Auditory Brainstem Responses (ABRs/BAEPs/BAERs)

what does BAEP stand for

brainstem auditory evoked potential

what does BAER stand for

brainstem auditory evoked response

what are auditory brainstem responses

auditory electrical potentials generated by the auditory nerve and brainstem following stimulation to the hearing pathway

what do we assess when monitoring ABRs

integrity of the auditory cranial nerve (CN 8) & brainstem b/c these structures are at risk

what type of stimulation is used for ABRs

broad-band click

for ABRs, we stimulate at the __ & record __

distal location; proximally

sound enters the __

external acoustic meatus

sound waves cause a vibration of the __

tympanic membrane

the tympanic membrane is connected to the __

ossicles

the ossicles connect to the __ via __

cochlea; oval window

___ movement leads to generation of the electrical potential that is relayed to the CN VIII

hair cell

where does the cranial nerve carry potential from? to?

from = cochlea

to = brainstem

ossicles include the __

malleus, incus, stapes

fluid movement at the oval window leads to __

depolarization

hearing pathway

sound --> tympanic membrane --> ossicles --> oval window --> cochlea --> distal auditory nerve --> cochlear nucleus --> superior olivary complex (SOC) --> lateral lemniscus --> inferior colliculus (IC) --> medial geniculate nucleus --> primary auditory cortex

cochlear nucleus

initial site that all auditory signals from CN VIII are obtained and processing begins

superior olivary complex (SOC)

contributed to the processing of sound timing & intensity --> projects UPWARDS to the pons

has CONTRALATERAL & IPSILATERAL side

lateral lemniscus

projection to higher brainstem structures & continued signal processing

on CONTRALTERAL side

inferior colliculus (IC)

receives all LOWER brainstem auditory input for processing and integration; contributes to spatial localization of sound

CONTRALATERAL

medial geniculate nucleus

processing of auditory input from BOTH ears

final processing and projection of MULTIPLE sound characteristics of the cortex

primary auditory cortex is part of the __ lobe

temporal

2 components of auditory nerve

1) distal = close to the cochlea

2) proximal = close to the brainstem

ABRs begin at the __

distal auditory nerve (AFTER the cochlea)

components of ABR come __ the cochlea

after

AFTER the sound goes through the ear pathway, it creates an __ that travels down to the __ to the __ & then to the __

action potential; cochlear nerve; brainstem; cortex

obligate ABR waveforms

wave I (WI)

wave II (WII)

wave III (WIII)

wave IV (WIV)

wave V (WV)

wave I is the?

distal auditory nerve

wave II is the?

cochlear nucleus (medulla-pons junction)

wave III is the?

superior olivary complex (medullla-pons junction)

IPSILATERAL auditory potentials are noted at BOTH the IPSILATERAL & CONTRALATERAL SOC

wave IV is the?

lateral lemniscus (contralateral)

wave V is the?

inferior colliculus (contralateral midbrain)

which wave is the MOST PROMINENT/BIGGEST

wave V

which waves are the most reliable/consistent

WI, WIII, WV

alert criteria is based on which waves?

I, III, V

which waves do we monitor

I, III, V

which waves do we not typically label even if they are presenting

II & IV

are there STANDARD latencies for ABRs

no just typical latencies

are ABR latencies shorter/longer than SSEPs?

shorter

interpeak latency for WI-WIII

~2 msec

interpeak latency for WIII-V

~2 msec

interpeak latency for WI-V

~4 msec

a tube inserts adds ___ msec to increase wave latency

0.9-1 msec

ABR amplitudes are typically __

1 uV or LESS

typical latencies:

WI

WII

WIII

WIV

WV

1.5

2.5

3.5

4.5

5.5

latencies WITHOUT tube compensation:

WI

WII

WIII

WIV

WV

2.5

3.5

4.5

5.5

6.5

WI is nearfield/farfield?

nearfield (distal auditory nerve potential)

WII-WV is nearfield/farfield?

farfield

what is the electrode that obtains a response at WI

Ai/Ac (Mi/Mc)

what is the electrode that obtains a response at WII-WV

Cz (CPz)

MPower Health ABR montages:

ipsilateral trace

contralateral trace

referential trace

non-cephalic (optional)

Ai/Mi-Cz (CPz)

Ac/Mc-Cz (CPz)

Ai-Ac (WI)

Erbs-Cz or Cs3-Cz (WII-WIV)

ACNS ABR montages (9C):

ipsilateral trace

contralateral trace

Cz-Ai/Mi

Cz-Ac/Mc

ACNS ABR montages (11C):

ipsilateral trace

contralateral trace

referential (optional)

Cz-Ai/Mi

Cz-Ac/Mc

Mi-Mc/Ai-Ac (optional)

what is the ground placed per MPower

can be placed with SSEP headpack on the shoulder/trap

what is the ground placed per ACNS

Fz

common types of surgery that may need ABR monitoring

microvascular decompression (MVD), acoustic neuroma, suboccipital craniotomy for tumor resection

microvascular decompression (MVD)

suboccipital craniotomy approach performed to remove abnormal compression of a cranial nerve by an artery near the brainstem --> retraction on cerebellum & brainstem

acoustic neuroma

tumor on the acoustic nerve (CN 8)

suboccipital craniotomy for tumor resection places retraction on the __

cerebellum

If the surgical approach places the auditory nerve at risk, we monitor ABRs to __

detect nerve stretch or nerve irritation --> leads to INCREASED latencies

If the surgical approach places the brainstem at risk or involves heavy retraction on the

brainstem/cerebellum, ABRs are used to __

assess brainstem integrity

CN __ is the shortest nerve

VIII

ABR kit includes?

ABR transducers

connection cable

ear inserts

tube

ABR transducer function

generates clicks using air pressure (stimulus generator)

what does the connection cable connect

ABR transducers to the base unit

what does the tube connect

transducer to the ear insert

the stimulation generated by the transducers are a series of broadband clicks that are delivered one/two ear/s at a time?

one

does the ipsilateral or contralateral ear ALWAYS get the white noise masking?

contralateral

white noise masking

prevents a crossover response from the ipsilateral ear being stimulated

what happens to the contralateral side if there is NO white noise masking

bone conduction crossover

do we run ABRs on interleaving stimulation

NO --> only ONE ear at a time

3 types of polarity

rarefaction

condensation

alternating

rarefaction polarity

produces NEGATIVE pressure at the tympanic membrane

OUTWARD pull like a suction

gives the BEST WI

condensation polarity

produces POSITIVE pressure at the tympanic membrane

gives the BEST WV

alternating polarity

alternates between rarefaction & condensation clicks

produces potentials

great for REDUCING stim artifact

stimulate one/two ears, record from one/both sides?

one; both

when we stimulate ABRs from one side, we record both __ & __ montages

ipsilateral & contralateral

in the IPSILATERAL montage, we expect to what waves?

I, III, V

in the CONTRALATERAL montage, we expect to what waves?

III & V

also gives a better separation of WIV & V --> helps identify WV

do we expect to see WI on the contralateral montage?

NO b/c the active electrode gives us a WI and Ai

interpeak latency

time between two waves

which wave will we see if there is NO white noise masking/stimulating too HIGH

I

ABR stimulation parameters

intensity = 80-100 dB SPL

contralateral masking = ½ of stim intensity

pulse width = 100 us

rep rate = 11.1-33.3 Hz

ABR recording parameters

LFF = 30-100 Hz

HFF = 100-150 Hz

amp gain = 10 uV/Div

trials = 500-3000

analysis time = 1-2 ms/div

smoothing filters = ABR1 & ABR2

dB SPL means?

decibels sound pressure level

patient set-up for ABRs

1) check ABR transducers BEFORE placing them to make sure they are working --> plug into base unit & test ONE at a time

2) check patient's ear to make sure there is NOT access cerumen

3) compress ear insert & firmly place into ear canal

4) seal around insert with bone wax

5) bend tube & secure it to the patient's skin with a tegaderm --> can use multiple layers

6) clip transducer to patient to prevent it from hanging

7) acquire baselines

1st & 2nd layer of protection around the ear for ABR set up to protect against fluid

1st = multiple tegarderm

2nd = bone wax

purpose of bone wax

helps with holding insert in place and prevents fluid from getting into the ear canal

when should ABRs be paused

when the surgeon is using bovie, bipolar, drill, burr b/c it will cause reject

effect of propofol & halogenated inhalational agents on ABR waveforms

MINIMAL latency prolongations & MINOR decreases in amp

effects of barbiturates on ABR waveforms

MINIMAL increase in latency without ANY effect on amp

does nitrous oxide have an effect on ABR waveforms

NO

effects of hypothermia on ABR waveforms

increase in WI latency; increase in interpeak latencies

significant cooling = COMPLETE LOSS of ABRs

cold irrigation in the field may contribute to __ along with __

increase in latency; facial nerve neurotonic EMG discharge

default to preferred/standard anesthetic reigmen if we are running ABRs with SSEPs or another anesthetic-sensitive modality?

preferred

what can cause a change in ABRs

mechanical trauma

thermal trauma

ischemia

mechanical trauma

stretching/cutting CN VIII or direct trauma to the brainstem

thermal trauma

CN 8 being heated up by cautery

ischemia

trauma to auditory (labyrinthine) artery or basilar artery

labyrinthine artery

perfuses cochlea

ABR alert criteria when monitoring to assess brainstem function

wave V latency INCREASE of 1 ms

DECREASE of wave III or V amplitude by 50%

report LOSS of any ABR waveforms