CLINICAL VISUAL OPTICS OPTO 223: WEEK VIfac.ksu.edu.sa/.../files/clinical_visual_optics_week_6.pdf · 2017-08-05 · CLINICAL VISUAL OPTICS OPTO 223: WEEK VI Dr Salwa Alsaleh. WEEK

CLINICALVISUALOPTICSOPTO223:WEEKVI

DrSalwaAlsaleh

WEEKIV :OUTLINE• Retinalelectrophysiologyreview• Electrophysiologicalvisiontests:• ERGtest• VECPtest

REVIEWOFRETINALANATOMY

HYPERPOLARIZATION ANDDEPOLARIZATION• Hyperpolarization isachangeina cell's membranepotential thatmakesitmorenegative.Itistheoppositeofa depolarization.Itinhibits actionpotentials byincreasingthestimulusrequiredtomovethemembranepotentialtotheactionpotentialthreshold.

RETINALELECTROPHYSIOLOGY• ►Receptorlevel:hyperpolarisation• ►Post‐receptorlevel• Bipolar cells :depolarisation orhyperpolarisation• Horizontalcells:hyperpolarisation• Amacrinecells:transitoryhyperpolarisation• Ganglionar cells :depolarisation orhyperpolarisation• ►Pre‐receptorlevel:depolarisation afterhyperpolarisation

ELECTROPHYSIOLOGICALVISIONTEST• Electrophysiologicaltestsofvisionarediagnostictestsfortheretinathaturesesoneormoreoftheelectrophysiologicalstepsofvisionprocesses,e.g.photoreceptorsinteractionwithincominglight.

• Thesetestsareimportantindiagnosingseveralretinaldysfunctionsortodetecttoxicologicaleffectsofsomeagentsontheretina.

• Therearemanyelectrophysiologicaltests,weshallonlydisscuss2mainones.

ERG&VEP

ELECTRORETINOGRAPHY(ERG)• Theelectroretinogram(ERG)isadiagnostictestthatmeasurestheelectricalactivitygeneratedbyneuralandnon‐neuronalcellsintheretinainresponsetoalightstimulus.Theelectricalresponseisaresultofaretinalpotentialgeneratedbylight‐inducedchangesinthefluxoftransretinalions,primarilysodiumandpotassium.Mostoften,ERGsareobtainedusingelectrodesembeddedinacornealcontactlens,whichmeasureasummationofretinalelectricalactivityatthecornealsurface.

COMPONENTSOFERG• a‐wave:initialcorneal‐negativedeflection,derivedfromtheconesandrodsoftheouterphotoreceptorlayers

• Thiswavereflectsthehyperpolarizationofthephotoreceptorsduetoclosureofsodiumionchannelsintheouter‐segmentmembrane.Absorptionoflighttriggerstherhodopsintoactivatetransducin,aG‐protein.Thisleadstotheactivationofcyclicguanosinemonophosphatephosphodiesterase(cGMPPDE)eventuallyleadingtoareductioninthelevelofcGMPwithinthephotoreceptor.Thisleadstoclosureofthesodiumionchannelsresultinginadecreaseofinwardlydirectedsodiumions,orahyperpolarizationofthecell.Thea‐waveamplitudeismeasuredfrombaselinetothetroughofthea‐wave.

COMPONENTSOFERG• b‐wave: corneal‐positivedeflection;derivedfromtheinnerretina,predominantlyMullerandON‐bipolarcells

• Thehyperpolarizationofthephotoreceptorcellsresultsinadecreaseintheamountofneurotransmitterreleased,whichsubsequentlyleadstoadepolarizationofthepost‐synapticbipolarcells.Thebipolar‐celldepolarizationincreasesthelevelofextracellularpotassium,subsequentlygeneratingatransretinalcurrent.ItisthistransretinalcurrentthatdepolarizestheradiallyorientedMullercellsandgeneratesthecorneal‐positivedeflection.Theb‐waveamplitudeisgenerallymeasuredfromthetroughofthea‐wavetothepeakoftheb‐wave.ThiswaveisthemostcommoncomponentoftheERGusedinclinicalandexperimentalanalysisofhumanretinalfunction.

COMPONENTSOFERG• c‐wave: derivedfromtheretinalpigmentepitheliumandphotoreceptors

• Thec‐waveisareflectionoftheresultingchangeinthetransepithelial potentialduetothehyperpolarizationattheapicalmembraneoftheRPEcellsandthehyperpolarizationofthedistalportionoftheMullercells.Thec‐wavegenerallypeakswithin2to10secondsfollowingalightstimulus,dependingonflashintensityandduration.Duetothec‐waveresponsedevelopingoverseveralseconds,itissusceptibletoinfluencesfromelectrodedrift,eyemovements,andblinks

COMPONENTSOFERG• Latencyofresponse referstotheonsetofthestimulustothebeginningofthea‐wave.

• Implicittime isameasureofthetimeintervalfromonsetofthestimulustothepeakoftheb‐wave.

TYPESOFRECORDINGELECTRODES• Burian‐AllenElectrode‐ (commonlyusedelectrodeforflashERG)variablelenssizesconsistingofanannularringofstainlesssteelsurroundingthecentralpolymethylmethacrylate(PMMA)contact‐lenscorewithalidspeculum

• Dawson‐Trick‐Litzkow Electrode‐ low‐massconductiveMylarthreadconsistingofindividualfibersimpregnatedwithmetallicsilver

• ERG‐JetElectrode‐ adisposableplasticlenswithagold‐platedperipheralcircumference

TYPESOFRECORDINGELECTRODES• MylarElectrode‐ aluminizedorgold‐coatedMylar• SkinElectrode‐ maybeusedasareplacementforcornealelectrodesbyplacinganelectrodeontheskinovertheinfraorbitalridgenearlowereyelid;duetodecreasedamplitudesandvariableresponses,theskinelectrodeisprimarilyusedforscreeningpurposesonl

• Cotton‐WickElectrode‐ Burian‐Allenelectrodeshellfittedwithacottonwickwhichisusefulforminimizinglight‐inducedartifact

• Hawlina‐Konec Electrode‐ Teflon‐insulatedthinmetalwire(silver,gold,platinum)withthreecentralwindows,3mminlength,moldedtofitintothelowerconjunctivalsac.

NORMALERGRESPONSES• TherodandconephotoreceptorfunctionresponsescanbeseparatedusingavarietyofERGtechniques.Scotopic(rod)responsesareisolatedbydark‐adaptationforaminimumof20minutesperISCEVstandardsfollowedbyashortwavelengthstimulusasasingleflashor10Hzflicker.Althoughtheresultingresponsehasrodandconecomponents,therodcomponentisdominantandtheprimarycontributortotheincreasedamplitudeandincreasedimplicittime.Photopic (cone)responsescanbeobtainedeitherbeforeorafterdark‐adaptation.

• Sincerodscannotfollowaflickerstimulusgreaterthan20Hz,conephotoreceptorfunctionisprimarilymeasuredunderlight‐adaptedconditionsforatleast10minuteswitheithersingleflash(stimuluswavelengthgreaterthan680nm)or30Hzflickerstimulus.Photopicresponsesresultinsmallb‐waveamplitudeswithashortlatency(30‐32ms),whereasscotopic(rod)conditionsproducemuchlargerb‐waveamplitudeswithalongerlatency(60ms).

FACTORSAFFECTINGTHEERG• DurationofstimulusSizeofretinalareailluminatedIntervalbetweenstimuliSizeofpupilSystemiccirculationanddrugsDevelopmentofRetinaClarityofOcularMediaAge,Sex,andRefractiveErrorAnesthesiaDiurnalFluctuations

ABNORMALITIESINSOMEDISEASESTATES

VISUALEVOKEDPOTENTIAL(VEP)• Recordofgrosselectricalsignalgeneratedatvisual(occipital)cortexinresponsetovisualstimulation

• Theamplitude ofVEP‐3to25microvoltisconsiderablysmallerthanthatofERG,whichcanbeaslargeas100microvolt

• Averagedandamplifiedrecordofactionpotentialsinvisualcortex

• VEPistheonlyobjectivetechniqueavailabletoassessclinicallythefunctionalstateofthevisualsystembeyondtheretinalganglioncells.

VEPabnormality

Visualpathwayabnormality

STEADY‐STATEVEP &TRANSIENTVEP• Atrapidratesofstimulationthewaveformbecomessinusoidal–STEADYSTATEVEPNotusedroutinelyduetoinferiorinformationonlatency

• Lowratesofstimulation– discretedeflections.KnownasTRANSIENTVEPThis iscommonlyemployed

VEPSTIMULI• FLASH:Occipitalcortexisrelativelyinsensitivetoflash

• PATTERN:Cortexissensitivetoedgesofcontrast

TYPESOFVEP

FLASHVEP• Responsetodiffuselyflashinglightstimulusthatsubtendsavisualfieldof20degrees

• CruderresponsethanpatternVEP• Merelyindicatesthatlighthasbeenperceivedbycortex• Indications– mediahaze,infants,poor patientcooperation

PATTERNREVERSALVEP• Responsetoapatternedstimulus‐ checkerboardorsquareandsinewavegratings

• Frequencyofgratingsisdecribed inCPD‐ cyclesperdegree• Forcheckpatternvisualanglesubtendedbyasinglecheckisused

• Preferredtechniqueformostclinicalpurposes,givesanestimateofformsenseandthusvisualacuity

PATTERNONSET/OFFSETVEP• Apatternisabruptlyexchangedwithanequilluminant diffusebackground

• Moreintersubject variabilitythanpatternreversalVEP• Usefulindetectionofptswithmalingering,ptswithnystagmus

Visualstimulus

Scalpelectrodes

Amplifier

Computer,read‐outsystem

TECHNIQUEOFRECORDINGVEP• Undilated pupil• Monocularrecording• Refractivecorrection• Relaxedposition• 1mdistancefrommonitor

COMPONENTSOFVEP

NORMALVEPDATA• P100LATENCY(msec)=102 5• R‐Ldifference(msec)=1.3 2.0• Amplitude(μV)=10 4.2• Duration=63 8.7

REMARKS• Ifacuity ofthepatientisinquestion,theamplitude ismoreimportant

• Ifdetectionofalesioninvisualpathwayisinquestion,latency ismoreimportant

• Latencyismorereliablethanamplitude(Variabilty – 5%ascomparedto25%)

• Presenceofreducedamplitudeisnonspecific,gainsimportanceonlyonserialtesting

• B/LsymmetryisseenbothwithflashandpatternVEP,thusanasymmetricalresponseismoreindicativeofanabnormality

FACTORSINFLUENCINGVEP• SIZEOFSTIMULUS– DecreaseinsizeofstimulusincreasesamplitudeofVEP

• Positionofelectrodesonscalp• AGE‐ amplitudedecreaseswithage• ATTENTIONOFPATIENT– Ifsubjectlookstosideofstimulus,thereisrapidfallinsizeofresponse

CLINICALAPPLICATIONSOFVEP• DELAYEDLATENCY:• Demyelinatingopticneuritis• Neurotransmitterdisorders• Glaucoma• Uncorrectedrefractiveerror• Ageing

CLINICALAPPLICATIONSOFVEP• REDUCEDAMPLITUDE:• Opticatrophy• Toxicagent• Compressive• Uncorrectedrefractiveerror• Ambylopia

EXAMPLE:GLAUCOMA• DecreasesinpatternVEPamplitudeandprolongedVEPlatenciesarefoundinmanypatientswithglaucomaandsomeglaucomasuspects

• Steady‐stateVEPsappeartobemoresensitivefordetectingglaucomatousdamagethantransient responses

EXAMPLE2:MULTIPLESCLEROSISIncreasedlatencyofP100Evenwhennodefectinvisualacuity,colourvisionorfieldofvision.About96%ofptswithMShavedelayedlatency

ENDOFLECTUREIV• FurtherreadingforthisweekincludesmoreelectrophysiologicaltestsotherthanERGandVEP.Moreover,moremedicalapplicationsofERGandVEP.

• HomeworkIII:CasepresentationsofdifferentpatientswithERGorVEPtestresults.

• Haveaniceweek!!