prescribing for astigmatism
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO
I recommend using computer programs to determine resultant cross-cylinder effects. These results, combined with your LARS findings, allow you to fine-tune your prescription. If you tend to tweak data, then fudge these data toward the manifest refraction axis. This is especially true if you base your prescription on only one finding, not on multiple visits.
Remember, for every 10 degrees the toric soft lens correcting cylinder misaligns with the manifest refraction axis, the resultant cylinder in the over-refraction will show one-third the amount of the correcting cylinder. If the toric lens is misaligned by 15 degrees, then the over-refraction will reveal one-half of the correcting cylinder; a 30-degree misalignment will result in the full amount of the cylinder in the over-refraction.
Torics: To Prescribe or Not
I preemptively educate patients regarding the quality of vision with toric soft contact lenses. I stress that vision will be "comparable" to the vision they achieve with their spectacles. I follow with positive reinforcement, informing the patient that he'll love his new contact lenses. (Any softening of vision will generally be more if the patient's refractive error has a large astigmatic component compared to the sphere.)
If a patient presents with a vertexed manifest refraction with high minus sphere power and low cylinder correction (9.00 1.00 x 90), then I don't feel compelled to prescribe a toric soft lens. When you combine the peripheral thickness of a high minus power prescription and the added thickness of the stabilizing prism, you risk substantially decreasing the oxygen available to the cornea. Eghbali et al (1996) showed that in some cases, the oxygen available to the inferior cornea is approximately one-half the oxygen available to the superior cornea. Either prescribe a soft sphere or a GP lens. I recommend a silicone hydrogel if the patient accepts the vision through the spherical equivalent.
Empirical or Diagnostic?
- If the patient traveled far to get to the exam, then trial fit
- If you knew the patient was coming for a soft toric fitting, then you could've ordered the initial pair empirically, used it as your first trial lens and dispensed it if the fit and vision were acceptable. This saves your patient one trip and saves you one office visit
- If you're not abundantly behind in your schedule, then trial fit
- If you have lenses that are "in the ballpark," then trial fit and dispense
Englehart et al (1996) found that more than 90 percent of patients empirically prescribed toric soft lenses achieved 20/20 acuity and were happy with their lenses.
The major advantage of trial fitting before ordering is that you can verify that the lens fits well and fine-tune the cylinder axis. A disadvantage is that the patient doesn't necessarily have a wonderful initial vision experience. It often seems that I have to explain that the trial lens provides extra information and that their lenses will be better in terms of vision. You could also maintain a huge inventory so most of your patients desiring toric soft contact lenses can wear a pair home from their initial visit.
If you do trial fit from a limited inventory, then verify that the lens brand you're using as diagnostic lenses is available in the sphere, cylinder and axis that your patient will need.
I tend to use GP toric lenses when corneal toricity exceeds 2.00D, even if a spherical base curve lens would provide satisfactory vision and comfort. A spherical GP lens may cause minor corneal distortion on a patient who has moderate to high corneal toricity, resulting in spectacle blur.
Prescribing a bitoric GP should minimize spectacle blur and enhance lens centration for these patients. Also, decreased edge lift and improved lid apposition to the peripheral cornea may minimize 3 o'clock and 9 o'clock staining.
Dr. Edrington is a professor at the Southern California College of Optometry. E-mail him at email@example.com.