1. Refractive error

2. Emmetropia
Adequate correlation between axial length and refractive
power
Parallel light rays fall on the retina (no accommodation)

3. Ametropia (Refractive error)
Mismatch between axial length and refractive power
Parallel light rays don’t fall on the retina (no
accommodation)
Nearsightedness (Myopia)
Farsightedness (Hyperopia)
Astigmatism
Presbyopia :NOT REFFERECTIVE ERROR

4. Accommodation
Emmetropic eye
object closer than 6 M send divergent light that focus behind retina ,
adaptative mechanism of eye is increase refractive power by accommod
ation
 theory
contraction of ciliary muscle -->decrease tension in zonule fibers --
>elasticity of lens capsule mold lens into spherical shape -->greater dio
ptic power -->divergent rays are focused on retina
contraction of ciliary muscle is supplied by parasympathetic third nerve

6. Myopia
Parallel rays converge at a focal point anterior to the retina
Etiology : not clear , genetic factor
Causes
excessive long globe (axial myopia) : more common
excessive refractive power (refractive myopia)

8. Myopia
Forms
Benign myopia (school age myopia)
onset 10-12 years , myopia increase until the child stops growing in
height
Progressive and malignant myopia
interchangeable
 myopia increase rapidly each year and is associated with vitreous
opacities , fluidity of vitreous and chorioretinal change
rate of increase in amount of myopia generally about 20 years of age

9. Myopia
Congenital myopia
Myopia > 10 D
Increase slowly each year

10. Myopia
Special forms : nuclear sclerosis , keratoconus , spherophakia
Symptoms
Blurred distance vision
Squint in an attempt to improve uncorrected visual acuity when
gazing into the distance
Headache
Amblyopia – uncorrected myopia > 10 D

11. Myopia
Morphologic changes
deep anterior chamber
atrophy of ciliary muscle
vitreous may collapse prematurely -->opacification
fundus change : loss of pigment in RPE , large disc and white crescent-
shaped area on temporal side , RPE atrophy in macular area , posterior st
aphyloma , retinal degeneration-->hole-->increase risk of RD
Treatment : concave lenses, clear lens extraction

13. Hyperopia
Parallel rays converge at a focal point posterior to the retina
Etiology : not clear , inherited
Causes
excessive short globe (axial hyperopia) : more common
insufficient refractive power (refractive hyperopia)

15. Hyperopia
Special forms : lens dislocation , postoperative aphakia
hyperopic persons must accommodate when gazing into
distance to bring focal point on to the retina
Symptoms
distance vision is impaired in high refractive error( > 3 D) and
in older patient

16. Hyperopia
Symptoms
visual acuity at near tends to blur relatively early
nature of blur is vary from inability to read fine print to near vision is clear but
suddenly and intermittently blur
blurred vision is more noticeable if person is tired , printing is weak or light
inadequate
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

17. Hyperopia
Fundus in axial hyperopia may reveal pseudooptic neuritis
(indistinct disc margin, no physiologic cup, may elevate disc)
DDx from optic neuritis by > 4 D , no enlarged blind spot, no
passive congestion of vein
Treatment : convex lenses, keratorefractive surgery, refreactive
lensectomy with IOL, phakic IOL

19. Astigmatism
Parallel rays come to focus in 2 focal lines rather than a single
focal point
Etiology : heredity
Cause : refractive media is not spherical-->refract differently
along one meridian than along meridian perpendicular to it-->2
focal points ( punctiform object is represent as 2 sharply defined
lines)

21. Astigmatism
Classification
Regular astigmatism : power and orientation of principle
meridians are constant
With the rule astigmatism , Against the rule astigmatism , Oblique
astigmatism
 Simple or Compound myopic astigmatism , Simple or Compound
hyperopic astigmatism , Mixed astigmatism
Irregular astigmatism : power and orientation of principle
meridians change across the pupil

24. Astigmatism
Symptoms
asthenopic symptoms ( headache , eyepain)
blurred vision
distortion of vision
head tilting and turning
Amblyopia – uncorrected astigmatism > 1.5 D
Treatment
Regular astigmatism :cylinder lenses with or without spherical
lenses(convex or concave), Sx
Irregular astigmatism : rigid CL , surgery

26. Presbyopia
Physiologic loss of accommodation in advancing age
deposit of insoluble proteins in lens in advancing age--
>elasticity of lens progressively decrease-->decrease accom
modation
around 45 years of age , accommodation become less than 3
D-->reading is possible at 40-50 cm-->difficultly reading fin
e print , headache , visual fatigue

27. Presbyopia
Treatment
convex lenses in near vision
Reading glasses
Bifocal glasses
Trifocal glasses
Progressive power glasses

28. Anisometropia
Difference in refractive power between 2 eyes
refractive correction often leads to different image sizes on the 2
retinas( aniseikonia)
aniseikonia depend on degree of refractive anomaly and type of
correction
closer to the site of refraction deficit the correction is made-->less
retinal image changes in size

29. Anisometropia
Glasses : magnified or minified 2% per 1 D
Contact lens : change less than glasses
Tolerate aniseikonia ~ 5-8%
Symptoms : usually congenital and often asymptomatic
Treatment
anisometropia > 4 D-->contact lens
unilateral aphakia-->contact lens or intraocular lens

30. Correction of refractive errors
Far point
point on the visual axis conjugate to the retina when
accommodation is completely relaxed
placing an object or imaging an object at far point will cause
a clear image of that object to be relayed to the retina
use correcting lenses to form an image of infinity at the far
point , correcting the eye for distance

32. Types of optical correction
Spectacle lenses
Monofocal lenses : spherical lenses , cylindrical lenses
Multifocal lenses
Contact lenses
higher quality of optical image and less influence on the size of
retinal image than spectacle lenses
indication : cosmetic , athletic activities , occupational , irregular
corneal astigmatism , high anisometropia , corneal disease

33. Contact lenses
disadvantages : careful daily cleaning and disinfection , expense
complication : infectious keratitis , giant papillary conjunctivitis ,
corneal vascularization , severe chronic conjunctivitis
Intraocular lenses
replacement of cataract crystalline lens
give best optical correction for aphakia , avoid significant
magnification and distortion caused by spectacle lenses

34. Surgical correction
Keratorefractive surgery :RK, AK, PRK, LASIK, ICR,
thermokeratoplasty
Intraocular surgery : clear lens extraction (with or without
IOL), phakic IOL