Errors 1
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This document discusses errors of refraction and accommodation. It defines key terms like diopter, focal length, real and virtual images. It describes different types of refractive errors like myopia, hyperopia and astigmatism. It discusses causes, characteristics and management of these refractive errors. Accommodation and presbyopia are also explained. Different refractive surgery procedures for correction of refractive errors are outlined.
Errors 1
- 1. ERRORS OF REFRACTIO NMOHAMED ABDELZAHER MD, FRCS
- 2. Change in the direction of rays as they pass between media of different refractive index.
- 3. Magnifying lens Minifying lens • Diopter is the unit power of lenses • Diopter = 1 / Focal length in meters Real Image Virtual Image
- 4. • Diopter is the unit power of lenses • Diopter = 1 / Focal length in meters
- 5. • A cylinder is specified by its axis • The axis of a cylindrical lens is parallel to the cylinder from which the lens cut • The power of a cylinder in its axis meridian is zero. • Maximum power is 90 degrees away from the axis. This is known as the power meridian. • The image formed by the power meridian is a focal line parallel to the axis. • There is no line focus image formed by the axis meridian, because the axis
- 6. • Carries both spherical & cylindrical correction
- 7. • Prism deviates light toward its base, thus the image is displaced toward its apex • Prism Diopter = Displacement of image for 1 cm at 1 m distance
- 8. Converging Diverging Magnifying Minifying Tow Prisms Base to Base Two Prisms Apex to Apex
- 9. The instrument used for measurement of lenses & prisms power is the Lensmeter.
- 10. • Change in the refractive power of the eye to maintain a clear focus on variable distances. • For near vision, the ciliary muscle contracts, reducing the zonular tension & allowing the elastic capsule of the lens to contract, causing a decrease in equatorial lens diameter and an increase in the curvatures of the anterior and posterior lens surfaces.
- 11. • Accommodation power decreases with age (Presbyopia)
- 12. • Far Point (Punctum Remotum, PR): Position of an object when its image clearly falls on the retina with no accommodation. • Near Point (Punctum Proximum, PP): The nearest point clearly seen with maximum accommodation • Range of accommodation: The distance between far & near point • Amplitude of Accommodation: The difference in Dioptric power between eye at rest & eye at maximum accommodation
- 13. • Far Point (Punctum Remotum): Emmetropic eye; at infinity Myopic eye; anterior to the eye Hyperopic eye; virtual point behind the eye
- 14. • Near Point (Punctum Proximum): Presbyopia is physiological recession of near point of accommodation N’ Normal near point N Near point in Presbyopia
- 15. • The amount of convergence measured in prism diopters per unit (diopter) change in accommodation • Normal Value: 3- 5 Prism Diopter / 1 D. • Abnormalities in AC/A ratio causes strabismus
- 16. • Visual Axis: Line passing between Object of fixation & Fovea. • Optic Axis: Line passing through the centre of refractive media of the eye. • Angle Alpha : Angle between visual axis & optic axis (Normally +5 degrees) • Nodal point of the eye: Point through which light passing undeviated
- 17. • Angle between visual axis & optic axis (Normally +5 degrees) • Normally, Visual axis cuts cornea NASAL to the optic axis
- 18. • Parallel light rays come into a focus on the retina, when accommodation is relaxed. • No refractive error is present.
- 19. • Parallel light rays come into a focus anterior to the retina, when accommodation is completely relaxed. • AKA Near sightedness • PR is in front of the eye, (MR & M’ are conjugate points)
- 20. A. Axial: Long axial length (Average AXL is 22 - 23 mm) B. Refractive: 1. Index; Increased RI of lens nucleus e.g. nuclear cataract or decrease RI of lens cortex e.g. DM 2. Curvature; increase curvature of cornea (KCN) or lens (Subluxation or lenticonus)
- 21. 1. Simple myopia 2. Degenerative (Progressive) myopia 3. Nocturnal myopia: Insufficient contrast, because of dim illumination for an adequate accommodative stimulus, the eye assumes the intermediate dark focus accommodative position rather than focusing for infinity 4. Pseudo-myopia: Increase in ocular refractive power due to overstimulation of the eye's accommodative mechanism e.g. during exams or ciliary spasm 5. Induced (acquired) myopia: Result from exposure to various pharmaceutical agents e.g. Phenothiazine, active ocular inflammation (uveitis), variation in blood sugar levels, nuclear sclerosis of the crystalline lens, or other anomalous conditions. This myopia is often temporary and reversible.
- 22. Nocturnal myopia Causes of Night Myopia: •Spherical aberration (The optical effect of the larger pupil decreases the depth of focus) •In dark adaptation the eye becomes more sensitive to shorter wave lengths (Blue) •Chromatic aberration: Shorter wavelengths come into focus in front of the retina •Ciliary spasm: the eye assumes the intermediate dark focus accommodative position rather than focusing for infinity
- 23. 1. Low Myopia: < 3D 2. Medium myopia: 3 -6 D 3. High myopia: > 6D 1. Congenital: Present since birth 2. Youth onset: < 20 years 3. Adult onset: > 20 years
- 24. Simple Myopia Progressive Myopia Common Less Common Starts at Puberty Starts at younger age Usually less than 6 D Up to 25 D or more Normal fundus Degenerative retinal changes
- 25. • Blurred far vision • Good near vision • Young age • Frowning in trial to improve vision • Defective night vision (Dilated pupil increases effect of aberrations) • Retinal symptoms: musca, photopsia (In high myopia > 6 D)
- 26. • Peri-papillary myopic annulus or temporal crescent Of High Myopia (Chorio-retinal degeneration)
- 27. • Tigroid (Tessellated) Fundus • Caused by atrophy of the retinal pigment epithelium • Underlying streaks of normal choroidal pigmentation are visible
- 28. • Lacquer Cracks Cracks in Bruchs membrane
- 29. • Fuchs spots Cracks in Bruchs membrane allows neovessels to proliferate under the retina & causes recurrent retinal haemorrhage
- 30. • Posterior staphyloma • Posterior protrusion of week sclera lined with choroid
- 31. • Vitreous floaters • Caused by vitreous degeneration or posterior vitreous detachment
- 32. • Peripheral Retinal Degeneration • e.g. Lattice degeneration • Predisopse to rhegmatogenous RD
- 34. • Nuclear cataract Of High Myopia • Open Angle Glaucoma
- 35. • Rhegmatogenous Retinal Detachment, caused by peripheral retinal degeneration Of High Myopia • Macular Hole
- 36. • Squin t Of High Myopia In high myopia, the fovea lies NASAL to the optic axis. So, the visual axis crosses temporal to the optic axis at the cornea. (-ve angle alpha)
- 37. • Squin t Of High Myopia Right exotropia when fixating at a 6 metres objection (A and B). Eyes straight with myopic correction (C). Exotropia disappeared when the eyes looked at a near object (D). Right exotropia worsened when the eyes looked at an object further than 6 metres (E).
- 38. Concave Lenses Concave Lenses Indications: A. Cosmetic B. Anisometropia Advantages: A. Less minification of image, 10% (nearer to the nodal point of the eye) B. Less aberration of lens periphery (wider field) Prescribe the lowest correctio n that gives the best vision
- 39. Indications: A. Cosmetic B. Anisometropia • A corneal flap is fashioned by an automated keratome • Excimer laser is used to ablate the central corneal stromal bed, thus reducing the curvature of the central part of the cornea • The flap is re-positioned • Can correct up to - 12 D according to corneal thickness & curvature
- 40. • A corneal flap is fashioned by an Femto laser • Excimer laser is used to ablate the corneal stromal bed, thus reducing the curvature of the central part of the cornea • The flap is re- positioned LASIK
- 41. • A lenticule of corneal storm is fashioned using the femto laser • The lenticule is extracted through a curvilinear small inferior corneal incision SMILE
- 42. Indications: A. Thin corneas B. Low refractive errors • Corneal epithelium is removed either mechanically or with alcohol • Excimer laser is used to ablate the corneal stromal bed, thus reducing the curvature of the central part of the cornea • No flap • Can correct up to - 4 D according to corneal thickness & curvature
- 43. Indications: A. Thin corneas B. High refractive errors
- 44. Indications: A. Thin corneas B. High refractive errors C. Old age > 40 years Disadvantages: A. Loss of accommodation B. High incidence of post-operative complications e.g. RD
- 45. • Radial Partial thickness cuts in corneal stroma • Increases the curvature of corneal periphery • Obsolete
- 46. • Parallel light rays come into a focus posterior to the retina, when accommodation is completely relaxed. • AKA Far sightedness • PR is a virtual point behind the eye, (MR & M’ are conjugate points)
- 47. A. Axial: Short axial length (Average AXL is 22 - 23 mm) B. Refractive: 1. Index; Decreased RI of lens e.g. cortical cataract 2. Curvature; decrease curvature of cornea (Cornea Plana) 3. Aphakia 4. Posterior Dislocation of the lens
- 48. 1. Total (Ht): Amount of hyperopia measured under the effect of Atropine 2. Manifest (Hm): Amount of hyperopia measured without Atropine 3. Latent (Hl): Amount of hyperopia caused by CB tone (1D) 4. Facultative (Hf): Amount of hyperopia corrected by Accommodation 5. Absolute (Ha): Amount of hyperopia NOT corrected by accommodation (to be corrected optically) • Ht = Hm + Hl • Hm = Hf + Ha • In young age with stronger accommodation, most, if not all Hm is Hf • Hf decreases with age as accommodation gets weaker
- 49. • None, in young age with small error because of strong accommodation • Poor near vision • Better far vision (later becomes poor, virtual PR) • Ocular asthenia, because of continuous accommodation • Early presbyopia
- 51. • Crowded Optic disc (Pseudo swelling) • with tortuous retinal vessels Small eye
- 52. • Narrow angle Glaucoma
- 53. • Squin t In high hyperopia, the fovea lies more Temporal to the optic axis than normal. So, the visual axis crosses nasal to the optic axis at the cornea. (large +ve angle alpha, > 5 degrees)
- 54. • Squin t Due to excessive accommodation
- 55. Convex Lenses Convex Lenses Indications: A. Cosmetic B. Anisometropia Advantages: A. Less magnification of image (nearer to the nodal point of the eye) B. Less aberration of lens periphery (wider field) Prescribe the highest correction that gives the best vision
- 56. Indications: A. Cosmetic B. Anisometropia • A corneal flap is fashioned by an automated keratome • Excimer laser is used to ablate the periphery of corneal stromal bed, thus increasing the curvature of the central part of the cornea • The flap is re-positioned • Can correct up to + 6 D according to corneal thickness & curvature
- 57. • A corneal flap is fashioned by an Femto laser • Excimer laser is used to ablate the corneal stromal bed, thus reducing the curvature of the central part of the cornea • The flap is re- positioned LASIK
- 58. Indications: A. Thin corneas B. Low refractive errors • Corneal epithelium is removed either mechanically or with alcohol • Excimer laser is used to ablate the periphery of corneal stromal bed, thus increasing the curvature of the central part of the cornea • No flap
- 59. Indications: A. Thin corneas B. High refractive errors Might not be applicable in some cases with very shallow AC & narrow anterior segment
- 60. Indications: A. Thin corneas B. High refractive errors C. Old age > 40 years Disadvantages: A. Loss of accommodation Might not be a better option than Phakic IOL, especially in cases with narrow AC angle
- 61. Indications: • Low refractive errors Principle: CTK delivers controlled-released radio-frequency current (within peripheral corneal stroma, raising the temperature of the peripheral collagen lamellae to the 65°C resulting in controlled shrinkage of the peripheral collagen lamellae & subsequent central corneal steepening.
- 62. • Parallel light rays do not come into a focus but rather a line.
- 63. A. Cornea: 1. Keratoconus 2. Corneal scars 3. Corneal stitches 4. Pressure over cornea B. Lenticular: Incipient cataract Lenticonus Lens subluxation C. Retinal: e.g. staphyloma in high myopia, because of posterior pole obliquity
- 64. • With (WTR) • Against (ATR) • Oblique • Simple • Compound • Mixed
- 65. The two Principal Meridians (of highest & lowest power) are at right angle to each others, and change of power from one meridian to the other is gradual & regular • Vertical meridian is more curved than Horizontal meridian • More common
- 66. • Vertical meridian is less curved than Horizontal meridian • Less common • The two principal meridians at at right angle to each other but they are not at 90 & 180 degrees • Less common
- 67. • One principal meridian is emmetropic & the other is ametropic, with accommodation relaxed • One principal meridian is Myopic & the other is emmetropic • One principal meridian is Hyperopic & the other is emmetropic
- 68. • Both principal meridians are ametropic, with same sign, with accommodation relaxed • Both Principal meridians are myopic • Both Principal meridians are hyperopic
- 69. • Both principal meridians are ametropic, with opposite sign, with accommodation relaxed • One Principal meridians myopic, the other is hyperopic