Refractive errors correction

For residents

1. Refractive Errors Correction By Desta Genete (MD, R l) Advisor Mr. Alemayehu ( consultant Optometrist) July 2018

2. Seminar outline Introduction to optical principles Correction of myopia Correction of hyperopia Correction of astigmatism Spectacle and accomodation Spectacle and binicularity

3. Introduction In a nonaccommodating emmetropic eye, a distant object at infinity is conjugate to the retina. Conversely, if the retina were the object, its image would be located at an infinite distance in front of it.

4. Far Point The far point denotes the location at which the far point plane intersects the optical axis. . For emmetropia it is at infinity Myopia…..finite distance infront of the eye The higher the degree of myopia, the closer the far point is to the eye Hypermetropia ….is virtual ; located behind the lens. 4

5. The far point forms the basis for optical correction of any ametropia Any lens brings light from an object located at a distance and images it at that lens' secondary focal plane. Therefore , if that lens is positioned in front of the eye so that its secondary focal plane is superimposed on the eye's far point plane, that lens becomes a “ corrective lens .”

6. Lens Effectivity Almost any lens can become a corrective lens if its secondary focal plane is placed to coincide with the ametropic eye's far point.

7. Vertex Distance The distance b/n the posterior surface of spectacle lens and anterior pole of the eye(cornea) M ust be given with all prescriptions over 4-5 diopters T he position in front of the eye affects the optical correction of ametropia . 7

8. The measurement may be made with a ruler held parallel to the arm of the trial frame . Vertexometer Add 2mm to correct lid thicknes s 8 Back Vertex Distance

9. Brain storm quizes An aphakic patient's right eye is refracted at a vertex distance of 14 mm and found to be +12.00D. What power contact lens would provide the same optical correction (i.e., the same effectivity ) Ans 14.4D

10. A patient who has been wearing -6.00D eyeglasses finds that, in order to see clearly, he must push his eyeglasses 1 cm closer to his eyes. What is his new refractive correction if he wants to use his current frame ? -5.66D

11. Myopia The incident parallel rays come to focus in front of the retina when accommodation is at rest. 11

12. Etiology 12

13. Treatment t Spectacle ( concave lens) Contact lens Refractive surgery Keratorefractive surgery like radial keratotomy(RK) Clear lens extraction for high myopia minus phakic IOL Excimer laser surgery 13

14. CORRECTION OF MYOPIA The far point plane of the myopic eye is located at a certain distance in front of the eye and determines the degree of myopia. The greater the myopia, the closer the far point will be . Eg . 1D=1m 0.25D= 4m

15. A “minus” lens, a lens that diverges light, is necessary to accomplish this task. The closer to the eye the corrective lens is placed, the less divergence power it needs to allow its secondary focal plane to be coincident with the eye's far point plane, therefore necessitating a weaker minus corrective lens A contact lens in myopia will not need to be as strong as the corresponding spectacle

16. Because of the small difference in power induced by changes in vertex distance in low refractive errors, it is not typically necessary to make corresponding prescriptive changes unless the myopia is 4D or greater.

17. Full Versus Partial Correction of Myopia To create clear retinal image, full extent of myopia must be corrected. Refraction without cycloplegia result slight under correction 0.25D Another factor is night myopia, 0.5-1D which may result in blurred vision or halos around lights at night

18. If a significantly myopic patient has never worn a correction before, prescribing the full correction may result in significant asthenopia Depends on clinical judgment whether or not to prescribe the full prescription. Eg in older patients, partial correction of myopia recommended.

19. Myopic Progression and Spectacles Remains topic of debate and continued research area. The COMET study (Correction of Myopia Evaluation Trial) was designed to evaluate this clinical practice and its outcome.

20. Children were fit with either progressive lenses or single vision lenses and were followed for 3 years using cycloplegic autorefraction . Study results revealed that the group wearing progressives glasses had developed only about 0.2D less myopia occurring primarily during the first year of lens wear

21. There was a corresponding greater increase in axial length in the single vision lens group compared to the progressive lens group prescribed bifocals showed a slowing of myopic development over the first two years (0.1D less development per year for the first 2 years ) Atropine/bifocal In one study showed lessen myopic progression of 0.15D/year

22. CORRECTION OF HYPEROPIA DEFINITION It is the refractive state of eye where in parallel rays of light coming from infinity are focused behind the sensitive layer of retina with accommodation being at rest . The posterior focal point is behind the retina which receives a blurred image

23. Etiology 23

24. Components of hypermetropia 24

25. Total hypermetropia is the amount of RE, which is estimated after complete cycloplegia with atropine Manifest hypermetropia -the strongest convex lens correction accepted for clear distance vision without cycloplegia . Components of hypermetropia 25

26. SYMPTOMS Principal symptom is blurring of vision for close work Symptoms vary depending upon age of patient & degree of refractive error 1.ASYMPTOMATIC small error produces no symptoms Corrected by accommodation of patient

27. 2.ASTHENOPIA Refractive error are fully corrected by accommodative effort Thus vision is normal Sustained accommodation produces symptoms Asthenopia increases as day progresses Increased after prolonged near work SYMPTOMS Tiredness Frontal or fronto temporal headache Watering Mild photophobia

28. BASIS FOR TREATMENT No Treatment Error is small Asymptomatic Visual acuity normal No muscular imbalance

29. Young children(<6 or 7yrs) Some degree of hypermetropia is physiological so no correction Treatment is required if error is high or strabismus is present

30. ADULTS If symptoms of eye-strain are marked , correct as much of the total hypermetropia as possible , trying as far as we can to relieve the accommodation.

31. Some patients with hypermetropia do not initially tolerate the full correction indicated by manifest refraction so we undercorrect them

32. MODE OF TREATMENT SPECTACLES CONTACT LENS SURGICAL

33. SPECTACLES Basic principle Prescribe convex lenses (Plus lenses) so that rays are brought to focus on the retina Advantages Comfortable Easier method Less expensive Safe

34. CONTACT LENS ADVANTAGES: Cosmetically good Increased field of view Less magnification Elimination of aberrations & prismatic effect

35. REFRACTIVE SURGERY Refractive surgery is not as effective as in myopia TYPES: (1)HEXAGONAL KERATOTOMY(HK) Low to moderate degrees of hypermetropiaIts risk /benefit ratio is not low enough to warrant its continued use.

36. LASER THERMAL KERATOPLASTY(LTK ) Procedure done using Thallium-Holmium-Chromium (THC:YAG) laser energy to heat the cornea ( contraction of collagen) and increase its curvature

37. The far point plane (the image of the retinal plane) of the hyperopic eye is located behind the eye and , so, is virtual A corrective lens for hyperopia is a “plus” lens

38. Many different plus lenses offer equal effectivity . It dependens on vertex distance Contact lens is closer to the far point plane so it must be of higher plus.

39. Full vs partial correction Prescription of spectacle depends on age of patient Degree of hyperopia Accomodative and binocular satus

40. Young patients will be uncomfortable initially if given full refractive correction due to habitual accomodative tone all times. Manifest refraction may be deceptive. As the patient ages , accomodative ability decreases and latent hyperopia may manifest .

41. In the presence of high hyperopia; especially if AC/A ratio is high may result accomodative esophoria / tropia Prescribing maximum tolerated plus lenses, often in the form of bifocals, is indicated to normalize binocularity.

42. As an under corrected hyperope ages, accomodative amplitude decreases, symptoms of early onset presbyopia In such cases correcting the hypermetropia usually alleviates symptoms

43. Astigmatism A refractive error in which incident light fail to focus to a single focal point and may focus at ,pre and post retina. The image is formed as a Sturm's conoid 43

44. 44

45. Etiology 45

46. Classification based on axis of principal meridians 46

47. Correction of astigmatism Unique because it has two far point planes from the two principal meridians of refractive errors In simple myopic astigmatism, one plane is located at infinity and another at finite distance in front of the eye In simple hyperopic astigmatism one at infinity another at the back of the eye.

48. In compound hyperopic astigmatism, both planes are located behind the eye at different distance In compound myopic both planes are located in front of the eye at different distance In mixed astigmatism one is located behind the other in front of the eye.

49. Cylindrical lenses are used A cylindrical lens has maximum dioptric power in one meridian, while the orthogonal (perpendicular) meridian has no dioptric power. They create two line images each with specific orientations.

50. A spherocylindrical lens’s meridians of maximum power and minimum power, which are perpendicular to each other, are called the major , or principal meridians

51. Plus and minus forms of cylinder

52. T o transpose a plus-cylinder prescription into minuscylinder form and vice versa. Keeping the signs, add the sphere and cylindrical powers to arrive at the new sphere value. • Reverse the cylinder sign to arrive at the new cylinder power. • Change the axis by 90 degrees to arrive at the new axis. (Remember that the axis is never greater than 180 degrees.)

53. C ircle of least confusion the circle is dioptrically centered between the focused images of the two principal meridians

54. circle of least confusion

55. POWER IN AN OBLIQUE MERIDIAN OF A CYLINDRICAL LENS Eg . pl −5.00 × 030 Power in the axis meridian(30) is zero and power meridian(120) is -5D. What about the other meridians? Can be calculated by using this formula F θ = ( F cyl )sin2 θ where Fθ is the power in the oblique meridian, F cyl is the power in the power meridian , and θ is the angle between the oblique meridian and the cylinder axis.

56. F θ = (−5.00 D)sin2(30) F θ = −1.25 D Oblique meridian doesnt have focal point but calculating it is useful In calculations that deal with lens thickness and the prism in decentered cylindrical lenses .

57. Let’s look at a spherocylindrical lens. What’s the power in the vertical meridian of + 2.00 − 3.00 × 160 lens ?

58. In compound or mixed astigmatism , two simple cylinders oriented 90 degrees apart are used to create image lines at each of the far point planes .

59. Combination of two cylinders result in spherocylindrical lens The spherical equivalent of that lens is the spherical power that is diopterically half way between the highest and lowest of the lens principle meridional powers.

60. In within the rule astigmatism The focusing element of the eye require more convergent power in the 180 degree meridian, which is corrected by plus cylinder axis 90 degree .

61. In against the rule astigmatism, the focusing elements of the eye require more convergent power in the 90degree meridian, which is corrected by plus cylinder axis 180 degree Or minus cylinder axis 90 degree . + 2.00 −3.00 × 90

62. Full versus partial correction of astigmatism The non uniform magnification or minification results in meredional anisokonia . This may result in; Spatial orientation and asthenopia . Common compliant is f lat surface appear tilted Usually patients adapt rapidly if not reducing astigmatic component may suffice to alleviate symptoms. Maintaining spherical equivalent ensures that the circle of least confusion will be maintained at the retina reducing retinal blur.

63. Effect of spectacle prescribing on accomodation Always keep in mind the effect of the spectacle on accomodation Especially on those elderly or with accomodative dysfunction. Careless refractive exam may result in prescribing more minus lens than actually needed.

64. Overminused prescriptions = excess accomodaive effort= asthenopia specially at near task. May result in early presbyopia ???slightly undercorrecting the myopia may delay the need for bifocals or progressive.

65. Hyperopes often benefits from the “the pushing of plus” Minimizes the adverse effects of accomodation from undercorrecting the refractive error.

66. Effects of spectacle prescribing on binocularity Accommodation and binocularity are neurologically linked. Important specially in esophoria and esotropia All esophoric / esotropic patients need full cycloplegic refraction. It will uncover any accomodative spasm or latent hyperopia

67. Once binocularity is maintained at distance, Additional plus power may need to be prescribed to lessen accomodative demand and any induced esotropia at near. In contrast inadvertant over correction of myopia or inadequate correction of hyperopia may result iatrogenic esophoria .

68. In exophoria / exotropia , the over correction with plus lenses or Undercorrection with minus lenses will likely exacerbate the condition because accomodation is relaxed Least plus or over minus may help to restore proper binocular posture

69. Correction of A nisometropia and A niseikonia Anisometropia in which patients refractive error significantly differs between the two eyes. The term significant is unclear Usually a difference of 2D Anisokonia the perception of an image size difference between the two eyes 1% image size difference per diopter 1-5% likely to result symptoms and interfere with fusion.

70. Aniesokonia may result in Headach Eye ach Tearing D ecreased reading stamina vertigo , and spatial distortions

71. The symptoms can be reduced by Minimizing the vertex distance By fitting contact lenses Axial anisomertopia can be minimized by placing the corrective lenses close to the anterior focal planes of the eyes(15.7mm in front of the eye) KnapPs rule

72. Anisophoria which is differential prism effect between the two eyes occurs with off axis viewing through the asymetric spectacle lenses. May result in both asthenopia and diplopia

73. VISUAL HYGIENE While reading or doing intensive near work take a break about every 30 min. When reading maintain proper distance Sufficient Illumination Place a limit spent watching television & watching videogames Sit 5-6 feet away from the television

74. Duanes ophthalmlogy 2012 edition Geometric and visual optics, second edition BCSC section 3 Onlinesources

75. Thank you