Prescribing for Astigmatism
Prescribing for Astigmatism
Old-School GP Problem Solving
BY KAREN LEE, OD; DIANA NGUYEN, OD; & TIMOTHY B. EDRINGTON, OD, MS, FAAO
As GP materials, designs, and fabrication technologies improve, sometimes it is advantageous to apply old-school basics to achieve present-day success. We will discuss three oldies, but goodies.
Improve Vision and Comfort
Even though most patients who present with refractive astigmatism are prescribed soft torics, a substantial segment of astigmats can benefit from wearing GP toric designs. Patients who have astigmatism of 3.00D and more generally report enhanced crispness of vision with GP lenses.
We suggest prescribing a bitoric GP design for such patients, even if a spherical GP provides optimal vision. The toric back surface of a bitoric design will contour the cornea, reducing induced corneal distortion and minimizing spectacle blur. On most toric corneas, a GP lens with a toric peripheral curve system will also improve lens comfort due to the uniform edge lift and decreased lens movement.
The Old Switcheroo
Even though we instruct patients to remove the right lens first, and we often order the right lens with a green tint and the left lens with a blue tint, patients still occasionally manage to switch their contact lenses. If patients telephone (or tweet) your office and report that their contact lens vision has suddenly decreased in one eye, first ask them to remove their lenses and put on their spectacles. If the vision is still blurred in that eye, they should immediately come to your office for an evaluation. If the vision is good through their spectacles, then check the patient record to see whether a contact lens switch would cause that eye to be blurred. If so, then have them reapply their lenses in the opposite eyes. If the vision is now optimal, the contact lenses were switched.
If a patient reports to your office with the same complaint, a quick evaluation of fit and vision would most likely reveal fluorescein patterns of touch in one eye and clearance in the fellow eye, as well as a plus over-refraction in one eye and a similar amount of minus over-refraction in the blurred eye.
Eyelid Versus Lens
The effect of the eyelids on GP lens comfort and fit is often overlooked. If a patient is reporting lens discomfort, pull away the eyelids to see whether the symptom dissipates. If so, then assuming the patient is adapted to GP lens wear, the discomfort is probably due to the contour of the lens edge. If the fit is optimal, then the lens edge needs to be recontoured—rounded if too sharp or thinned if too rounded. This modification can be readily performed in-house or by sending the lens to the laboratory.
Excessive edge lift could also contribute to GP lens discomfort. A new lens with a steeper peripheral curve system would need to be ordered.
We regularly manipulate the eyelids to assess GP lens fitting relationships for our patients who have irregular corneal surfaces. It is important to observe the fitting relationship by fluorescein pattern with the lens centered and also in its “natural” resting position. Sometimes, despite your best efforts, lens centration cannot be achieved. If so, verify that the lens fit in its “natural” location is not too steep or too flat. That is, the areas of bearing do not have sharp borders and there are no (or only small) bubbles in the areas of lens clearance. Also, verify the fit by assessing areas of mechanical staining (too flat) and dimple veiling (too steep). CLS
Dr. Edrington is a professor at the Southern California College of Optometry at Marshall B. Ketchum University. You can reach him at firstname.lastname@example.org. Drs. Karen Lee and Diana Nguyen are the current cornea and contact lens residents at the Southern California College of Optometry at Marshall B. Ketchum University. Dr. Lee received her OD degree from Indiana University and Dr. Nguyen received her OD degree from University of California, Berkeley.
Contact Lens Spectrum, Volume: 28 , Issue: October 2013, page(s): 14