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