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.