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Refractive Errors OPTICS When light rays passes through space 3 things can happen 1-If it strikes on opaque medium e.g. black object- absorbs light 2-If it strikes a mirror, it is reflected 3-If it strikes a transparent medium, it changes its path. This is called refraction OPTICS THIS BENDING OF LIGHT DEPENDS ON 3 FACTORS: 1-The difference of refractive indices of the two media 2-The angle at which the light rays strikes the medium 3-The wavelength of light (blue light bends more as compared to red light) REFRACTION EMMETROPIA AMMETROPIA EMMETROPIA When incident parallel rays of light from infinity come to focus on the retina (fovea centralis) With accommodation at rest, the eye is considered to be emmetropic. It is the normal optical condition of eye with no errors of refraction. AMMETROPIA ►Myopia ►Hypermetropia ►Astigmatism ERRORS OF REFRATION AMETROPIA When incident parallel rays of light from infinity do not focus on the retina with accommodation at rest, the condition is called ametropia AMETROPIA 1-AXIAL AMETROPIA: abnormal length of eyeball. 2-CURVATURE AMETROPIA: abnormal curvature of refracting surface of cornea and lens. 3-INDEX AMETROPIA: abnormal refractive index of media 4-ABNORMAL POSITION OF LENS: forward or backward displacement of lens. MYOPIA When the incident parallel rays of light coming from infinity focus at a point in front of retina ,with the accommodation at rest ,eye is said to be myopic. (Refractive power is more than required for the available axial length) MYOPIA ETIOLOGY: Axial: increase length of eyeball Curvature: increase curvature of cornea of lens Index: increase refractive index of lens nucleus Forward displacement of lens MYOPIA TYPES: 1-Simple 2-Pathological SIMPLE MYOPIA: No degenerative changes in fundus Doesn't progress after adolescence Doesn't progress more than -5 or -6 D PATHALOGICAL MYOPIA: Degenerative and progressive Begins at the age of 5-10 years May attain a degree of -15 to -20 D It is strongly hereditary MYOPIA SIGNS: Prominent eyes, large pupil, deep Ant. Chamber Apparent divergent squint may be present MYOPIA SIGNS: Fundus: 1-optic disc: temporal crescent posterior staphyloma 2-macula: chorioretinal degeneration foster fush’s flecks 3-periphery: holes and tears cystoid degeneration of ora serrata tesselated fundus MYOPIA • COMPLICATIONS: Vitreous degenerations, opacities, detachment Tears and hemorrhage Retinal detachment Chronic simple glaucoma MYOPIA TREATMENT: Spectacles: suitable spherical concave lenses Operative: Radial keratotomy Excimer laser Epikeratophakia Keratomileusis LASIK HYPERMETROPIA When the incident parallel rays of light coming from infinity focus at a point behind the retina, with the accommodation at rest, eye is said to be hypermetropic. (Refractive power is less than required for the given Axial Length) All new borns are hyperopic (average 2.5 D) HYPERMETROPIA ETIOLOGY: Axial: shorter length of eyeball Curvature: flat cornea curvature Index: Decreased refractive index of lens Backward displacement of lens Aphakia HYPERMETROPIA 1- LATENT HYPEROPIA: Overcome by normal tone of ciliay muscle Detected by using atropine to paralyze ciliary's muscle (Cycloplegia) 2-MANIFEST HYPEROPIC: Detected without ciliary's muscle paralysis HYPERMETROPIA SYMPTOMS: Blurring of vision Frontal headache and eye strain In adults, presbyopia commences earlier SIGNS: Shallow Ant. Chamber, prone to close angle glaucoma Accommodative convergent squint Fundus may be normal or pseudopapillitis may be present TREATMENT: ASTIGMATISM It is a condition in which a point of light cannot be made to produce a punctate image upon the retina by any spherical correcting lens ETIOLOGY: Unequal curvature of cornea in different meridians Decentring of lens ASTIGMATISM TYPES: 1-Regular a) simple myopic hypermetropic b)compound myopic hypermetropic c)mixed 2-Irregular ASTIGMATISM 1-REGULAR ASTIGMATISM: It is present when two principle meridians are at right angle to each other, it can be corrected by lenses WITH THE RULE ASTIGMATISM: vertical meridian is more curved ,as in normal cornea AGAINST THE RULE ASTIGMATISM: horizontal meridian is more curved ASTIGMATISM 1-REGULAR ASTIGMATISM: a- simple: one focus is on the retina other is in front of retina in simple myopic behind the retina in simple hyperopic b-compound: both focus are in front of retina in compound myopic behind the retina in compound hypermetropic c-mixed: one focus is in front of retina Other behind the retina ASTIGMATISM 2-IRREGUALR ASTIGMATISM: It is present when corneal surface is irregular, it can not be adequately corrected by lenses ASTIGMATISM SYMPTOMS: Diminished visual acuity Eye strain and headache after near work Letters in the book appear to be running together TREATMENT: Suitable cylindrical lenses for regular astigmatism For irregular astigmatism hard contact lens keratoplasty APHAKIA • Absence of lens is called aphakia OPTICAL DEFECTS: • Eye is highly hypermetropic • Complete loss of accommodation • Presence of astigmatism SYMPTOMS: • Gross dimness of vision due to acquired high hyperopia SIGNS: • • • • Deep anterior chamber Iridodonesis Jet black pupil Corno-scleral scar mark at upper limbus in case of surgical aphakia APHAKIA TREATMENT: • Correction by spectacles: by prescribing suitable spherical convex lens (about +10 D) • Contact lens • Intraocular lens implantation: after calculating the power, standard power is +20 D, it can be implanted in anterior or posterior chamber PRESBYOPIA It’s the insufficiency of accommodation due to advancing age, usually 40 years. It is not an error of refraction. CAUSE Physiological failure of the lens to accommodation due to hardening of lens with age weakness of ciliary’s muscle and suspensory ligament PRESBYOPIA SYMPTOMS: blurring of vision on near work People involve in close work appreciate the symptom earlier Depends on existing error of refraction i.e. early onset of hypermetropia, delayed onset in myopia. SIGNS: Impaired near vision, tested by near chart TREATMENT: By prescribing suitable convex spherical lens for near work This correction is added to the correction for distant vision