|Dailies AquaComfort Plus Toric from Alcon provides astigmats with a convenient daily disposable option.|
|The Flexlens ARC from X-Cel Specialty Contacts is a custom soft lens that fits like a regular toric.|
|The design of PureVision2 for Astigmatism from Bausch + Lomb stabilizes the lens to reduce blur.|
Correcting ‘normal’ and irregular astigmatism with contact lenses is easier than ever. Here’s how to confidently fit your patients.
The first thing many patients say when they sit in my exam chair is how difficult a fit they are going to be. When asked why, their response is, “I have the astigmatism.” So, why is this simple refractive error a feared condition among our patients””and even some doctors? It has the stigma as a debilitating condition when it should be the bread and butter of our livelihood. Fitting astigmatic lenses gives us the opportunity to not only impress our patients but also develop long-term relationships with them when we solve their vision problems.
Astigmatism greater than 1.0D affects 36% of the U.S. population over the age of 20 and increases to greater than half the population over the age of 60. Since patients benefit from a correction of 0.75D of astigmatism, there is a significant opportunity to fit toric lenses. (More than one in three patients benefit from toric lens fitting.) Toric lens correction is especially beneficial with against-the-rule and oblique astigmatism. To this end, most, if not all, lens manufacturers have lenses that start at 0.75D of astigmatism and offer around-the-clock axes. With all of the soft toric lens varieties available, almost every motivated astigmat should have a lens that works for them. While we have many different lens modalities to choose from (soft toric, custom toric, gas permeable, hybrid, and scleral lenses), soft toric lenses generally offer patients the best initial comfort.
THE FITTING PROCESS
Regardless of the amount of astigmatism, all you need to fit these lenses are a spectacle prescription, keratometry values, and a slit lamp to properly evaluate the on-eye lens fit. Since astigmatism prevalence and dry eye disease increase with age, it is also important to assess the overall health of the corneal surface, especially the quality of the tear film. If a patient suffers from dry eye symptoms, it is imperative that the condition is managed either prior to, or concurrent with, the contact lens fitting, especially if you are fitting a toric multifocal.
After ensuring a healthy corneal surface, select an initial lens that closely matches the spectacle prescription adjusted for vertex. Choosing a lens modality option depends on personal preference, but more manufacturers are offering daily disposables for toric lenses.
Options include Dailies AquaComfort Plus Toric from Alcon Laboratories, Clariti 1 Day Toric from CooperVision, Inc., 1-Day Acuvue Moist Brand Contact Lenses for Astigmatism from Johnson & Johnson Vision Care, Inc., and ClearSight 1 Day Toric from CooperVision.
Place the lens on the eye, check vision, over refract, and check for centration, movement, rotation, and stability. Allowing the lens to settle on the eye for 5 minutes will yield the most accurate results. If the lens is stable and rotates more than 5Â°, apply the LARS (left add, right subtract) rule to the lens prescription to properly align the lens powers. Lens stability is an important factor and indicates whether the lens fits properly. If lens rotation is unstable, vision will fluctuate throughout the day and, as the astigmatism correction increases, there will be a greater effect on vision. For example, a lens that rotates 30Â° off axis induces the full cylinder power at an axis oblique to the desired axis; and as little as 10Â° of rotation results in one-third of the correcting cylinder appearing in the over-refraction at some axis.
Crossed cylinder calculators can be found on the internet to assist in identifying lens power adjustments to optimize vision.
STABILIZE LENS ROTATION
There are two fundamental techniques to stabilize on-eye lens rotation. The first is prism ballasting. This method utilizes a lens thickness differential to stabilize the lens. The principle behind prism ballasting is that the upper lid presses on the thinnest portion of the lens squeezing the thicker portion of the lens into the bottom lid where rotation is minimized. This is referred to as the “watermelon seed” effect and is one of the oldest techniques used to stabilize a lens.
Periballasting is a modification to prism ballasting where the prism is restricted to the edge of the lens. Vision improves by eliminating prism from the optic zone of the lens. Biofinity Toric from Cooper-Vision, PureVision2 for Astigmatism from Bausch + Lomb, and Alcon’s Air Optix for Astigmatism lenses utilize varying forms of prism and periballasting for stabilization.
The second method is dynamic or thin zone stabilization. Originally, two thin zones at the lens periphery were utilized to orient the lens between the upper and lower lids orienting the thicker part of the lens centrally between the lids. Acuvue toric products utilize an adapted form of this approach called Blink Stabilized Design. This lens design uses four zones of accelerated slope at the lens periphery that, upon blinking, interact with both the lids to stabilize the lens. Generally, prism ballast lenses tend to rotate more with head and eye movement than the Blink Stabilized Design.
Sometimes “˜off the shelf’ parameters are insufficient to correct patients’ vision. Fortunately, many manufacturers can lathe cut a soft lens to any power or axis required. Until recently, this option was limited to only low Dk materials.
Custom toric lenses are also available for irregular corneas, especially keratoconus. Examples include Flexlens ARC-Atypical Refractive Correction from X-Cel Specialty Contacts and Kerasoft IC from Bausch + Lomb. These designs rely on increased central lens thickness to mask the corneal astigmatism or utilize asphericity which can offer patients with irregular corneas a solution to both correct vision and improve eye comfort. In addition to using central thickness to mask irregular astigmatism, NovaKone from Alden Optical uses Dual Elliptical Stabilization and full cylinder power to correct the astigmatism and ensure rotational stability.
With these “˜specialty’ lenses, managing increased astigmatic power, on-eye rotation, and lens stability are more critical to achieve success. Alternatively, in these situations, rigid gas permeable, hybrid, and scleral lenses may provide superior vision at the expense of some reduced comfort.
Fitting the astigmatic patient is of utmost importance to the optometric practice. Practitioners should view toric lens fitting as an opportunity, and this confidence should be shared with patients to solve their vision problems.
Kenneth A. Lebow, OD, FAAO, and Mark MacMillan, OD, are in private practice in Virginia Beach, VA.
WHERE TO FIND IT:
Alcon Laboratories, Inc.
800.451.3937 | alcon.com
Alden Optical, Inc.
800.253.3669 | aldenoptical.com
Bausch + Lomb
800.828.9030 | bausch.com
800.341.2020 | coopervision.com
Johnson & Johnson Vision Care, Inc.
800.843.2020 | ACUVUEProfessional.com
X-Cel Specialty Contacts
800.241.9312 | xcelcontacts.com