1. Refractive Errors
Prof Dr.Şule Ziylan

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

4. 􀂉 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.

5. 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)

6. 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)

7. 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.
Sorsby et al. 1957

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

9. 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

10. 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

11. 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

12. Accomodation
Von Helmholtz theory
Near objects
Far objects

13. Presbyopia

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

16. 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)

17. 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").

18. Myopia
Nature
Genetic factors
Interactive with environment

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

20. 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)

21. Astigmatism

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

23. Pin - hole

24. Correction
Spectacles
Myopia : Concave lenses

25. Correction
Spectacles
Hyperopia : Convex lenses

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

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

28. 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

29. Radial keratotomy

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

31. LASEK

32. Refractive Surgery

33. Any question ?