Test: Historically, the Watch Test was a common method used by clinicians before the development of audiometers. In this test, the examiner brings a ticking watch close to the patient’s ear. The distance at which the patient can hear the ticking sound is measured, providing insight into their auditory sensitivity.

(iii) Free field voice/ Speech Test: The Free Field Voice/Speech Test is a structured assessment designed for clinical environments. This test evaluates each ear separately while masking the opposite ear to prevent sound interference. Masking can be achieved by having an assistant rubbing the index finger on the patient’s tragus or by using a Barany noise box.

Procedure: Speech test must be done in quiet surroundings. 

• Patient Positioning: The patient stands 6 meters away from the examiner, facing the test ear, with their eyes closed to eliminate visual cues from lip reading.
• Masking: An assistant blocks the non-test ear by applying pressure on the tragus, or alternative masking methods are employed.
• Testing: The examiner speaks spondee words (e.g., “iceberg,” “sunlight,” “bathroom”) or alphanumeric combinations (e.g., “Y3G,” “6BZ”) while gradually moving closer to the patient. The distance at which the patient can hear the conversational voice is recorded.
• Whispered Voice Test: The examiner then whispers spondee words, and the distance at which the whispered voice is heard is also measured.

Interpretation: The results of the Free Field Voice/Speech Test can indicate the presence of hearing loss. A patient with a hearing loss greater than 30 dB, does not hear a whispered voice from a distance of 2 feet from the test ear. The sensitivity of this test is reported to be 95%, with a false-positive rate of 10%. If a patient can hear a voice from 2 feet away, it suggests that their pure tone threshold is better than 30 dB HL.

Disadvantage: Despite its utility, the Free Field Voice/Speech Test has limitations. The standardization of the intensity and pitch of the voice used during testing is often questioned, and external ambient noise can interfere with the accuracy of results. These factors can affect the reliability of the test outcomes, highlighting the need for careful consideration during clinical assessments.

TUNING FORK TESTS

Selection of Tuning fork: The selection of an appropriate tuning fork is crucial, with frequencies such as 128, 256, 512, 1024, 2048 and 4096 hertz are available. In clinical practice, a tuning fork of 512 Hz is preferred due to its optimal decay time and minimal overtones, making it ideal for accurate testing. Lower frequency forks tend to produce sense of bone vibrations, while higher frequencies have shorter decay times, which can affect test results.

Setting tuning fork into vibration : To initiate the test, the practitioner should hold the tuning fork by its stem and strike it gently against a stable surface, such as the examiner’s elbow or the heel of their hand. For optimal results, it is recommended to strike the prong approximately one-third of its length from the free end to minimize overtones and produce a pure tone.

Pre-requisites for tuning fork tests: Before conducting the tests, it is essential to explain the procedure to the patient, instructing them to raise a finger when they can no longer hear the sound. Additionally, the practitioner should stabilize the patient’s head to ensure accurate results.

Testing Methods. The tuning fork tests primarily assess two types of conduction: air conduction (AC) and bone conduction (BC).

• Air Conduction (AC) Test: The vibrating tuning fork is placed vertically about 2 cm from the external auditory canal. Sound waves travel through the tympanic membrane, middle ear, and ossicles to the inner ear, allowing evaluation of both the conducting mechanism and cochlear function. Typically, sound heard through air conduction is louder and lasts longer than through bone conduction.
• Bone Conduction (BC) Test: The footplate of the vibrating tuning fork is placed on the mastoid bone. This method stimulates the cochlea directly through vibrations transmitted via the skull bones, measuring cochlear function alone.

The clinically useful tuning fork tests include:

1. Rinne’s Test: In this test, the base of the vibrating tuning fork is placed on the mastoid bone. When the patient can no longer hear the sound, the fork is moved 2.5 cm in front of the external auditory canal. If the patient hears the sound again, it indicates that air conduction is superior to bone conduction.

1. • Interpretation:

1. • • Positive Rinne: AC > BC, indicating normal hearing or sensorineural deafness.
     • Negative Rinne: BC > AC, indicating conductive deafness.
     • False Negative: In cases of severe unilateral sensorineural hearing loss, the patient may hear the sound on the mastoid but not in front of the ear. This can be confirmed with masking techniques and the Weber test.

The degree of air-bone gap can be assessed using tuning forks of 256, 512, and 1024 Hz, with specific interpretations for each frequency.

• • A Rinne test equal or negative for 256 Hz but positive for 512 Hz indicates an air-bone gap of 20–30 dB.
  • A Rinne test negative for 256 and 512 Hz but positive for 1024 Hz indicates an air-bone gap of 30–45 dB.
  • A Rinne negative for all the three tuning forks of 256, 512 and 1024 Hz indicates an air-bone gap of 45–60 dB.

Remember that a negative Rinne for 256, 512 and 1024 Hz indicates a minimum AB gap of 15, 30, 45 dB, respectively.

2. Weber’s Test: This test involves placing the vibrating tuning fork in the middle of the forehead / vertex/ central incisors or mandibular symphysis from where it will be conducted directly to the cochlea. The patient is asked to identify which ear hears the sound better.

• • Interpretation:

1. • • Normal hearing results in equal sound perception in both ears.
     • In conductive hearing loss, sound lateralizes to the affected ear (this is due to loss of ambient noise or failure to dissipate sound because of the ossicular discontinuity), while in sensorineural loss, it lateralizes to the unaffected ear (as sound travels directly to the cochlea via bone).

3. Absolute Bone Conduction Test: This test compares the bone conduction of the patient to that of the examiner, assuming the examiner has normal hearing. With the patient’s ear occluded by the examiner by pressing over the tragus, the tuning fork is placed on the mastoid bone.

1. • Interpretation:

1. • • Normal or conductive deafness: Both the patient and examiner hear the sound for the same duration.
     • Sensorineural deafness: The examiner continues to hear the sound longer than the patient.

4. Schwabach Test: Similar to the absolute bone conduction test, but without occluding the ear canal.

5. Bing Test: The tuning fork is placed on the mastoid process while altering air pressure in the ear canal by closing/ opening the ear canal by alternatively pressing on the tragus.

1. • Interpretation:

1. • • Normal or sensorineural hearing loss: Sound increases when the ear canal is blocked (Bing positive). 
     • Conductive hearing loss: No change in sound perception occurs in patients with fixed or disconnected ossicular chain (Bing negative).

6. Gelle’s Test: This test assesses the functional status of the ossicular chain by altering air pressure in the ear canal while the tuning fork is placed on the mastoid process.

1. • Interpretation:

1. • • Normal subjects experience a decrease in sound perception with increased air pressure, while patients with conductive hearing loss show no change.

Tuning fork tests remain a valuable tool in clinical practice for evaluating hearing loss, providing quick and effective assessments that can guide further audiometric testing when necessary.

 

TUNING FORK TESTS AND THEIR INTERPRETATION

Test	Normal	Conductive deafness	SN deafness
Rinne	AC > BC (Rinne positive)	BC > AC (Rinne negative)	AC>BC
Weber	Not lateralized	Lateralized to poorer ear	Lateralized to better ear
ABC	Same as examiner’s	Same as examiner’s	Reduced
Schwabach	Equal	Lengthened	Shortened

Special Tuning Fork Tests for Malingering and Non-Organic Deafness

1. Stenger’s Test. Stenger’s test is based on the principle that when two identical sounds are presented to a person with one healthy ear and one deaf ear, the individual will only perceive the sound in the ear that is closer to the sound source.

Procedure: In this test, the patient is blindfolded. The examiner uses two similar tuning forks, typically of 512 Hz, struck to moderate intensity and held approximately 25 cm from each ear. If the patient is malingering, he will only hear the sound in the normal ear. The tuning fork on the deaf side is then moved closer by 3 inches. A malingerer will not perceive the sound at all, confirming the suspicion of non-organic deafness.

2. Chimani Moos Test. The Chimani Moos test is a modification of the Weber test, designed to identify non-organic hearing loss.

Procedure: When a vibrating tuning fork is placed on the forehead, a malingerer will often report hearing the sound in their better ear, which simulates sensorineural deafness. If the ear canal of the better ear is occluded, a genuinely deaf patient will still hear the sound in the occluded ear, while a malingerer will claim they cannot hear it.

3. Teal Test. The Teal test is specifically used for patients who assert that they can only hear through bone conduction. This test evaluates the validity of such claims.

Voice Tests for Malingering and Non-Organic Deafness

1. Erhard’s Test. Erhard’s test is utilized to detect total unilateral hearing loss. In this procedure, the ear canal of the normal ear is occluded, which reduces speech perception by 30 dB or less. The suspected malingerer is instructed to close their eyes and repeat words they hear. The examiner occludes the normal ear by pressing on the tragus and then speaks words into the suspected ear. If the patient fails to repeat the words, it suggests malingering, as even with the head shadow effect, the other ear should still be able to hear.
2. Lombard’s Test. Lombard’s test is grounded in the observation that individuals typically raise their voice when speaking in noisy environments. During this test, the patient is asked to read prose aloud. Noise is then introduced to the good ear. If there is an organic loss in the suspected ear, the patient will raise their voice. Conversely, if the hearing loss is feigned, the patient will show no change in speech volume, indicating normal monitoring of their voice.

AUDIOMETRY TESTS

Audiometry tests are essential assessments used to evaluate an individual’s hearing capabilities. Various types of audiometry tests are employed, including Pure Tone Audiometry, Speech Audiometry, Bekesy Audiometry, and Impedance Audiometry. Among these, Pure Tone Audiometry (PTA) is considered the gold standard for measuring hearing thresholds.

1. Pure Tone Audiometry (PTA)

PTA is a subjective test designed to determine the hearing threshold for pure tones, which are sinusoidal waves characterized by a single frequency, amplitude, and phase. The test utilizes an instrument called an audiometer, which is an electronic device which produces pure tones. The intensity of audiometer can be increased or decreased in 5 dB steps. During the test, the threshold intensity for each frequency is measured and recorded in a graphical representation known as an audiogram.

Components of an Audiometer. An audiometer is composed of several key parts:

• • Electronic Oscillator: This component generates pure tones across a range of frequencies, from low to high.
  • Intensity Dial: This dial allows the examiner to adjust the threshold intensity of hearing for each tone.
  • Headphones: These deliver pure tones of various frequencies to each ear separately.

Prerequisites for PTA. For accurate results, PTA should be conducted under specific conditions:

• • Sound-Proof Environment: The test should take place in a sound-proof room, with ambient noise levels accepted at less than 35 dB.
  • Minimizing Distractions: Transient noises, such as sneezing or coughing, should be avoided, as they can interfere with the test results.
  • Examiner-Patient Interaction: The patient sits in a sound-proof booth while the examiner conducts the audiometry from outside. Communication can occur via intercom or face-to-face to observe the patient’s reactions.

Testing Procedure

The testing process typically begins with the better ear. The thresholds for pure tones are measured in the following order: 1000 Hz, 2000 Hz, 4000 Hz, and 8000 Hz, followed by 500 Hz and 250 Hz. If there is a difference of 20 dB or more between contiguous frequencies, inter-octave frequencies (750 Hz, 1500 Hz, 3000 Hz, and 6000 Hz) will also be tested. The procedure is then repeated for the other ear, starting with air conduction (AC) and, if necessary, followed by bone conduction (BC) testing. Test frequencies for BC are those in the range 500–4000Hz, with no retest at 1000Hz required.

Bracketing Technique

The bracketing technique, also known as the 10 dB down, 5 dB up method, involves delivering each test signal at an audible level, then reducing the volume in 10 dB steps until the signal becomes inaudible. The volume is then increased in 5 dB increments until the patient can perceive the sound again.

Pure Tone Audiogram is a graphic representation of hearing detection thresholds in each ear.

MASKING. Masking is done to produce inaudibility of one sound by the presentation of another sound. During masking, one ear is kept busy by a narrow band noise when the other is being tested. Masking of the non-test ear is done in all bone conduction tests because BC signals are transmitted to both cochleae, regardless of the side on which the transducer is placed.  However, for air conduction tests,  a continuous masking noise (or masker) is required only when the interaural hearing difference (AB gap) exceeds 40 dB or more (when using supra or circum-aural headphones), or 55 dB or more (when using insert earphones). This is done to avoid transcranial transmission/ interaural attenuation/ cross-hearing and accurately measure hearing in each ear.

Interpretation of PTA Results. The results of PTA can indicate different types of hearing loss:

• • Conductive Hearing Loss: Identified when there is a difference of more than 10 dB between air conduction and bone conduction thresholds at any frequency.
  • Sensorineural Hearing Loss: Indicated when bone conduction thresholds are raised above 20 dB HL.
  • Mixed Hearing Loss: Occurs when bone conduction thresholds are raised above 20 dB HL, accompanied by a significant air-bone gap (ABG).

Utilization of PTA. PTA serves multiple purposes in audiology:

• • Determining Hearing Loss: It helps identify the type, degree, and configuration of hearing loss. The air conduction pure tone audiogram primarily measures the degree of hearing loss, while the bone conduction audiogram differentiates between conductive and sensorineural hearing loss.
  • Medicolegal Assessment: PTA can confirm the degree of hearing impairment for legal and insurance purposes.
  • Hearing Aid Prescription: The audiogram is crucial for accurately prescribing hearing aids.
  • Speech Reception Threshold: PTA also assists in determining the speech reception threshold of the subject.

Disadvantages of PTA. Despite its advantages, PTA has some limitations:

• • Subjectivity: The test results are based on the patient’s responses, which can introduce variability.
  • Potential for Malingering: Results may be inaccurate if the patient is feigning hearing loss.
  • Age Considerations: The test may yield unreliable results in children under the age of seven, as it depends on cognitive development and cooperation.

2. Speech audiometry. (Assessing Hearing Ability and Speech Comprehension). Speech audiometry is an essential tool for evaluating a patient’s hearing ability and understanding of spoken words. The results of this test are plotted on a graph.  The test is also useful for detecting retrocochlear pathology and non-organic hearing loss.

1. Speech Reception Threshold (SRT):
   • Also known as the Speech-awareness threshold (SAT), SRT is the lowest speech intensity at which a patient can correctly repeat or detect 50% of the words.
   • The test involves delivering recorded tapes or monitored voice of spondee words (two-syllable words with equal stress on each syllable, such as “pancake,” “hardware,” “playground”) through headphones or speakers in a soundproof room.
   • The intensity of these words is varied in 5 dB steps until the patient correctly identifies half of them.
   • For a normal adult, the SRT is within 10 dB of the pure tone average (500, 1000, and 2000 Hz). An SRT that is better than the pure tone average by more than 10 dB suggests functional hearing loss or an unreliable pure tone audiogram.

2. Speech Discrimination Score (SDS):

• Also known as the speech recognition or word recognition score, SDS measures the patient’s ability to understand speech.
• A list of 25 to 50 phonetically balanced words (single-syllable words such as “pin,” “sin,” “day,” “bus”) is presented to each ear separately at 30–40 dB above the patient’s SRT using pre-recorded material.
• The percentage of words correctly identified by the patient is recorded.
• In normal hearing and conductive hearing loss, a high score of 90–100% is typical.

Table : Ability to understand speech and its relation to speech discrimination (SD) score.  A list of 50 PB words is presented and the number correctly heard is multiplied by 2 the number

SD score	Ability to understand speech
90–100%	Normal (Excellent)
76–88%	Slight difficulty (Good)
60–74%	Moderate difficulty (Fair)
40–58%	Poor
<40%	Very poor

Interpretation of Speech Discrimination Scores

• Conductive Hearing Loss: Patients typically show improved recognition scores when the speech signal intensity is increased.
• Sensorineural Hearing Loss: Patients usually do not show improved recognition scores with increased intensity because louder sounds can distort the speech signal.
• Retrocochlear Pathology: Patients may exhibit a reduction in recognition scores with increased intensity, known as the “rollover” effect, which suggests a lesion in the eighth cranial nerve. In this scenario, as speech intensity increases beyond a certain level, the word recognition score declines rather than stabilizing as it does in cochlear sensorineural hearing loss.

Performance Intensity Function for PB Words

• PB Max: It is beneficial to plot PB scores against various speech intensity levels to determine the maximum score (PB max) a person can achieve, rather than using a single suprathreshold intensity of 30–40 dB above the SRT. The intensity at which PB max is achieved is noted, and the maximum volume of a hearing aid should be set below this level.

Benefits of speech audiometry :

• To measure the speech reception threshold which determines actual disability & not hearing impairment.
• To differentiate organic from nonorganic(functional) hearing loss.
• To find the intensity at which a hearing aid/cochlear implants fits and assessing rehabilitation outcome.
• To differentiate a cochlear from a retrocochlear sensorineural hearing loss.

3. Bekesy Audiometry. Bekesy audiometry is a self-recording audiometric technique based on sweep and fixed frequency testing. In this method, various pure tone frequencies automatically shift from low to high, while the patient controls the intensity using a button. Two tracings are obtained: one with a continuous tone and the other with a pulsed tone. These tracings help differentiate between cochlear and retrocochlear hearing loss as well as between organic and functional hearing loss. However, Bekesy audiometry is seldom performed in contemporary practice.

The types of tracings obtained in Bekesy audiometry include:

• Type I: Continuous and pulsed tracings overlap, indicating normal hearing or conductive hearing loss.
• Type II: Continuous and pulsed tracings overlap up to 1000 Hz, after which the continuous tracing falls. This pattern is seen in cochlear loss.
• Type III: The continuous tracing falls below the pulsed tracing at 100–500 Hz, sometimes up to 40–50 dB. This pattern is indicative of a retrocochlear or neural lesion.
• Type IV: The continuous tracing falls below the pulsed tracing at frequencies up to 1000 Hz by more than 25 dB. This pattern is also seen in retrocochlear or neural lesions.
• Type V: The continuous tracing is above the pulsed tracing, which is characteristic of nonorganic hearing loss.

4. Impedance Audiometry. 

Impedance audiometry is an objective test used to assess middle ear function and mobility of the tympanic membrane. The word ‘impedance’ means resistance to the flow of acoustic energy, expressed in ohms. and compliance is expressed in cubic centimetre of air. It provides valuable otological and neurological information about the nature and site of a lesion. The middle ear functions as the ‘impedance matching device’. Any pathology in the middle ear may cause impedance mismatching and vary the amount of sound reflected back from the tympanic membrane leading to conductive deafness. Impedance audiometry measures the efficiency of the middle ear to perform this function.

Impedance audiometer consists of a handheld probe which is hermetically sealed into the external auditory meatus forming a leak-free seal from the probe tip to the eardrum and has three channels: (i) first channel delivers a tone of 220 or 226 Hz, (ii) the second channel allows the reflected sound from the tympanic membrane to pass through a microphone amplifier assembly for processing (iii) the third channel is attached to air- pump-manometer system to bring about changes in air pressure in the ear canal from positive to normal and then negative(+300 to -600 mm of water pressure).

(i) TYMPANOMETRY. (one of the acoustic immittance tests) is the measurement of the change of impedance in the middle ear due to varying air pressure in the sealed external auditory meatus. A non-compliant/ stiff tympanic membrane tends to reflect back more sound energy as compared to a compliant TM. This reflected sound pressure is measured at the tip of the probe and charted in the graphical form of with impedance of the middle ear/compliance of TM on Y-axis and the air pressure on X-axis, called the tympanogram.

Impedance is the opposition/ resistance to the flow of energy, admittance is the ease of movement with which the flow of energy occurs. For ease of measurement most clinical impedance audiometers measure the acoustic admittance. Admittance is reciprocal of impedance just like compliance is the reciprocal of stiffness.

Procedure:

Types of tympanogram: 

Type A	Normal tympanogram (the peak is near zero pressure)
Type As	Compliance is lower at or near ambient air (zero) pressure. Hypo-mobile admittance is observed in otosclerosis, tympanosclerosis or malleus fixation.
Type Ad	High compliance at or near ambient pressure. Hypermobile admittance is observed in discontinuity of ossicular chain or thin, flaccid tympanic membrane.
Type B	A flat or dome-shaped graph. No change is seen in compliance with changes in pressure. This curve is seen in middle ear fluid, adhesive otitis media, thick or perforated TM, or grommet in situ.
Type C	Maximum compliance occurs with negative pressure in excess of 100 mm H2O. Seen in retracted tympanic membrane, fluid in middle ear.
Type D	A notched graph, seen in scarred and flaccid TM
Type E	An undulating graph, seen when 8000 Hz sound is presented . This is seen in thick grafts after myringoplasty or in any mass.

(ii) EUSTACHIAN TUBE (ET) FUNCTION TEST. Tympanometry can be used to assess eustachian tube function and confirm the integrity of the auditory system. Eustachian tube dysfunction plays a dominant role in the pathogenesis of suppurative and non-suppurative otitis media. The prognosis and treatment of both types of otitis media are dependent upon the eustachian tube function. Tests which can be done are: (a) William’s test (b) Toynbee’s test (c) Acoustic immittance.

(a) William’s test. is done in an intact tympanic membrane. The impedance audiometer is programmed to measure the middle ear pressure in three conditions (i) the middle ear pressure at the beginning of the test (resting pressure), (ii) after swallowing (with the nose pinched and mouth closed) and (iii) after performing Valsalva. 

Inference:

• Normal ET function: Middle ear pressure is same as atmospheric pressure at rest, during swallowing or Valsalva.
• Impaired ET function: Pressure becomes negative during swallowing. It does not become positive on Valsalva or vice versa.
• Grossly impaired ET function. Pressure does not change at all in either of the situations.

(b) Toynbee’s test. is done in patients having a perforation in the tympanic membrane or grommet-in -situ. The test is similar to William’s test where negative or positive pressure (−250 or +250 mm H2O) is created in the middle ear and the person is asked to swallow five times in 20 seconds. The test is carried out for a fixed duration of time, e.g., 40 seconds (minimum) or 160 seconds (maximum). The ability to equilibrate the pressure with every act of swallowing indicates whether the patient is having normal tubal function or not.

(c) Acoustic immittance can also measure the physical volume of air between the probe tip and the tympanic membrane. Normally it is up to 1.0 mL in children and 2 mL in adults. Any increase in volume >2 mL in children and >2.5 mL in adults indicate perforation of the tympanic membrane (because middle ear volume is added up to the volume of the external ear canal). This has also been used to find patency of the ventilation tube.

(iii) ACOUSTIC REFLEX.  is based on the fact that a loud sound 70–100 dB above hearing threshold of one ear, causes bilateral contraction of the stapedius muscle (which pulls the stapes slightly outward and upward) & and tensor tympani muscle (pulls the tympanic membrane slightly inward). The effect is more pronounced on the stapedius muscle than on tensor tympani. Stapedial reflex threshold (SRT): Minimal intensity of sound that produces stapedial reflex. (bilateral reflex)

Reflex arc:    

Ipsilateral ear: CN VIII → ventral cochlear nucleus → CN VII nucleus → ipsilateral stapedius muscle.

Contralateral ear: CN VIII → ventral cochlear nucleus → contralateral medial superior olivary nucleus → contralateral CN VII nucleus → contralateral stapedius muscle

Uses of acoustic reflex: It is very simple to perform, requires a few minutes, is a non-invasive objective test (does not depend upon subjective responses from the patient) 

Acoustic reflex tests help the otolaryngologist/neurologist : 

• To test the hearing in infants, young children and uncooperative patients. 
• To assess middle ear function accurately.
• To differentiate cochlear and retro-cochlear pathology. In cochlear lesion, the stapedial reflex is present at lower intensities, e.g. 40–60 dB than the usual 70 dB(recruitment phenomenon)
• To detect brain-stem pathologies: If ipsilateral reflex is present but the contralateral reflex is absent, the lesion is in the area of crossed pathways in the brainstem.
• To detection non-organic hearing loss: A malingerer does not give any response on pure tone audiometry but shows a positive stapedial reflex.
• To identify the level of lesion in facial nerve paralysis: Absence of stapedial reflex when hearing is normal indicates a lesion of the facial nerve, proximal to the nerve to stapedius. The reflex can also be used to find prognosis of facial paralysis as the appearance of reflex after it was absent, indicates return of function and a favourable prognosis. 
• To detect VIIIth nerve lesion: If a sustained tone of 500 or 1000 Hz, delivered 10 dB above acoustic reflex threshold, for a period of 10 s, brings the reflex amplitude to 50%, it shows abnormal adaptation and is indicative of VIIIth nerve lesion (stapedial reflex decay). 

SPECIAL TESTS OF HEARING 

1. Recruitment
2. SISI Test
3. Threshold Tone Decay Test
4. Auditory Evoked Potential
5. Auditory Steady State Response (ASSR)
6. Otoacoustic emmisions (OAE’s)
7. Central Tests

1. Recruitment

Recruitment is a phenomenon where a sound of a particular intensity becomes abnormally loud and intolerable to a patient. Patients with recruitment experience an inability to hear low-intensity sounds in the affected ear, but high-intensity sounds are perceived as equally loud or even louder than in a normal hearing ear. Recruitment is commonly seen in patients with cochlear dysfunction, making them poor candidates for hearing aids.

The Alternate Binaural Loudness Balance (ABLB) test is used to detect recruitment in unilateral cases. In this test, a tone, such as 1000 Hz, is played alternately to the normal and affected ear, with the intensity in the affected ear adjusted to match the loudness in the normal ear. The test begins at 20 dB above the threshold of the deaf ear and is repeated at every 20 dB increase until the loudness is matched or the limits of the audiometer are reached. In cases of conductive and neural deafness, the initial difference is maintained throughout, while in cochlear lesions, partial, complete, or over-recruitment may be observed.

2. Short Increment Sensitivity Index (SISI)

The Short Increment Sensitivity Index (SISI) test assesses a patient’s ability to detect small changes in the intensity of pure tones. Patients with cochlear lesions can better appreciate these small changes compared to normal subjects or those with conductive or retro-cochlear dysfunction. Thus, the SISI test is used to differentiate cochlear from retro-cochlear lesions.

Procedure: A continuous, supra-threshold tone (20 dB above the hearing threshold) is presented to the patient for 2 minutes. The tone is then increased by 1 dB every 5 seconds, with 20 such increments. The patient is instructed to count the number of increments they hear.

Interpretation:

• • Conductive deafness: SISI score < 15%
  • Nerve deafness: SISI score 0–20%
  • Cochlear deafness: SISI score 70–100%

Disadvantages: Patients with severe deafness (above 85 dB) cannot be tested with most clinical audiometers, and the test requires active cooperation from the patient.

3. Threshold Tone Decay Test

The Threshold Tone Decay Test is used to detect ‘decibels of decay’ and measure auditory nerve fatigue. Normally, a tone slightly above the absolute hearing threshold can be heard continuously for 60 seconds. However, a patient with nerve fatigue will stop hearing the tone before 60 seconds elapse.

Procedure: A tone at a frequency of 4000 Hz is presented for 60 seconds at an intensity 5 dB above the patient’s hearing threshold. If the patient stops hearing the tone before 60 seconds, the intensity is increased by another 5 dB, and the process is repeated until the patient hears the tone continuously for 60 seconds, or no level exists above the threshold where the tone is audible for the full duration. The resultant measure indicates the decibels of decay.

Interpretation: A decay between 15 and 20 decibels indicates cochlear hearing loss, while a decay of more than 25 dB is diagnostic of a retro-cochlear lesion.

4.  Auditory Evoked Potentials. It records the potentials from the auditory pathways in response to a brief auditory stimulation using special equipment with an averaging computer. Though several components of evoked electric response have been reported only two have gained clinical acceptance. 

(i) Electrocochleography (EcoG): It measures the summated intracellular potentials of the outer hair cells seen in response to an auditory stimulus. The electrical response is either due to cochlear microphonic potentials(electrical activity in the cochlea), summating potentials(a complex measurement of many electrophysiological parameters taken together) or electrical activity due to action potential of 8th nerve. It is an objective test that provides a reasonably accurate measurement of the hearing threshold between 1000 to 8000 Hz. No masking in the contralateral ear is required.

Procedure: The electrical activity can be detected by inserting a needle-like thin, active recording electrode through the tympanic membrane and placing onto the promontory or round window. The reference and ground electrodes are placed on the mastoid process and on the forehead respectively. It is usually done under sedation or local anaesthesia.

Uses of EcoG:

• To diagnose Meniere’s disease.
• To detect hearing threshold in the young infants and children within 5–10 dB
• To differentiate cochlear lesions from eighth nerve lesions. 
• To monitor cochlea and cochlear nerve during neurotological surgical procedure.

(ii) Brainstem auditory evoked response (BAER) or BERA (brainstem evoked response audiometry). It is an objective, non-invasive test to assess the structural integrity and functional status of the auditory pathway from the spiral ganglia to the level of the lateral lemniscus in the midbrain. The test was introduced by Jewitt & Williston.

Principal : The series of electrical potentials generated by the activation of different parts of the auditory system are recorded by placing electrodes on the scalp. But since these cortical potentials are small and buried in the background of spontaneous electrical activity (EEG waves), an amplifier is required to study the waves in detail.

Procedure :

• The test is conducted in a sound-proof room. 
• The patient is asked to be in a supine posture with eyes closed, relaxed and preferably asleep(to reduce myogenic potentials). However, accurate assessment in children may require sedation.
• Test one ear at a time. 
• Three surface electrodes are used: 
• Active electrode kept on the vertex (best location) and if not possible may be placed on the lop of the forehead just below the hairline.
• Reference electrode kept on ear lobe/mastoid of the tested ear.
• Ground electrode kept on ear lobe/mastoid of the opposite ear. 
• The electrical potentials generated in response to a series of 1000-1200 clicks (given at a rate of 5-50/sec) at an intensity of 50 to 60dB above the average pure tone hearing level are picked up from the vertex by the active electrode and is plotted graphically. 
• These neurogenic potentials elicited are recorded for the first 10 milliseconds. This is the time taken for the electrical responses to be carried to the brainstem alone
• In a normal person, seven waves are produced in the first 10-15 milliseconds. The first, third and fifth waves are most stable and are used in measurements. 

Characteristics of waves:

• Latency  – absolute, inter-wave (usually between wave i and v)  and interaural.
• Absolute latency is the time interval (in millisec) between the onset of the stimulus and the peak of the wave. The absolute latency of the wave V is most important, as it is common, easily identifiable and is least affected when intensity increases.
• lnterwave latency is the time interval between two different waves in the same ear and in the same BERA tracing. 
• Interaural latency is the difference in the time interval of the same wave between the two ears e.g. the latency of wave IV of left ear and right ear 

Upper limits of normal values 

• Latency of wave V – 5.9 m.s. 
• I – V interval – 4.4 m.s. 
• I – V interaural difference – 0.29 m.s. 

Wave I	Distal part of CN VIII
Wave II	Proximal part of CN VIII near the brainstem
Wave III	Cochlear nucleus
Wave IV	Superior olivary complex
Wave V	Lateral lemniscus
Waves VI and VII	Inferior colliculus

Remember as mnemonic EE COLI (eight, eight, cochlear nucleus, olivary complex, lateral lemniscus, inferior colliculus) compare E COLI-MA in pathways of hearing.

Uses:

• Objective hearing assessment test in infants, young children, non-cooperative adults, in malingerers, comatose and unconscious patients.
• To diagnose the site of lesion in retro cochlear pathologies particularly acoustic neuroma.
• To diagnose brainstem pathology, e.g. multiple sclerosis or pontine tumours.
• To monitor  and preserve the auditory nerve intraoperatively during surgery of acoustic neuromas 

Disadvantages :

• There is no standardisation at present for BERA. 
• Wave V is not recorded if the hearing level is 75 dB at 3 kHz. 
• Normally, the latency of wave V increases in old age, conductive hearing loss and pure sensorineural hearing loss. 
• Wave I is not easily identifiable in BERA. 

Limitations :

• ABR testing is highly sensitive(90 %) for detecting large tumours; however, small tumours (<1 cm) may be missed. MRI is a more sensitive and specific test for an acoustic neuroma.
• ABR waves are usually absent when a patient has a severe or profound hearing loss. 
• A conductive hearing loss also attenuates cochlear stimulation and increases ABR wave latencies. 
• It does not give frequency-specific information. Low-frequency hearing losses are undetectable by the BERA. Hence additionally ASSR is needed.
• The test is practically insensitive for hearing losses above 75·dB.
• This test can only be done on awake, conscious and cooperative adult patients. It can be done in infants and young children under sedation as the latter has no effect on BERA.

5. Auditory Steady State Response (ASSR)

It is an electrophysiological test to predict the frequency-specific hearing thresholds. The steady-state pure tone signals(modulated in amplitude and frequency) are used in ASSR unlike, the transient signals of tone bursts or clicks used in ABR producing a frequency-specific audiogram. 

Uses : 

• Hearing loss of more than 80 dB can be detected. It can help in the early detection of the children who need cochlear implantation.
• ASSR can be done in all ages, any mental state, any degree of hearing loss. 
• Multiple frequencies can be assessed at the same time, as long as their carrier frequencies utilize different modulation rates.
• ASSR is an objective test that can be analysed and interpreted easily using statistical methods.

6. Otoacoustic Emissions (OAEs). An ever-evolving, fast, easy to obtain, minimally-invasive screening test, used for screening for hearing loss in early life. OAE tests play an important role in monitoring ototoxicity and noise-induced hearing loss (NIHL) because OAEs can detect outer hair cell dysfunction earlier than a pure tone audiogram can.

Pathophysiology: OAE are the sounds of low intensity produced by the outer hair cells of a normal cochlea and recorded from the external auditory canal with the help of a sensitive microphone placed in the external ear canal. The sound produced by the outer hair cells travels in a reverse direction: outer hair cells → basilar membrane → perilymph → oval window → ossicles → tympanic membrane → ear canal. 

OAEs are present in healthy outer hair cells, thus helps to test the functional status of the cochlea. OAE’s are absent in 50% of normal individuals, lesions of the cochlea, middle ear disorders (as sound travelling in the reverse direction cannot be picked up) and when the hearing loss exceeds 30 dB. 

Types of OAEs:

(i) Spontaneous OAE’s are seen in the persons with normal hearing or when hearing loss < 30 dB. 

(ii) Evoked OAE’s. Depending on the sound stimulus used, evoked OAE may be:

• Transient evoked OAEs (TEOAEs): A series of click stimuli are presented at 80–85 dB SPL (sound pressure level) and the response is recorded. TEOAEs can be recorded from 500–4000Hz but are more sensitive for hearing loss at 500 and 1000 Hz.
• Distortion product OAEs (DPOAEs): Two continuous tones of moderate-intensity, e.g. 55 and 65dB SPL, are presented to the cochlea at the same time to produce distortion. They are used to test hearing in the range of 1000–8000 Hz but have a better clinical performance for hearing loss at 4000 Hz. 

Uses of OAE’s

• OAEs is used to distinguish between deafness caused by cochlear and retro-cochlear lesions. OAEs are absent in cochlear lesions, e.g. ototoxic sensorineural hearing loss. 
• DPOAEs may be used to monitor ototoxicity effects earlier than pure tone audiometry.
• It may also be used in non-cooperative or mentally unwell patients, in non-organic hearing loss after giving sedation. Sedation does not affect OAEs.

Disadvantages:

• It is advisable to do tympanometry with OAEs because the middle ear pathology may interfere with the recording of OAEs. There are chances that the clinicians may misinterpret the absence of OAEs as a sensorineural hearing loss if they are unaware of the existence of middle ear pathology.
• OAEs may not diagnose the auditory neuropathy in the neonates. Auditory neuropathy is a hearing disorder characterized by abnormal or absent auditory brainstem response in the presence of normal outer hair cell function. Therefore, AABR is the best method in the NICU/SCBU population to detect auditory neuropathy.

OAE test results are affected by the ambient noise level and the patient’s internal noise level (e.g. breathing or body movements. Therefore, the results of OAE tests should be interpreted in conjunction with pure tone audiometry, tympanometry etc. 

7. Central Auditory Tests, Auditory processing disorder, Central deafness

The patients with the central auditory pathology have difficulty in hearing a distorted, unclear speech and also when there is some background noise like in a party. They have normal pure-tone hearing thresholds and other hearing tests but cannot follow instructions. These problems are due to poor processing of the auditory cues in the higher auditory pathways.

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Types of speech discrimination tests :

• A message is presented in a distorted speech. Patients with cortical lesions will face difficulty in understanding the message.
• Two different speech messages (a pair of spondaic words along with digits or nonsense words) are presented at the same time, one to each ear and the patient is asked to identify both. The patients with temporal lobe lesions will have difficulty identifying these words when presented to the ear opposite to that of the side of the lesion.
• Binaural tests. They are used to identify the integration of information from both ears. Such tests are normal in cortical lesions but affected in lesions of brainstem and thus help to localize the site of the lesion. A most common test used is the binaural masking level difference test. 

——– End of the chapter ——–

Reference Textbooks.

• Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
• Glasscock-Shambaugh, Textbook of Surgery of the Ear.
• Susan Standring, Gray’s Anatomy.
• Frank H. Netter, Atlas of Human Anatomy.
• B.D. Chaurasiya, Human Anatomy.
• P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
• Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
• Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck Surgery
• Hans Behrbohm, Textbook of Ear, Nose, and Throat Diseases With Head and Neck Surgery.
• Salah Mansour, Middle Ear Diseases – Advances in Diagnosis and Management.
• Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
• Rob and smith, Textbook of Operative surgery.
• Anirban Biswas, Textbook of Clinical Audio-vestibulometry.
• Arnold, U. Ganzer, Textbook of  Otorhinolaryngology, Head and Neck Surgery.
• Ganong’s Review of Medical Physiology.
• Guyton & Hall Textbook of Medical Physiology

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