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Contact Lens Spectrum

Article

PRESCRIBING FOR ASTIGMATISM

HEADS OR TAILS?

By: TOM ARNOLD, OD
Contact Lens Spectrum April 1, 2022 Vol 37, Issue April 2022 Page(s): 12

Will you choose a corneal GP or scleral lens?

The goal of any contact lens evaluation is to provide patients with the best possible combination of visual acuity and comfort. When a patient presents with an ectatic cornea, the practitioner is confronted with some decisions.

For many practices, topography has replaced keratometry as the principal diagnostic tool. The three common maps—axial, tangential, and elevation—are all useful in the contact lens evaluation. These were covered in detail in a previous column (Arnold, 2021).

The ability to measure the entire corneal surface makes topography a valuable tool (van der Worp et al, 2020.). Additionally, topographers can accurately measure the visible iris diameter (VID) (Kojima and Caroline, 2009).

The first step is to locate and measure the amount of astigmatism present. Axial maps will show the areas of greatest curvature using a common radius. These maps are valuable in determining corneal shape, whether the astigmatism is “with-the-rule” or “against-the-rule” or is irregular (Anderson and Kojima, 2007).

Axial maps can be used to determine whether corneal astigmatism matches refractive astigmatism. Tangential maps use instantaneous radii to calculate each area of curvature (Anderson and Kojima, 2007). This is useful in measuring both the size of the cone and its exact location. Cones that are small and well-centered, like a nipple cone, may do very well with a corneal GP or hybrid lens.

Decentered, oval cones, or pellucid marginal degeneration (PMD) may present a greater challenge for a corneal fit and require a scleral lens (Arnold and Sindt, 2019).

Evaluate the axial map by moving the cursor around the meridians to locate an axis that has the least curvature change. The intent is to determine whether there is somewhere to “land” a corneal lens.

Next, switch to the elevation map. The elevation map compares corneal curvature to a “best fit” reference sphere (BFS). Areas that are higher than the BFS are shown in red (flatter) while areas that are below (steeper) are indicated in blue. This is the opposite of the color scheme of axial and tangential maps.

Find the location of the most elevated and most depressed areas of the cornea, which are in the same meridian, 180º apart. Place the cursor of the topographer on the periphery of the map indicating the most elevated area of the cornea (red) and record the height in microns. Repeat on the most depressed area (blue). Previous studies have shown that if this difference in elevation is less than 350µm, corneal GPs may be fit with a high rate of success (Zheng et al, 2015) (Figure 1). Eyes that have a greater degree of asymmetry may be better candidates for a scleral lens (Caroline and André, 2015) (Figure 2).

Figure 1. An elevation difference of 150µm makes this eye an ideal candidate for a corneal GP lens.
Figure 1. An elevation difference of 150µm makes this eye an ideal candidate for a corneal GP lens.

Figure 2. This asymmetric eye could benefit from a scleral lens.
Figure 2. This asymmetric eye could benefit from a scleral lens.

There are a couple of tips to keep in mind:

  1. Take the VID measurement along an oblique axis. Studies have shown that the vertical diameter is often shorter than the horizontal diameter (Fadel, 2018).
  2. Start with an overall diameter (OAD) that is 2.0mm less than the VID (e.g., 11.8mm VID ≥ 9.8mm OAD corneal lens) (Kojima and Caroline, 2009).
  3. When the astigmatism is limbal-to-limbal, bitoric base curves are indicated (Anderson and Kojima, 2007). CLS

References

  1. Arnold T. A deeper dive into mapping astigmatism. Contact Lens Spectrum. 2021 April;36:17. Available at https://www.clspectrum.com/issues/2021/april-2021/prescribing-for-astigmatism . Accessed March 9, 2022.
  2. van der Worp E, Kojima R, Johns L. Differentiating regular from irregular corneas. Contact Lens Spectrum. 2020 Sept;35:20-25,27,28,30.
  3. Kojima R, Caroline, P. Designing GPs from corneal topography. Contact Lens Spectrum. 2009 Oct 1;24. Available at https://www.clspectrum.com/issues/2009/october-2009/designing-gps-from-corneal-topography . Accessed March 9, 2022.
  4. Anderson D, Kojima R. Topography: A clinical pearl. Optometric Management. 2007 Feb;42. Available at https://www.optometricmanagement.com/issues/2007/february-2007/topography-a-clinical-pearl . Accessed March 9, 2022.
  5. Arnold T, Sindt C. PMD Versus Keratoconus: Clinical Differentiation. Contact Lens Spectrum. 2019 Aug;34:24-26,51. Available at https://www.clspectrum.com/issues/2019/august-2019/pmd-versus-keratoconus-clinical-differentiation . Accessed March 9, 2022.
  6. Zheng F, Caroline P, Kojima R, André M, Lampa M. Corneal elevation differences and the initial selection of corneal and scleral contact lens. Poster presented at the 2015 Global Specialty Lens Symposium, Las Vegas. January 2015.
  7. Caroline P, André M. Initial Selection of Corneal Versus Scleral GP lens. Contact Lens Spectrum. 2015 Apr;30:56. Available at https://www.clspectrum.com/issues/2015/april-2015/contact-lens-case-reports . Accessed March 9, 2022.
  8. Fadel, D. Scleral Asymmetry: Also in diameters. Contact Lens Spectrum. 2018 Dec;;33:46-48. Available at https://www.clspectrum.com/issues/2018/december-2018/reader-and-industry-forum . Accessed March 9, 2022.

Dr. Arnold is a regional vice president for AEG Vision and has a long-standing interest in specialty contact lenses. He is a Fellow of the Scleral Lens Society and is a consultant for CooperVision Specialty EyeCare and Eaglet-Eye. He often speaks internationally and now makes his home in Loveland, CO.

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