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

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 +0.25DS/-0.50DCx180 = + Axis for + cyl 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 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

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 3 900 2 4 1350 450 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 “That looks awful” Increases in size Does not change position! “That looks awful” Interval of Sturm Longer

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

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 –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

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

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

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 –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

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

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

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