Refractive Errors - KSUMSC

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RefractiveErrors

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Colorindex:

432Team–Important–433Notes–Notimportant

433 Ophthalmology Team Refractive Errors

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Contentsoflecture

Physiology Inves0ga0ons Refrac0veerrors

Myopia

Hyperopia

As0gma0sm

Presbyopia


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Physiology-Tohaveaclearpictureintheretina&tobeseeninthebrain,thereshouldbeaclearcornea,clearanteriorchamber,clearlens,clearvitreouscavitythenthepictureshouldbefocusedontheretinawithnormalrefractiveindex.

-Theretinaisresponsiblefortheperceptionoflight.Itconvertslightraysintoimpulses;sentthroughtheopticnervetoyourbrain,wheretheyarerecognizedasimages.-Normalrefractivepoweroftheeyeis60diopters.(Thecorneaaccountsforapproximatelytwo-thirdsofthisrefractivepower(about40diopters)andthecrystallinelenscontributestheremaining)

-Thenormalaxiallengthis22.5mml(it’smeasuredfromthetipofthecorneatothesurfaceoftheretina)

-Iftheaxiallengthislonger=thepicturewillbeinfrontoftheretina

-Iftheaxiallengthisshorter=thepicturewillbebehindtheretina


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Refractions:

• Refractionindicatesthediopter,andthediopteristhefocalrateoflens.Themorerefractivepowerthelenshastheshorterthefocalpointwillbe.

• Inoptics,refractionoccurswhenlightwavestravel

fromamediumwithagivenrefractiveindextoamediumwithanother.Attheboundarybetweenthemedia,thewave'sphasevelocityisaltered,itchangesdirection.

• Theamountofbenddependsontherefractiveindexofthemediaandtheangleofincidence

• Therefractiveindexofamediumisdefinedastheratioofthephasevelocityofawavelightinareferencemediumtoitsvelocityinthemediumitself.

Unitofrefraction-ThepowerofthelensismeasuredbytheDiopter(D)Theunitofrefraction.

THEEYE’SOPTICALSYSTEM

• Cornea• Mainrefractingsurface(2/3rdthepoweroftheeye,40diopter)

• Thepowerofthecorneaisfixed,itreachesitsmaximumpowerattheageof18.That’swhyit’sNOTrecommendedtodoanyrefractivesurgerybeforeageof18.

**Attheageof40=theywillhavepresbyopia**Poweroflensattheageof60=0


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• Lens

• Thelensprovides20dioptersofrefractivepower.

• Therelaxedlens=20diopter.Inaccommodativestageitcanincreasetherefractivepowerupto15dmorelikeinchildren(withtimeitbecomesless)

• Accommodation• Emmetropic(normal)eye

Objectscloserthan6meterssenddivergentlightthatfocusbehindretina;adaptivemechanismofeyeistoincreaserefractivepowerbyaccommodation

• Helm---holtztheory• Whentheeyelookatcloseobject->Contraction

ofciliarymuscle------>decreasetensioninzonulefibers------>elasticityoflenscapsulemoldlensintosphericalshape------>greaterdiopticpower------>divergentraysarefocusedonretina

• contractionofciliarymuscleissuppliedbyparasympatheticthirdnerve

• Visualacuity(VA)• VAisthevitalsignoftheeyewithIOP(intraocularpressure).

• ToassesstheeffectofpathologyonVA.Youmusteliminatetheeffectofrefractiveerror.Thisisachievedbymeasuring:thepatient’sbestspectaclecorrectionorviewingthetestchartthroughapinhole.

• Thepinholeisusedtoeliminatethemildrefractiveerrorofthepatient.Thepinholewillcausemusclespasm.Pinholeglassestypicallyhaveaholediameterofabout1mmto1.2mm.Thepinholecorrectsforaboutthreediopters.

• Whenexaminingthepatientyoushouldexamineeacheyealone(andcovertheothereye)

Note:Afterprolonged

readingtheremightbeciliaryspasmassociated

headache


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• Aftervisualacuitydopinhole:√ Ifimproved20/20=>meansrefractiveerror√ Noimprovement=>othercauses√ Ifimprovedbut20/80=>refractiveerror+othercauses.

• Howtotestthevision• Centralvisualacuityo Displayofdifferent–sizedtargetsshownatastandarddistancefromtheeye.

• Inthefirst2monthsoflife:dolightobjectiontest(ifthebabyobjectingorclosingtheeyeinresponsetolightitmeanshe/sheisseeing)

• From2months–3years:dofollowandfixatetest.Atthisage,babieswillstarttofollowtheobjects,sobringatoyinfrontofthemanddothetest.(Iffollowingthetoy------>goodvision).ORyoucandoCSM“CentralSteadyMaintained”(central=seeingcentrally.Steady=nonystagmus.Maintained=babyisfollowingobject&afterblinkinghe/shecontinuesfollowingthesameobject)

• Age3–6years:Allen’schart• Morethan6years:Snellen’schart• Thevisionmaturationisacquiredskillforthebrain,sobabieswhenthey’refirstborntheywillbelegallyblind.

• Theaxiallengthoftheeyewillgrowquicklyinthefirst6months.Soifanythingstopsthegrowingtheywillhaveamblyopia(lazyeye)E.g:vitreoushemorrhage,congenitalcataract.


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Testingpoorvision• Ifthepatientisunabletoreadthelargestletter<(20/200)

• Movethepatientclosere.g.5/200

• Ifpatientcannotread:

o Countfingers(CF)o Handmotion(HM)o Lightperception(LP)ORNolightperception(NLP)Legalblindness:(heneedsassistance)Legalblindness:Thecriteriausedtodetermineeligibilityforgovernmentdisabilitybenefitsandwhichdonotnecessarilyindicateaperson'sabilitytofunction.IntheUS,thecriteriaforlegalblindnessare:*Visualacuityof20/200orworseinthebettereyewithcorrectivelenses.*Visualfieldrestrictionto20degreesdiameterorless(tunnelvision)inthebettereye.NotethatthedefinitionoflegalblindnessdiffersfromcountrytocountryandthatthecriterialistedabovearefortheUS.Testingnearvisualacuity

• Itisdoneatastandardworkingdistance~30---40cm

• Avarietyofchartsareavailable

Refractiveerrors

• Amismatchbetweentherefractivepowerandthefocusingdistanceoftheeye

• Inabilitytoseeclearlyisoftencausedbyrefractiveerrors.

• Threetypesofrefractiveerrors(Ametrpia)Myopia(nearsightedness),Hyperopia(farsightedness)andAstigmatism.• Emmetropia(normal)• Ametrpia=Refractiveerror

Whatdoes20/200mean?Itmeansthatthepatientseesat20feetwhatanormalpersoncouldseeat200feet


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1)Emmetropia• AdequatecorrelationORmatchingbetweenaxiallengthandrefractivepoweroftheeye• Raysoflightfromadistantobjectarebroughttoapinpointsharpfocusontheretina(noaccommodation).Allrefractiveerrorsaresomedeviationfromemmetropia.

2)Myopia• Raysoflightfromadistantobjectsconvergeinfrontoftheretina,causingablurredimageontheretina

• Myopescanseecloseobjectsclearly,myopiaiscommonlyknownas“near---sightedness”

• MostprevalentamongAsians(80---90%),followedby25%ofAfricanAmericansand13%ofCaucasians.

• Averageageofonset:8years• Etiology:notclear,geneticfactors

• Diabeticspatientshavebothmyopiaandhyperopiadependingonthelevelofthebloodsugar,thismeanstheirDMisnotcontrolled!!

v Causesofmyopia1.Increasedrefractivepower:a) Changeinlensnucleusorshape:cataract,spherophakia,diabetesb) Lensrepositioning:ciliarymuscleshifte.g.miotics.OrLensmovemente.g.anteriorlensdislocationc) Ciliarymuscletone*:excessiveaccommodatione.g.medicalstudents*Readingalotataneardistance.d) Increasecornealpower:keratoconus,congenitalglaucoma2.Increaseaxiallength:congenitalglaucoma,posteriorstaphyloma• Symptoms1. Blurreddistancevision2. Squintinanattempttoimproveuncorrectedvisualacuitywhengazingintothedistance3. Headacheduetoeyestrain.


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o MyopiaForms:–Benignmyopia(schoolagemyopia)• Onset8---12years,myopiaincreasesuntilthechildstopsgrowinginheight• Generallytapersoffatabout20yearsofage–Progressiveormalignantmyopia:(alsocalledDegenerativeorpathologicalmyopia)• Myopiaincreasesrapidlyeachyearandisassociatedwith,fluidityofvitreousandchorioretinalchange.

o Morphologiceyechangesinpathologicalmyopia:– Deepanteriorchamber– Atrophyofciliarymuscle– Vitreousmaycollapseprematurely–leadingtoopacification– Funduschanges:LossofpigmentinRPE(Retinalpigmentepithelium),largediscandwhitecrescent---shapedareaontemporalside,RPEatrophyinmaculararea,posteriorstaphyloma,andretinaldegeneration------>hole------>increaseriskofRRD(rhegmatogenousretinaldetachment).

o Myopiaiscorrectedby(negative)concavelenses.

3)Hyperopia• Raysoflightfromadistantobjectnowfocusbehindtheretina.•Hyperopicpeoplemustaccommodatewhengazingintodistancetobringfocalpointontotheretina•However,thisreducestheiraccommodativereservewhentheywanttoviewcloseobjects.Thismeanstheirdistancevisionisgenerallybetterthantheirnearvision,hencetheterm“long---sightedness”• Etiology:notclear,inherited,traumamaycausedislocationofthelensCausesofHyperopia1.Decreasedrefractivepoweroftheeye:

• Absent(aphakia)orposteriorlyrepositionedlens• Weakaccommodation:trauma,marijuana(marijuanacausesaweakaccommodationafterusingit)2.Decreasedeffectiveaxiallength(retinapushedforward):tumor,orbitalmass

Uncorrectedmyopiaanduncorrectedhyperopiainchildrenmightleadtoamblyopia(seelateranisometropia)


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Symptoms

• Visualacuityatneartendstoblurrelativelyearly‘’inabilitytoreadfineprint’’

• Asthenopicsymptoms:eyepain,headacheinfrontalregion.• Accommodativeesotropia:becauseaccommodationislinkedtoconvergenceleadingtoeasotropia.

Hyperopiaiscorrectedby(positive)convexlenses.

4)Astigmatism• Astigmatismisacommonandgenerallytreatableimperfectioninthecurvatureoftheeyethatcausesblurreddistanceandnearvision.

• Astigmatismoccurswheneitherthecorneaorthelens,hasmismatchedcurves.Insteadofhavingonecurvelikearoundball,thesurfaceiseggshaped.Thiscausesblurredvisionatalldistances.

• Inastigmatism,surfaceofcorneaisnothomogenous.Usuallyitiscongenital.

• Parallelrayscometofocusin2focallinesratherthanasinglefocalpoint

Causesofastigmatism

• Cornealcauses(majority):Simplecornealastigmatism,Keratoconus(causesMyopicastigmatism),Massese.g.lidtumor,Ptosis

• Lenticularcauses:Lensdislocation,lenticonus


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Symptoms

– Asthenopicsymptoms(headache,eyepain)– Blurredvision– Distortionofvision– Headtiltingandturning– Uncorrectedastigmatism>1.5Dioptersmightleadtoamblyopiainchildren

Classification–Regularastigmatism:(2meridians)powerandorientationofprinciplemeridiansareconstant.Theprincipalmeridiansare90degreesapart(perpendiculartoeachother).– Withtheruleastigmatism,Againsttheruleastigmatism,Obliqueastigmatism

–Irregularastigmatism:(differentmeridians)powerandorientationofprinciplemeridianschangeacrossthepupil.Theprincipalmeridiansarenotperpendicular.

Astigmatismiscorrectedbycylindricallenses

Anotherclassification:

Threeprimarytypesofastigmatism:

Myopicastigmatism:Oneorbothprincipalmeridiansoftheeyearenearsighted.

Hyperopicastigmatism:Oneorbothprincipalmeridiansarefarsighted.

Mixedastigmatism:Oneprinicipalmeridianisnearsighted,andtheotherisfarsighted.


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√ Cylindricallens=>tocorrectastigmatismwitheithermyopiaorhyperopia.

√ Biconcavelens=>tocorrectmyopia(-lens)√ Biconvexlens=>tocorrecthyperopia(+lens)

4)Presbyopia• AgerelatedPhysiologicallossofaccommodation• Depositionofinsolubleproteinsinthelenswithadvancingageleadstoprogressivedecreaseintheelasticityofthelensanddecreaseaccommodation.

• Around40yearsofage,accommodationbecomelessthan3D.readingispossibleat40-50cm->difficultlyreadingfineprint,headache,visualfatiguePresbyopiaiscorrectedbyconvexlenses

5)Anisometropia• Anisometropiaistheconditioninwhichthetwoeyeshaveunequalrefractivepower. Generallyadifferenceinpoweroftwodioptersormoreistheacceptedthresholdtolabeltheconditionanisometropia.

• Morethan3dioptersdifferenceifnotdetectedinpediatricsandcorrecteditcancauseunilateralamblyopia“intheweakereye”.

• Individualscantolerateupto2-3Dioptersofanisometropiabeforebecomingsymptomatic.

6)AniseikoniaAnocularconditioninwhichtheimageofanobjectinoneeyediffersinsizeorshapefromtheimageofthesameobjectintheothereye.

Ifthedifferencebetween2eyes:(D=Diopter)-Lessthan3D->it’soktowearglasses

-Morethan3butlessthan7D->patientcan’ttolerateglassesbutcanusecontactlenses

-Morethan7D->refractivesurgery


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Causes

-Correctionofarefractiveerror

-Anisometropia

-Antimetropia(beingmyopic(nearsighted)inoneeyeandhyperopic(farsighted)intheother.)

-Meridionalaniseikoniaoccurswhentheserefractivedifferencesonlyoccurinonemeridian(seeastigmatism).

-Refractivesurgery

v Typesofopticalcorrection

• Spectaclelenses

– Monofocallenses:sphericallenses,cylindricallenses

– Multifocallenses

• Contactlenses

–Higherqualityofopticalimageandlessinfluenceonthesizeofretinalimagethanspectaclelenses

–Indication:cosmetic,athleticactivities,occupational,irregularcornealastigmatism,highanisometropia,andcornealdisease

–Disadvantages:carefuldailycleaninganddisinfection.

–Complications:infectiouskeratitis,giantpapillaryconjunctivitis,cornealvascularization,andseverechronicconjunctivitis

v RefractivesurgeriesA. Keratorefractivesurgery:(workonthecornea.Doesn’tcorrect

highpower)• Refractivesurgery–flattenscornealsurfaceformyopiaorincreasesit’scurvatureinHyperopia.

• Improvesunaidedvisualacuitybutmayhavecomplications

• Examples:PRK,LASIK,LASEK,EPILASIK


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B. Intraocular surgery: for high power

• Givebestopticalcorrectionforaphakia;avoidsignificantmagnificationanddistortioncausedbyspectaclelenses.

• Clearlensextraction.• PhakicIOL( intraocular lenses): lensesmadeofplasticorsiliconethatareimplantedintotheeyepermanentlytoreduceaperson'sneedforglassesorcontactlenses.

• Oneofthesideeffectsofintraocularlensprocedure=>lossofaccommodation.

Photorefractivekeratectomy(PRK)

(noflap.Wejustremovetheepithelium applylaser thentheepitheliumwillgrow)

Advantages:saferonthelongrun

Disadvantages:severepainfor1week,blurredvisionfor2 3weeks

laser-assisted-in-situkeratomileusis(LASIK)(thinflap)

Advantages:immediate20/20vision,nopain,goodvisualrehabilitation,cancorrecthighnumbers(upto–8)

Disadvantages:severetrauma theflapcanfalldown


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MCQ

1.AladywantsLASIKsurgeryforherdaughter.Sheasksforyouropinion.AllthefollowingthingsaresuitableforperformingLASIKexcept:A. Myopiaof4Diopters.B. Ageof15years.C. Stablerefractionfor1year.D. Cornealthicknessof600microns

2. Inmyopia:A. LengthofeyeballisshortB. ImageformsinfrontofretinaC. LensislesssphericalD. Patientcanseefarobjectsclearly

3. ThemostcommoncauseofreducedvisionintheworldisA. TrachomaB. DiabeticretinopathyC. RefractiveerrorsD. Glaucoma

Answers:1-B2-B3-C

CONTRAINDICATIONSofLASIK(source:http://lessons4medicos.blogspot.com/2008/09/mcqs---opthalmology.html)

Unstablerefractiveerror.

Agelessthan21years.

Activecollagenvasculardisease(especiallyinthepresenceofiritisorscleritis).

Pregnancy.

Presenceofapacemaker.

Anyongoingactiveinflammationoftheexternaleye(eg,conjunctivitis,severedryeye).

Refractiveerroroutsidetherangeoflasercorrection.

Keratoconus


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OthmanAbid KhalidAlsuhaibani

DoneBy:

RevisedBy:AwatifAlenaziSaraHabis

Date post:	04-Apr-2023
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Refractive Errors - KSUMSC

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