Download Auditory Brainstem Response Test

Auditory Evoked Potential
(AEP)Testing
Lecture 11
Outline
 Principles/Background
 Purpose/Features
 View video
 Clinical application/limitations
 ABR Interpretations

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Threshold
Differential Diagnosis
 Other Evoked Potentials
Underlying Principles
 Normal auditory system

Transmit acoustic stimuli into an electrical
response
 Stimulus – triggers “Action Potentials”
 Response to the stimulus is extremely small
 Repeated stimuli –response patterns
“averaged” over time result in a robust tracing
that’s observed
Background
 Computerized test of hearing (cochlear) and
auditory nerve (neurological) functioning
 Used in evaluation of:


Hearing Integrity
Neurologic Integrity
 First application 1971(Jewett and Williston)
 Hundreds of published clinical studies on
applications
 Does not provide info about cortical areas
Purpose
 To convert acoustic stimuli into electrical
stimuli and measure associated brain wave
activity
 Brain wave activity is monitored by measuring
computer-averaged changes in EEG activity
 Electrical responses recorded from the scalp
in response to an auditory stimulus
 Time that it takes for sounds to travel can be
measured on the acquired waveforms
 http://www.youtube.com/watch?v=8ZXnN9AVL0o
Features
 Objective test

BAER. BSER, BAEP,ABR
 Noninvasive
 Ear specific
 Performed in quiet or sleep state
 Performed with AC and/or BC
 Performed with frequency specific stimuli
Clinical Application of evoked
potential testing
 Differential diagnosis (cochlear and retrocochlear)
 Screening procedure for newborns
 Diagnostic tool to ID HL in infants and children
 Estimate hearing in difficult to test population
 Intra-operative monitoring
 Prognostic indicator with head trauma
Generator sites of ABR
 Major peaks in waveforms: labeled by Roman
numerals I-V
 Response originates in the VIIIth cranial nerve
 Wave I: VIII distal
 Wave II: CN
 Wave III: SOC
 Wave IV: LL
 Wave V: IC
Generator Sites
Synchronous
activation
Reflects synchronous
activation
(onset type neurons)
Synchronous
activation

Frequency range of ABR response
Response dependent
on activation of basilar
region of the cochlea in
response to click (2K-4K)
Stimulus
 Click: brief duration signal, broad band signal
______through AC headphones
 Tone burst : provides more frequency specific
info
 Rate of stimulus: _______/sec
 Intensity : Start w high intensity and decrease
Lowest level at which a repeatable waveform
observed (of wave _______) is called
threshold
Preparation
 Application of recording electrodes



Vertex
Forehead
Earlobe/mastoid
 Placement of earphones
 Patient lying still or asleep
 30 minutes or longer
Electrode Set up
ABR clinical measures (time)
 Two Metrics
• Inter-wave Interval
• Absolute latency value
of wave
ABR clinical Measures (intensity)
 One Metric
• Absolute threshold
Factors that influence ABR
 Age of subject (<18 mos., >60 yrs)

Longer latency values for older and younger
clients
 Gender
 Not affected by most drugs (including
sedatives)
 Movement
 Temperature
Clinical limitations
 Frequency range 2000-4000 Hz, most
important to ABR
 Does not estimate hearing levels in lower
frequency ranges
 ABR is NOT a test of hearing
 Response provides no information on the
auditory system above the brainstem level
ABR mostly reflects higher freq
hearing
Normal Hearing:
Wave V threshold
@ 20 dB:
Suggests normal
hearing
I-V interval
ABR used to determine cochlear
function
I
II
III
IV V
Lower intensity
associated with
increased
latency values
V
V
V
Try to ID the dB HL
level where you
still observe
Wave V
Threshold ABR - Moderate HL
 Wave V responses
observed down to 55
dB HL
 Results suggest a
possible moderate
hearing loss
Threshold ABR: Severe HL
What is threshold?
ABR to determine function of
VIIIth nerve
 Multiple Sclerosis
 VIII nerve tumors
 Meniere’s Disease
 Auditory Neuropathy
Calculate
Interwave
Interval
Calculate IWI
Hearing: WNL
Right sided tinnitus
Intolerance to loud sounds
Acoustic Neuroma
Reporting results of ABR
What would ABR look like?
Differential Diagnosis
Examples of Differential Diagnosis
Screening ABR
 Prevalence of HL
NICU: .5-5%
 Well baby <1%
 Conducted on newborns prior to discharge
 Test at 2 levels: 60 dB HL and 30 dB HL
 Factors influencing outcome:

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Neurologic abnormalities
Poor health
Transient conductive problems
Muscle artifact
Collapsing canals
Earphone placement
Screening ABR
60 dB HL
30 dB HL
Cochlear Microphonic (CM) (Wever
and Bray, 1930)
 Summary of classic research




Electrode in the auditory nerve of a cat
Speech through loudspeaker
The resulting electrical activity when
transduced back to sound (thru a telephone
receiver) and amplified, was transmitted as
clear speech
Pure tones were reproduced accurately as
well up to 3000 Hz.
CM defined
 An electrical response from the cochlea
reflects a combination of IHC and OHC
function in response to acoustic stimulation
 CM Mimics the form of the sound pressure
waves that arrive at the ear … (aka a
“stimulus following response”
 Reverses in phase with changes in the
stimulus polarity from rarefaction (-)to
condensation (+)(Ferraro & Krishnan, 1997).
 looks like the waves "flip" or invert
Cochlear Microphonic in AN
 AN is defined as absent or severely distorted
ABR with preserved OAE’s and cochlear
microphonics
 CM “reverses or flips” when the stimulus
reverses polarity (from + to -)
 Can only be seen if ABR done with both +
and – polarity
Other Auditory Evoked Responses
 Middle Latency Response (MLR)
 Auditory Late Response (ALR)
MLR
 Documentation of CNS
dysfunction above
brainstem through
thalamus
 estimation of auditory
sensitivity in older
children / adults
(malingerers)
 State of arousal
ALR
 Assesses higher
cortical processing
from A1 and A2
areas
 P300 – information
processing


Latencies slow with
age
Auditory processing
Calculate Interwave intervals