Step 4: Refine Cylinder Axis and Power
Once you have confirmed that an astigmatic refractive error is present, refine the cylinder axis as follows.
(Directions for use with a minus cylinder phoropter)
With the initial estimate of the cylindrical correction dialed into the phoropter, place the Jackson cross cylinder device in the “axis” position; that is, with an imaginary line connecting the 2 small thumb-wheels aligned parallel to the axis of the cylinder battery (Figure I-25). That alignment is indicated by the arrows on the axis knob and by indicator lines or arrowheads near the aperture of the phoropter. The cross cylinder should click into place. Present a binary comparison between the 2 positions of the cross cylinder by rotating 1 of the thumb-wheels. Notice how this interchanges the indicator dots (red and white) on the cross cylinder lens. Continue the binary comparisons, and after each, turn the (minus) cylinder axis toward the orientation of the red dots on the cross cylinder lens in the position where the patient reports clearer vision. The endpoint is often vague (“about the same,” “equally bad,” and so on). Watch for increased hesitation by the patient, or a reversal of the progression of the axis as you make successive binary comparisons. The process is self-correcting—if you go too far in 1 direction, the patient’s responses will indicate that the cylinder axis should be brought back to a previous position.
Next, refine the cylinder power.
(Directions for use with a minus cylinder phoropter)
Turn the cross cylinder turret 45° to the “power” position, as indicated by an imaginary line connecting the “P” marks on the cross cylinder lens rim running parallel to the (newly determined) cylinder axis (Figure I-26). Flip the cross cylinder to provide a binary comparison. Notice how either the red dots or the white dots on the cross cylinder will line up along the cylinder axis. If the patient’s preferred position of the cross cylinder places the red dots along the cylinder axis, increase the minus cylinder power. (If the preferred position of the cross cylinder places the line through the red dots perpendicular to the cylinder axis, so the line connecting the white dots lies parallel to the cylinder axis, reduce the minus cylinder power.) Repeat until you reach an endpoint, where the views through the alternate positions of the cross cylinder are “about the same.” [At this stage, the view may still not be very good!] When in doubt, go with the numerically weaker choice of minus cylinder power.
KEY STEP: For each 0.5 D (2-click) increase in minus cylinder power as determined by these cross cylinder flips, add +0.25 D (1 click) of plus sphere power. After you have located the endpoint, remove the cross cylinder from the optical pathway. If all is well, the patient should experience a considerable improvement in visual acuity.
Once the optimal cylinder axis and power have been determined, proceed to step 5.
Once the optimal cylinder axis and power have been determined, proceed to step 5.
After optimizing the cylinder power, you may find it useful to recheck the cylinder axis. If the cylinder axis is substantially changed, recheck the cylinder power as well.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.