An Astigmatism Action Plan for Your Practice
An Astigmatism Action Plan for Your Practice
Soft toric success is a matter of knowing the options, evaluating the refraction and considering comfort.
By Susan Kovacich, OD, FAAO
Dr. Kovacich graduated from Indiana University School of Optometry and completed a hospital-based residency at the St. Louis VAMC. In 1998 she returned to IU as a clinical assistant professor, and is current co-director of the Cornea and Contact Lens Clinic. She has worked as a researcher and consultant for for Allergan, Bausch & Lomb and Vistakon.
In this article, we'll review important factors to consider in successful soft toric contact lens fitting. These factors include:
• Keeping updated on the existing soft toric lens designs and materials.
• Analyzing the refraction with regard to the sphere/cylinder ratio, the total amount of cylinder, and the location of the cylinder axis; then making the appropriate soft toric lens selection from the available choices.
• Taking into consideration patients who may have problems with contact lens wear due to dry eyes, allergies or who may be better candidates for an occasional wear schedule.
Soft Toric Lens Designs
The rotational stability of a toric lens — and therefore the stability of vision — is the key to success in fitting soft toric lenses. Historically, soft toric lens designs were prism-ballasted, with the thickness and weight of the lens concentrated at the 6 o'clock location. This "watermelon seed" design had the upper eyelid sliding over the thinner upper portion of the lens and pushing the thicker lower portion of the lens down into the correct position. Most soft toric lens designs today utilize this principle for success.
Two of the four silicone hydrogel toric lenses are modified ballasted designs. PureVision Toric (Bausch & Lomb) utilizes a ballasted design with a bevel 360 degrees along the circumference of the lens (comfort chamber) to reduce mass and allow the lens to tuck under the lower eyelid. Air Optix for Astigmatism (CIBA Vision) utilizes the Precision Balance 8|4 design, where the thickest points in the ballasted lens are at the 8 o'clock and 4 o'clock positions to improve rotational stability by minimizing interaction with the lower lid.
Vistakon uses a design that is not ballasted. The Accelerated Stabilization Design features four thickened areas of the lens which remain between the eyelids when the eye is open to stabilize the lens. This design minimizes interaction with the lower lid and is used in both the Acuvue Advance for Astigmatism and the Acuvue Oasys for Astigmatism lens designs.
Silicone Hydrogel Toric Lenses versus Hydrogel Toric Lenses
Vision and comfort are the most important concerns for contact lens patients; you also need to consider corneal physiology. Silicone hydrogel lenses provide the best oxygen transmission to the cornea, and these are my initial lenses of choice if they are available in the appropriate parameters (Table 1).
Refitting Existing Hydrogel Toric Patients If there is any time when the adage, "If it ain't broke, don't fix it" applies to contact lenses, it probably applies to toric soft lenses. Many astigmatic patients have attempted several different toric lenses before achieving success. Therefore, if a patient is satisfied with vision and comfort, both you and the patient may be unwilling to change. However, because the ballast design is thicker at the 6 o'clock position, vessel encroachment and/or neovascularization often manifests in this area with the use of hydrogel toric lenses. If the patient is wearing a hydrogel toric lens and the eye is manifesting any signs of hypoxia including injection, microcysts or vascular encroachment, you should consider a refit with silicone hydrogel toric lenses.
Astigmatism Action Plan There are currently four silicone hydrogel toric lenses available, and if the appropriate parameters are available, this option should be your first choice for new wearers. When an astigmatic patient wearing a hydrogel toric lens has signs of corneal hypoxia, consider silicone hydrogel toric lenses.
The important factors to note in an astigmatic patient's refraction include the amount of cylinder power in relation to the amount of sphere power, the total amount of astigmatism and the cylinder axis.
Sphere/Cylinder Relationship One of the most challenging scenarios in fitting soft toric lenses occurs when the cylinder power is greater than the sphere power, especially if the patient is plano or approximates plano in his spherical power. The stability of the lens rotation will be more critical in these patients who will be more sensitive to any change in cylinder axis. These patients often experience better vision from spectacles than from soft toric lenses.
Case Study #1 A 20-year-old male reported being previously told that he would not be able to wear soft lenses because of his astigmatism, but wanted to see if there were any new lenses available. His manifest refraction is OD –0.50 –1.25 ×180 20/20 and OS –0.50 –1.25 ×175 20/20. We tried several lenses but all rotated slightly with the blink. The patient was not satisfied with the vision in any of these lenses.
He was ultimately successful with Air Optix for Astigmatism in the following parameters: 8.6mm BCR; OD –0.50 –1.25 ×180 20/20, no rotation and OS –0.50 –1.25 ×180 20/20, no rotation.
Astigmatism Action Plan It's important to educate these patients from the beginning that their prescription is more challenging to fit, and it may be necessary to try more than one lens design. If a lens is available in more than one base curve radius, attempt the steeper base curve to minimize rotation while maintaining adequate lens movement.
Total Amount of Astigmatism (High Cylinder) It's important to be aware of the available lens parameters before selecting a particular brand of lenses. Due to their superior oxygen transmission, I recommend silicone hydrogel materials as the first choice when making a lens selection. However, at the time of this writing, no silicone hydrogel soft toric lens is available in cylinder powers greater than –2.25D.
Numerous hydrogel toric lens designs are available with cylinder powers which extend much greater than –2.25D. CooperVision has several extended range (XR) products including the monthly Proclear, Frequency 55 and Preference lenses, as well as the quarterly Hydrosoft Toric, which has a very large range in both cylinder (up to –10.00D) and sphere powers (±20.00D).
Astigmatism Action Plan It's beneficial to have an updated chart with the available cylinder powers posted so that — with a quick glance — you can select an appropriate trial lens. If no lenses are available in the fitting set, select the lens that most approximates the final power so that you can assess rotation before ordering a trial lens. Select the parameters in order of cylinder axis first, cylinder power second and sphere power last.
Total Amount of Astigmatism (Low Cylinder) Twenty years ago, toric contact lens manufacturing was less precise. It often seemed as if every time we re-ordered a toric lens in the same parameters, the new lens was different from the original, making toric soft lenses almost as individual as snowflakes. Today manufacturing techniques are better and the reproducibility is more reliable. However, to avoid some of the problems associated with fitting soft toric lenses, many low cylinder (–0.75D or –1.00D) patients were fit in spherical lenses with the spherical equivalent of the refraction used for the power.
Today, there is much interest in correcting for higher-order aberrations (HOAs), particularly the fourth-order HOA of spherical aberration, in both spectacle and contact lenses. However, lower-order aberrations (LOAs), such as the second-order aberration of astigmatism, have a much higher impact on vision than do HOAs.
Why correct HOAs when it's not uncommon to fit spherical lenses rather than correct for astigmatism? Fitting a low-cylinder patient with a spherical lens is easier for you and often less expensive for the patient, but the trade-off in vision may not always be acceptable. The patient's visual demands should be the primary driver when selecting a contact lens.
While a spherical lens may be adequate for an astigmatic weekend athlete who wears soft contact lenses for soccer, it may not be sufficient for a patient who has taxing near demands. The lowest cylinder correction in most soft toric lenses is –0.75D, but the Extreme H2O LC (low cylinder) design from Hydrogel Vision Corporation has a cylinder power of –0.65D. This lens also comes in a medium cylinder (MC) option of –1.25D.
Case Study #2 A 19-year-old female complained of eye strain and blurred vision, especially at near after reading for more than 30 minutes. She was a freshman in college and was studying more than she did in high school. She had worn spherical soft contact lenses successfully for the past four years.
The patient's spectacle Rx was OD –3.50 –0.75 ×180 20/20 and OS –3.25 –0.75 ×175 20/20. Her habitual contact lens Rx was SofLens 38 (Bausch & Lomb) 8.3mm base curve; OD –3.75DS 20/20 and OS –3.50DS 20/20.
We refitted her with the following lenses: PureVision Toric (B&L) 8.6mm base curve; OD –3.50 –0.75 ×180 20/15 (rotates 5 degrees temporally) and OS –3.25 –0.75 ×180 20/15 (no rotation).
At the two-week follow-up, she was much more satisfied with her vision and no longer felt her eyes strain when she read or worked on the computer.
Astigmatism Action Plan If an astigmatic patient is complaining of asthenopia and is wearing spherical soft lenses, you should attempt a toric lens design. As the patient is not customarily provided with the "spherical equivalent" in spectacles, the spherical equivalent may not be adequate in soft lenses.
The Cylinder Axis Due to the interaction of both the upper and lower eyelids with a contact lens, the location of the cylinder axis is also a factor when fitting soft toric lenses. With-the-rule corneas are easier to fit than against-the-rule corneas, and an oblique axis is the most difficult to fit as the lens will be more susceptible to rotation.
Astigmatism Action Plan Not all cylinder powers are available in oblique axes, so options may be more limited.
Dry Eye Patients, Allergy Sufferers or Occasional Contact Lens Wearers
Contact lens comfort is often an issue when fitting any contact lens, including soft toric contact lenses. If a patient reports poor comfort or reduced wearing time, you should explore the cause(s) of these symptoms. Silicone hydrogel toric lenses, toric lenses approved by the United States Food and Drug Administration (FDA) for dry eye and daily disposable toric lenses may help improve comfort for these patients and allow them to wear lenses successfully.
Dry Eye Complaints Dry eye patients often have problems with comfort, especially toward the end of the day. Silicone hydrogel lenses transmit more oxygen to the corneal surface, and a healthier eye may lead to more comfortable contact lens wear. The hydrogel Proclear (omafilcon A, CooperVision) lenses have FDA clearance for the claim that they "may provide improved comfort for contact lens wearers who experience mild discomfort or symptoms relating to dryness during lens wear."
Daily disposable lenses may also help these patients, and currently there are two daily disposable toric options (Table 2).
Allergic Individuals Patients with allergies tend to be problematic, as they may also be sensitive to contact lens disinfection solutions. Replacing multipurpose solutions with peroxide disinfecting solutions can help. However, daily disposable lenses are my first choice for these patients because they eliminate both allergens that adhere to the lens and the need for disinfection solutions.
Occasional Contact Lens Wearers Some patients cannot successfully wear contact lenses or choose to wear their lenses on an occasional wear basis. These patients may desire to wear contact lenses only for social or athletic pursuits and also may benefit from daily disposable lenses.
Case Study #3 A 38-year-old female patient reported problems with dry eyes and lens intolerance in the past. The patient was a research optometrist who said she unsuccessfully attempted "every" available soft toric lens. She desired to be fit in a spherical soft contact lens that she could wear a few hours at a time when attending her children's sporting events. We informed her that a new daily disposable toric option was available, and she was willing to try it.
Her spectacle Rx was OD –7.50 –1.00 ×165 20/20 and OS –7.25 –1.75 ×025 20/20.
She was successfully refit with ClearSight 1 Day Toric lenses (CooperVision) in the following parameters: OD –6.50 –1.25 ×180 20/20 (no rotation) and OS –6.50 –1.25 ×020 20/20 (no rotation).
The patient wore the lenses successfully to sporting events several times a week and was thrilled to wear lenses again on an occasional wear schedule.
Astigmatism Action Plan When an astigmatic patient has dry eyes, allergies, sensitivity to disinfection solutions, contact lens intolerance or desires to wear lenses on an occasional wear schedule, consider silicone hydrogel toric lenses, toric lenses approved for dry eyes and daily disposable toric lenses.
While astigmatic patients can be challenging, you can successfully fit them by learning about the soft toric lenses available, carefully evaluating the patient's refractive status and considering comfort issues. CLS
Contact Lens Spectrum, Issue: December 2008