PC - Jackson's Cross Cylinder - Week 2 Flashcards
Does the JCC technique require fogging?
No
What does the success of the JCC technique rely on regarding the COLC?
It must be on the retina throughout the procedure.
Describe the design of the x-cyl lens, including all surface powers, and the spherical equivalent.
It is a lens with two orthogonal cyl powers, both at 0.25, but one at - and the other at +.
The spherical equivalent is a plano lens.
What power for the x-cyl lens is used if VA is better than 6/12 with BVS?
0.25
What shapes represent the axis on a x-cyl lens, and what colour represents what?
Dots are used, red represents the - power, white for + power.
How are views compared with a x-cyl lens, and what view would be preferred in an emmetropic eye vs an ametropic one?
The x-cyl lens is flipped so the two axes swap.
An emmetropic eye would see no improvement, while an ametropic eye will prefer one view.
Consider the power of the x-cyl lens. Does the position of the COLC change?
No, the mean power of the lens is still 0, and the spherical equivalent is plano, so the COLC doesnt change, and hence why emmetropic eyes see both views as equal.
What is the fixation target for the JCC technique? What VA is it equivalent to?
A circle with concentric rings inside it, or 4 illuminated circles arranged cardinally.
Equivalent to 6/12.
Consider asking a patient to compare two views during JCC. What instructions should they be given on the appearance of the circle?
Whether the circle appears sharper, and clearer (or more round).
Describe how the axis of astigmatism is initially checked, and describe what the patient preferring each view means in terms of the axis of their astigmatism.
The axis is set to 0/180, and the handle of the JCC is aligned with the axis.
The patient is asked to compare the two views as the handle is turned. This compares 90 and 180.
If both are equal, the patient is likely emmetropic or oblique.
If they prefer 180, the axis is likely 0-45 or 135-180.
If they prefer 90, the axis is likely 45-90 or 90-135.
The handle of the JCC is rotated 45 degrees until it clicks, and should be aligned with the 45 degree axis.
45 and 135 are compared.
If they prefer 180 and 45, the axis will be between 0-45.
If they prefer 180 and 135, the axis will be between 135 and 180.
If they prefer 90 and 45, the axis will be between 45-90.
If they prefer 90 and 135, the axis will be between 90-135.
If 45 and 135 appear equal, their axis is likely 90 or 180, depending which view they preferred previously.
Describe the steps to take once an axis has been bracketed during JCC. Which direction should the axis be set towards if the patient prefers a given view?
The handle of the x-cyl lens is realigned with the phoropter axis. -0.50D is added to the cyl power, and +0.25D is added to the spherical component.
The phoropter axis is aligned approximately midway between the axis bracket obtained.
For example, if 0-45 was obtained, the axis could be set to 20-25.
When the patient prefers a given view, turn the axis in the direction of the red dot/negative axis (usually 10 degrees), until they report equal preference for both views.
Consider a situation where you add an additional -0.50D tot he phoropter cyl power. What must be done in addition to this, and why?
For every -0.50D added to the cyl component, +0.25D must be added to the spherical component to keep the COLC on the retina. In this way, the interval of sturm is collapsed.
Once the patient’s correct axis of astigmatism is obtained during JCC, what should be done next?
The power axis of the JCC is aligned with the axis of the phoropter. This is done by looking at the JCC lens for a letter P, and aligning that with the phoropter axis. It should click into place.
The two views are compared. If they prefer the view with the red dot and P aligned, they require more negative power.
If their preferred view is the white dot aligned with the P, they require less power.
Power should be adjusted in 0.25 steps, and for every 0.50D adjustment, the spherical component must be adjusted by 0.25D.
Describe the final checks stage of the JCC technique.
Power should be adjusted until reversal is obtained.
As with spherical power, always select the least minus, most plus option if applicable.
Once the power of the astigmatism is determined, what is advisable to do after?
The axis of the astigmatism can be refined, and this is especially important in mid-high cylindrical components, but not so much in low powers.
It can be justified to skip this step therefore, if the power is low.
The axis is checked by realigning the handle with the phoropter axis, and comparing views.
Adjust the axis in the direction of the red dot/negative cyl axis.
Once JCC has been completed, including axis refinement post power determination, what are two endpoint checks?
Take off the JCC lens, and test their VA once again.
Confirm using the duochrome.
Why are letter charts typically not used as a fixation target for JCC?
Letter orientation presents a bias for certain axes over other.
Between ring charts and dot targets, which is less susceptible to the optical distortions of the x-cyl lens?
Both are, but dot targets are less so.
Name two possible errors that can occur during JCC.
- The COLC is not on the retina
- The x-cyl power is too strong
Describe the ‘first mentality’ that some patients can develop, and two possible solutions.
The patient cant tell the difference, and so always says the first view.
Mitigated by allowing longer to view each presentation
Can also use larger steps (in axis change and the power of the cyl) if there is poor discrimination
Consider a situation where you are short on time, and use retinoscopy to determine the axis of astigmatism. How can you proceed in JCC?
Align the JCC handle to 45 degrees of the axis found.
Flip and modify the power accordingly.
What three techniques can be employed if both fogging and JCC let you down?
Retinoscopy
Stenopaic slit
Keratometry - Javal’s rule
Briefly describe Javals rule.
Ao = 1.25 (Ac) - 0.50
Briefly describe the stenpaic slit technique in 5 steps.
- Determine BVS
- Fog by +1.00DS, and insert slit
- Rotate the slit until maximum acuity is found (is the minus cyl axis)
- So spherical refraction with the slit in place
- Rotate slit 90 degrees and repeat
