The Brainstem Auditory Response
Recording the response of brainstem nuclei and tracts in response to auditory stimuli consisting of short duration clicks. These responses are very small and occur within 10 msec of the stimulus. They are most easily recorded from near the vertex. Up to six separate peaks can be seen with a good recording, each peak resulting from the activity of one of the brainstem nuclei or tracts involved in auditory processing. Because of the nature of this response, a large number of trials must be averaged to obtain a satisfactory signal to noise ratio.
- I Cranial Nerve VIII
- II Cranial Nerve VIII
- III Cranial Nerve VII
- IV SOC; Superior Olivary Complex
- V LL
- VI Inferior Colliculus
The Ear
outer ear - funnels sound, middle ear - transmits vibrations, inner ear - controls fluid movements into neural firing
bones - incus - malleus - stapes
cochlea - oval window, scala vestibuli, scala media, helicotrema, scala tympani, round window, *fluid path
basilar membrane has very tiny vibrations, different sounds produce vibrations at different places
Resonance properties of the Basilar Membrane
- Basal - Thick, Stiff, High Frequency
- Apical - Thin, Floppy, Low Frequency
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*LH*
Anatomy of the auditory system; outer ear, auricle, external auditory meatus
Inner Ear, Middle Ear, Tympanic Membrane, Malleus, Incus, Stapes, Eustacian tube, Inner Ear, cochlea, cochlear nerve
an imbalance of pressure can cause a conductive hearing loss, small muscles will tense up
compression - condensation - stimulation
rarefaction - negative pressure; transducer pulls out creates negative are pressure ear canal and pulls out of tympanic membrane
longitudinal; the way the tympanic membrane click pulling out rarefaction
brainstem auditory pathway, medial geniculate body, inferior colliculus, etcetera
physiology of the auditory system; outer ear, ear canal, tympanic membrane, middle ear, air filled cavity, provides an impedance match from the outside air to the fluid of the inner ear; inner ear the traveling wave moves along the scala media, the basilar membrane of the scala media is stiffer at its base and more flexible at it's tip
differences in flexibility serve to separate the traveling wave into component frequencies, gross sound filter; high frequencey vibrations displace the basal portion low frequency vibrations displace the apical portion
Hair cells; the inner and outer hair cells are displaced as the scala media moves which causes depolarization of the hair cells resulting in neural firing
Physics of sound; sound propagation, rarefactions, condensation, normal air pressure, frequency, wavelength, velocity, speed of sound in dry air is 769 mph @ 21C
Recording Parameters; Montages A1-Cz, A2-Cz, direct nerve action potential, filters low-cut 10 to 30 Hz, high-cut 2500-3000hZ, 12 dB/octave filter roll-off for low and 24 db/octave for high frequency filters; sweep 10 to 15 msec, number of averages 1000 to 4000
stimulus parameters, click types, condensation rarefaction, alternating, duration of clicks 100 microsecond broadband click; effective frequency range of clicks, types of stimulators; supraaural earphones used in clinical setting, insert earphones used in surgical setting
insert earphones, inserted into the ear canal to prevent fluid from entering the canal, the inserts are attached to a short tube, causes 0.9 msec delay in all waveforms; delay serves to separate waveforms from stimulation artifact
click duration 100 microsecond broadband click, rate 5 to 12/s optimal up to 50/s can be used; stimulus rate; repeatable responses can be obtained at repetition rates above 30 Hz, at higher rates responses may increase in latency slightly, rate should be high enough to acquire well resolved data in a timely matter, masking noise, a white noise (static) should be presented to the contralateral ear to eliminate sound conduction through the bone to the contralateral side
intensity should be calibrated in decibels peak equivalent sound pressure level; intensity of 100 db pe SPL is recommended, in HL recommended 60-70 HL
click polarity; rarefaction; tympanic membrane initially deflected outward, produces earlier waveform latencies, clearer definition of waves I, III and V: Condensation, tympanic membrane initially deflected inward, produces slightly later waveform latencies, better definition of wave IV- complex
Alternating polarity; alternation of rarefaction and condensation stimuli helps to eliminate stimulus artifact; removes cochlear microphonic revealing summating potential, results in poorer definition of waveforms
Measures of sound intensity; dB SPL (sound pressure level), dB HL (hearing level) pure tones defined as the mean hearing threshold established in a group of normals with the equipment to be used for monitoring; dB nHL (normal hearing level) non pure tones; dB SL (sensation levels); dB pe SPL (peak equivalent sound pressure level)
probably anatomic generators
Wave I - Distal Auditory Nerve
Wave II - Transition from IAC to Brain Stem
Wave III - Cochlear Nucleus/Superior Olive
Wave IV - Lateral Lemniscus
Wave V - Inferior Colliculus
*Wave I is negative, all other waveforms are positive
*Aage Moller says that there are six peaks
Anesthetic affects on baers; minimal effect from most anesthetic agents (BAERs are highly robust); propofol can cause increased interpeak latencies, N2O can increase the middle ear pressure causing a conductive hearing loss
Other non surgical factors; temperature, cooling will cause slowed conduction; hypotension, bilateral decreased amplitude wave V, conductive loss, increased absolute latencies no change in interpeak latencies
Applications of BAER Monitoring
Intracranial, preservation of VIII function during skull based surgeries; differentiation of vestibular and auditory branches of VIII during vestibular nerve sectioning, AP/SP ratio evaluation during endolymphatic sac surgeries. Extracranial, determination of hearing sensitivity in uncooperative patients (children)
Interpretive features of the ABR; absolute latencies, interpeak latencies, I-III, III-V, I-V, interaural latencies, wave V amplitude
criteria for significant changes; absence of waves I through V, absence of all waves following I, II or III, abnormal prolongation of I-III, III-V, and I-V interpeak latencies, I-III or III-V can be prolonged even with normal I-V interpeak latencies, abnormal decrease of the V/I amplitude ratio, abnormal intraaural latency differences
Intervention Criteria; intervention should occur if wave III or V amplitudes drop more than 50%; alert surgeon if wave I-V interpeak latency increases by more than 0.5 msec, note opening the dura may cause as much as a 0.5 msec I-V interpeak latency increase; intervention should occur when wave I-V interpeak latency increase is 1 msec or greater; intervention should occur if wave I-III interpeak latency increases by 0.5 msec or greater
Causes of ABR changes; irrigation and/or cooling of nerve, stretching of nerve, auditory nerve trauma, brainstem trauma, cerebellar retraction, cauterization or compromise of cochlear blood supply
retrosigmoid view of the posterior fossa cranial nerves; facial is anterior to CN VIII
Anatomy of the auditory system; outer ear, auricle, external auditory meatus
Inner Ear, Middle Ear, Tympanic Membrane, Malleus, Incus, Stapes, Eustacian tube, Inner Ear, cochlea, cochlear nerve
an imbalance of pressure can cause a conductive hearing loss, small muscles will tense up
compression - condensation - stimulation
rarefaction - negative pressure; transducer pulls out creates negative are pressure ear canal and pulls out of tympanic membrane
longitudinal; the way the tympanic membrane click pulling out rarefaction
brainstem auditory pathway, medial geniculate body, inferior colliculus, etcetera
physiology of the auditory system; outer ear, ear canal, tympanic membrane, middle ear, air filled cavity, provides an impedance match from the outside air to the fluid of the inner ear; inner ear the traveling wave moves along the scala media, the basilar membrane of the scala media is stiffer at its base and more flexible at it's tip
differences in flexibility serve to separate the traveling wave into component frequencies, gross sound filter; high frequencey vibrations displace the basal portion low frequency vibrations displace the apical portion
Hair cells; the inner and outer hair cells are displaced as the scala media moves which causes depolarization of the hair cells resulting in neural firing
Physics of sound; sound propagation, rarefactions, condensation, normal air pressure, frequency, wavelength, velocity, speed of sound in dry air is 769 mph @ 21C
Recording Parameters; Montages A1-Cz, A2-Cz, direct nerve action potential, filters low-cut 10 to 30 Hz, high-cut 2500-3000hZ, 12 dB/octave filter roll-off for low and 24 db/octave for high frequency filters; sweep 10 to 15 msec, number of averages 1000 to 4000
stimulus parameters, click types, condensation rarefaction, alternating, duration of clicks 100 microsecond broadband click; effective frequency range of clicks, types of stimulators; supraaural earphones used in clinical setting, insert earphones used in surgical setting
insert earphones, inserted into the ear canal to prevent fluid from entering the canal, the inserts are attached to a short tube, causes 0.9 msec delay in all waveforms; delay serves to separate waveforms from stimulation artifact
click duration 100 microsecond broadband click, rate 5 to 12/s optimal up to 50/s can be used; stimulus rate; repeatable responses can be obtained at repetition rates above 30 Hz, at higher rates responses may increase in latency slightly, rate should be high enough to acquire well resolved data in a timely matter, masking noise, a white noise (static) should be presented to the contralateral ear to eliminate sound conduction through the bone to the contralateral side
intensity should be calibrated in decibels peak equivalent sound pressure level; intensity of 100 db pe SPL is recommended, in HL recommended 60-70 HL
click polarity; rarefaction; tympanic membrane initially deflected outward, produces earlier waveform latencies, clearer definition of waves I, III and V: Condensation, tympanic membrane initially deflected inward, produces slightly later waveform latencies, better definition of wave IV- complex
Alternating polarity; alternation of rarefaction and condensation stimuli helps to eliminate stimulus artifact; removes cochlear microphonic revealing summating potential, results in poorer definition of waveforms
Measures of sound intensity; dB SPL (sound pressure level), dB HL (hearing level) pure tones defined as the mean hearing threshold established in a group of normals with the equipment to be used for monitoring; dB nHL (normal hearing level) non pure tones; dB SL (sensation levels); dB pe SPL (peak equivalent sound pressure level)
probably anatomic generators
Wave I - Distal Auditory Nerve
Wave II - Transition from IAC to Brain Stem
Wave III - Cochlear Nucleus/Superior Olive
Wave IV - Lateral Lemniscus
Wave V - Inferior Colliculus
*Wave I is negative, all other waveforms are positive
*Aage Moller says that there are six peaks
Anesthetic affects on baers; minimal effect from most anesthetic agents (BAERs are highly robust); propofol can cause increased interpeak latencies, N2O can increase the middle ear pressure causing a conductive hearing loss
Other non surgical factors; temperature, cooling will cause slowed conduction; hypotension, bilateral decreased amplitude wave V, conductive loss, increased absolute latencies no change in interpeak latencies
Applications of BAER Monitoring
Intracranial, preservation of VIII function during skull based surgeries; differentiation of vestibular and auditory branches of VIII during vestibular nerve sectioning, AP/SP ratio evaluation during endolymphatic sac surgeries. Extracranial, determination of hearing sensitivity in uncooperative patients (children)
Interpretive features of the ABR; absolute latencies, interpeak latencies, I-III, III-V, I-V, interaural latencies, wave V amplitude
criteria for significant changes; absence of waves I through V, absence of all waves following I, II or III, abnormal prolongation of I-III, III-V, and I-V interpeak latencies, I-III or III-V can be prolonged even with normal I-V interpeak latencies, abnormal decrease of the V/I amplitude ratio, abnormal intraaural latency differences
Intervention Criteria; intervention should occur if wave III or V amplitudes drop more than 50%; alert surgeon if wave I-V interpeak latency increases by more than 0.5 msec, note opening the dura may cause as much as a 0.5 msec I-V interpeak latency increase; intervention should occur when wave I-V interpeak latency increase is 1 msec or greater; intervention should occur if wave I-III interpeak latency increases by 0.5 msec or greater
Causes of ABR changes; irrigation and/or cooling of nerve, stretching of nerve, auditory nerve trauma, brainstem trauma, cerebellar retraction, cauterization or compromise of cochlear blood supply
retrosigmoid view of the posterior fossa cranial nerves; facial is anterior to CN VIII