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Astigmatism

Astigmatism. Walter Huang, OD Yuanpei University Department of Optometry. Definition. When parallel rays of light enter the eye ( with accommodation relaxed ) and do not come to a single point focus on or near the retina. Optics.

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Astigmatism

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  1. Astigmatism Walter Huang, OD Yuanpei University Department of Optometry

  2. Definition • When parallel rays of light enter the eye (with accommodation relaxed) and do not come to a single point focus on or near the retina

  3. Optics • Power in the horizontal plane projects a vertical focal line image • Power in the vertical plane projects a horizontal focal line image

  4. Optics • Refraction of light taking place at a toric surface: the conoid of Sturm

  5. Etiology • Cornea • The cornea has an unequal curvature on its anterior surface

  6. Etiology • Lens • The crystalline lens has an unequal curvature on its surface or in its layers

  7. Etiology • It is due to a distortion of the cornea and/or lens • The refracting power is not uniform in all meridians • The principal meridians are the meridians of greatest and least refracting powers • The amount of astigmatism is equal to the difference in refracting power of the two principal meridians

  8. Classification • Based on etiology • Based on relation between principal meridians • Based on orientation of meridian or axis • Based on focal points relative to the retina • Based on relative locations of principal meridians or axes when comparing the two eyes

  9. Corneal Astigmatism • When the cornea has unequal curvature on the anterior surface

  10. Lenticular Astigmatism • When the crystalline lens has an unequal on the surface or in its layers

  11. Total Astigmatism • The sum of corneal astigmatism and lenticular astigmatism

  12. Regular Astigmatism • When the two principal meridians are perpendicular to each other • Most cases of astigmatism are regular astigmatism • The three types are with-the-rule, against-the-rule, and oblique astigmatism

  13. Irregular Astigmatism • When the two principal meridians are not perpendicular to each other • Curvature of any one meridian is not uniform • Associated with trauma, disease, or degeneration • VA is often not correctable to 20/20

  14. With-The-Rule (WTR) Astigmatism • When the greatest refractive power is within 030 of the vertical meridian (i.e., between 060 and 120 meridians) • Minus cylinder axisaround horizontal meridian • The most common type of astigmatism based on the orientation of meridians

  15. With-The-Rule (WTR) Astigmatism

  16. Against-The-Rule (ATR) Astigmatism • When the greatest refractive power is within 030 of the horizontal meridian (i.e., between 030 and 150 meridians) • Minus cylinder axisaround vertical meridian

  17. Against-The-Rule (ATR) Astigmatism

  18. Oblique (OBL) Astigmatism • When the greatest refractive power is within 030 of the oblique meridians (i.e., between 030 and 060 or 120 and 150)

  19. Oblique (OBL) Astigmatism

  20. Simple Astigmatism • When one of the principal meridians is focused on the retina and the other is not focused on the retina (with accommodation relaxed)

  21. Simple Myopic Astigmatism • When one of the principal meridians is focused in front of the retina and the other is focused on the retina (with accommodation relaxed)

  22. Simple Hyperopic Astigmatism • When one of the principal meridians is focused behind the retina and the other is focused on the retina (with accommodation relaxed)

  23. What Patient Sees One meridian is out of focus

  24. Compound Astigmatism • When both principal meridians are focused either in front or behind the retina (with accommodation relaxed)

  25. Compound Myopic Astigmatism • When both principal meridians are focused in front of the retina (with accommodation relaxed)

  26. Compound Hyperopic Astigmatism • When both principal meridians are focused behind the retina (with accommodation relaxed)

  27. What Patient Sees Both meridians are out of focus

  28. Mixed Astigmatism • When one of the principal meridians is focused in front of the retina and the other is focused behind the retina (with accommodation relaxed)

  29. Symmetrical Astigmatism • The principal meridians or axes of the two eyes are symmetrical (e.g., both eyes are WTR or ATR) • The sum of the two axes of the two eyes equals approximately 180

  30. Symmetrical Astigmatism • Example • OD: pl -1.00 x 175 • OS: pl -1.00 x 005 • Both eyes are WTR astigmatism, and the sum of the two axes equal approximately 180

  31. Asymmetrical Astigmatism • The principal meridians or axes of the two eyes are not symmetrical (e.g., one eye is WTR while the other eye is ATR) • The sum of the two axes of the two eyes does not equal approximately 180

  32. Asymmetrical Astigmatism • Example: • OD: pl -1.00 x 180 • OS: pl -1.00 x 090 • One eye is WTR astigmatism, and the other eye is ATR astigmatism, and the sum of the two axes do not equal approximately 180

  33. Prevalence • Age • Infants are born with ATR astigmatism, where the cornea is the source of the astigmatism • Preschool children have little or no astigmatism • Teenage children demonstrate a shift towards WTR astigmatism • Older adults show a shift towards ATR astigmatism

  34. Prevalence • Gender • In general, there are no significant differences between males and females

  35. Prevalence • Ethnicity • Higher prevalence in North Americans, Latinos • Asian infants tend to be WTR astigmatism • Caucasian infants tend to be ATR astigmatism

  36. Incidence • General trend • For older adults, the average rate of change towards ATR astigmatism is less than or equal to 0.25D every 10 years

  37. Visual Acuity • Theoretically, at NO distance does an uncorrected astigmat have a sharp retinal image • Clinically, if astigmatism is small (less than 0.50DC), the patient may not notice blur

  38. Visual Acuity • Simple or compound myopic astigmatism • Accommodation may make the retinal image even more blurry • Simple or compound hyperopic astigmatism • Accommodation may improve VA to some extent • Mixed astigmatism • VA is relatively good • May not need much accommodation

  39. Spherical and Astigmatic Ametropia

  40. Spherical and Astigmatic Ametropia • Spherical refractive error (D)* • Myopia or absolute hyperopia • When multiplied by a factor of two, it equals astigmatism (D)

  41. Symptoms • Distorted vision at distance and near • Letter confusion • Asthenopia or ocular fatigue • Due to constantly squinting to clear up distorted vision • Headaches • Squinting

  42. Signs Decreased visual acuities at distance and near

  43. Clinical Tests • Visual acuity tests – distance and near • Autorefraction • Keratometry • Retinoscopy • Most reliable source of information for cylinder power and axis • Monocular subjective refraction, including Jackson cross cylinder

  44. Management • Cylindrical lenses and spherocylindrical lenses in spectacles and contact lenses for simple astigmatism and compound astigmatism, respectively • Refractive surgery

  45. Management • Spectacles • Single vision glasses with cylinder

  46. Management • Contact lenses • Toric soft contact lenses • Toric rigid gas permeable contact lenses

  47. Management • Refractive surgery • Photorefractive keratectomy (PRK) • Laser in-situ keratomileusis (LASIK)

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