AEPs

Ahmed Khater, MD, PhDAss. Prof. of audio-vestibular

medicineConsultant audiologist

Head of cochlear implant Program

Evoked potentials (EPs), or evoked

responses, measure the

electrophysiological responses of the

neural cells to a variety of stimuli.

Audio-vestibular Evoked Potentials

From the vestibular

nerve

From the auditory nerve

From the OHCs

VEMP

OAEs

Early, middle & late AEPs

Auditory evoked potentials span

activity from the full length of the

auditory pathway, from cochlear hair

cells to cerebral cortex, as well as

activity associated with cognitive

processing auditory stimuli.

Indications Threshold detection

For intra-operative monitoring

Diagnosis of site-of-lesion.

Neonatal screening

CAPD

AEP

Early(the first 10 ms),

Middle(10–80 ms)

Late(80 ms to 500+ ms).

Auditory brainstem response (ABR) peaks denoted by Roman numerals; I, II, III, IV, V, VI, VII

Middle latency response (MLR) are indicated by Po, Na, Pa, Nb, and Pb

Auditory late response (ALR) are indicated by P1, N1, P2, and N2.

AEP

Exogenous

Endogenous

Mesogenous

Exogenous: conditions external to the listener (e.g. stimulus intensity or duration). Mesogenous AEPs fall somewhere in between exogenous and endogenous as both external and internal conditions can affect the response eg. MLR. Endogenous conditions internal to the listener (e.g. attention, vigilance)

Latency & Amplitude

The more caudal structures in the

auditory nervous system would

have shorter onset latencies than

the more rostral structures. This

latency increase for more rostral

structures is the result of the finite

action potential conduction

velocity and the delay as the

activity passes through chemical

synapses.

Following presentation of an acoustic

stimulus, We can record activity in the

cochlea, auditory nerve, auditory

brainstem, medial geniculate body,

and auditory cortex.

Acoustic stimulus

Subject

Bioamplifier

4- Signal

Averaging

1- Filtering

2- Artifact

rejection

3- A-C converter

Computer

Traces

Diagram of AEP

Stimulus 1- Tone Burst

(long duration, long rise/fall times, and

a very narrow spectrum)

2- Click

(short duration, very fast rise/fall times,

and very broad spectrum).

Recording1- Scalp preparation (Water)2- Electrode connection: - Noninverting lead is the positive or the active lead. - Inverting lead is the negative or the reference lead - The common lead is the ground

All are measured through scalp

electrodes EXCEPT

electrocochleography, the ECoG

responses are typically NOT measured

with scalp electrodes, but rather from

electrodes placed in the ear canal, on

or near the tympanic membrane, or on

the promontory or round window of

the inner ear

NoiseAny unwanted electrical activity…..Noise

Biologic Non-Biologic

Muscle EEG

BioAmp.

Power

Noise Reduction1- Low electrode impedances.2- Making the subject comfortable and encouraging them to sleep.3- Filtering the output of the bioamplifier can reduce noise. 4- Signal averaging reduces unwanted background noise.5- Light off.

Filters

Low pass filter

High pass filter

Band pass filter

Band rejectfilter

Electrocochleography (ECoG) refers to the responses from the cochlea and auditory nerve, using a recording electrode located in close proximity to the inner ear.

Labeling: CM, SP, AC

The auditory brainstem response

(ABR), as its name indicates, is a

series of five to seven peaks arising

from auditory nerve and brainstem

structures occurring within 10 ms of

the onset of a moderate-intensity click

stimulus.

Labeling: Wave I to VII

MLRs are usually recorded over a time window of 80 to 100 ms. Generators are thought to include thalamus and auditory cortex.

Labeling: (e.g., Pa, Nb, etc.).

LLR: AEP responses occurring beyond roughly 75 ms. These responses are commonly labeled with a capital N or P (e.g., P1 is the first positive SVP peak). OR N200 would be the negative peak with a mean latency of 200 ms.

There are differences in the responses to the frequent and infrequent stimuli used as an index of a cognitive construct that is related to attention.

If attending………P300

If not attending….MMN

http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&docid=uPk9-h-EqosxDM&tbnid=GsbPDTe08wYQeM:&ved=0CAUQjRw&url=http://www.oxanis.com/about.php&ei=6qwoUtmsF-rb0QWKsYDwBw&bvm=bv.51773540,d.ZG4&psig=AFQjCNEwqxCnO-ZTPGxVX-YT0H6qlz619Q&ust=1378483746117769

VEMP

OAEs

Otoacoustic emissions (OAEs) are sounds measured in the external ear canal that reflect movement of the outer hair cells in the cochlea.

http://www.google.com.sa/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=7GzEhED3q4LCnM&tbnid=fLjtah--zOBDjM:&ved=0CAUQjRw&url=http://www.neuroreille.com/promenade/english/corti/hcells/ohc/ohcoaes/fohcoaes.htm&ei=CaYsUu3fJ4G60QWMoYGYAg&psig=AFQjCNHNuoQWQAyy2efBhEkTJoM844Hybw&ust=1378744194245180

EP & MRIThe basic difference between the 2

studies is as follows: MRI largely remains an imaging,

structural, or anatomic test and therefore gives more accurate information about structural problems

EP testing assesses functionality and thus supplies information about the physiology of a certain anatomic pathway.

In some circumstances, MRI and EP studies may be complementary.