Evoked Potentials.akhil

8/14/2019 Evoked Potentials.akhil

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VISUAL

EVOKED POTENTIALS

AKHIL.B.S

08/08/09


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INTRODUCTION

Evoked responses measure theelectrophysiological response of

the nerve system to a variety ofstimuli.

In clinical practice only a feware used on a routine basis.

They are VEP,SSEP,BAEP,MEPolfactory and taste evokedresponses.


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VI SUA L EVOK EDPO TENT IA L VEP tests the function of the

visual pathway from the retina

to the occipital cortex.


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Anatomy and physiology

of VEPThe light signal enters into the eye

It is converted to an electrical signal

in the retina.It travels through the optic nerves to

the visual sensory area of the brain

The ESG in the retina is calledelectroretinogram (ERG)


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It passes through the lens,

The right half retina sees the left half VF. The left half retina sees the right half VF.

RT &LT optic nerves next to the nose

cross at optic chiasm.

RT &LT optic nerves next to the ear do

not cross.

After the optic chiasm the optic nerves

are called optic tract.


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It pass through lateralgeniculate body.

Spread out and reach the visualsensory area .

Back of brain as opticradiations.

There are two optic radiations

for each side. Upper half VF& lower half VF

image.


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The potential is generated in the

VSA can be recorded as VEP.

By the help of VS.

EP is recorded from theelectrode at the same side of the

VS.

Main component of VEP is P100. In VEP recording the abn of the V

Pathway can be evaluated by

changing the VF by pattern.


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vis ual evokedpote ntials


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Stimulus parameters of

VEP Two types of stimuli are used

for recording VEP.

They are pattern and flash(Goggle).


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Techni cal aspe cts Checkerboard pattern or flash

is used as stimulation.

Responses are collected over

Oz, O1, and O2 and with hemi

field studies at T5 and T6electrodes using the standard

EEG electrode placement.


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Sedation should not be used. Impedance should be kept

below 5k.

Montage used in VEP is

OL-FZ

OR-FZ

OZ-FZ


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Pa ttern VEP Visual stimulation by alternating the black

and white checkered Pattern.

Triphasic evoked potential is recorded

from the occipital area. It is known as pattern VEP.

Pattern VEP has three peaks, they areN75,P100,N145.

N75&P100 potentials are generated in thecortical visual sensory area.

P100 with the peak latency of about

100msec.


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The visual angle

=(360/2)*(w/d)=57.3*(w/d).

Where w= horizontal screen

size.

Where d=distance from the pt

&display.


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In full vfs have the pt watch the

centre of the screen.

In half vfs have pt watch thenon stimulation side about

1degree from the fixation point.


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Stimulation settings

Contrast ratio of w&b:50% or

higher.

Frequency: less than 1 HZ(b&wcounted as one cycle).


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waveforms

Usual waveform

Initial negative peak ( N75)

Large positive peak ( P100) Another negative peak (N145)

Maximum value for P100 is 110

milliseconds (ms) in patients < 60years

Thereafter> 120 ms in females; 125ms in males


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Flash VEP

Under anesthesia and coma

In young children

Non cooperative patients

When checking for integrity of

visual pathways

White flashes are recommended

1 flashes/sec


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Variable in flu encing VEP Age in flu ence the l atency of p 100. The p 100 la tency i s longer inadult ma les c ompared tofemale. Eye dominance(le ss prolonged&higher amplitu de).


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Eye movement re ducesampli tude n o change inlatency. Visu al Acuity-wit h signifi cantreduction in acuity p100 latencycan be prolonged. Drugs can i ncreased p100latency. During me nta l activation p100la tency can decrease and theampli tude can increases.


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Basics of VEP

abnormalities Latency prolongation

Amplitude reduction

Combined latency andamplitude abn


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The commonest prolongation of

p100 is demyelination in the

optic pathways.

Axonal lose in the optic nerve

reduces the amplitude.


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Multiple sclerosis(10-30ms). Optic neuritis.

Ischemic optic neuropathy.

HIV infections. Nutritional &toxic optic

neuropathy.

Hereditary &degenerativediseases.


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ABNORMAL VEP


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SUJJET KUMAR PATHAK 27/M

Patient with IIH

DV 6/6 bl

Vep -pattern


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Shajahan.M.56y/M

Demyelination

DV normal


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John lukose 52/m

Bi optic neurotic


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Sruthy.b 15/f

Optic neuropathy


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NORMAL VEP


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Abraham .M.G 47/m

Migraine.


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