VSP 2: Electrophysiology Part 5

VEP

visual evoked potential

Visual evoked response VER

Visually Evoked Cortical Potential VECP

VEP is the the only electrophysiologic test that measures

visual cortical activity

VEP measures

the functional integrity of the visual pathway

retina to optic nerve to the visual cortex

VEP signal

small

5 uV

embedded in the electro-encephalogram EEG

EGG

electrical potentials of 60 UV

Generated by the brain in response to stimuli

VEP waveforms are extracted from the EEG through the process of

Signal averaging

signal to noise ratio

ratio of the cortical visual response to the underlying EEG

The VEP is triggered by a

precise and defined repetitive visual stimulus

Vs the EEG signal which is

random relative to the visual stimulus

Averaging the signals recorded while filtering out the noise

VEP visual stimuli

light

VEP usually recorded from

occipital lobe using electrides

We can measure

the amplitude o how strong the response

the latency - how quickly it gets there

VEP is a clincal measure of vosual pathway funciton that is used

when there is unexplaied vision loss

Response of the VEP reflects activity in the

primary visual/striate cortex at the posterior tip of the occipital lobe

VEP measures primarily activity from the _______ degrees of the VF only

central 10

due to the topographical representation of the VF in the striate cortex (macular fibers in the back)

VEP asses

gross integrity of the macular cortical pathway

CENTRAL VISION

VEP findings must be clincally correlated with other tests infcluding

clinical exam (retina, VA)

ERG

VF/OCT

Neurologic Exam/Imaging

VEP electrodes

1. active (on occipital region)

2. Ground

3. Reference

VEP signals from the active and and reference go into a differential amplifier

electrical activity that is common to both electrodes is filtered out

leaving just the visual response

Waveform

Amplitude - strewnght of sigal

Latency - measures time to peak response

The latency of the VERP is most similar to what measurement of the ffERG

Multiple types of VEP stimuli

Flash VEP

Pattern VEP

Multifocal VEP

Sweep VEP

Flash VEP used when

- cant get pattern VEP

which can happen if

- very poor VA

- media opacity

- Poor pt fixation

Flash VEP results in

complex series of waves

N2

latency of 70 to 90 msec

P2

latency of 100 to 120ms

Pattern VEP

Checkerboard Pattern reverses every 1/2 second

Better inter-subject reliability

preferred test

Always recorded with best refractive correction in place

Pattern VEP waves

N1 or N70

P1 or P100

N2 or N140

Check sizes affect VEP

Smallest check size he pt can see clearly

largest amplitude

fastetst peak latency

we can estimate VA using check sizes

Checks are typically high contrast

high contrast checks > amplitude than low contrast checks

Therefore can use patter VEPS to emasure

contrast sensitivity

low contrast VEP check --> absent or increased latency in multiple sclerosis

Multifocal VEP

Variant of flash VEP

assess small or localized optic nerve pathway problems

changing pattern of discrete flashing

reversing check patter

measures approx 25 degrees of VF

produces spatially discrete pattern of responses

Mf VEP - simulataneous recordings of different locations of VF may provide more accurate assesment of visual field defects than a tuypical pattetnVEP

due to emasurements of alrger cross sectional area of the optic nerve

mfVEP helpful in

glaucomatous defects

optic neuritis

Sweep VEP

rapid measure of vision

multiple pattern sizes are presented in quick sucession

checks or stipes

amplitude of sVEP is measured

as pattern is reduced in secons

Used to asses

grating/VA

contrast

Vernier acuity

Objective and fast asses,net of VA or cotrast

VEPS can be impaired by

malingerers

purposeful poor fixation

VEPs clincally

can over estimate VA obtained with other methods

does not localize the lesion (except mfVEP)

any lesion affecting central vision may result in abnormal VEP

see slides for when to use electodiagnostic tesing

- pediatr

ambylopia: EOG

normal

(retina not involved)

ambylopia: ERG

normal

(retina not involved)

ambylopia: VEP flash

normal

ambylopia: pattern VEP

ambnormal

Multiple sclerosis: ERG

Normal

multiple sclerosis: VEP flash

abnormal

increased latency

multiple sclerosis: VEP pattern

abnormal

increased latency

Retinitis Pigmentosa: EOG

abnormal

Retinitis Pigmentosa: ERG

abnormal

scoptopic absent

photopbic present but reduced

Retinitis Pigmentosa: VEP

normally eary and if pt retains central vision

Macula lesion: EOG and ERG

normal

Only small portion of the retina

Macula lesion: multi focal ERG and P50 wave of PRG and pattern erg

abnormal