Anderson, Dianne OD
Uncorrected astigmatism is often the reason for difficulty or failure in achieving success with presbyopic contact lenses. Attempting to correct both distance and near vision with monovision or multifocal lenses can result in compromised acuities. Even mild amounts of residual astigmatism can add to this consequence.
When to Correct Astigmatism
Keep in mind the cylinder value of a patient's spectacle refraction. Does it make a significant difference? Is it greater than the spherical value? Is it helpful in the patient's distance-only contact lens prescription? Answers to these questions can help predict the success of spherical multifocal lenses that have small amounts of refractive cylinder. When fitting spherical multifocals, it's best to follow the manufacturers' fitting guidelines as to what amount of astigmatism is acceptable.
Monovision with Soft Torics
Difficulty adjusting to monovision may result from uncorrected astigmatism. Using spherical equivalents can reduce acuity or cause a greater sense of imbalance. Toric lenses may be necessary to improve acuities and to restore balance. If the spectacle refraction calls for 0.75D to 1.00D of cylinder correction in both eyes, try applying a toric to the dominant eye to improve distance VA or to the non-dominant eye to restore near VA. Patients may need toric lenses OU to improve their sense of balance and for adaptation and in patients who have 1.00D or more of cylinder OU.
Soft Multifocal Toric Lenses
Soft multifocal toric lenses work well in astigmatic patients who have critical distance and near demands and for those who can't adapt to monovision. Following are a few examples.
CibaSoft Progressive Toric (CIBA Vision) is a front-surface, dual thin zone toric multifocal. The back surface consists of a center-near progressive add zone. Once the spectacle prescription is properly vertexed to the corneal plane, obtain the sphere power by adding one-half the add power to the distance sphere. Round off the cylinder axis to the nearest 5 degrees. Start with the 8.6mm base curve if the flat K reading is 42.00D or greater and order the 8.9mm base curve for flat Ks less than 42.00D. This daily wear conventional lens is a good choice for astigmatic patients who are emerging presbyopes.
Proclear Multifocal Toric (CooperVision) is a back-surface, prism-ballasted, monthly replacement toric available as a D lens for the dominant eye and N lens for the non-dominant eye. The D lens has a 2.3mm spherical central distance zone. The N lens has a 1.7mm spherical central near zone. The Proclear Multifocal Toric is based on the design of the UltraVue PC Multifocal Toric (CooperVision), which is available as an annual replacement lens in a wider range of parameters (Table 1). CooperVision recommends determining the dominant eye via fogging with a +2.00D hand-held trial lens. The eye that is blurred most by the +2.00D lens at distance is the dominant eye.
TABLE 1 Proclear Multifocal vs. UltraVue PC Parameters
Fitting Soft Multifocal Toric Lenses As with spherical multifocal lenses, these four trends will help you succeed with fitting soft multifocal torics:
1. Binocular multifocal toric: multifocal toric on both eyes, balanced binocular vision.
2. Enhanced multifocal toric: multifocal toric OU, one eye enhanced for either distance or near.
3. Modified toric monovision: multifocal toric on one eye, single-vision toric (distance or near) on the other eye.
4. Standard toric monovision: single-vision toric OU, one for distance and one for near.
Monovision with GP Lenses
GP lenses mask greater amounts of cylinder than soft lenses do. Aspheric GP designs work well if the cylinder is corneal as well as refractive. Most designs can correct up to 2.75D of cylinder. Toric GP designs are recommended for cylinder values greater than 2.75D.
Materials with mid-Dk values will maximize the refractive index (effective add power) and wetting ability of a multifocal GP. Some good choices are SGP 3 (The LifeStyle Company), Paragon HDS (Paragon Vision Sciences) and Boston ES (Bausch & Lomb). These materials have higher refractive indices and are known to produce better results in patients who have higher add requirements.
Figure 1. Corneal molding secondary
to long-term wear of a back-surface
aspheric multifocal GP lens.
Aspheric Multifocal GPs
Aspheric multifocal GP lenses are a good option for current GP wearers. The new aspheric lens designs incorporate the asphericity on the anterior surface. This simplifies the fitting process and reduces corneal molding issues. These designs, however, aren't recommended for corneas that currently exhibit molding secondary to long-term wear of back-surface aspheric multifocal GPs (Figure 1). Such corneas aren't spherical, and fitting with a spherical GP will result in decentration. Following are a few examples of this lens option.
Renovation (Art Optical) features front-surface eccentricity, controlled lens mass and increased add powers. The center-dis-tance/intermediate zone is aspheric and can be customized for the patient's pupil size (Figure 2). The add power is a spherical addition on the periphery of the front surface. Controlled lens mass helps to maintain centration and to decrease aberration with increased add powers. You can fit Renovation on corneas with up to 3.00D of cylinder. This design doesn't induce corneal molding because it's fit no more than 0.50D steeper than flat K. Order an add power 0.25D greater than the spectacle add.
Figure 2. Variations of the
Zone of a Renovation Multifocal.
ContinuVu (Essilor) is a progressive multifocal with a 2mm-diameter central distance zone on the back surface. The front surface consists of a central spherical distance zone available from 3.0mm to 5.0mm to accommodate pupil size. The back surface generates approximately +1.50D of the add power, with up to +2.00D additional add power incorporated onto the front surface. Fit the alignment curve essentially on K to minimize spectacle blur (Table 2). Fit the central base curve (distance zone) 2.00D steeper than the alignment curve.
TABLE 2 ContinuVu Empirical Fitting Guide
Other GP Multifocal Options
Following are some of the many other GP multifocal options available.
Tangent Streak Translating Bifocal and Tangent Streak No-Line (both Fused Kontacts) are available in toric designs. These are good choices for presbyopic patients who currently wear a toric GP lens. The no-line, back-surface aspheric progressive toric is an easy transition from monovision.
The Essential GP Multifocal (Blanchard Contact Lens) is a back-surface aspheric lens with low eccentricity to prevent corneal molding. A proprietary manufacturing process called S-Form Technology produces a smooth posterior power gradient shift from the center distance zone into the peripheral near zone. This lens is available as a front- or back-surface toric. Series I features the largest distance zone and allows up to a +1.25D power gradient for near and intermediate vision. Series II has an add zone that begins closer to the center distance zone, with increased eccentricity to generate a +1.50D to +2.00D power gradient for near and intermediate vision. Series III has the smallest center distance zone and the greatest amount of add correction (+2.25D to +2.50D) generated by greater back-surface eccentricity. The Concentric S-Form Addition (CSA) Enhancement is another option for patients requiring additional add power. CSA cuts a spherical annular near power curve on the anterior surface of the Essential lens and allows for up to +2.00D of additional add power. This option works best over a Series I or II Essential GP to increase the effective add power while preserving the distance vision. It works well when dealing with small pupils or poor lens translation.
TABLE 3 SynergEyes M Parameters
SynergEyes M (SynergEyes, Inc.) is the newest GP multifocal available. It's based on the SynergEyes A lens, but with a front-surface near segment in the center. Once you achieve a good fit and good distance acuity with the SynergEyes A lens, determine the parameters of the center-near segment using a patient's pupil size in normal room illumination and his add power as well as his lifestyle needs. The center design allows for customized positioning of the add power while the soft skirt keeps the lens well centered. It works well in presbyopes new to contact lenses as well as in seasoned GP wearers. It controls large amounts of astigmatism provided that the refractive cylinder is less than or equal to the corneal cylinder. Fit the SynergEyes lens 1.50D to 2.00D steeper than flat K. The back surface is spherical so minimal corneal molding occurs. Table 3 lists the parameters.
Dr. Anderson is a graduate of Indiana University School of Optometry. She practices in the western suburbs of Chicago, specializing in orthokeratology, keratoconus and post-surgical lens fits and anterior segment disease. She may be reached at Dianne.Anderson@comcast.net.
Special thanks to the following consultants for providing lenses and expert advice: Mike Johnson (Art Optical), Dr. Tim Giles (CIBA), Louise LeLerc (CooperVision), Keith Adams (Essilor), Dave Rusch (Fused Kontacts), Richard Dorer (Blanchard) and John Davis (SynergEyes).
For references, visit www.clspectrum.com/references.asp and click on document #136.