2. Astigmatism LOGO
The refractive error of an optical system that creates
two principal focal planes instead of a single one.
3. Astigmatism LOGO
When meridians are perpendicular to each other
then astigmatism is considered as regular, otherwise
is considered irregular.
4. Corneal astigmatism LOGO
Against the rule
Vertical meridian
flatter than horizontal.
Corrected with
(-) cyl. X 90°
With the rule
Vertical meridian
steeper than
horizontal
Corrected with
(-) cyl. X 180°
5. Astigmatism LOGO
Total astigmatic error occurs by integration of
corneal and lens astigmatism.
Usually, corneal with the rule astigmatism
+ lenticular against the rule astigmatism.
6. Incidence of astigmatism among population LOGO
83% of prescriptions incorporate an astigmatic component
15%
<0.50
45%
10%
0.50-1.00 60%
1.00-1.50
20% 7%
3% 1.50-2.00
2.00-3.00
>3.00
Hartstein J. - Review of eye terminology
16. Fan Chart (Clock Dial) Test LOGO
Advantages
No special equipment
Easy to perform for screening
Easy to communicate with high amounts of
astigmatism
Disadvantages
Difficult discrimination in small amounts of
astigmatism
Difficult axis orientation in small amounts of
astigmatism
Difficult to communicate in small amounts of
astigmatism
21. “V” Chart LOGO
Advantages
More accurate than Fan Chart
Easier to communicate than Fan Chart
Easier to check for the right amount of cylinder
Disadvantages
Difficult discrimination in small amounts of
astigmatism
Difficult axis orientation in small amounts of
astigmatism
50. Determination of power LOGO
-0.75 DS -1.00 DC x 170
About the same.
2?
…or about the
same?
Cross Cyl. +/- 0.25 DC
51. Notes on axis LOGO
If your endpoint for the determination of axis is
alternating between +/- 5°, you can choose an
intermediate or lay on the one closer to a major
meridian.
If changes in axis exceed 90° from your starting
point, then consider removing the cylinder and
replace it with the spherical equivalent lens.
52. Notes on power LOGO
If your endpoint for the determination of power is
alternating between +/- 0.25 DC, keep the lower
amount of cylinder.
If changes in cylindrical power exceed 0,50 DC then
it is suggested that you recheck the axis.
53. Cross Cylinder LOGO
Advantages
Accurate for power and cylinder
Direct effect on perception
Easy to communicate. Comparison
Disadvantages
Stability with hand-held cross cylinder
May need more than one power of cross
cylinder
Time consuming
54. Perceptual issues of astigmatism LOGO
Astigmatism with the rule creates less visual
disability compared to astigmatism against the rule.
Xiguana Indians have been great hunters besides of
their high uncorrected astigmatic refraction (10%
population >3.0 DC).
Pensyl CD, et al. 1997 Focused Defocused
With the Rule Against the Rule
55. Perceptual issues of astigmatism LOGO
Brain is searching for information that can be useful
in dealing with gravity. This means that vertical
orientation enriches visual perception.
Focused Defocused
With the Rule Against the Rule
56. Effect of cylindrical error and axis on V.A. LOGO
V.A. Oblique Against With
the Rule the Rule
20/25 0.25 0.50 0.50
20/30 0.75 1.00 1.00
20/40 1.00 1.25 1.50
20/50 1.50 1.75 2.00
20/70 1.75 2.00 2.50
20/100 2.25 2.50 3.00
20/150 2.75 3.00 3.50
20/200 3.50 4.00 4.50
61. When to prescribe LOGO
When patient is symptomatic (reduced acuity,
fatigue, photosensitivity, conjuctivity, etc.)
Rarely small (< -0,75 DC) uncorrected astigmatism
induce symptoms, especially @ 180°.
But it is not uncommon for small (<0,75 DC)
uncorrected astigmatism, especially oblique or @ 90°
to induce asthenopic symptoms since they tend to
guide accommodation all the time towards the circle
of least confusion.
63. When to prescribe LOGO
Amblyopia risk
Astigmatism in children under the age of 3 years is
liable.
Astigmatic refraction < 2,00 DC need not to be
compensated. Premature prescribing may interfere
with emmetropization.
Astigmatic refraction > 1,25 DC, especially in oblique
axis, persisting after the age of 2 years, should be
prescribed.
Astigmatic amounts that could reduce acuity,
especially at near, or create symptoms need to be
prescribed directly. Plasticity in young children is
huge.
64. When not to prescribe or under-correct LOGO
History of dizziness, disorientation, ear infections,
etc.
Small amounts, first time. Re-check, especially if
habitual Rx had always been spherical.
High cylindrical corrections. Check axis and amount
at near. Consider under-correcting according to this
findings.
When complaints are mainly at near and are
unrelated to spherical equivalent, suspect
cyclotorsion.
65. 3 strategies for better tolerance LOGO
If changes in astigmatic refraction are >0.50 DC,
consider decreasing cylinder by adjusting sphere.
Hab. Rx
• -0.50 -1.25 X 180 O.U.
New Refraction
• -0.50 -2.25 X 180 O.U.
New Rx (check)
• -0,75 -1,75 X 180 O.U.
66. 3 strategies for better tolerance LOGO
Approach equality in the cylindrical power,
especially when axis are oblique.
Hab. Rx
• O.D. -1.25 -0.50 X 145 / O.S. -1.25 -0.50 X 35
New Refraction
• O.D. -0.75 -1.50 X 145 / O.S. -1.25 -0.75 X 35
New Rx (check)
• O.D. -1,00 -1,00 X 145 / O.S. -1.25 -0.75 X 35
67. 3 strategies for better tolerance LOGO
When patient reports failure of previous attempts
with astigmatic correction or axis are very different
from the habitual, consider prescribing spherical
equivalent.
Hab. Rx
• O.D. -5.00 -0.50 X 170 / O.S. -5.25 -0.50 X 180
New Refraction
• O.D. -5.00 -0.50 X 115 / O.S. -5.25 -0.50 X 60
New Rx (check)
• O.D. -5,25 / O.S. -5.50
68. Before you prescribe LOGO
Keep axis as symmetrical as possible.
Keep axis as close to main meridians as possible.
Consult sensitive patients not to make significant
changes in frame’s shape.
Changes in base curve, pantoscopic angle and
refractive index of material can alter magnification
effects but usually patients adapt to in a few days.
