Transcript auditory brainstem evoked response (abr)

AUDITORY BRAINSTEM
EVOKED RESPONSE (ABR)
DR. Fawzia AlRouq
PHD. (NEUROPHYSIOLOGY)
What is evoked potential?
Electrical potentials that occur in the group of
neuron in response to stimulation of a sense
organ which can be recorded by surface
electrodes is known as Evoked Potential.
eg. SEP, ABR and VEP
Introduction
• Auditory brainstem response (ABR) is a
neurologic test of auditory brainstem
function in response to auditory (click)
stimuli.
• It’s a set of seven positive waves recorded
during the first 10 seconds after a click
stimuli. They are labeled as I - VII
PHYSIOLOGY
• Auditory brainstem response (ABR)
typically uses a click stimulus that
generates a response from the hair cells of
the cochlea, the signal travels along the
auditory pathway from the cochlear
nuclear complex to the inferior colliculus in
mid brain generates wave I to wave V.
ORIGIN OF BAEP
Origin of each wave
Wave
Origin
I
Cochlear nerve
II
Dorsal & Ventral cochlear nucleus
III
Superior olivary complex
IV
Nucleus of lateral lemniscus
V
Inferior colliculus
VI
Medial geniculate body
VII
Auditory radiation(cortex
Objectives
• At the end of this session the students should be
able to:
Acquire the principles involved in the test.
 Identify & describe the genesis of the brainstem
potentials in order to use them to study
brainstem conductions in health & disease.
 Construct evoked response audiograms to
identify conductive & sensorineural hearing loss.
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Requirements
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Clinical averager
Ear phone
Silver cup electrodes
Electrode jelly
Patient leads
Electrode placement (Montage)
• Cz (at vertex) (recording electrode)
• Ipsilateral ear lobule or mastoid process
(reference electrode).
• Contra lateral ear lobule (act as a ground)
Procedure
• Subject lying supine with a pillow under his
head.
• Room should be quite.
• Clean the scalp & apply electrode.
• Check the impedance.
• Apply the ear phone (red for the right ear
& blue for the left ear)
• Select the ear in the stimulator & apply
masking to the opposite ear.
Contd..
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Stimulation rate : 11/sec.
Repetition : 2000
Find out the threshold of hearing.
ABR should be done at around 80dB.
Start averaging process & continue until
the required repetition accomplished.
• Calculate the peak – interpeak latencies
for the ABR waves.
Normal values
• Peak latency of a wave =
less than the next higher
no. wave
• Or just add 1 to that
wave, latency will be less
than that.
eg. Latency of wave 1 is
less than 2.
Wave
Latency
I
<2mSec.
II
<3 m.sec
III
<4 m.sec
IV
<5 m.sec
V
<6 m.sec
VI
<7 m.sec
Identification of waves
• Identify wave V which is the most persitent
wave. It comes as IV-V complex, and
wave V comes to the base line.
• Go in reverse order, wave IV, III, II, & I.
• Also observe their latencies, eg. latency of
wave I will be less than 2mSec.
Calculation & Analysis
• Write down the absolute peak latencies for
the waves
• Find out the interpeak latencies of I – III,
III – V and I – V.
Interpretation
• Wave I : small amplitude, delayed or absent may
indicate cochlear lesion
• Wave V : small amplitude, delayed or absent
may indicate upper brainstem lesion
• I – III inter-peak latency: prolongation may
indicate lower brainstem lesion.
• III – V inter-peak latency: prolongation may
indicate upper brainstem lesion.
• I – V inter-peak latency: prolongation may
indicate whole brainstem lesion. Shortening of
wave the interval with normal latency of wave V
indicate cochlear involvement.
APPLICATIONS
• Identifying the hearing loss
• Classification of type of deafness
(conductive or sensorineural)
Contd…
• Identification of retro choclear patholgy
Auditory brainstem response (ABR)
audiometry is considered an effective
screening tool in the evaluation of
suspected retrocochlear pathology such
as an acoustic neuroma or vestibular
schwannoma.
NEWBORN SCREENING
• Several clinical trials have shown auditory
brainstem evoked response (ABR) testing
as an effective screening tool in the
evaluation of hearing in newborns, with a
sensitivity of 100% and specificity of 9698%.
IN SURGERY
• Intraoperative monitoring
• Auditory brainstem response (ABR), often used
intraoperatively, provides early identification of
changes in the neurophysiologic status of the
peripheral and central nervous systems. This
information is useful in the prevention of
neurotologic dysfunction and the preservation of
postoperative hearing loss. For many patients
with tumors of CN VIII or the cerebellopontine
angle, hearing may be diminished or completely
lost postoperatively, even when the auditory
nerve has been preserved anatomically.
EVOKED RESPONSE
AUDIOMETRY
• The procedure of evoked response
audiometry is same as that of ABR.
• Here the evoked response is obtained with
different intensities of click stimuli.
• First start with a stimulus intensity of 80 dB.
• Put the record in memory
• Change the stimulus intensity to 60 dB.
• Put the record in memory.
Cont…
• Continue the test with stimulus intensity of
40 dB, & 20 dB.
• In all these tracings identify wave V and
find out its latency at all intensities of
stimuli.
• Plot latency – intensity graph for wave V.
Interpretation
• Normal : There is a shaded area for the
normal person. If your points fall in this
area then the person is having normal
hearing.
• Conductive Deafness: the latency –
intensity graph plotted will be above &
parallel to the shaded area.
• Sensorineural Deafness: the graph plotted
will be irregular & not forming a curve.
CASES