the Correct Lens for Each Astigmatic Patient
TIMOTHY B. EDRINGTON, OD, MS, FAAO
Matching the best contact lens option to each
patient is a skill that improves patient satisfaction and practice success. I'll
discuss pearls related to astigmatic patients and contact lens selection.
Sphere, Low Cylinder (for example, –7.50 –1.00 x 180)
Soft and silicone hydrogel spheres, soft aspherics, soft and silicone hydrogel
torics and GP lenses all work well for most patients who have high sphere and low
Even though I've become more aggressive in prescribing torics
for my soft lens patients who have low amounts of astigmatism, I'm more conservative
when the spherical component is highly plus or minus because of the extra lens thickness
and resulting decreased oxygen transmission. As available parameters expand, this
refractive group would be good candidates to upgrade into silicone hydrogels.
High Sphere, High Cylinder (for example, +6.50 –4.00
x 90) These patients are ideal candidates for GP lenses, which generally
provide sharper vision and better corneal physiology compared to the soft toric
lenses available today. If a patient's corneal toricity exceeds 2.00D, I generally
prescribe a bitoric GP lens design. Even though a spherical or aspheric base curve
design may provide excellent vision, bitoric designs enhance lens centration, decrease
peripheral cor-neal desiccation and minimize lens awareness by decreasing lens movement.
Low Sphere, Low Cylinder (for example, –150 –0.25
x 90) These patients are great candidates for spherical soft and spherical
silicone hydrogel lenses. Spherical GP lenses may also provide good vision, but
many of today's patients balk at the initial adaptation.
Low Sphere, High Cylinder (for example, +1.00 –4.50
x 180) To optimize vision, I prefer bitoric GP lenses for the majority
of these patients. For a bitoric to succeed, you need a sufficiently toric cornea
(at least 1.50D of toricity) to stabilize lens rotation. Bitoric (as opposed to
spherical GP) lenses minimize physically induced localized corneal distortion for
highly toric corneas.
Practitioners often prescribe toric soft contact lenses for these
patients with mixed results. Vision may be decreased because of lens misalignment
or rotation. For a patient who has 4.50D of refractive astigmatism, 10 degrees of
cylinder axis misalignment will result in 1.50D of over-refraction (OR) cylinder
at an axis oblique to the prescribed axis (10 degrees of misalignment results in
one-third of the lens' correcting cylinder in the OR; 15 degrees of misalignment
results in one-half of the cylinder in the OR; and 30 degrees of misalignment results
in the full amount).
Medium, Medium (for example, –2.50 –1.50 x 90)
Many of your patients won't fall into any of the groups I've mentioned.
If a patient has –1.00D to –2.00D of cylinder in the manifest refraction,
yet the cornea is relatively spherical, rush to your soft toric fitting set. Spherical
or low toricity corneas don't provide enough of an anchor to stabilize lens rotation
with a toric back-surface GP. Textbooks suggest prism-ballasted, front-surface toric
GP lenses for these patients. (Ergo, soft toric.)
Don't overlook patients who could benefit from part-time lens
wear. Part-time wearers generally do better with soft lenses because of faster adaptation
and readaptation. Part-time wearers may also be less critical about the quality
of their vision through soft torics.
Of course, you should also weigh other considerations such as
the individual patient's vocation, hobbies and past contact lens experience in deciding
which option to prescribe. CLS
Dr. Edrington is a professor
at the Southern California College of Optometry. E-mail him at
Contact Lens Spectrum, Issue: December 2005