Correcting Astigmatism with Orthokeratology
BY EEF VAN DER WORP, BSC, FAAO, FIACLE
Astigmatism can sometimes rule an orthokeratology fit simply because many rules are involved. However, new toric ortho-k lens designs have recently become available, and the future of ortho-k treatment of astigmatism looks bright.
Consider Astigmatism Type
The first rule that applies is that you can correct only corneal astigmatism with standard ortho-k lenses. Any residual refractive astigmatism due to internal astigmatism can possibly complicate the refractive outcome.
Rules also apply when it comes to the type of astigmatism: With-the-rule corneal astigmatism is preferable, because against-the-rule and oblique astigmatism are more difficult to correct. In addition, Mountford and Pesudovs (2002) demonstrated that standard ortho-k lenses reduce only about 50 percent of corneal astigmatism meaning that astigmatism of about 1.50D is usually the limit.
Another rule is that central corneal astigmatism is easier to correct than limbus-to-limbus corneal astigmatism. Collins et al (2006) at the University of Brisbane, Australia, recently categorized this phenomenon. They found that 38 percent of corneas have a spherical center with a spherical periphery and 22 percent showed a toric central cornea with a decreasing toricity towards the periphery. These should be relatively simple to correct. But another 21 percent of corneas have a toric center with a stable toric periphery, 15 percent showed a spherical central toricity with a toric periphery and 4 percent demonstrated toric central astigmatism with an increase in toricity towards the periphery. These represent potential fitting challenges.
Improved Toric Designs
The good news is that two independent groups in Europe (Beerten et al, 2005; Baertschi et al, 2005) have now shown that correcting corneal astigmatism with toric ortho-k lenses is feasible. They used corneal topography to calculate the sagittal height of the cornea in both meridians (which includes the increase or decrease in astigmatism towards the periphery) and created custom lenses based on this. In fact, this method is so successful that corneas with low astigmatism of about 1.00D are now also fit with these toric back surface designs. Alternatively, you can choose to design your own toric ortho-k lenses based on corneal topography with the use of additional software such as the Wave Contact Lens System (Wave Contact Lens System LLC).
The results from the European studies show that correction of with-the-rule corneal astigmatism up to 3.00D might be feasible, and that there are also possibilities for correcting against-the-rule and limbus-to limbus corneal astigmatism. You must take into account that the maximum amount of correctable ametropia with orthokeratology cannot exceed 6.00D (so a –3.50D spherical correction with –2.50D of astigmatism is pushing the limits). Furthermore, the amount of astigmatism should not exceed the spherical component of the refraction (a –2.00D sphere with –3.00D of astigmatism is not an ideal candidate).
But all in all, we can now accommodate the majority of patients who have corneal astigmatism and who are interested in ortho-k, which is another step towards the adolescence of orthokeratology. We and an increasing number of patients can all benefit from the availability of these toric ortho-k designs.
For references, please visit www.clspectrum.com/references.asp and click on document #128.
Dr. van der Worp is a lecturer at the school of optometry of the Hogeschool Utrecht and a researcher at the University of Maastricht department of ophthalmology in the Netherlands.