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Distance Lateral Phoria by von Graefe Technique
PURPOSE To determine the relative horizontal position of the visual axes of the eyes at distance when fusion has been broken．
EQUIPMENT
·Phoropter．
·A distance acuity chart that can isolate letters.
SET-UP
·The phoropter should contain the patient’s distance correction and be set for the patient’s distance PD．
·A single letter on the distance chart，one line larger than the patient’s best corrected visual acuity in the poorer eye, should be displayed．
·Place the Risley prisms before both eyes．Instruct the patient to close his eyes while you are adjusting the prisms．Set the prism before the right eye at 12Δ base in and the prism before the left eye at 6Δ base ．The 12Δ base in prism serves as a measuring prism and the 6△ base prism serves as a dissociating prism for the lateral phoria test(see Figure 4-1)．
STEP-BY-STEP PROCEDURE
1. Instruct the patient to open both eyes．Ask the patient how many targets he sees and where they are in relation to one another．The patient should see two targets：one and to the right，one down and to the left．
a．If the patient sees only one target，check to see if one eye is occluded or alternately occlude each eye to help the patient locate each of the images in space．An alternative method is to change the dissociating prism to 6Δ base down on the left eye or change the amount of base in prism in front of the right eye．
b．If the patient sees two targets，but one is and to the left and the other is down and to the right, increase the amount of base in prism on the right eye．Do this until the targets are seen in the appropriate relationship，one and to the right，one down and to the left．
2．Instruct the patient to look at the lower target，and to keep it clear．
3．Instruct the patient to look at the lower target，but think about the other，or per one．Tell him that you will make the per target move．Ask him to tell you when the two images or targets are vertically lined in a straight line，one directly above the other．Inform him that he should always see double，to keep the images clear，and to continue to look at the lower target．
4．At about 2Δ per second, reduce the base in prism until the patient reports vertical alignment of the two targets．Note the amount of prism and the direction of the base of the prism when the patient reports that the two targets are aligned．
5．Continue changing the measuring prism in the same direction (ie，overshoot the point of alignment), until the patient sees the two targets as one and to the left, one down and to the right．
6．Bring the measuring prism back in the other direction until the patient again reports vertical alignment of the two targets．Note the amount of prism and the direction of the base of the prism when the patient reports that the two targets are aligned (see Figure 4-2)．
7．The result is the average of values in steps 4 and 6, if they are within 3Δ of each other．If the two values are not within 3Δ of one another, repeat the measurement, emphasizing the instructions to the patient，and average the two closest values．
RECORDING
·Record the size of the deviation in prism diopters and the type of deviation observed．
EXAMPLES
·DLP：ortho
·DLP：2Δexo
·DLP：4Δeso
EXPECTED FINDINGS
1Δ XP(±3Δ)：1Δ EP(±1Δ) for presbyopes |
