BERA

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DR. SUPREET SINGH NAYYAR, AFMC

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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.

Changes produced by the passage of electrical stimulus generated in the cochlea through the neural pathway

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HISTORY

First described by Jewett and Williston in 1971, ABR audiometry is the most common application of auditory evoked responses .

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USES OF BERA

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

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USES OF INTRAOPERATIVE AUDITORY BRAINSTEM RESPONSE 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

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PRINCIPLE OF BERA

Processing at different levels Generates electrical activity Monitored by surface electrode Graphic recording presents a waveform Depends on the functional integrity of the pathway

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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 Electrical activity of brain: occurs randomly

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NEUROPHYSIOLOGIC BASIS OF BERA

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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 Neural conduction time

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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 Applied Importance

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AUDITORY EVOKED POTENCIALS

Electrical activity in brain elicited by sound stimulus Recorded upto 500 millisecs 3 responses are recorded:

• • • Short Latency Response (10ms) i.e BERA Middle Latency Response (10-50ms) Late Latency Response (50-500ms) 16-07-2012 www.nayyarENT.com

MIDDLE LATENCY RESPONSE Wave peaks: N 0 (10ms), P 0 (10-15ms), N a (16-30ms) ,P a (25-45ms) and N 3 (50ms) Most consistent waves: N a , P a 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

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40 HZ STEADY STATE POTENCIAL

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

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LATE LATENCY RESPONSE

Recordable between 50-500ms Originates in the cerebral cortex Cortical Evoked Response Audiometry (CERA) 5 wave peaks: P 1 , N 1 , P 2 , N 2 & P 3 Tone pips of 1000-2000 Hz Rate: 1 stimulus every 2-3 secs P 300 Wave peak Important to neuropsychiatrist & neurotologists

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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 Good earthing\ Faraday cages

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PRE-REQUISITES OF RECORDING BERA

Position of patient Relaxed Sedation in infants & children Prior PTA Sound stimulus: Broad Band Clicks (100 microsecs duration)

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ADVANTAGES OF BBC

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)

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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 Numbered I-VII

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SITE OF NEURAL GENERATOR

Wave I II III IV V VI & VII Site of Neural Generator Cochlear nerve (distal end) Cochlear nerve ( proximal end) Cochlear nucleus Superior Olivary Complex Lateral Leminiscus & Inferior Colliculus Not definitely known www.nayyarENT.com

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WAVE V

Identified first Most reliable & easily identifiable Sharp negative deflection following the peak Appears at 5.6-5.85 millisecs Largest & most robust wave

WAVE IV

Preceding wave V Maybe superimposed on wave V Distinct wave present in 50-60% subjects

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WAVE III

Upward peak between wave II & IV Maybe bifid Maybe fused with II Preceding wave IV Around the 3.8 msec Amplitude: 0.2-0.25 microvolt

WAVE II

Immediately preceding wave III Latency: 2.8 msec

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WAVE I

Sharp peak beyond 1msec mark Importance of identification:

• • • 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 16-07-2012 www.nayyarENT.com

NORMAL BERA TRACING

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PARAMETERS STUDIED

Latency of the wave(s)- absolute, interwave, interaural Amplitude of the wave(s)- absolute & relative (amplitude ratio) Wave-form morphology Latency-intensity functions of wave V

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LATENCY STUDIES

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

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AMPLITUDE STUDIES

Variable Studies are not very reliable Used as supplementary evidence Measured in microvolts Known as Absolute amplitude of a wave Relative Amplitude Ratio

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STUDY OF WAVE MORPHOLOGY

Shape of the graph Normal graph Graph in newborns Conditions altering the morphology of the graph:

• • • Acoustic neuroma Leshion in the auditory pathway Variation in rate\intensity of stimulus 16-07-2012 www.nayyarENT.com

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 Sex\age of the patient

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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 High stimulus rate: Multiple sclerosis

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STIMULUS PHASE OR POLARITY

Condensation & rarefaction phase Affects latency, amplitude, morphology of waves Routine studies: rarefaction waves are used Alternate phase: reduces the artifacts & also the sharpness of waves

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INTENSITY OF SOUND STIMULUS

60 dB suprathreshold Low intensity: increased absolute latency & decreased amplitude First to disappear: wave I Most stable: wave V

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FILTER CHARACTRISTICS

Recording of fixed range of frequencies Low fq filter: 100-150 Hz High fq filter: 3000-5000 Hz Frequencies of the recorded electrical stimulus

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NORMAL VALUES & CRITERIA FOR ABNORMALITY

Parameter measured I to III IPL III to V IPL I to V IPL Interaural difference of wave V Morphology of wave V Normal value (ms) 2 2 4 Less than 0.3

Present Criteria for abnormality (ms) More than 2.4

More than 2.4

More than 4.4

More than 0.3

Absent www.nayyarENT.com

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Clinical uses of BERA

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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 Frequency specific audiogram cannot be obtained

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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 Neural : steep sloping graph

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IDENTIFICATION OF RETROCOCHLEAR PATHOLOGIES

Most reliably identified Parameters:

• Increased interaural latency difference of wave V • Increase interaural interwave/interpeak latenct between wave I to V • Interwave latency between wave I & III/V 16-07-2012 www.nayyarENT.com

DERIVED BAND STACKED BERA

Elicit response from several discrete regions of cochlea Composite picture of neural activity Increases sensitivity of the test Cochlea is divided into 5 segments & response from each is noted

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DERIVED BAND STACKED BERA

1 st segment: sounds above 8000Hz (extreme basal end) 2 nd segment: 4000-8000Hz (basal end of cochlea) 3 rd segment: 2000-4000Hz (between basal & mid-portion) 4 th segment: 1000-2000Hz (mid portion of cochlea) 5 th segment: 500-1000Hz (apical part of cochlea)

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DERIVED BAND STACKED BERA

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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 Useful in patients with U/L SNHL with normal BERA

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ASSR

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Auditory Steady State Response Objective determination of frequency specific hearing threshold Overcomes the limitations of BERA:

• Idea of hearing threshold for higher frequencies (2000 4000 Hz) • Insensitive for hearing loss above 75-80 Db

Importance in providing hearing aid Advantage over LLR & LLR

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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 Frequency specific threshold is determined

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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 Modulation frequency: no. of times CF is modulated

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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 Phase coherence is commonly measured

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Consistency of response Individual presentation of different frequencies Measurement of fq dependent hearing threshold Time taken: 45 mins Determination of behavioral threshold:

• • Click evoked BERA: 10 dB more Tone evoked BERA: 20-30 dB more 16-07-2012 www.nayyarENT.com

MEASUREMENT OF RESPONSE

Discrepancy is more in the lower than higher frequencies Better correlation in high frequency hearing loss Regression formula for overcoming this Overcomes the subjective visual factor

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BERA VS ASSR Similarities Deliver an auditory stimulus Differences BERA stimulus: click or a tone burst presented at a slower rate; ASSR: amplitude or frequency modulated sounds presented rapidly Stimulate the auditory system Record bioelectric responses from the auditory system via electrodes 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.

Patient does not have to respond volitionally.

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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THANK YOU

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