Transcript BERA - nayyarENT

BERA
DR. SUPREET SINGH NAYYAR, AFMC
16-07-2012
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Recording of the synchronous electrical
activity recorded by a far-field electrode
placed on the scalp in response to a sound
presented to the cochlea.
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Changes produced by the passage of
electrical stimulus generated in the cochlea
through the neural pathway
HISTORY
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First described by Jewett and Williston in
1971, ABR audiometry is the most
common application of auditory evoked
responses.
USES OF BERA
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Detection & quantification of deafness in
difficult to test patients
Detection of the nature of deafness
Identification of the site of leshion in
retrocochlear pathologies
Study of central auditory disorders
Study of maturity of nervous system in
newborns
Objective identification of brain death
Assessing prognosis in comatose patients
USES OF INTRAOPERATIVE AUDITORY
BRAINSTEM RESPONSE
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Monitoring cochlear function directed at hearing
preservation:
Cerebellopontine angle tumor resection (acoustic neuroma
surgery)
Vascular decompression of trigeminal neuralgia
Vestibular nerve section for the relief of vertigo
Exploration of the facial nerve for facial nerve
decompression
Endolymphatic sac decompression for Mèniére disease
Monitoring brainstem integrity:
Brainstem tumor resection
Brainstem aneurysm clipping or arteriovenous
malformation resection
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PRINCIPLE OF BERA
PRINCIPLE OF BERA
Processing at different levels
Generates electrical activity
Monitored by surface electrode
Graphic recording presents a waveform
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Depends on the functional integrity of the pathway
PRINCIPLE OF BERA
Process becomes difficult due to the
background potential generated by the brain
Separation of the 2 activities by summation &
averaging
Sound evoked electrical potential: time
specific
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Electrical activity of brain: occurs randomly
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NEUROPHYSIOLOGIC BASIS OF
BERA
PROCESSING OF THE SOUND
STIMULUS
‘Sound conduction time’
‘Cochlear transport time’
• Less for high fq sound
• High for low fq sound
Passage through cochlear filters
Cochlear filter –build-up time
• Broadening of cochlear filters
Synaptic delay
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Neural conduction time
MECHANISM OF ACTIVATION IN BERA
Click sound presented to the ear
Earlier stimulation by the high fq sounds
The middle & apical parts don’t contribute
much to BERA response
Changes in high fq loss
Relation of intensity of sound stimulus to the
latency & amplitude of the waves
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Applied Importance
AUDITORY EVOKED POTENCIALS
Electrical activity in brain elicited by sound stimulus
Recorded upto 500 millisecs
3 responses are recorded:
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• Short Latency Response (10ms) i.e BERA
• Middle Latency Response (10-50ms)
• Late Latency Response (50-500ms)
MIDDLE LATENCY RESPONSE
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Wave peaks: N0(10ms), P0(10-15ms), Na (16-30ms)
,Pa(25-45ms) and N3(50ms)
Most consistent waves: Na, Pa
Neurogenic & myogenic origin
Affected by sleep, anaesthesia
Origin: Proximal to the midbrain
Assess hearing level between 250-500 Hz
Fitting hearing aid
Elicited by tone pips
Limitations
40 HZ STEADY STATE POTENCIAL
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Also known as ASSR
Superimposition of BERA & some MLR waves
Recorded as continuous sinusoidal wave
Sound stimuli: 500 Hz of 15 ms duration at 40
stimuli/sec
Indicates a state of arousal
Objective determination of hearing threshold
Used for objective frequency dependent
hearing threshold estimation
LATE LATENCY RESPONSE
Recordable between 50-500ms
Originates in the cerebral cortex
Cortical Evoked Response Audiometry (CERA)
5 wave peaks: P1, N1, P2, N2 & P3
Tone pips of 1000-2000 Hz
Rate: 1 stimulus every 2-3 secs
P300 Wave peak
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Important to neuropsychiatrist & neurotologists
PRE-REQUISITES OF RECORDING
BERA
Elicited by click stimulus
50-60dB above avg. pure tone threshold
Location of electrodes: active, reference & ground
Air conditioned room
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Good earthing\ Faraday cages
PRE-REQUISITES OF RECORDING
BERA
Position of patient
Relaxed
Sedation in infants & children
Prior PTA
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Sound stimulus: Broad Band Clicks (100 microsecs duration)
ADVANTAGES OF BBC
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Synchronous stimulation of large no. of
neurons
Clear, sharp well- marked tracing
Very rapid onset & fall
Easy latency & amplitude measurement
Lowest fq: 100-150Hz
Highest fq: 3000-5000Hz
Total recordings: 2000-4000
Stimulus rate: 10-40 clicks\sec (11.1/sec)
RECORDING
Graph plotted with amplitude (in microvolts) on the ordinate &
time (in msec) on the abscissa
5-7 peaks\waves within 8-10 millisecs
BERA waves: 5 prominent & 2 small
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Numbered I-VII
SITE OF NEURAL GENERATOR
Wave
Site of Neural Generator
I
Cochlear nerve (distal end)
II
Cochlear nerve ( proximal end)
III
Cochlear nucleus
IV
Superior Olivary Complex
V
VI & VII
Lateral Leminiscus & Inferior
Colliculus
Not definitely known
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WAVE V
Identified first
Most reliable & easily
identifiable
Sharp negative
deflection following
the peak
WAVE IV
Preceding wave V
Maybe superimposed
on wave V
Distinct wave present
in 50-60% subjects
Appears at 5.6-5.85
millisecs
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Largest & most robust
wave
WAVE III
WAVE II
Upward peak between
wave II & IV
Immediately
preceding wave III
Maybe bifid
Latency: 2.8 msec
Maybe fused with II
Preceding wave IV
Around the 3.8 msec
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Amplitude: 0.2-0.25
microvolt
WAVE I
Sharp peak beyond 1msec mark
Importance of identification:
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• Presence of wave I in the absence of others: leshion beyond
distal nerve end
• Delayed wave I: conductive/cochlear pathology
• Abolition of wave I: severe peripheral leshions
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NORMAL BERA TRACING
PARAMETERS STUDIED
Latency of the wave(s)- absolute, interwave, interaural
Amplitude of the wave(s)- absolute & relative (amplitude ratio)
Wave-form morphology
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Latency-intensity functions of wave V
LATENCY STUDIES
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Time interval between onset of stimulus &
peak of the wave
Measured in millisecs
Also known as Absolute Latency
Most important for clinical measurements
Latency of wave V depends on intensity of
sound stimulus
Interwave Latency
Interaural Latency
AMPLITUDE STUDIES
Variable
Studies are not very reliable
Used as supplementary evidence
Measured in microvolts
Known as Absolute amplitude of a wave
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Relative Amplitude Ratio
STUDY OF WAVE MORPHOLOGY
Shape of the graph
Normal graph
Graph in newborns
Conditions altering the morphology of the graph:
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• Acoustic neuroma
• Leshion in the auditory pathway
• Variation in rate\intensity of stimulus
NON CLINICAL FACTORS AFFECTING
BERA
Stimulus rate
Stimulus phase or polarity
Intensity of sound stimulus
Binaural\monoaural stimulation
Filter characters of BERA machine
Nature of sound used
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Sex\age of the patient
STIMULUS RATE
No. of clicks presented to the ear/sec
Recommended rate: 10-40/sec
Normally used: 1.1 clicks/sec
Rate >25/sec: increased latency & decreased
amplitude
Children: >50/sec
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High stimulus rate: Multiple sclerosis
STIMULUS PHASE OR POLARITY
Condensation & rarefaction phase
Affects latency, amplitude, morphology of
waves
Routine studies: rarefaction waves are used
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Alternate phase: reduces the artifacts & also
the sharpness of waves
INTENSITY OF SOUND STIMULUS
60 dB suprathreshold
Low intensity: increased absolute latency &
decreased amplitude
First to disappear: wave I
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Most stable: wave V
FILTER CHARACTRISTICS
Recording of fixed range of frequencies
Low fq filter: 100-150 Hz
High fq filter: 3000-5000 Hz
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Frequencies of the recorded electrical
stimulus
NORMAL VALUES & CRITERIA FOR
ABNORMALITY
Parameter
measured
Normal value
(ms)
Criteria for
abnormality (ms)
I to III IPL
2
More than 2.4
III to V IPL
2
More than 2.4
I to V IPL
4
More than 4.4
Interaural
difference of
wave V
Less than 0.3
More than 0.3
Morphology of
wave V
Present
Absent
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Clinical uses of BERA
ESTIMATION OF HEARING THRESHOLD
Useful in newborns, infants, difficult patients
Estimation of hearing threshold
Estimation of type & degree of hearing loss
Avg. pure tone threshold = 0.6 (BERA threshold)
Comparison of latency of wave V at different
intensity sounds
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Frequency specific audiogram cannot be obtained
IDENTIFICATION OF NATURE
OF DEAFNESS
Analysis of latency-intensity function
Conductive, sensory or neural
Latency of wave V is recorded for different
intensities
Plotted graphically
Conductive loss: upward & parallel shift
Sensory loss: shallow configuration
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Neural: steep sloping graph
IDENTIFICATION OF
RETROCOCHLEAR
PATHOLOGIES
Most reliably identified
Parameters:
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• Increased interaural latency difference of wave V
• Increase interaural interwave/interpeak latenct
between wave I to V
• Interwave latency between wave I & III/V
DERIVED BAND STACKED BERA
Elicit response from several discrete regions
of cochlea
Composite picture of neural activity
Increases sensitivity of the test
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Cochlea is divided into 5 segments &
response from each is noted
DERIVED BAND STACKED BERA
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1st segment: sounds above 8000Hz (extreme
basal end)
2nd segment: 4000-8000Hz (basal end of
cochlea)
3rd segment: 2000-4000Hz (between basal &
mid-portion)
4th segment: 1000-2000Hz (mid portion of
cochlea)
5th segment: 500-1000Hz (apical part of
cochlea)
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DERIVED BAND STACKED BERA
STACKED BERA
Improvement of derived band BERA
Increases the sensitivity & specificity of
BERA for small tumours
Aligning 5 wave Vs of derived band BERA &
adding the amplitudes
Reduced in presence of tumours
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Useful in patients with U/L SNHL with normal
BERA
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ASSR
Auditory Steady State Response
Objective determination of frequency specific
hearing threshold
Overcomes the limitations of BERA:
• Idea of hearing threshold for higher frequencies (20004000 Hz)
• Insensitive for hearing loss above 75-80 Db
Importance in providing hearing aid
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Advantage over LLR & LLR
MODULATION OF SOUND
Modulation of pure tone sound:
• Amplitude domain (alternate off & on)
• Frequency domain (warbling of tone)
Amplitude modulation of 100% is used
Frequency modulation of 20% is used
Restricted narrow area of basilar membrane is
stimulated
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Frequency specific threshold is determined
RATE OF MODULATION
Rate of modulation:
• <20 per sec: response from cortical areas
• 20-50 per sec: subcortical areas
• >60 per sec: brain stem
Recording in sedated infants
Carrier frequency: test frequency
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Modulation frequency: no. of times CF is
modulated
METHOD OF RECORDING
Pure tone sounds (500/1000/2000/4000Hz)
Modulation: 90 times/sec
Evoked neural response is pre-amplified,
filtered, sampled & analyzed
90 Hz component of evoked response is
measured
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Phase coherence is commonly measured
Consistency of response
Individual presentation of different frequencies
Measurement of fq dependent hearing threshold
Time taken: 45 mins
Determination of behavioral threshold:
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• Click evoked BERA: 10 dB more
• Tone evoked BERA: 20-30 dB more
MEASUREMENT OF RESPONSE
Discrepancy is more in the lower than higher
frequencies
Better correlation in high frequency hearing
loss
Regression formula for overcoming this
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Overcomes the subjective visual factor
BERA VS ASSR
Deliver an auditory
stimulus
Stimulate the auditory
system
Record bioelectric
responses from the
auditory system via
electrodes
Patient does not have to
respond volitionally.
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Differences
BERA stimulus: click or a tone burst
presented at a slower rate;
ASSR: amplitude or frequency
modulated sounds presented
rapidly
BERA is dependent on a relatively
subjective analysis of amplitude
versus latency. ASSR is
dependent on a statistical
analysis of the probability of a
response, usually at a 95%
confidence level.
The BERA response is measured in
millionths of a volt (microvolts),
and the ASSR is measured in
billionths of a volt (nanovolts)
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Similarities
THANK YOU
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