Post on 07-May-2015
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