1. Walter Huang, OD Yuanpei University Department of Optometry
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 axis around 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 axis around 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 x 175
OS: pl 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 x 180
OS: pl 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
Uncorrected VA
Spherical Refractive Error (D)*
Astigmatism (D)
20/30
0.50
1.00
20/40
0.75
1.50
20/60
2.00
20/80
3.00
20/120
4.00
20/200
2.50
>4.00

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)