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Anatomy, optics & refractive errors of eye
- 1. FUNCTIONAL ANATOMY OF EYE;OPTICS OF EYE & REFRACTIVE ERRORS DR.K.AMUDHA LAKSHMI M.B.B.S.,D.O.,M.D
- 2. SYNOPSIS INTRODUCTION HISTORY STRUCTURE OF EYEBALL OPTICS OF EYE REFRACTIVE ERRORS APPLIED ASPECTS
- 3. INTRODUCTION Vision- important special sense Performed by eye balls Situated in a quadrilateral pyramid shaped bony cavity- orbit Well protected by eyelids and extraocular muscles
- 5. There are some species in animal kingdom which doesn’t have eyes to see
- 8. SEA URCHINS
- 10. HISTORY A.D. Galen – Retina Euclid – Geometrical optics Benjamin Franklin – Spectacles Leonardo Da Vinci – Lens Thomas Young, Von Helmholtz - Color vision
- 16. EYE DEVELOPMENT Begins at 3 weeks & continues upto 10 weeks. Derived from neuroepithelium, surface ectoderm, neural crest and mesoderm. Neuroepithelium - retina, ciliary body, iris, optic nerve. Surface ectoderm- lens, corneal epithelium, eyelid. Extra cellular mesenchyme- sclera, cornea, blood vessels, muscles, and vitreous.
- 17. STRUCTURE OF EYEBALL The anteroposterior diameter- 24 mm Volume : 6.5 ml Weight : 7 g Shape- oblate spheroid It has 3 layers : Outer- fibrous sclera and cornea Middle- uveal tract- vascular and pigmented Inner – retina- nervous coat
- 19. Limbus Sclerocorneal junction Anterior chamber 2.5 mm deep 0.25 ml of aqueous humor Posterior chamber 0.06 ml of aqueous humor Posterior cavity Vitreous humor
- 20. Sclera Avascular fibrous coat Gives protection for eyeball contents Bulbar conjunctiva Palpebral conjunctiva Conjunctivitis Cornea Avascular Transparent More convex Trigerminal nerve
- 22. The average diameter 11 – 12 mm Thickness center 0.52 mm periphery 0.67 mm Has 5 layers Epithelium Stratified squamous Highly proliferative Regenerate & heal rapidly Bowman’s membrane Heals with a scar Stroma Collagen fibrils 90% of thickness
- 23. Descemet’s membrane Resistant to infection Can regenerate Endothelium Prevents excess hydration of cornea Keratitis Corneal abrasion Corneal ulcer Corneal transplant-most successful one
- 24. Lacrimal gland Secretes tear Keeps cornea moistened Contain lysozymes, antibodies Blinking- spread tears Basal secretion-1ml in 24 hrs Regulation of stromal water- clarity of vision
- 26. Choroid Lines the inner aspect of sclera Provide nourishment to eye structures Membrane of bruch- inner side Supra choroidal space-b/w choroid & sclera Chromatophores Albinism Choroiditis Choroidal detachment
- 27. Ciliary body Ciliary muscle & ciliary processes Ciliary muscle-meridional,radial & circular Ciliary body attaches to suspensory ligaments Ciliary processes secrete aqueous humor Cyclitis
- 28. Iris Characteristic colour to eye Central aperture- Pupil Highly vascular Circular & radial muscles Myosis-circular muscle- sphincter pupillae- parasympathetic-occulomotor nerve Mydriasis-radial muscle- dilator pupillae- sympathetic- cervical sympathetic chain remember- MaRS
- 29. Intrinsic muscles of eye Ciliary & iris muscles
- 30. Aqueous humor Protein free clear fluid Source-ciliary process Ultrafiltration, diffusion, active transport Nutrient for lens & cornea Rich in ascorbic acid Formed in posterior chamber poured into anterior chamber through pupil
- 31. Maintains IOP Absorbed by trabecular meshwork & drained into schlemn canal Open angle glaucoma Closed angle glaucoma Lens Biconvex Avascular Highly elastic-collagen fibrils Converge light rays & focus on retina
- 32. Protein- alpha crystalline At rest- lens ligaments stretched- anterior surface is less convex Opacity of lens- cataract- immature /mature Absence of lens- aphakia
- 33. Vitreous humor Gelatinous Transparent avascular Protein- vitrein Old age & high myopia- liquefied Persistent hyaloid artery
- 34. Retina Very thin- 200 μm 10 layers, from outer to inner 1.Layer of pigment epithelium 2. Layer of rods and cones 3. External limiting membrane 4. Outer nuclear layer 5. Outer plexiform layer 6. Inner nuclear layer 7. Inner plexiform layer 8. Layer of ganglion cells 9. Layer of nerve fibers 10. Internal limiting membrane.
- 36. Surrounds the vitreous Terminates at ora serrata Pigment epithelium Rich in melanocytes Prevents scattering of light Phagocytosis Store vitamin A Albinism- lack of melanin Nystagmus, photophobia, defective vision
- 37. Cells in retina Rods- 120 million Cones- 6 million Ganglion cells- 1 million Horizontal cells- connect rods & cones Bipolar cells-connects photoreceptors & ganglion cells Amacrine cells- connects bipolar cells & ganglion cells Retinal glial cells/ Muller cells- supporting cells
- 38. Diabetic retinopathy Retinal detachment Retrolental fibroplasia/ Retinopathy of Prematurity (ROP) Macula lutea Yellow spot 1-2 mm Fovea centralis 0.4 mm Only cones Maximum visual acuity Ratio of ganglion to cone cell 1:1
- 40. Optic disc 3mm medial to & above posterior pole No photoreceptors Blind spot- at temporal visual field White in colour- myelinated nerves Fundus Red background Arteries readily visible Diagnosing HT,DM,atherosclerosis
- 41. IOP 10-20 mm Hg Maintains shape of eyeball Tonometry Glaucoma
- 42. Extraocular muscles (LR 6 SO 4 O 3)
- 49. OPTICS OF EYE
- 51. OPTICS OF EYE Image formed on retina by Refraction of light rays by cornea and lens Ciliary muscle activity- accommodation of lens Change in pupil size by iris muscles
- 54. Principal axis of the lens The line joining the centers of the two lens surfaces Optical center/ Nodal point of lens Point on the principal axis through which light rays pass without being deviated. 7 mm behind anterior surface of cornea
- 55. Principal focus When parallel rays of light strike the surface of the lens, they converge to meet at a point behind the lens Focal distance The distance between the optical center of a lens and the principal focus
- 56. The refractive power of lens It is the reciprocal of the focal distance of a lens in meters. expressed in diopters (D) greater the lens curvature, greater the refractive power Refraction takes place at cornea both surfaces of the lens Total refractive power of eye = 60 D 43 D by cornea 17 D by lens
- 57. When the head is immersed in water, the refractive power by cornea= zero, since water has the same refractive index as cornea. Schematic eye/reduced eye Nodal point 7mm Eyeball length=24 mm Focal length=24-7 =17 Refractive power of lens= 1 Focal length( in metres) =1000 17 =59 D
- 59. On retina real Inverted Small image Left to right reversal of image In brain Neural processing helps the brain to perceive visual images in upright position.
- 60. ACCOMMODATION
- 61. Accommodation Cornea-fixed structure Normal resting eye- ciliary muscle relaxed During accommodation Ciliary muscle contracts Suspensory ligaments relax Lens becomes more convex Visual axis converges Pupil constricts
- 62. To focus an object present within 6 m, the eye has to increase its refractive power, a process called accommodation, in which the curvature of the lens is increased. Near point- The nearest point to the eye, at which an object can be clearly seen The maximum accommodation ability of the eye decreases throughout life lens-harder, progressive loss of elasticity.
- 65. Chromatic aberrations Long wavelength(red)-refracted more Short wavelength(violet)-refracted less Lights of different colours focussed at different points on retina Spherical aberrations Non uniform refractive index of lens Periphery-refraction more
- 66. Size of pupil decreases chromatic and spherical aberration by eliminating the peripheral rays increases the depth of focus Bright light-pupil constricts-2 mm Dark- pupil dilates- 8 mm
- 67. ERRORS OF REFRACTION Emmetropia The refractive condition of a normal eye, in which the parallel rays of light from a distant object are focused on the retina without accommodation Emmetropic eye Ametropia If the refractive state differs from the normal, in which parallel rays are focused in front or behind the retina resulting in a blurred image.
- 68. Amblyopia ex anopsia Occurs in children, one of the image is suppressed, causing loss of vision in one eye. STRABISMUS/ SQUINT
- 69. Ametropia is of three types Myopia Hypermetropia Astigmatism May be due to abnormality in Axial length of the eye Refractive power − Curvature of the surface of the cornea or the lens Refractive indices of the media Position of the lens
- 71. Myopia/ Short sightedness A refractive error in which parallel rays of light from a distant object are brought into a focus in front of the retina when the accommodation is at rest Causes Genetic Acquired More close work AP diameter of eye increased Correction Biconcave lens- minus lens
- 72. Hypermetropia (Far-sightedness) A refractive error in which parallel rays of light from a distant object are brought into a focus behind the retina when the accommodation is at rest Decreased anteroposterior diameter of the eye Correction Biconvex lens- plus lens
- 73. Astigmatism A refractive error in which parallel rays of light from a distant object cannot converge to a point focus on the retina due to unequal refraction at different meridians.
- 75. Causes Corneal surface is not perfectly spherical, the curvature of one meridian differs from the other. Light rays are focused at different points on the retina causing blurring of image. Lens curvature- not uniform or lens- pushed out of alignment. Correction- Cylindrical lens.
- 76. Presbyopia Near point of vision recedes beyond the normal reading or working distance due to progressive loss of plasticity of the lens Physiological phenomenon Above 40 years Normal Near point of accommodation 25-33 cm Correction Convex lens-plus lens For both distant & near vision Bifocal lens