specialty contact lenses
Breaking Down Bitoric GPs
Practical tips for patient selection, lens design and a comfortable fit
BROOKE MESSER, O.D., EDINA, MINN.
GH, a 10-year-old female soccer player, presented for a contact lens evaluation. Her manifest refraction was +2.50-2.50x167 OD and +5.25-4.25x169 OS. She was correctable to 20/20 in each eye. Corneal topography revealed 2.50D of corneal astigmatism OD and 4.25D OS. After I discussed the visual and ocular health benefits of GP bitoric lenses, GH and her parents were enthusiastic about the lens fitting and her vision potential.
When many of us hear “GP contact lenses,” we immediately think of their benefits to patients who have corneal diseases, such as keratoconus. That said, we must not forget the other patient populations who can achieve successful wear in these lenses, particularly those like GH who have high refractive errors and high astigmatism and are, therefore, often able to improve both their vision and quality of life with bitoric GP contact lenses.
Here, I discuss the ideal candidates for bitoric GP contact lenses, lens selection and design, how to determine an optimum fit and GH’s current status.
Bitoric GP lenses are designed for patients who have moderate (2.00D) to high amounts (2.75D) of corneal astigmatism. To determine whether a patient who falls into one of these categories would benefit from this lens, use a corneal topographer, or insert a spherical GP lens.
► Corneal topography. For example, a patient who has 2.50D of corneal toricity that topography reveals extends limbus-to-limbus is much more likely to need a bitoric lens vs. a patient who has the same toricity, but with a toric central cornea alone. (The latter patient can achieve successful wear in spherical GP lenses that provide high amounts of toricity because the patient’s peripheral cornea can align with the lens’ spherical back surface.)
► Spherical GP lens. Use a lens that has a base curve of the patient’s flat keratometry reading, and evaluate the fluorescein pattern. If excessive edge lift is in the steep corneal meridian alone, the toricity likely extends to the limbus, making the patient a good candidate for a bitoric lens. (See Figure 1, left.)
Figure 1: Spherical GP on a 3.50D toric cornea. Note the excessive edge lift in the steep cornea meridian.
In addition to meeting the criteria described above, ideal candidates for bitoric GPs can be new to contact lenses or frustrated soft lens wearers looking for an improved visual experience. Also, patients who have more astigmatism than sphere power in their refraction are especially good candidates because they are particularly sensitive to even the smallest amount of lens rotation that may occur while wearing a soft toric contact lens. In addition, spherical GP wearers who experience lens decentration are candidates for bitoric lenses because decentration can cause glare and haloes due to the pupil approaching the edge of the lens’ optical zone, and it can cause spectacle blur after lens removal due to a misshapen cornea. A bitoric lens may improve the lens-wearing experience in these patients by enhancing vision, increasing comfort and decreasing the risk of lens ejection during critical daily activities, like driving.
Lens selection and design
Once the decision to fit a bitoric GP lens has been made, the right lens can be acquired via fitting trial lenses, empirical fitting or by corneal topographer-assisted lens design programs.
To minimize the chair time, check out the following resources, which are available at the GP Lens Institute website (gpli.info):
► The Mandell-Moore Guide for Empirical Design. This “tried and true” lens calculator efficiently designs a lens by using patient information, such as keratometry and spectacle prescription. I use this guide on busy clinic days when I need a quick and accurate lens design.
► The GPLI Toric and Spherical Lens Calculator. For the visual learner in all of us, this guide has graphics of the cornea, tear layer and contact lens base curves for a demonstration of the optical system. In addition, you have the option to select for overall larger and smaller lens diameters, and the calculator compensates the lens curvatures automatically.
► The Newman GP Toric Guide. If you like to control all lens parameters, this is the guide for you. The Newman GP Toric guide assists you in choosing overall diameter, peripheral curve radii and other lens aspects.
► Lens lab. Consider measuring your patient’s corneal diameter (HVID), lid positioning and toricity to support the lab in parameter selection.
If you have a corneal topographer that has a lens design program, the predicted fluorescein pattern feature can be beneficial in making fine adjustments to lens size and curvatures after you utilize an empirical method. Ideal parameters of an empirical bitoric GP lens have the flat base curve nearly matching the corneal flat K and the toricity of the lens (difference between the base curves) at 0.75D to 1.0D less than the corneal toricity. A small amount of mismatch between the lens and corneal toricity facilitates tear exchange and a healthy cornea.
Determining an optimum fit
Assessing a bitoric GP lens fit can be challenging. Whether you have performed a trial fitting or empirically ordered the lens, be sure to examine the central fluorescein pattern of the lens when it is centered on the cornea. If you see a circular pattern of central touch or clearance, the bitoric lens matches the corneal toricity, and to achieve an optimal fit, each base curve of the lens must be steepened or flattened by the same amount, respectively. (See Figure 2, left.)
Figure 2: A bitoric GP lens with appropriate toricity, but showing central touch. Both the flat and steep base curves of this lens should be steeper to achieve an appropriate fit.
If the fluorescein pattern is a band of corneal touch on the flat corneal meridian, termed “residual tear lens toricity,” the lens does not provide enough lens toricity, and the curvatures must be altered to increase the difference between the two meridians, usually by adjusting both base curves. If base curve changes are needed, remember to alter the lens powers by the steeper-add-minus, flatter-add-plus (SAM-FAP) rule to compensate for the tear layer changes. (See Figure 3, page 45.)
Figure 3: Note the residual tear toricity in this bitoric lens.
Once the central fluorescein pattern has been evaluated, the other lens aspects, such as lens movement, peripheral curve clearance and over-refraction, must be documented. If the lens is not moving, the lens-cornea relationship might be “too perfect,” and decreasing the lens toricity to prevent lens binding is recommended. If you note too much movement, there may be residual tear lens toricity, or consider increasing the lens’ overall diameter if the fluorescein pattern appears ideal. The peripheral curve system should now appear spherical and be adjusted similarly to a spherical GP fitting. A sphero-cylindrical over-refraction should be completed, and if there is a significant amount of astigmatism noted (≥0.75D), optical crosses and cross-cylinder calculators are helpful in adjusting lens powers.
You can find a helpful calculator and other GP resources at www.therightcontact.com. If there is less than 0.75D of astigmatism in the over-refraction, add the spherical equivalent to both meridians of power.
When I dispensed the bitoric GP lenses to GH, the lens fit was nearly perfect — there was a small myopic over-refraction in both eyes, which was easily corrected by an adjusted lens order. GH now experiences 20/20 vision OU during all of her daily activities. OM