Transcript File - Optometry Peer Tutoring

Subjective refraction
OP1201 – Basic Clinical Techniques
Astigmatism
Dr Kirsten Hamilton-Maxwell
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
Subjective refraction
RE
LE
Check sphere
Check sphere
Check cyl
(Axis and power)
Check cyl
(Axis and power)
Recheck sphere
BE
Recheck sphere
Binocular balance
Final prescription (Rx)
Today’s
topic
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)
Background
What is a cross-cylinder?
Power: +0.25DC
Power: -0.25DC
=
+
Axis for + cyl
+0.25DS/-0.50DCx180
Axis for - cyl
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
Procedure
Starting point – finding the right sphere power
Finding the axis
Finding the power
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
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
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
With BVS
Circle of Least Confusion
Has moved, is now on the retina
Interval of Sturm
“That’s better but it still isn’t clear”
Length unchanged
Reason the vision is still blurred
All blur is now due to uncorrected cyl: We will learn how fix that next week
Finding the axis
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!
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
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
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
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
Axis
Position 1
Resultant axis
Axis
Position 2
Resultant axis
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
Axis
Position 1
Resultant axis
Axis
Position 2
Resultant axis
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
“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
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!
I don’t have a ret result
Finding the axis from scratch
Axis without retinoscopy
900
450
1350
3
4
2
1800
00
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
Finding cyl power
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
Option 1
Circle of Least Confusion
Increases in size
Does not change position!
“That looks awful”
Interval of Sturm
Longer
Option 1
Circle of Least Confusion
Decreases in size
Does not change position!
“That is much better”
Interval of Sturm
Shorter
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
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
Example - power
Position 1
Axis
–ve cyl axis
Example - power
Position 2
Axis
–ve cyl axis
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
Why change the sphere power?
 CLC needs to stay on the retina – how it should be
Option 1
CLC
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.
What if it goes wrong?
 What if the CLC is in the wrong place?
Option 1
One meridian crystal clear
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.
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”
Example - power
Position 1
Axis
–ve cyl axis
Example - power
Position 2
Axis
–ve cyl axis
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
 Patient response “2”
 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
Refraction routine
So far…
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!
Things that can go wrong
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
Further reading
Read Elliott, Section 4.13-4.14