We done experiments based on: Gordon’s Glazing Window Company. Convenient & common method for fine-tuning astigmatic element found in retinoscopy. Requires the retinoscopy to be within ~1.00DC. It Cannot be relied upon if remaining cyl to be corrected is >1.00DC. Either 0.25 or 0.50 cyls depending on level of astigmatism that needs correction.
Handle is at 45° to axes. The number written on the handle indicates the astigmatic interval between the two meridians. E.g. a cross cyl marked 1.00D has meridians of power +0.50D & -0.50D
The markings on the lens are at plus & minus 45° to the handle, & indicate the axes meridians, not the power meridians.
Cross cyls are available in ±25, 0.50, 0.75 & 1.00 D powers, which result in astigmatic intervals of twice these amounts.
Choose a cross cyl that has an astigmatic interval equal to or less than value of Px’s estimated cyl
- For the target, most practitioners use double black rings on a white background as targets. (One concentric circle, say which one to look at)
- The circles subtend 6/12 & 6/4.5 so most patients can resolve larger circles with their best sphere acuity
- Sometimes letter “O” from Snellen chart is used – letter size must be just better than the patient’s best sphere acuity for each eye
- Cross cyl method is divided into two parts:
- To find & refine the cylinder axis
- To refine the cylinder power
- To find cyl axis:
- Occlude one eye
- Start with cyl obtained from ret in the trial frame (of other eye)
- Hold cross cyl handle parallel to axis of cylinder in trial frame in front of other eye & twirl (flip over, still keeping handle in that position)
- Ask for preference while looking at the rings: “Are the circles blacker, clearer & rounder with lens 1 or lens 2?”
- Make sure to notice which position you’re holding cross cyl, & whichever “lens” they prefer, (flip back if lens 1) move the axis 10° towards the –ive/minus axis meridian
- Then if patient tells you to go the other way, only move it by 5°, & when alternating keep decreasing the interval until you get to 2.5°/1°
- To find cyl power:
- Turn cross cyl through 45° so that cross cyl axes are parallel with & at right angles to the trial case cyl axis
- Ask again for preference while looking at rings: “Are the circles blacker, clearer & rounder with lens 3 or lens 4?”
- Put in the power they prefer
- If no cyl has been found with ret
- Hold cross cyl axes at 90 & 180 & twirl (flip)
- If for e.g. prefers 90 then turn through 45° & twirl (essentially asking would you like minus cyl a bit >90 or a bit <90) – first preference narrowed it down to vertical rather than horizontal, the second preference could be from 45 to 90, or 90 to 135)
- Then put axis meridians at suitable axis (middle of interval) & ask which power they prefer, plus or minus & give
- Then refine axis again
- If there’s no preference on the first twirls with cross cyl to refine axis then either they don’t have any astigmatism or by chance the axes are 45 & 135. Try again with the handle at 90°, if still no preference – no astigmatism. If there is a preference don’t forget that axes must be 45 or 135
- After retinoscopy check VA & do +1.00 blur test
- If ~6/9 & blurs to ~6/18 do cross cyl
- If ~6/9 & does not blur with +1.00, optimise sphere (best sphere) & then do cross cyl
- If worse than 6/9, optimise sphere
- Still poor VA? Pinhole? F&B?
- Patients need to have a small amount of active accommodation ~0.25 for cross cyl i.e. +1.00 only blurs to ~6/12 – R = G or just on green on duochrome. So before doing cross cyl check that VA is on green (can add more -ive spheres but not more than -1.00DS, then take it out after finishing). Distance VA should be R = G or just on red whereas near VA should always be on green.
- After cross cyl optimise best sphere again
- Explain to Px that even if VA may be best without cross cyl you want to know if lens one is better than lens two.
- Particularly with small cyls, check the power as soon as possible rather than wasting time finding axis of cyl with trivial power, as axis can be found best when power is correct, & the power when axis is correct.
- For cross cyl method to work, disc of least confusion must be kept on retina, therefore make sure that for every -0.50DC you add +0.25DS & vice versa. (not so much for F&B). This can be checked at any time during the routine by use of duochrome (R = G)
- Especially in older patients with little accommodation as they will not be able to reposition the remaining circle of least confusion onto the retina after each change in cyl.
- Cross cyl method is usually only used with negative cyls
- If plus cyls are used, it’s essential to modify sphere with changes in cyl power to keep circle of least confusion on retina
- Advantages over F&B:
- Quicker, simpler method
- Relatively unaffected by head tilt
- Many modern charts do not include a fan & block but all have circular target of Some form
- Can be performed for both distance & near vision
- Alternate choice test
- Good for lower power cyls
- However F&B is good as it is simultaneous choice & is good for high power cyls. (Professional decision)