Refractive E rror s and management

1. Refractive Errorsand management

2. Vergence Parallel Beam Convergence Divergence

3. VERGENCE • All naturally occurring sources of light are divergent • Light rays traveling parallel have zero vergence • Light rays that focus on a point are convergent • The unit of measurement of vergence is the diopter D= Vergence (Diopters)=___________1_____________ Distance from the source in meters

4. BASIC OPTICS • Therefore: • The closer the light is to its source, the greater the vergence • The farther the light is from its source, the lower the vergence, approaching zero as distance goes toward infinity. • Diverging light has negative power (-) • Converging light has positive power (+) • Lens power is calculated by : using the biometry by measuring the axis length and the corneal convexity.

5. REFRACTION • Refraction of light occurs when light passes from one medium to another of different refractive index (ie. density)

6. Refractive Components of the Eye • Cornea: responsible for the majority of the refractive power of the eye (40 D) / constant. • Lens: 20 D of refractive power, changes with accommodation. • Axial length is constant except under certain conditions

7. These two refracting elements in the eye converge the rays of light because: 1-The cornea has a higher refractive index than air; the lens has a higher refractive index than the aqueous and vitreous humours that surround it. 2-The refracting surfaces of the cornea and lens are spherically convex

8. Fovea Light rays

9. Emmetropia (normal vision) • Adequate correlation between axial length and refractive power. • Parallel rays of light from a distant object are brought to focus on the retina with the eye at rest (no accommodation),such an individual can see sharply in the distance without accommodation.

10. Ametropia (Refractive error) • Mismatch between axial length and refractive power. • Parallel light rays don’t fall on the retina with the eye at rest (a change in refraction is required to achieve sharp vision). • Ametropia may be divided into: • Nearsightedness (Myopia) • Farsightedness (Hyperopia) • Astigmatism • Presbyopia

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

12. 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). • Increase in the curvature of the cornea or the surfaces of the crystalline lens.

15. Uncorrected, light focuses in front of fovea Corrected by divergent lens, light focuses on fovea

16. Myopia • The patient is able to focus on objects near but not far away (blurred distance vision). • Typical complaint is difficulty focusing on road signs or the black board. • Usually detected by the young when they discover they cannot see things at a distance as well as their friends do . • The teacher complains that the child makes too many mistakes copying things from the black-board. • The lens is unable to flatten enough to prevent conversion of images before reaching the retina. • Frequently squinting is compensatory mechanism in an attempt to improve uncorrected visual acuitywhen gazing into the distance.

17. MYOPIA • Increases with age roughly until the person stops growing in height. • A myopic person can still see some objects clearly, provided the object is closer than the far point • Pseudomyopia: accomodative spasm.The patient cannot relax accomodation when looking in the distance. For example, an over anxious student

18. Myopia • Forms • Benign myopia (school age myopia): Benign myopia usually starts in school, develops during adolescence period and finally stabilizes in the adulthood. It rarely grows beyond -7.0 D and it is never associated with any pathologic changes on the eye fundus. • Progressive and malignant myopia • interchangeable • myopia increase rapidly each year and is associated with vitreous opacities , fluidity of vitreous and chorioretinal change.

19. Myopia • Congenital myopia • Myopia > 10 D • Increase slowly each year Special forms cataract nuclear sclerosis , keratoconus , spherophakia

20. Myopia • Symptoms • Typically do not have “eye-strain”, “watering” of the eyes or headaches as often as hypermetropes do • Usually detected by the young when they discover they cannot see things at a distance as well as their friends do • The teacher complains that the child makes too many mistakes copying things from the black-board • Blurred distance vision • Squint in an attempt to improve uncorrected visual acuity when gazing into the distance • Headache • Amblyopia – uncorrected myopia > 10 D

21. Myopia • Morphologic changes • deep anterior chamber • atrophy of ciliary muscle • vitreous may collapse prematurely --> opacification • fundus change : loss of pigment in RPE ,largedisc and white crescent-shaped area on temporal side ,RPE atrophy in macular area , posterior staphyloma ,retinal degeneration-->hole-->increase risk of RD • Treatment : concave lenses, clear lens extraction • *Staphyloma : depression on retina.

22. PATHOLOGICAL CAUSES OF MYOPIA • KERATOCONUS

23. PATHOLOGICAL CAUSES OF MYOPIA * Nuclear Cataract *Diabetes

24. PATHOLOGICAL CAUSES OF MYOPIA Marfan’s Staphyloma

25. Complication of myopia 1-RD2-Macular degeneration.3-Cataract 4-Open angle glaucoma.

26. 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) • Hyperopia forms a stage in normal development of the eyes—at birth eyes are hypermetropic (2.5 to 3.0 Diopters). When persists in adulthood it represents an imperfectly developed eye. • Lens changes (cortical cataract). • Special forms : lens dislocation , postoperative aphakia

27. HYPEROPIA • • hyperopic persons must accommodate when gazing into distance to bring focal point on to the retinamore than normal people . • Hyperopia may be partially compensated for by using the eyes’ accommodative ability this is possible during the first two to three decades of life • • When accommodative ability cannot keep up with demand, hyperopia is manifest and images are blurred in the distance and for near and the need for reading glasses arises earlier than in the normal person. • Typical complaint is difficulty reading.

28. Hyperopia • Symptoms • Typical complaint is difficulty reading • 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 or inadequate light . • Extremely good distant vision (6/4) • asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis. • Young children with significant hypermetropia can also develop a convergent squint. • accommodative esotropia : because accommodation is linked to convergence -->ET • Amblyopia – uncorrected hyperopia > 5D

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

31. Uncorrected, light focuses behind fovea Corrected by convergent lens, light focuses on fovea

33. PATHOLOGICAL CAUSES OF HYPEROPIA DISLOCATED LENS

34. PATHOLOGICAL CAUSES OF HYPEROPIA RETINAL DETACHMENT RETINAL FLUID 1 mm = 3D CHOROIDAL TUMOR

35. Complication . They are more susceptible to closed angle glaucoma because there smaller eyes are more likely to have shallow A.C and narrow angles.

36. Astigmatism • Parallel rays come to focus in 2 focal lines rather than a single focal point • Etiology : hereditary • Cause : Astigmatism present when the cornea curvature is irregular (Refractive power of the cornea in different planes is not equal). • In short, astigmatism occurs whenever the vertical curvature of the cornea is different from the horizontal.

38. 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.

39. Keratoconus is pathological cause of irregular astigmatism • Simple astigmatisum : one focal point on retina other in front • Compound astigmatism : 2 points in front or 2 behind • Hypermetropic : 2 points behind retina. • Mixed : 1 point in front other behind. • With rule astigmatism : when the steeper meridian is vertical. • Against rule : when steeper meridian is horizontal .

41. 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

42. Treatment of astigmatism • 1. Laser, Intraocular toric lenses, Limbal relaxing incision • Contraindicated to do LASIK as it needs good thickness of cornea and regular curvature .

43. Pathologic Causes of Astigmatism • Corneal: post surgical, traumatic, infectious • External pressure on cornea: lid masses • Lens: pressure on lens from tumors

44. Presbyopia • Physiologic loss of accommodation in advancing age due to age related loss of elasticity of the capsule. • deposit of insoluble proteins in lens in advancing age-->elasticity of lens progressively decrease-->decrease accommodation. • Around 45 years of age , accommodation become less than 3 D-->reading is possible at 40-50 cm-->difficultly reading fine print , headache , visual fatigue. • This occurs earlier in hypermetropes than myopes.

45. Presbyopia • Treatment • convex lenses in near vision • Reading glasses • Bifocal glasses • Trifocal glasses • Progressive power glasses

46. 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 ??

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

48. Diagnosis OF refractive errors : not required Diagnosis of refractive errors is made by an optician or ophthalmologist. Instruments used to diagnose refractive errors include: - pinhole glasses - Autorefractor (measures how light changes as it enters the eye). Retinoscope (measures the refractive condition of the eye). - Phoropter.

49. Autorefractor Pinhole glasses Phoropter View through an autorefractor

50. Pinhole Test • Light rays that are perpendicular to the lens do not refract , but go straight forward. • If visual acuity improves with the pinhole test, then this is a refractive error; if it worsens, then this is a central field defect.