assessment of hearing
TRANSCRIPT
Assessment of Hearing
Dr. Maria Sajjad
AUDITORY SYSTEM
Organ of Corti
o Sense organ of hearing, It is situated on the basilar membrane.
o Important components:• Tunnel of Corti: Formed by inner and outer rods.• Hair cells: Receptor cells of Hearing Transfers sound energy into electrical energy. Inner hair cells are supplied by afferent cochlear
fibres. Outer hair cells receive efferent innervation from
olivary complex.
• Supporting Cells: Dieters` cells: provide support to outer hair
cells. Cells of Hensen: lie outside Dieters` cells.• Tectorial Membrane: Overlies the organ of
corti. Shearing force between the hair cells and tectorial membrane produces the stimulus to hair cells.
Nerve supply of Hair cells
• 95% of Afferent fibres of spiral ganglion supply the inner hair cells, while 5% supply the outer hair cells.
• Efferent fibres to the hair cells come from olivocochlear bundle whose cell bodies are situated in superior olivary complex.
Auditory Pathway
Hair cells
Dendrites of bipolar cells of spiral ganglion
Axons of bipolar cells form cochlear division of CN VIII
Cochlear Nuclei
Superior olivary complex
Lateral lemniscus
Nucleus of lateral lemniscus
Inferior colliculus
Medial geniculate body
Auditory radiations
Auditory cortex (area 41)
Mechanism of Hearing
• Mechanical conduction of sound Pinna EAC Tympanic Memberane• Chain of Ossicles Stapes Foot Plate Pressure changes in inner ear fluid Movement of basilar membrane Stimulation of hair cells.
• Tranduction of mechanical energy to electrical impulses:
Movement of basilar membrane sets up shearing force between tectorial membrane and hair cells.
Distortion of hair cells gives rise to cochlear microphonics which trigger the nerve impulse.
• Neural Pathways.
Frequently used terms
• Frequency: Number of cycles per second• Pure tone: Single frequency sound• Complex sound: Sound with more than one
frequency.• Decibel (dB): Unit to determine intensity of
sound. It is 1/10th of a bel, and named after Graham Bell.
• Masking: Phenomenon to produce inaudibility of one sound by the presentation of the other.
Types of Hearing Loss
• Conductive hearing loss: caused by interference of sound conduction from external ear to stapediovestibular joint.
• Sensorineural hearing loss: results from either lesion of cochlea (sensory type) or from lesion of VIIIth nerve and its connections (neural type). Neural can be further divide as:
Retrocochlear: lesion of VIIIth nerve, Central: Lesion of central auditory connections
• Mixed Hearing loss: Elements of both conductive and
sensorineural deafness are present in the same ear.
• NOHL: No organic lesion present. It is either
psychogenic or due to malingering (usually involved in litigation)
Assessment of Hearing
Clinical tests:
Tuning Fork tests: • Performed with tuning forks of different
frequencies (128,256,512,1024,2048,4096)• Most Commonly used tuning fork has a
frequency of 512Hz.• AC measures both the conducting mechanism
and cochlea.• BC measures cochlear function only.
Tuning forks
Rinne Test
• AC is compared with BC• Vibrating tuning fork is placed on patient`s
mastoid and when he stops hearing, it is placed erect and in line with the EAC, about 2cm from the orifice.
• If he still hears AC is more than BC.
• It is called positive when AC is longer and louder than BC. It is seen in normal persons or those having sensorineural deafness.
• Negative Rinne ( BC > AC) is seen in conductive deafness.
• False negative Rinne test: • patient has no hearing in test ear.• BC stimulus may be perceived by contralateral
ear.• No hearing by AC the result is labeled as Rinne
negative. • This mistaken impression of function in a non
functioning ear is called false negative Rinne test.
Weber Test
• Tuning fork is struck and base placed on either the forehead, vertex or upper incisor teeth.
• Patient is asked where the sound is heard the loudest.
• Normal hearing person: Sound is heard equally in both ears.
• Conductive deafness: Lateralized to worse ear.• SN deafness: Lateralized to better ear.• Lateralized weber in conductive deafness may
indicate a hearing loss of 10-15 dB.
Modified Schawabach Test(ABC test)
• Compares the bone conduction of patient with that of a normal hearing person(examiner).
• Tuning fork is placed on the patients mastoid with the meatus blocked and when the patient no longer hears it, it is placed on the normal hearing person`s mastoid.
• Conductive deafness + Normal hearing person: Patient and examiner hear the fork for the same duration.
• SN deafness: Patient hears the fork for a shorter duration.
Bing Test
• Test of BC, and examines the effect of occlusion of ear canal on hearing.
• Vibrating tuning fork is placed on the mastoid while the examiner alternately closes and opens the ear canal by pressing tragus inward.
• Normal person+ SN hearing loss: Hears louder when ear canal is occluded and softer when canal is open. (Bing +)
• Conductive hearing loss: No change appreciated ( Bing -).
Gelle`s test
• Test of BC and examines the effect of increased air pressure in ear canal on hearing.
• Vibrating tuning fork is placed on the mastoid while air pressure in ear canal is changed by Siegle`s speculum.
• Normal person + SN hearing loss: Hearing is decreased ( Gelle`s +).
• Conductive deafness: No change (as ossicular chain is fixed or disconnected) (Gelle`s -).
Audiometric Tests
Pure Tone Audiometry
PTA
• An audiometer is an electronic device which produces pure tones, intensity of which can be increased or decreased in 5 dB steps.
• AC thresholds are measured for tones of 125,250,500,1K,2K,4K,8K Hz.
• BC thresholds are measured for tones of 250,500,1K,2K,4K Hz.
• It is charted in the form of a graph called audiogram.
Precautions
• Patient should be visible to the tester, and respond by signaling.
• No visible or tactile clues should be available to the patient regarding the hearing stimulus.
• Test should be conducted in a sound proof room.
• Duration of presentation should be 1-3 sec.
• The head phones should be properly seated over the external auditory canal.
• The audiometer should be properly checked before performing the test.
• Before placing the ear phones on the patient, the patient's ear should be examined for the presence of wax. If present it should be cleaned.
Procedure
• Pure tone air conduction threshold is tested using head phones.
• “Up 5-down 10" method of threshold estimation: This technique is based on Hughson - Westlake ascending technique.
• Tones of short duration is used for threshold estimation. This method of threshold estimation involves the following steps:
• Step I : The better ear is tested first in order to determine the need for masking.
• Step II : Start with a 1000 Hz tone at a level above the threshold to allow easy identification of the tone. If the patient is suspected to be having a profound hearing loss then the testing should be started with 250Hz frequency. This is because of the fact that the individuals with profound hearing loss often have testable hearing only in the low frequency range.
• Step III : The patient's understanding of the listening task should be checked by using both short and long duration test tones. The patient should be instructed to raise the index finger as soon as the sound is heard.
Step IV : During testing, the examiner should vary the interval between tone presentations to avoid telegraphing the stimulus.
• Step V : The starting intensity of the test tone is reduced in 10 dB steps following each positive response, until a hearing threshold level is reached at which the subject fails to respond. Then, the tone is raised by 5 dB until tone is heard.The threshold is defined as the faintest tone that can be heard 50% or more of the time, and is established after several threshold crossings.
• Second ear is then test in the same way with the retest beginning at 1K Hz.
• If there is difference in AC threshold exceeding 40dB at any frequency masking should be used.
• Bone Conduction level are measured between 250Hz and 4K Hz.
• BC is tested using a bone vibrator.• The auditory threshold is assessed as
described for air conduction assessment. The only difference is that the better hearing ear should be masked using a masking tone delivered via a head phone.
Scale of hearing threshold estimation in audiometry:
dB loss Description
- 10 to 15 Normal limits
16 - 19 Mild hearing loss
30 - 44 Moderate hearing loss
45 - 59 Moderately severe hearing loss
60 - 79 Severe hearing loss
80 and above Profound hearing loss
Nomenclature on the pure tone audiogram
• O right air conduction• X left air conduction• [ masked right bone conduction• ] masked left bone conduction
Conductive hearing loss, low tone
Otitis media with effusion:A conductive loss maximal in the low frequencies with now sensorineural element.
Conductive hearing loss
Conductive hearing loss across the audiometric range with an increase in the BC threshold (caharts`notch) at 2kHZ.Caharts` notch reflects a loss in the inertial component of stapes footplate.
Sensorineural hearing loss, low tone (meinere’s disease)
Sensorineural hearing loss with a notch at 6 kHz (noise induced hearing loss)
Speech Audiometry
• Patient`s ability to hear and understand speech is measured.
• Two parameters are studied:a. Speech Reception thresholdb. Discrimination score
Speech Reception Threshold(SRT)
• Minimum Intensity at which 50% words are repeated correctly by the patient.
• Set of spondee words is delivered to each ear.• Intensity is varied in 5dB steps till half of them are
correctly heard.• Normally SRT is within 10dB of the average of
pure tone threshold of 3 speech freqeuncies(500, 1K, 2K Hz).
• SRT better than pure tone average by more than 10db suggests a functional hearing loss.
Discrimination score
• Measure of patient`s ability to understand speech.
• List of phonetically balanced (PB) words (single syllable) is delivered to each ear separately at 30 to 40 dB above his SRT and % of words correctly heard is recorded.
• In normal persons and with CHL score of 90 to 100% can be obtained.
Bekesy Audiometry
• Self recording audiometry where various pure tone frequencies automatically move from low to high while patient controls intensity through a button.
• Two tracings one with continuous and other with pulsed tone are obtained.
• They help to differentiate a cochlear from retrocochlear and organic from functional hearing loss.
• It is rarely done these days.
Impedence audiometry
• The primary purpose of impedence audiometry is to determine the status of the tympanic membrane and the middle ear.
• It is divided into 2 parts:a. Tympanometryb. Acoustic reflex measurements.
Tympanometry
• Altering the pressure in the EAC results in changes in compliance because the TM is tense and ossicular chain stiffened.
• Point of maximum compliance occurs when the pressure in the meatus is equal to that in the middle ear.
• If the pressure exceeds or falls below the middle ear pressure, compliance will be reduced.
• Indirectly middle ear pressure can be calculated from point of maximum complaince.
Procedure• Probe is inserted into the external auditory canal till a air
tight seal is obtained. Probe tone is presented typically at 226Hz into the ear canal while the air pressure of the canal is altered between +200 and - 400 decapascals. The maximum compliance occurs when the pressure of the external auditory canal and the middle ear becomes equal.
• The compliance peak indicates the pressure of the middle ear.
• The height of the compliance peak indicates the mobility / stiffness of the tympanic membrane or the middle ear cavity.
• By charting the compliance of tympanoossicular system various pressure changes different types of tympanograms are obtained.
Types of tympanograms:
• Type A: Normal Tympanogram• Type As: Reduced compliance at or near
ambient air pressure, seen in fixation of ossicles.
• Type Ad: Increased compliance at or near ambient pressure seen in ossicular discontinuity or thin or lax TM.
• Type B: Flat or dome shaped graph. No change in compliance with pressure changes. Seen in middle ear fluid or thick TM.
• Type C: Maximum compliance at pressures more than -100mmH2O (negative pressure in middle ear).
Acoustic(Stapedial) reflex Measurements
• A loud sound, 70-100dB above the threshold of hearing of a particular ear causes bilateral contraction of the stapedial muscle which can be detected by Tympanometry.
• Reflex arc is the VIIIth nerve, cochlear nucleus and complex brainstem internuclear connections to the ipsilateral and contralateral facial nuclei, facial nerve and the nerve to stapedius.
• Minimal auditory stimulus that produces a contraction of stapedius muscle is known as acoustic reflex threshold and indicated on an audiogram by the letter ‘Z’.
Physical volume of ear canal
• Acoustic immitance can measure the phsyical volume of air between probe tip and TM.
• Normally it is upto 1ml in children and 2ml in adults.
• If > 2ml in children and > 2.5ml in adults, indicates perforation of TM.
Special Tests
Short Increment Sensitivity Index (SISI Test)
• The SISI test is still widely used to determine whether the patient is having cochlear pathology. This test is based on a phenomenon known as recruitment (abnormal loudness growth).
• Recruitment: The ear which does not hear low intensity sounds begins to hear greater intensity sounds as loud or even louder than normal hearing ear.
• A continous tone is presented 20dB above the threshold and sustained for two minutes.
• Every 5 sec tone is increased by 1dB and 20 such blips are presented. Patient indicates the blips heard.
• In conductive deafness SISI score is more than 15%, 70-100% in cochlear deafness, 0-20% in nerve deafness.
Evoked Response Audiometry
• It measures electrical activity in the auditory pathways in response to auditory stimuli.
• Two most important component of evoked electrical response are:
a. Electrocochleographyb. Auditory brainstem response (ABR).
Electrocochleography
• It measures electrical potential arising in the cochlea and CN VIIIth in response to auditory stimuli within first 5 milliseconds.
• There are three classes of potentials that can be recorded .They are compound action potential of auditory nerve (AP), Summating potential (SP), and cochlear potential (CP) also known as cochlear microphonic.
• Recording electrode is usually a thin needle passed through the TM onto the promontory.
• In Adults it can be done under LA, but in children or anxious individuals sedation or GA is required.
• It is useful:a. To find threshold of hearing in young infants
and children.b. To differentiate lesions of the cochlea from
those of CN VIII.
Auditory brainstem response (ABR)
• Also known as Brain stem evoked response audiometry.
• BERA is resistant to the effects of sleep, sedation, sleep and anesthesia.
• BERA is an objective way of eliciting brain stem potentials in response to audiological click stimuli. These waves are recorded by electrodes placed over the scalp.
• Since the electrodes should be placed over the head, the hair must be oil free.
• The standard electrode configuration for BERA involves placing a non inverting electrode over the vertex of the head, and inverting electrodes placed over the ear lobe or mastoid prominence.
• One more earthing electrode is placed over the forehead.
• In brain stem evoked response audiometry, the impulses are generated by the brain stem.
• These impulses when recorded contains a series of peaks and troughs.
• The positive peaks (vortex positive) are referred to by the Roman numerals I - VII.
• These peaks are considered to originate from the following anatomical sites:
1. Cochlear nerves - waves I and II2. Cochlear nucleus - wave III3. Superior olivary complex - wave IV4. Nulclei of lateral lemniscus - wave V5. Inferior colliculus - waves VI and VII
• Uses of BERA: I. As a screening procedure for infants.II. To determine the threshold of hearing in children and
adults who do not cooperate and in malingerers.III. To diagnose retrocochlear pathology particularly
acoustic neuroma.IV. To diagnose brainstem pathology e.g multiole
sclerosis or pontine tumors.V. To monitor CN VIII intraoperatively in surgery of
acoustic neuroma.
Otoacoustic emissions (OAE)
• Sounds produced by motile elements of cochlear outer hair cells.
• Can be elicited by a very sensitive microphone placed in EAC.
• Sounds produced by outer hair cells travel in a reverse direction: outer hair cells Basilar Memberane perilymph oval window
ossicles TM EAC.
• OAEs are present when outer hair cells are healthy and absent when they are damaged, thus help test function of cochlea.
• Uses:1. To screen children and neonates for hearing
disabilities2. Estimate hearing sensitivity within a limited range
of frequencies3. To differentiate sensory and neural components in
sensorineural hearing loss4. To rule out malingering (functional hearing loss).
Hearing assessment in Infants and Children
Screening Procedures
• They are employed to test hearing in high risk infants and are based on infants behavioral response to sound stimuli.
• Arousal test: High frequency noise is presented for 2 secs to the infant when he is in light sleep. Normal hearing infant can be aroused twice when 3 such stimuli are presented.
• Auditory response cradle: screening device for newborns where baby is placed in a cradle and his behaviour in response to auditory stimulation are monitored by transducers.
Behaviour Observation Audiometry
• Auditory signal presented to an infant produces a change in behaviour e.g alerting, cessation of an activity or widening of eyes.
• Moro`s reflex: sudden movement of limbs and extension of head in response to sound of 80-90 dB.
• Cochleopalpebral reflex: Child responds by a blink to aloud sound.
• Cessation reflex: Infant stops activity or starts crying in response to a sound of 90 dB.
Behaviour Observation Audiometry
Visual Reinforcement Audiometry(VRA)
• This test technique is suited to infants aged 7 or 8 months to 3 years developmentally.
• The child is taught (i.e., conditioned) to turn their head when a sound is heard.
• Initial conditioning is achieved by the introduction of stimuli at moderately high levels
• When child looks for source of sound they are shown a colourful, moving puppet or toy under illumination as a reward.
• This puppet or toy "reinforces" the child's turning behaviour or orientation and gives rise to the term Visual Reinforcement Orientation Audiometry. Once this conditioned response is reliably observed, the stimuli can be presented at ever decreasing levels until auditory threshold or minimum audible levels have been reached.
Visual Reinforcement Audiometry(VRA)
Play Audiometry
• Play audiometry is suited to children aged around 3 to 7 years developmentally.
• The child is taught to respond, using a pre-determined task, whenever they hear tonal stimuli that are introduced through headphones or through a bone conductor placed behind the ear on the mastoid.
• Given sufficient cooperation from the child it is usually possible to produce a complete and accurate "audiogram" that illustrates their threshold of hearing for a pre-determined frequency range.
Play Audiometry
Speech Audiometry
• Child is asked to repeat the names of certain objects or to point them out on the pictures.
• Voice can be gradually lowered.• In this way hearing level and speech
discrimination can be tested.
Objective Tests
• Evoked Response Audiometry:a. Electrocochleography.b. Brainstem Evoked Response Audiometry
(BERA).• Otoacoustic Emmisions.• Impedance Audiometry.
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