ABR

Event related potentials are responses that

are time locked to some events, the events

possibly could be Sensory stimuli like visual

flashes or it could be mental events like

recognition tasks or other events.

When the used event is the sound, the

evoked potentials are known as Auditory

Evoked Potentials (AEP).

It is s one of these Auditory evoked potentials.

Sound used as a stimulus could be click, Pure tone, Tone burst or pipes or speech.

ABR Magnitude < 1 μv

ABR waves may be picked up by superficial electrodes. The first 5 waves will be recorded and used for diagnosis.

They are transient responses.

It occurs 1 to 15 seconds after the

stimulus.

The major measures used to diagnose are

the latencies and amplitudes.

ABR is not a test for hearing, it’s a test for

neural synchrony instead.

It is a test for the auditory pathway from

the cochlea up to the brainstem level, it

does not go beyond that.

So, if there is a cortical hearing loss ABR

waves may become present and the

results will be normal!

The ABR waves originating from different

sites along the pathway from cochlea up

to the brainstem :

Wave I : from the proximal part of the 8th

nerve (auditory nerve).

Wave ǁ: from the distal part of the 8th

nerve (Auditory nerve).

Wave III: from the cochlear nucleus ( in

the brainstem).

Wave VI: from Superior Olivary Complex

(SOC) ( a nucleus within the brainstem).

Wave V: from Lateral Lemniscus (LL) and

Inferior Colliculus (IC).

Waves’ recordings contain peaks and

troughs, some are more identifiable than

the others.

Peaks I, III & V are the most identifiable

and clinically more reliable.

Amplitudes, latencies and the

relationship between the peaks usually

used for differential diagnosis.

Latency:

From stimulus onset to the emergence

of peak or trough.

The measured latencies are:

Absolute latencies of waves I, III, V

And the inter-peak latencies ( latencies’

differences) I-III, I-V and III-V.

IT5 ( Inter-aural peak latency of wave v),

the difference between wave V absolute

latency between both side, it should be

within normal limits (not more than 0.3 to

0.4 ms ).

When IT5 measures, a symmetrical or

unilateral hearing loss should be

considered.

Amplitude:

It is the size of peak to peak measure or

baseline to peak for waves I, III and V (V

usually larger than I).

1. Age:

Prolonged latencies found in infants up to

2 years-old >> due to anatomical reasons.

Correction factor need to be used in the

preterm babies >> delivered 8 weeks

earlier , for instance at 4 month the

results should be compared to 2 months

norms.

2. Gender:

Females have shorter latencies and higher

amplitudes >> difference of 0.1 to 0.2 ms.

There is a positive relationship between

head size and ABR peaks’ latencies.

It was being thought that the Brainstem

size is the reason for such difference

between sexes ( BS pathway shorter)..

But more likely the reason behind is the

speed of the traveling wave ( faster in

female ) >> female cochlea 13% shorter

than male one ( Sato et al, 1991)

There is no difference between the

children from 5 and 7 years from different

sex but the difference become apparent

from 8 to 11 years.

3. Stimulus:

Increase in the stimulus results in higher

amplitudes and shorter latencies >> for

example at 80-90 dBnHL wave V latency may

be at 5-6 ms and high amplitude.

Decrease in the intensity results in increasing

latencies and decreasing amplitudes and may

result in loss of earlier waveforms >> for

instance

At 30 dBnHL wave V latency may be at 7-9 ms,

other waves may not be identified and the

amplitude will be short.

It has 2 systems: stimulating and recording system.

Stimulating system

Recording system

Stimulus

generator Attenuator Stimulus

Amplifier

Transducers Patient

Electrodes

Physiologic Amplifier &

pre-amplifier

Averages

Filter Display Printer

Headphone or insert earphones used to deliver the stimulus. Bone oscillator also can be use to deliver the signal in the Bone conduction testing.

Ear canal needs to be clear, especially when the inserts used.

The type of the transducer should be clearly reported as this leads to differences.

They are used for recording.

Typical electrode placement (Montage):

Non-inverting (+): Vertex (Cz) or high forehead, active ( maximal response activity).

Inverting, Reference (-): Earlobe or the mastoid where there is a lower activity( ipsilateral mastoid).

Ground: required for proper amplifier

functioning, on the forehead or the

contralateral mastoid.

ABR recorded by measuring electrical

activity between 2 electrodes, averaging.

Testing environment should be comfortable

and quiet with low lighting level.

Whether the patient sleep, sedated or

under GA all better for a proper test.

Parents need to be relaxed and

comfortable and able to feed the baby,

once the parents comfortable the baby can

sleep well.

Test can be done while the baby in his/her

travel or car seat.

Take care of any interference , ensure that

all mobile phones or other devices are off

and not on silent!.

Location of the ABR room within the

hospital is critical. For example, next to

the MRI is not an ideal place

Epoch:

Time base or recording window.

It started in correspondence with the

starting of the stimulus. Usually it’s about

15 to 20 ms. (ABR usually the first 10 ms)

Amplifier gain:

ABR peaks are small therefore the electrical activity needs to be amplified.

Stimulus type:

ABR requires short duration stimuli for synchrony across neurons to record the response.

Clicks, tone pips can be use depending on what test you are applying and what you are looking for.

Clicks usually used in adults (oto-

neurological, diagnostic).

Tone pips or frequency-specific chirp used

in threshold test for infant and children to

give more frequency-specific information.

However, low frequency sounds (250-500

Hz) have longer onset and responses are

harder to record around threshold.

High rate used when you want to reduce

the recording time as much as you can >>

60 /second in threshold ABR.

Low rate used when you want to maintain

the responses’ characteristics >> 11.1-

17.1 / second in oto-neurological ABR.

At different intensities from high to low

in threshold ABR >> for example 80,

60,40,30.

At one and high intensity (e.g. 80-90

dBnHL), make it twice in diagnostic ABR.

To improve signal to noise ratio.

ABR is small response hidden by noise signals.

ABR is time locked but the noise occurs randomly.

2000 sweeps usually used.(this number is not standard)

Types of ABR test

Neuro-otological ABR Threshold

ABR

Uses:

Detection of Retro-Cochlear lesions Auditory nerve tumor, Intra-axial tumors

(within the B/S) or extra-axial tumors ( Outside the B/S)

Demyelnating diseases, e.g. MS.

Vascular lesions of the B/S, blockage or hemorrhage of the blood vessels.

D.D of comatose patients: whether it is

Metabolic or toxic cause coma ( drug

overdose) vs coma as a result of structural

lesion.

Status of auditory nerve and B/S

pathways.

Possible effects:

Site of lesion affects the ABR results of

the ear in which the abnormalities

manifested.

In general:

Auditory nerve lesion, Ipsilateral effect.

Large CPA tumors, may see also

contralateral effects due to B/S

compression.

B/S lesions may cause either Ipsilateral,

contralteral or bilateral abnormalities of

ABR .

It’s not finding threshold, so wave V is not

the only one to be checked. All waves

from I to V need to be clear.

Intense stimulus level needed ( between

60 & 90 dBnHL) and just to be done at one

intensity.

Must be repeated. At least there should be

2 similar waveforms clear before marking

the waves I to V.

Click used as a stimulus.

IPL

IPL of I-V, normal is about 4 ms.

IPL of I-III and III-V can be obtained as

well. This will help to pinpoint the location

of the lesion more precisely.

IPL can be also compared between right

and left side of the patient.

IT5

It is very important measure especially when wave I cannot be elicit and therefore IPL I-V cannot be measured.

It may be more sensitive than absolute latency of wave V ( may be within normal range ) but when both sides compared, a clear difference presents.

There should not be 0.3 to 0.4 ms difference between the ears.

beware of

Unilateral or a symmetrical hearing loss as

this may account for IT5 difference .

So ensure that patient’s PTA is obtained

before neuro-oto ABR to be administer.

This help in choosing the stimulus

intensity and interpreting the results.

In other word, with severe Peripheral

hearing loss and Absent ABR , Oto-neuro

ABR cannot be used as part of assessment

( the machine limit).

Uses:

From its name , it estimates hearing threshold.

Interpretation:

Wave V most robust.

Well correlated with behavioral audio threshold.

As intensity decreases , most of ABR

waves disappear except wave V which

can be still elicit within 10-20 dB of

behavioral threshold.

Normally begin the test at 60 dBnHL or higher and continues to decrease until wave V no longer seen.

Once you think the threshold has reached, you have to repeat at that level ( intensity)

Click usually used, for now being there is no longer agreement as it gives information from 2to 4 KHz only.

In normal and CHL, click threshold is usually 10-20 dB higher than the best HF audio threshold.

For cochlear loss, this value is reduced to about 5-10 dB.

Maximum output of the click is between 90-100 dB, so it will be difficult to differentiate between severe and profound cases.

So, absence of wave V at the maximum output does not mean no hearing.

In many centers nowadays tone burst ABR

used rather than click to give more

frequency specific info.

Usually 1 and 4 KHz tested.

Latency differences at different

frequencies. At 500Hz different sittings

required.

Using ABR in patients with proved

neurological damage is faulty, as wave V

may not be a valid indicator of peripheral

hearing sensitivity.

ABR is a sub-cortical measure, it does not

truly measure the whole hearing.

It assesses the integrity of the peripheral

auditory system and B/S pathways

BEFOREE reaching the cortex.

Cortically deaf patients will have normal ABR

ABR may overestimates the degree of HL.

Some patients with no ABR at maximum

machine limits may have normal hearing on

behavioral hearing tests. In those patients

ART and OAEs need to be done >> Auditory

neuropathy.

Even if ABR considered as the gold standard

of audiology, it needs to be done as part of

test battery and should not relied upon in

isolation.

Tested using BV and it should be away

from auricle and electrodes

It’s used to detect CHL

1 frequency testing is enough and 500 Hz

is recommended

It’s recommended to use wide-band click

or chirp and correction factor should be

applied (subtract 5 dB at low frequency

from the level at which wave v detected)

Do not go beyond 55 dB as the

interferences will be greater at higher

levels

Threshold ABR Neuro-Oto ABR

- Hearing test in DT population - Diagnostic

- Wave V - wave I to V

Latency and amplitude - Latencies

- Take longer time - IPL and IT5

- Mdico-legal - Site of lesion

Which ABR and Why?