Instrumentation Used In The Measurement of Acoustic Signals And Auditory Function

Dr. Ghulam SaqulainM.B.B.S., D.L.O., F.C.P.S

Head of Department of ENTCapital Hospital, Islamabad

Auditory Evoked Potentials

Instrumentation Auditory Brainstem Response Audiometry (ABR) Other AEPs

Instrumentation

AIMS OF ABR

Diagnosing Auditory impairment in the newborn.Testing auditory sensitivity in the young and difficult to test

child.Assessing neurologic diagnosis of cochlear nerve disorders.Monitoring the seventh nerve and brain stem pathways

during surgery.

During surgery of the posterior cranial fossa.Prognosis in coma patients. Establishing brain death.

AUDITORY EVEOKED POTENTIALSTransient responsesSustained responsesPerceptual responses.

UTILIZATION OF THESE RESPONSESTransient and sustained responses – more objective.Perceptual responses – less objective

ABR DIAGNOSTIC LIMITS AND ADVANTAGES.LIMITS:

1. No information for frequencies below 1000 Hz.

2. The information provided on central auditory pathways have and upper limit at the inferior border of mid brain.

3. Influence of age gender and body temperature.

ADVANTAGES:

1. ABR is recorded through non invasive electrodes.

2. It is not influenced by: A state of awake fullness or sleep. Drugs acting on central nervous system including

sedatives and anesthesia.

ABR BASIC BIO-ELECTRIC PHENOMENAThere are two evoked potentials defined ‘far-field’ and

‘near-field’ depending on the distance of the recording electrodes from the nervous generator

In near field, wave 1,2 and 3 are generated from hair cells and cochlear nerve fibers.

The far field waves 4 and 5 are recorded through surface electrodes recording from vertex, mastoid and forehead.

THE BIO ELECTRIC SIGNALSThe route of travelling of electric signal:The bioelectric output from each neural generator

propagates through the encephalic tissues, the meninges, CSF, cranial bones and soft tissues thus reaching electrodes.

These bio electric oscillations on their way from generators to the electrodes face an opposing impedance of the tissues through which they propagate.

PLACEMENT OF ELECTRODES FOR ABR RECORDINGThe electrodes may be placed at vertex – recording

electrodes.Ear lobe or mastoid- reference electrode.Forehead- common electrode.

ACOUSTIC STIMULATION

The stimulus of the choice is the click, with a duration of 100 micro second.

Its acoustic spectrum contains all frequencies with equal energy.

The intensity of the click is measured in physical units, dB SPL.

STIMULATION RATE

The rate per second is an important parameter in ABR testing. Odd numbers are best

11 clicks/s are used to establish normal 7 nerve and auditory pathways function. No modification in the response up to around 20/s.

For neurodiagnosis the common strategy is to start recording at 11/s to establish a reference response, which will be compared with pathologic ABR responses.

STIMULATION BY AC AND BY BC

BC stimulation is seldom used, for several purposes:

Because of the lower intensity levels of the stimulus by BC it may be difficult to detect a clear-cut supra threshold reference ABR, only N V might be clearly detectable.

The BC vibrator as mastoid, close to the ear lobe electrode, might engender electromagnetic artefacts, especially with refraction click.

ABR by BC may help diagnosis only in few cases. Its routine use is time consuming and scarcely efficient.

ABR ELICTED BY THE CLICK

The response elicited by a 100 us click with 10 ms analysis time consists of 7 waves:

Wave 1-5 are constant,Wave 6-7 are less constant .

Intensity/latency function

Decreasing the intensity level of the stimuli, includes the following phenomena:

1. Increased latencies of the ABR waves.

2. Inter-peak latencies remain constant.

3. The first components (N I,N II and N III) progressively disappear with decreasing levels.

4. N V will remain present even at threshold and/or slightly above threshold levels.

NORMAL LATENCIES

According to james W. Hall (handbook of Auditory Evoked responses, 1992) for a 100 dB pe SPL these are the normal latency values. N I latency=1.5 msEach following wave: add 1ms, therefore:N I-N III interval: NIII latency: 1.5 + 2 = 3.5 ms.N V latency: 3.5ms + 2 ms = 5.5 ms.

ABR seems to be characterized by a rule of “5”At high intensity, N V amplitude is 0.5 mV. N I – N V inter

peak latency is normally 4.5 ms

( 5 ms in the new born)

ABR NEURAL GENERATORS

Interpertation for ABR

latency

amplitude

morphology

N I originates from the portion of the VIII nerve in the modiolus of the cochlea, in the internal auditory canal and in the crebello-pontine angle.

N II originates from the cochlear nerve portion in proximity of the brain stream.

(These first two components are those typical of ECochG)

N III reflects the activity of the cochlear nucleus.

N IV and N V are generated by the superior olivary complex, the lateral lemniscus and its nuclei, and the inferior colliculus in the brain stem

N VI and N VII, less constant represent the activity of the inferior colliculus and the geniculate body.

VARIABLES IN OTOLOGICALLY NORMAL SsThere are three main variables:

CHILDHOODSEX AGE

Childhood:In the normal newborn (1 month or less from birth),

N I is delayed by 0.3 ms with respect to the adult, N III is just detectable and N V is clear cut and delayed.

From 3 months onward, the ABR response acquires its standard shape.

At 1 year of age, the latency of the various waves reaches values of the adult.

GENDER: latencies are 1.5ms shorter than in males, with

0.075 uV greater in amplitude.

AGE: N V latency increases 0.2ms from 25 to 55 years.

Body Temperature

N I – N V inter peak latency may vary when body temperature varies of 1 degree with respect to normal (37 C = 98.6 F)