RefractionandRetinoscopyHowtoPasstheRefractionCertificate

JONATHANCPARKBSc(Hons),MBChB(Hons),FRCOphthOphthalmicSpecialtyTrainingRegistrar

SouthWestPeninsulaDeanery

and

DAVIDHJONESMA,BMBCh,FRCOphthConsultantOphthalmologistRoyalCornwallHospital

Illustrationssupervisedby

SalmanWaqarBSc,MBBS,MRCS(Ed)

OphthalmicSpecialtyTrainingRegistrarSouthWestPeninsulaDeanery

Forewordby

AnthonyQuinnConsultantOphthalmologistHeadofSchool,OphthalmologyNHSSouthWestPeninsulaDeanery

RadcliffePublishingLondon•NewYork

RadcliffePublishingLtd33–41DallingtonStreetLondonEC1V0BBUnitedKingdom

www.radcliffehealth.com_____________________________________

©2013JonathanCParkandDavidHJonesIllustrations©JessicaLiWanPo

JonathanCParkandDavidHJoneshaveassertedtheirrightundertheCopyright,DesignsandPatentsAct1988tobeidentifiedastheauthorsofthiswork.Everyefforthasbeenmadetoensurethattheinformationinthisbookisaccurate.Thisdoesnotdiminishtherequirementtoexerciseclinicaljudgement,andneitherthepublishernortheauthorscanacceptanyresponsibilityforitsuseinpractice.

Allrightsreserved.Nopartofthispublicationmaybereproduced,storedinaretrievalsystemortransmitted,inanyformorbyanymeans,electronic,mechanical,photocopying,recordingorotherwise,withoutthepriorpermissionofthecopyrightowner.

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ISBN978-184619-860-1

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ContentsForewordAbouttheauthorsListofabbreviations

1Introduction:abookforophthalmologists

2TheRefractionCertificateExamination

3Whatdoesrefractiveerrormean?AmetropiaNotation,transpositionandsphericalequivalent

4HowtorefractOverviewHistoryInter-pupillarydistance,trialframeandbackvertexdistanceVisualacuityRefractionestimationVisualacuitytestingofachildRetinoscopy(objectiverefraction)RetinoscopybasicsRetinoscopytechniqueWorkinginplus/minuscylsorspheresPowercrossesInterpretingtheinitialretinoscopysweepsCycloplegicversusnon-cycloplegicretinoscopySubjectiverefractionRefiningthesphereRefiningthecylaxisRefiningthecylpowerwithspherecompensationDuochrometest

BinocularbalanceCoverandalternatecovertestsPrismcovertestMaddoxrodtestNearvision

5Retinoscopyofamodeleye

6Howtouseafocimeter

7Lensneutralisation

8Finaltipsfortheexam

Appendix1:Typicalrefractiverecordingsheet

Appendix2:Theretinoscope

ForewordRefractionisaskillthatallophthalmologistsneedtomasterearlyintheircareers.Asthisexcellentbookshows,itisnotadifficultarea,butconsiderablepractiseisneededtoreachtheconsciouslycompetentlevelrequiredinexaminations.Theauthorsaretobecongratulatedforprovidingtheirreaderswitha

clear,conciseguidetolearningtheartofrefractionanditsprinciples.Theyprovideawell-signpostedpathwaytosuccessintheRoyalCollegeofOphthalmologists’RefractionCertificateExamination.Traineeswillwelcomethisbookandexperiencedpractitionerswill

alsofindmanypearlsofwisdominside.Icommendthisbooktoyou.

AnthonyQuinnConsultantOphthalmologistHeadofSchool,OphthalmologyNHSSouthWestPeninsulaDeaneryNovember2012

AbouttheauthorsJonParkgraduatedfromtheUniversityofBristol,EnglandwithdegreesinAnatomicalSciencesandMedicine&Surgery.HecurrentlyworksasanOphthalmicSpecialistRegistrarintheSouth

WestPeninsulaDeanery.HeisinterestedintrainingandhopesthatthisbookwillbeusefultoallOphthalmicRegistrars.Jonhasastronginterestinresearchandwasafoundingmemberof

theSouthWestOphthalmicResearchandDevelopmentgroup(SWORD–pleasevisitwww.myeyesurgery.org.uk).Hehasbeeninvolvedwithusingvirtualrealitysimulationtoaidtrainingandtotestresearchquestions.Heisalsothechiefinvestigatorforanationwidestudyinvestigatingsight-threateningeyeinfectionsafterretinalsurgery,inassociationwiththeRoyalCollegeofOphthalmologists’BritishOphthalmicSurveillanceUnit.

DavidJonesisafull-timeconsultantophthalmologistinCornwall.HestudiedmedicineatCambridgeandOxfordUniversitiesandundertookmostofhisophthalmologytraininginGlasgow.HeiseducationalsupervisorandCollegeTutorforophthalmictraineesinCornwall.

ListofabbreviationsBDbasedownBIbaseinBObaseoutBUbaseupBVDbackvertexdistancecylcylinderIPDinter-pupillarydistanceJCCJacksoncrosscylinderMRMaddoxrodOSCEobjectivestructuredclinicalexaminationPCTprismcovertestpdprismdioptre

1

Introduction:abookforophthalmologistsRefraction,likemostpracticalskills,isanartwithascientificbasis.Oncemastered,itissatisfyingforthepractitionerandpatient.However,learningtorefractisinitiallyoftenbewilderingforjuniorophthalmologists.Whenstartingtolearnrefractionourselves,everyonetoldusthatpractisewasthekey.Weagreestronglywiththis,buttheobviousproblemiswhattopractise.Theaimofthishandbookistoprovideaconciseandsimple

understandingoftherefractiveprocess.Asfarastheauthorsareaware,thisisthefirstpublishedbooktofocusontherelativelynewformatoftheRefractionCertificateExamination.Thisbookhasbeenwrittenforjuniorophthalmologistswhohave

passedtheRoyalCollegeofOphthalmologists’Part1ExaminationandareabouttopreparefortheRefractionCertificateExamination.ItwillalsoaidthoserevisingfortheopticssectionofthePart1Examination,sinceitwillhelplinktheorytopractice.Wealsohopethatthisbookwillbeusefultojunioroptometristsand

anyseniorophthalmologistwhohaslettheirrefractiveskillslipandneedsabriefreminderofthistechnique.

JonathanCParkandDavidHJonesNovember2012

2

TheRefractionCertificateExaminationThecurriculumforophthalmicspecialitytraineeswasupdatedbytheRoyalCollegeofOphthalmologistsinAugust2007,andtherewasamajorchangeintheexaminationsrequiredtobecomeaFellowoftheRoyalCollegeofOphthalmologists.Therefore,theRefractionCertificateExaminationisarelativelynewexaminationand,asfarastheauthorsareaware,thisisthefirstpublishedbooktohelpcandidatesprepare.AlthoughtheRefractionCertificateExaminationisunlikelytochange

intheforeseeablefuture,itisvitalthatyouobtainthemostrecentguidancefromtheRoyalCollegeofOphthalmologists.ThedetailsinthisbookarecorrectasofNovember2012.TheCollegeassessescompetenceinrefractionusingamulti-station

objectivestructuredclinicalexamination(OSCE).Thisisapracticalexaminationthatyouwillnotpassunlessyouhaverefractedmanyadultsandchildren.Theexaminationaimstoassessyourabilityinthefollowingskills:

assessmentofvisionandocularmotilityuseofspectaclelensesandprismsperformanceofarefractiveassessmentandprovisionofanopticalprescriptionformationofamanagementplanfollowingassessmentandinvestigationsestablishmentofagoodrapportwiththepatientmaintenanceofaccurateclinicalrecordsunderstandingoftherelevantopticsandmedicalphysics.

YouwillbeexaminedonanumberofdifferentOSCEstations,soitisnecessarytobecompetentinallofthefollowingareas.

1. Refractionofanadult

History.

Trialframefittingandinter-pupillarydistance(IPD)measurement.Visualacuityandrefractionestimation.Non-cycloplegic(andsometimescycloplegic)retinoscopy.Subjectiverefractionofthesphere.Subjectiverefractionofthecylinder.Duochrometestandbinocularbalance.MusclebalancewiththeMaddoxrod(MR)testandprismcovertest(PCT).Nearaddition.

2. Refractionofachild

Visualacuitytestinginachild.Cycloplegicretinoscopy.Refractionofamodeleye.

3. Establishingtheprescriptionofapairofspectacles

Focimetry.Lensneutralisation.

Sincetheexaminationconsistsofmultiplestations,theseareascouldbeexaminedinanynumberofdifferentorders,buttheyarelistedaboveinanorderthatmakesclinicalsense.Forexample,thestationslistedunder‘Refractionofanadult’arethosetypicallyusedtorefractanadultinsequencefromstarttofinish,whichshouldtypicallytake15to20minutes.Atpresent(November2012),theOSCEstationiscomposedof12stations,withthreestationsineachofthefourrooms,aslistedfollowing.

Room1Station1:Cycloplegicretinoscopy1–firsteyeofapatient.Station2:Cycloplegicretinoscopy2–secondeyeofthesamepatient.Station3:Subjectiverefractionofcylinder–oneeyeofadifferentpatient.

Room2

Station4:Cycloplegicretinoscopy3–firsteyeofapatient.Station5:Cycloplegicretinoscopy4–secondeyeofthesamepatient.Station6:Lensneutralisationwithorwithoutafocimeter.

Room3Station7:Non-cycloplegicretinoscopy1–firsteyeofapatient.Station8:Non-cycloplegicretinoscopy2–secondeyeofthesamepatient.Station9:Visualacuityandtrialframefittingonadifferentpatient.

Room4Station10:Subjectiverefractionofthesphere–botheyesofapatient.Station11:Subjectiverefraction:binocularbalance–botheyesofthesamepatient.Station12:Nearaddition–botheyesofadifferentpatient.

Asdetailed,thefirsttwostationsineachroominvolveexaminationsconductedonthesamepatientandthethirdstationiseitherconductedonadifferentpatientoratasknotinvolvingapatient(suchasusingafocimeter).Youareallowedupto5minutestoorientateyourselfbeforethestationsformallybegin.Youthenhaveatotalof16minutestocompleteallthreestations–10minutesforthefirsttwostationsfollowedbya1-minutechangeoverperiodthen5minutesforthethirdandfinalstationintheroom.

GuidanceregardingcommonerrorsWhilstrevisingfortheRefractionCertificate,itisimportanttoconsiderthefeedbackprovidedtotheophthalmictraineesgroupinApril2011.Themostcommonerrorsincluded:

indecipherablenumbersincorrectnomenclature(forexample,notusing+or–signs)forgettingtorecordvisualacuityincorrecttranspositionfromretinoscopyprescriptionfinalrefractionwrittendownincorrectly,despitecorrectinitialworkingsinabilitytorefractquicklyunderpressure(reflectingthatfailureisrelatedtoinexperience).

Therefore,itwouldbewisetopractiseadequatelypriortotheexaminationandensurethatyourrecordingsarepreciseandcorrect.

3

Whatdoesrefractiveerrormean?

AmetropiaEmmetropia‘Emmetropia’meanstheabsenceofarefractiveerror,solightfromadistantsourceisperfectlyfocusedontheretina(seeFigure3.1).Anemmetropewillhavenormaldistanceacuitywithnospectacles(uncorrectedSnellenacuityof6/6orbetter)–provided,ofcourse,thereisnoamblyopia,ocularpathologyorcerebralvisualimpairment.

Figure3.1Emmetropia:lightfromadistantobjectformsanimageontheretina

Refractiveerror(ametropia)‘Refractiveerror’(ametropia)meansthataneyedoesnotallowlightfromadistantsourcetobefocusedperfectlyontheretina.Approximatelyone-thirdofthepopulationhasarefractiveerrorofmorethan1dioptre,andthusmayneedspectacles.Myopiaisjustascommonashypermetropia.Therefractivepowerofaneyeisafunctionofthecornealcurvature

(accountingfortwo-thirdsofthepower;thiscannotbealtered)andlens(accountingforone-thirdofthepower;thiscanbealteredby

accommodation,providedthereisnopresbyopia).Thisisasurprisetomostpeople,sincemostassumethatthelensisthemostpowerfulrefractiveelement.Theair–corneainterfaceisinfactthemostpowerfulrefractiveelement–thisbecomesquiteobviouswhenyoudiveintowaterwithoutanygoggles.

Refractiveerror(ametropia)occurswhentherefractivepoweroftheeyedoesnotcorrelatewiththeaxiallengthoftheeye,soanimagefromadistantobjectdoesnotfallontheretina.

Myopia‘Myopia’(short-sightedness)meansthattherefractivepoweroftheeyeistoogreatrelativetotheaxiallengthoftheeye;asaresult,theimageofadistantobjectliesinfrontoftheretina(seeFigure3.2).Therefore,myopiawillresultiftherefractivepoweristoohighoriftheeyeistoolong.Myopiaiscorrectedbyaminus(concave)lens,whicheffectivelyweakenstherefractivepowertoallowtheimagetobeshiftedbackontotheretina(seeFigure3.3).

Figure3.2Myopia:lightfromadistantobjectformsanimageinfrontoftheretina

Figure3.3Myopiacorrectedbyaminus(concave)lensthatdivergesrays

Hypermetropia‘Hypermetropia’(long-sightedness)meansthattherefractivepoweroftheeyeistooweakrelativetotheaxiallengthoftheeye;asaresult,theimageofadistantobjectliesbehindtheretina(seeFigure3.4).Therefore,hypermetropiawillresultiftherefractivepoweristoolow,oriftheeyeistooshort.Hypermetropiaiscorrectedbyaplus(convex)lens,whicheffectivelystrengthenstherefractivepowertoallowtheimagetobeshiftedforwardsontotheretina(seeFigure3.5).

Figure3.4Hypermetropia–lightfromadistantobjectformsanimagebehindtheretina

Figure3.5Hypermetropiacorrectedbyaplus(convex)lensthatconvergesrays

Astigmatism‘Astigmatism’referstotherefractivepoweroftheeyebeingdifferentindifferentmeridians.Therefore,lightfromapointofadistantobjectcannotformasinglepointofanimage(seeFigure3.6).

Figure3.6Astigmatism:lightfromthesamedistantpointobjectdoesnotformasinglepointimage,aslightisrefractedbydifferentamountsindifferentmeridians

Aneyewithastigmatismbehavesasasphero-cylindrical(toric)lens.TheprincipalmeridiansformseparatelinefociandbetweenthemisSturm’sconoid.Wheretheselinesintersectisthecircleofleastconfusion,whichissituatedatthefocalpointforthelenssphericalequivalentvalue(seeFigure3.7).

Figure3.7Aneyewithastigmatismbehavesasasphero-cylindricallens,withthecircleofleastconfusionofSturm’sconoidlyingatthefocalpointforthelenssphericalequivalentvalue

Thesphericalequivalentofasphero-cylindricallensisequaltothesumofthesphereplushalfthecylinder(cyl)andisusedtoestablishwhetheraneyewithastigmatismcanbeconsideredtobemyopicorhypermetropicoverall(formoreinformation,see‘Notation,transpositionandsphericalequivalent’,p.17).Thismaysoundquitecomplex,but,basically,ifaneyewithastigmatismbehavesasasphero-cylindricallens(i.e.aspherelenswithacylindricallenssuperimposeduponit),itfollowsthattocorrectastigmatismasphero-cylindricallensisrequired.Thisisdifferenttomyopiaorhypermetropia,whichcansimplybecorrectedwithasphericallensalone.Theoriginofastigmatismisusuallycorneal,wherethecornealcurvatureand,therefore,therefractivepowerisdifferentindifferentmeridians.Thisiswhyweoftenexplaintopatientsthatastigmatismimpliesthattheireyeisshapedlikearugbyballratherthanafootball.Thedegreeofcornealastigmatismcanbeassessedbyakeratometer,whichgives‘K’valuesfortherefractivepowerindifferentmeridians.Thesteeperthecorneainagivenmeridian,thegreaterthenumericalvalueoftheKvalueforthatmeridian.Ifcataractsurgeryisproposed,itisimportanttoconsiderthekeratometerKvalues,sincethesiteoftheincisionwillflattenthe

corneainthismeridian.ByplacingtheincisiononthesteepestKmeridianthedegreeofcornealastigmatismisreduced,whichcanbebeneficialtothepatientsinceastigmatismhasnorefractiveadvantage.Ifastigmatismispresentdespitehavingasphericalcornea,itwillbe

duetothelens(lenticularastigmatism).Lenticularastigmatismiseliminatedbytheplacementofasphericalintra-ocularlensimplantatthetimeofcataractsurgery.Somefurthertermsusedtodescribeastigmatismfollow.

RegularastigmatismThisapplieswhenthemeridiansofmaximumandminimumrefractivepowerareperpendiculartoeachother.Thisisfurtherdividedinto:‘withtherule’regularastigmatism,inwhich:

thecorneaissteepestintheverticalmeridianandflattestinthehorizontalmeridianthemaximalrefractivepowerofthecorneaactsvertically(thesteepestKmeridianwillbenear090)theweakestrefractivepowerofthecorneaactshorizontally(theflattestKmeridianwillbenear180)theaxisofacorrectingpluslenswillbevertical(090),sinceitspowerneedstoacthorizontallytostrengthentherelativelyweakerhorizontalmeridianthisisanexampleofaneyethathaswiththeruleregularastigmatism:+1.00/+1.50@090.Thereisa+1.50dioptrecyl,whichhasitsaxisat090.Thiscylissuper-imposedona+1.00dioptresphere.IntheabsenceofanylenticularastigmatismonewouldexpecttheKvalueintheverticalmeridiantobegreaterthanthehorizontalmeridian.

‘againsttherule’regularastigmatism,inwhich:

thecorneaissteepestinthehorizontalmeridianandflattestintheverticalmeridianthemaximalrefractivepowerofthecorneaactshorizontally(thesteepestKmeridianwillbenear180)theweakestrefractivepowerofthecorneaactsvertically(theflattestKmeridianwillbenear090)

theaxisofacorrectingpluslenswillbehorizontal(180),sinceitspowerneedstoactverticallytostrengthentherelativelyweakerverticalmeridianthisisanexampleofaneyethathasagainsttheruleastigmatism:+2.00/+1.75@180.Thereisa+1.75dioptrecylwithitsaxisat180.Thiscylissuperimposedona+2.00dioptresphere.IntheabsenceofanylenticularastigmatismonewouldexpecttheKvalueinthehorizontalmeridiantobegreaterthantheverticalmeridian.

‘oblique’regularastigmatism,inwhich:

themaximalandminimalmeridiansareperpendiculartoeachotherbuttheyarenotactingintheverticalorhorizontalplane;forexample,amaximalmeridianalong070andaminimalmeridianalong160.

IrregularastigmatismThisapplieswhenthemeridiansofmaximumandminimumrefractivepowerarenotperpendiculartoeachother.Themostcommoncauseforthisiskeratoconus(whichgivesascissor-likeretinoscopereflexthatisdifficulttoneutralise).Retinoscopyisoflimitedvalueforirregularastigmatismanditisusefultomoreaccuratelymapthecornealcurvaturewithcornealtopography.Irregularastigmatismisalsocommonfollowingcornealsurgerysuchas‘penetratingkeratoplasty’(full-thicknesscornealgraft).SimpleastigmatismThisiswhentheeyeisplano(emmetropic)inonemeridian(i.e.theraysinthismeridianfocusontheretina)andcylindricalinanother(i.e.theraysinthismeridiandonotfocusontheretina).Forexample,0.00/+1.50@055impliesthatnosphericalcorrection

isrequired,buta+1.50cyllensisrequiredwithanaxisat055(poweractingperpendicularlyat145)tocorrecttherefractiveerror.CompoundastigmatismThisapplieswhenbothmeridiansarehypermetropic(i.e.theraysinallmeridianscometofocusbehindtheretina)orbotharemyopic(raysinallmeridianscometofocusinfrontoftheretina).

Forexample,+1.00/+2.00@090impliesthata+1.00sphericallenswitha+2.00cyllenswithaxisat090(poweractingperpendicularlyat180)isrequiredtocorrecttherefractiveerror.MixedastigmatismApplieswhenonemeridianismyopic(raysfallinfrontoftheretina)theotherishypermetropic(raysfallbehindtheretina).Forexample,–1.50/+2.50@040impliesthata–1.50sphericallenswitha+2.50cyllenswithaxisat040(poweractingperpendicularlyat130)isrequiredtocorrecttherefractiveerror.

Notation,transpositionandsphericalequivalentArefractiveerrorisexpressedbythespectacle(orcontactlens)prescriptionrequiredtocorrecttherefractiveerrorintheform:

Refractiveerror=sphere/cyl@000(angleofcylaxis).

Itisimportanttodenote+or–forthesphereandthecylvalue,andthesevaluesshouldbeexpressedtotwodecimalplaces(e.g.+0.75,–3.25).Theangleofthecylaxisisexpressedasavaluefrom000to180(fromrighttoleft,anticlockwise,foreithereye),andshouldalwaysbethreesignificantfigures;thedegreesymbolshouldbeomitted.Forexample,insteadofwriting‘40°’,write‘040’.Ifyougetconfusedorcannotrememberthattheangleofthecylaxisrunsfrom000to180fromrighttoleft(anticlockwise)foreithereye,simplypickupatrialframe,astheanglesareclearlydemarcatedonthis(seeFigure3.8).

Figure3.8Thetrialframeprovidesdemarcationforthecylaxis

Aspectacleprescriptionmaybewritteninpluscylnotationorminuscylnotation–thesearethetwoequivalentwaysinwhichanysinglerefractiveerrorcanbecorrected.Bothplusandminuscylnotationsareacceptable,soeithermaybeused.Alwaysensurethatforanysinglepatientbotheyesareinthesamenotation(thatis,botheyesinpluscylnotationorbotheyesinminuscylnotation–neverusepluscylforoneeyeandminuscylfortheothereye).Thenotationthatischosenbysomeoneusuallyreflectstheirtraininginretinoscopy.Toobtaintheequivalentnotation,oneformhastobetransposedtotheotherform(transposethepluscylnotationtotheminuscylnotation,orviceversa).

Transpositioninvolvesthreesteps:

1. addthecyltothespheretogivethenewsphere2. changethesignofthecyltogivethenewcyl3. thenewaxisisperpendiculartotheoldaxis.

TranspositionexampleTranspose–8.00/+3.00@165.Newsphere=(–8.00)+(+3.00)=–5.00(addingthecyltothe

spheretogivenewsphere)Newcyl=–3.00(changingthesignofthecyltogivethenewcyl)Newaxis=165–090=075(thenewaxisisperpendiculartotheold

axis)Togive–5.00/–3.00@075.Therefore,–8.00/+3.00@165(pluscylformat)willcorrectthesame

refractiveerrorasthetransposedequivalentprescription–5.00/–3.00@075(minuscylformat).Itdoesnotmatterifyouchoosetorecordinplusorminuscyl

notation,butitiscrucialthatyouareconsistentandfortheeyesofanysinglepatientalwaysuseeitherpluscylnotationthroughoutforbotheyesorminuscylformatthroughoutforbotheyes.Donotusepluscylforoneeyeandminuscylfortheothereyeofthesamepatient,sincesuchinconsistencyisconfusingandunacceptableintheRefractionCertificateExamination.Giventhatrefractiveprescriptionscanbewritteninbothplusor

minuscylformat,itcanbeconfusingatfirsttoappreciatewhetherornotsomebodyismyopicorhypermetropicoverall.Forexample,consider–1.50/+4.00@020,whichisequivalentto

+2.50/–4.00@110.Thispersonobviouslyhasasignificantamountofastigmatism,butaretheymyopicorhypermetropicoverall?Thiscanbesimplifiedbytheconceptofthe‘sphericalequivalent’,whichcombinestheeffectofthesphereandcyltodecideiftheeyeismyopicorhypermetropicoverall.

Sphericalequivalent=sphere+(cylinder/2).

Thesphericalequivalentcanbeobtainedfromtherefractiveprescriptionineithertheplusortheminuscylformat.So,fortheaboveexample(–1.50/+4.00@020,whichisequivalent

to+2.50/–4.00@110),thesphericalequivalentwouldbe–1.50+(+4.00/2)=+0.50.Notethatthisisequalto+2.50+(–4.00/2)=

+0.50.Therefore,inthiscase,theeyecanbeconsideredtobemildlyhypermetropicoverall.Thisconceptofsphericalequivalentisparticularlyimportantto

understandwhenchoosingtheintra-ocularlenspowerincataractsurgerybecauseiftheincorrecttargetsphericalequivalentischosenanisometropiamayresult.‘Anisometropia’iswhenthedifferenceinrefractiveerrorsbetweenthetwoeyesissufficientlylargetoresultintroublesomesymptomssuchasaniseikonia(differentimagesizeofsingleobjectbetweeneyes)andasthenopia(eyestrain–patientsareoftennon-specificbutcomplainoffatigue,blurredvisionandheadache).Thisisdifferentfordifferentpatientsbutisasignificantriskwhenthe

differenceinsphericalequivalentbetweenthetwoeyesismorethan1.5dioptres.Therefore,itiscrucialtodiscusswiththepatientundergoingcataractsurgery:

theirtargetrefraction(oftenthetargetisemmetropia,butmyopesmayliketobeleftalittlemyopic,whereasthereisnorefractiveadvantageofbeinglefthypermetropicunlessthisisdonetoavoidanisometropiainhypermetropicpatientskeenforcataractsurgeryinoneeyeonly)theplanfortheothereye(sinceapatientundergoingsequential,bilateralcataractsurgerywilloftenchoosetobeemmetropicbutshouldbewarnedofanisometropiawhilstawaitingsecondeyesurgery).

4

Howtorefract

OverviewTherearedifferentwaystorefractapatient(i.e.toobtainaspectacleprescriptiontocorrectrefractiveerror).WedetailasystemthatcanbepractisedtocorrectlyrefractapatientandobtainallthenecessaryinformationrequiredtocompletetheRefractionCertificateExamination(atthetimeofwriting).Refractingapatienttakesaslongasittakes;however,themajorityof

casescanberefractedwithin15to20minutes.Practiseisrequiredandthesystemfollowingprovidesaframeworkforthis,whichyoucanmodifyifnecessary,accordingtotheadviceyouareprovidedwithwhilsttraining.Youwillneedtorefract70to100patientsbeforefeelingcomfortablewithmostsituationsandhencebeforeyoucanpasstheRefractionCertificateExamination.RememberthatthecertificateOSCEconsistsofmultiplestations,so

differentpartsoftherefractiveprocessmaybeexaminedinvariousdifferentorders.However,asalreadydiscussed,wehavearrangedthesepartsinanorderthatmakesclinicalsense.Forexample,thestationslistedunder‘Refractionofanadult’arethosetypicallyusedtorefractanadultinsequencefromstarttofinish,which,asnoted,typicallytakes15to20minutes.Notethat‘objectiverefraction’impliesobtainingarefractive

prescriptionthatdoesnotrequireanyresponsefromthepatient–thisisobtainedbyretinoscopy;forchildrenoradultswithlearningdisability,thismaybethesolebasisforaspectacleprescription.‘Subjectiverefraction’relatestofine-tuningtheprescriptionobtained

fromretinoscopybyaskingthepatientanumberofclear,closedquestionswhilstavoidingfatigue.Thisiswheretheartofrefractionbecomesevident!

TherefractiveprocessThefollowingisausefultemplateoftherefractiveprocessundertaken

withanadult,whichshouldtakeapproximately20minutes.Onceexperienced,itcantakeconsiderablylesstime,astheexaminationcanbetailoredtofitthepatient;however,forthepurposeoftheRefractionCertificateExamination,allcomponentsmustbewellrehearsed.

History(2minutes).IPD/Trialframe/Backvertexdistance(BVD)(1minute).Visualacuity(2minutes).Objectiverefraction–retinoscopy(5–10minutes).

Typicallywithoutcycloplegiainanadult.

Subjectiverefraction(5–10minutes).

Sphere.Cylaxis.Cylpowerandspherecompensation.Duochrome.Binocularbalance.MRandPCT.Nearvision.

Recordingresults(1minute).

HistoryThisshouldbebrief–about2minutes.Introduceyourselfthenaskthepatientfortheirnameandage.Clinically,itisusefultoaskthefollowing:

‘Doyouwearspectaclesorcontactlenses?’‘Areyourspectaclessinglevision,bifocalorvarifocal?’

Ifbifocalorvarifocal,presbyopiaisrelevant,soyouwillneedanearadd.

‘Atwhatagedidyoustartwearingspectacles?’

Theyoungertheage,oftenthegreatertherefractiveerrorandhigherthechanceofamblyopia.

‘Whendoyouwearyourspectacles–whenlookingintothedistance(suchaswhendriving/watchingtelevision)oratthingscloseby(e.g.whenreading)?’

Amildmyopemayonlywearthemfordistance.Anemmetropeormildhypermetropewhoisolderthan35years(presbyopiamaystarttomanifestfromthispoint)mayonlywearthemforreading.

‘Areyouadriver?’

Ifso,theirbestcorrectedbinocularvisualacuityshouldbebetterthan6/12,whichapproximatestheDriverandVehicleLicensingAgency’slegalrequirementofbeingabletoreadanumberplatewithbotheyesopenatadistanceof20metresaway.

‘Whatisyouroccupation/hobby?’

Computerworkmayrequireaspecificintermediatecorrection(aweakernearaddtothedistanceprescriptionthanthatrequiredforreading).

‘Doyoudoanythingthatrequiresyoutoseeobjectscloserthanatnormalreadingdistance,suchassewing/modelmaking?’

Astrongernearaddmaybeneededforsuchcloserwork.

‘Haveyouhadanyeyeproblemsinthepast?’

Hastherebeenanysurgery,laser,traumaordrops?Havetherebeenanyproblemswithalazyeye/useofpatchasachild?

Amblyopiaorpreviouseyediseasemaylimitbestcorrectedvisualacuity,sodonotpanicif6/6isnotobtainedinthesecases.

‘Doyouhaveanydoublevision–whereyouseetwoimages?’

Patientsmayreportblurredvisionasdoublevision–always

establishiftwoseparateimagesareseen(truediplopia)andwhetherthisisbinocular(suggestingasquintwithoutsuppression)ormonocular(suggestingunilateralocularpathologysuchasacataractorcornealscar).Forbinoculardiplopia,itisimportanttoassessthesquintanglewiththecovertestandMR,andthepatientmayrequireprismsfortheirsymptoms.

Inter-pupillarydistance,trialframeandbackvertexdistanceItshouldonlytakeaminuteortwotomeasuretheinter-pupillarydistance(IPD),fitthetrialframeandmeasurethebackvertexdistance(BVD).

Inter-pupillarydistanceAskthepatienttolookatadistanttargetandmeasurethedistancefromtherightnasallimbustotheirlefttemporallimbususingarule(whichyoushouldbringyourselftotheexam).TheIPDtypicallyliesbetween55and75mm.

Inter-pupillarydistancenearCheckyouareatthesameheightasthepatient.Facethepatientandaskthemtolookatyouropeneye(closeyourrighteye;withyourlefteye,measurefromtheirrightnasallimbus)thenaskthemtolookatyourothereye(nowcloseyourlefteye,openyourrighteyeandmeasuretotheirlefttemporallimbus).Typically,theIPDfornearis2to4mmlessthanfordistanceduetotheconvergencethatoccurswithnearstimulation.

FittrialframeSetyourtrialframeIPDtothedistanceIPDvalueyouhavejustmeasured.Makethesidearmsaslongaspossiblethenplaceontheframeonthepatient’sface,checkingthatthesidearmshookaroundtheearsandtightenthesidearmsuntilstableandcomfortable.Checkthatthepupiliseasilyseen–ifitisobscuredinthehorizontalplane,youwillneedtore-checkyourIPD;ifitisobscuredintheverticalplane,youwillneedtoadjustthenasalrest(ifthepupilistoohigh,lowerthe

centralframebrackettoelevatethetrialframe,seeFigure4.1).

Figure4.1Correctfittingofatrialframewitheachpupilinthecentreofeachaperture,bothhorizontallyandvertically.

BackvertexdistancePlacealens(ofanyvalue)inthetrialframe.Askthepatienttofixateonadistanttarget,andusearuletomeasurefromthepatient’scorneatothebackofthelens(thesurfaceofthelensnearestthecornea).AnormalBVDis10to12mm.Thepowerofalenssystemdependsuponthedistanceofthelensfromthecornea.Thisconceptisknownas‘lenseffectivity’andexplainswhyamyope’scontactlensprescriptionwillbenumericallyweakerthantheirspectacleprescription.Italsoexplainswhypatientswithpowerfulprescriptionsgetablurredviewwhentheirspectaclesslipdowntheirnose.Therefore,theBVDisimportantwhenaframeistobeconstructed,sincethefunctionofthelenssystemdependsnotonlyonthelenspowerbutalsoonthelenspositionrelativetothecornea.Practically,thisis

relevantforprescriptionsofmorethan4dioptres,butitisgoodpracticetoalwaysrecordtheBVD.FormulaeexisttoallowcorrectionofanygivenprescriptionaswellasBVDtoadifferentprescriptionandBVDthatwillhaveanequivalenteffect.

Visualacuity‘Acuity’isameasureoftheresolvingpoweroftheeye–theabilitytodiscriminatebetweentwopoints.DistancechartsthatyoushouldbecomfortablewithincludetheSnellenandtheLogMAR.NearvisionchartsthatyoushouldbecomfortablewithincludetheN-series.Inanyclinicalsetting,itisimportanttocheckthedistancevisual

acuityforeacheye(unaided,aidedandpinhole)andthenearacuityforeacheye(unaidedandaided).Ifaided,itisusefultostateifthisiswithspectaclesorcontactlenses.Theeyenotbeingtestedshouldbecorrectlyoccluded.Forthepurposeoftheexam,thepatient’sspectacleswillnotbe

available,sothefollowingwillneedtobeestablishedforeacheye:

distanceacuityunaided(SnellenorLogMAR)distanceacuitywithpinholenearacuityunaided(N-series;remembertouseabrightlamp).

Pinholesonlyallowaxialraysthroughtotheeye,hencereducetheeffectofrefractiveerror.Rememberthatthepinholevisiongivesagoodideaofpotentialvisionforthateyeoncetherefractiveerrorhasbeencorrected.Ideally,yourtargetend-refractionvisualacuityshouldbeatleastasgoodasthepinholeacuity.Rememberthateyeswithreducedpinholevisionorreducedvision

despiteadequaterefractivecorrectionhaveacuitythatislimitedbyamblyopia,ocularpathologyorcerebralvisualimpairment.Pinholeacuitytendstopartiallyimprovewithcornealorlenspathologybutwillnotimprovewithamblyopia,retinal,nerveorcerebralpathology(pinholeacuitycanbeworsethanunaidedacuityinpatientswithmacularpathology,sinceitprecludeseccentricfixation).

Alwaysconsider–whyisthevisionpoor?

Refractiveerror:…improveswithpinhole.

Amblyopia:…noimprovementwithpinhole.

Ocularpathology:…ifofretinaornerveorigin,willnotimprovewithpinhole…ifofcorneaorlensorigin,mayimprovewithpinhole.

Cerebralvisualimpairment:…noimprovementwithpinhole.

Note,ofcourse,amixtureofthesereasonscommonlycoexist.

RefractionestimationCheckingthevisualacuitywillgiveyouanideaoftherefractiveerror:

1dioptreofsphericalerrorgives6/122dioptresofsphericalerrorgive6/24to6/363dioptresofsphericalerrorgive6/60.

However,notethatthisguideisforsphericalerrorandignoresthatthepatientmayhaveastigmatism.Theimpairmentinacuityisabouthalfthatforcylindricalerrorsrelativetosphericalerrors.Therefore,apatientwith0.00/+2.00@080wouldbeapproximately6/12unaided.Thisguideshouldonlybeusedasanapproximation,sincepatients

willhaveamixtureofsphericalandcylindricalerror.Thisrefractionestimationalonedoesnot,however,suggestwhether

thepatientismyopicorhypermetropic.Forexample,iftheyare6/24unaided,theirrefractioncouldbe–1.75or+1.75sphericaldioptres.Toestimateifthepatientismyopicorhypermetropic,comparetheirunaideddistanceacuitywiththeirunaidednearacuity.Thisconceptismoreusefulifthepatientispresbyopic,sinceotherwisetheeffectofaccommodationconfoundstheestimation.Ifapatienthaspoordistancevisionbutgoodnearvision,youknowtheyaremyopic.Forexample,ifapresbyopehasanunaidedSnellendistanceacuityof6/60,yetisN5atreadingdistance(onthenearvisionN-seriesreadingchart),theirrefractionisprobablyaround–2.00to–3.00sphericaldioptres.Iftheyhavepoordistancevisionandpoornearvision,youknowthey

arehypermetropic(ortheyhaveamblyopia,orocularpathologyorcerebralvisualimpairment–thisshouldbeclearfromyourhistory).

VisualacuitytestingofachildAlthoughchildrencanbeunpredictable,whichaddsstresstoanexaminationsettingsinceitissomethingyoucannotcontrol,thereareanumberofusefulwaysofhandlingthisthatcomewithexperienceinassessingthevisualbehaviourofchildren.Itisimportanttospendtimewithorthopticstaff,sincethisisthebestwaytolearntobecomfortablewiththefollowing:

patchingasameansofocclusion(notethatobjectiontoocclusionimpliespooracuityintheothereye)assessingifachild’svisioniscentral(i.e.nosquint),steady(i.e.conjugatemovementswithnonystagmus)andmaintainedthroughthedurationofablink(i.e.thereissufficientacuitytofixateonandfollowanobjectofinterest,demonstratingthatitisseen)preselectedtests,suchasCardiffCards,KayPictures,singleoptotypeorcrowdedcharts,usedtoassessbinocularandmonoculardistanceacuity.

Retinoscopy(objectiverefraction)RetinoscopybasicsTheaimofretinoscopyistoobtainanobjectiverefraction–thatis,anestimationofthepatient’sspectacleprescriptionusingaprocessthatdoesnotrequireanydecisionstobemadebythepatient.Retinoscopyalsogivesagoodbenchmarkfromwhichtheprescriptioncanbefine-tunedusingsubjectivetechniques(usingsubjectiveratherthanobjectiverefractionfromthebeginningtakesconsiderablylonger).Retinoscopyisaninvaluableprocessforchildrenoradultswithlearningdisability,asthesepatientswillnotbeabletoanswerthequestionsrequiredforsubjectiverefraction.Forthesepatients,yourspectacleprescriptionwillbebasedonyourretinoscopyalone.Aretinoscopeproducesalight,which,withthecufffullydown,islinear(thescopeslit).Formoreinformationontheretinoscope,seeAppendix2.Quitesimply,thescopeslitlightispassedacrossthe

patient’spupilandalightwithinthepupil(thereflex)isobserved.Bynotingthequalityofthisreflex,variouslensesarethenplacedinthetrialframetoneutralisethereflex.Asneutralisationisapproached,thereflexwillbecomefasterandbrighter.Adull,slowrefleximpliesneutralisationisnotclose.Atneutralisation,thereflexisaglowingbrightpupil;atthispoint,thelensesinthetrialframeprovidetheobjectivespectacleprescription(oncecorrectedforworkingdistance).Thescopeslitisheldatacertainangle(say,vertically)thensweptacrossthepupilinadirectionperpendiculartotheorientationofthescopeslit(inthiscase,horizontally).Asthescopeslitpassesacrossthepupil,thereflexcanbenotedtohavecertaincharacteristics:(a)direction,(b)orientation,and(c)brightnessandspeed.

Characteristicsofretinoscopereflex

Direction:

withoragainstorneutralised.

Orientation:

vertical,horizontalorobliquescissorreflex.

Brightnessandspeed:

brightandfastdullandslow.

DirectionofreflexA‘with’reflexisseenif,asyourslitpassesacrossthepupil,alightwithinthepupil(thereflex)movesinthesamedirection(seeFigure4.2).Apluslensmustbeaddedtothetrialframetoapproachneutralisation.An‘against’reflexisseenif,asyourslitpassesacrossthepupil,alightwithinthepupil(thereflex)movesintheoppositedirection(seeFigure4.3).Aminuslensmustbeaddedtothetrialframetoapproachneutralisation.

Figure4.2A‘with’reflex.Thescopeslitisorientatedverticallyandswepthorizontallyacrossthepupiltogiveawithreflex

Figure4.3An‘against’reflex.Thescopeslitisorientatedverticallyandswepthorizontallyacrossthepupiltogiveanagainstreflex

Toneutralise:withreflex…addpluslensagainstreflex…addminuslens.

Therefore,toapproachneutralisation,eitheraplus(ifwithreflex)orminus(ifagainstreflex)mustbeaddedtothetrialframe.Ifthereflexisalreadyquitefastandbright,only0.25or0.50maybesufficienttoreachneutralisation.Toconfirmneutralisation,youcanleanbackwards,furtherawayfromthepatient(reflexbecomesagainst)orleanforwardsclosertothepatient(reflexbecomeswith).Thisisbecausethecloseryouare,themoreminusmustbeaddedtocorrectfortheworkingdistance(see‘Correctionforworkingdistance’,p.34).Alternatively,toensure

theendpointhasbeenreached,adda+0.25lens,whichshouldgiveanagainstreflex.Suchreversalofthereflexisimportanttoachieve,sinceithighlightsthatthetrueendpointofneutralisationhasbeenestablished.Notethatthelensesaddedtoapproachneutralisationareeither

sphericalorcylindrical.Ifasphereisaddedtoneutralisethereflex,itwillalsoalterthesubsequentlensesrequiredintheperpendicularaxistoobtainneutralisation.Ifacylindricallensisadded(withtheaxisorientatedthesamewayasthescopeslit,sothepowerofthecylindricallenswillactinthesameplaneasthescopesweep),neutralisationinthisplaneisapproachedandhasnoeffectontheotherprincipalmeridian.

Orientationofreflex

Theorientationoftheretinoscope’sslitlightshouldbeparalleltothepupilreflex.

Ifthereisnoastigmatism,oriftheastigmatismiseitherwiththeruleoragainsttherule,thereflexwillbeorientatedverticallyandhorizontally.Inthesesituations,ensuretheslitisverticalthenhorizontal(rotatetheslitbyrotatingthecuffslightly)toneutralisethesemeridians.Withobliqueastigmatism,theprincipalmeridiansarestill

perpendicularbutdonotlieverticallyandhorizontally.Therefore,whenahorizontalscopesweepismadewiththeslitorientatedvertically,theorientationofthepupilreflexwillbeobliqueandnotlievertically(itwillliebetween045and090or090and135)–seeFigure4.4.Similarly,ifthescopeslitwasorientatedhorizontallyandasweepmadevertically,theorientationofthepupilreflexwillagainbeobliqueandnotbehorizontal(itwillliebetween000and045or135and180).Forobliqueastigmatism,thescopeslitshouldberotatedbyturningthecuffslightlysotheslitisparalleltothepupilreflextoaidsubsequentneutralisation.Theperpendicularmeridiancanthenbeneutralisedbyrotatingtheslit90degrees(e.g.ifonemeridianisat110,theotherwillbeat020).

Figure4.4Withobliqueastigmatism,theorientationofthereflexwillnotbehorizontalorverticalbutoblique

Anothertypeofreflexisthe‘scissorreflex’,whichoccurswithahighdegreeofirregularcornealastigmatism,suchaskeratoconus.Thesereflexescanbedifficultorsimplynotpossibletoneutralise.Keratoconusisacornealectasia,characterisedbyprogressivestromalthinningandconicaldistortion,associatedwithincreasingirregularastigmatismandmyopia.Itisappropriatetoexaminetheeyeontheslitlampforothersignsofkeratoconus(stromalthinning/cone,Vogt’sstriae,Fleischerring).Investigationsincludecornealtopographysothedegreeofirregularastigmatismcanbequantifiedandmapped.Thisaidstheconsiderationofthevariousavailabletreatmentoptionsforkeratoconus,includingcontactlenses,scleralcontactlensesorsurgicalintervention(riboflavinwithultravioletA/collagencross-linking,intra-stromalimplants,deeplamellarorpenetratingkeratoplasty).

BrightnessandspeedofreflexAsmentioned,thebrighterandfasterthereflex,theclosertoneutralisation.Inthesesituations,useasmallmagnitudeoflenspoweralteration(0.25or0.50dioptres)sinceneutralisationisclose.Therefore,adull,slowreflexisfarfromneutralisationandsometimes

itpaystobeginwitha±5or±10sphericallenstostartoffwith.Remember,adullreflexalsooccurswithmedialopacity(suchaswith

acataractorvitreoushaemorrhage).Adullreflexcanalsooccurasaresultofflatretinoscopebatteries!

Correctionforworkingdistance‘Workingdistance’isthedistancefromthepatient’scorneatoyourretinoscope.Itisnecessarytoalterthesphereofthelensesinthetrialframeto

giveacorrectedfullprescriptionbaseduponthevalueoftheworkingdistance.Theretinoscopeisconstructedsothatifretinoscopyisperformedat1

mfromthepatient,thelensesinthetrialframetogiveneutralisationareequaltothespectacleprescription.However,wedonotdoretinoscopyat1m,butratherat66cm(whenworkingwithtrialframes)or50cm(ifyouhaveshorterarmsorwhenworkingwithouttrialframes–forexample,withchildren,examinationunderanaesthesiaoramodeleye).Therefore,onceneutralisationisobtained,toconverttothecorrectedprescription,itisnecessarytoadda–1.50spheretothetrialframe(tocorrectfora66cmworkingdistance)ora–2.00sphere(tocorrectfora50cmworkingdistance).Notethatthecylremainsunchanged.Therefore,a–1.50myopewillneutralisewithoutanylensesif

workingat66cm.A–2.00myopewillneutralisewithoutanylensesifworkingat50cm.Herearesomeotherexamples:

neutralisationoccurswith+4.25/–1.75@030at66cm,sothecorrectedrefractionwillbe+2.75/–1.75@030,since+4.25plus–1.50=+2.75neutralisationoccurswith–3.75/+0.75@044at50cm,sothecorrectedrefractionwillbe–5.75/+0.75@044,since–3.75plus

–2.00=–5.75.

Therefore,theworkingdistancecorrectionfactoristhereciprocaloftheworkingdistanceinmetresandthismustbesubtractedfromtheretinoscopyresult.

Wheneveraresultisrecorded,itisvitaltostatewhetherthisisuncorrectedorcorrectedfortheworkingdistanceandwhatthatworkingdistanceis.Therefore,adda–1.50sphericallensforaworkingdistanceof66cmandadda–2.00sphericallensforaworkingdistanceof50cm.

Thecorrectionofworkingdistancecanbedoneattheendoftheretinoscopyonceneutralisationhasbeenachieved,whilstworkingat66cmor50cm.However,itcanbedoneatthestartofretinoscopy.Inthiscase,beforeusingtheretinoscope,youmustadd+1.50(for66cm)or+2.00(for50cm)tothetrialframe(oryourfingers,ifworkingwithnoframe),andtheresultantlenssummationatneutralisationwillgivethecorrectedprescription.Whetheryoudecidetocorrectforworkingdistanceattheendorthestartofretinoscopydoesnotmatter–butitmustbedoneandyourresultsshouldbeclearlyrecordedtodemonstrateatwhatstageacorrectionforworkingdistancewasmade.

Staticversusdynamicretinoscopy‘Static’retinoscopymeansthattheworkingdistanceisfixedthroughoutretinoscopy.Thisiswhatmostpracticeandiswhatisdetailedinthisbook.Experiencedpractitionerscanusetheconceptofworkingdistancetotheiradvantagebyvaryingtheirworkingdistancetoobtainneutralisation(ratherthanchangingthelenses).Thisisknownas‘dynamic’retinoscopy.Forexample,anemmetropeneutralisesat1m,a–1.50myopeat66cm,a–2.00myopeat50cm,a–5.00myopeat20cmandsoon.Imagineyougetanagainstmovementat66cm–ratherthanaddingaminuslens(inthecaseofstaticretinoscopy),youinsteadleanforwardto50cmandneutralisationoccurs–thepatient’srefractioninthatmeridianistherefore–2.00.Dynamicretinoscopyislesspracticalforhypermetropes,since

hypermetropesneutralisewithaworkingdistanceofmorethan1m.Dynamicretinoscopytakesconsiderablepractisebutisextremelyusefulforrefractingchallengingpatients(suchaschildren)becauseitissorapid.

RetinoscopytechniqueIdeally,theroomshouldbedim.Thedarkertheroom,theeasieritistonotethereflexcharacteristics;iftheroomistoodark,youwillstruggletofindyourlenses.Ausefultrickistouseyourretinoscopelightasatorchifyoucannotseethelensmarkingseasily.Ensurethatyourretinoscopecuffisallthewaydownontheshaftoftheretinoscope.

Keypointsforretinoscopy

Establishadimroom.Fog(orocclude,ifnecessary)thefelloweye.Scopethepatient’srighteyewithyourrighteye/righthand.Scopethepatient’slefteyewithyourlefteye/lefthand.Keepyourscopeascloseaspossibletotheirvisualaxis,withoutinterruptingcontinuousdistantfixation.Correctforworkingdistance(add–1.50sphereifat66cm;add–2.00sphereifat50cm).Recordineitherpositivecylnotationforbotheyesornegativecylnotationforbotheyes(neverpositiveforoneeyeandnegativefortheother).

Thefirststepistoexaminethepatient’srighteyewiththeretinoscope.Fornon-cycloplegicrefractionofpatientswhoarenotpresbyopic(especiallyiftheyaremyopic),itisnecessarytofog(blur)thefellowlefteye.Thisinvolvesplacinga+1.50or+2.00sphericallensontopofthepresumedrefraction(estimatedfromtheiracuity,whichyouhavejustchecked),sothattheacuityispoorerthanthatoftheeyebeingexaminedwiththeretinoscope.Adequatefoggingcanbeconfirmedbyensuringthattheretinoscopy

reflexisagainstor,alternatively,checkingtheacuityineacheyewiththefoginplaceandensuringthefoggedeyehaspooreracuitythantheeyeabouttobeobjectivelyrefracted.Ifthepatientis6/6withthepresumedrefraction,a+1.50or+2.00sphericaldioptrefogtypicallyrenderstheeyeto6/12to6/24.Thereasonwhythefelloweyeshouldbefoggedistoreduceaccommodation,whichwouldgiveafalseresultwhenexaminingthefelloweyewiththeretinoscope.Withcycloplegicrefraction(typicallyinchildren),thereisnoneedtofog,sincetheaccommodativecomponentisremovedbythecycloplegia.Fornon-cycloplegicrefraction(mostadults),foggingisrequiredtoreduceanyaccommodativedrive(especiallyifthepatientisamyopewhoisnotyetpresbyopic).Thisfogginginduceslessaccommodationthansimpleocclusionwithablackoccluder–hence,theeffortmadetofogratherthansimplyocclude.Occlusion,ratherthanfogging,shouldbeavoided,asitstimulatesmoreaccommodation.However,occlusionisrequiredinthefollowingsituations:

whentheeyebeingtestedisdenselyamblyopic(sincetheeyenotbeingtestedmusthaveapooreracuitytohelpavoidaccommodationanda+2.00lenswillprobablybeinsufficienttoachievethis)ifthepatientmarkedlyobjectstofoggingduetodiplopiaorasthenopiaifyouareunabletoestimateacuityandprovideanadequatefoglens.

Onceyouhaveadequatelyfogged(or,ifnecessary,occluded)thefelloweye,askthepatienttofixateonthewhitelightorgreentargetinthedistance.Explaintothemthatitisimportantthattheycontinuetolookintothedistanceandnotatyourownwhitelight.Askthemtoletyouknowifyourheadobscurestheirviewofthedistantfixationtarget.Itisvitaltoensurethatyourheadisascloseaspossibletotheirvisualaxis,withoutactuallyobscuringtheirdistantfixationtarget–thisensuresthatyourretinoscopelightwillbeclosetotheirvisualaxis(seeFigure4.5).Failuretobe‘onaxis’inthiswaycanresultinspurious

astigmatism,thusitisimportanttobewaryofthiswhenrefractingchildrenwhoshifttheirposition.

Figure4.5Useyourlefthandtoperformretinoscopyofthepatient’slefteye(leftphoto),sinceincorrectlyusingyourrighthandwillobstructtheirview(centralphoto).Checkworkingdistancewitharm(rightphoto).

Useyourrighthandandrighteyetoscopetheirrighteye.Scopefirstwithavertical,thenahorizontalandfinallyadiagonalslittolocatetheprincipalmeridians.Ifonlyadull,slowreflexisseen,tryusinga±5orevena±10lens.Thenproceedbyrefractinginplusorminuscylsorspheresalone(see‘Workinginplus/minuscylsorspheres’,p.39).Onceyouhaveobjectivelyrefractedtherighteye,correctforyourworkingdistance(adda–1.50sphereifat66cm)andrecordyourresult(state‘correctedforworkingdistance’).Thenfogtherighteyeanduseyourlefthandandlefteyetoscopetheirlefteye.Onceyouhaveobjectivelyrefractedthelefteye,againcorrectforworkingdistanceandrecordthis.Youshouldnowturnthelightson,checkthevisualacuityandmoveontosubjectiverefraction.Rememberthatifawithreflexisseen,thenapluslensshouldbeaddedandifanagainstreflexisseenthenaminuslensshouldbeaddedtoapproachneutralisation.Thebrighterandfasterthereflex,thecloseryouaretoneutralisation(theentirepupillightsupwhentheslitentersthepupil),whereasadullandslowrefleximpliesyouarenotclosetoneutralisation.

Workinginplus/minuscylsorspheresItispossibletorefractwithyourretinoscopeinthreedifferentways:

1. usingpositivecyls2. usingnegativecyls3. usingspheresonly.

UsingpositivecylsThismeansthatyourretinoscopyresultwillbeinapluscylformat.Identifytheorientationofthetwoprincipalmeridians,whichwillbe

perpendiculartoeachother.Theprincipalmeridianthathasanagainstreflex–or,ifbothreflexesarewith,itwillbetheleastwithreflex(whichisfastestandbrightest,asitisnearestneutralisation)–isneutralisedfirstwithspheres.Thiswillresultintheotherprincipalmeridiangivingawithreflex,whichisthenneutralisedwithpositivecyls(theaxisonthelensinthesameorientationasthescopeslit).Theresultantprescriptionwillbethelensesinthetrialframe(whichmustthenbecorrectedforworkingdistance).Forexample,youidentifyanagainstreflexwithscopeslitat135anda

withreflexat045.Addminusspheresuntiltheagainstreflexat135isneutralised(say,–3.00causesneutralisation).Thenaddpluscyls(withtheaxisinthesameorientationasthescopeslitat045)toneutralisethewithreflex(say,+1.50at045causesneutralisation).Theaxislineonthecyllensshouldbeparalleltothescopeslitandlightreflex(perpendiculartoitspower).Thelensesinthetrialframethengivetheretinoscopyresultinpluscylformat:–3.00/+1.50@045,whichmustthenbecorrectedforworkingdistance(ifat66cm,thisgives–4.50/+1.50@045).Thismaysoundcomplicated,butsimplyconsiderthatapatientwith

regularastigmatismrequiresaspherewithacylsuperimposeduponittocorrecttheirrefractiveerror.Thesphereisfoundbyneutralisingthemostagainstreflex,andtheperpendicularmeridianwillthengiveawithreflex,whichcanbeneutralisedwithpluscylstogivethesphero-cylindricalcorrection(whichmustbecorrectedforworkingdistance).

UsingnegativecylsThismeansthatyourretinoscopyresultwillbeinaminuscylformat.Identifytheorientationofthetwoprincipalmeridians,whichwillbe

perpendiculartoeachother.First,neutralisethemostwithreflexwithplusspheresthenneutralisetheperpendicularagainstreflexwithminus

cyls.Thelensesinthetrialframewillgivetheretinoscopyresultinminuscylformat,whichmustthenbecorrectedforworkingdistance.

UsingspheresonlyItispossibletoobtainanobjectiverefractiveresultwithoutusinganycylindricallenses.Identifythetwoprincipalmeridians.Neutraliseoneofthemeridianswithasphere,recordtheresultandorientationofreflexthenremovethesphere.Followingthis,neutralisetheperpendicularmeridianwithasphereandrecordtheresultandorientationofthereflex.Therefractiveresultcanthenbeexpressedineitherplusorminuscylformat;inbothcases,themagnitudeofthecylisthedifferencebetweenthetwospheres.Itcanbeusefultouseapowercrosstogeneratetheresultantprescription.

PowercrossesAsnoted,ifworkinginplusorminuscyls,theresultantrefractionobtainedbyretinoscopywillsimplybethelensesinthetrialframe(thisdoesnotapplyifworkinginspheres).Thiscanthenbecorrectedforworkingdistance.Therefore,itisnotnecessarytodrawpowercrossesandpower

crossesarenotrequiredfortheRefractionCertificateExamination(atthetimeofwriting).However,sincesomepractitionersusepowercrossesitisgoodpracticetounderstandthem.Furthermore,ifyouworkonlyinspheres,itisusefultouseapowercrosstoobtainyourresultantrefraction.Eacharrowedarmofapowercrossrepresentsthedirectionof

movementoftheretinoscopesweep.Forexample,whensweepinghorizontallywiththescopeslitorientatedvertically,thepowerinthehorizontalplane(180)isexamined.Therefore,ifaspherewithpower+3.50dioptresneutralisesahorizontalsweep,thisimpliesthepowerinthehorizontaldirectionis+3.50dioptres.Ifaspherewithpower+2.00dioptresisthenrequiredtoneutraliseaverticalsweepwithahorizontallyorientatedscopeslit(toassessverticallyactingpower),theresultantpowercrosswouldbe:

Correctingforworkingdistancewouldgive:

Toobtaintheprescriptionfromthepowercrossinpositivecylnotation:

recordtheleastpositivesweepasthesphererecordthecylasthedifferencebetweenthetwosweepsrecordtheaxisasthesameaxisofthemostpositivesweep(rememberingthattheaxisisperpendiculartothedirectionofactionofthepowerarrow).

Therefore,thisexamplegivestheprescription+0.50/+1.50@090,which,whentransposed,mayalsobewritten+2.00/–1.50@180.Hereisanotherpowercrossexample:Withslitat045,powersweepat135,asphereofpower–1.50

dioptresisrequiredforneutralisation.Withslitat135,powersweepat045,asphereofpower+0.25dioptresisrequiredforneutralisation.Thisgivesthepowercross:

Which,whencorrectedforworkingdistance,gives:

Whichgivestheprescription–3.00/+1.75@135.

Therefore,ifworkinginplusorminuscyls,powercrossesarenotnecessarysincetheresultantprescription,oncecorrectedforworkingdistance,issimplythelensesintheframe.However,ifrefractinginspheres,powercrossesareusefulforobtainingtheprescription.

InterpretingtheinitialretinoscopysweepsWhenyouarejuststarting,itisusefultohaveaclearideainyourmindofhowtointerprettheinitialretinoscopysweeps,sinceitisfromherethatyouwillmakesequentialdecisions.Thelevelofyourexperiencewillbecomepainfullyobvioustothe

examinersatthisearlystage,soitisimportanttobeconfidentanddecisiveatthispoint.Itisusefultomakethreesweeps:onewiththeslitvertical,onewith

ithorizontalandonethatisobliquelyorientatedatameridianthathasbecomecleartoyoufollowingtheverticalandhorizontalsweeps,ifthereisanobliquereflex.Assumingyouareworkingat66cmandhavedecidedtoworkinpluscyls

format,considerthesevenpossibleinitialscopesweepresults:

1. Neutralisedinallmeridians.Thepatienthasasphericalrefractiveerrorof–1.50dioptres(nocyl).

2. Adull,slowreflexthatisdifficulttointerpret.Providedyourretinoscopebatteryhasnotbeenexhaustedfromallyourenthusiasticwork,thepatienthasahighdegreeofametropia,sotryinterposinga±5or±10sphericallens.Remember,aphakiaisacommoncauseofhighhypermetropia.

3. Anagainstreflexinallmeridiansthatisequallyfastandbright.Thepatientismoremyopicthan–1.50dioptres,andthereisnosignificantastigmatism(neutralisewithminusspheres).

4. Awithreflexinallmeridiansthatisequallyfastandbright.Thepatientismoreplusthan–1.50dioptres,andthereisnosignificantastigmatism(neutralisewithplusspheres).

5. Anagainstreflexinonemeridianbutmoreagainst(slowerandduller)inanother.Thepatienthascompoundmyopicastigmatism.Addminusspheresuntilthemostagainstcylisneutralised,leavinga

perpendicularwithreflexthatcanbeneutralisedwithpluscyls.6. Awithreflexinonemeridianbutmorewith(slowerandduller)inanother.Thepatienthascompoundhypermetropic(orrathermoreplusthan–1.50dioptres)astigmatism.Addplusspheresuntiltheleastwithcyl(fasterandbrighterreflex)isneutralisedleavingaperpendicularwithreflexthatcanbeneutralisedwithpluscyls.

7. Awithreflexinonemeridianandanagainstreflexintheperpendicularmeridian.Thepatienthasmixedastigmatism.Addminusspherestoneutralisetheagainstreflexthenaddpluscylstoneutralisethewithreflex.

Cycloplegicversusnon-cycloplegicretinoscopy‘Cycloplegia’referstoparalysisoftheciliarymuscle,sothataccommodationisnotpossible.Cycloplegics,suchastopicalcyclopentolate,willcausemydriasis(pupildilatation)inadditiontocycloplegia.Non-cycloplegicretinoscopyisoftensufficientforthemajorityof

adultpatients,especiallyiftheyarepresbyopic(noeffectiveaccommodation).However,inthefollowingsituationsitisusefultoperformcycloplegicrefraction:

inchildrenandyoungadults(especiallyiftheyaremyopic)toremoveaccommodation,whichgivesafalselymyopicrefractionifnotremovedinadultswithsmallpupilsoropaquemedia(suchasacornealscarorcataract)whohaveapoor-qualityretinoscopicreflexwithoutpupildilatation.

Ensurethatthecycloplegiaiscompletebyinstillingthecycloplegicandwaitingatleast30minutes.Checkthatthereisnomiosisfollowingilluminationofthepupil.Sincecyclopentolatecansting,considerfirstgivingatopicalanaestheticforchildren.Notethatthepupildilatationoccursbeforethefullcycloplegiceffect,soitisnecessarytowaitthefull30minutes,evenifthepupilisdilatedafter10minutes.

Otheraspectsofcycloplegicretinoscopy

Ifthepatientisachild(oranadultwithlearningdisability),trialframesarenotalwaystolerated.Tryhalf-aperturechildtrialframesorsimplyplacelensesinyourownfingersinfrontofthechild’seye.Childrenarelesslikelytoremainstill.Thechallengehereistoensureyourretinoscopelightisonthevisualaxisofthechild,sincespuriousastigmatismisnotedifyouarenotco-axialtotheeye.Itisalsohardertokeepaconstantworkingdistance,whichistypicallyshorterforachild(50cm,witha–2.00dioptreworkingdistancecorrection)thanforanadult(66cm,witha–1.50dioptreworkingdistancecorrection).Neutralisationcanbehardertoappreciate.Thedirectionoftheinitialretinoscopicreflexcanbeeasiertodetermineindilatedeyes,butthiscangiveafalsesenseofsecurity,astheneutralisationpointcanbemoredifficulttoestablish.Itmayseemthatneutralisationoccursoverawiderrangeoflensesrelativetonon-cycloplegicrefraction–itisimportanttowatchthecentralreflexofthedilatedpupiland‘push’thelensesuntilclearreversalisseen.Forexample,itmayseemthatneutralisationoccursat+2.00dioptres,butdonotsettleforthis–pushtheplus.Itwillthenbecomeapparent,forexample,thatthecentralreflexgivesabetterneutralisationreflexat+3.00dioptresandreversalisseenwith+3.25dioptres.Accommodationisnotactive–hence,thereisnoneedforthepatienttocomplywithdistantfixationandthereisnoneedtofogthefelloweye.

Otheraspectsofnon-cycloplegicretinoscopy

Trialframesaretypicallytoleratedinadultnon-cycloplegicretinoscopy,andthesecanhelpwithestablishingamoreaccurateangleofanastigmaticmeridian.Patientsaregenerallystill.Thismakesiteasierforyourretinoscope’slighttoremainco-axialwiththepatient’seye,thusreducingtheriskofspuriousastigmatism.

Withsmallpupilsoropaquemedia(suchasacornealscarorcataract),thereflexcanbedifficulttointerpret.Adimroomwilldilatethepupilandhelpwiththis.Accommodationisactiveinpre-presbyopes(especiallyifmyopic);thiscanbereducedbyfoggingthefelloweyeadequately,maintainingdistantfixationandavoidingprolongedretinoscopybursts(trytomakeadecisionwithinthefirstcoupleofsweepsandalwayswithinafewseconds).

Reducingaccommodationinnon-cycloplegicretinoscopy:

1. fogfelloweye2. ensurepatientmaintainsdistantfixation3. avoidprolongedretinoscopybursts.

Failuretoreduceaccommodationgivesaspuriouslymyopicresult.

Finally,someimportantretinoscopytips.

Keepyourlensestidy(itwillinfuriatetheexaminershavingtotidyupafteryou).Putyournextlensintothetrialframebeforetakingalensout(thiswillhelptominimiseanyaccommodation).Noretinoscopysweepshouldlastmorethanafewseconds.Prolongedsweepsnotonlyinduceaccommodation(ifnon-cycloplegic)butalsodemonstratetotheexaminersthatyoudonotknowhowtoactinresponsetowhatyousee.Therefore,ifyouarenotsureafterafewseconds,comeaway,putadifferentlensinandtryagain.Ifthereflexistoodulltointerpret,checkyourretinoscopebattery.IfthebatteryisOK,youaredealingwithhighametropia.Tryinterposinga±5or±10sphere.Iftheresultsaretoominus,checkthatthepatientisnotaccommodating,eitherbecausetheyarenotlookingatthedistanttarget(patientsneedconstantreminderstodothis)orbecause

youhaveoccludedratherthanfoggedthefelloweye.Occludethefelloweyewhencheckingvisualacuity,butwhenusingyourretinoscopeandforsubjectiverefractionfogthefelloweye(witha+2to+4addonyourestimatedprescriptiontoreduceaccommodation).Ifthepatientisamblyopicordiplopic,avoidfoggingandsimplyoccludethefelloweyeforretinoscopyandsubjectiverefraction.Ifaccommodationisanissue(asitiswithallchildren),cycloplegicrefractionisrequired.Iftheresultsaretooplus,remembertosubtracttheworkingdistancecorrectionfactor.

SubjectiverefractionSubjectiverefractioninvolvesthepatientmakingconsciousdecisionssothataprescriptionthathasbeenapproximatedbyobjectivemeans(retinoscopy)canbefine-tuned.Therefore,thisisnotalwayspossibleinchildrenorpatientswith

learningdisability,soyourretinoscopyresultwillprovidethebasisforspectacleprescriptioninthesepatients.Theprocessofsubjectiverefractionshouldstartwithin10minutesof

therefractiveprocessandtakenolongerthan10minutes.Theprocessincludesthefollowingstages:

1. refiningthesphere2. refiningthecylaxis3. refiningthecylpowerwithspherecompensation4. duochrometesting5. binocularbalancetesting6. MRandPCT7. nearvisiontesting.

Therefinementofthesphereandcylandduochrometestiscompletedfirstfortherighteyethenforthelefteye.Binocularbalanceisthentestedwithbotheyesopen.TheMRtest(andpossiblyPCT)isusedtoassessthetendencyofthe

eyestodissociate,toestablishifprismsarerequiredtocontrolasymptomatictropia.Followingthis,thenearvisioniscorrectedandtestedwith

appropriatecorrectionfortherightthenthelefteye(testeacheyeindependently).Retinoscopyshouldbeconductedindimlight.Subjectiverefraction

shouldbeconductedingoodlight–so,whenyouputyourretinoscopedown,turnthelightsbackon.Ensureyouhaverecordedyourretinoscopyresults(correctedfor

workingdistance)andthevisualacuitythatwasobtainedwiththese.Aswithretinoscopy,duringsubjectiverefraction,itremainsimportant

tofogthefelloweye(or,ifappropriate,occludethefelloweye–seep.37).Thisnotonlyreducesaccommodationinnon-cycloplegicrefractionbutalsoensuresthatthepatient’sanswerstoyoursubjectiverefractionquestionsarebasedentirelyontheeyebeingexamined.Inaddition,aswithretinoscopy,whenchangingalens,alwaysputthe

nextlensintothetrialframebeforetakingalensout,tominimiseaccommodation.

RefiningthesphereAskthepatienttofixateononeofthelettersonthelowestlineoftheacuitychartthattheycanseecomfortably.Askthepatient:

‘Isthatletterclearerwith[placea+0.25sphereinfrontoftheireye]orwithoutthelens[removethe+0.25sphere]oraboutthesame?’

Ifaresponseisnotimmediatelygiven,afteronlyacoupleofsecondsremovethelens,waitacoupleofseconds,thenre-offerthemthelensandthequestion.Donotsimplyholdthelensupwaitingforadecision,sincethequalityoftheanswerdiminishesrapidlywithtime.Ifnoresponseissuccinctlygiven,itislikelythattheletterremainsaboutthesame.Ifthepatientreportsthattheletterisbetteroraboutthesame,add

thepluslenstotheframeandrepeat.Iftheyreportthattheletterisworsewiththepluslens,donotgive

thepluslens.Instead,nowofferthema–0.25sphereandaskthem:

‘Isthatletterbetter,orjustsmalleranddarker?’

Thisminuslensshouldonlybeofferedforabriefmomenttoavoidaccommodation.Iftheyimmediatelyreportthattheletterisbetter,add

the–0.25spheretothetrialframeandrepeat.Iftheyreportthattheletterissmalleranddarker,checktheacuityandmoveontorefiningthecyl.Iftheyreportthattheletterisworse(eventhoughyoudidnotaskthemthis),alsochecktheacuityandmoveontorefiningthecyl.Noticingthataletterissmalleranddarkerratherthanactuallybettercanbedifficult,andthereisthedangerofovercorrectingaccommodatingmyopes.Therefore,beslightlyreluctanttokeepgivingminusspherestoamyope(suchexperiencecomeswithpractise).Notethatwhenthe–0.25sphereisoffered,onlyholdthisupforacoupleofseconds.Ifthepatientdoesnotmakeadecisionquickly,removethe–0.25sphereandre-offerthemthelensandthequestion.Donotsimplyholdthelensupwaitingforadecision,sincethequalityofthedecisionwilldecreasewithtimeand,inthecaseofthisminuslens,thepatientwillaccommodate.Usinga±0.25spheretorefinethesphereisappropriateiftheacuityis6/9orbetter.Iftheacuityisbetween6/12and6/18,usea±0.50sphere,andconsiderusinga±1.00sphereifitisworsethan6/18.Atthisstage,donotpaniciftheacuityispoorandcannotbeimproved.Itmaybethatthepatienthasalargecyl(ahighdegreeofastigmatism).Therefore,moveontorefiningthecylwhenanendpointisreached,ratherthanperseveringonlywithspheresinthepursuitofperfectacuity.

RefiningthecylaxisRefiningthecylfollowsrefiningthesphere.Thefoggingofthefelloweyeshouldremaininplaceand,forthepurposeoftheRefractionCertificateExamination,ifdemonstratingsubjectiverefractionofthecylinderonly,adequatefoggingmustfirstbeensured(seep.37).Thecylindricalcomponentofthespectacleprescriptionisfine-tunedsubjectivelyusingtheJacksoncrosscylinder(JCC),whichwaspopularisedbyEdwardJackson(1893–1929).TheJCCisasphero-cylindrical(toric)lensinwhichthepowerofthecylinderistwicethepowerofthesphereandoftheoppositesign.TheJCCisequivalenttosuperimposingtwocylindricallensesofequalpowerbutoppositesignwiththeiraxesperpendiculartoeachother.ThehandleoftheJCCis45degreestotheaxesofthecyls.Sincethereare

twoperpendicularopposingcyls,anaxisfortheJCCisnotdenoted.Thesphericalequivalent(equaltothesphereplushalfthecyl)ofaJCCisthereforezero.Ifa–0.25cylissuperimposedperpendicularlywitha+0.25cylthenetresultisequivalentto–0.25/+0.50(whentransposedequivalentto+0.25/–0.50).Thiswouldbea0.50JCC,sincetheJCCisdefinedbythepowerofthecylnotation.JCCsareavailableinvariouspowers,typically0.50and1.00,andthisisusuallywrittenontheshaft(seeFigures4.6and4.7).Thepowerisnamedafterthepowerofthecylgivenbyitsnotation.Hencea–0.25/+0.50(sameas+0.25/–0.50)isa0.50JCCanda–0.50/+1.00(sameas+0.50/–1.00)isa1.00JCC.The0.50JCCisusedifacuityis6/12orbetterwhereasthe1.00JCCisusedifacuityisworsethan6/12.

Figure4.6A0.50JCC(–0.25/+0.50)

Figure4.7A1.00JCC(–0.50/+1.00)

DonotrelyonthecolouroftheJCCaxestoconfirmwhichisplusandwhichisminus–theonlywaytobesureistolookatthelensmarkings.A0.50JCCwillhave+0.25writtenonthelensand,perpendiculartothis,–0.25willbedenoted.A1.00JCCwillhave+0.50writtenonthe

lensand,perpendiculartothis,–0.50willbedenoted.ItshouldbeacceptabletotakeyourownJCCstotheexaminationifyouwish.Tocheckthecylaxis(establishedbyretinoscopy)withtheJCC,holdthehandlealongtheproposedplusaxis.AskthepatienttolookattheletterO(orothertypesofcirculartargetssuchastwodoublerings).Askthem:

‘DoestheOlookrounderandclearerwithlens1[position1–handlealongaxis]orlens2[position2–twist180degrees]oraboutthesame?’

NotethatthisquestionforcesacomparisonbetweentheJCCinposition1andtheJCCposition2,notacomparisonwithouttheJCC.Ifthepatientreportsthatbothareequallyasbad,thisshouldbeinterpretedasmeaningthatposition1isthesameasposition2.Whenworkinginpluscyls,ifthepatientprefersposition1,rotatethecylsotheaxismovestowardsthepluscyloftheJCCwheninposition1.Ifthepatientprefersposition2,rotatethecylsotheaxismovestowardsthepluscyloftheJCCwheninposition2.Theamountofrotationrequired(range2to20degreesinanyalteration)dependsupontheacuityandthestrengthofthecyl.Ifacuityisalreadygood,onlymovethecylbysmallamountstoavoidlosingthegoodacuity.Ifthecylislarge,avoidlargemovements,sinceonlyacoupleofdegreesofmovementofalargecylcanmakequiteadifference.Thisappreciationcomeswithpractise.Ifunsure,applythe‘bracketing’technique,inwhichyouinitiallymovetheaxisby20degrees,thenre-checkandmoveby10degrees,then5degrees,then2degreestoreachthedesiredendpoint.Neverunderestimatehowimportantitistoobtainthecorrectaxisforahigh-poweredcyl.Ifthepatientreportsthatposition1isthesameasposition2(or,asisquitecommon,appearstorejectbothofthem)anendpointhasbeenreachedandasatisfactoryaxishasbeenobtained.Nowmoveontorefiningthecylpower.

RefiningthecylpowerwithspherecompensationAskthepatienttofocusagainonthedistantcirculartarget.Whenworkinginpluscyls,holdtheplusJCCaxisoverthepluscylaxisinthetrialframe(position3–thisincreasesthecylpower).Askthepatientto:

‘LookattheO–doestheOlookrounderandclearerwithlens3[position3]orlens4[position4–twist180degrees,thisplacestheminusJCCaxisoverthepluscylaxisinthetrialframetodecreasethecylpower]oraboutthesame?’

Again,theforcedcomparisonisbetweenthetwopositionsofthecrosscyl,andnotacomparisonwithnocrosscyl.Ifposition3ispreferred,add+0.50cyltothepluscylandadd–0.25sphere.Thisspherecompensationwhenadjustingthecylensuresthesphericalequivalentofthelensesismaintained(sphericalequivalent=sphere+cyl/2).Tomaintainthesphericalequivalent,thespheremustbechangedbyhalftheamountofthecylandinoppositedirection.Ifposition4ispreferred,reducethepluscylpowerby0.50cylandadd+0.25spheretomaintainthesamesphericalequivalent.Ifthecylpowerischanged(andspherecompensated),itisnecessarytore-checktheaxis,thenagainchallengethecylpower.Ifyoudonottrustthecylobtained,reducethecyl(orremoveifsmall)andseeifthepatientprefersthis(i.e.testforrejectionofcyl),sincepatientsaremorelikelytopreferunderratherthanoverastigmaticcorrection.Continuethisprocessuntilanendpointisreachedforboththecylaxisandcylpower(i.e.untilthepatientreportsthatposition1issameas2,andposition3issameas4).Re-checktheacuitythenproceedtotheduochrometest.

DuochrometestThisisamonocularsubjectivetesttominimiseaccommodationwhilstthedistanceprescriptionisworn,whichisespeciallyimportantinmyopes.Ifamyopeisovercorrected(prescriptiontoominus),theyareeffectivelyrenderedhypermetropicandmayexperienceasthenopia(eyestrain)duetoprolongedaccommodation.Theprincipleoftheduochrometestreliesonchromaticaberration,whichiswherewhitelight,whenrefractedatanopticalinterface,isdispersedintoitsdifferentcolours(wavelengths).Anemmetropiceyefocusesdistantyellow-greenlight(555nmwavelength)perfectlyontotheretina.Redandgreenlightareusedfortheduochrome,sincetheirwavelengthfocistraddleyellow-greenlightbyequalamounts(about0.4dioptresoneitherside),withgreenbeingdeviatedmorethanred,sinceredhasthelongerwavelength(seeFigure

4.8).

Figure4.8Dispersionandthehumaneye.Yellow-greenlight(555nm)isfocusedperfectlyontotheretina(R)byanemmetropiceye,whenlightisdispersedbytheprincipalplane(P)oftheopticalinterface.Greenlightfallsinfrontoftheretinaandredlightfallsbehindtheretinabyequalamounts

Theduochromeconsistsofaringofblackcirclesorlettersonaredandgreenbackground(seeFigure4.9).

Figure4.9Theduochrome

AftertheJCCtest,whilstthefelloweyeisstillfogged,askthepatienttolookatthedistantduochromeandaskthemifthecircles/lettersarecleareronthered,greenoraboutthesame.Iftheyprefergreen,add+0.25sphereandrepeatthequestion.Addingplusspheresshouldshiftthepreferencefromgreentoindifferenttored,andshouldrelieveanyaccommodationwithsacrificingtheacuity.Mostpractitionerswouldagreetoleavemyopesjustonthered.Formyopes,greenisgenerallyconsideredunacceptable,indifference(equalredandgreen)acceptableandjustontheredpreferable.Thereasonwhymyopesshouldnotbeleftonthegreenisthattheywillbeaccommodating,astheprescriptionistoominus(i.e.overcorrected,renderingthemhypermetropic).Thistestislessimportantforhypermetropes–leavethemindifferentorjustonthegreen.Notethatthetestcanalsobedoneinpatientswhoarecolourblind,sincethetestisdependentonthepositionoftheimagewithrespecttotheretina.Therefore,colour-blindpatientscanbeaskediftheleftorright(orupperorlower)rankisclearer,ratherthantheredorgreenrank.Onceadjusted,re-checktheacuity.

Asanextrastepinmyopes,itisusefultotrythe+1.00blurbacktest,inwhicha+1.00sphereisaddedthatshouldblurtheacuityto6/12.Ifthemyoperemains6/6,theprescriptionistoominus(overcorrected)andthis+1.00sphericallensshouldbeaddedtotheirprescriptiontoremovetheiraccommodation,whilstretainingdistanceacuity.Theduochrometestisthenrepeatedforthelefteye(remembertofogtherighteye).

BinocularbalanceThisisafinalsteptobalanceanyaccommodationandisdoneoncebotheyeshaveindependentlybeensubjectivelyrefracted.Itisparticularlyusefulinyoungmyopestoensurethattheirprescriptionistoominus(overcorrected)andisanalternativetothe+1.00blurbacktestalreadydescribed(seeabove).Checkthebinocularacuity(removeanyfoggingoroccludinglenses).Nowaskthepatienttofixateonaletteronthelowestlinethattheycansee.Thenplacea+1.00sphereoverthelefteyeanda+0.25sphereovertherighteyeandask:

‘Istheletterbetter,worseoraboutthesame?’

Iftheletterisbetteroraboutthesame,addthe+0.25spheretotherighteyeandrepeat.Donotgivethepluslensiftheletterappearsworse(blurred).Repeattheprocesswiththe+1.00sphereovertherighteyeandthe+0.25sphereoverthelefteye.Ifanylensesareadded,re-checkthebinocularacuitytoensurethatithasnotreduced.Ifacuityhasfallen,removethepluslens.

CoverandalternatecovertestsThesetestsareusefulinassessingtheangleofdeviationineyesthathaveasquintoratendencytodrift.Itisimportanttounderstandthese,sincetheyareveryquicktoperformandoftenyieldinvaluableinformation.TheyalsoformabasicstandpointfromwhichthePCTorMRtestprogressesfromsothatthesquintcanbequantifiedwithprismsandprismaticincorporationcanbeconsideredinthespectacleprescriptionforsignificantlysymptomatic

patients.

CovertestThisisaquicktestthatisusedtodetectamanifestsquint(tropia).Rememberthatchildren(oradultswithuntreatedchildhoodsquint)withamanifestsquintwillsuppresstheimagefromtheweaker,non-fixatingeyeandthereforenotcomplainofdiplopia.Incontrast,adultswitharecentlyacquiredsquintwillcomplainofbinoculardiplopiathatisworsewhentheylookinthedirectionofextra-ocularmuscleunder-action.Thecovertestshouldbeperformed:

withandwithoutspectacleswithandwithoutanycompensatoryheadposturefordistanceandnear(totorchlightandanaccommodativetarget)alwaysintheprimarypositionand,ifnecessary,inthedifferentdirectionsofgaze.

Forthedistancecovertest,askthepatienttofixateonadistant(6m)target.Remembertofirstgentlyguidethepatient’sheadintotheprimarypositiontoremoveanycompensatoryheadposture.Coverthelefteyeandobserveforanymovementintherighteye(seeFigure4.10).

Esotropia(convergentsquint)whentherighteyeinitiallyispointingnasallyandthenmovestemporallyoncovertest.Exotropia(divergentsquint)whentherighteyeinitiallyispointingtemporallyandthenmovesnasallyoncovertest.Righthyper-/hypo-tropia(verticalsquint)whentherighteyeinitiallyishigher/lowerthantheleftandthenmovesdownwards/upwardsoncovertest.

Figure4.10Thecovertestdemonstratingarightesotropia(middlepicture)andexotropia(bottompicture)

Nowrepeatthecovertestwiththerighteyeoccluded,observingthemovementoftheleft.Iftherewasmovementwithlefteyeocclusionbutnotforrighteye

occlusion,thetropiawillbea‘righteye’tropia(e.g.rightesotropiaiftherighteyemovedtemporally),sincethelefteyeisthemorestableeyethatisadoptingfixation(andviceversaformovementwithrighteyeocclusionbutnotlefteyeocclusion).Repeatthecovertestwithspectaclesandwithanycompensatoryhead

posture.Thenrepeatthecovertestwithaneartorchlightfollowedbyanearaccommodativetarget,suchasasmallattention-holdingimageat33cm(readingdistance).

AlternatecovertestThealternatecovertestisadissociativetestthatdissociates,oruncouples,theeyes.Aseacheye‘sees’adifferentfixationtarget,theirtruetendencytodriftisreleased.Asthealternatecovertestcontinues,thistendencytodriftoftenbecomesmoremarked.Therefore,theamountofdeviationnotedwiththealternatecovertest

isthesumoftheboththemanifestsquint(detectedwiththecovertest)andthelatentcomponentofthesquint(thetendencyoftheeyestodriftoncedissociated).Ifthedeviationisobservedwiththecovertestalone,thisisknownasa‘-tropia’.Ifthereisnodeviationwiththecovertestbutthereiswiththealternatecovertest,thisisolatedlatentcomponentisknownasa‘-phoria’.Aswiththecovertest,thealternatecovertestshouldbeperformed:

withandwithoutspectacleswithandwithoutanycompensatoryheadposturefordistanceandnear(totorchlightandanaccommodativetarget)alwaysintheprimarypositionand,ifnecessary,inthedifferentdirectionsofgaze.

Forthedistancealternatecovertest,askthepatienttofixateonadistant(6m)target.Remembertofirstgentlyguidethepatient’shead

intotheprimarypositiontoremoveanycompensatoryheadposture.Coverthelefteyeandobserveforanymovementintherighteye.

Thenswiftlymovetheoccludertocovertherighteyeandobserveforanymovementasthelefteyebecomesuncovered.Repeatthisafewtimes,untilthedegreeofmovementhassettled(sinceitwillincreasewithtime)andonceyouhavenotedthedirectionofmovement.

Atemporalmovement(frominitialnasal,convergentposition)impliesanesodeviation.Anasalmovement(fromaninitialtemporal,divergentposition)impliesanexodeviation.Adown/upmovement(fromaninitialhigh/lowposition)impliesahyper-/hypo-(vertical)deviation.

Iftheeyesrapidlytakeupfixation,thissuggeststheacuityandsubsequentneurallinkwiththevisualpathwaysissimilarforeacheye.Ifoneeyeisslowtotakeupfixation(sometimesrequiringverbalencouragement),itislikelythattheacuityinthiseyeispoor.Repeatthealternatecovertestforneartorchlightthenanear

accommodativetargetat33cmreadingdistance.

PrismcovertestThePCTallowsthemeasurementoftheangleofdeviation,whichallowsobjectivequantificationofthesquintandsubsequentprescriptionoftheprismforsymptomaticcontrolifnecessary.Aswiththecovertest,thePCTshouldbeperformed:

withandwithoutspectacleswithandwithoutanycompensatoryheadposturefordistanceandnear(thepatientcanholdthenearaccommodativetarget)alwaysintheprimarypositionand,ifnecessary,inthedifferentdirectionsofgaze.

NotethatthePCTshouldbeperformedfordistantandnearaccommodativetargetsanddifferentprismsmayberequiredfordistanceandnearprescriptions,sincepatientstendtoconvergeonnear

fixation.Sincetheexaminerrequiresonehandtoholdtheprismbarandonehandtomovetheoccluder,whentestingtheangleforanearaccommodativetarget,itisnecessarytoaskthepatienttohold,andlookat,theaccommodativetarget.Aprismcanbeheldinfrontofeithereye,sincetheangleofdeviation

relatestotheanglebetweentheeyes.Ifyouareright-handed,youmayfinditeasiertoholdtheoccluderinyourlefthandandtheprismsinyourrighthand.Theprismscanbeheldindividuallyorintheformofaprismbar–whicheveryoufeelmorecomfortablewith.Acombinationofhorizontalandverticalprismsmaybeneeded.First,

establishthehorizontalangle.Oncethisiscorrected,lookspecificallyforaverticaldeviationandsuperimposeverticalprismsonthehorizontalprismtocorrecttheverticalcomponent.Verticaldeviationsaretypicallysmallerthanhorizontaldeviations

but,intheabsenceofsuppression(suchaswithanacquiredsquintinanadultinthecaseofthyroideyediseaseorcranialnerve4palsy),theyareoftenmoresymptomaticduetothebinocularfusionrangebeingsmallerverticallyratherthanhorizontally.Notethatprismswillhaveaformofdemarcation,suchasacross,at

theirbasetohelporientation.ForthedistancePCT,askthepatienttofixateonadistant(6m)

target.Remembertofirstgentlyguidethepatient’sheadintotheprimarypositiontoremoveanycompensatoryheadposture.Performanalternatecovertestasdescribed(seep.59).Repeatthealternatecovertestwithaprisminplace:

forexodeviations,abase-in(BI)prismisneededforesodeviations,abase-out(BO)prismisneededforhyper-/hypodeviations,abase-down(BD)/base-up(BU)prismisneeded.

Thereisnoneedtoremembertheselistedpoints–justrememberthatthecorrectingprismmusthaveitsapexpointinginthedirectionofdeviation.Ifthemovementisinthesamedirectionwiththiscorrectiveprism,

thestrengthoftheprismmustbeincreased.Ifthemovementhasreverseddirection,theprismstrengthmustbereduced.Theaimistoaltertheprismsuntilreversalisnoted,toobtainasatisfactoryendpoint,

whichiswhentheeyesremainstillduringthealternatecovertestsincetheprismshaveneutralisedanydeviation.Thiscanbeconfirmedbyaskingthepatientiftheirdoublevisionhasbeeneliminated.Asmentioned,firstcorrectthehorizontalanglethenlookspecifically

foraverticalcomponentandcorrectthis,ifpresent,bysuperimposingverticalprismsuponthecorrectinghorizontalprism.Nowrepeatthetestforanearaccommodativetarget(heldbythe

patientat33cmreadingdistance).Thepatientshouldweartheirnearspectacles(albeitwithoutprismsatthisstage).Whenincorporatingprismsintothespectacleprescription,theterm

‘prismdioptre’canbedenotedbyatriangle(∆).However,asthiscanbemistakenforazero,itissafertousetheabbreviation‘pd’inthespectacleprescription.Theamountofdeviationindegreesiscorrectedbyaprismwithapowerdoublethatmagnitudeinprismdioptres.Forexample,a15-degreeangleofdeviationiscorrectedbya30pdprism.Typically,theprismaticcorrectionishalvedbetweenthetwolenses

andthebaseswillbeinthesamedirectionforhorizontaldeviationsandinoppositedirectionsforverticaldeviations.Forexample,a13pdexodeviationwillbecorrectedbya6pdBIcorrectioninfrontoftherighteyeanda7pdBIcorrectioninfrontofthelefteye.A4pdrighthyperdeviationwillbecorrectedbya2pdBDcorrectioninfrontoftherighteyeanda2pdBUcorrectioninfrontofthelefteye.

Rememberthattheapexofthecorrectingprismisalwaysinthedirectionofsquintdeviation.

MaddoxrodtestTheMaddoxrod(MR)testisasubjectiveassessmentofextra-ocularmusclebalanceandestimatesthedegreeofphoria(tendencyofeyestodriftsotheyarenotdirectedatthesametarget).Themajorityofpatientsdonotneedsprisms.Prismsshouldonlybe

incorporatedintothespectacleprescriptionif:

thereisahistoryofdoublevision,orsignificantasthenopic(eyestrain)symptoms,associatedwithademonstrablephoria(latentsquint)

ifamanifestsquint(tropia)isnoticedwiththecovertestrestorationoforthophoriaisachievedwiththeproposedprismsusingtheMRtest(andPCT).

Ifthereisnodoublevisionandifnotropiaisseenwithacovertest,theMRtestisunnecessary,as,regardlessofwhatitshows,therewillbenoneedforprismaticcorrection.Therefore,intheRefractionCertificateExamination,alwaysconsiderthatthecorrectanswermightbetoordernoprisms.Youmayfindthatsomepatientswithoutanyrefractivecorrection

haveacuitythatistoopoortoallowthemtoappreciatemultipleimages.Withouttheirspectacles,theydonotcomplainofdiplopiasinceeverythingisjustsimplyblurred.Inthesecases,youwillnoticethatonceyouhaveimprovedtheacuityofbotheyesthepatientwillstarttocomplainofdoublevision.ThesepatientswillbenefitfromtheMRtestandprismaticcontrol.TheMRconsistsofaseriesofstrong,concave(plus)cylindricalred

glassrodsthatconverttheappearanceofawhitespotoflightintoaredstreak(seeFigure4.11).Whentherodsareorientatedvertically,thestreakwillbehorizontal,andviceversa.Lightfromadistantsourcepassesthroughtheredcylswithnodeviationinthesamemeridianastheaxisofthecyls(sincetheyhavenopowerinthedirectionoftheiraxis).Sincetherearemultipleredrods,thisgivesasingleredlineontheretinaandisperceived.Lightraysinothermeridiansareconvergedbythesepowerfulrodstoapointfocusjustinfrontoftheeyethatistoocloseforittobeappreciated(thisisnotseen).

Figure4.11TheMaddoxrodconsistsofredcylinders.HeretheMaddoxrodisheldwiththerodsorientatedvertically,andthisresultsinahorizontalredlineoflight,whenlightfromadistantsourceisviewedthroughtheMaddoxrod

ByplacingtheMRinfrontofoneeyewhilstthepatientfixatesatadistantwhitelight,thetwoeyesaredissociated,sinceoneeyestaresattheredlinewhilsttheotherstaresatthewhitelight.Iforthophoric,theredlinewillappeartopassthroughthewhitelightwhentheredlineisorientatedeitherverticallyorhorizontally.Ifthereisahorizontalphoria,whentheredlineisorientatedvertically(rodshorizontal),theredlinewillappeartooneside.Ifthereisaverticalphoriawhentheredlineisorientatedhorizontally(rodsvertical),theredlinewillappeareitheraboveorbelowthewhitelight.Thismaysoundcomplex,but,withpractise,theMRtestcanbe

completedinlessthan1minutewithease.Rememberthatcorrectiveprismshavetheirapexesdirectedinthedirectionofeyedeviation.ThedistantcovertestisusefultodopriortotheMRtest,sinceit

givesanobjectivestartingpointthattheMRtestsubjectiveresult

shouldmatch.Nowturntheroomlightsdown.Ensurethepatientiswearingtheir

binocular,distanceprescriptionandaskthemtofixateatadistantwhitedotlight(somebodyholdingapentorchattheendoftheroomissufficientifnowhitedotlightisinthelightbox).HoldtheMRinfrontoftherighteyewiththebarsorientated

horizontallyandaskthepatientiftheycanseeaverticalredline.Iftheycannot,occludetheirlefteyemomentarilyandtheywillusuallyseetheredline.Askthemiftheredlineistotheright,leftorstraightthroughthewhitedot.Ifthelinegoesthroughthewhitedot,noprismaticcorrectioninthe

horizontalplaneisrequired.Ifthelineistotheright,theyhaveanesophoria,soBOprismsshould

beplacedinfrontofthelefteyeuntiltheredlineisthroughthewhitespot.Intheory,aBOprismcouldalsobeplacedinfrontoftherighteyetocorrectanesophoria,butbecausetheMRisinfrontoftherighteye,itiseasiertoplaceprismsinfrontofthelefteye.Ifthelineistotheleft,theyhaveanexophoria,soBIprismsshould

beplacedinfrontofthelefteyeuntiltheredlineisthroughthewhitespot.Again,thiscouldalsobecorrectedwithaBIprisminfrontoftherighteye,butastheMRisinfrontoftherighteye,itiseasiertoplaceprismsinfrontofthelefteye.A3pdlenscanbeusedfirsttotrytoshiftthepositionoftheredline

topassthroughthewhitespotor,ifovercorrected,topassovertotheotherside.Inpatientswithoutdiplopia,3pdisusuallysufficienttoshiftthelineandconfirmsthatnoprismsneedtobeincorporatedintothespectacleprescription.Inpatientswithdiplopia,morethan3pdwillprobablyberequiredtoshifttheredlinetopassthroughthewhitespot.Theresultantprismaticcorrectionshouldthenbesharedbetweenthetwoeyes.Forexample,if8pdBOisrequiredtocorrectanesodeviation,4pdBOinfrontoftherighteyeand4pdBOinfrontofthelefteyeshouldbeprescribed.If13pdBIisrequiredtocorrectanexodeviation,7pdBIinfrontoftherighteyeand6pdBIinfrontofthelefteyeshouldbeprescribed.NowholdtheMRinfrontoftherighteyewiththerodsorientated

verticallyandaskthepatientiftheycanseeahorizontalredline.Askthemiftheredlineisabove,beloworstraightthroughthewhitelight.

Ifthelinegoesthroughthewhitedot,noprismaticcorrectionintheverticalplaneisrequired.Ifthelineliesabovethewhitedot,theyhavealefthyperdeviation,

whichcanbecorrectedwithaBDprisminfrontofthelefteye.ThiscouldalsobecorrectedwithaBUprisminfrontoftherighteye.Ifthelineliesbelowthewhitedot,theyhavealefthypodeviation,

whichcanbecorrectedwithaBUprisminfrontofthelefteye(oraBDprisminfrontoftherighteye).Again,forverticaldeviations,a3pdlenscanbeused,butnotethat

patientsaregenerallymoresensitivetoverticaldeviations.Forexample,a3pddeviationinthehorizontalplaneisusuallyfusedanddoesnotresultinsymptomaticdiplopia,whereas3pdintheverticalplanemaynotbefusedandthepatientmayhavediplopia.Ifaverticalprismaticcorrectionisrequired,againthisshouldbesharedbetweenthetwoeyes;however,unlikeforhorizontaldeviations,inverticaldeviationstheprismsareorientatedinoppositedirections.Forexample,a5pdlefthyperdeviationcanbemanagedwith3pdBDinfrontofthelefteyeand2pdBUinfrontoftherighteye.YoumayhaverealisedthatiftheMRisplacedinfrontoftheright

eyeandthecorrectiveprismsarethenplacedinfrontofthelefteye,theapexoftheprismisalwaysinthesamedirectionthatthepatientreportstheredlinetoappear,relativetothewhitedot:

linetotheleft:placeprismwithapextoleftlinetotheright:placeprismwithapextorightlineabove:placeprismwithapexupwardslinebelow:placeprismwithapexdownwards.

Therefore,itissimpletoplacetheMRinfrontoftherighteyeandusecorrectiveprismsinfrontofthelefteyewiththeapexpointingtowheretheredlinelies.Theonlysituationinwhichthisisnotpossibleiswhentherighteyehasrelativelypoorbestcorrectedacuity(duetoamblyopiaorocularpathology).Inthiscase,theMRshouldbeheldinfrontofthelefteye.

Nearvision‘Accommodation’referstotheprocessofthefocalpointoftheeye

shiftingfromadistanttargettoaneartarget.Patientswhoarepresbyopicareunabletoreadclearlywhilstwearing

theirdistantspectacleprescriptionduetoaninabilitytoaccommodate.Presbyopiamanifestsatanearlierageinhypermetropes(fromage

35years)thaninemmetropes(fromage40years)andmaynotevermanifestinmyopes.Nearvisionisalsoimprovedbypupillaryconstriction,whichincreases

thedepthoffocus.Adequatemacularfunctionisalsovitalforsatisfactorynearvision.Forthesereasons,checkingnearvisionwithgoodilluminationismosthelpful.Giventhatpatientswillconvergewithneartargets,theymayalso

requireaprismaticcorrectiondifferenttotheirdistantcorrection(see‘Prismcovertest’,p.60).Toestimateaninitialnearadd,obtainabriefrelevanthistory:

theiragewhethertheyhavehadpreviouscataractsurgerywithanintra-ocularlensimplant(pseudophakia)theiractivitiesofdailylivingthatinvolvenearvisualtasks–reading,needlework,modelmaking,etc.,sincethiswillaltertheirnearworkingdistance.

Thefollowingguideshouldbeausefulstartingpoint.AgeNearadd40–50years+1.00to+1.5050–60years+1.50to+2.00>60years+2.00to+3.00Pseudophakia+2.50to+3.00

Toassessnearvision,askthepatienttoholdthereadingchartatthecomfortablenearworkingdistancefortheneartasktheywouldlikecorrectionfor;forexample:

reading–typically,about33cmneedlework,modelmaking,etc.–maybemuchcloserand,therefore,requireagreaternearaddcomputerwork–suchanintermediatedistancemayrequirea

weakeraddtothedistantprescription,relativetofullnearcorrectionrequiredforreading.

OccludethelefteyeandaskthemtoreadthesmallestprinttheycanontheN-seriesreadingchartheldattheirworkingdistance.Nowaddtheappropriatenearpluslensandrecordthecorrectednearacuity(aimingforN5orN6intheabsenceofoculardisease).Askthepatienttolookataletterthenask:

‘Istheletterclearerwith[placea+0.25sphereinfrontoftheireye]orwithoutthelens[removethe+0.25sphere]oraboutthesame?’

Iftheyreportthattheletterisbetterwiththelensoraboutthesame,addthe+0.25sphereandrepeatuntilacuityisoptimal.Repeattheprocessforthelefteye(occludetheright)thencheckthat

thereadingspeedisgoodwithbotheyesnotoccluded.Thepatient’snearaddistypicallythesameforbotheyes,butthis

shouldstillbecheckedbecausepre-presbyopesthathavehadunilateralcataractsurgerywillrequireahighnearaddintheirpseudophakiceyeandperhapsonlyasmallnearaddintheirphakiceye.

5

RetinoscopyofamodeleyeTheRefractionCertificateExaminationmayrequireyoutocompleteobjectiverefraction(retinoscopy)ofamodeleyewithin5minutes.Themodeleyeisonasimplestandand,fortunately,hasbeenmadeto

provideascopereflexthatiseasiertointerpretthanrealreflexes.However,sincenotrialframecanbeusedhere,youneedtotakecarewhenjudgingtheworkingdistanceandinbeingawareofthecylindricalaxis.Thissituationissimilartoperformingretinoscopyonchildren(whoareaversetotrialframes).SeeChapter4tolearnhowtousetheretinoscope.Ifworkinginpluscyls,refractthemodeleyeusingspheresuntilthe

leastwithmovementisneutralisedandleavearesidualwithmovementintheperpendicularaxis.Thenplaceapluscyllensinfrontofthesphere(holdingbothlensesflushtogether)androtatethecylaxislinesothatitisorientatedparalleltoyourscopeslit.Continuetorefractinthismeridianuntilneutralised.Greatcaremustnowbetakenwhenrecordingyourresults.An

approximationofthecylaxismustbemade,sincethereisnotrialframetoaidyourrecordingofthecylaxis.Furthermore,evenifyourworkingdistanceistypically66cmfor

refractingadultsintrialframes,youwillprobablyfindthatyourworkingdistanceforrefractingchildrenwithouttrialframes(and,therefore,modeleyes)isreducedto50cm.Ifyourworkingdistanceis50cm,itisnecessarytoadd–2.00spheretoyourprescriptiontocorrectforworkingdistance(ratherthanthe–1.50spherethatisaddedforaworkingdistanceof66cm).Remembertostateyourworkingdistanceanditscorrectionforthe

examiners.Forexample,iftheretinoscopygives–3.50/+1.75@130,recordyourresultas–5.50/+1.75@130,correctedforaworkingdistanceof50cm.

6

Howtouseafocimeter

FocimeterprinciplesThefocimeterisusedtomeasurethebackvertexpowerofalens.Itispossibletoestablishthesphere,cyl(powerandaxis)andnearaddofapairofbifocalspectacles.Itcanalsobeusedtomeasureanyprismsthatmayhavebeenincorporatedintothelens.Itisnotsoaccurateatmeasuringthestrengthofvarifocalspectacles.TheRefractionCertificateExaminationrequiresyoutousethe

focimeter.In5minutes,youwillbeexpectedtorecordthedistanceandnearprescriptionforapairofbifocalspectacles.Focimetershaveadiverginglightsourcethatpassesthroughacard

thathasaringofholesinandthentoacollimatinglensthatconvergeslight,which,oncefocused,givesaringofdots.Thisringofdotsisobservedthroughaviewingsystem.Whenthelenstobetestedisplacedonthefocimeter,thedistanceofthecardfromthecollimatinglenscanbealtereduntilthedotsarefocused,andthisgivesapowervaluethatisnotedfromacalibratedscale.Althoughtherearedifferenttypesoffocimeter,essentially,theyall

workaccordingtothisprinciple.Ifpossible,trytobecomeacquaintedwithatleasttwodifferenttypesoffocimeter(seeFigure6.1).Intheexamination,thereisusuallyacoupleofthecommonlyusedfocimeterstochoosefrom.Beforeusingthefocimeter,lookatthespectaclesandnotethatifthey

arebifocalanearaddvaluewillalsoberequired.Quicklynotethatiftheyminifyanobject,theywillbethespectaclesofamyope(minuslens),whereasiftheymagnifyanobjectthentheywillbethespectaclesofhypermetrope(pluslens).

Figure6.1Afocimeter

RecordingdistanceprescriptionTurnthefocimeteronandsetthefocusingwheeltozero.Thenturntheviewingeyepiecefullyanticlockwiseandlookdowntheeyepiece,turningitclockwiseuntilthedotsandgraticuleareinfocus(thisreducesinstrumentaccommodation,whichwillgiveafalserecording).Placethespectaclesonthefocimeterwiththearmsfacingbackwards,toensurethatthefocimetermeasuresthebackvertexpowerofthelens.Conventionally,thedistancethennearprescriptionsareestablishedfortherightlensandthenfortheleftlens.Ifthespectaclesarebifocals,checkthatitistheupperdistancesegmentthatisorientatedonthefocimeter.Youmayneedtomovethelensarounduntiltheringofdotsiscentralisedonthegraticule.Ifthisisnotpossible,thisisduetoaprisminthelens(seep.75).Oncethespectaclesareplacedonthefocimeter,aringofdotsisonlyseenifthelensonlycontainsasphereandwhenthecollimatinglensisfocused.Therefore,rotatethefocusingwheeluntilacrispringofdotsisseenthennotethepowervalueandsign(+or–)onthewheel.This

willgivethedistancesphericalprescription.Aswithmostcases,theprescriptionwillhaveanastigmaticelement,so,ratherthanaringofdotsbeingobserved,aringoffinelinesisobserved.Turningthefocusingwheelwillbringtheselinesintofocusandturningthewheelfurtherwillbringasetofperpendicularlinesintofocus(thepreviouslineswillbecomeblurredorwilldisappear).Itisnecessarytoadjusttheaxisofthegraticulesothatthelinesaremadelinear.Oncetheaxishasbeencorrected,turnthefocusingwheeltobringthelinesintosharpfocus.Failuretofirstmatchtheaxiswillresultinaninabilitytosharplyfocusthelines.Recordthepowerandtheaxis–thisisthevalueofthecylindricalcomponentinoneofthetwoprincipalmeridians.Thenturnthefocusingwheeluntiltheperpendicularlinesappear.Again,fine-tunetheaxisofthegraticuleuntilthelinesarelinearthenalterthefocuswheeluntilinsharpfocus.Recordthepowerandaxisofthisperpendicularprincipalmeridian.Itisquitesimpletoconvertthetwocylrecordingsintoaspectacleprescription.Ifworkinginpluscyls:

thesphereisthemostnegativerecordingthecylisplusandisthedifferencebetweenthetworecordingstheaxisisthesameasthemostplusrecording.

Someexamplesfollow.Twocylrecordingsfromfocimeter Prescription

+3.00@030,–2.00@120 –2.00/+5.00@030

–1.75@145,–3.25@055 –3.25/+1.50@145

+1.50@060,+6.25@150 +1.50/+4.75@150

Notethatthefocimeterrecordsthecylaxisandnottheorientationofthecylpower(perpendiculartotheaxis).Thisisimportanttoappreciateifusingpowercrossestoobtaintheprescription,ratherthanthesimplethree-stepprocessdescribedhere.Forexample,ifthetwocylrecordingsfromthefocimeterare+3.00@135and–1.75@045,thiswouldgivethefollowingpowercross:

Thisgives–1.75/+4.75@135(equivalentto+3.00/–4.75@045).See‘Powercrosses’,inChapter4,toseehowtoobtainthe

prescriptionfromthepowercross.Althoughacademicallyitisusefultoappreciatepowercrosses,youmaywellfinditpracticallysimplertousethethree-stepmethodalreadydetailed.

RecordingnearaddvalueTomeasurethenearaddofthebifocalsegment,movethespectaclessothatthelowernearsegmentisorientatedonthefocimeter.Rotatethefocuswheeluntilthedots(orlinesinthecaseofastigmatism)areinfocusandrecordthepower.Subtractthedistanceprescriptionfromthisnearvaluetogivethenearadd.Forexample,ifthedotsareinsharpfocusat–3.00sphereforthe

distancesegmentand–1.50sphereforthenearsegment,thenearaddwillbe+1.50sphere.Whenestablishingthenearaddforasphero-cylindricallens(usedtocorrectastigmatism)ensurethatthelinesbroughtintofocusareatthesameorientationasthoselinesusedtogivethepowervaluefordistancethatissubtractedfromthenearrecording.Forexample,ifthelinesareinfocusforthedistancesegmentat+3.00@030and–2.50@120andthe030lineisinfocusat+5.00forthenearsegment,thenearaddis+2.00sphere.The120lineswouldthenbeinfocusat–0.50forthenearsegment.Inmostcases,thenearaddvaluewillbethesameforeacheye.

However,donotassumethis,sincetheymaybethespectaclesofayoung(pre-presbyopic)patientthathashadunilateralcataractsurgery.Inthiscase,anearaddmaynotberequiredontheeyethathasnothadsurgery;however,anearaddmayberequiredonthesidethathashadcataractsurgery.

RecordingtheprismaticcorrectionWhentryingtocentrethedotsonthegraticule,itmaybecomeapparentthatthedotscannotbecentralised.Thisisduetoaprismbeing

incorporatedintothelens.Thedotswillbedeviatedtowardsthebaseoftheprismbecause,althoughprismsdeviateimagestowardstheirapex,thefocimetereyepieceviewingsysteminvertsthisview.Thepoweroftheprismisequaltothenumberofspaces(denotedbythegraticule)thatthedotsaredeviated.Forexample,ifthedotsaredeviatedbytwospacesupwards,thereisa2pdBUprisminthatlens.Ifthedotsaredeviatedbyfourspacestotherightwhentherightlensisbeingassessed,thereisa4pdBIprisminthatlens.Typically,theprismaticcorrectionishalvedbetweenthetwolenses,andthebaseswillbeinthesamedirectionforhorizontaldeviationsandoppositedirectionsforverticaldeviations.Forexample,a13pdexodeviationwillbecorrectedbya6pdBIcorrectioninfrontoftherighteyeand7pdBIcorrectioninfrontofthelefteye.A4pdrighthyperdeviationwillbecorrectedbya2pdBDcorrectioninfrontoftherighteyeanda2pdBUcorrectioninfrontofthelefteye.Theapexofthecorrectingprismisalwaysinthedirectionofdeviation.Symptomaticoculardeviationscanbecorrectedbyincorporatingprismsintothespectacleprescription,whichcanbemeasuredbythefocimeterasdescribedearlier.However,itisimportanttonotethatoculardeviationscanalsobecontrolledinanotherway–through‘lensdecentration’.Thisiswheretheopticalaxisofthelensispurposefullydecentredrelativetothepatient’spupil.Theprismaticpower(pd)isequaltothepowerofthelens(dioptres)multipliedbythedistanceofdecentration(cm).Ifthishasbeendone,itwillstillbepossibletocentretheimageonthefocimeter.Suchprismaticcorrectioncould,therefore,beoverlooked.Theonlywaytodetectlensdecentrationisbycheckingthelensforamarkingthatindicatesthisorbyusingalensmarkertomarkthepositionofthepupilcentrewhilstthepatientiswearingthespectacles.Thismarkshouldthenbeplacedinthecentreofthefocimeterstopandanydecentrationwillbeevident.Fortunately,intheRefractionCertificateExamination,sinceyouareonlyprovidedwithapairofbifocalspectaclesandnotwiththeirowner,youarenotexpectedtomarkthepupilcentreandassessforlensdecentration.

7

LensneutralisationItispossibletoestablishthespectacledistanceandnearprescription(andalsoprismaticcomponent)ofapairofbifocalspectaclesusingalensboxaccordingtotheprincipleoflensneutralisation.Thisestimatemaynotbeasaccurateasthatestablishedusingafocimeter,butlensneutralisationisstillausefulskilltohaveandonethatisassessedintheRefractionCertificateExamination.‘Lensneutralisation’meansusinglensesthatareequalinmagnitude

butoppositeindirectiontoneutralisethespectacles,sothereisnooveralleffect.Forexample,a+2.50sphereinaspectaclelensisneutralisedwitha–2.50sphericaltrialframelens.A2pdBOprismwillbeneutralisedbya2pdBIprism.First,toestablishifthelensisminusorplus,completethetransverse

test.Passthelenshorizontallyfromrighttoleftacrossaverticalline.Iftheimageofthelinemovesinthesamedirection(righttoleft)asthesweep(‘with’),thelensisminus.Iftheimageofthelinemovesintheoppositedirectiontothesweep(‘against’),thelensisplus.Youmayalsonoticethatminuslenseswillminifyobjectsandpluslenseswillmagnify.Ifthetransversetestimpliesthetestlensisminus,placeapluslens

(say,+3.00sphere)indirectcontactandseeifthiseliminatesthemovementoftheverticallineimage.Ifthemovementisstillwiththesweep,tryamorepluslens;ifitisagainst,tryalesspluslens.Thereverseistrueforplustestlenses.Neutralisationoccurswhentheimageoftheverticallineremainsstillasthelensesaresweptacrossthemhorizontally.Notethatitisvitalthatlensesareheldinclosecontact,sinceifthey

areheldaparttheireffectivepowerisaltered.Oncethelenshasbeenneutralised,theprismaticcomponent(if

present)canbeneutralisedinthesamefashionbytheapplicationofequalandoppositeprisms.Imagesviewedthroughaprismaredisplacedtowardstheirapex.Hence,a3pdBUprismwillshifttheimagedownwardsandbeneutralisedbya3pdBDprism.Neutralisationoccurs

whentheimageisnotshiftedatallbythecombinedprisms.

8

Finaltipsfortheexam

Morethan2monthsbeforetheexamReadthisbook!ReadtheRefractionCertificateExaminationapplicationdetailsvery

carefullyandcontacttheRoyalCollegeifyouhaveanyuncertaintiesaboutwhatisexpectedofyou.Considerattendingacourseonrefraction.Thiswillnodoubtbe

helpful,buttheyareexpensiveandabsolutelynosubstituteforrefractingyourself.Getstudyleave–notjustfortheexambutfortheweekbeforethe

exam,duringwhichtimeyoumustrefractintensively.Getrefracting!PrintoutAppendix1,‘Typicalrefractiverecording

sheet’,andfillinforeachpersonyourefract.Keepallthesesheetssoyoucankeeparecordofhowmanyyouhavedoneandwhatyouhavelearntfromeachone.Ideally,getyourownretinoscopeandborrowadecenttrialframeand

JCCssothatyouarefamiliarwiththeequipmentthatyouwilluseintheexamination.

OnemonthbeforetheexamBynow,youwillprobablyhaverealisedthatthemainlimitationtopractisingisobtainingafreeroomandasubjecttorefract.Itdoesnottakelongtorefracteverybodyinthedepartment,soyouwillneedtolookelsewhere.Tryallstaff–thisincludesmedical,nursing,healthcareassistants,

studentsandadministrativestaff.Anotheroptionistorefractpatientswhilsttheyarewaitingtobeseenduringclinic.Aimtorefractpatientsofallages(children,pre-presbyopicadults,

presbyopicadults)andwithdifferentcharacteristics(highmyopia,highhypermetropiaoraphakia,thosewithsignificantastigmatism,thosethatneedprismaticcontrol,smallpupils,clearphakiclenses,thosewithcataractandpseudophakicpatients).

Remembertopracticebothnon-cycloplegicandcycloplegicrefraction.Finally,considerbookingpeopleinadvanceinto30-minuteslotstorefractduringyourstudyweektoensureafinalburstofresources!

OneweekbeforetheexamConcentrateongettingyournumbersupbyrefractingthepeoplethatyouhavebookedintoyourfreestudyweek.Re-confirmthatyouunderstandtheexaminationformat.Checkyouhaveallthethingsyouwillneedfortheexamination:

yourownretinoscope(placefreshbatteriesinthisandtakeasparepair)aborrowedtrialframeandJCCsanoccluder,ruleandpentorch(forthecovertest)yourpassportordrivinglicence(requiredbyexaminerstoconfirmyouridentity)alltheexamination,accommodationandtraveldetails

OnthedayPrepareforstartingwithanystationfirst.Remembertobeconsistentwhenrecordingyourresults–alwaysuseonlypositivecylsoronlynegativecyls(donotusebothpositiveandnegativecylnomenclature).Alwaysspecifytheworkingdistanceandcorrectforthis.Alldioptricpowersshouldbetotwodecimalplacesandhaveaclear+or–sign(e.g.+0.25,–1.50).Thedegreesymbol(°)shouldbeavoidedandallaxesshouldbetothreesignificantfigures(e.g.045,010,135).Keepyourlensestidy–itwillannoytheexaminersiftheyhavetotidyupafteryou.Ifyoufindyourselfstrugglingwitharetinoscopyreflex,donotjustsittherepersisting,asprolongedretinoscopysweepingisuncomfortableforthepatient,inducesaccommodationanddemonstratestotheexaminersyouruncertainty.Noretinoscopysweepburstshouldtakelongerthanafewseconds,sotrytomakeadecisionandsimplycomeawayandtryadifferentlensifyouareunsure.

Beforeformallystarting,checkthatyouarecomfortablewiththeroomset-up(lighting,recordsheetandequipment)thenaskquestionsifyouareuncertainbeforethebellactuallygoes.Goodluck!

JonathanCParkandDavidHJones

Appendix1

Typicalrefractiverecordingsheet

Name:

Age:

Occupation/Hobbies/Ophthalmichistory:

RIGHT LEFT

Unaidedvisualacuity(VA)

PinholeVA

UnaidednearVA

IPDdistance

IPDnear

BVD

RETINOSCOPY

Workingdistance

SUBJECTIVEREFRACTION

PRESCRIPTION

RIGHT Sph Cyl Axis Prism Base Sph Cyl Axis Prism Base LEFT

Dist.

Near

CORRECTEDVADISTANCENEAR

Appendix2

TheretinoscopeTherearetwotypesofretinoscope–slitandspot.Slitretinoscopesarefarmorecommoninophthalmicoutpatientdepartments,sotheprinciplesoftheslitretinoscopearedetailedhere.Aretinoscopeconsistsofalightsourceandamirrorwithaviewing

holeinitsotheobservercanobservewhateverisilluminatedwhenlookingthroughthehole.Whenthecuffoftheslitretinoscopeisfullydown,alinearlightis

produced(thescopeslit).Withthecuffdown(correctposition),acondensinglensbetweenthelightandmirrorallowsdivergingraystoexittheretinoscope(seeFigureA2.1).Withthecuffupwards,thecondensinglensismovedtoadifferentpositiontogiveconvergingrays.Asthescopeslitissweptacrossthepupil,lightenteringthepatient’s

eyeisreflectedbytheretinathenrefractedbytheireyebeforebeingobservedbythepractitionerthroughtheviewingholeoftheretinoscope.Thequalityofthelightreflexdependsuponthefollowingfactors:

cuffpositionworkingdistancerefractivestateofthepatient’seyeorientationofretinoscopeslitanddirectionofsweep.

Byensuringthatthecuffisfullydown,theworkingdistanceisknownandthescopeslitorientationanddirectionofsweeparecontrolled,itispossibleforthepractitionertoobtainanobjectiverefractionforthepatient’seyebyinterspersingvarioustriallensestoneutralisetheretinoscopyreflex(seeChapter4).Theopticsoftheretinoscopecanbedetailedfurtherbyconsidering

howtheretinaisilluminated(illuminationstage),howanimageoftheilluminatedretinaisformedatthepatient’sfarpoint(reflexstage)andhowtheimageatthefarpointislocatedbymovingtheillumination

acrosstheretinaandnotingthereflexquality(projectionstage).Werecommendthatforfurtherdetailedopticsinformationyourefertothisexcellentbook,whichisalsoveryusefulfortheRoyalCollegeofOphthalmologists’Part1FellowshipExamination:ElkingtonAR,FrankHJ,GreaneyMJ.ClinicalOptics.3rded.London,Malden,MA,andVictoria:Blackwell;2006.

FigureA2.1Aretinoscopewiththecuffdown