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ERROR OF REFRACTION

RefractionDEFINITIONRefraction is defined as the ratio of the refractive power of the lens and cornea (the refractive media) to the axial length of the globe.Emmetropia is absence of refractive error, and ametropia is the presence of refractive error.

EMETROPIA (NORMAL EYE)Light cornea aqueous humor pupil aqueous humor Lens Vitreous Humor Retina Optic Nerves Brain

REFRACTION OF NORMAL EYEThere are four transitional refraction media trought by light :

Transition between air anterior surface of cornea

Transition between posterior surface of cornea - humor aqueous

Transition between humor aqueous-anterior surface of lens

Transition between posterior surface of lens-vitreous humor

ERROR OF REFRACTIONMyopia Hyperopia(Hypermetropia): Astigmatism Presbyopia

MYOPIA

Myopia A discrepancy between the refractive power and axial length of the eye such that parallel incident light rays converge at a focal point anterior to the retina

MyopiaParallel rays converge at a focal point anterior to the retinaEtiology : not clear , genetic factorCausesexcessive long globe (axial myopia) : more commonexcessive refractive power (refractive myopia)

The etiology of myopia is not clear. Familial patterns of increased incidence suggest the influence of genetic factors.

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FormsBased on desease developmentStasioner Myopia/school age myopia persistent when adultProgressif Myopia progressif altought in adult, because increase eye ball lenghtMalignant Myopia progressif, can make retinal ablasion and blindness

SymptomsBlurred distance visionSquint in an attempt to improve uncorrected visual acuity when gazing into the distanceSpace between palpebra is decreasedHeadacheAmblyopia uncorrected myopia > 10 D

Myopia Correction The choosen lens should be the biggest positive which can give the visus measurement 6/6.For example, if correction with -3.00 can give visus 6/6 and the correction with -3.25 can give visus 6/6, we chose the the first one.

Hypermetropia

HypermetropiaHyperopia: there is a discrepancy between the refractive power and axial length of the eye such that parallel incident light rays converge at a focal point posterior to the retina.

SymptomsVisual acuity at near tends to blur relatively early blurred vision is more noticeable if person is tired , printing is weak or light inadequateasthenopic acommodative symptoms : eyepain, headache in frontal region, burning sensation in the eyes.accommodative esotropia : because accommodation is linked to convergenceAmblyopia uncorrected hyperopia > 5D

Special forms of refractive hyperopia:Absence of the lens (aphakia) due to dislocation.Postoperative aphakia following cataract surgerywithout placement of an intraocular lens

Hypermetropia CorrectionThe choosen lens should be the biggest positive which can give the visus measurement 6/6.For example, if correction with +3.00 can give visus 6/6 and the correction with +3.25 can give visus 6/6, we chose the the second one.

Astigmatism

AstigmatismAstigmatism : The disorder is characterized by a curvature anomaly of the refractive media such that parallel incident light rays do not converge at a point but are drawn apart to form a line.

AstigmatismEtiology : heredity Cause : refractive media is not spherical refract differently along one meridian than along meridian perpendicular to it 2 focal points ( punctiform object is represent as 2 sharply defined lines)

ClassificationAstigmatism can be classified as follows:External astigmatism: astigmatism of the anterior surface of the cornea.Internal astigmatism: the sum of the astigmatic components of the other media.

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Classification

Astigmatism can also be classified according to the location of the meridian of greater refraction:With-the-rule astigmatism (most common form): The meridian with the greater refractive power is vertical, i.e., between 70 and 110 degrees.Against-the-rule astigmatism: The meridian with the greater refractive power is horizontal, i.e., between 160 and 20 degrees.

ClassificationOblique astigmatism: The meridian with the greater refractive power is oblique, i.e., between 20 and 70 degrees or between 110 and 160 degrees.

TreatmentTreatment of regular astigmatism: The purpose of the correction is to bring the focal lines of two main meridians together at one focal point. This requires a lens that refracts in only one plane. Cylinder lenses are required for this application.Once the two focal lines have been converged into a focal point, additional spherical lenses can be used to shift this focal point on to the retina if necessary.

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TreatmentTreatment of irregular astigmatism: This form cannot be corrected with eyeglasses. External astigmatism may be managed with a rigid contact lens, keratoplasty, or surgical correction of the refractive error.Irregular internal astigmatism is usually lens-related. In this case, removal of the lens with implantation of an intraocular lens is indicated.

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Presbyopia

PresbyopiaPresbyopia happens naturally in people as they age. The eye is not able to focus light directly on to the retina due to the hardening of the natural lens. Aging also affects muscle fibers around the lens making it harder for the eye to focus on up close objects. The ineffective lens causes light to focus behind the retina, causing poor vision for objects that are up close.

PresbyopiaPhysiologic loss of accommodation in advancing agedeposit of insoluble proteins in lens in advancing age elasticity of lens progressively decreasedecrease accommodationDecrease cilliary muscle contraction

PresbyopiaTreatmentconvex lenses in near visionReading glassesBifocal glassesTrifocal glassesProgressive power glasses

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