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Fused Cross Cylinder
PURPOSE To eva luate the accommodative posture of a patient while viewing a near target under binocular conditions. The fused cross cylinder test contributes to a functional analysis when over- or under-accommodation at near is suspected. It is sometimes used to find a tentative add for a presbyope.
EQUIPMENT
·Phoropter with Jackson Cross Cylinders.
·Cross cylinder target (see Figure 4-13).
·Near point rod.
·Illumination source.
SET-UP
·The phoropter should contain the patient’s best distance correction from the most recent refraction or his habitual correction.
·Put the cross cylinder grid on the near point rod at 40 cm in dim illumination.
·Place the Jackson Cross Cylinders in front of both of the patient’s eyes, with the minus cylinder axis set to 90°(red dots vertical). Be sure not to change the axis of the correcting cylinder when making this adjustment.
·Set the phoropter to the patient’s near PD and make sure that both of the patient’s eyes are unoccluded.
STEP-BY-STEP PROCEDURE
1. Ask the patient to report which lines are sharper or more distinct, the lines going and down or the lines going across.
2. If the patient reports that the vertical lines are sharper than the horizontal lines before any lenses have been added, reduce the illumination. If the patient then reports that the horizontal lines are sharper or that the two sets of lines appear equal, proceed to step 4. Otherwise, proceed to step 3.
3. If the patient continues to report that the vertical lines are sharper with reduced illumination, flip the JCC axes so that the red dots (minus cylinder axis) are now along the horizontal meridian and again ask the patient which lines are sharper.
a. If the patient then reports that the vertical lines are still sharper, stop the procedure and record “vertical preference.”
b. If the patient reports that the horizontal lines are sharper, record “lead of accommodation,” “minus add indicated,” or “minus projection.”
4. If the patient reports that the horizontal lines are sharper or that the two sets of lines appear equal, add plus lenses binocularly, +0.25 at a time, until the patient reports that the vertical lines are sharper.
5. Reduce the plus power binocularly until equality in the sharpness of the lines is produced. If equality is not reached, stop at the highest plus lens through which the patient reports that the horizontal lines are sharper.
RECORDING
·Record the net amount of plus added, relative to the patient’s distance prescription, at the endpoint of the test (step 5 above).
·When appropriate, record “minus add indicated,” “lead of accommodation” or “vertical preference”(see step 3).
EXAMPLES
·FCC = +1.OO
·FCC minus add indicated
EXPECTED FINDINGS
·+0.50 (population mean)
·+0.25 to +0.75 (for nonpresbyopes)
·The lag of accommodation (amount of plus measured on the FCC) is expected to increase gradually as a function of the patient’s age. |
