Refractive error

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Refractive error

  1. 1. Refractive error
  2. 2. Emmetropia Adequate correlation between axial length and refractive power Parallel light rays fall on the retina (no accommodation)
  3. 3. Ametropia (Refractive error) Mismatch between axial length and refractive power Parallel light rays don’t fall on the retina (no accommodation) Nearsightedness (Myopia) Farsightedness (Hyperopia) Astigmatism Presbyopia :NOT REFFERECTIVE ERROR
  4. 4. Accommodation Emmetropic eye object closer than 6 M send divergent light that focus behind retina , adaptative mechanism of eye is increase refractive power by accommod ation  theory contraction of ciliary muscle -->decrease tension in zonule fibers -- >elasticity of lens capsule mold lens into spherical shape -->greater dio ptic power -->divergent rays are focused on retina contraction of ciliary muscle is supplied by parasympathetic third nerve
  5. 5. Myopia Parallel rays converge at a focal point anterior to the retina Etiology : not clear , genetic factor Causes excessive long globe (axial myopia) : more common excessive refractive power (refractive myopia)
  6. 6. Myopia Forms Benign myopia (school age myopia) onset 10-12 years , myopia increase until the child stops growing in height Progressive and malignant myopia interchangeable  myopia increase rapidly each year and is associated with vitreous opacities , fluidity of vitreous and chorioretinal change rate of increase in amount of myopia generally about 20 years of age
  7. 7. Myopia Congenital myopia Myopia > 10 D Increase slowly each year
  8. 8. Myopia Special forms : nuclear sclerosis , keratoconus , spherophakia Symptoms Blurred distance vision Squint in an attempt to improve uncorrected visual acuity when gazing into the distance Headache Amblyopia – uncorrected myopia > 10 D
  9. 9. Myopia Morphologic changes deep anterior chamber atrophy of ciliary muscle vitreous may collapse prematurely -->opacification fundus change : loss of pigment in RPE , large disc and white crescent- shaped area on temporal side , RPE atrophy in macular area , posterior st aphyloma , retinal degeneration-->hole-->increase risk of RD Treatment : concave lenses, clear lens extraction
  10. 10. 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)
  11. 11. Hyperopia Special forms : lens dislocation , postoperative aphakia hyperopic persons must accommodate when gazing into distance to bring focal point on to the retina Symptoms distance vision is impaired in high refractive error( > 3 D) and in older patient
  12. 12. Hyperopia Symptoms visual acuity at near tends to blur relatively early nature of blur is vary from inability to read fine print to near vision is clear but suddenly and intermittently blur blurred vision is more noticeable if person is tired , printing is weak or light inadequate asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis accommodative esotropia : because accommodation is linked to convergence -->ET Amblyopia – uncorrected hyperopia > 5D
  13. 13. Hyperopia Fundus in axial hyperopia may reveal pseudooptic neuritis (indistinct disc margin, no physiologic cup, may elevate disc) DDx from optic neuritis by > 4 D , no enlarged blind spot, no passive congestion of vein Treatment : convex lenses, keratorefractive surgery, refreactive lensectomy with IOL, phakic IOL
  14. 14. 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 points ( punctiform object is represent as 2 sharply defined lines)
  15. 15. Astigmatism Classification Regular astigmatism : power and orientation of principle meridians are constant With the rule astigmatism , Against the rule astigmatism , Oblique astigmatism  Simple or Compound myopic astigmatism , Simple or Compound hyperopic astigmatism , Mixed astigmatism Irregular astigmatism : power and orientation of principle meridians change across the pupil
  16. 16. Astigmatism Symptoms asthenopic symptoms ( headache , eyepain) blurred vision distortion of vision head tilting and turning Amblyopia – uncorrected astigmatism > 1.5 D Treatment Regular astigmatism :cylinder lenses with or without spherical lenses(convex or concave), Sx Irregular astigmatism : rigid CL , surgery
  17. 17. Presbyopia Physiologic loss of accommodation in advancing age deposit of insoluble proteins in lens in advancing age-- >elasticity of lens progressively decrease-->decrease accom modation around 45 years of age , accommodation become less than 3 D-->reading is possible at 40-50 cm-->difficultly reading fin e print , headache , visual fatigue
  18. 18. Presbyopia Treatment convex lenses in near vision Reading glasses Bifocal glasses Trifocal glasses Progressive power glasses
  19. 19. Anisometropia Difference in refractive power between 2 eyes refractive correction often leads to different image sizes on the 2 retinas( aniseikonia) aniseikonia depend on degree of refractive anomaly and type of correction closer to the site of refraction deficit the correction is made-->less retinal image changes in size
  20. 20. Anisometropia Glasses : magnified or minified 2% per 1 D Contact lens : change less than glasses Tolerate aniseikonia ~ 5-8% Symptoms : usually congenital and often asymptomatic Treatment anisometropia > 4 D-->contact lens unilateral aphakia-->contact lens or intraocular lens
  21. 21. Correction of refractive errors Far point point on the visual axis conjugate to the retina when accommodation is completely relaxed placing an object or imaging an 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
  22. 22. 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
  23. 23. Contact lenses disadvantages : careful daily cleaning and disinfection , expense complication : infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe chronic conjunctivitis Intraocular lenses replacement of cataract crystalline lens give best optical correction for aphakia , avoid significant magnification and distortion caused by spectacle lenses
  24. 24. Surgical correction Keratorefractive surgery :RK, AK, PRK, LASIK, ICR, thermokeratoplasty Intraocular surgery : clear lens extraction (with or without IOL), phakic IOL

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