refractive error

Refractive error

Emmetropia Adequate correlation between axial length a

nd refractive power Parallel light rays fall on the retina (no

accommodation)

Ametropia (Refractive error) Mismatch between axial length and refractiv

e power Parallel light rays don’t fall on the retina (no

accommodation) Nearsightedness (Myopia) Farsightedness (Hyperopia)AstigmatismPresbyopia :NOT REFFERECTIVE ERROR

Accommodation Emmetropic eye

object closer than 6 M send divergent light that foc us behind retina , adaptative mechanism of eye is i

ncrease refractive power by accommodation theory

-- contraction of ciliary muscle >decrease tension in -- zonule fibers >elasticity of lens capsule mold lens

-- --into spherical shape >greater dioptic power >div ergent rays are focused on retina

contraction of ciliary muscle is supplied by parasympathet ic third nerve

Myopia

Parallel rays converge at a focal point anterior to the retina

Etiology : not clear , genetic factorCauses

excessive long globe (axial myopia) : more common

excessive refractive power (refractive myopia)

MyopiaForms

Benign myopia (school age myopia) - 1012onset years , myopia increase until the chi

ll lllll lllllll ll llllll Progressive and malignant myopia

lllllllllllllll myopia increase rapidly each year and is associated

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rate of increase in amount of myopia generally about 20 years of age

Myopia Congenital myopia

Myopia > 10 DIncrease slowly each year

Myopia

Special forms : nuclear sclerosis , keratocon us , spherophakia

SymptomsB lurred distance visionS quint in an attempt to improve uncorrected v

isual acuity when gazing into the distanceHeadacheAmblyopia – uncorrected myopia > 10 D

Myopia Morphologic changes

deep anterior chamber atrophy of ciliary muscle --vitreous may collapse prematurely >opacification fundus change : loss of pigment in RPE , large disc an

- d white crescent shaped area on temporal side , RPE atrophy in macular area , posterior staphyloma , retin

-- -- al degeneration >hole >increase risk of RD Treatment : concave lenses, clear lens extraction

Hyperopia Parallel rays converge at a focal point poster

ior to the retina Etiology : not clear , inheritedCauses

excessive short globe (axial hyperopia) : morecommon

insufficient refractive power (refractive hyperopia)

Hyperopia Special forms : lens dislocation , postoperati

ve aphakia hyperopic persons must accommodate whe

n gazing into distance to bring focal point on to the retina

Symptoms distance vision is impaired in high refractive e

rror( > 3 D) and in older patient

HyperopiaSymptoms

visual acuity at near tends to blur relatively early nature of blur is vary from inability to read fine print to near vi

sion is clear but suddenly and intermittently blur blurred vision is more noticeable if person is tired , printing is

l lll ll lllll llllllllll asthenopic symptoms : eyepain, headache in frontal

region, burning sensation in the eyes, blepharoconjunctivitis

accommodative esotropia : because accommodation --is linked to convergence >ET

Amblyopia – uncorrected hyperopia > 5D

Hyperopia

Fundus in axial hyperopia may reveal pseudoo ptic neuritis (indistinct disc margin, no physiolo

gic cup, may elevate disc) DDx from optic neuritis by > 4 D , no enlarged bli

nd spot, no passive congestion of vein Treatment : convex lenses, keratorefractive

surgery, refreactive lensectomy with IOL, phakic IOL

Astigmatism

Parallel rays come to focus in 2 focal lines rath er than a single focal point

Etiology : heredity --Cause : refractive media is not spherical >refr

act differently along one meridian than along -- meridian perpendicular to it >2 focal points (

punctiform object is represent as 2 sharply def ined lines)

AstigmatismClassification

Regular astigmatism : power and orientation of principle meridians are constant

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ll lll lllll l lllll llllll lllll l lll lll ll , Compoundhyper opi c ast i gmat i sm, Mi xed ast i gmat i sm

Irregular astigmatism : power and orientation of principle meridians change across the pupil

AstigmatismSymptoms

asthenopic symptoms ( headache , eyepain) blurred vision distortion of vision head tilting and turningAmblyopia – uncorrected astigmatism > 1.5 D

Treatment Regular astigmatism :cylinder lenses with or witho

ut spherical lenses(convex or concave), Sx Irregular astigmatism : rigid CL , surgery

Presbyopia Physiologic loss of accommodation in advan

cing age deposit of insoluble proteins in lens in advan

-- cing age >elasticity of lens progressively d-- ecrease >decrease accommodation

around 45 years of age , accommodation be -- come less than 3 D >reading is possible at

- -- 40 50 cm >difficultly reading fine print , he adache , visual fatigue

PresbyopiaTreatment

convex lenses in near visionReading glassesBifocal glassesTrifocal glassesProgressive power glasses

Anisometropia

Difference in refractive power between 2 eyes refractive correction often leads to different imag

e sizes on the 2 retinas( aniseikonia) aniseikonia depend on degree of refractive anom

aly and type of correction closer to the site of refraction deficit the correctio

-- n is made >less retinal image changes in size

Anisometropia

Glasses : magnified or minified 2% per 1 DContact lens : change less than glassesTolerate aniseikonia ~ 5-8% Symptoms : usually congenital and often asym

ptomaticTreatment

-- anisometropia > 4 D >contact lens -- unilateral aphakia >contact lens or intraocular le

ns

Correction of refractive errors Far point

point on the visual axis conjugate to the retina when accommodation is completely relaxed

pl aci ng an obj ect or i magi ng an obj ect a t f ar poi nt wi l l cause a cl ear i mage of t h

at obj ect t o be r el ayed t o t he r et i na use correcting lenses to form an image of inf

inity at the far point , correcting the eye fordistance

Types of optical correction Spectacle lenses

Monofocal lenses : spherical lenses , cylindrical lenses

Multifocal lenses Contact lenses

higher quality of optical image and less influenc e on the size of retinal image than spectacle lens

es indication : cosmetic , athletic activities , occupa

tional , irregular corneal astigmatism , high aniso metropia , corneal disease

Contact lenses disadvantages : careful daily cleaning and disinfe

ction , expense complication : infectious keratitis , giant papillary

conjunctivitis , corneal vascularization , severe c hronic conjunctivitis

Intraocular lenses replacement of cataract crystalline lens give best optical correction for aphakia , avoid si

gnificant magnification and distortion caused by spectacle lenses

Surgical correctionKeratorefractive surgery :RK, AK, PRK, LASIK,

ICR, thermokeratoplastyIntraocular surgery : clear lens extraction

(with or without IOL), phakic IOL