Astigmatic contact lens options offer our patients an alternative to glasses for vision correction. By embracing these patients and utilizing all of the contact lens technologies available, we can offer patients who may have previously been unsuccessful with lens wear the ability to wear contact lenses comfortably. This, in turn, will minimize dropouts and benefit both your patients and practice.
Astigmatism may be the least understood word by patients in eye care. They know if they have it, but many are unfamiliar with what it means and how it affects their vision. It is of concern when patients feel that they can’t wear contact lenses because of astigmatism.
Choosing Astigmatic Lenses
There are multiple options available to correct residual astigmatism for our patients. Although at one time a spherical equivalent for someone with a diopter of astigmatism would suffice, the options today are remarkably predictable in their performance and fitting characteristics. We have a number of silicone hydrogel toric lens options for patients with comprehensive fitting sets available. This allows for most astigmatic patients to be able to walk out of the office wearing their precise prescription.
Silicone hydrogel materials are critically important and have been embraced by eye care practitioners because of their high level of oxygen permeability. Studies have shown a decrease in bulbar and limbal hyperemia, as well as a reduction in end-of-day dryness.1,2
There are also significant differences in design. The Air Optix for Astigmatism (CIBA Vision), Biofinity Toric (CooperVision) and Purevision Toric (Bausch + Lomb) lenses are all stabilized with a prism ballasted design.3-5 The Acuvue Advance for Astigmatism (Vistakon) and Acuvue Oasys for Astigmatism (Vistakon) both utilize an accelerated stabilization design. This incorporates a dual thin zone superiorly and inferiorly, with additional stabilization areas where the lid margins interact with the lens.6 The most important factors—on eye stabilization and rotation predictability—are still hard to predict until the fitting process occurs, the lenses are assessed on the eye and the contacts are worn in a real environment.
The traditional philosophy for astigmatism correction said the minimum amount of refractive astigmatism that can be corrected is 0.75D, but today we have additional options. Extreme H2O Toric LC (Hydrogel Vision) lenses, for example, can correct astigmatism as low as 0.50D.7 It is our role to demonstrate this level of astigmatism and let our patients determine whether they appreciate the improved vision.
Silicone hydrogel toric lenses are arguably more important for prism ballasted toric lens designs because of their increased thickness inferiorly. Lathable silicone hydrogel—Definitive material that can be custom-made for high astigmatic prescriptions—is now available. The Definitive material is produced by Contamac and is composed of efrofilcon A with a Dk of 60.8 Manufacturing partners that utilize the Definitive material include Art Optical, Metro Optics, Unilens and X-Cel/Walman.
The Rigid GP Option
RGPs are a great resource for patients with high amounts of corneal astigmatism that require sharp, stable vision. When a patient’s corneal astigmatism coincides with refractive astigmatism, a spherical back surface can be used in an attempt to correct the corneal cylinder utilizing the tear lens that is created between the back surface of the RGP and the cornea. As the corneal cylinder increases, so does the likelihood that flexure of the lens will occur. When lens flexure occurs, the effective cylinder power delivered by the tear lens will decrease. Here, utilizing an aspheric design or a slightly increased center thickness may mask or prevent some of the effects of lens flexure.
Additionally, the fit of the lens and stability of a spherical back surface RGP over a highly toric cornea may be a non-ideal fit. In these instances, a back surface toric contact lens is favored to stabilize the RGP and provide better visual outcomes. Back surface toric RGP’s have limitations: They work best when the spectacle cylinder is approximately 1.5 times the corneal astigmatism. Although this is a quick rule for calculation purposes, the actual number will vary based on the refractive index of the RGP—which is usually slightly less than 1.5.
A bitoric lens may be warranted when other options are suboptimal. Having access to a good resource is key for successful patient outcomes. The RGP lab that you work with will be a valuable asset to consult with on very specific cases. The Gas Permeable Lens Institute (www.rgpli.org) is an additional resource.
When soft torics and traditional RGPs do not meet the needs of your astigmatic patient, consider the hydbrid lenses and large diameter RGP alternatives.
Hybrid lenses provide patients with the visual quality of a RGP without the lens edge awareness that often initially accompanies these lenses.
They are composed of a RGP center and are surrounded by a soft skirt. The SynergEyes A lens is composed of a Paragon HDS 100 rigid center and a non-ionic 27% water hydrogel skirt. This lens is intended for normal corneas with low to moderate amounts of corneal astigmatism. These lenses are typically fit approximately 1.5D steeper than flat K and they require large molecular weight fluorescein for proper fit assessment so that the fluorescein does not absorb into the soft skirt component of the lens.9
SynergEyes has recently introduced a new hybrid lens to the market—the Duette lens. Its RGP portion is a proprietary design (MaxVu), which has a Dk of 130. Its soft skirt is made of silicone hydrogel, hem-larafilcon A, which has a Dk of 84. In addition to the design and a simplified fitting process, this lens is unique in that regular fluorescein can be used to assess the fit of the lens. The initial base curve of the lens is selected based on their fitting guide and is near an on-K RGP fit. This lens is not fit as steep as the SynergEyes-A design and will account for larger astigmatic prescriptions. The ideal fit of the Duette shows an alignment fluorescein pattern and the fit is manipulated through changing the base curve of the silicone hydrogel skirt. For a lens that is fitting flat, the practitioner would not manipulate the base curve of the RGP portion but would steepen the base curve of the silicone hydrogel skirt.
Additionally, large diameter RGPs may be an option for patients who would benefit optically from a RGP design but have difficulties adapting to the lens. The SO2Clear design is a 13.3mm to 15.5mm lens that bears on the sclera. The fit is meant to have an alignment fit with the central cornea with a light touch present. This lens works remarkably well for those patients who have corneal astigmatism that matches their refractive astigmatism.10
Clinical considerations of the SO2Clear lens suggest that a slight amount of movement is desired. Additionally, careful inspection of the lens edge and its interaction with the conjunctiva should avoid any impingement of the blood vessels. To most accurately assess the fit on patients with diagnostic lenses, make sure to let the lenses settle for 15 to 30 minutes after placement on the eye, as the lens tends to tighten up as it is allowed to settle on the eye. Patients with an acceptable fit shortly after insertion, may have an unacceptably tight fit after the lens settles.
For patients being fit with either hybrid lenses or large diameter RGPs, make sure to have patients fill the bowl of the lens with either non-preserved saline or preservative-free artificial tears before insertion. This will ensure proper fluid behind the lens and will minimize bubbles between the lens and the cornea. Preservative-free products minimize cornea exposure to excessive preservatives. Patients should have their faces horizontally positioned over the mirror to minimize spilling of the solution out of the bowl of the lens before insertion.
Case In Point
A 26-year-old female patient presented to my office complaining of reduced vision, eye fatigue at the end of the work day and increasing headaches with her current contact lenses. She recently started a new job and is on the computer more than eight hours a day. Her visual acuity was 20/25- O.D. and 20/30+ O.S. with her current spherical contact lenses. Her manifest refraction was +2.00-0.75 x 109 O.D. and +3.25-1.25 x 092, with a best-corrected visual acuity of 20/20 O.U. She was wearing a silicone hydrogel +1.75D O.D. and +2.75D O.S. After demonstration of the cylindrical correction and discussion of visual compromise affecting her visual acuity and eyestrain, the patient decided to be refit with silicone hydrogel toric contact lenses.
She returned for her follow-up appointment ecstatic with her improved vision and relief from her eye fatigue.
1. Dumbleton K, Keir N, Moezzi A, et al. Objective and subjective responses in patients refitted to daily-wear silicone hydrogel contact lenses. Optom Vis Sci. 2006 Oct;83(10):758-68.
2. Dillehay SM. Does the level of available oxygen impact comfort in contact lens wear?: A review of the literature. Eye Contact Lens. 2007 May;33(3):148-55.
3. CIBA Vision. 2010. Available at: www.mycibavision.com (Accessed May 2011).
4. CooperVision. Available at: www.coopervision.com (Accessed May 2011).
5. Bausch + Lomb. 2011. Available at: www.bausch.com (Accessed May 2011).
6. Johnson & Johnson Vision Care Inc. 2011. Available at: www.jnjvisioncare.com (Accessed May 2011).
7. Hydrogel Vision. 2009. Available at: www.hydrogelvision.com (Accessed May 2011).
8. Contamac. Available at: www.contamac.com (Accessed May 2011).
9. SynergEyes. Available at: www.synergeyes.com (Accessed May 2011).
10. SO2Clear. Available at: www.so2clear.com (Accessed May 2011).