1. Subjective refraction
OP1201 – Basic Clinical Techniques
Astigmatism
Dr Kirsten Hamilton-Maxwell

2. Measuring astigmatism
So far, we have looked at
Estimation of sphere and cyl power using retinoscopy
Refinement of sphere power by subjective methods
Today, we will add to your routine by showing you how to refine cyl power by the cross-cylinder method

3. Subjective refraction
Check sphere
Check cyl
(Axis and power)
Recheck sphere LE
Subjective refraction
Today’s
topic BE
Binocular balance
Final prescription (Rx)

4. Today’s goals By the end of today’s lecture, you should be able to
Explain the basic optical principles of the Jackson cross cylinder (x-cyl)
Explain how to determine the astigmatic correction using the x-cyl
From your ret result
If no ret result is available
By the end of the related practical, you should be able to
Demonstrate that you can determine the amount and axis of astigmatism using the x-cyl
10min time limit for both eyes (including refinement of sphere after retinoscopy)

5. Background

6. What is a cross-cylinder?
Power: +0.25DC
Power: -0.25DC
+0.25DS/-0.50DCx180 = +
Axis for + cyl
Axis for - cyl

7. Jackson cross-cylinder
-0.25
+0.25
Typically a lens with a handle attached at 45deg to the cylindrical power of the lens (in this case, the power is +0.25DS/-0.50DC)
These also come in different powers: ±0.50DC, ±0.75DC, ±1.00DC
Spherical equivalent is always plano
Also known as JCC, cross-cyl, x-cyl

8. Procedure Starting point – finding the right sphere power
Finding the axis
Finding the power

9. Starting point for x-cyl
Following retinoscopy…
Circle of least confusion must be on retina, so check sphere first, as we discussed last week
Balanced or green clearest on duochrome
In other words, over-minus slightly
Avoid over-plussing!
Will show you why shortly

10. Some phrases to remember
Axis
This is the angle at which the negative correcting cyl is positioned
Measured in degrees between 0 and 180deg
(You will hear this described in other ways in other modules, particularly during discussions of meridian vs axis; all are correct)
Interval of Sturm
This is the distance between the two focal lines
This is equal to the power of the “cyl”
Circle of least confusion (CLC)
Exactly halfway (measured in D) between the two focal lines (or in the Interval of Sturm), there is a place where there is equal blur in both meridians
This is called the circle of least confusion, or CLC
This is equal to the spherical equivalent, or best vision sphere

11. Simple myopic astigmatism
Circle of Least Confusion
Focal lines are equally blurred
“It’s very blurred”
Blur is due to combination of…
CLC in front of the retina
Focal lines being separated
Interval of Sturm
Distance between the focal lines

12. With BVS Circle of Least Confusion Interval of Sturm
Has moved, is now on the retina
Interval of Sturm
Length unchanged
Reason the vision is still blurred
“That’s better but it still isn’t clear”
All blur is now due to uncorrected cyl: We will learn how fix that next week

13. Finding the axis

14. Optical principles of x-cyl
The combination of two cylindrical lenses at different axes (obliquely crossed) will produce a “resultant cyl” that has its own axis and power
First axis = the axis of the patient’s uncorrected astigmatism
Second axis = the axis of the x-cyl
Resultant axis is located somewhere between the two, and is a proportion of the sum of their powers
The x-cyl is then presented in 2 different orientations resulting in 2 different resultant axes
Your patient compares 2 images and reports which one is clearer
The resultant axis that is closer to the true axis of the eye will appear clearer
I will give examples a little later
The axis must always be checked before power!

15. Choice of x-cyl power Vision 6/9 or better: use 0.25DC x-cyl
If results unreliable, then change up to 0.50 x-cyl. and see if more reliable
Vision 6/12 or worse: use 0.50DC x-cyl
If results reliable and vision improves, change down to 0.25
Use a larger target until the vision improves!
Vision 6/24 or worse: try 0.75DC x-cyl
If results unreliable, use alternative method of astigmatic correction (Fan and block, keratometry)
If retinoscopy reflex distorted or hazy, start with 0.50DC x-cyl, then try to refine with 0.25DC

16. Finding the axis Illuminate the circles on the white background
Vision must be 6/12 or better
Hold the x-cyl with the handle pointing in the same direction as the trial lens axis from retinoscopy
The trial lens axis will be exactly halfway between the red and white markings on the x-cyl
Flip the x-cyl so that the position of the red and white markings is reversed

17. Finding the axis Ask the patient
“Are the circles clearer and rounder with lens 1 or 2?”
Remind the patient that both images may be slightly blurred
Rotate the trial lens towards where the red marking was when the image was clearer
Repeat until the two images are equally blurred
Start with a rotation of 10 to 15deg, then refine

18. Example You have performed retinoscopy on a patient You have found
-1.00DS/-1.00DCx180, 6/7.5
You have checked the sphere power – they are now one lens into the green on duochrome
You would now like to check the axis of the cyl

19. Axis
Position 1
Resultant axis

20. Axis
Position 2
Resultant axis

21. Axis
Patient response
“Lens 1 was clearer and rounder”
So, we rotate towards the position of the red markings for “Lens 1”
In this case, position 1
Initially, move by steps of about 15deg, then use smaller steps as you get closer

22. Axis
Position 1
Resultant axis

23. Axis
Position 2
Resultant axis

24. Patient response
Note that I rotated the axis of the trial lens AND the handle of the x-cyl by 10 deg
“Both lenses are equally blurred”
This means that the cyl axis of the trial frame now matches the patient’s cyl axis
The true axis is 170deg
In real life, you would continue until the patient sends you in the other direction (reversal)
There is usually a range where the images appear equal, and you need to find the limits
Choose the axis mid-way between the two reversals

25. “They look the same”
May be on axis, therefore move cyl axis by about 20deg and check to see if it returns
May be within range of uncertainty (next slide)
0.25DC x-cyl may give insufficient difference
Try 0.50DC
0.50DC x-cyl may give too much distortion
Move down to 0.25DC
If none of the above help, use alternative technique

26. Range of uncertainty
In real life, most patients will report that both lenses are equally blurred over a range of axes
This is more common with low cyl power
You need to identify the range
Find where the patient tells you to rotate in the opposite direction at each end
Select the axis in the middle of the range
Note that the point exactly 90deg from the true axis will also behave the same way!

27. I don’t have a ret result
Finding the axis from scratch

28. Axis without retinoscopy3
900 2 4
1350
450
1800 00

29. Axis without retinoscopy
Hold handle horizontal. Ask patient “Which is roundest and clearest: position 1, or position 2” in time with twirling the x-cyl
Will now know which quadrant cyl axis is in, i.e. 0deg to 90deg or 90deg to180deg
Repeat with handle at 45deg
Now 45deg to 135deg or <45deg or >135deg
Overlap of quadrants narrows axis down to 45deg sector
The limits of this sector are given by the position of the minus-cyl axis (red) in the preferred orientations
Remember these two orientations and insert cyl midway between
Now have trial frame cyl axis, so proceed as normal

30. Finding cyl power

31. Optical principles of x-cyl
When determining the power, x-cyl will either increase or decrease residual cyl, either expanding or collapsing the astigmatic interval and circle of least confusion
Thereby making the target less or more clear
One option will extend this interval (interval of Sturm), the other will shorten it

32. Option 1 Circle of Least Confusion “That looks awful”
Increases in size
Does not change position!
“That looks awful”
Interval of Sturm
Longer

33. Option 1 Circle of Least Confusion “That is much better”
Decreases in size
Does not change position!
“That is much better”
Interval of Sturm
Shorter

34. Finding the power
This is a similar procedure as finding the axis except that the handle is now held at 45deg to the trial lens axis
This means that the white or red lines will be aligned with the trial lens axis in position 1 and 2
If you have a ret result, start with the cyl power that is in your trial frame

35. Estimating astigmatism
If you have no ret result…
Find the axis, using the procedure described earlier
When the BVS is in place, you can estimate the amount of astigmatism that is still uncorrected and use this as a starting point
-0.50DC per line of vision
(This doesn’t work if you have already put cyl in the trial frame)
Eg. A patient has the potential for a best corrected visual acuity is 6/6 but is currently seeing only 6/9 through their BVS. What is the estimated astigmatism?
2 lines = -1.00DC

36. Example - power Position 1 –ve cyl axis Axis
This adds +cyl to the patient’s axis – if this view is clearer, then we need to remove some minus cyl

37. Example - power Position 2 Axis –ve cyl axis
This adds –cyl to the patient’s cyl power. If the prefer this choice, add more –ve cyl

38. Add -0.25DS for each +0.50DC change
Example - power
If the patient prefers the lens
When red marks are aligned with trial cyl axis, add more negative cyl
When white marks are aligned with trial cyl axis, reduce the negative cyl
Equally clear: you have the right power
For each -0.50DC change, you need to add +0.25DS, to keep the circle of least confusion on the retina
Add -0.25DS for each +0.50DC change

39. Why change the sphere power?
CLC needs to stay on the retina – how it should be
Option 1
CLC

40. Why change the sphere power?
Option 2
CLC
You are asking your patient to compare the size of 2 different CLCs
Here, the clearest option is directing you towards correcting the cyl.

41. What if it goes wrong? What if the CLC is in the wrong place? Option 1
One meridian crystal clear

42. What if it goes wrong? What if the CLC is in the wrong place? Option 2
Blur circle
Here, you are asking your patient to choose between one crystal clear astigmatic line and a blur circle created by simple myopia.
Accepting the clearer lens will lead you away from the correct cyl.

43. The remedy
Always confirm the sphere power before you start the cross cyl so that you know that the circle of least confusion is on the retina
Slight accommodation is preferred
Ask your patient which lens is“clearer and rounder”

44. Example - power Position 1 –ve cyl axis Axis
This adds +cyl to the patient’s axis – if this view is clearer, then we need to remove some minus cyl

45. Example - power Position 2 Axis –ve cyl axis
This adds –cyl to the patient’s cyl power. If the prefer this choice, add more –ve cyl

46. Example - power “Which lens is clearer and rounder, 1 or 2?”
Patient response “2”
This means we need to add another -0.25DC (new power -1.25DC)
Assuming that options 1 and 2 are the same as before
We need to add another -0.25D (-1.50DC), this time add +0.25DS (-0.75DS) because the total cyl change is -0.50DC
Repeat options 1 and 2
Patient response “same”
Note that you should find where the response reverses
It is usual practice to recheck the cyl axis again now
Final refraction is -0.75DS/-1.50DCx170, 6/6

47. Refraction routine
So far…

48. The routine Check sphere Check axis Check power Recheck axis
Remove cross-cylinder and check visual acuity
It should be the same or better than following spherical refraction
Any improvement should make sense!
Repeat for the other eye
Next week, we will look at the final sphere power and how to ensure that both eyes are equally corrected – your distance Rx will be complete!

49. Things that can go wrong

50. Common errors Not keeping the circle of least confusion on the retina
Starting with the wrong sphere power
Forgetting to change sphere power if cyl is changed by 0.50DC or more
Assuming the axis is correct if the patient says “they look the same” without checking
Could be no astigmatism at all
Could be 90deg off
Incorrect presentation time – esp too quick
Poor alignment of x-cyl and trial frame axis

51. Further reading
Read Elliott, Section