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Near Lateral Phoria by von Graefe Technique
PURPOSE To determine the relative horizontal position of the visual axes of the eyes at near when fusion has been broken．The von Graefe phoria test can also be used to measure the AC/A ratio．
EQUIPMENT
·Phoropter．
·Near point rod．
·Near point card with a small block of letters approximately 20/30 in size.
SET-UP
·The phoropter should contain the patient’s near correction and be set for the patient’s near PD．
·Place the near point card in good illumination on the near point rod at 40 cm from the patient．
·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 6Δbase prism serves as a dissociating prism for the lateral phoria test(see Figure 4-7)．
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．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．1f the patient is still unable to see two targets, change the dissociating prism to 6Δ base down on the left eye or change the amount of the base in prism in front of the right eye．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 until the targets are seen in the appropriate relationship．
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．Inform him that you will make the per target move．Ask him to tell you when the two images or targets are vertically lined , one directly above the other．Tell 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, 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 4-8)．
7．The result is the average of values in steps 4 and 6, if they are within 3Δof 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．
8．The von Graefe phoria test can be used to measure the gradient AC/A ratio．The near lateral phoria test is repeated through +1.00 or -1.00 over the patient’s near correction．The finding is then compared to the phoria finding through the near correction．The phoria will become more exo or less eso through +1.00 and more eso or less exo through -1.00．
RECORDING
·Record the size of the deviation in prism diopters and the type of deviation observed．
·For the gradient AC/A, record whether the test was done through +1.00 or -1.00，record the phoria finding，and the AC/A ratio (the difference between the two phoria findings)．
EXAMPLES
·NLP：ortho
·NLP：2Δ exo，-1.00 2 eso, AC/A 4/1
·NLP：5Δ eso，+1.00 3 exo, AC/A 8/1
EXPECTED FINDINGS
·Near lateral phoria：3Δ XP(±3Δ)；8Δ XP(±3Δ)for presbyopes
·AC/A：4/1(±2Δ) |
