Auditory Brainstem Response Monitoring

BAEP Electrode Sites —  CZ – Central Vertex —  Midway between nasion & inion and left and right preauricular points —  A1 – Left auricle —  A2 – Right auricle —  Ground —  Anywhere on head —  Montage: —  A1-­‐CZ —  A2-­‐CZ Normal Interpeak Latencies (IPL) —  I – III – 2.0 msec —  Auditory nerve conduction Peak
Ear inserts Headphones
I
2.4
1.5
—  III – V – 2.0 msec —  Brainstem conduction II
3.4
2.5
III
4.4
3.5
—  I – V – 4.0 msec —  Auditory nerve and brainstem IV
5.4
4.5
V
6.4
5.5
Obligate waveforms highlighted yellow Instrument Parameters —  Recording Parameters —  Sensitivity -­‐ 0.5 uv/div —  Bandpass – 100-­‐150 to 2500 – 3000 Hz —  Analysis Time – 15 msec/div —  Trials – 500-­‐1000 —  Stimulating Parameters —  Stimulus Intensity – 60-­‐70 dB HL; 100 peSPL —  Masking – 30 – 35 dB HL; 60 dB SPL —  Stimulus Type – Alternating —  Rate – 5-­‐12/sec From guidelines 11C pg1-­‐2 S;mulus —  Hardware —  Headphones —  Ear Inserts — 
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Adds 0.9 msec to normal peak latencies Benefit is that waveforms are more seperated from the stimulus artifact —  Type of stimulus —  Broad Band clicks preferred — 
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100usec square wave pulse Delivered Monaurally (one ear at a time) —  Extras —  Stim rates of 30/sec can be used for faster acquistion of waves 1 and 5, but wave 3 is usually degraded —  Stim rates greater than 50 can cause degradation of amplitude in all waveforms and increased latencies S;mulus Intensity Measures —  dB SPL —  Sound Pressure Level — 
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Physical measure of lowest sound intensity What we consider to be O dB —  dB HL —  Hearing Level — 
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Hearing threshold averaged for a group of individuals 40 dB HL means that it is 40 dB above the averaged group hearing threshold regardless of what is the individual’s hearing threshold. —  dB nHL —  Normal Hearing Level — 
Same at dB HL, only the number of test subjects is smaller
—  dB SL —  Sensation Level — 
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Individual hearing threshold
40 dB SL means the it is 40 dB above the lowest sound intensity a subject can hear —  dB peSPL —  Peak Equivalent Sound Pressure Level — 
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Compare peak-­‐to-­‐peak amplitude of the stimulus (click) with amplitude of a sine wave of a pure tone with known SPL The same as SPL only calibrated from a loud stimulus If you get a CNIM question regarding peSPL, just treat it like SPL S;mulus Intensity Measures Click Polarity —  Rarefaction —  NEGATIVE —  Condensation —  POSITIVE Click Polarity —  Rarefaction (NEGATIVE) —  AWAY from the eardrum —  Shorter Latencies —  Better definition of Wave I —  Condensation (POSITIVE) —  TOWARD the eardrum —  Longer Latencies —  Better definition of Wave V — Alternating —  Alternates between rarefaction and condensation —  Reduces stim artifact and cochlear microphonics Beginning of BAERs Pathway —  Start with audible tone clicks —  Ear inserts — 
Add 0.9 msec —  Headphones Soundwave enters ear canal Soundwave arrives at the tympanic Membrane, also called the eardrum. The eardrum vibrates causing Movement in the ossicles (bones of Middle ear). The ossicles send the vibrations into the Cochlea where tiny hair cells convert the physical movement into an electrical sensory impulse. Wave I: Distal Auditory nerve Wave II: Proximal Auditory nerve Wave III: Superior Olivary Complex At the junction of the Pons and Medulla Oblongata. This is the beginning of the Lateral Lemniscus. Wave IV: Lateral Lemniscus The Lateral Lemniscus travels upward through the pons and terminates at the Inferior Colliculus Wave V: Inferior Colliculus Located in the midbrain. Acts as a relay station to send the sensory impulse to the correct area in the brain. End of the Lateral Lemniscus. Normal BAERs —  Wave I only present on ipsilateral side —  Wave V typically has large descending morphology —  Wave V typically is very prominent in contralateral side Abnormality Criteria for Calling Changes —  1 msec increase in IPL of waves I -­‐ V OR —  50 % decrease in Wave 5 amplitude Prac;ce Exam: 84 —  When Monitoring BAERs intraoperatively it is recommended to warn the surgeon when an increase of the following occurs? —  1. The latency of wave V of less than 1.0 —  2. The latency of wave I of 1 msec —  3. The latency of wave V of 1.0 or greater —  4. The interpeak latency of I-­‐V of 1.0 or less Abnormal BAERs —  Most common causes of injury to auditory system —  Compression, Traction, Thermal Injury, Ischemia —  Specific Examples in ACNS Guidelines (11.C pg 4) Injury Result Cochlear ischemia Loss of all waveforms Injury to Distal Auditory nerve Wave I: dec. amp, Inc. Latency Waves III and V: Proportional delayed; IPL I-­‐V normal Proximal nerve injury( acoustic Normal I; Prolonged and decreased amp neuroma, microvascular decomp.) waves III and V Brainstem injury Prolonged and decrease amp of waves V and III if near the cochlear nucleus Lesion in Rostral Pons Affects only wave 5 Abnormal BAERs —  Wave 1 delay —  Disruption of pathway from ear canal to cochlea — 
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Fluid in ear Kinked Tubing Cochlea infarction —  IPL I –III changes —  Disruption to Auditory Nerve — 
Mechanical or ischemic damage to nerve —  IPL III-­‐V changes —  Disruption to pathway through the pons — 
Mechanical or ischemic damage to brainstem Abnormal BAERs —  Increased wave I latency —  Conductive hearing loss — 
Fluid in Outer or Middle Ear —  Decreased stimulus intensity — 
Problem with stimulator or tubing —  Conductive Problems DO NOT AFFECT interpeak latencies —  Normal I-­‐III IPL —  Normal III-­‐V IPL —  Normal I-­‐V IPL Abnormal BAERs-­‐Factors —  Loss of All waveforms —  Destruction of Wave I generator —  Physical destruction —  Ischemia (AICA, Internal auditory artery) —  Hypotension —  Drilling (masking effect) —  Increased I-­‐III interpeak latencies — 
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Slowing in the nerve Manipulation of the nerve Tumor resection Local nerve cooling —  Increased III-­‐V interpeak latencies —  Cerebellar retraction —  Brainstem slowing —  Increased I-­‐V interpeak latencies —  Decreased body temperature —  BP changes —  Cerebellar retraction Other AEPs (PAMR) —  Post Auricular Muscle Response (PAMR) —  MUSCLE Response —  Recorded from mastoid —  Downward going deflection at 12-­‐15 msec Other AEPs (NAP) —  Auditory Nerve Action Potential (NAP) —  Records from Proximal Auditory nerve and is placed by surgeon after exposure —  Benefits: — 
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Quick identification of neural compromise Large SNR so few trials are needed —  Disadvantages: — 
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Cannot be monitored until after exposure of nerve Cannot be monitored during closing Abnormal BAERs Condi;ons —  Cerebellopontine Angle Tumor (CPA) —  Increased I-­‐III IPL; Possible absence of waveforms after I —  Tumor within the angle of the cerebellum and pons, often affects cranial nerve V, VII, and VIII —  Acoustic Neuroma —  Increased I-­‐III IPL; Possible absence of waveforms after I —  Also called Vestibular Schwannoma —  Tumor of the vestibular branch of cranial nerve VIII — 
Most often caused by Neurofibromatosis Type II Abnormal BAERs Condi;ons —  Lesion in lower brainstem —  Increased IPL I-­‐III if near superior olivary complex —  Increased IPL III-­‐V Abnormal BAERs Condi;ons —  Lesion in Upper Brain stem (Pons) —  Increased III-­‐V IPL —  Wave V may be absent Auditory Diseases and Symptoms Menieres Cochlear fluid imbalance Vertigo, Tinnitus Normal Tinnitus Ringing in ear Normal Vertigo Sensation of loss of balance Normal Otitis Media Middle ear infection Eardrum may bulge outward Prolonged wave I Otosclerosis Fusion of Ossicles Conductive hearing loss Prolonged wave I ECochG —  Recording montage —  Active Electrode —  Needle through tympanic membrane —  Reference Electrode —  Needle at ipsilateral earlobe or mastoid —  Some sites reference to CZ or contra-­‐ear —  3 components —  AP: Action Potential —  Same as Wave 1 —  CM: Cochlear Microphonics —  SP: Summating Potential —  Waveforms obtained by switching between, rarefaction, condensation and alternating —  Contralateral masking not necessary —  Indicated when recording wave 1 is essential, but unable to be obtained with BAERs (Spehlmann) ECochG AP CM Rarefaction Condensation Sum I V SP 10 msec. ECochG —  CM —  Cochlear Microphonics —  Generated by hair cells in the cochlea —  Series of rhythmical low amplitude deflections that coincide with the peaks of the sound wave stimulus —  Rarefaction and Condensation stimulation only —  AP —  Action Potential —  Same as Wave I in BAERs —  Generated by acoustic nerve fibers —  Latency increases with decreased stim intensity —  Decreased amplitude with increasing rep rate ECochG —  SP (Summating Potential) —  Alternating Stimulus only —  Represents a steady component of the receptor potential of cochlear hair cells Other AEPs —  Middle Latency Auditory Evoked Potentials (MLAEP) —  Waveform recorded 10-­‐50 msecs following stimulus —  Long Latency Auditory Evoked Potentials (LLAEP) —  Waveforms recorded 50-­‐500 msecs following stimulus Prac;ce Exam: 18 —  In BAEPs, wave 1 can most easily be differentiated from cochlear microphonics based upon —  1. Amplitude —  2. Morphology —  3. Click Polarity —  4. Absence in contralateral recording Anesthe;c effects on BAERs —  Unaffected by anesthetics Surgeries related to auditory nerve Surgery Nerves monitored Clival surgeries V, VII, VIII Pontine surgeries V, VII, VIII Microvascular Decompression of Trigeminal N. V, VII, VIII i.e. Tic Delaroux Microvascular Decompression of Facial N. VII, VIII CPA surgeries VII, VIII Troubleshoo;ng —  Tension —  Recognized by Viewing — 
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Patient Raw Data —  High frequency random activity —  Shoulders / Neck / Scalp / Jaw —  Uncomfortable Headphones —  60 Hertz —  Sinusoidal activity occurring at six times per 100 msec —  Causes — 
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High Electrode Impedance Imbalanced Electrode Impedance Other Electrical Equipment Fluorescent Lights Troubleshoo;ng —  Stimulus Artifact —  Large deflection at onset of sweep —  Causes — 
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High intensity stimulus High impedance electrodes —  Correction — 
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Impedance equally matched and below 5K Reposition ear electrode to back of ear or mastoid Alternating polarity Troubleshoo;ng: Waveform Enhancement —  Wave I —  Increase Stimulus Intensity & Decrease Rate —  Change Click Polarity —  Alternating click polarity when using high intensity stimulus —  Check Ear canal for obstruction —  Wave II —  May be absent in normal subjects —  Identify on contralateral side —  Wave III —  May be bifurcated — 
Choose highest amplitude or middle of peak —  Change click polarity Troubleshoo;ng: Waveform Enhancement —  Wave IV —  May be Absent in Normal Subjects —  Often Fuses with Wave V —  Compare latency with contralateral side —  Change click polarity —  Wave V —  Often fuses with Wave IV —  Identified by high amplitude trough following wave —  Non-­‐stimulated ear may be resolved better —  Decrease stim intensity CNIM QUESTIONS —  Expect to see a BAERs recording from a surgery with multiple waveforms compiled on one another and random changes throughout. Arrows may point to various times of changes and they may ask what is happening at that point. Anesthetic changes? Physiological changes? Drilling? Etc.