Refractive Errors Prof Dr.Şule Ziylan

Aim Learning the types of refractive errors and mechanisms of occurrence in the optical system of the eye Learning the emetropisation and accommodation mechanisms Objective To be able to completely classify the refractive errors and completely list the mechanism of occurence of each refractive error of the eye To be able to completely list the concept, mechanism of occurrence and aim of emmetropisation process To be able to completely list the elements of near vision reflex triad To be able to completely list the changes in the eye when looking at distance and at near, according to vonHelmholtz theory 02.06.2018

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􀂉 The eye is an amazing organ that functions somewhat like a camera. 􀂉 Think of the cornea as a camera lens, the iris as the shutter of a camera, and the retina as the photographic film. 02.06.2018

Refractive State of The Eye Corneal power (mean, 43 D) Anterior chamber depth (mean, 3.4 mm) Crystalline lens power (mean, 21 D) Axial length (mean, 24 mm) 02.06.2018

Light enters the eye through the cornea and travels through the anterior chamber, pupil and the crystalline lens İris is the colored muscular ring that surrounds the pupil and functions like a cameras shutter Light exits the crystalline lens and travels through the vitrous jel, where the images are inverted (turned upside down). Ideally, light comes to focus on the center of the macula, called the fovea. Signals travel from the retina to the brain through the optic nerve. The brain then translates light into vision. The brain can be thought of as the film processing center because it converts electrical signals from the retina into vision. Vision takes place in the occipital lobe of the brain (not in the eye) 02.06.2018

Distribution of refraction components Average sagittal diameter of the eye is app 18 mm at birth. By the age of 3 years, the axial length increases to about 23 mm. This elongation would theoretically yield a state f myopia of about 15 D. However, almost 75% of these young eyes are hyperopic. Between 3 and 14 years old, elongation increases by an additional millimeter. This would also theoretically yield another 3 D. However, a strong clustering around emmetropia is seen around 14 years old. Cornea and ACD change vey little during these periods, but the power of the crystalline lens changes to maintain emmetropia. This process is coordinated by the retina-brain complex. Emmetropization is largely programmed on a genetic basis. Average sagittal diameter of the eye is app 18 mm at birth. By the age of 3 years, the axial length increases to about 23 mm. This elongation would theoretically yield a state f myopia of about 15 D. However, almost 75% of these young eyes are hyperopic. Between 3 and 14 years old, elongation increases by an additional millimeter. This would also theoretically yield another 3 D. However, a strong clustering around emmetropia is seen around 14 years old. Cornea and ACD change vey little during these periods, but the power of the crystalline lens changes to maintain emmetropia. This process is coordinated by the retina-brain complex. Emmetropization is largely programmed on a genetic basis. Experimental procedures which significantly degrade the retinal image (eg. Suturing lids together) influence the axial length process. The axial length increases significantly and produces myopia upto 12 D. 02.06.2018 Sorsby et al. 1957

Refractive Error A mismatch between the refractive power and the focusing distance of the eye 02.06.2018

Refractive Error Clinically, it is the variation from perfect coincidence of the principal focus of the eye with the retina Parallel rays of light fail to converge to a sharp focus on the retina Affects all ages, sex, race, ethnic groups and religions but with variable prevalence and distribution 02.06.2018

Refractive status Emmetropia Ametropia Hyperopia Myopia Astigmatizm Simple myopic Simple hyperopic Compound myopic Compound hyperopic Mixed Emmetropia: good vision, no corrective lenses needed. Ametropias: eye does not focus properly but vision can be MOSTLY (not always) corrected with lenses or ref. Surg. Hyperopia=Farsighted Myopia= Nearsighted Astigmatism=Blurred vision. The cornea or eye is shaped like an ellipse 02.06.2018

Etiology of Ammetropia Axial ammetropia Curvature ammetropia Index ammetropia Stenstom’s study (Uppsala, Sweden) 29% low myopia (2D or less), 7% have moderate myopia (2-6 D), 2.5% high myopia (>6 D) App 70% are clustered btw emmetropia and 2D hyperopia The rest: high hyperopia and aphakia 02.06.2018

Accomodation Von Helmholtz theory Near objects Far objects 02.06.2018

Presbyopia 02.06.2018

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Hyperopia The refractive power of the eye is too weak so that light focuses behind the retina Total Manifest Latent 02.06.2018

Myopia The refractive power of the eye is too much so that light focuses in front of the retina Physiologic (Simple) Pathologic (Degenerative) 1. 2-3% of the population have pathologic myopia (enlargement of the eyeball with a lenghthaning of the posterior segment. Marked choroidal & retinal degeneration, high incidence of retinal detachment, glaucoma, increased occurrence of staphyloma development; sex-linked recessive inherited disorder) 2. Physiologic or school myopia (a normal, physiologic response to a stress, ie, increased time spent reading from early teenage to the mid-20s. However, near work is not the sole cause) 02.06.2018

Nature vs Nurture Myopia Causes of myopia ?? The nature versus nurture debates concern the relative importance of an individual's innate qualities ("nature") versus personal experiences ("nurture") in determining or causing individual differences in physical and behavioral traits. The view that humans acquire all or almost all their behavioral traits from "nurture" is known as tabula rasa ("blank slate"). The nature versus nurture debates concern the relative importance of an individual's innate qualities ("nature") versus personal experiences ("nurture") in determining or causing individual differences in physical and behavioral traits. The view that humans acquire all or almost all their behavioral traits from "nurture" is known as tabula rasa ("blank slate"). 02.06.2018

Myopia Nature Genetic factors Interactive with environment 02.06.2018

Myopia Nurture ? Visual task No definite conclusions yet Eg. Reading, writing, computer, TV, etc. No definite conclusions yet 02.06.2018

Astigmatism The refractive power is different in different meridians 50% of full-term infants in their first years of life show astigmatism over 1D. About 15% of the adult population have astigmatism >1 D, and only 2% have astigmatism >3 D (mostly due to some form of intraocular surgery) 02.06.2018

Astigmatism 02.06.2018

Correction By changing the eyeball length By changing the refractive power Changing the power of the existing structure Adding extra refractive medium 02.06.2018

Pin - hole 02.06.2018

Correction Spectacles Myopia : Concave lenses 02.06.2018

Correction Spectacles Hyperopia : Convex lenses 02.06.2018

Correction Contact lenses Commonly used types Hard lens Rigid gas-permeable lens (RGP) Soft lens Daily wear Extended wear 02.06.2018

Contact lens - Complications Change ocular surface environment Hypoxia Allergy & Toxic Mechanical trauma Infection 02.06.2018

Correction Refractive surgery Radial keratotomy Photorefractive keratectomy (PRK) Laser assisted subepithelial keratomileusis (LASEK) Laser in situ keratomileusis (LASIK) In myopia, the refractive power is too strong for the axial length Light focuses in front of the retina Correction: Cornea is flattened Chinese were among the first to put stones on the eye at night to achieve a flatter cornea in the morning.....Today “Orthokeratology” In hyperopia, light focuses behind the retina Corretion: Cornea is steepened 02.06.2018

Radial keratotomy 02.06.2018

LASIK Refraction +6D to -8D Astigmatism <6D Corneal thickness and biomechanical properties Pupil size 02.06.2018

LASEK 02.06.2018

Refractive Surgery 02.06.2018

Any question ?