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Optics of human eye & refractive errors


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Optics of human eye & refractive errors

  2. 2. Optics of human eye Eye as a camera Components Schematic eye and reduced eyes Axes and visual angles Optical aberrations
  3. 3. Eye as a camera Eyelids- shutter Cornea- focusing system Lens- focusing system Iris- diaphragm Choroid- dark chamber Retina-light sensitive film
  4. 4. Components The cornea The anterior chamber The iris and pupil The crystalline lens The retina
  5. 5. Cornea  Reasons of refraction:  Curvature.  Significant difference in refractive indices of air and cornea.
  6. 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. 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. 8. Iris and Pupil • 2-4mm Average size: • depth of focus increases • Concept used as pin hole test in refractionSmall pupil • Retinal image quality improves • Size of blue circle increasesLarge pupil •Regulate amount of light entering the eye •At 2.4mm pupil size, best retinal image obtained, as aberration and diffraction are balanced.
  9. 9. The crystalline lens • Birth 3.5 – 4 mm • Adult life 4.75 – 5 mm Thickness • Ant surface 10 mm • Post surface 6 mm Radius of curvature • Nucleus 1.41 • Pole 1.385 • Equator 1.375 Refractive index of lens • 15 -18 d.Total power • At birth- 14-16 D • At 25yrs- 7-8D • At 50yrs- 1-2D Accommodative power
  10. 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. 11. Vitreous  Refractive index same as aqueous.
  12. 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. 13. Schematic eye and reduced eyes • Two principal foci • Two principal points • Two nodal points Gullstrand’s schematic eye: • Two principal foci • Single principal • Single nodal point Listing’s reduced eye:
  14. 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. 15. Axes and visual angles
  16. 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. 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. 18. Optical aberrations  Diffraction of light  Spherical aberrations  Chromatic aberrations  Decentering  Oblique aberrations  Coma
  19. 19. Diffraction  Bending of light caused by edge of the aperture or rim of the lens.
  20. 20. Spherical aberrations A convex spherical lens refracts peripheral rays more strongly than paraxial rays, so peripheral rays are focused closer.
  21. 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. 22. Chromatic aberrations Different color rays have different focal length, as refractive indices of media varies with wavelength of incident light.
  23. 23. Why less in human eyes???  Due to narrow spectral sensitivity bands of long and mid wavelength cones.  Foveal lack of blue cones.
  24. 24. Decentering  Lens is usually slightly decentred.  Corneal centre of curvature is situated about 0.25mm below the axis of lens.  Clinically insignificant.
  25. 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. 26. Oblique aberrations
  27. 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. 28. Emmetropia 24-25mm 43 diopters 18 diopters Accomodation at rest
  29. 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. 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. 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. 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. 33. Clinical types of myopia  Congenital  Simple or developmental  Degenerative or pathological  Acquired
  34. 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. 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. 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. 37. Pathophysiology Genetic factors More growth of retina Stretching of sclera Increased axial length Degeneration of choroid Degeneration of retina Degeneration of vitreous General growth process Degenerative changes in sclera Decrease d vision
  38. 38. Myopic Crescent
  39. 39. Lacquer Cracks Breaks in Bruch’s membrane
  40. 40. Sub retinal neovascularization
  41. 41. Sub retinal hemorrhage
  42. 42. Foster – Fuchs's spots
  43. 43. Posterior staphyloma
  44. 44. Complications  Macular hemorrhage  Retinal tears, detachment  Vitreous hemorrhage  Choroidal hemorrhage  Complicated cataract  Nuclear sclerosis  Primary open angle glaucoma
  45. 45. Clinical features - Symptoms  Distant blurred vision  Half shutting of eyes  Asthenopic symptoms  Muscae volitantes  Night blindness  Divergent squint
  46. 46. Signs  Prominent eyeballs  Large cornea  Anterior chamber is deep  Large & sluggishly reacting pupil  Fundus examination-changes seen only in pathological myopia
  47. 47. Optical treatment  Concave lenses  Children  Adults  Contact lenses
  48. 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. 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. 50. Surgical treatment  Radial keratotomy  Lamellar corneal refractive procedures  Laser based procedures  PRK  LASIK  LASEK  C-LASIK  E-LASIK
  51. 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. 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. 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
  55. 55. SIMPLE HYPERMETROPIA  Commonest form  Results from normal biological variations in the development of eyeball  Include axial and curvatural HM  May be hereditary
  56. 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. 57. FUNCTIONAL HYPERMETROPIA  Results from paralysis of accommodation  Seen in patients with 3rd nerve paralysis & internal ophthalmoplegia
  58. 58. TOTAL HYPERMETROPIA  It is the total amount of refractive error,estimated after complete cycloplegia with atropine  Divided into latent & manifest
  59. 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. 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. 61. Total hyper metropia Manifest hyper metropia Facultative hyper metropia Absolute hyper metropia Latent hyper metropia
  62. 62. 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
  63. 63. 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
  64. 64. 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
  65. 65. FUNDUS: A) DISC: Dark reddish color, irregular margins ,confused with Papillitis so termed as PSEUDO-PAPILLITIS B) MACULA: Situated further from the disc than usual, large positive angle alpha, apparent divergent squint C) BLOOD VESSELS: Show undue tortuosity & abnormal branchings D) BACKGROUND: SHOT- SILK RETINA
  66. 66. 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
  67. 67. TREATMENT BASIS FOR TREATMENT  No Treatment Error is small Asymptomatic Visual acuity normal No muscular imbalance
  68. 68. 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
  69. 69. 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
  70. 70. 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
  72. 72. 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
  73. 73. CONTACT LENS ADVANTAGES Cosmetically good Increased field of view Less magnification Elimination of aberrations & prismatic effect
  74. 74. 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
  75. 75. PRESBYOPIA The physiologic loss of accommodation in the eyes in advancing age
  76. 76.  Physiologic loss of accommodation in advancing age  deposit of insoluble proteins in lens in advancing age-->elasticity of lens progressively decrease-->decrease accommodation  around years of age , accommodation become less than D-->reading is possible at - cm-->difficultly reading fine print , headache , visual fatigue
  77. 77. Increasing Near Point of Accommodation with Age Age (years) Distance (cm) 10 7 20 10 30 14 40 20 50 40
  78. 78. SYMPTOMS  The need to hold reading material at arm's length.  Blurred near vision  Headache  Fatigue  Symptoms worse in dim light.
  79. 79. CORRECTION
  80. 80. SPECTACLES Plus lens (or) Convex lens
  81. 81. Surgery  Monovision LASIK  Monovision & CK  IntraCor  Refractive lens exchange  Corneal Inlays & Onlays
  82. 82. 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.
  83. 83.  Conus of Sturm:- Geometric configuration of light rays emanating from single point source & refracted by spherocylindrical lens
  84. 84.  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
  85. 85. 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
  86. 86.  Types of regular astigmatism  Simple astigmatism  Simple hyperopic astigmatism  Simple myopic astigmatism  Compound astigmatism  Compound hyperopic astigmatism  Compound myopic astigmatism  Mixed astigmatism
  87. 87. 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]
  88. 88. • Symptoms : Blurring of vision Asthenopic symptoms Tilting of head Squinting [Half closure of eyelid]
  89. 89. Investigations:  Retinoscopy  Keratometry  Computerized corneal Tomography  Astigmatic fan test  Jackson cross cylinder
  90. 90. Treatment  Optical treatment  Spectacles  RGP contact lenses  Toric contact lenses  Surgical correction
  91. 91. 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
  92. 92. Irregular Astigmatism • Etiology : Corneal -[ Scars , Keratoconus , flap complications, marginal degenration ] Lenticular -[Cataract maturation] Retinal-[scarring of macula,tumours of retina,choroid]
  93. 93. Symptoms : Defective vision Distorsion of objects Polyopia Investigations: - Placido's disc test reveals distorted circles - Computerized corneal topography
  94. 94. Treatment :  Optical treatment : - RGP contact lenses -Hybrid contact lenses -Scleral lenses - Piggyback lens  Surgical treatment: - penetrating keratoplasty
  95. 95.  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
  96. 96. 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 .
  97. 97. Refractive surgeries  Astigmatic Keratotomy  Photoastigmatic refractive Keratectomy [PRK]  Relaxing incisions with compression sutures  LASIK surgery
  98. 98. 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.
  99. 99. Anisometropia  Difference in refractive power between eyes  refractive correction often leads to different image sizes on the 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
  100. 100. 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 > D-->contact lens  unilateral aphakia-->contact lens or intraocular lens