ASTIGMATISM

1. ASTIGMATISM

2. DEFINITION
 Astigmatism by definition is a condition where the
parallel beam of light rays incident on the cornea
after refraction are not focused to form a point image
near or on the retina

3. ASTIGMATISM

4. ASTIGMATISM

5. ETIOLOGY
Irregular curvature or RI in the anterior surface of
cornea
Irregular curvature or RI and also the position of
lens/IOL
The refracting power is not uniform in all meridians
of both lens or cornea
The principal meridians are the meridians of greatest
and least refracting powers

6. Classification of astigmatism
• 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

7. Based on etiology
Etiology
Corneal Lenticular

8. • When the cornea has unequal curvature on the anterior
surface
Reasons:
• Asymmetric cornea
• Meridional difference in RI
• Varying RI
Corneal astigmatism

9. Lenticular Astigmatism
When the crystalline lens has an unequal
on the surface or in its layers
Reasons:
•Asymmetric crystalline lens
•Asymmetric lens layers
•Tilted crystalline lens
•Varying RI of the lens layers

10. Total Astigmatism
The sum of corneal astigmatism and
lenticular astigmatism

11. •Regular
•Irregular
Based on relation between principal
meridians

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
Can be corrected with Cyl or Sph- Cyl lens

13. Irregular Astigmatism
When the two principal meridians are not
perpendicular to each other –more than two
principal meridians
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
• Against the rule
• Oblique
• Bi oblique
Based on orientation of meridian or axis

15. Based on relation between principle
meridian
 WTR-With The Rule Astigmatism
 Vertical meridian of cornea is more curved
 Myopic power is more in vertical meridian

16. With-The-Rule (WTR)
Astigmatism
• When the greatest refractive power is within ±30 of the vertical meridian
(i.e., between 060 and 120 meridians)
• Vertical meridian is more myopic than horizontal
 Eg :- -3.0D cyl x 180
• Minus cylinder axis around horizontal meridian
• The most common type of astigmatism based on the orientation of
meridians

17. With-The-Rule (WTR) Astigmatism

18. 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
• Horizontal meridian of cornea is more curved
• Myopic power is more in horizontal meridian
 Eg :- -3.0D cyl x 90˚

19. Against-The-Rule (ATR)
Astigmatism

20. Based on orientation of meridian
 OBLIQUE
 axis is not on or near 90˚ or 180˚
 Range of axis lies in between 120˚-150˚ & 60˚- 30˚
 Eg :- -3.0D cyl x 45˚

21. Orientation of meridian

22. Oblique (OBL) Astigmatism
• When the greatest refractive power is within 030 of the
oblique meridians (i.e., between 30 and 60 or 120 and 150)

23. Oblique (OBL) Astigmatism

24.  BIOBLIQUE
 The meridians are not perpendicular
 Eg :- -3.0D cyl x 90˚/ -2.0 D cyl x 110˚

25. Based on focal points relative to retina
 Simple astigmatism
 One focal point on retina and the other front or behind
 Of 2 types simple myopic and simple hyperopic
 Compound astigmatism
 Both the 2 focal points are in front or behind the retina
 Of 2 types compound myopic and hyperopic
 Mixed astigmatism
 One focal point in front and other behind the retina

26. 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)

27. 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)

28. 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)

29. What Patient Sees
One meridian is out of focus

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

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

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

33. What Patient Sees
Both meridians are out of focus

34. 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)

35. Based on relative locations of principal meridians
or axes when comparing the two eyes
1. Symmetrical
2. Asymmetrical

36. 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

37. Symmetrical Astigmatism
Example


OD: Plano -1.00 x 175
OS: Plano -1.00 x 05
Both eyes are WTR astigmatism, and the
sum of the two axes equal approximately
180

38. 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

39. 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

40. 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

41. Prevalence
Gender

In general, there are no significant differences between
males and females

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

43. 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

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

45. Signs
Decreased visual acuities at distance and
near

46. 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

47. Refining astigmatism
 Astigmatic fan
 Clock dial
 Jackson’sc0rosscylinder
 Stenopic slit

48. Management
 Spectacles
 Cylindrical lenses and spherocylindrical lenses in spectacles
 Contact lens
 Toric soft contact lenses
 rigid gas permeable contact lenses
 Refractive surgery
 Photorefractive keratectomy (PRK)
 Laser in-situ keratomileusis (LASIK)

49. Guidelines for optical
treatment
Small astigmatism- treatment is required
In presence of asthenopic symptoms
Decreased vision
• High astigmatism- full correction
• Better to avoid new astigmatic correction in
adults because of intolerable distraction
• Bi-oblique,mixed,high astigmatism arebetter
treated by contact lenses
• Correction of spherical component

50. Residual astigmatism
The amount of astigmatism that still
remains after correction of a refractive error.
In the case of correction of corneal
astigmatism using rigid contact lens
,lenticular residual astigmatism is exposed.

51. Thank you