Prescribing for Astigmatism

Add Two Quick Steps to Confirm Soft Toric Rotational Stability

By Thomas G. Quinn, OD, MS, FAAO

Several studies have shown that patients who have low astigmatism (0.75DC to 1.00DC) benefit visually when fitted with toric soft lenses instead of with spherical soft lenses (Richdale, 2007; Bayer and Young, 2005). In one survey, however, 30 percent of astigmatic patients reported that they had not tried toric soft lenses because their eye doctors did not recommend them (Decision Analyst, 2008). When I ask colleagues who prescribe spherical lenses for patients who have low astigmatism why they do so, many say that patients tolerate a little bit of blur all of the time better than intermittent blur from a rotationally unstable toric lens. How often is variable vision with a toric lens an issue? In one survey, 37 percent of patients wearing toric soft lenses reported fluctuating vision (Millward Brown, 2010). All of this leads to the conclusion that consistently clear vision is key to a satisfactory toric soft lens-wearing experience, but how can we know if a diagnostic lens being fit on the eye will deliver?

Attack the Issue Straight On

Typically, when we assess a soft lens fit, we evaluate centration and movement at the slit lamp by having the patient look straight ahead and blink. When evaluating a toric soft lens, we generally also note the location and stability of the lens marking during the blink as the patient continues to look straight ahead. This is a good first test, but two quick additional tests will tell us more about rotational stability.

Look Here, There, Everywhere

After assessing lens performance in primary gaze, ask the patient to look up, blink, then return to primary gaze. Note the position of the lens marking. Repeat this procedure, having the patient change gaze in various directions. Zikos et al (2007) and Young and McIlraith (2008) have demonstrated significant changes in lens orientation with changes in gaze. Chamberlain et al (2008) directly assessed visual acuity with gaze changes during toric soft lens wear and found that diagonal versions induced the greatest visual disturbance. Further, diagonal versions best highlighted differences in performance among lens types. Lenses that demonstrate large swings in lens marking position with gaze changes are less likely to provide consistently clear vision. If variable positioning is noted, consider changing base curve, center thickness, or lens design. If the position of the lens marking remains constant in straight-ahead gaze and with gaze changes, proceed to the third and final stability test.

Give a Little Push

Ask the patient once again to look straight ahead as you view the lens through the slit lamp. Note the position of the lens marking. Then, with your clean finger, manually rotate the lens out of position. Immediately move the slit lamp to the fellow eye and repeat the process. Once completed, return to the first lens and observe the position of the lens marking. A lens that has returned to the position observed before it was manually rotated should deliver reliable visual performance during day-to-day activities, even if the eye is rubbed or the lens is forcefully rotated by any means.

Rest Assured: You Can Deliver

Patients love clear vision, but it needs to be consistent. Use the techniques described here to ensure the toric lenses you dispense will deliver on both counts. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #195.