AEPs Ahmed Khater, MD, PhD Ass. Prof. of audio-vestibular medicine Consultant 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 OHCs From the auditory nerve 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 Middle Late (the first 10 ms), (10–80 ms) (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 Mesogenous Endogenous

Exogenous: conditions external to the listener (e. g 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.

Diagram of AEP Subject Traces Bioamplifier Acoustic stimulus Computer 4- Signal Averaging 1- Filtering Traces 2- Artifact rejection 3- A-C converter

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).

Recording 1- 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

Any unwanted electrical activity…..Noise Non-Biologic Biologic BioAmp. Power Muscle EEG

Noise Reduction 1- 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 Band pass filter Band reject filter Low pass filter High pass filter Band pass filter Band reject filter

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 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

VEMP

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

EP & MRI The 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.

How often are we missing it? If somebody comes in with DKA, head CT is probably not the first thing we would do 4 cases cited under Pubmed