This document discusses various types of refractive errors including emmetropia, ametropia, myopia, hyperopia, astigmatism, and presbyopia. It describes the causes and characteristics of each condition as well as common symptoms. Treatment options are also outlined including spectacle lenses, contact lenses, and various surgical procedures to correct refractive errors.

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