assr, vemp, vng, oae

7/23/2019 ASSR, VEMP, VNG, OAE

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Dr. Supreet Singh Nayyar, AFMC 2009

www.nayyarENT.com 1

AUDITORY STEADY STATE RESPONSE (ASSR)

(for more topics & ppts visitwww.nayyarENT.com)

Introduction

A far-field evoked auditory potential testPrinciple

Similarity to ABRo Sound stimulus converted to electrical impulse pathway EE COLI

recording via electrodes on scalp

Difference from ABRo Evoked using continuous rather than transient stimulationo Stimulus is amplitude or frequency modulatedo Place of maximal stimulation on the cochlea is determined by the choice of

carrier frequencyo Continuous, sinusoidal nature of the response lends itself to analysis in

frequency rather than the time domaino ASSR is converted to frequency domain using FAST FOURIER

TRANSFORM (FFT) techniques.o Frequency spectrum is considerably narrower than the frequency spectrum

of the tone bursts typically used to elicit the ABRo Additionally continuous rather than a transient stimulus possible to

achieve higher stimulation easier to distinguish between severe andprofound hearing loss

Procedure

Headphone,computer for analysis Electrodes

o Inverting (-ve) two Ipsilateral mastoid Scalp of neck

o Non inverting (+ve) Vertex

o Ground Forehead

Historical perspective

When the ASSR was originally described, modulation frequencies of approximately

40 Hz were generally used to evoke the response, and relatively strongcorrelations between ASSR thresholds and audiometric thresholds were reported

Initial optimism about the correlation between ASSR thresholds and audiometricthresholds waned, however, when it was noted that these responses wereadversely affected by sleep and sedation

Interest in ASSR was revived when research showed that these problems could be
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avoided if a modulation frequency of approximately 80 Hz rather than 40 Hz wasused

There have been a number of studies that have reported finding good correlationsbetween ASSR thresholds and audiometric thresholds in both children and adults

Uses & advantages

Analysis of the ASSR does not require as much training as is currently needed withABR

For estimating frequency-specific thresholds give results similar to PTA

To record the ASSR in ears with no measurable ABR at the limits of the equipment

Low frequency hearing loss can be recorded

For evaluation of children being considered for cochlear implantation

VEMP(for more topics & ppts visitwww.nayyarENT.com)

Principle : Sudden Changes in Saccular Activity Evoke Changes in Postural Tone

Anatomic and Physiologic Basis of normal saccular function

Saccule lies in a parasagittal plane

Hair cells of the saccule, are polarized so that they are excited by otoconial mass

displacements away from the striola, can sense accelerations up and down Only the sacculus can sense linear accelerations up or down

When the head is upright in the gravitational field, the acceleration resulting from gravity(9.8 m/sec2) constantly pulls the saccular otoconial mass toward the earth

Afferents in the inferior half of the saccule, whose hair cells are excited by this downwardacceleration, have lower firing rates and lower sensitivities to linear accelerations than dothose afferents in the upper half

Afferents in the upper half are excited by relative upward acceleration of the otoconialmass, such as might occur when the head drops suddenly (e.g., when one is falling)

Thus, sudden excitation of hair cells across the saccular macula would likely beinterpreted by the brain as a sudden loss of postural tone (i.e., falling)

The appropriate compensatory reflex would be one that activates the trunk and limbextensor muscles and relaxes the flexors to restore postural tone

Accordingly, the saccular afferents project to the lateral portions of the vestibular nuclei,which give rise to the vestibulospinal tract, in contrast to the utricular afferents, whichproject more rostrally to areas involved in the VOR


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Clinical Importance Saccular excitation underlies the test of VEMP

VEMPs are transient decreases in flexor muscle electromyographic (EMG) activity

evoked by loud acoustic clicks or tones applied to the ear Sufficiently loud sounds applied to the ear excite saccular afferents

The predicted reflexive response would include relaxation of flexor muscles

EMG activity averaged over multiple acoustic stimuli from a tonically contracting flexormuscle will demonstrate a biphasic short-latency relaxation potential

EMG activity can be recorded in many different flexor muscles, but SCM responseshave been best described

Because the saccule is the only end organ that mediates VEMP responses, absence ofVEMP responses may indicate saccular dysfunction

However, transmission of the VEMP acoustic stimulus is very sensitive to any cause ofconductive hearing loss in the middle ear

Interestingly, the preservation of VEMP responses in the face of conductive hearingloss implies an abnormally low acoustic impedance of the labyrinth, such as occurs insuperior canal dehiscence syndrome or with enlarged vestibular aqueduct syndrome

Method An intense click or tone pip is delivered to an earphone, stimulating sensory tissue

(otolith organ) in the saccule

This is interesting since the saccule is part of the balance system and not normallythought of as being sensitive to sound.(work on pigeons was done by pierre fluorence)

Neural impulses travel from saccule up to inferior division of the vestibular nerve(cranial nerve VIII), to the lateral vestibular nucleus, to the lateral vestibulospinal tract,

to the accessory nerve (XI), to the sternocleidomastoid muscle (SCM). To increase sensitivity, the head is turned away from the ear tested (right ear) and

elevated to tense the SCM

SCM contracts producing a large amplitude potential (compared to ABR) with positiveand negative peaks at 13 and 23 ms (P13 and N23)

This pathway is called the vestibulo-collic reflex. Presence of the VEMP indicatesintegrity of the pathway

VEMPs may be abnormal (absent, low amplitude, high or enhanced amplitude, ordelayed latency) in Meniere's disease, superior canal dehiscence, vestibular neuritis,multiple sclerosis, migraine, spinocerebellar degeneration. See table.

If RE and LE represent the VEMP amplitude for the right and left ears, then a 30-47%asymmetry is clinically significant:

Asymmetry = 100*(LE - RE)/(LE + RE)


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VIDEONYSTAGMOGRAPHY (VNG) TESTING


Videonystagmography (VNG) is often used in the evaluation of a patient who presents with vertigo anduses the vestibular-ocular reflex to indirectly measure vestibular function. VNG tests for nystagmus

using infrared light and video technology to monitor eye movements during testing.

VNG testing is considered the new standard for testing inner ear

functions over Electronystagmography (ENG), because VNG

measures the movements of the eyes directly through infrared

cameras, instead of measuring the mastoid muscles around the

eyes with electrodes like the previous ENG version. VNG testing is

more accurate, more consistent, and more comfortable for the

patient. By having the patient more comfortable and relaxed,consistent and accurate test results are more easily achieved.

VNG testing is used to determine if a vestibular (inner ear) disease

may be causing a balance or dizziness problem, and is one of the

only tests available today that can decipher between a unilateral

(one ear) and bilateral (both ears) vestibular loss. VNG testing is a

series of tests designed to document a persons ability to follow

visual objects with their eyes and how well the eyes respond to

information from the vestibular system.

This test also addresses the functionality of each ear and if a

vestibular deficit may be the cause of a dizziness or balanceproblem. To monitor the movements of the eyes, infrared goggles

are placed around the eyes to record eye movements during

testing. VNG testing is non-invasive, and only minor discomfort is

felt by the patients during testing as a result of wearing goggles.

Appointments usually last about 1.5 hours.


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There are 4 main parts to a VNG test:

1. Occular Mobility

You will be asked to have your eyes follow objects that jump from

place to place, stand still, or move smoothly. The technician will belooking for any slowness or inaccuracies in your ability to follow

visual targets. This may indicate a central or neurological problem,

or possibly a problem in the pathway connecting the vestibular

system to the brain.

2. Optokinetic Nystagmus

2. You will be asked to view a large, continuously moving visual

image to see if your eyes can appropriately track these movements.

Like the occular mobility tests, the technician will be looking for any

slowness or inaccuracies in your ability to follow visual targets. This

may indicate a central or neurological problem, or possibly a

problem in the pathway connecting the vestibular system to thebrain.

3. Positional Nystagmus

The technician will move your head and body into various positions

to make sure that there are no inappropriate eye movements

(nystagmus), when your head is in different positions. This test is

looking at your inner ear system and the condition of the

endolymph fluid in your semi-circular canals. The technician is

verifying that small calcium carbonate particles called otoconia are

not suspended in the fluid and causing a disturbance to the flow of

the fluid.

4. Caloric Testing

The technician will stimulate both of your inner ears (one at a time)

with warm and then cold air. They will be monitoring the

movements of your eyes using goggles to make sure that both of

your ears can sense this stimulation. This test will confirm that your

vestibular system for each ear is working and responding to

stimulation. This test in the only test available that can decipher

between a unilateral and bilateral loss.

Optokinetic Testing using

Micromedical Visual Eyes System
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Otoacoustic emissions


Otoacoustic emissions are low energy sounds produced by the cochlea. They are thought to beacoustic byproducts of the outer hair cells, which are thought to underlie the amplification of the basilar

membrane. Clinically, they are most often evoked using transient and distorted product stimulation.

The evoking response causes outer hair cell motility which results in a mechanical wave that travelsfrom the cochlea through the middle ear and tympanic membrane to the ear canal where it is recorded.

Spontaneous emissions are not present from the cochlea when there is a greater than 25dBhearing loss. Unfortunately, they are not present in all normal ears, which does not make this the test of

choice to clinically assess cochlear functioning.

Transient stimuli such as clicks evoke emissions from a large portion of the cochlea. Theemissions are then sampled and signal-averaged to extract them from background noise. These

alternating samples are then stored in one of two memory banks and compared. Reproducibility,expressed as a percentage, is the cross correlation between these two waveforms. A reproducibility

score of 50% or greater indicates that a response is present. Waveforms may vary significantly

between people, but they are highly reproducible within a given individual. When hearing thresholdsare better than 35dB, TEOAEs are generally present. The advantages of TEOAE are that it can

separate normal from abnormal ears at 20-30dB and that it is quick. The specificity of clean, dry ears

of infants is 95%. The main disadvantage is that it fails to extract responses at higher frequencies.

Distorted products are additional tones which are created when two tones, f1/lower frequency &

f2/higher frequency, are presented simultaneously to a healthy cochlea. The most robust DPOAEoccurs at the frequency determined by the equation 2f1-f2. Due to a nonlinear process within the

cochlea, the DPOAE assesses the cochlear integrity of the region near f2. When hearing thresholds are

better than 50dB, DPOAEs are generally present. The main advantage is that DPOAEs can recover

OAEs above 6000Hz.

The transmission properties of the middle ear directly influence the OAE characteristics. Thepresence of a middle ear effusion, as in otitis media, affects both the forward and backward

transmission. Although otitis media often eliminates OAEs, it is possible to record OAEs in some

patients with middle ear effusion. OAE characteristics increase significantly over the first few days of

life likely as a result of changes in the ear canal and middle ear. Small tympanic perforations willimpede the forward transmission. This can usually be overcome with DPOAEs by increasing the

amplitude.


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Central Auditory Processing


There is no accepted definition of Central Auditory Processing (CAP). In its simplest form, it iswhat we do with what we hear. The Task Force on CAP Consensus Development defines CAP as the

auditory system mechanisms and processes responsible for the following behavioral phenomena:

Sound localization

Auditory discrimination

Auditory pattern recognition

Temporal aspects of audition, including temporal resolution and masking

Auditory performance decrements with competing and degraded acoustic signals

Deficiencies in any of these behaviors are considered central auditory processing disorders

(CAPD). Results of CAPD testing have revealed clustering of test results and characteristic behaviors.

These four categories are decoding, tolerance-fading memory, integration, and organization. Each ofthe four categories has been associated with a specific region of the brain. The Buffalo model of

CAPD assessment and management takes into account the classification of CAPD as well as speech

language evaluation and academic characteristics. It is important to understand that there is no one test

that is sensitive enough to detect CAPD, especially in children where the variability of the tests is verywide. Therefore a battery of tests is recommended. In the Buffalo model, the CAP battery always

includes the Staggered Spondaic Word (SSW) test, the Phonemic Synthesis (PS) test, a speech-in-noise(SN) test, and the masking level difference (MLD) test.

Most patients will have weaknesses in more than one category and the categories are notmutually exclusive.

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assr, vemp, vng, oae

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