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Prism-Dissociated Duochrome Test
PURPOSE To equalize the stimulus to accommodation for the two eyes.
INDICATIONS
Perform the prism-dissociated duochrome test at the conclusion of the monocular subjective refraction of the left eye only if the two eyes did not reach equal VA’s during their monocular refractions and there is reason to believe that the accommodation differs in the two eyes．
STEP-BY-STEP PROCEDURE
1．Make sure neither eye is occluded and that both eyes can see the visual acuity chart on the screen．
2．Isolate a line of letters one line above the VA of the poorer seeing eye at the end of the monocular subjective refractions．Fogging lenses are not added．
3．Place the red-green filter over the isolated line of letters．
4．Using the phoropter’s Risley prisms，place 3 to 4Δ BU over the right eye and 3 to 4Δ BD over the left eye．
5．Inform the patient that he should see two lines of red-green letters．Make certain that this is the case．
6．Direct the patient’s attention to the lower line of letters．
7．Tell the patient to look from the green side to the red side and back to the green side．Have him state which side has the sharper, clearer (not “better,” darker, or brighter) letters or to state if the two sides are equally clear.
8．If the letters on the red side are clearer or if the letters on both sides are equally clear, introduce an additional 0.25 D of minus spherical power．If the letters on the green side are clearer，take away 0.25 D of minus (or add another 0.25 D of plus sphere)．
9．Direct the patient’s attention to the per line of letters and repeat step 7 and 8．
10．Repeat steps 6 through 9 until the patient is one into the green，the minimum amount of minus power at which the patient reports the green side has the clearer letters, for both the per and the lower line letters．As an alternative endpoint use a red-green balance，the point at which both sides appear equally clear, rather than one into the green．
11．Remove the red-green filter and recheck the VA in each eye separately and in both together．
Note：Some patients always seem to choose either the red or the green side regardless of the lens powers in place．Be alert to the possibility that your patient is unresponsive to the duochrome test and abandon it in favor of the smaller/darker endpoint for each eye separately (see step 31 b)．
If this side trip was taken, the subjective distance refraction is completed．Record the results． |
