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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 < optic nerves next to the nose
cross at optic chiasm.
RT < 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 °enerativediseases.
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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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