Flashcards in Jackson Cross Cylinder - Week 2 Deck (23):
What is a cross cylinder examination?
An examination used for fine tuning of the axis and strength of astigmatism after its determination by retinoscopy
(though it can also be done without having done retinoscopy prior)
If you aren't doing ret, what do you need to do before cross cyl?
Need to determine BVS
Through which 2 instruments can you perform x-cyl? How do they differ
- trial frame
When used with a trial frame, the JCC lens is mounted in a handle which is at 45deg tot he axes. If used with phoropter, the handle is a knob thingy
Explain the design of the X-cyl lens?
Positive cyl on one surface, negative cyl of numerically equal power on the other surface of the axes of the cylinders at right angles
- the powers are numerically equal but opposite in sign
What colours on the JCC lens are used for -ve and +ve cyl? Is this always the case?
-ve = red, +ve = black
Exception: In the U.K it's the other way around
How do you compare 2 views using the X-cyl?
The lens is 'twirled' or 'flipped' to enable the px to compare view with the lens in 2 different orientations
- this changes the axis of the cross-cyl lens
What V.A is the circular target equivalent to?
When determining axis location, why do we use a circular target?
Because otherwise, the px may be biased towards numbers or letters
How do we determine axis with the x-cyl?
Step 1: Horizontal/vertical axis --> place CC handle at 90 or 180deg, flip between view 1/2
Step 2: Oblique axis --> place CC handle at 45 or 135 deg, flip between view 3/4
Step 3: Insert a -0.50DC trial lens power, continue presenting 2 views
Step 4: From best view, move in large then gradually smaller steps away or back towards (e.g. 30deg, then 15, then 5, then 2.5). Repeat until both views look the same for px.
What instruction should you give when first asking a px to compare 2 views in x-cyl?
"I'm going to show you 2 views. Both views may be blurry, but I want you to tell me which view makes the circles rounder, clearer and more like circles"
When determining axis with x-cyl, do you always use -0.50DC trial lens?
Only if V.A is better than 6/9
If worse, work out from retinoscopy, tables, rule of thumb
To keep the COLC on the retina, what sign and power spherical lens should be added for every ___ sign and power cyl lens?
To keep COLC on the retina:
a +0.25DS lens must be added for every -0.50DC added
What sign axis should we be looking at/is preferred to be closer to the axis of astigmatism?
The -ve axis
That way it's already in negative cyl form. Easy script writing. Simple as that.
When adding a trial cylinder, what power do we normally use?
We normally use a low power trial cylinder, like -0.50
What happens if the trail lens cylinder is at the precise axis of required direction
Both views will be the same
In terms of power determination, what view will patients prefer?
Px will prefer the view which reduces the interval of sturm
(note: we are talking about the size and clarity, as position of COLC will not be affected)
After JCC procedure, how can you confirm your results?
Confirm with spheres/duohrome
What targets can be used with the X-cyl?
1) Letter Charts: note this may bias responses toward one position of the focal lines compared to another
2) Ring charts: symmetrical - no orientation preference
3) Dot targets: no orientation preference, less susceptible to optical distortions of x-cyl
What possible error conditions can occur for x-cyl procedure?
- COLC not on retina
--- px may prefer apparently clearer focus when one of the focal lines is on the retina
- X-cyl too strong
--- if small amnt of astig relatively small changes in residual astig with flip. Px may be insensitive: all "same"
What are potential problems for JCC?
- px develops a "first mentality" (i.e. px becomes a youtube commenter)
- px can't tell the difference b/w 1st and 2nd presentation, ever!
= allow longer to view each presentation
= poor discrimination maybe, therefore use larger steps (axis change and power of correcting trial cyl)
What is the 'Simultan Test'
It's literally like the X-cyl test EXCEPT, it displays the 2 cross cyl views side by side instead of consecutively as 2 separate independent views
Hence the name, 'simultan' for simultaneous
What if fogging and X-cyl don't work to find astig? What do?
You could try:
- stenopaic slit (meridional refraction)
- keratometry: Javal's rule: Ao = 1.25 (Ac) - 0.50