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TBL 1~ Refractive Error

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Prepared by:

Mohammad Daniel bin Md Shamuddin (1008-1749)Nurul Afidza binti Noor Allim (1008-1898)Ammar Zulqarnain bin Ahmad Denil(1008-1604)Lim Ping Hoong (1008-1738)

(i) Refractive Surgery for Myopia(1) Cornea-based proceduresRadial keratotomy (RK)Photorefractive keratectomy (PRK)Laser in-situ keratomileusis (LASIK)Intercorneal ring (ICR) implantationOrthokeratology

(2) Lens based proceduresRefractive lens exchange (RLE)Phakic refractive lens (PRL)/ implantable contact lens (ICL)50STATEMENT:WHO estimates that 153 million people worldwide live with visual impairment due to uncorrected refractive errors.3

Light RefractionDefinition : The bending of the path of light wave as it pasesses between mediums of different densities

Why do this happen?

4Refraction occur at the boundary due to change in the speed of light when passing through different medium5Light Refraction Of The Eye

6Pathway of Light Refraction In the Eye.

Cornea (refractive index : 1.38) - 1st surface of which refraction occur. - 80% of light will be refrated. - largest change in index medium density

No refraction in pupilNo significant refration occur in aqueous humour

Lens (refractive index : 1.40) - 20% of light will be refractedMade of fibrous materialAble to change thickness to tune the light refraction (accommodation)

No significant refraction in vitreous humour

RetinaImage form and focused at the retina after refration processThe virtual image on retina is inverted from the original image,

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8Refractive Power of The EyeIt is measured in dioptres (D).It describes the power of a structure has to focus parallel rays of light.The higher the value, the stronger its focusing ability.The refractive power will affect the overall accuracy of the eye to focus an image.It is equal to the reciprocal of the focal length of the device : P = 1/fDefinition of focal length : the distance between the center of structure of which light is refracted to at site where light is focus.High optical power correspond to short focal length.

The overall accuracy depends on : - Integrity of these structure (mainly corne and lens) - shapes of cornea and lens - depth of anterior chamber of the eye- The length of the eye from front to back (axial length)9THE CORNEAThe cornea contributes to approximately 2/3 of the refracting power of the eye along with the tear film. It contributes 43 diopters.

THE LENSThe lens contributes to 1/3 of the refractive power of the eye ( 20 diopters). By itself it is more powerful than the cornea as a convergent surface but because of less difference in indices of refraction between the aqueous versus the cornea, less convergence exists at the level of the lens.10

11Types of refractive errors12Definition Refractive errors occur when the shape of the eye prevents light from focusing directly on the retina.

The length of the eyeball (longer or shorter), changes in the shape of the cornea, or aging of the lens can cause refractive errors.

Emmetropia is absence of refractive error. It is the normal condition of the eye.

Ametropia is presence of refractive error. Is divided into two:Spherical myopia, hyperopiaastigmatic13

The light rays in focus directly on the retina.14Types Myopia When the image of distant object focuses in front of the retina in the unaccommodated eye

Nearsightedness/short-sightedness difficulty in seeing distant objects clearly

Pathophysiology: The eyeball is too long or the cornea has too much curvature, so the light entering the eye is not focused correctly. Images focus in front of the retina, rather than directly on the retina, causing blurred vision.15

16Types of myopiaSimple myopiaResults from correlation mismatch between corneal power, lens power and axial length of the eye. These 3 are the basic determinants of refractive status of eyeAll of these 3 factors are within normal range but their sum total lack of perfect correlation.

Pathological myopiaResults from specific pathological condition of one of the 3 basic determinants of refractive status of eye; usually the axial length of the eye.The mechanisms proposed for axial elongation myopia subsequent to excessive reading or other fine work include:Tonically increased ciliary muscle contractionThe pressure of long tendons of the recti muscles against the globe as in convergenceBoth of the mechanisms cause subtle increase in intraocular pressure (IOP) and produce elongation of the globe.17Etiology:Inheriting short sight If one of your parents is short-sighted, you have around a 40% chance of developing the condition yourself. The risk increases to around 60% if both of your parents are short-sighted.

Close work There certainly seems to be some connection between children or young adults who spend a lot of time doing close work, such as reading, writing and computer work, and an increased risk of developing short-sightedness.

Symptoms: Complain of headaches, eyestrain, squinting, or fatigue when driving, playing sports, or looking more than a few feet away. Children commonly complain of not being able to see the board at school.18Management:Glasses concave (minus) lenses

Contact lenses work by becoming the first refractive surface for light rays entering the eye, causing a more precise refraction or focus.Refractive surgery aims to change the shape of the cornea permanently. E.g LASIK (Laser-assisted in situ keratomileusis.)

19Hyperopia/hypermetropia

Is the state in which the unaccomodated eye would focus the image behind the retina.

Farsightedness/long-sightedness difficulty in seeing close objects clearly.

Pathophysiology: Hyperopia develops in eyes that focus images behind the retina instead of on the retina, which can result in blurred vision. This occurs when the eyeball is too short, which prevents incoming light from focusing directly on the retina. It may also be caused by an abnormal shape of the cornea or lens.20

21Etiology:Genetic Hyperopia often runs in families and is often present at birth; however, many children outgrow it.Short eyeball

Symptoms:HeadachesEye strainDifficulty concentrating or focusing on nearby objectsFatigue or headache after performing a close task such as reading22Management Glasses convex (plus) lenses

Contact lensesRefractive surgery

23Presbyopia

Is a normal condition associated with age that causes problems with our near vision.

The loss of accommodation the comes with aging to all people. As the eye ages, the lens can no longer change shape enough to allow the eye to focus close objects clearly.

A person who previously has emmetropic eyes will begin to notice inability to read small print or discriminate fine close objects at about age 44 46.

It is worse in dim light and usually worse early in the morning or when the person is fatigued.

These symptoms increase until about age 55, when they stabilize but persist.24

25Symptoms: The need to hold reading material at arm's length.Blurred vision at a normal reading distance.Headaches or fatigue from doing close work.Eyestrain

Management Prescription glasses Contact lenses multifocal lenses, which come in soft or gas permeable versions, and monovision lenses, in which one eye wears a lens that aids in seeing objects at a distance, while the other has a lens that aids in near vision.Reading glassesProgressive addition lenses similar to bifocals but they are made to have a gradual or blended transition between the two prescriptions.Bifocals are eyeglasses that have two different prescriptions in one spectacle lens. The top part of the lens corrects for distance vision and the lower portion of the lens is designed to help a person see objects up close.Conductive keratoplasty uses radio waves. The doctor uses a small instrument to apply the radio waves to the eye (usually just one eye) to reshape the cornea and improve the patient's vision of nearby objects.26Astigmatism

Astigmatism is caused by an eye that is not completely round. This is one type of refractive error. Astigmatism occurs in nearly everybody to some degree.

Slight amounts of astigmatism usually don't affect vision and don't require treatment.

Astigmatism frequently occurs with other vision conditions like nearsightedness (myopia) and farsightedness (hyperopia). 27Etiology: Is unknown. In most cases, astigmatism is present at birth. However, it sometimes develops after an injury to the eye or as a complication of an eye operation.

Pathophysiology: Astigmatism occurs due to the irregular shape of the cornea or the lens inside the eye. Instead of the cornea having a symmetrically round shape (like a baseball), it is shaped more like a football, with one meridian being significantly more curved than the meridian perpendicular to it.

(To understand what meridians are, think of the front of the eye like the face of a clock. A line connecting the 12 and 6 is one meridian; a line connecting the 3 and 9 is another.)

The steepest and flattest meridians of an eye with astigmatism are called the principal meridians.

28The curvature of the cornea and lens causes light entering the eye to be bent in order to focus it precisely on the retina at the back of the eye. In astigmatism, the surface of the cornea or lens has a somewhat different curvature in one direction than another. In the case of the cornea, instead of having a round shape like a basketball, the surface of the cornea is more like a football. As a result, the eye is unable to focus light rays to a single point causing vision to be out of focus at any distance.

In some cases, astigmatism is caused by the shape of the lens inside the eye. This type of astigmatism is called lenticular astigmatism, to differentiate it from the more common corneal astigmatism.

Sometimes astigmatism may develop following an eye injury or eye surgery. There is also a relatively rare condition called keratoconus where the cornea becomes progressively thinner and cone shaped. This results in a large amount of astigmatism resulting in poor vision that cannot be clearly corrected with spectacles. Keratoconus usually requires contact lenses for clear vision, and it may eventually progress to a point where a corneal transplant is necessary.

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34Types of astigmatism:Myopic astigmatism. One or both principal meridians of the eye are nearsighted. (If both meridians are nearsighted, they are myopic in differing degree.)Hyperopic astigmatism. One or both principal meridians are farsighted. (If both are farsighted, they are hyperopic in differing degree.)Mixed astigmatism. One prinicipal meridian is nearsighted, and the other is farsighted.

Astigmatism also is classified as regular or irregular. In regular astigmatism, the principal meridians are 90 degrees apart (perpendicular to each other). In irregular astigmatism, the principal meridians are not perpendicular. Most astigmatism is regular corneal astigmatism, which gives the eye a football shape. Astigmatism may be regular or irregular.

Regular astigmatism is called astigmatism with the rule. The cornea is more curved in one meridian which is often vertical.

Irregular astigmatism is associated with numerous foci. It can result from an eye injury that has caused scarring on the cornea, from certain types of eye surgery or from keratoconus, a disease that causes a gradual thinning of the cornea. It cannot be corrected with glasses.

35Symptoms:HeadachesEyestrainSquintingDistorted or blurred vision at all distancesDifficulty driving at night

Managementeyeglassescontact lensesOrthokeratology (Ortho-K) involves the fitting of a series of rigid contact lenses to reshape the cornea, the front outer cover of the eye. The contact lenses are worn for limited periods, such as overnight, and then removed. laser and other refractive surgery procedures

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37Anisometropia

Is a difference in refractive error between the two eyes. One eye may be myopic (nearsighted) and the other hyperopic (farsighted) or one eye may be markedly stronger than the other.

Anisometropia is a serious concern in newborns and young children because it can lead to amblyopia (impaired vision in one eye).

With a major degree of anisometropia, the brain cannot reconcile the difference in images coming from the two eyes. It develops a preference for the image coming from one eye and suppresses the image from the other eye and, in time, the brain loses the ability to "see" the image from the suppressed eye.38Etiology: In normal vision, the retinal image for each eye is roughly the same size and shape. In most cases, the difference between the size of the images perceived by each eye is less than 0.5%, although your brain can cope with differences of up to 5% before your vision is adversely affected.

When you suffer from anisometropia, the difference in size of your retinal images is greater than 5%. People with this condition may have a difference of as much as 20% between each eye

Symptoms Amblyopia reduced vision in an eye that has not been used adequately during childhood.Strabismus not able to align both eyes simultaneously under normal conditions. One or both eyes may turn out, in, down, or up.Diplopia

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40Management

Eyeglasses produces a difference in retinal image size approximately 25%, which is rarely tolerable.

Contact lens reduces the difference in image size to approximately 6%, which can be tolerated.

Intraocular lenses produce a difference of less than 1%.

41TYPE OF LENS42

43Concave Lens

44Convex lens

45Astigmatism

46PresbyopiaMultifocal

Bifocal

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Refractive Surgery increasingly popular; /eliminate dependency on glasses/ contact lenses 48Refractive Eye SurgeryShould be performed after error has stabilized.Preferably after 20 y/o.

Surgical techniques that will be discussed:Refractive surgery for myopiaRefractive surgery for hyperopiaRefractive surgery for astigmatismRefractive surgery for presbyopia 49Radial keratotomy (RK)Make deep radial incisions (90% of corneal thickness) in peripheral part of cornea leaving central 4 mm optical zone.Flatten central cornea, reducing its refractive power.

Photorefractive keratectomy (PRK) Central optical zone of anterior corneal stroma is photoablated using excimer laser to cause flattening of central cornea.

Both procedures good correction for -2 to -6 D of myopia. D/t disadvantages & introduction of safe techniques (PRK & LASIK) RK x recommended.

51Laser In-situ Keratomileusis (LASIK)A flap of 130-160 micron thickness of anterior corneal tissue is raised with the help of an automated microkeratome (femtosecond laser used recently for more accurate and smooth flaps).

After creating corneal flap mistromal tissue is ablated directly with an excimer laser beam flattening the cornea

Current refractive surgery of choice for myopia of up to -8 D. 52Laser In-situ Keratomileusis (LASIK)Patient selection criteria: Above 20 y/o.Stale refraction for at least 12 months.Motivated pt.Absence of corneal pathology.

Ectasia/ other corneal pathology/ corneal thickness < 450 m- absolute contraindication53

54Advantages of LASIKMinimal/ no postoperative pain.Recovery of vision is very early as copared to PRK.No rick of perforation during surgery & later global rupture d/t trauma (unlike RK).No residual haze unlike PRK where subepithelial scarring may occur. Effectively correct myopia up to -8 D. 55DisadvantagesExpensiveRequire greater surgical skill

Potential risk of flap-related complications, including: intraoperative flap amputationwrinkling of the flap on repositioningpost-operative flap dislocation/ subluxationepithelization of flap-bed interface Irregular astigmatism.

56Intercorneal ring (ICR) implantation into peripheral cornea at approximately 2/3 stromal depth flatten central cornea decreasing corneaAdvantage: reversible

Orthokeratology Non-surgical reversible method.Molding cornea with overnight wear of unique rigid gas permeable contact lenses.For correction of myopia up to -5 D.Pt < 18 y/o.

57Lens-based proceduresRefractive lens exchange Removal of clear crystalline lens (Fucalas operation) for unilateral very high myopia (-16 to -18 D).Clear lens extraction + intraocular lens (IOL) implantation of appropriate power recommended as refractive surgery for myopia > 12 D.

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Phakic Refractive Lens (PRL)A.k.a implantable contact lens (ICL).

Correction of myopia > 8 D.

IOL implanted in anterior chamber/ posterior chamber anterior to natural crystalline lens.59

(ii) Refractive Surgery for Hyperopia(1) Cornea-based proceduresThermal laser keratoplasty (TLK)Hyperopic PRK- using excimer laser (disadvantages: regression effect & prolonged epithelial healing)Hyperopic LASIK- correction up to +4 D.Conductive keratoplasty (CK)

(2) Lens based proceduresRefractive lens exchange (RLE)- good option for high hyperopia, esp in presbyopic age.Phakic refractive lens (PRL)/ implantable contact lens (ICL)- surgical option for hyperopia > + 4D.

60Thermal laser keratoplasty (TLK)For low degree of hyperopia 8 laser spots are applied in a ring at the periphery to produce central steepening with mid-infrared energy from Thallium-Holmium-Chromium (THC): YAG laser. Disadvantage: Regression effect & induced astigmatism

*YAG = neodymium-doped yttrium aluminum garnet61

Conductive keratoplastyNon-ablative, non-incisional procedure.

Cornea is steepened by collagen shrinkage through the radiofrequency energy applied through a fine tip inserted into peripheral corneal stroma in a ring pattern.

This technique is effective for correcting hyperopia of up to 3D.

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Conductive keratoplasty(iii) Refractive Surgery For AstigmatismAstigmatic keratotomy (AK) making transverse cuts in the mid-periphery of the steep corneal meridian

Photo-astigmatic refractive keratotomy (PARK)- using excimer laser

LASIK- can be adapted to correct up to 5D.64Astigmatic Keratotomy (AK)65

(iv) Refractive Surgery For Presbyopia (under trial)(1) Cornea-based proceduresMonovision LASIKMonovision conductive keratoplasty (CK)Presbyopic bifocal LASIK/ LASIK PARM (LASIK by Presbyopic Avalos Rozakis Method)

(2) Lens based proceduresBifocal/ multifocal/ accommodating IOL implantation after lens extraction Monovision with intraocular lenses

(3) Sclera-based procedures Anterior ciliary sclerotomy (ACS), with tissue barriersSclera spacing procedures and scleral ablation with erbium

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