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OCULAR REFRACTION

OCULAR REFRACTION. Visual anal y ser performances. Phisical status of refractive components (correct focus on the retina); Retinian neurons quality; Brain neurons quality. Ocuar refraction is defined by the anatomical components which contribute on light array optical modulation.

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OCULAR REFRACTION

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  1. OCULAR REFRACTION

  2. Visual analyser performances • Phisical status of refractive components (correct focus on the retina); • Retinian neurons quality; • Brain neurons quality.

  3. Ocuar refraction is defined by the anatomical components which contribute on light array optical modulation

  4. transparentmediums • lacrimal layer • cornea • aqueoushumour • lens • vitrious

  5. Ocular refraction • static refraction • dinamic refraction

  6. Subjects • Ametropia • Hyperopia • Myopia • Astigmatism • Anizometropia • Accomodation • Presbyopia

  7. EMETROPIA • emetropia is that optical condition in which there is no refraction error so that rays of parallel to the visual axis appear entering the eye are brought to a focus on the fovea centralis when no accommodation is used

  8. AMETROPIA classification • spherical or stigmic disorders in which enter: • myopia • hyperopia

  9. asferical or astigmic disorders in which we include astigmatisms: • regular • iregular

  10. HYPEROPIA • refractive condition of the eye in which (with accommodation suspended) parallel rays of light are intercepted by the retina before coming to focus

  11. Clasification of hyperopia • Axial = when the globe is too short to the amount of the refractive power present; • Refractive = when the power of the cornea and lens is inadequate for the length of the globe

  12. Type of hyperopia • Total hyperopia= amount of the hyperopia with all accommodation suspended (by using the cycloplegic drugs); • Manifest hyperopia= is the maximum hyperopia that can be corrected with a convex lens when accommodation is active; • Latent hyperopia= is the difference between total and manifest hyperopia.

  13. Signs and symptoms • Visual acuity is reduced when accommodation is not able to fully substitute the hyperopia; • Vision is blurred, especially after near work; • The increased of the accommodation may stimulate an excessive degree of convergence, manifested as a tendency of for the eyes to deviate inward (esodeviation); • The cornea is smaller than normal, and even the globe itself may be smaller;

  14. Oftalmoscopy • optic disk congestive, with indistinct margins and the absence of the physiologic cup (pseudopapiledema)

  15. Treatment • convex lenses (glases or contact lenses) • corneal refractive surgery: • Laser thermal keratoplasty • Lamellar keratoplasty • Photorefractive keratectomy • Laser in situ keratomileusis (LASIK) • phakic intraocular lenses using iris-claw IOL; • clear lens extraction with posterior chamber lens implantation.

  16. LASIK

  17. Myopia • optical condition in which rays of light entering the eye parallel to the visual axis come to focus in front of the retina

  18. Clasification of myopia • Axial = because the refractive power of the cornea and lens is too great for the length of the eye; • Refractive = because the eye is to long for the refractive power present

  19. physiologic myopia • refractive (lenticularmyopia – is refractive myopia related to the lens power) • axial • pathologic or degenerative myopia – is axial myopia

  20. physiologic myopia • is the most common type • onset usually between 5 and 10 years of age but may begin as late as 25 years age • gradually increase until the eye is fully grown about 18 years of age • seldom exceeds 6 diopters.

  21. pathologic or degenerative myopia Symptoms • decreased vision • axial length of the eye is excessive, primarily because of overgrowth of the posterior two thirds of the globe

  22. oftalmoscopy • crescent of the optic disc that begins at the temporal side and progresses to surround the disc. • Staphyloma of the posterior pole . • hyperpigmented spot in the macula (Fuchs spot). • degeneration of the retinal pigment epithelium • choroidal sclerosis. • peripheral retinal thinning with lattice degeneration and retinal breaks.

  23. Treatment • is neutralized by concave lenses • contact lenses (in high myopia, anizometropia); highly fitted contact lenses may temporarily reduce the corneal curvature and thus may show an apparent decrease in myopia

  24. Refractive surgery • radial keratotomy acts through peripheral radial incision that flatten the central cornea • photorefractive keratectomy (PRK) ablates tissue directly from central cornea. • laser in situ keratomileusis (LASIK) excimer application (PRK) after creation of a hinged flap • posterior chamber phakic intraocular lens (implantable ocular lens) • intracorneal implants

  25. For high myopia • laser photocoagulation for symptomatic retinal breaks • extrafoveal or juxtafoveal laser photocoagulation therapy • combination between LASIK and phakic intraocular lens = “BIOPTIC” • scleral enforcement to stop or to retard the progression of disease (controversial and under investigation

  26. ASTIGMATISM • optical condition in which the refractive power of the eye is not the same in all meridians. • variation in the curvature of the cornea or lens at different meridians induces light rays focusing to more than one point.

  27. Classifications • irregular = when the meridian are not at right angle to each other • regular = when the meridian of minimal and maximal refraction are at right angle to each other. Regular astigmatism can be: • with the rule – when the vertical meridian is steepest and a correcting plus cylinder should be placed at axis 90° • against the rule – when the horizontal meridian is steepest and a correcting plus cylinder should be placed at axis 180° • oblique astigmatism – the principal meridian do not lie at or close to 90° and 180°

  28. Classifications II • simple: when one meridian is emetropic and other is ametropic (myopic and hypermetropic) • composed: when both meridian are ametropic • myopic = both meridian are in front of the retina • hypermetropic = both meridian are intercepted by retina before coming to a focus • mixed astigmatism: one meridian is myopic and other is hypermetropic

  29. Classifications III • congenital astigmatism • acquired astigmatism: • surgical incision o the cornea • trauma and scarring of the cornea • tumors of the eyelid pressing upon the globe

  30. Pathogenic aspects • parallel rays of light do not focus at a point. Each meridian has a focal line; the distance between these focal lines is the “interval of Sturm”. Between the focal lines, in the circle of least diffusion (conffusion), the diverging and converging tendency of the light rays is the same. In this area the image is clear enough to satisfy the patient

  31. Symptoms and signs • the image is not clear • the accommodation is interfered • severe astigmatism may cause the optic disc to appear oval rather then near circular

  32. Treatment • Minor degree of astigmatism with good VA and without symptoms – correction is not indicated • Simple astigmatism – cylindrical lens placed in the meridian 90 degrees away from the axis of the ametropic meridian • Composed and mixed astigmatism – sphero-cylindric lenses (combination of a sphere and a cylinder -plus or minus- )

  33. Hard contact lenses may be used to correct regular astigmatism and especially the irregular astigmatism • Surgery • incisional surgery (relaxing incisions, block resection, compressive suture) is indicated in congenital regular astigmatism • excisional surgery (excimer laser – PRK and LASIK) is recommended both in regular and irregular astigmatism

  34. ACCOMMODATION • process by which the refractive power of the anterior lens segment increases so that a near object may be distinctly imaged upon the retina

  35. complex reflex • the stimulus is a blurred retinal image • this is coming to brain • different area of the brain send order to the periphery • stimulation of the short ciliary branch of the oculomotor nerve constricts or relaxes the circular ciliary muscle • the eye almost instantly adjust to provide clear vision.

  36. For near vision • contraction of the ciliary muscle causes • the zonulae fibers to relax • then lens become more convex • the power is increased • the focal line is projected onto retina.

  37. For long distance • ciliary muscle relaxes • the zonulae fibers are pull and tensioned • the convexity of the lens surface becomes minimal

  38. amplitude of accommodation • is distance between far point of the eye and nearest point at which the eye can maintain focus • amplitude of accommodation is properly a monocular expression and is measured for each eye independently

  39. accommodation associated reactions • convergence of the eyes • pupilary constriction (miosis)

  40. presbyopia • with ageing • lens capsule becomes less elastic • nucleus becomes harder and less compressible • this causes a gradual loss of accommodation

  41. Symptomes • inability to see near work distinctly • patient places reding mater farther away from the eyes than previously • ocular discomfort

  42. Evolution of accomodation • 10 years age - 14Dpt. • 45 years age- 1Dpt., • 50 years age - 2Dpt. , • 55 years age - 2.5Dpt. , • 60 years age - 3Dpt. , • 70 years age - 3.5Dpt

  43. Special situations • Myopic patients may compensate for presbyopia by removing the lens that corrects the distance vision. • Presbyopia is aggravated in a hypermetropic patients if the lens that corrects the hypermetropia is removed

  44. Treatment • convex lenses added to the distance correction; • Rules: • the weakest possible convex to permit the individual to carry an vocational and avocational tasks; • if a subject requires lenses for distance, bifocal, trifocal or multifocal lenses should be worn as soon as are indicated

  45. Convergence insufficiency (CI) • the inability to maintain fusion at near as a result of a reduced amplitude of fusional convergence power. • Symptomes: • eye discomfort • headache • blurred vision from reading or doing near work • Signs: • reduced amplitude of accommodation • exoforia

  46. Accomodative insufficiency (AI) • symptoms develops after 20-40 min of reading (same as CI) but: • these patients have normal fusion capacities when a 4 diopters base in prism is placed in front of the eye while reding • patients with AI benefit from reading glases

  47. Accomodative spasm (AS) • inability to relax ciliary muscle (involuntary) • is associated with stressful situations or functional nevroses • Symptoms: • bilateral blurred distance vision • headache, • fluctuating vision • Sings: • cycloplegic refraction reveals hyperopia • abnormally close near point of focus • miosis

  48. ANISOMETROPIA • condition in which the refractive error of each eye is different • the different is more than 2 diopter there is a different in image size of the two eyes named anizeiconia • total amount of aniseiconiasuported by patient must be less then 8%

  49. several problems • the difference of the power of the two lenses (bifocal lenses) induce a vertical prism, so that the image from each eye is on a different level (anisophoria); • severe anisometropia may cause ambliopia because of the developing infant`s failure to use the eye with greater refractive error; • failure of central vision leads to strabismus, to absence of binocular vision or to deficiency of binocular vision

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