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Associated Phoria
PURPOSE To measure the amount of prism required to neutralize a misalignment of the visual axis under binocular viewing conditions. The associated phoria may be manifested either horizontally or vertically.
INDICATIONS
The associated phoria test is of great value in determining the amount of prism to prescribe for patients with vertical imbalances. It is a valuable diagnostic procedure for binocular problems in general, although its value may be greatest for the presbyopic population. The associated phoria is a single data point on the forced vergence curve.
EQUIPMENT
·Associated phoria targets generally come in three designs:
a. without a central fusional stimulus.
b. with a central fusional stimulus.
c. with a peripheral fusional stimulus (see Figure 4-18).
Depending on the manufacturer, the target may have separate nonius lines for the vertical versus horizontal function. Others are unified as one target.
·Phoropter with polarizing filters.
Note: The associated phoria may also be measured out of the phoropter using polarized glasses over the patient’s correction and a prism bar.
SET-UP
·The phoropter should contain the patient’s PD and the induced or habitual correction for the distance being tested.
·The target should be placed at the appropriate testing distance. For near this may be at either 40 cm or the patient’s customary working distance.
·The target should be well illuminated.
·The polarizers should be positioned in front of both eyes.
STEP-BY-STEP PROCEDURE
1. Instruct the patient to look at the target and ask him what he sees (see Figure 4-19).
2. If the patient reports he sees 2 lines (1/2 the target), the patient is spressing and the test is over. Determine which eye is spressing and record your results.
3. If the patient sees 4 lines, test for the existence of a vertical fixation disparity. Ask the patient if the horizontal lines form a perfect line, if they are exactly aligned with one another, or if one line looks higher than the other.
a. If the horizontal lines appear aligned, go to step 6.
b. If the horizontal lines are not aligned, a vertical fixation disparity exists (see Figure 4-20). Continue with step 4.
4. Determine the direction of the patient’s vertical associated phoria.
a. If the right line is too high, use base prism in front of the right eye or base down prism in front of the left eye.
b. If the left line is too high, use base down prism in front of the right eye or base prism in front of the left eye.
5. Determine the minimum amount of prism to achieve alignment of the horizontal nonius lines. This prism value is the vertical associated phoria.
6. Test for the existence of a horizontal fixation disparity. Ask the patient if the vertical lines form a perfect line, if they are exactly aligned with one another, or if one line looks off center.
a. If the vertical lines appear aligned, there is no horizontal associated phoria and the test is ended.
b. If the vertical lines are not aligned, a horizontal fixation disparity exists. Go to step 7.
7. Determine the direction of the patient’s horizontal associated phoria (see Figure 4-21).
a. If the top line is to the left, this is a crossed or exo fixation disparity. Measure the horizontal associated phoria by using base in prism over either the right eye or the left eye to align the nonius lines.
b. If the top line is to the right, this is an uncrossed or eso fixation disparity. Measure the horizontal associated phoria by using base out prism over either the right eye or the left eye to align the nonius lines.
8. Measure the size of the associated phoria by determining the minimum amount of prism needed to achieve alignment of the vertical nonius lines. This prism value is the horizontal associated phoria.
RECORDING
·Record the technique used and at what distance the test was done.
·Record the amount and direction of the prism required to achieve alignment.
·If a vertical associated phoria exists, specify the eye over which the prism was placed.
EXAMPLES
·Associated phoria: ortho/ortho, D + N
·Associated phoria: 2Δ BO/2Δ BD OD at N
·Associated phoria: Spression OS |
