Auditory Brainstem Response ABR
Jan 05, 2016
Auditory Brainstem Response
ABR
What is ABR??
It is short latency, small amplitude,
far field electrical potentials
responses of the auditory pathways
that occur within 10—15 ms of an
appropriate acoustic stimulus in
normal subjects.
Why important?-The stability of these potentials over subject state.
-The relative ease with which they may be recorded.
-Their sensitivity to dysfunctions of the peripheral and brainstem auditory systems
The ABR consists of a series of 5–7 waves. Labeled: wave I to VIIThe potentials comprising the ABR arise from the auditory nerve, as well as brainstem structures
OriginWave I arise from the central end of the eighth nerve.
Wave II arise from the central end of the eighth nerve.
Waves I and II arise from structures ipsilateral to the side of stimulation.
Later waves may come from structures that receive ipsilateral, contralateral, or bilateral inputs
Basic ABR Measures
AmplitudeAmplitude typically is measured between a positive peak and the following negative trough Peak-to-peak measures are favored because they avoid the difficulty of determining the baseline of the potential.
LatencyAbsolute latency: The time difference between stimulus onset and the peak of the wave, such as wave I , Wave III and Wave V.
Interwave latencies (or interpeak intervals) are the differences between absolute latencies of two peaks, such as I–V, I–III, and III–V
Stimulus Parameters
IntensityIt has effects on:Latencies: increase as stimulus intensity decreases.Amplitudes : decrease as the intensity decreases. Waves : diminish and ultimately vanish, whereas Wave V often remains discernible down to levels approximating the behavioral thresholds for the same stimulus.
Stimulus intensities employed generally range between 40 and 120 dB pe SPL.
dBnHL: “decibels above normal hearing level” or dB HL (dB above the average hearing threshold of a group of normal young adults tested by the same laboratory under conditions identical to those used for recording BAEPs clinically)
SpectrumIt is influenced by the stimulus plateau and rise/fall durations.
The response is insensitive to the stimulus duration but quite dependent on the rise/fall times.
Response amplitudes decrease and latencies increase as rise time increases.
Clicks: are the most commonly used
stimuli for eliciting the ABR. The abrupt
onset and broad spectrum of a click
result in synchronous excitation of a
broad population of neurons. The click is
usually the most effective stimulus and
can provide high frequency
information
Tone Burst: are frequency-specific,
transient stimuli, so there is a
spread of energy around the central
frequency. The effective rise time
may become progressively longer as
the frequency decreases. This may
reduce synchrony in the apical end
of the cochlea, making it more
difficult to measure.
PolarityNegative pressure in front of the earspeaker diaphragm are referred to as rarefaction clicks
Positive pressure in front of the earspeaker diaphragm are referred to as condensation clicks
In certain pathologic conditions
associated with severe, steep high-
frequency hearing loss, BAEPs
elicited by rarefaction clicks may
differ in latency and, to a degree, in
morphology from BAEPs evoked by
condensation clicks
Many examiners prefer to use stimuli
of alternating polarity, which help
to minimize stimulus artifact and the
CM, both of which can obscure Wave
I. This approach can reduce or
eliminate the need for
electromagnetic shielding of the
earphone.
Stimulus RateStimulus rates employed vary widely
from 5 to 200/s. depending on test
applications. As the stimulus rate is
increased, the latencies of all the waves
are prolonged and the amplitudes of the
early waves are decreased.
MaskingIt is recommended that the contralateral
(nonstimulated) ear be masked by white
noise at 60 dB SPL to eliminate
“crossover” responses, i.e., bone-
conducted responses originating in this
ear.
When??
Recording Parameters
BandPass
The recommended system bandpass for
BAEP recording is 10-30 to 2,500-3,000
Hz
Analysis timeAn analysis time of 10-15 ms from
stimulus onset is suggested. An analysis
time of no less than 15 ms is sometimes
required to demonstrate extremely
delayed responses in certain pathologic
conditions. Analysis times of 15 ms are
also essential for neonatal and
intraoperative recordings.
Signal Averaging
It is suggested that about 1,000-4,000
individual trials be averaged until good
waveform resolution has been achieved.
Two or more responses must be
obtained and superimposed to
demonstrate replicability.
Electrode placement
1- Positive: on the scalp at the vertex
2-Negative: over the left and right mastoid processes
3- Ground electrode may be placed anywhere on the body.
Patient StateBAEPs can be obtained during either wakefulness or sleep.
Sedation may occasionally be indicated with very young or tense patients,
Natural sleep OR hypnotics OR anaesthesia
Effect of Patient Age
ABR evaluations in premature infants
and newborns require the use of age-
adjusted norms and necessitate the use
of a wider analysis window (e.g., 15–20
ms) than is typically used for adults
(e.g., 10 ms).
Effect of Patient gender
Start in adolescence, males begin to develop longer Wave V latencies than females, which by adulthood amounts to an average intersex difference of approximately 0.2 ms.
Females display slightly larger Wave V amplitudes than do males.
Conductive hearing loss Effect
Conductive hearing losses cause prolong the latencies of all the waves of the ABR due to the effective lowering of the stimulus level.
Otoscopic examination, immittance testing are valuable.
It prolong the latencies of the waves without affecting interpeak latency value and cause essentially the same degree of latency shift at all stimulus levels
Cochlear hearing loss Effect
Sloping high frequency losses of moderate severity, and more, cause increased latencies.
The pure tone audiogram would be useful for accurate interpretation of the ABR evaluation..
Various corrections for Wave V latency have been suggested to take into account degree of peripheral loss
Analysis of the Results
Analysis
Amplitude
LatencyResponse
Absolute
Interpeak
Ratio
Absolute
Yes
No