postpiggycase

5THEROYALCOLLEGEOFOPHTHALMOLOGISTSFocusAutumn2010AnoccasionalupdatecommissionedbytheCollege.
Theviewsexpressedarethoseoftheauthor.
5RefractiveSurpriseafterCataractSurgeryThelastdecadehasseentheemergenceofrefractivecataractsurgery.
Thisisdenedascataractsurgerywhichnotonlyrestoresthetransparencyoftheocularmediabutalsoattemptstocorrectanyrefractiveaberrationsoftheeye,withtheobjectiveofachievingthebestpossibleuncorrectedvisualacuity.
Thisreducesthespectacledependenceofpatientswithconsequentqualityoflifeandeconomicbenets.
AsphericMonofocal,Toric,MultifocalandAccommodativeIOLsprovidegoodoptionstoachievereducedspectacledependence.
Itisnotunreasonabletoexpectalmostallpatientsfreeofco-morbiditytoachieveuncorrectedvisionequaltoorbetterthanthelegalstandardtodriveacarwithasphericmonofocalIOLs.
MultifocalandAccommodativeIOLscaninadditionprovidegoodintermediateandnearvisionwithoutspectacles.
Thesuccessofrefractivecataractsurgerydependsonachievingapredictablerefractiveoutcomefordefocus(sphericalequivalent)andastigmatism.
Refractivesurprisescanseriouslycompromisepatientsatisfactionandalsogiverisetopotentialproblemsofanisometropia,dominanceswitchinwhichthedominanteyeendsupwiththeweakeruncorrectedvisionand,aboveall,giveriseasenseoffailureinpatientsexpectinggooduncorrectedvisualacuity.
Thisarticlefocusesonthepreventionandmanagementofrefractivesurprisesincataractsurgery.
Prevention:Percivaletal1usingultrasoundmeasurementsandcustomisedlensconstantsreported97%ofeyesachievingarefractiveoutcomewithin1dioptreoftarget.
Galeetal2suggestabenchmarkforNHScataractsurgeryistoachieve85%within1dioptre.
Thesegureshavetobeviewedwithintheper-spectiveofthenormaldistributionofrefractiveerrorinthepopulationwith66%ofeyeswithin1dioptreofemmetropia.
ItfollowsthatifoneistouseastandardpowerIOLwithinthepopulationwithoutanybiometry,66%ofeyeswouldfallwithin1dioptreoftarget.
ItisinterestingtolookatthecausesofrefractivesurpriseafterMilindPande,MedicalDirectorVisionSurgery&ResearchCentre,Hull.
cataractsurgery.
In1992priortotheadventofopticalbiometry,Olsen3reportedthat54%ofrefractivesurpriseswereduetoerrorsinaxiallengthmeasurement,38%wereduetoerrorsinpredictingthepostoperativeIOLpositionand8%wereduetoerrorsinkeratometrymeasurements.
TheadventofopticalbiometryimprovedtheaccuracyandconsistencyofaxiallengthmeasurementstosuchadegreethatasimilarstudybyNorrby4in2008showedthatthecommonestsourceoferrorisinthepredictionofpostoperativeIOLposition(36%),followedbyerrorsinpostoperativerefraction(27%),axiallengthmeasurement(17%),keratometry(10%),pupilsize(8%),variationinrefractionacrossthepupilandIOLpower1%.
OpticalbiometryisanessentialtoolforimprovingtheaccuracyofIOLpowercalculation.
Inpatientswithdensecataractwhereopticalbiometryisnotfeasible,immersionultrasoundbiometryprovidessimilarlevelsofaccuracy.
Therearevariousprotocolsavailabletoimprovetheaccuracyofmeasurementsandallofthemarebasedonrecheckingthemeasurementswhentheprobabilityoftheseoccurringinthepopulationisverylow.
Theseprotocolsareimplementedwithinthenewerversionsofsoftwareforopticalbiometrymachines.
Althoughtheseprotocolsalerttheoperatortounusualmeasurementstheydonotidentifyerrors,whichdonotappeartobeunusualinpatientswithunusualeyes.
ItisthuscriticaltonotonlyusetheseprotocolsbuttosupplementthemwithastrategyofreconcilingtheIOLpowermeasurementswiththepatient'srefractivehistorypriortothedevelopmentofcataracts.
Acruderuleofthumbistoexpectadifferenceof3dioptresintheIOLpowerbetweeneyeswithadifferenceinpre-cataractrefractionof2dioptres.
Reducingtheriskofrefractivesurpriserequiresaconsistentapproachtomeasuringeyes,reconcilingthemeasurementswiththepatient'srefractivehistory,usingamoderntheoreticalformulaliketheSRK-T,HaigisortheHolladay2and66customisingformulaconstantsforsurgeonsaswellasdifferentlenses.
Smallhyperopiceyes,largemyopiceyes,eyeswithverysteeporatcorneas,shallowanteriorchamberdepths,priorhistoryofrefractivesurgery,vitrectomy,cornealectasia,peripheralcornealmeltsyndromesandcontactlensuse(whenmeasuredwithoutanadequatecontactlensholiday)areatsignicantriskofrefractivesurprises.
Itisimportanttowarnthesepatientsoftheincreasedriskofrefractivesurpriseaspartoftheinformedconsentprocessandpreparethepatientsforasecondstageenhancementprocedure.
ClinicalAssessmentofRefractiveSurprise:Amethodicalapproachiscriticalinidentifyingthecauseofarefractivesurprise.
Thisconsistsofthefollowing:1.
Refraction:Inaccuraterefraction4isthesecondmostcommoncauseofrefractivesurpriseaftercataractsurgery.
Anaccuratesubjectiverefractionisessential.
Auto-refractormeasurementswhilerepeatablearenotconsistentwithsubjectiveassessments.
Arepeatableconsistentstrategytorefractpostoperativepatientsisessentialinordertoreduceerrorsaswellcustomiselensconstants.
Thepost-operativerefractionalsoformsthebasisforcalculatingthecorrectionneededinasecondaryenhancementprocedure.
2.
RepeatBiometryMeasurements:Opticalbiometrymakesiteasytomeasuretheaxiallengthandkeratometryinpseudophakiceyes.
Thiswillidentifyanymeasurementerrorsintheoriginalbiometry.
3.
CalculatingIOLpowerwiththenewmeasurementsallowsforacomparisonwiththepreviouscalculation.
ThedifferenceinIOLpowerbetweentheoriginalandrecalculationshouldbeconsistentwiththemagnitudeoftherefractivesurprise.
IfthefullmagnitudeoftherefractivesurprisecannotbeexplainedbythedifferencebetweentheoriginalandrecalculatedIOLpowerotherfactorsapartfrommeasurementerrorlikepredictionofpostoperativeIOLpositionoralenspowererrormaybesignicantcontributorstotherefractivesurprise.
Thecauseofarefractivesurprisecaninuencethemethodchosentocorrecttherefractivesurprise.
Anexamplecaseworkupisshownintheboxbelow.
CorrectionofRefractiveSurprise:Identifyingthecauseofarefractivesurpriseiscriticalinpickingthecorrectrefractiveenhancementproceduretocorrectthesurprise.
Notallsurprisesneedtobecorrected.
Priortoanysuchenhancementitisimportanttoidentifyanddemonstratethebenetsaswellasthepotentialrisksapatientmayexpectfromanenhancementprocedure.
Itisimportanttokeepinmindthetrade-offsapatientmayhavetoacceptbycarryingoutanenhancementprocedure.
Patientswhoendupmyopicintheirnon-dominanteyemaywellprefertheaccidentalmonovision.
Similarlypatientswithmultifocallensesmaywellpreferalongerworkingdistanceattainedbyasmallhyperopicsurprise.
Laservisioncorrection,SecondaryPiggybackIOLsandIOLExchangearethecommonmethodsforcorrectingrefractivesurprises.
Itisimportanttodemonstrateastablerefractionbeforeattemptingacorrection.
LaservisioncorrectionusingeitherLASIKorLASEKwillgivethemostpredictablerefractiveoutcome.
Acompletelynewtypeofprocedurewithconsiderablecostcancreatesignicantanxietyespeciallyinelderlycataractpatients.
SecondaryPiggybackIOLsplacedintheciliarysulcusisasimpleprocedurewithinthecomfortzoneofmostcataractsurgeons.
ThetraumaandrisksofremovinganIOLisavoidedandpiggybackingcoversforanIOLpowererror.
Sphericalerrorsarerelativelyeasytocorrectbutsphero-cylindricalerrorscanalsobetreatedwithToricpiggybacklenses.
ThecalculationsforchoosingthepoweroftheselensesisbasedontherefractionusingavertexingformulaliketheRefractiveVergenceformula5.
IOLExchange:Thisisamethodoflastresortwhenallothercorrectiveoptionshavebeenconsideredanddiscarded.
RemovinganIOLfromaneyecanbeatechnicalchallengedependingonthelensdesignandthetimeperiodthelenshasbeenintheeye.
Removinglensesmonthsoryearsafterprimarysurgerycanbefraughtwiththedangerofrupturingthecapsule.
ThereplacementIOLcalculationsusethesamemethodasusedfortheprimaryIOL.
IOLexchangeisnotagoodmethodtocorrectrefractivesurpriseduetoanerrorinpredictingthepostoperativeIOLpositionoranerrorintheactualIOLpower.
Summary:Refractivesurprisesaftercataractsurgeryareacommoncauseofpatientdissatisfaction.
Preventionrequiresaconsistentmethodofbiometry.
Amethodicalassessmentwithrepeatmeasurementisneededtoidentifythecause.
Ariskbenetassessmentiscriticaltoestablishtheneedforarefractiveenhancement.
LaservisioncorrectionandsecondarypiggybackIOLscarrylowerriskandaremorepredictablemethodsforcorrectingrefractivesurprises.
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GaleRP,SaldanaM,JohnstonRL,ZuberbuhlerB,McKibbinM.
BenchmarkstandardsforrefractiveoutcomesafterNHScataractsurgery.
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OlsenT.
Sourcesoferrorinintraocularlenspowercalculation.
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NorrbyS.
Sourcesoferrorinintraocularlenspowercalculation.
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HolladayJT:RefractivePowerCalculationsforIntraocularLensesinthePhakicEye.
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