1) The auditory brainstem response (ABR) is an objective test that assesses auditory function from the peripheral auditory system to the lower brainstem by recording electrical potentials in response to auditory stimuli. 2) ABR waves are generated as the signal travels along the auditory pathway. Wave I originates in the cochlear nerve and wave V originates in the inferior colliculus. 3) ABR is used clinically to evaluate hearing thresholds, detect retrocochlear lesions, and monitor the brainstem during surgery. Abnormalities can indicate conditions like tumors or demyelination.

1. Auditory Brainstem Response
ABR
Alia El-Karaksy
Lecturer AudioVestibular Medicine

2. 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

3. • AEPs are classified according to the time domain they
are occupying into: First, Fast, Middle, Slow & Late.
• AEPs are also classified according to the time
characteristics of the evoking stimulus to:
• 1- Transient responses: generated by short duration
stimuli.
• 2- Sustained or steady state potentials generated by
continuous stimuli.

4. Human Auditory Evoked Potentials
Transient Responses Sustained (steady-state) responses
First 8th CN Action potential Cochlear microphonic &
Summating potentials
Fast Auditory Brainstem Freq-following response (FFR)
Responses steady state responses (ASSR)
Middle Middle latency response 40-Hz Potentials
Slow Slow vertex potentials Sustained cortical potentials
(P1, N1, P2, N2)
Late N2-P3 complex Contingent negative variation

5. • The auditory brainstem response (ABR) is a far-field
recording of stimulus-locked synchronous neural events.
• ABR is an objective, early-latency neurologic test of
the auditory brainstem function in response to
auditory stimuli.
• ABRs assess the auditory function from the peripheral
auditory system to the level of the lower brainstem.

8. 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.

10. ORIGIN OF BAEP

11. Origin of each wave
Wave Origin
I Proximal Cochlear nerve
II Distal Cochlear nerve
III Dorsal & Ventral cochlear nucleus
IV Superior olivary complex
V
Nucleus of lateral lemniscus/
Inferior colliculus
VI Medial geniculate body
VII Auditory radiation(cortex)

12. The most widely used ABR measure is the latency of a
component peak. The latency of a peak is the time from the
onset of the stimulus to the point of maximum deflection of a
given peak.
 interpeak latencies of about 2
msec for I-III and III-V and 4 msec
for I-V.

13. Instrumentations
• Clinical averager (enhance a low amplitude response on the
expense of a relatively higher amplitude noise.)
• Ear phone (insert/ headphones)
• Electrodes
• Patient leads

14. Electrode placement (Montage)
• Cz (at vertex) (recording electrode)
• Ipsilateral ear lobule or mastoid process
(reference electrode).
• Low forehead/ chin (act as a ground)

15. ABR CLINICAL APPLICATION
• 1) Determination of hearing thresholds
• In infants and young children
• In uncooperative patients/malingrers
• Patients whose intellectual capacities are too severely impaired, mentally
retarded.
• 2) Oto-neurological diagnosis (Retrocochlear pathologies)
• 3) intraoperative monitoring

16. 1) ABR in threshold estimation
• To estimate auditory sensitivity, ABR waveforms are obtained at a series of
click intensity levels, beginning at a moderate to high intensity (e.g. 60-70 dB
nHL) and continuing down to lower levels until wave V is no longer seen. If no
response occurs at the starting level, intensity is increased.
• If wave V is present at 20 dB nHL, this is considered normal.
• At least 2 waveforms should be obtained at each stimulus level so repeatability
can be examined.
• wave V of the ABR is used because it is the most robust of the waves
and the one best correlated with behavioral thresholds.
• As stimulus intensity decreases, the amplitude of wave V decreases and
its latency increases.
After the wave v intensity series is obtained, two factors are used in
hearing estimation: wave V threshold and the LIF.

18. Wave V latency intensity function (LIF)
Clinicians have used the shape and slope of wave V latency-magnitude function
to derive some insight into the possible nature of the patient's hearing
impairment, for example, conductive, sensorineural, and if sensorineural is it
likely to reflect a cochlear or a neural lesion.

20. 2) Oto-neurological diagnosis
Different hypotheses explain the occurrence of ABR
changes in space occupying lesions:
1- A tumor may stretch or compress nerve fibers which may
slow conduction velocity.
2- de-synchronization of the firing of nerve impulses.

21. ABR Indices in Retro-cochlear Lesions
I.latency measures
• 1- Interpeak latencies (IPL): The most powerful index. I-V IPL is usually 4.0
msec (sd 0.2 msec). I-III IPL is slightly above 2.0 msec & III-V IPL is slightly
below 2.0 msec (sd 0.1 to 0.2 msec). I-III IPL is mainly affected in acoustic
neuroma, other CPA lesions and in low brainstem lesions, whereas III-V IPL
is mainly altered in high brainstem lesions.
• 2- Absolute latencies: If wave I is absent or ambiguous, one has to rely on
absolute latency of wave V by comparing it with norms. If wave V latency is >
2 sd from the norms, one should diagnose retrocochlear lesion.
3- Interaural latency difference (ILD): it compares absolute wave V latencies
between both ears, assuming normal value for one ear. If ILD is greater
than 0.4 msec, one should be concerned about retro-cochlear lesion.

22. II. Waveform Morphology
1- Total absence: 25 – 50% of tumor cases showed absent waveforms, depending
on tumor size. Generally speaking, one should be able to elicit ABR waveforms
if stimulus presentation level is 20 dB above 3000-4000 Hz-thresholds.
2- Partial absence: Absent wave V or waves III & V in presence of wave I is
indicative of low BS or CPA lesion.
III. Amplitude Ratio
It was found that in 50% of acoustic neuroma Amplitudes of V/I is less than one
(normally should be more than one, since wave V amplitude is always bigger
than wave I amplitude). To adopt this index, this amplitude ratio should be
repeatable in different replications.

23. A middle-aged patient with a leftsided vestibular schwannoma
(acoustic tumor) with the waveform showing a poor morphology Wave
I and no replicable later waves

24. IV. Repetition Rate
The use of high RR stresses the auditory system and may uncover a hidden
lesion. Neural refractory period is increased in lesions and therefore the
neuron is not ready to react to high rates of stimulation.
V. Opposite Ear effects
Latency changes in non-tumor side with absent waveforms in the tumor side
were found in big tumors with shift of neural structures to the other side.

25. • In patients with eighth nerve tumors, the most reliable
diagnostic measurements to use are the I–III and I–V IPL
measurements and the ILD, either alone or in
combination.
• Using these indices in the interpretation of the ABR
results in hit rates of 90% or better and false-positive
rates of 20% or less (Musiek et al., 2007).

26. ABR in Demyelinating diseases
• Myelin is neurolipid that wraps the axons of all nerve fibers in the CNS
acting as insulator to prevent exchange of ions between the exterior & the
interior of the axons except at specific areas called nodes of Ranvier.
• At these nodes the process of Na influx into the cell occurs leading to
depolarization. Thus, the action potential the travels through the nerve is
transmitted by high speed since it jumps between successive nodes
(sultatory conduction). The lack of myelin results in slowing of impulse
conduction affecting different evoked potentials in respect.
ABR abnormalities in MS
• 25% of MS patients have latency prolongation, 45% have amplitude
diminution for waves II to V, and 30% have both amplitude & latency
changes. The most commonly affected parameter is the III-V IPL. Poor
repeatability of responses were reported in 80%.

27. ABR in BRAINSTEM LESIONS
• it appears that ABR is not as sensitive to brainstem lesions as it is to 8th
nerve tumors.
• A study that investigated the sensitivity for a variety of brainstem lesions
found the hit rate was approximately 80%, which was about 10% lower.
• However, it appears that the ABR hit rate for brainstem involvement is
dependent on the type of disorder, ABR abnormalities for intrinsic brainstem
lesions are high (>95%), whereas for various degenerative disorders the hit
rates are only moderate, the precise location of the lesion (sometimes the
auditory pathways are not involved when a brainstem lesion exists

28. • Tumors of the brainstem are divided: extra- and intra-axial.
• Extra-axial lesions encroach on the brainstem from outside. For example,
tumors of cranial nerves IV through VIII and of the cerebellum as well
meningiomas.
• vestibular schwannoma that grows large enough (usually 2 cm or greater) to
affect the brainstem. analyses infer hit rates of around 80%
• Intra-axial tumors arise from within the brainstem with the most common
being gliomas, ABR is highly sensitive to intra-axial tumors (>90%)

29. 3)ABR in Intra-operative Monitoring
• The merit of ABR here is that it is not affected by Anesthetics like halothane or by
nitrous oxide, unlike all evoked potentials at 50 msec latency and up.
ABR Applications
1- Posterior cranial fossa operations: especially when hearing preservation is an
aim in CPA tumor removal
2- Cardiovascular surgery: In open heart with hypothermic technique and under
respiratory arrest to monitor the brainstem condition.
• ABR performance in comatose patients: (not affected by drugs as barbiturates and muscle
relaxants).
1- Monitoring brainstem status.
2- As a prognosticator of comatose patients: absent ABR indicates poor prognosis, while intact ABR
indicates a good one.
3- Diagnosis of brainstem death

30. Limitations of ABR
1- The Issue of Frequency Specificity
• click-evoked ABR stimulates the entire cochlea due to the fact that the click is
a broad band stimulus. However, due to number of factors the surface
recorded potentials represent activities generated from the area 3000 to 4000
Hz of the cochlea with no available sensitivity information from the low
frequency region.
• Tone burst ABR: is solution for poor frequency specificity. However, the poor
agreement between burst ABR & behavioral thresholds especially for low
frequency stimuli is a real difficulty.

31. 2)STANDARD ABR MEASURES CANNOT
DETECT SMALL TUMORS
• Two requirements for any ABR measure used for tumor detection:
(a) the tumor exerts sufficient pressure to desynchronize, block, or alter the
conduction properties of 8th nerve elements
(b) the tumor affects a sufficient number of those neural elements.
• The high failure rate in detecting small intracanalicular (in the internal auditory
meatus) tumors is not surprising because normal standard ABR latencies are
possible if the synchronous activity of the high-frequency fibers that determine
the latency is not sufficiently compromised by the tumor.