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
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
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.
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
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
Multifocal Toric Lenses As with spherical multifocal
lenses, these four trends will help you succeed with fitting soft
Binocular multifocal toric: multifocal toric on both eyes, balanced
Enhanced multifocal toric: multifocal toric OU, one eye enhanced for
either distance or near.
Modified toric monovision: multifocal toric on one eye,
single-vision toric (distance or near) on the other eye.
Standard toric monovision: single-vision toric OU, one for distance
and one for near.
Monovision with 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.
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
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
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.
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
TABLE 2 ContinuVu Empirical Fitting Guide
Other GP Multifocal Options
are some of the many other GP multifocal options available.
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
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.