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BINOCULAR BALANCE
PURPOSE To equalize the stimulus to accommodation for the two eyes．The primary purpose of binocular balance is to match the accommodative stimulus for the two eyes．It serves a secondary purpose of relaxing the accommodation．With both eyes open，the accommodative responses of the two eyes should be maximally relaxed and equal．For many patients, binocular balance serves the additional function of matching the visual acuity in the two eyes through the new prescription．
INDICATIONS
Since this binocular balance procedure calls on the patient to match the VA under fog in the two eyes, perform it only if the two eyes achieved the same VA during their monocular refractions．If the best corrected VAs of the two eyes differ after the distance monocular subjective refractions, the binocular balance procedure should be skipped．If the best corrected VAs of the two eyes differ and there is reason to believe that the accommodation differs in the eyes following their distance monocular subjective refractions，perform the prism dissociated duochrome test．
34．Make sure that neither eye is occluded and that both eyes can see the projector screen．
35．Fog each eye by +0.75 D sphere relative to the endpoint of its respective monocular refraction．Measure the patient’s binocular VA and continue adding +0.25 sphere in front of both eyes until the binocular VA is 20/25 or worse，if necessary．
36．Isolate a line of letters one line above the VA found at the end of step 35.
37．Place 3 to 4Δ base over the right eye and 3 to 4Δ base down over the left eye using the phoropter’s rotary (Risley) prisms，as shown in Figure 3-13．
38．Inform the patient that he should see two lines of letters，both of which should be blurry, as in Figure 3-14．Make certain that this is the case．
39．Have the patient look back and forth between the two lines of letters and have him tell you which line, the per or the lower，is less blurry．
40．Add +0.25 (take away -0.25) sphere to the eye which sees the clearer line(ie，make it blurrier)．For example，if the patient reports that the lower line is clearer, as in Figure 3-14, add +0.25 sphere (take away -0.25) to the clearer seeing right eye．
41．Repeat steps 39 and 40 until the patient reports equal blurriness of the two lines or until the patient is simply switching back and forth between the two．If it is not possible to achieve a close match between the blurriness of the two eyes，perform the sighting-dominance check．Then leave the sighting-dominant eye with the subjectively clearer vision．
Note：Since the letters must remain legible at all times, after the first or second repetition of steps 39 and 40，make the blurrier eye clearer．Do this by adding -0.25 D sphere to it (taking away +0.25 D) rather than continuing to blur the clearer eye．Take care to assure that both eyes remain fogged until the end of the binocular balance procedure．
42．When equality of the vision of the two eyes is reached，or the sighting-dominant eye is left with the slightly clearer vision , remove the Risley prisms to allow fusion．
Binocular MPMVA
PURPOSE
To determine the maximum plus spherical power which provides the patient with his maximum visual acuity through both eyes simultaneously．
43．The patient’s eyes should still be equally fogged or the sighting-dominant eye should be slightly clearer from step 42．Check the patient’s binocular VA through the fog.
44．Predict the final sphere by comparing the patient’s VA under fog to Egger’s chart．Remember that the patient should obtain approximately one additional line of VA for each 0.25 D of minus sphere added (or each 0.25 diopter of plus sphere removed) during the MPMVA．
45．Reduce the plus (or add minus) 0.25 D at a time over both eyes simultaneously，checking VA and encouraging the patient to read the next smaller line each time．
46．Keep in mind that each 0.25 D should allow the patient to read smaller letters．The patient reporting that the chart looks “better” is not sufficient justification to give him more minus．
47．Reach an appropriate stopping point．To stop the binocular MPMVA，choose one of the following endpoints．The order of presentation here does not imply order of preference．
a．The smaller/darker endpoint
(1) Introduce one more 0.25 D of minus (less plus)．
(2) Ask the patient whether the change makes the letters clearer or smaller, darker, or “better．”
(3) If the change makes the letters subjectively clearer to the patient，accept the change and repeat steps (1) and (2)．If the change makes the letters smaller, darker, or “better.” do not accept the change and take away the 0.25 D added in step (1)．
Note: When this technique is used，patients often accept 0.25 D more minus than the lens power that provided them with maximum visual acuity．While this outcome is acceptable，never give a patient more than 0.25 D beyond the lenses that gave him maximum VA．
b．The DUOCHROME endpoint
(1) Put the projector’s red-green filter over the chart of letters．
(2) Direct the patient’s attention to the 20/25 line or to the letters one line above his best VA so far．For some patients it may be necessary to isolate this line of letters．
(3) 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” or darker or brighter) letters or to state if the two sides are equally clear．
(4) If the letters on the red side are clearer or if the letters on the two sides appear equal, introduce an additional 0.25 D of minus (take away 0.25 D of plus) spherical power over both eyes．If the letters on the green side are clearer, take away 0.25 D of minus (or add 0.25 D of plus sphere) from both eyes．
(5) Repeat steps (3) and (4) until the patient is one into the green，the minimum amount of minus power at which the patient reports that the green side has the clearer letters．As an alternative, use a red-green balance，the point at which both sides appear equally clear, rather than one into the green，as the endpoint．
(6) Remove the red-green filter and recheck the VA．
Note：Some patients always seem to choose one side or the other，regardless of the lens powers in place．Be alert to the possibility that your patient is unresponsive to this test and abandon the duochrome test in favor of one of the other endpoints．
c．The 20/20 endpoint
If the patient has subjectively clear 20/20 vision，and if the lenses in the phoropter at step 46 match the prescriptions that were predicted from the starting information for both eyes, the binocular MPMVA may be stopped．The correct refraction must yield a close match between the actual and the predicted changes in lens power．
RECORDING
·Record the sphere power，cylinder power，and cylinder axis in the phoropter for each eye．
· Measure and record the VA for the right eye, the left eye, and for both eyes together．
Note：The patient’s final eyeglass prescription may differ from the results of the distance subjective refraction with the phoropter．Some examples will illustrate this point．If the prescription is greater than ±40 D, it is necessary to adjust it for the vertex distances of the phoropter versus the patient’s spectacles．Be conservative about changing the cylindrical components (particularly the axis) of a prescription that is comfortable for the patient．For high amounts of cylinder, it may be necessary to prescribe a different axis and power in glasses that will be used for reading as opposed to distance viewing．
EXAMPLE
Monocular Subj.
0D +1.75= -1.O0×165 20/15
0S +1.75= -1.00×10 20/15
Binocular Bal.
0D +2.O0= -1.O0×165 20/15
0S +2.O0= -1.OO×10 20/15
OU 20/15 |
