2. Definition
It is the measurement of electrical potential that originate in the
auditory nerve and brainstem that originate in response to sound
stimulation.
TYPES OF CLICK:-
• Acoustic click
• C.E- Chirp
• NB C.E-Chirp
• Tone burst2-1-2
3. Electrode
Placement
• Active electrode-Over the vertex
• Reference electrode-over the mastoid process
• Ground electrode-over the mastoid process of other side
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BY-SA
4. 7 waves which appear within the
first10-15 msec.
Each wave represents
synchronized neuro electrical at
one or more of the synaptic sites
along the auditory brainstem
pathway.
Each wave has a peak and is
given latin numeral.
To
waves from 1 to 5 are
important ones..
This Photo by Unknown Author is licensed under CC BY-SA
This Photo by Unknown Author is licensed under
CC BY-SA
This Photo by Unknown
Author is licensed under CC
BY-SA
5. In all waves with increase in intensity of stimulus there will be increase
in wave amplitude and decrease in wave latency.
Sites of origin of ABR WAVES:-
• WAVE I and II - auditory nerve
• WAVE III - cochlear nucleus
• WAVE IV - superior olivery nucleus
• WAVE V - lateral lemniscus and inferior colliculus
6. CLINICAL APPLICATION
A. Estimation of threshold of hearing:-
-especially in children and newborn where sedation is required.
-it is detected as the lowest intensity at which wave V can be identified.
-accuracy of ABR in estimating hearing threshold lies within 10-20 dB of
behavioural threshold (esp. at frequencies 2000-4000 HZ)
B. Diagnosis of retrocochlear pathology(e.g. acoustic neuroma):-
The following finding are suggestive of acoustic neuroma-
-delay in the latency of wave V
-interaural latency delay of wave V greater than 0.2 msec
-delay wave I-V interpeak interval(IPI)(normally 4 msec)
7. -no recognizable wave from (absent ABR) in the affected ear.
Advantages- 95% diagnostic rate
Disadvantages-
Less accurate in diagnosing small tumors
Less accurate in diagnosing other CPA lesion which may not cause compression of the VIII
th Nerve.
MRI is better
C. Diagnosis of brainstem lesions:-
diagnosis of:-
brain stem tumors
dissiminated sclerosis
there will be:-
distortion or absence of ABR inspite of normal PTA and speech audiometry
8. D. Intraoperative monitoring:-
-intraoperative monitoring during surgeries that may endanger the VIIIth Nerve.
Ex—
surgery for acoustic neuroma and other CPA lesions,
Vestibular nerve selection,
Facial nerve decompression
-surgeries that may endanger brainstem:-
Tumor resection
Cardiopulmonary bypass
E. Monitoring comatosed patients ICU
10. Disadvantage:-
• ABR can produce a clear wave form only with hearing threshold of 74 dB or
better
• At higher threshold levels there will be absent ABR regardless the cause
• Individuals with hearing losses restricted to particular frequencies may
show normal ABR
• Of limited value in Hearing aid fitting
• In conductive deafness-
Wave latencies are prolonged i.e. mistaken for retrocochlear lesion, but
interwave latencies are normal.
• Temperature below 36 deg. Celsius or above 38 cause prolonged latency.
