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Refractive Errors

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

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Refractive Errors

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  1. Refractive Errors Prof Dr.ŞuleZiylan

  2. 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

  3. 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)

  4. 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. Sorsby et al. 1957

  5. Refractive Error • A mismatch between the refractive power and the focusing distance of the eye

  6. 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

  7. Refractive status • Emmetropia • Ametropia • Hyperopia • Myopia • Astigmatizm • Simple myopic • Simple hyperopic • Compound myopic • Compound hyperopic • Mixed

  8. Etiology of Ammetropia • Axial ammetropia • Curvature ammetropia • Index ammetropia

  9. Accomodation • Von Helmholtz theory Near objects Far objects

  10. Presbyopia

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

  12. Myopia • The refractive power of the eye is too much so that light focuses in front of the retina • Physiologic (Simple) • Pathologic (Degenerative)

  13. Myopia • Causes of myopia ?? Nature vs Nurture 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").

  14. Myopia • Nature • Genetic factors • Interactive with environment

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

  16. Astigmatism • The refractive power is different in different meridians

  17. Astigmatism

  18. Correction • By changing the eyeball length • By changing the refractive power • Changing the power of the existing structure • Adding extra refractive medium

  19. Pin - hole

  20. Correction Spectacles Myopia : Concave lenses

  21. Spectacles Correction Hyperopia : Convex lenses

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

  23. Contact lens - Complications • Change ocular surface environment • Hypoxia • Allergy & Toxic • Mechanical trauma • Infection

  24. Correction • Refractive surgery • Radial keratotomy • Photorefractive keratectomy (PRK) • Laser assisted subepithelial keratomileusis (LASEK) • Laser in situ keratomileusis (LASIK)

  25. Radial keratotomy

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

  27. LASEK

  28. Refractive Surgery

  29. Any question ?

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