VISUAL EVOKED POTENTIAL

A presentation on Visual evoked potential

1. VISUAL EVOKED POTENTIAL
Dr Saurabh Kushwaha
Resident (Ophthalmology)

2. SCOPE
Introduction
VEP stimuli
Types of VEP
Equipments required
Prerequisites
Recording of VEP
Properties of VEP waveform
Interpretation
Clinical applications

3. VISUAL EVOKED POTENTIAL
EEGisarecordoftheelectricalactivityofthebrain,
obtainedbyplacingsurfaceelectrodesonthescalp
VEPisan'evoked'electrophysiologicalpotential
recordedfromscalpinresponsetovisualstimuli
Itassesstheintegrityofthevisualpathwaysfromthe
opticnervetotheoccipitalcortex

4. ANATOMIC BASIS OF VEP
VEPgrosslyover-represent
themacularregionprimarily
becauseanatomically:
•Macularfibersprojectto
theoccipitallobecortex,
andthosefromthe
peripheralretinaproject
deeperwithinthecalcarine
fissure
•Also,overthecourseof
thevisualpathway,the
macular fieldgets
‘amplified’asitreachesthe
cortex

5. VEP STIMULI
VEPcanbeevokedbyeitheraflashoflightora
pattern
FlashStimuli
•TheflashVEPiselicitedby
flashesoflightproducedbya
xenonarcphotostimulator
•Occipitalcortexisrelatively
insensitivetoflash

6. PatternStimuli
•Thesearepresentedinachecker-boardpattern
•‘Patternonsetoffset‘formwhereinthepatternis
shownforabriefperiodandthenreplacedwitha
blankscreen(ofthesameintensity)
•‘Patternreversal’typewhereintheblackandwhite
checksreversetheirorientation
•Luminancecontamination:Theevokedpotential
shouldbeinresponseonlytothechangingpattern
andnotduetoachangeinlightintensity(brightness)
Therefore,theaverageluminancemustbekept
constantthroughoutthetest

7. TYPES OFVEP
FlashVEP
•Lesscommonlyused
•Usedinuncooperative
andunconsciouspatients
PatternVEP
•Mostcommonlyusedin
clinicalpractice.
Chromaticpatternedstimuli
•Helpfulindetectingcolor
blindness.

8. TRANSIENT AND STEADY -STATE
VEP
Ifthevisualstimulusisintermittent,thusallowingthe
braintorecoveritsrestingstateinbetween,thenthe
VEPobtainediscalledtransientVEP.
•TransientVEPisusedforallpracticalpurposes.
Ifhowever,thestimulusisprojectedfastersothatthe
braindoesnotregainitsrestingstate,asinusoidal
waveformcalledsteady-stateVEPisobtained.
•Notusedroutinelyduetoinferiorinformationon
latencyoramplitudecomponents.

10. FLASH VEP
FlashResponsetodiffuselyflashinglightstimulusthat
subtendsavisualfieldof20degrees
Itisperformedinadimlyilluminatedroom
CruderresponsethanpatternVEP
Merelyindicatesthatlighthasbeenperceivedbycortex
Indications-mediahaze,infants,poorptcooperation

11. PATTERN REVERSAL VEP
Responsetoapatternedstimulus-checkerboardor
squareandsinewavegratings
FrequencyofgratingsisdescribedinCPD-cycles
perdegree
Forcheckpatternvisualanglesubtendedbyasingle
checkisused
Preferredtechniqueformostclinicalpurposes,gives
anestimateofformsenseandthusvisualacuity

12. PATTERN ONSET/OFFSET VEP
Apatternisabruptlyexchangedwithan
equiilluminantdiffusebackground
MoreintersubjectvariabilitythanpatternreversalVEP
Usefulindetectionofptswithpoorfixation,
malingering,deliberatedefocusing,ptswithnystagmus

13. EQUIPMENTS REQUIRED
Visualstimulusproducingdevice
Scalpelectrodes
Amplifier
Computerandreadoutsystems

14. PREREQUISITES
Thereshouldbenodistractingsoundorlightwaves
Patternandflashmustbothbedoneinallpatientsas
patterncannotbedetectedinptswithmediaopacities
PatternVEPfollowedbyflashVEP
Procedureissignificantlyaffectedbyeccentric
fixation,excessiveblinkingofeyesandpartialclosure
ofeyes

15. RECORDING OF THE VEP
Recordedmonocularlywithundilatedpupils
Refractivecorrectionandinarelaxedpositionatthe
calibratedviewingdistance(1mdistancefrommonitor)
Thepupillarysizeshouldbenotedforeach
evaluation
Whilemonocularstimulationisstandard;inchildren
orotherspecialgroups,binocularstimulationmaybe
usedtoassessvisualpathwayconductionfromeither
eye
WhileperformingflashVEP,amechanicalpatch
shouldbeappliedovertheunstimulatedeye

16. PROPERTIES OF THE VEP
Amplitude:
•Itistheheightofthewave(vertical)measuredin
microvoltsfromtheprecedingtrough
•Absoluteamplitudeisdifficulttocomparebecause
ofthelargevariationbetweennormalpersonsand
variationsinsensitivitiesoftherecordingequipment
•Relativeamplitude(differencebetweenthetwo
eyes)ismoresensitivewhenlookingforunilateralor
asymmetricdisease
•Ingeneral,absoluteamplitudeofP100lessthan
05microvoltsisabnormal

18. Latency:
•Measuredinmilliseconds,itisthedelaybetween
thestimuluspresentationandthepeakofthewave
inquestion.
•Latencyshowsmuchlessvariationbetween
subjects
•However,itisalsoaffectedbyanumberof
factors,includingpupilsize,refractiveerror,ageand
stimulusfactors(patternsize,luminanceand
contrast)
•LatencyofP100waveshouldnotexceed110ms
inpatientsundertheageof60years.

20. Waveform:
•Agedependentandarestandardizedfora
populationbetween20-60yearsofage.
•Inastandardwavepattern,thetimefromstimulus
onsettothemaximumpositiveornegativeexcursion
oftheVEPisrecordedas"peaktime“
•TheflashVEPpatterncomprisesaseriesofpositive
andnegativedeflections,withapeaktimevarying
between30msto300ms,themostrobustpeaks
beingtheN2andtheP2peaksatabout90msand
120mspeaktimerespectively

21. •PatternreversalVEPwaveformscomprisetheN75,
P100andN135peaks(Fig.A)
•Standardpatternonset-offsetVEPsdemonstrate
threemainpeaksinadults;theC1positivepeakat
about75ms,theC2negativepeakatabout125ms
andC3,anotherpositivepeakatabout150ms(Fig.B)

22. FACTORS INFLUENCING VEP
Sizeofstimulus-Decreaseinsizeofstimulusincreases
amplitudeofVEP
Positionofelectrodesonscalp
Age-amplitudedecreaseswithage
Gender-P100latencyislongerinadultmalesand
meanamplitudeisgreaterinfemales
Pupilsize-PupillaryconstrictionincreaseP100latency
whichisattributedtodecreasedareaofretinal
illumination
Eyemovements-reducestheamplitudeofP100but
latencyisnotaffected
Attentionofpatient-Ifsubjectlookstosideofstimulus,
thereisrapidfallinsizeofresponse

23. INTERPRETATION
EacheyeprojectstoB/Loccipitalcortexviaoptic
chiasma
UnilateralVEPabnormality-Anteriorvisualpathway
lesion(prechiasmallesion)
BilateralVEPabnormality-Nolocalizingvalue
Latencyprolongation
•P100Latencyprolongation>3SDorinterocular
latencydifference>10msecissignificant
•ProlongedP100latency-demyelinatinglesions,
retinopathiesandglaucoma.

24. Amplitudereduction
•AmplitudeofP100showswideindividualvariation
•Hence,Interocularamplituderatioisusedto
detectabnormalities
•InterocularP100amplituderatio>2issignificant
•Reducedamplitudeindicatesaxonallesionslike
AION
Combinedlatency&amplitudeabnormalities
•Opticnervecompressionproduceresultsin
segmentaldemyelinationandaxonalloss
•Henceitproducescombinedlatencyand
amplitudeabnormalities

25. CLINICAL APPLICATIONS

26. PATTERN VEP
Opticneuritisandmultiplesclerosis(MS):
•CharacteristicallyshowsadelayedP100latency
•Findingpersistsevenafterrecoveryofvisualacuity,
hence,itcanbeusefulinconfirmingapreviousattackof
opticneuritis
•However,itmaynotbeabletodelineateafreshattack
•PatientswithMSbutwithoutahistoryorclinical
featuresofopticnerveinvolvementcanshowabnormal
VEPresponsestopatternstimulation,suggesting
subclinicalvisualpathwayinvolvement.

27. Left retrobulbaroptic neuritis showing a delayed P100component

28. Nonarteriticanteriorischemicopticneuropathy
(NAAION):
•Typicallyshowsreducedwaveamplitudebut
latencyisnotsignificantlydelayed
•VEPintheclinicallyuninvolvedeyeisinvariably
normal
Compressivelesions:
•Showsprolongedlatencyatanearlystage,though
notasmuchasinopticneuritis
•Muchhigherincidenceofwaveformabnormalities
thaninpatientswithdemyelinatingdisease
Functionalvisualloss:
•Distinguishesbetweenorganicandfunctional
visualloss
•AnormalpatternVEPestablishesthepresenceof
anintactvisualpathway

29. Optic nerve Ethambutoltoxicity showing slow P100 peaktimes

30. Patientswithmotordisorders:
•Suchpatientsmayappearvisuallyimpaired
becausetheeyescannottrackamovingtarget.
•inchildrenwithcerebralpalsy,isapreferred
modality,butmaybedifficulttoperformitbecauseof
seizureactivity,wanderingeyemovementsor
depressedcorticalactivityduetoanticonvulsants.
Nystagmus:
•Useofhorizontalgratingsproduceslessblur.
•Patternonsetpreferredratherthanpatternreversal
providemoreaccurateinformation.

31. THANK YOU
Childrenwithneurofibromatosistype1arevulnerableto
developmentofopticnerveglioma
VEPcanbeamoresensitiveandcosteffectivetestto
followtheprogressofnervepathologythanMRItestsalone

32. MalingeringandHysteria:
•PatientswithHystericalBlindness.
•VEPremainsnormalwithvisionaslowas1/60.
•VEPcanbeenhancedbyusinglargefields,large
checksandbinocularvision.
DuringOrbitalorNeurosurgicalProcedures:
•Continuousrecordofopticnervefunctioninform
ofVEPtopreventinadvertentdamagetothenerve
duringsurgicalmanipulation

33. FLASH VEP
Canbeperformedinshortertime
Noneedforactivepatientparticipation
Usedinadultsandinfantswithdensemedia
opacitiestotestthevisualpathwayintegrity
•AgoodcorrelationbetweenVEPpredictionand
actualpostoperativevisualacuityhasbeenseenin
patientswithdensecataracts.
Usedasaprognostictoolpriortovitrectomyin
diabeticvitreoushemorrhage
Toevaluatethecentralnervoussysteminhigh-risk
neonates,especiallythosebornprematurely,with
intraventricularhemorrhage,orhydrocephalus.

34. MULTIFOCAL VEP
mfVEPrecordedwiththesameequipmentasformfERG
VEPsrecordedsimultaneouslyfrommultipleregionsof
thevisualfield
Visualfielddividedinto60sectors,eachhaving16
checks(8blackand8white)
Sectorsandchecksarescaleddifferently(peripheral
sectorslarger)sothattheyareallofapproximatelyequal
effectivenessforcorticalstimulation.
Pseudo-randomsequencesandasoftwarealgorithm
allowtheon-boardcomputertorapidlyextractinformation
simultaneouslyfromeachofthestimulatedsectors.
mfVEPprovidesaprobabilityplotlikeautomated
perimeters

35. Usedtodetectsmallabnormalitiesinvisualsignal
transmissionfromcentricandeccentricfieldandprovides
atopographicaldisplayofthesedeficits
Toruleoutnon-organicvisualloss
Todiagnoseandfollow-uppatientsofopticneuritisand
multiplesclerosis
Toconfirmunreliableorquestionablevisualfield
examinations

36. MULTICHANNEL VEP
Thistechniqueusesmultipleactive(parasagittal)
electrodes
Thistechniqueprovideslocalizingvalue
Chiasmallesionsshowacrossedasymmetry,i.e.
findingsofoneeyeshowanasymmetricaldistributionthat
isreversedwhentheothereyeisstimulated.
Retrochiasmallesionsshowuncrossedasymmetry,
whereinfindingsofeacheyeshowanasymmetrical
distributionacrossthehemispheresthatissimilarwhen
eithereyeisstimulated

37. THANK YOU