OP1201 – Basic Clinical TechniquesAstigmatism

Dr Kirsten Hamilton-Maxwell

Measuring astigmatismSo far, we have looked at

Estimation of sphere and cyl power using retinoscopyRefinement 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

BE

Today’stopic

Today’s goalsBy 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 toDemonstrate that you can determine the amount and axis of

astigmatism using the x-cyl10min time limit for both eyes (including refinement of sphere

after retinoscopy)

Axis for + cyl

Power: +0.25DC

+ =

+0.25DS/-0.50DCx180

Axis for - cyl

Power: -0.25DC

What is a cross-cylinder?

Jackson cross-cylinder

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 planoAlso known as JCC, cross-cyl, x-cyl

-0.25

+0.25

Starting point – finding the right sphere power

Finding the axis

Finding the power

Starting point for x-cylFollowing retinoscopy…Circle of least confusion must be on retina, so check

sphere first, as we discussed last weekBalanced or green clearest on duochromeIn other words, over-minus slightlyAvoid over-plussing!

Will show you why shortly

Some phrases to rememberAxis

This is the angle at which the negative correcting cyl is positionedMeasured 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 linesThis 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 CLCThis is equal to the spherical equivalent, or best vision sphere

Simple myopic astigmatism

Interval of SturmDistance between the focal lines

Circle of Least ConfusionFocal lines are equally blurred

Blur is due to combination of…CLC in front of the retinaFocal lines being separated

“It’s very blurred”

With BVSCircle of Least Confusion

Has moved, is now on the retina

Interval of SturmLength 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

Optical principles of x-cylThe 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 powersThe x-cyl is then presented in 2 different orientations resulting

in 2 different resultant axesYour 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

clearerI will give examples a little laterThe axis must always be checked before power!

Choice of x-cyl powerVision 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-cylIf results reliable and vision improves, change down to 0.25Use a larger target until the vision improves!

Vision 6/24 or worse: try 0.75DC x-cylIf 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 axisIlluminate 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 retinoscopyThe 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 axisAsk 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 blurredStart with a rotation of 10 to 15deg, then refine

ExampleYou have performed retinoscopy on a patient You have found

-1.00DS/-1.00DCx180, 6/7.5You have checked the sphere power – they are now

one lens into the green on duochromeYou would now like to check the axis of the cyl

AxisPosition 1

Resultant axis

Axis

Resultant axis

Position 2

AxisPatient 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

Resultant axis

Position 1

Axis

Resultant axis

Position 2

Patient responseNote 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 axisThe 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 limitsChoose 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 returnsMay 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 distortionMove down to 0.25DC

If none of the above help, use alternative technique

Range of uncertaintyIn real life, most patients will report that both

lenses are equally blurred over a range of axesThis 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 rangeNote that the point exactly 90deg from the true

axis will also behave the same way!

Finding the axis from scratch

2

3 4

00

450

900

1350

1800

Axis without retinoscopy

Axis without retinoscopyHold handle horizontal. Ask patient “Which is roundest

and clearest: position 1, or position 2” in time with twirling the x-cylWill now know which quadrant cyl axis is in, i.e. 0deg to 90deg or

90deg to180degRepeat with handle at 45deg

Now 45deg to 135deg or <45deg or >135degOverlap 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 orientationsRemember these two orientations and insert cyl midway between

Now have trial frame cyl axis, so proceed as normal

Optical principles of x-cylWhen 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 1Circle of Least Confusion

Increases in sizeDoes not change position!

Interval of SturmLonger

“That looks awful”

Option 1Circle of Least Confusion

Decreases in sizeDoes not change position!

Interval of SturmShorter

“That is much better”

Finding the powerThis 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 astigmatismIf you have no ret result…Find the axis, using the procedure described earlierWhen 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–ve cyl axis

Position 1

Axis

Example - power

–ve cyl axis

Position 2

Axis

Example - powerIf 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 CLCsHere, 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 remedyAlways confirm the sphere power before you start

the cross cyl so that you know that the circle of least confusion is on the retinaSlight accommodation is preferred

Ask your patient which lens is“clearer and rounder”

Example - power–ve cyl axis

Position 1

Axis

Example - power

–ve cyl axis

Position 2

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 -0.50DCRepeat options 1 and 2

Patient response “same”Note that you should find where the response reversesIt is usual practice to recheck the cyl axis again nowFinal refraction is -0.75DS/-1.50DCx170, 6/6

So far…

The routineCheck sphereCheck axisCheck powerRecheck axisRemove 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 eyeNext 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!

Common errorsNot keeping the circle of least confusion on the

retinaStarting with the wrong sphere powerForgetting to change sphere power if cyl is changed by

0.50DC or moreAssuming the axis is correct if the patient says

“they look the same” without checkingCould be no astigmatism at allCould be 90deg off

Incorrect presentation time – esp too quickPoor alignment of x-cyl and trial frame axis

Read Elliott, Section 4.13-4.14