Refractive Errors

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Slide 1: 

Refractive Errors

FACTS : 

80% of the blindness in the world is avoidable……and could be prevented or cured through cost effective means The treatments available for the prevention and cure of blindness are among the most successful of all health interventions FACTS

FACTS : 

75% of avoidable blindness is due to: • Uncorrected refractive error • Cataract • Trachoma Blindness due to refractive errors is a substantial public health problem in many parts of the world. FACTS

FACTS : 

• The presence of RE implies inadequate eye care services in the population concerned, because its treatment probably the simplest and most effective of eye care interventions. • Intervention at this level can help not only the individual but indeed the whole family. FACTS

Slide 5: 

World Health Organization Programme for Prevention of Blindness released 2004: 37 million blind >124 million low vision 181 million with significant uncorrected refractive errors (Without further intervention, by 2020 these numbers will have increased to 75 million blind and 250 million visually impaired)

How The Eye Works? : 

How The Eye Works?

The healthy eye : 

The healthy eye Light rays enter the eye through the clear cornea, pupil and lens. These light rays are focused directly onto the retina in the same way a camera focuses light onto a film. (the light sensitive tissue lining the back of the eye) The retina converts light rays into impulses; sent through the optic nerve to your brain, where they are recognized as images.

Slide 9: 

The eye requires about 60 dioptres of power to focus the light from a distant object precisely onto the retina.

Slide 12: 

The power of the lens is measured by the diopter (D) The unit of refraction

Slide 13: 

REFRACTION In optics, refraction occurs when light waves travel from a medium with a given refractive index to a medium with another. At the boundary between the media, the wave's phase velocity is altered, it changes direction.

Slide 14: 

The amount of bend depends on the refractive index of the media and the angle of incidence The refractive index of a medium is defined as the ratio of the phase velocity of a wave light in a reference medium to its velocity in the medium itself. REFRACTION

Unit of refraction : 

Unit of refraction Dioptre = 1 focal length of a lens Therefore the power is 1 Dioptres 1m

Slide 16: 

THE EYE’S OPTICAL SYSTEM CORNEA Main refracting surface The cornea provides 40 dioptres, or 75% of the total refracting power of the eye. CRYSTALINE LENS Double purpose: balancing eye’s refractive power and providing a focusing mechanism The lens provides 20 dioptres of refractive power

Accommodation : 

Accommodation Emmetropic (normal) eye object closer than 6 meters send divergent light that focus behind retina , adaptative mechanism of eye is to increase refractive power by accommodation Helm-holtz theory contraction of ciliary muscle -->decrease tension in zonule fibers -->elasticity of lens capsule mold lens into spherical shape -->greater dioptic power -->divergent rays are focused on retina contraction of ciliary muscle is supplied by parasympathetic third nerve

Slide 18: 

6 meters

VISUAL ACUITY : 

VISUAL ACUITY VA is the vital sign of the eye To assess the effect of pathology on VA the effect of refractive error must be eliminated This is achieved by measuring: the patient’s best spectacle correction or viewing the test chart through a pinhole

PINHOLE : 

PINHOLE Optimal size 1.2mm Correct 3D of RE

How to test the vision? : 

How to test the vision? Central visual acuity: display of different –sized targets shown at a standard distance from the eye. Snellen chart. 20/20, 6/6

Slide 23: 

Testing poor vision If the patient is unable to read the largest letter <(20/200) Move the patient closer e.g. 5/200 If patient cannot read: - Count fingers (CF) - Hand motion (HM) - Light perception (LP) - No light perception (NLP)

NEAR VISUAL ACUITY : 

NEAR VISUAL ACUITY At a standard working distance ~ 30-40 cm A variety of charts are available

Refractive errors : 

Refractive errors A mismatch between the refractive power and the focusing distance of the eye Inability to see clearly is often caused by refractive errors. • Four types of refractive errors: Myopia (nearsightedness) Hyperopia (farsightedness) Astigmatism

Slide 26: 

REFRACTIVE ERROR Emmetropia (normal) Ametrpia=RE Myopia Hyperopia Astigmatism

Emmetropia : 

Emmetropia Adequate correlation OR matching between axial length and refractive power of the eye Rays of light from a distant object are brought to a pin-point sharp focus on the retina (no accommodation) All refractive errors are some deviation from emmetropia

MYOPIA : 

Most prevalent among Asians (80-90%) followed by 25% of African Americans and 13% of Caucasians. Average age of onset: 8 years Etiology : not clear, genetic factor Causes: excessive refractive power (refractive myopia) excessive long globe (axial myopia) : ‘’more common’’ MYOPIA

MYOPIA : 

Rays of light from distant objects converge in front of the retina, causing a blurred image on the retina The myopes can see close objects clearly, myopia is commonly known as “short-sightedness” MYOPIA

Causes of myopia : 

Causes of myopia Increased refractive power: a) change in lens nucleus or shape cataract, spherophakia, diabetes b) lens repositioning: ciliary muscle shift e.g miotics lens movement e.g anterior lens dislocation c) ciliary muscle tone: excessive accommodation e.g medical students d) increase corneal power: keratoconus, congenital glaucoma

Slide 31: 

2. Increase axial length: congenital glaucoma, posterior staphyloma

Myopia : 

Myopia Myopia Forms: Benign myopia (school age myopia) onset 10-12 years , myopia increase until the child stops growing in height generally tapers off at about 20 years of age Progressive or malignant myopia myopia increases rapidly each year and is associated with , fluidity of vitreous and chorioretinal change

Myopia : 

Myopia Symptoms Blurred distance vision Squint in an attempt to improve uncorrected visual acuity when gazing into the distance Headache Amblyopia – uncorrected myopia > -10 D

Myopia : 

Myopia Morphologic eye changes: Deep anterior chamber Atrophy of ciliary muscle Vitreous may collapse prematurely -->opacification Fundus changes: loss of pigment in RPE , large disc and white crescent-shaped area on temporal side , RPE atrophy in macular area , posterior staphyloma , retinal degeneration-->hole-->increase risk of RD

Hyperopia : 

Hyperopia Parallel rays converge at a focal point posterior to the retina Etiology : not clear , inherited Causes excessive short globe (axial hyperopia) : more common insufficient refractive power (refractive hyperopia)

HYPEROPIA : 

Rays of light from a distant object now focus behind the retina hyperopic persons must accommodate when gazing into distance to bring focal point on to the retina However, this reduces their accommodative reserve when they want to view close objects. This means their distance vision is generally better than their near vision, hence the term “long-sightedness” HYPEROPIA

Causes of Hyperopia : 

Causes of Hyperopia 1. Decreased refractive power of the eye: a) absent (aphakia) or posteriorly repositioned lens b) weak accommodation trauma, marijuana 2. Decreased effective axial length(retina pushed forward): tumor, orbital mass

Hyperopia : 

Hyperopia Symptoms visual acuity at near tends to blur relatively early inability to read fine print asthenopic symptoms : eyepain, headache in frontal region accommodative esotropia : because accommodation is linked to convergence -->ET Amblyopia – uncorrected hyperopia > +5D

Slide 41: 

ASTIGMATISM Cornea is usually shaped like half a football. In these eyes there will be no astigmatism. Parallel rays come to focus in 2 focal lines rather than a single focal point Etiology : heredity Cause : refractive media is not spherical-->refract differently along one meridian than along meridian perpendicular to it-->2 focal

Astigmatism : 

Astigmatism Classification Regular astigmatism : power and orientation of principle meridians are constant With the rule astigmatism , Against the rule astigmatism , Oblique astigmatism Irregular astigmatism : power and orientation of principle meridians change across the pupil

Causes of astigmatism : 

Causes of astigmatism Corneal causes: a) simple corneal astigmatism b) Keratoconus c) Masses e.g lid tumor d) Ptosis Lenticular causes: Lens dislocation, lenticonus

Astigmatism : 

Astigmatism Symptoms asthenopic symptoms (headache , eye pain) blurred vision distortion of vision head tilting and turning Amblyopia – uncorrected astigmatism > 1.5 D

Slide 46: 

ANISOMETROPIA A difference in refractive error between the two eye Individuals can tolerates up to 2-3D of anisometropia before becoming symptomatic Refractive correction often leads to different image sizes on the 2 retinas (aniseikonia) Aniseikonia depend on degree of refractive anomaly and type of correction

Presbyopia : 

Presbyopia Physiological loss of accommodation in advancing age Deposit of insoluble proteins in the lens with advancing age-->elasticity of lens progressively decrease-->decrease accommodation around 40 years of age , accommodation become less than 3 D-->reading is possible at 40-50 cm-->difficultly reading fine print , headache , visual fatigue

Correction of refractive errors : 

Correction of refractive errors Far point: a point on the visual axis conjugate to the retina when accommodation is completely relaxed placing the imaging of the object at far point will cause a clear image of that object to be relayed to the retina use correcting lenses to form an image of infinity at the far point , correcting the eye for distance

Types of optical correction : 

Types of optical correction Spectacle lenses Monofocal lenses : spherical lenses , cylindrical lenses Multifocal lenses Contact lenses higher quality of optical image and less influence on the size of retinal image than spectacle lenses indication : cosmetic , athletic activities , occupational , irregular corneal astigmatism , high anisometropia , corneal disease

Slide 50: 

Contact lenses disadvantages : careful daily cleaning and disinfection , expense complication : infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe chronic conjunctivitis

Surgical correction : 

Surgical correction Keratorefractive surgery : Refractive surgery – flattens corneal surface for myopia Improves unaided visual acuity but may have complications e.g PRK, LASIK,LASEK Intraocular surgery : give best optical correction for aphakia , avoid significant magnification and distortion caused by spectacle lenses clear lens extraction (with or without IOL), phakic IOL

Slide 52: 

LASIK – State of the Art Laser Assisted Stromal In-situ Keratomileusis

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