www.journalofoptometry.org
ORIGINAL
ARTICLE
Measurement
of
visual
acuity
with
a
digital
eye
chart:
optotypes,
presentation
modalities
and
repeatability
Giada
Campo
Dall’Orto
a,
Alessio
Facchin
b,c,d,∗,
Alessia
Bellatorre
a,
Silvio
Maffioletti
a,d,
Marina
Serio
a,eaDegreeCourseofOpticsandOptometry,UniversityofTorino,Italy bDepartmentofPsychology,UniversityofMilanoBicocca,Milano,Italy
cOpticsandOptometryResearchCenter,UniversityofMilanoBicocca,Milano,Italy dInstituteofResearchandStudiesinOpticsandOptometry,Vinci,Italy
eDepartmentofPhysics,UniversityofTorino,Italy
Received18July2019;accepted17August2020 Availableonline31October2020
KEYWORDS Visualacuity; Optotypes; Eyechart; QUEST; Crowding Abstract
Purpose:Digitalorcomputerisedeyechartsarebecomingstandardintheexaminationofvisual acuity.Eachinstrumentallowstheselectionofdifferentoptotypes,presentationmodalities, andcrowding.Theaimofthisstudywastoexaminethedifferencesinvisualacuity(VA) mea-surementusingadigital eyechart,comparingdifferentoptotypesandprocedures,together withanevaluationoftherepeatabilityofthemeasurement.
Methods:Twogroups of52 participantsaged between 18 and 31 years participatedin the study.Inthefirst experiment,VAthresholdsweremeasuredusingLEASymbols,TumblingE, andLandoltRings inmonocular andbinocular conditionsusing singleline presentation and QUESTpresentation.Inthesecondexperiment,wehavecomparedallmodalitiesofpresentation togetherwithapapereye-chartandtesttherepeatability.
Results:TheresultsshowedthatthresholdsforLEASymbolsarelow.Themodalityof presen-tationaffectsthesethresholds.ForLandoltRingsandTumblingE,theQUESTproceduregave significantlybetterthresholdsthanlinepresentation,whilethisdifferencewasabsentforLEA Symbols.Incomparingallmodalitiesofpresentation,singleletterandlinepresentationshowed similarvalues,slightlybetterthanblockpresentation.Papereye-chartsshowedbettervalues ofVA.Repeatabilityandagreementweregoodforallpresentations,butbestforQUEST.
∗Correspondingauthorat:DepartmentofPsychology-UniversityofMilano-Bicocca,Piazzadell’AteneoNuovo1,20126,Milano,Italy.
E-mailaddress:alessiopietro.facchin@gmail.com(A.Facchin).
https://doi.org/10.1016/j.optom.2020.08.007
1888-4296/©2020SpanishGeneralCouncilofOptometry.PublishedbyElsevierEspa˜na,S.L.U.ThisisanopenaccessarticleundertheCC BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Conclusions: TheQUESTmodalityofpresentationprovidesabetterthresholdthanline presen-tationexceptforLEASymbols.Examinersusingdigitaleyechartsmusttakeintoaccountthat notallmodalitiesofpresentationandoptotypesareequivalentandgivedifferentVAthresholds. Specificthresholdsneedtobeusedforeachoptotypeandpresentationmodality.
©2020SpanishGeneralCouncilofOptometry.PublishedbyElsevierEspa˜na,S.L.U.Thisisan openaccessarticleundertheCCBY-NC-NDlicense( http://creativecommons.org/licenses/by-nc-nd/4.0/).
Introduction
Visualacuity(VA)expressesthelevelofabilitywithrespect tothespatialresolutionofthevisualsystem,anditisthe firsttestintheanalysisofthevisualfunction.1Anumberof
techniquesareusedtomeasurevisualacuityusingdifferent instruments,charts,andmethods.2,3Dependingontheage
ofthesubject,variousoptotypes(i.e.,thesymbolsusedin the assessment) areavailable for measuringvisual acuity, each one having specific characteristics. In its evidence-basedclinicalpracticeguidelines,theAmericanOptometric Association suggests the use of letter charts (Snellen or ETDRS) withadults and symbol charts (e.g.LEA Symbols) or smallsetsof letters(HOTV) withpreschoolchildren.4,5
In recording data, the letter-by-letter scoring method is preferred becauseit takes into account each symbol and providesapreciseevaluationofVA.1,2
During vision screening aimed at detecting amblyopia, refractiveerrorsandotherconditions,itisnecessarytouse acuitychartswithstandardizedoptotypesandmethods.6---9
The selection of the most appropriate VA charts depends ontheageof theparticipant.Specialcaremust betaken with toddlers and preschool children.4 The use of
sym-bolsoptotypes(e.g.,LEASymbols)andsmallsetsofletters (e.g.,HOTV)areconsideredtobebestpracticesfor visual acuity testing of children between 3 and 6years old.5,10
However,despitethis,differentoptotypesandcharts/cards maybeusedforpreschoolchildren,suchasKayPictures,11
Cambridge crowded cards,12 Glasgow acuity charts13 and
Patti-Pics.14 Withschoolchildren,afulllogMARchartwith
differentoptotypescanbeused.15---17
Orientedoptotypes,suchasTumblingE,canbeusedwith preschool children18 but this optotype requires left-right
comprehension.19 This involvesacomplexmentalprocess,
especiallyat agesbetween3and4years.20 Aprintedcard
that a child can rotate, 21 or the use of a three-option
forced-choice task (i.e., Up, Down, Left/Right), without the problem of left-right orientation, represent possible solutions.22
LEASymbolsareavalidalternativetothetumblingE.19
These symbols werecreated toimprove thetestability of preschoolchildren(3---5yearsold)whoarenotalways suf-ficientlyfamiliar withletters ordirectional symbols. They can answer by naming a picture or pointing to a symbol onaresponsecard.Theoptotypesarefoursimplefigures; squareand circleandtheir derived apple andhouse.The design of the test is such that each symbol has a similar thresholdfor VA, and,consequently, thischartmeets the
criteria for a good VA test. It should be noted that the size of each symbol was slightly modified to meet these criteria.19
Digital or computerised eye charts are becoming stan-dard in vision examinations. With digital eye charts, it is possible tomodify each parameter inthe presentation of optotypessuchascontrast,timeofpresentation, presenta-tionmodalities,thespacingbetweentargets(i.e.,tocontrol thecrowdingphenomena),andobviously,optotypes.Using the advantage of the random presentation of optotypes, memorizationproblemsareabolishedevenforasinglelevel ofVA.Withtheseinstrumentsandspecificprocedure, sim-ple, accurate and repeatable measurements can also be madeeasilywithlowexpertiseinVAmeasurementpossessed bytheexaminer.1
In fact, a simple way of achieving an increased stan-dardizationofthemeasurementistousedigitaleyecharts basedonalgorithmsthateliminatetheexaminer’s involve-mentfromtheinterpretationofthesubject’sanswersand thescoring oftheresults.Italsoallowsaprecise compar-ison of the results measured by different professionals.23
Furthermore,computerizedVAsystemscanreducethe test-retestvariability through repeatedmeasurements. In this way, it is simple to detect real clinical changes in indi-vidualsubjects.24 Theautomatedscoringmethoddoesnot
allowtheexaminertomakemistakesormisunderstandthe answers.25
Digital eye charts allow a QUEST function to be used, which is a psychometric procedure that defines the relationship between physical stimuli, in this case, optotypesdimensions,andtheprobabilityofa psychophys-icalresponse.26 This method identifiesthe most probable
VA threshold value by using an adaptive process. The size of the stimulus in the next trial is determined on the basis of the subject’s answer in the previous trials.27
Differentdigitaleyechartspresentdifferent character-isticsinthepossiblemodalitiesofpresentation,crowding, andflankerused.Forthesereasons,thevariousdigitaleye charts could present different results even for the same instrument.Theaimsofthisstudyaretouseadigitaleye chart:(i) to examine the differences in VA measurement usingdifferentoptotypes (i.e.LEASymbols, LandoltRings andTumblingE);(ii)tocomparedifferentprocedures(i.e., singlepresentation,line, blockandQUEST); (iii)toassess thevalidityandrepeatabilityofthemeasurement.Inorder toavoidresponsefactorspresentinthetestingofchildren,28
onlyadultsweretested.
JournalofOptometry (2021)133---141
Materials
and
methods
Experiment1
Subjects
A groupof 52 adultsparticipated in thefirst experiment. Theage(mean±SD)was24±3years,from18to30years old.Therewere22maleand30femaleparticipants.Each participantwasaskedtowearhis/herownusualrefractive correction duringtheevaluation (13wore glasses; 5wore contactlenses,and34requirednocorrection).Thesingle exclusioncriterionwasthepresenceofanyocular disease that wasreported by a subject. None of the participants reportedanyeyedisease.Thestudywascarriedoutin accor-dancewiththeguidelinesgivenintheDeclarationofHelsinki andparticipantsgavetheirwrittenconsenttoparticipatein thestudy.
Test
Visualacuitywastestedat5mdistance withadigitaleye chart (CSO--- Vision Chart,v 1.3.0).This instrumentused wasaPC-baseddigitaleyechartwithascreenwitha reso-lutionof1280×1024,whichhasaVAstandardpresentation optotypesrangefrom1.30to-0.3LogMAR.Optotypeswere presentedinblackonawhitebackgroundandhada lumi-nanceof2.6and135cdm−2,respectively.Thecontrastwas set at themaximum level of500:1. Threedifferent opto-types, Landolt Rings, Tumbling E, and LEA Symbols, were presented in twomodalities: singleline presentation and QUEST.Bothmodalitiespresentedoptotypeswith logarith-mic sizescaling. Single line presentation wasused in the digitaleyechartbecauseitrepresentsasimpleform,andit isreadilyacceptedbytheobserverforitssimplicityof read-ing.Thesequenceofpresentationofeachoptotypeusedwas random.Fiverandomoptotypeswerepresentedspacedby thesamesizeofeachsymbol(100%spacingasreferredto othercharts15).Spacingwasproportionaltothelettersize. TheQUESTpresentationusedthesameoptotypesviewed above,inwhich thesizeof thesingletargetwaschanged activelyonthebasisoftheresponse(correctornot)ofthe previous trial.QUESTpresentationwasunflankedbyother optotypes.
Procedure
Testing was performed in an optometry laboratory of the UniversityofTurin,withauniformilluminanceof approxi-mately350lux.Monocular(rightandlefteye)andbinocular visualacuityweremeasuredinthatorder.Thefirst modal-ity of presentation was by a single line. A single row of fivetargets(fourpossibleoptionstochoosefrom)was pre-sented,andtheparticipantswererequiredtoidentifythe different optotypes presented. Participants were encour-aged to guess, and the first response was recorded (no self-correctionasQUESTmode).Thescoringprocedurewas theletterbylettervisualacuity.1,15Thevaluecorresponding
tothelineinwhichtheparticipantcorrectlyreadatleast3 outof5targetswasrecorded.Thenumberofsymbols rec-ognized and verballyreported (i.e.,3/5, 4/5 or 5/5)was thenrecordedplusthenumberoflettersreadatthe subse-quentsmallestsize(uptotwo).1,15ThefinalvalueoftheVA
takesintoaccountallthelettersreadcorrectly.Thesecond
modalityofpresentationwastheQUEST,whichpresenteda seriesoftargetsofvariablesize,whichwasdeterminedby thecorrectorincorrectresponsesgivenbytheparticipant. WetookthethresholdVA reportedbythesoftwareasthe VAvalue.AllvalueswererecordedonalogMARscale.The modalityofpresentationwasbalancedacrossparticipants. Theparticipantsweredividedequallyaccordingtotheorder inwhichthethreedifferentoptotypeswerepresented.
Results
DescriptiveresultsarereportedinTable1.
A 2×3×3 repeatedmeasure ANOVA wasperformed to comparealltheVAvalues.ThefactorsappliedintheANOVA wereMethod withtwolevels(Lineand QUEST),Optotype withthreelevels(LandoltRings,TumblingEandLEA Sym-bols),andEye withthreelevels,Right eye,Left eye,and Binocular. The results showed a significant main effect of Method (F(1,51)=13.62, p<0.0005, 2p=0.21), a signifi-cant main effect of Optotype (F(2,102)=216.11, p<0.0001, 2
p=0.81) and a main effect of Eye (F(2,102)=105.98, p<0.0001, 2
p=0.68). Significant second-order interac-tionsareonlyMethod×Optotype(F(2,102)=24.76,p<0.0001, 2
p=0.33). No other interactions were significant. The resultsareshowninFigs.1and2.
Since in the ANOVA, the factor Eye was significant only as the main factor, we have compared the single meanvaluesusingthePost-HocBonferronitest.Theresults show a significant difference between the right eye and Binocular(p<0.0001),betweenthelefteyeandBinocular (p<0.0001),butnosignificantdifferencebetweenleftand righteye.Theoverallimprovementbetweenmonocularand binocularVAis0.084logMAR(i.e.,aboutfourletters;Fig.1). Inordertoillustratethedifferencebetweenoptotypes better,post-hoc analyses withBonferroni correctionshow that there is not a significant difference between Lan-doltRingsandTumblingE.However,therearedifferences between Landolt Rings and LEA Symbols and between E and LEA Symbols (both p<0.0001), which corresponds to 0.101 logMAR(about 5letters). In comparingthe second-orderinteraction,Method×Optotype,therearesignificant differences between Line and QUEST for Landolt Rings (p<0.0001),andforTumblingE(p<0.0001),butnotforLEA Symbols (p=0.11).The meandifferencefor Landolt Rings andTumblingEbetweenLineandQUESTwas0.040logMAR (i.e.,twoletters).
Discussion
Inthisfirstexperiment,weexaminethe differencesinVA measurementusingdifferentoptotypes(i.e.,LEASymbols, LandoltRingsandTumblingE)andprocedures(i.e.,lineand QUEST)withadigitaleyechart.
Regardingthethresholdsfor theleftandrighteyes,as expected,we foundthatthe overallbinocularVA was sig-nificantlybetterthanmonocular,intheorderofabout0.08 logMAR(i.e.,fourletters).
Incomparingmethod,wefoundthattheQUEST presenta-tionforLandoltRingsandTumblingEoptotypesshowbetter valuesofVAcomparedtothesingle-linepresentation.This differencedoes notappearfor LEASymbols optotypes. In
Table1 Mean±SDvaluesofVAobtainedbydifferentoptotypes,eyetestedandprocedureoftestingforexperiment1.
Righteye Lefteye Binocular
Singleline presentation LandoltRings −0.005±0.086 −0.004±0.098 −0.079±0.104 TumblingE −0.007±0.082 −0.017±0.079 −0.089±0.075 LEASymbols 0.069±0.096 0.067±0.093 −0.018±0.076 QUEST mode LandoltRings −0.034±0.145 −0.056±0.115 −0.135±0.090 TumblingE −0.040±0.095 −0.049±0.091 −0.124±0.093 LEASymbols 0.090±0.118 0.088±0.119 −0.007±0.103
Figure1 Meanresultsformonocularandbinocularacuitytestedinexperiment1.Barsrepresent+/-1SEM.
comparingQUEST andline for LandoltRings andTumbling E,theQUESTthresholds arebetterthanline, andthe dif-ference couldsimplybe causedby thedifferentcrowding conditionsbetweenthetwomodalitiesofpresentation.This lastpointillustratesperfectlythedifferenceinthecrowding of the twoconditions.While theline presentation is par-tially crowded on twosides(except for thefirst and last symbol), QUEST is totallyuncrowded and consequently, a better thresholdvalue wasfound. WithLEA Symbols, this differencedidnotoccur,probablyduetothenatureofthe optotypesitself(i.e.,linethicknessandspecificdrawings).19
Incomparingoptotypes,wefoundthatLandoltRingsand Tumbling E givethesame thresholds. Conversely, we also replicatedwithadigitaleyecharttheresultthatLEA
Sym-bolsshowworse thresholdsgivinginferior VAof about0.1 logMAR,whichwasequivalentto5lettersoroneline.This resultconfirms a previous comparisonthat used standard eye charts.28 It is essential to underline that this
differ-ence of 0.1 logMAR was obtained by directly comparing theoptotypesusingthesamemodalityofpresentationand crowding. Differenteye charts, withdifferent flankers or spacing(crowding), couldgivedifferentresults.3,29---31 The
differenceinVAduetothepresenceofflankerscouldonly betested directlybycomparingseveralmodalitiesof pre-sentation.Moreover,inordertoteststabilityasafunction of time,the repeatability of the measurement shouldbe analysed.Theseissueshavebeenaddressedinexperiment 2.
JournalofOptometry (2021)133---141
Figure2 MeanresultsfortheinteractionbetweenMethod×Optotypeinexperiment1.Thedifferentpointsexpressthemean acuityvaluesseparatedbymethod(LineandQUEST)andoptotype(LandoltRings,TumblingEandLEASymbols).Barsrepresent +/-1SEM.
Experiment
2
Inthesecondexperiment,theaimwastocomparethe dif-ferentpresentationmodalityofthedigitaleyechartwitha classicpapereyechartandtheirtest-retestrepeatability.
Materialsandmethods
Subjects
Asecond sampleof 52adults(independentfromthefirst) participatedin thisstudy.The age(mean±SD)was23±2 years, witha range from20 to 31 yearsold. There were 17 male and 35 female participants. Inclusion and exclu-sion criteriawere the same asthose described abovefor experiment 1. Participants woretheir habitual correction (22woreglasses;6worecontactlensesand24requiredno correction).
Tests
Visualacuitywasmeasuredwithtwoinstruments.Ata dis-tanceof 5m,VA wastested withadigital eyechart(CSO ---Vision Chart,v1.3.0)asreportedabovefor experiment 1.Randomly-selectedSLOANletters(C,D,H,K,N,O,R,S, V,andZ)werepresentedinfourdifferentmodalities: sin-gleletter,singleline,ablockofletters(similartoLogMAR chart) and QUEST. The block of letters consist in a pre-sentation ofthreelinesof differentsizeoffiveoptotypes identicaltoalogMARcharts.Participantshavetoreadthe centralrow.TheSLOANletterswereusedforadirect com-parisonwithapapereyechart.Inthecaseofsingleletter modality,amultipleof5randompresentationswereused.
Allotherparameterswereidenticaltoexperiment1.Visual acuitywasalsotestedwithMilanEyeCharts15usingonlythe
standard100%spacingchart(100A).Inthiscase,adistance of3mwasused,and thesame letter-by-letterprocedure described abovefor experiment 1wasapplied. The chart hasauniformilluminanceofapproximately350lux.
Procedure
Sincethe aim of this second experiment was tocompare the different modalities and their repeatability, VA was measuredonlybinocularly.Thescoring ofVAandallother experimentalparameterswerethesameasthosedescribed aboveforexperiment1.Theorderofpresentationofeach modalityplusthepaperchartwasbalancedbetween parti-cipants.Theretestsessionforrepeatabilitywasperformed afteroneweekusingthesamesequenceofpresentation.
Results
DescriptiveresultsarereportedinTable2.
Table2 Mean±SDvaluesofVAobtainedbydifferent pro-cedureoftestingandsessionforexperiment2.
Method Test Retest Difference Single −0,105±0087 −0,120±0086 −0,014±006 Line −0,108±0075 −0,120±0088 −0,012±0042 Block −0087±0072 −0101±0096 −0014±0056 Quest −0,132±008 −0,134±0085 −0,002±0038 Paper −0,206±008 −0,210±0078 −0,004±0055
Figure3 Meanresultsfor theinteractionbetweenMethod×Sessioninexperiment 2.The differentpointsexpressthemean acuityvaluesseparatedbymethod(Single,Line,Block,QuestandPaper)andSession(firstandsecond).Barsrepresent+/-1SEM.
Firstly,a2×5repeatedmeasureANOVAwasperformed tocompareallVAvalues.ThefactorsappliedintheANOVA wereSessionwithtwolevels(firstandsecond),andmethod with five levels (single, line, block, QUEST, and paper). The results showed a significant main effect of Session (F(1,51)=4.19, p<0.05, 2p=0.08), Method (F(1,51)=72.21, p<0.0001, 2
p=0.59)andno significantinteractions were found. Inordertoexaminethe differencebetween meth-ods of presentation in the bestway, a repeated measure ANOVAwasperformedonlyusingthefirstsessionof admin-istration.Resultsweresignificant(F(4,204)=66.71,p<0.0001, 2
p=0.57). Post-hoc comparisons with Bonferroni correc-tionshowsignificantdifferencesbetweenQUESTandblock (p<0.05)andbetweenpaperandalltheotherfour modal-ities (all ps<0.0001). The paper presentation shows the bestvaluesofVA. ThemeandifferencebetweenLine and Questpresentationwas0.024;thisdifferencebetweenLine andQuestpresentationwassignificantlylower[t(102)=6.4, p<0.0001] comparedtoexperiment one,but inthe same direction.TheresultsareshowninFig.3.
Regarding the repeatability, thefirst analysistookinto accountthedirect meancomparisonsof each modalityof presentationbetweensessions.EveninthepreviousANOVA Session was significant; direct comparison with paired t-test (withoutcorrectionformultiple comparisons)showed a significant result only for line presentation (t(51)=2.12, p<0.05). In contrast,all other comparisons between ses-sionsdidnotshowsignificantdifferences.Repeatabilityof measurementalsoneedstobeanalysedwithcorrelationand BlandandAltmanlimitsofagreement(LoA).Correlationof each VA method of presentation was performed with the singlescoreICC[ICC(A,1)].32 Theresults showeda
moder-atetohighcorrelationforsingle,block,andpapermodality of presentation, but high correlation for line and QUEST (Table3).
The meanbiasbetweensessionsis smallereventhana single letterand the QUEST modalitypresents the small-estvalue.Thelimitsofagreement(LoA)werecalculatedas 1.96*SDofdifferences. Thebroader results(single,block, andpaper)areintheregionof5---6letters,andthenarrower (line and QUEST)were approximately4 letters.Graphical representationsoftheseresultsareshowninFig.4.
Table3 ResultsofrepeatabilityofVAforallmodalitiesof presentation.Thecorrelationwastheintraclasscorrelation (ICC);p-valueswere referredtothe significanceof corre-lation;Bias=meandifferencebetweensessions;LoA=limits ofagreement.
Method Correlation(95%CI) p-value Bias LoA Single 0.75(0.6to0.85) <0.0001 −0,014 0.118 Line 0.86(0.77to0.92) <0.0001 −0,012 0.082 Block 0.78(0.64to0.87) <0.0001 −0,014 0.109 Quest 0.90(0.83to0.94) <0.0001 −0,002 0.074 Paper 0.76(0.61to0.85) <0.0001 −0,004 0.107 Discussion
Theaimofthesecondexperimentwastocomparethe dif-ferentprocedures(i.e.,singlepresentation,line,block,and QUEST)toapapereyechartinordertoassessthevalidity andrepeatabilityofthemeasurement.
Themajorityoftheresultsshowthattheeffectof crowd-ing,althoughsmall,waspresentintheexpecteddirection. VAwassimilarforsingleandlinepresentationandreduced for a block presentation of approximately a value of one letter.Forthedifferentpresenceofflankers,alarger dif-ference between single and line presentation would be expected. Many participants showed values of VA in the orderof-0.3logMAR;thepresentationofthislineof opto-typein block presentation wasnotfully flanked because, in the lower part, it was not surrounded by other small optotypes (e.g., -0.4 logMAR). Compared tothe line pre-sentation,the QUEST presentation gavea bettervalue of 0.024.Thisvaluedifferenceissignificantlylowerthanthe oneobtainedinexperiment1,albeitinthesamedirection. Differentoptotypes andparticipants mayexplain this dif-ference.However,thereisadifferencebetweenQUESTand singlepresentationthatdidnotcorroboratetheinfluenceof crowdingingivingbetterresultsforQUEST.Thetwo thresh-oldmethods,thefullpsychophysicalQUESTandthesimple letterbyletterscoring couldexplainthisresult.Themost unexpected result was obtained in comparing the results forthedigitaleyechartwiththoseforapapereye-chart.
JournalofOptometry (2021)133---141
Figure4 BlandandAltman limitsofagreementrepresentation for testandretest for thefivemodalitiesofpresentation in experiment2.ValuesareinlogMAR.Thexaxisrepresentthemeanofthemeasurementofthetwosessions.Theyaxisrepresent thesecondmeasurementminusthefirstmeasurement.Thesmalldottedlinerepresentsthemeanbiasandthelongdottedlines representthepositiveandnegativeLoAs.
ThelatterpresentVAvaluesaresignificantlybetterthanall othermodalitiesofpresentation.
Therepeatabilityandtheagreementofthemeasurement ofVAgenerallyshowedgoodvalues.Specifically,themean biaswassmallinthedirectionofaminorimprovement,on theedgeofsignificance.Thiswasvisibleonlyfortheoverall test-retestcomparison(ANOVA)andinadirectcomparison tolinepresentation.However,thesizeofthiseffectissmall anddidnotreachthevalueofasingleletter.ICCcorrelations andLoAsresultsweregenerallygoodandsmallerthanother studies.Theircomparisonsonlyallowdifferencesbetween modalityofpresentationtobeshown.QUESTpresentation presentsthehighcorrelation,thesmallbias,andthesmall LoAsasasignofhighrepeatabilityandagreement.
General
discussions
Digitaleyechartsaremoderninstrumentsthatareintended to allow differenttypes of presentations at a distance in visionexaminations.However,fewstudieshavetakeninto accountitsuseandmadecomparisonbetweenvarious pre-sentation modality and between different optotypes and haveassessedtherepeatabilityofthemeasurement.
Specifically,theaimsofthisstudyweretouseadigital eye chart: (i) toexamine the differences in VA measure-ment usingdifferentoptotypes(i.e. LEASymbols, Landolt RingsandTumblingE);(ii)tocomparedifferentprocedures (i.e. single presentation, line, block, and QUEST); (iii) to
assessthe validityand repeatabilityof themeasurement. Thestudyhasproducedsomeinterestingfindings,andthese arediscussedbelow.
SinceweareinterestedincomparingVAbetween differ-entmodalitiesofpresentation, wehaveusedthehabitual correction.Consequently,ouracuitycouldbebetterdefined ashabitual correction visual acuity (HCVA) and not best-correctedvisual acuity(BCVA),andwe, therefore,cannot generalizeandcomparetheseresults.Theresultsofastudy thatcollectedBCVAvaluesatvariousageshavereported val-uesthatareslightlybetterthanours(i.e.,whenreferredto thesinglelinepresentation).33
AstudyaimedatcomparingLEASymbolswithTumbling E on a group of preschool children found comparable VA results. However, the difference between the two charts wasstatisticallysignificantonly in the3---4and notinthe 5---6yearage group.20 Basedonagroupofadultsbetween
17and31yearsold,anotherstudyfoundthatLEASymbols showedmeanVAvaluesthatwereweakerthanTumblingE, withaboutavalueof0.042logMAR,whichisapproximately twosymbols.16
Inagroupofchildrenof4---9yearsofage,thecomparison betweenLEASymbolsandotherpediatrictests(i.e.,single lineor multiple linessurroundedby ablackbox) withthe fulllogMARETDRSchartshowedthattheETDRSchartsgave worseVAresultscomparedtoLEASymbols(i.e.about1---2 lines).Sincetheauthorshaveuseddifferenteye-chart lay-outs,thecrowdingeffecthadacrucialroleandinteracted withdifferentlayoutsandoptotypes.29
DifferentmodalitiesofcrowdingaffecttheVAthresholds. Sailoganathanetal.3founddifferentresultsforLEASymbols
surroundedbyflankingoptotypesorflankingbars.
The crowding phenomenon is an important factor in measuring VA because of the different spacing between optotypes produces different VA values.15 Visual acuity
charts, cards, and testsoffer several modalities that can beusedtocontrolthecrowdingeffectbymeansof differ-entflankersandspacing.28 Asaresult,datafromdifferent
chartsarenotdirectlycomparableandthisshouldbetaken intoaccountwhencomparingcharts.3,16,29
In thefirstexperiment,whencomparingoptotypes,we foundthattheLEAsymbolsshowaworseVAofabout1line. WealsofoundthattheQUESTpresentationforLandoltRings andTumblingEoptotypesshowbettervaluesofVAcompared tothesingle-linepresentationusingaletterbyletter scor-ing.Thisresultisalsoconfirmedinthesecondexperiment, butwithasmallerdifference.Comparingsingleletter,line and blockpresentation directly,we found that only block presentationshowslowervaluesofVA.Crowdinghasagreat influencewhenatargetisflankedinfourdirectionsrather thantwo.
Theunexpectedresultisthesignificantdifference(0.07 logMAR)betweentheotherformatsandthepaperchartsin thesecondexperiment.Thereasonthatapapereye-chart shows better resultsis an interesting question. All instru-mentswerecorrectlysetatthecorrectdistancefromthe examiner.Contrastdoesnotseemanacceptableexplanation becauseitisgreaterforthedigitaloptotypethanthepaper chart.Themostrealisticexplanationseemstobelinkedto thedifferentdistancesusedfortesting.TheVAat5m(and notat3m),canbeinfluencedbyasmalldegreeofmyopia/ pseudo-myopia,andthisfactorcouldexplainthedifference. Infact,oneofthelimitationsofthisstudyisthetestingof HCVAandasopposedtoBCVA,andconsequently,theroleof therefractivestatusisunknown.
Digital eye charts based on different algorithms have been compared with standard charts. Harris et al.25 did
not find any significant difference when comparing back-litETDRSwithanautomatedETDRSsystem(A-ETDRS).The A-ETDRS measures VA in two phases: range finding and thresholding.ThesametwophasesareusedbythecompLOG system.Nosignificantbiaswasfoundbetweenthegold stan-dardETDRSandthecompLOGsystem,thetimerequiredfor testingbeingsimilar.34ThecompLOGwasalsousedwithKay
PicturesandtocomparetheprintedKayPictureschartwith compLOGKayPictures,resultinginthedetectionofasmall bias.However,thedifferencewassmallandnotclinically significant.35
The agreementfor all modalitiesofpresentation ofVA wasexcellent,witha smallmeanimprovementover time (withavalueofhalfofasymbol).Thisbiasbetweensessions isalignedwithastudythatreportslittlebias36butsmaller
than the previously reported data of about one line.37,38
Compared to previous studies,36---38 which report LoAs of
about0.15−0.18,theLoAsfoundweresmaller(0.07−0.12) withthesmallestvaluesfor QUEST andline presentation. The high correlation andsmall LoAis a sign ofconsistent and stable performance over time.The correlation value hasanessentialroleforitsconnectiontothestandarderror ofmeasurement,test-retestvariability,and,consequently, thesmallestvisualacuitychangesthatcanbedetected.39
However,oncomparingthedifferentmethods,QUESTshows betterresultsintermsofsmallerLoAandbias.Basedonthe overallresultsofexperiment2,excludingQUEST,thesingle lineseemstobethebettermodalityofpresentation, bear-inginmindadifferencebetweenthisandblockpresentation ofaboutoneletter.
Fromapsychophysicalpointofview,theQUEST presen-tationrepresentsthebestmodalityofletterpresentation, preventing the response factor and the influence of the experimenter.However,the line presentation presents an advantagebecausebyusingtheremotecontrolofthe dig-italeyechart,itis simpletoswitchfromthesingleletter presentationtoreachthethresholdzonequickly,andthen switchtolinepresentationinordertomeasurethethreshold preciselyusingletter-by-letterscoring.Thisisperfectly con-sistentwiththepresentationmethodusedintheA-ETDRS,25
andtheFrACTsystems.40
Thelimitationsfoundinthedigitaloptotypeusedarein thetotally un-flankedpresentation during QUEST andthe lack of line of -0.4 logMAR in theblock presentation. We hopethatfuturerevisionofthesoftwarecouldincludethe QUESTpresentationwithflankers(usingthesameoptotype) andanotherlineofoptotypes intheblockpresentation.A limitationofourevaluation isthe useof onlyadult parti-cipants. This point couldbe solved infuture studies with childrenparticipantstoprovideclinicalinformationspecific forthispopulation.
Conclusions
InevaluatingVA,digitaleyechartspresentdifferent modali-tiesofpresentationandoptotypes.Clinicianswhousedigital eye charts have to take into account that notall modal-ities and optotypes are equivalent and give different VA thresholds. Landolt Rings and Tumbling E present similar thresholdswhilethoseofLEASymbolsareworse.The modal-ity of presentation also affects the measurement of VA, whichwasstableovertime.Takentogether,theseaspectsof themeasurementsbecomeessentialinpatientsthatpresent VA nearthe cut-off when specific thresholds arerequired (e.g.,driverlicensing,specificjobs,etc.).Underthese con-ditions,thedifferencesinthetypeofpresentationandthe optotypesusedplayanimportantrole.
Conflicts
of
interest
Theauthorshavenoconflictsofinteresttodeclare.
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