Optics of human eye & refractive errors

1. OPTICS OF HUMAN EYE & REFRACTIVE ERRORS - SAHITHI GANESHULA

2. Optics of human eye

3. Eye as a camera Eyelids - shutter Cornea - focusing system Lens - focusing system Iris - diaphragm Choroid - dark chamber Retina -light sensitive film

4. Components

5. Cornea Reasons of refraction: Curvature. Significant difference in refractive indices of air and cornea.

6. Vertical diameter slightly less than horizontal Front apical radius 7.7 mm K= 48.83 D Back apical radius 6.8 mm K=-5.88 D Actual refractive index cornea= 1.376 Power of cornea +43D (2/3 of total eye power) Not optically homogenous (ground substance)=1.354, n(collagen)=1.47

7. The anterior chamber Cavity between cornea and iris Filled with aqueous humor. Depth of AC – about 2.5-4.0 mm Change in AC depth change the total power. 1mm forward shift of lens- increase about 1.4D in power Refractive index of aqueous humor= 1.336

8. Iris and Pupil Regulate amount of light entering the eye At 2.4mm pupil size, best retinal image obtained, as aberration and diffraction are balanced.

9. The crystalline lens

10. Lens accounts for about one third of the refraction of the eye . ACCOMODATION Provides a mechanism of focusing at different distances. OPTICAL CHANGES IN CATARACTOUS LENS Visual Acuity reduction. Myopic shift. Monocular diplopia . Glare. Color shift.

11. Vitreous Refractive index same as aqueous.

12. Retina Maximum resolving power at fovea. A concave spherical surface with r =-12 mm. Advantages of curvature of retina over plane image forming surfaces of cameras and optical instruments: The curved images formed by the optical system is brought in the right order. A much wider field of view is covered by the steeply curved retina

13. Schematic eye and reduced eyes

14. Schematic and reduced eyes It is a theoretical optical specification of an idealized eye, retaining average dimensions but omitting the complications. Useful for understanding ophthalmologic problem and conceptualizing the optical properties of the human eye. To calculate cardinal points, the radii of curvature and distances separating the refracting surfaces must be known.

15. Axes and visual angles

16. OPTICAL AXIS: line passing through centre of cornea, lens and meets retina on nasal side of fovea VISUAL AXIS: line joining fixation point, nodal point and fovea FIXATION AXIS: line joining fixation point and centre of rotation

17. Visual angles ANGLE ALFA:angle formed between visual axis and optical axis at the nodal point ANGLE GAMMA:angle between optical axis and fixation axis at the centre of rotation ANGLE KAPPA:angle between visual axis and pupillary line. Point on centre of cornea is considered as centre of pupil.

18. Optical aberrations Diffraction of light Spherical aberrations Chromatic aberrations Decentering Oblique aberrations Coma

19. Diffraction Bending of light caused by edge of the aperture or rim of the lens.

20. Spherical aberrations A convex spherical lens refracts peripheral rays more strongly than paraxial rays, so peripheral rays are focused closer.

21. Why less in human eyes??? Human cornea is periphery is flatter than centre. In human lens, central portions have greater density and greater curvature. Iris blocks the peripheral rays, only paraxial rays enter the system.

22. Chromatic aberrations Different color rays have different focal length, as refractive indices of media varies with wavelength of incident light.

23. Why less in human eyes??? Due to narrow spectral sensitivity bands of long and mid wavelength cones. Foveal lack of blue cones.

24. Decentering Lens is usually slightly decentred . Corneal centre of curvature is situated about 0.25mm below the axis of lens. Clinically insignificant.

25. Oblique aberrations When the object is in peripheral visual field, a thin incident narrow pencil of rays enter limited by the pupil. Peripheral portion of lens forms Sturm’s conoid , so two line foci are formed. Minimum in human lens due to curvature of retina. Significant in biconvex/biconcave lens.

26. Oblique aberrations

27. Coma Different areas of lens form foci in planes other than chief focus, producing a coma effect in image plane, from a point source of light.

28. Emmetropia 24-25mm 43 diopters 18 diopters Accomodation at rest

29. REFRACTIVE ERRORS • Ametropia : a refractive error is present • Myopia : Near sightedness • Hyperopia ( Hypermetropia ): Far sightedness • Presbyopia : Loss of accommodative ability of the lens resulting in difficulties with near tasks • Astigmatism : the curvature of the cornea and/or lens is not spherical and therefore causes image blur on the retina

30. REFRACTIVE ERRORS • Anisometropia : a refractive power difference between the 2 eyes (> 2D) • Aniseikonia : a difference of image size between the 2 eyes as perceived by the patient • Aphakia : ( Phakos =lens), aphakia is no lens • Pseudophakia : artificial lens in the eye

31. Myopia A form of refractive error in which parallel rays of light entering the eye are focused in front of retina with accommodation being at rest.

32. Etiological types Axial(MC)-increased AP length of eyeball Curvatural -increased curvature of cornea, lens or both Index-increased refractive index of lens with nuclear sclerosis Positional-anterior placement of lens Myopia due to excessive accommodation

33. Clinical types of myopia Congenital Simple or developmental Degenerative or pathological Acquired

34. Congenital myopia Common in premature babies or with birth defects Stationary(8-10D) Associated with Increase in axial length Esotropia Other congenital anomalies of eye Early and full correction under cycloplegia Poor prognosis in unilateral cases with severe myopia and anisometropia

35. Simple myopia Physiological/school myopia Commonest type Results due to normal biological variations in development of eye Age of onset- 7-10yrs Moderate severity-<5D,never exceeds8D No degenerative changes

36. Degenerative myopia Progressive in nature Related to heredity, general growth process Heredity linked growth of retina Factors affecting general growth process Age of onset-early adult life Severe->6D

37. Pathophysiology General growth process Degenerative changes in sclera Decreased vision

38. Myopic Crescent

39. Lacquer Cracks Breaks in Bruch’s membrane

40. Sub retinal neovascularization

41. Sub retinal hemorrhage

42. Foster – Fuchs's spots

43. Posterior staphyloma

44. Complications Macular hemorrhage Retinal tears, detachment Vitreous hemorrhage Choroidal hemorrhage Complicated cataract Nuclear sclerosis Primary open angle glaucoma

45. Clinical features - Symptoms Distant blurred vision Half shutting of eyes Asthenopic symptoms Muscae volitantes Night blindness Divergent squint

46. Signs Prominent eyeballs Large cornea Anterior chamber is deep Large & sluggishly reacting pupil Fundus examination-changes seen only in pathological myopia

47. Optical treatment Concave lenses Children Adults Contact lenses

48. Optical treatment Minimum acceptance providing maximum vision Low myopia(<6D): Young children : glasses required only if Isometropia <2years ≥ -4.0D 2-3years ≥ -3.0D Anisometropia : ≥ -2.5D Give full correction under cycloplegia Avoid overcorrection

49. Adults: <30years-full correction >30years-less than full correction with which patient is comfortable for near vision. HIGH MYOPIA under correction is done to avoid near vision problem minification of images contact lenses are better(to avoid image minification )

50. Surgical treatment Radial keratotomy Lamellar corneal refractive procedures Laser based procedures PRK LASIK LASEK C-LASIK E-LASIK

51. Miscellaneous corneal refractive procedures Orthokeratology Intracorneal contact leses Intra stromal corneal ring segments Gel injectable adjustable keratoplasty Intraocular refractive procedures Phakic refractive lenses Refractive lense exchange

52. Hypermetropia 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

53. Etiological types Axial( m.c )-decreased AP diameter of eyeball Curvatural -flattening of cornea, lens or both Index –old age, diabetics under treatment Positional- posteriorly placed lens Absence of lens- aphakia

54. CLINICAL TYPES SIMPLE HYPERMETROPIA PATHOLOGICAL FUNCTIONAL HYPEROPIA

55. SIMPLE HYPERMETROPIA Commonest form Results from normal biological variations in the development of eyeball Include axial and curvatural HM May be hereditary

56. PATHOLOGICAL HYPERMETROPIA Anomalies lie outside the limits of biological variation Acquired hypermetropia Decrease curvature of outer lens fibers in old age Cortical sclerosis Positional hypermetropia Aphakia Consecutive hypermetropia

57. FUNCTIONAL HYPERMETROPIA Results from paralysis of accommodation Seen in patients with 3 rd nerve paralysis & internal ophthalmoplegia

58. TOTAL HYPERMETROPIA It is the total amount of refractive error,estimated after complete cycloplegia with atropine Divided into latent & manifest

59. LATENT HYPERMETROPIA Corrected by inherent tone of ciliary muscle Usually about 1D High in children Decreases with age Revealed after abolishing tone of ciliary muscle with atropine

60. MANIFEST HYPERMETROPIA Remaining part of total hypermetropia Correct by accommodation and convex lens Consists of facultative & absolute FACULTATIVE HYPERMETROPIA Corrected by patients accommodative effort ABSOLUTE HYPERMETROPIA Residual part not corrected by patients accommodative effort

61. NORMAL AGE VARIATION At birth +2+3D HM Slightly increase in one year of life, Gradually diminished by the age 5-10 years In old age after 50 year again tendency to HM Tone of ciliary muscle decreases Accommodative power decreases Some amount of latent HM become manifest More amount of facultative HM become absolute

62. SYMPTOMS Principal symptom is blurring of vision for close work Symptoms vary depending upon age of patient & degree of refractive error Asymptomatic Asthenopic symptoms Defective vision with asthenopia Defective vision only

63. SIGNS VISUAL ACUITY : Defective EYEBALL: small or normal in size CORNEA : may be smaller than normal. There can be CORNEA PLANA ANTERIOR CHAMBER : may be shallow LENS: could be dislocated backwards A Scan ultrasonography (biometry) reveal short axial length

64. FUNDUS: DISC: Dark reddish color, irregular margins ,confused with Papillitis so termed as PSEUDO-PAPILLITIS MACULA: Situated further from the disc than usual, large positive angle alpha, apparent divergent squint BLOOD VESSELS: Show undue tortuosity & abnormal branchings BACKGROUND: SHOT- SILK RETINA

65. COMPLICATIONS Recurrent styes,blepharitis or chalazion Accomidative convergent squint Amblyopia Anisometropic Strabismic Uncorrected bilateral high hypermetropia Predisposition to develop primary narrow angle glaucoma Care should be taken while instilling mydriatics

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

67. Young children(<6 or 7yrs) Some degree of hypermetropia is physiological so no correction Treatment required if error is high or strabismus is present working in school small error may require correction In children error tends normally to diminish with growth so refraction should be carried out every six month and if necessary the correction should be reduced, ortherwise a lens which is overcorrecting their error may induce an artificial myopia No deduction of tonus allowance in strabismus

68. Adults If symptoms of eye-strain are marked,we correct as much of the total hypermetropia as possible,trying as far as we can to relieve the accommodation When there is spasm of accommodation we correct the whole of the error Some patients with hypermetropia do not initially tolerate the full correction indicated by manifest refraction so we under correct them Exophoria hyperopia should be under correct by 1 to 2D

69. Patients with absolute hypermetropia are more likely to accept nearly the full correction because they typically experience immediate improvement in visual acuity In pathological hypermetropia the underlying cause rather than the hypermetropia is chief concern

70. MODE OF TREATMENT SPECTACLES CONTACT LENS SURGICAL OPTICAL TREATMENT

71. 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 idea

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

73. REFRACTIVE SURGERY Refractive surgery is not as effective as in myopia TYPES Hexagonal keratometry Laser thermal keratoplasty Photo refractive keratectomy LASIK Photorefrctive keratectomy Phakic IOL and clear lens extraction

74. PRESBYOPIA The physiologic loss of accommodation in the eyes in advancing age

75. Physiologic loss of accommodation in advancing age 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

76. Increasing Near Point of Accommodation with Age Age (years) Distance (cm) 10 7 20 10 30 14 40 20 50 40

77. SYMPTOMS The need to hold reading material at arm's length. Blurred near vision Headache Fatigue Symptoms worse in dim light.

78. CORRECTION

79. SPECTACLES Plus lens (or) Convex lens

80. Surgery Monovision LASIK Monovision & CK IntraCor Refractive lens exchange Corneal Inlays & Onlays

81. ASTIGMATISM  A defect of an optical system causing light rays from a point source to fail to meet in a focal point resulting in a blurred and imperfect image.

82. Conus of Sturm :- Geometric configuration of light rays emanating from single point source & refracted by spherocylindrical lens

83. Focal interval of Sturm :- Distance between 2 focal lines Circle of least diffusion At the dioptric mean of focal lines the cross section of sturms conoid appears as circular patch of light rays – best overall focus

84. Types Regular astigmatism – change in refractive power is uniform from one meridian to another With-the-rule astigmatism Against-the-rule astigmatism Oblique astigmatism Bi-oblique astigmatism Irregular astigmatism –Irregular change of refractive power in different meredia

85. Types of regular astigmatism Simple astigmatism Simple hyperopic astigmatism Simple myopic astigmatism Compound astigmatism Compound hyperopic astigmatism Compound myopic astigmatism Mixed astigmatism

86. Regular Astigmatism : Correctable by Spherocylindrical lenses Etiology : 1. Corneal - abnormalities of curvature [common] 2. Lenticular is rare. It may be: i . Curvatural - abnormalities of curvature of lens as seen in lenticonus . ii. Positional - tilting or oblique placement of lens , subluxation . 3. Retinal - oblique placement of macula [rare]

87. Symptoms : Blurring of vision Asthenopic symptoms Tilting of head Squinting [Half closure of eyelid]

88. Investigations: Retinoscopy Keratometry Computerized corneal Tomography Astigmatic fan test Jackson cross cylinder

89. Treatment Optical treatment Spectacles RGP contact lenses Toric contact lenses Surgical correction

90. Guidelines for optical treatment Small astigmatism- treatment is required In presence of asthenopic symptoms Decreased vision High astigmatism- full correction Better to avoid new astigmatic correction in adults because of intolerable distraction Bi- oblique,mixed,high astigmatism are better treated by contact lenses Correction of spherical component

91. Irregular Astigmatism Etiology : Corneal -[ Scars , Keratoconus , flap complications, marginal degenration ] Lenticular -[ Cataract maturation ] R etinal - [scarring of macula,tumours of retina,choroid ]

92. Symptoms : Defective vision Distorsion of objects Polyopia Investigations: - Placido's disc test reveals distorted circles - Computerized corneal topography

93. Treatment : Optical treatment : - RGP contact lenses -Hybrid contact lenses - Scleral lenses - Piggyback lens Surgical treatment: - penetrating keratoplasty

94. Astigmatism correction requires prescription of convex cylindrical lenses at 180 +/- 20 deg or concave cylindrical lenses at 90 +/- 20 deg with the rule and vice versa

95. Contact lenses Toric contact lenses Soft lenses [SL] Rigid gas permeable lenses [RGP] RGP do not conform to the asymmetry of corneal surface but replaces it totally and also provides clarity of vision ,more durable. Soft lenses are more comfortable to wear ,easy to fit, adhere more tightly to cornea .

96. Refractive surgeries Astigmatic Keratotomy Photoastigmatic refractive Keratectomy [PRK] Relaxing incisions with compression sutures LASIK surgery

97. Residual astigmatism : The amount of astigmatism that still remains after correction of a refractive error. In the case of correction of corneal astigmatism using rigid contact lens , lenticular residual astigmatism is exposed.

98. 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

99. 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