Myopia Profile

Clinical

Fitting orthokeratology successfully on a patient with astigmatism

 Posted on June 10th 2020 by Connie Gan

In this article:

When a patient has significant astigmatism, it can be difficult to decide whether it is better fitting a spherical or toric orthok lens.

In this post, PC shared with the Myopia Profile community a successful orthokeratology fitting on a patient who had moderate myopia with moderate astigmatism. PC fitted this patient with a custom designed toric orthoK lens. Here are some orthokeratology fitting tips from the Facebook community.

PC How quickly does OrthoK work for high myopia and astigmatism? Here’s a patient referred to me recently for myopia management. This is her worse eye, which had been progressing by around a dioptre per year wearing SVD glasses. No family history. Now -5.00/-1.25x10. AXL 25.15mm. Fitted with custom-designed toric OK. Bullseye topography showing ideal centration at week 1 and VA 6/6. No corneal staining. She said she didn’t need glasses after the third night. When fitted properly, OrthoK works very quickly indeed! ok.jpg

Before fitting orthokeratology, ensure there is no irregular astigmatism or any corneal disease. From the topography results, this patient’s astigmatism is mainly contributed by corneal toricity. The discussion turned to the type of lens design to achieve this ideal fitting.

Spherical vs Toric orthokeratology lens?

LN I would have thought a cyl this low could try out a spherical OK lens to start?.. at what degree do u start using these custom torics?...PC Where the elevation difference in the 2 meridians at the landing zone of the lens is 30 microns or greater, a toric lens gives a better fit and sealing. Which is essential for generating the hydraulic pressure required for a high correction. If it was say a -2.00 then may get away with a spherical OK.PG It’s the shape of the corneal toricity rather than the refractive astigmatism that decides whether or not to fit a toric OK lens. Toricity is physical shape and astigmatism the refractive error the toricity creates. In this case the toricity extends across the cornea (limbus to limbus). Think of this being like a horse saddle. If you tried to fit a spherical lens (saucer shape onto the horse saddle) it would rock and decenter. Using a toric lens fixes this problem... The rules are not fully agreed but I use 45µm as the difference between height of the corneal steep and flat meridian as the point where toric lenses should be used. Below 45µm use a spherical and above use a toric. It is only corneal toricity that defines whether a spherical or toric lens should be used. Once this is decided the next question is how best to correct the astigmatism.PG Part 2: Having decided which lens type (see previous comment), the amount of astigmatism that can be corrected depends on the lens type chosen - remembering that the lens type has been chosen only based on corneal toricity and not astigmatism . If a spherical lens is chosen (from step 1 - see previous post) you should be able to correct up to -1.50D astigmatism quite easily, but beyond this might be difficult - the research is limited. If a toric lens is being fit then this gives the option of also using a toric optic (a toric optic cannot be used in a spherical lens). The toric optic should allow correction of higher amounts of astigmatism - though again the research is limited. The toric Paragon CRT option uses a spherical optic, but custom deigns like WAVE and Forge will allow a toric optic and thereby possibly correct a greater amount of astigmatism
  • The decision to prescribe a toric orthoK lens depends on the amount corneal toricity instead refractive astigmatism. A spherical orthoK lens can typically be fit on up to 1.50D corneal astigmatism with-the-rule. However, higher amounts of corneal toricity lead to increased likelihood of treatment zone decentration.1
  • Hence, toric orthoK lenses are the better option in cases of moderate to high corneal astigmatism as it helps in achieving good lens centration compared to spherical orthoK.2 One could also choose a toric orthoK lens when there is an elevation difference of 30-45μm or more between the steep and flat corneal meridians.
  • The toricity of the orthoK is designed on the return curve and alignment curve for better alignment. This will ensure a good fit.
  • After deciding on a good lens fit, residual astigmatism can be corrected using either spherical or toric optics. Spherical optics can generally correct up to 1.50D astigmatism, while toric optics are ideal for correcting levels beyond that.

Do younger corneas respond faster to orthokeratology?

JS I have been thinking for a while now about what factors seem to affect the speed of treatment. The younger corneas are quicker than older is my theory. PG There is a nice paper on this from Jaikishan Jayakumar of the UNSW ROK Group. (see reference Jayakumar J. et al)PC Yes definitely I’ve noticed younger corneas are easier/quicker to shape. Some of the presbyopic-age patients take a bit of time.PP Good study looking at the relationship between corneal hysteresis and axial length change progression. (See reference Wan K. et al)PP Also a paper talking about short term changes to corneal hysteresis in early part of orthokeratology. (See reference Mao XJ. et al)
  • This patient was a 5D myope who achieved unaided vision of 6/6 (20/20) after only one week. With such a swift response, there was a small thread that discussed how quickly a cornea responds to orthoK treatment. Jayakumar et al3 found a reduced epithelial response to orthoK with increasing age - children and young adults (mean age 25 yrs) responded similarly while older adults (mean age 44 yrs) responded more slowly.
  • OrthoK does not permanently change the corneal biomechanical properties such as corneal hysteresis (CH) and corneal resistance factor (CRF).4 Research has indicated, however, some baseline biomechanical differences in corneal stiffness and modulus in poor-responders compared to good-responders wearing orthoK.5 Low CH and CRF may also be risk factor of myopia progression. Research has indicated that patients with low CH may benefit from OrthoK to slow myopia progression.6 

Take home messages

  • When deciding between spherical or toric orthoK lenses, check for corneal toricity rather than refractive astigmatism to achieve a good seal and lens fit.
  • Refractive astigmatism can then be corrected with either spherical or toric optics to allow for good visual correction.
  • Younger eyes tend to respond faster to orthoK treatments - you can expect patients over 40 to take longer to reach full correction.
  • Corneal biomechanical factors also appear to influence orthoK results - but since these are not measured in practice we can expect to see some variation between individuals in the swiftness and maximum treatment level of their orthoK response.

Meet the Authors:

About Connie Gan

Connie is a clinical optometrist from Kedah, Malaysia, who provides comprehensive vision care for children and runs the myopia management service in her clinical practice.

Read Connie's work in many of the case studies published on MyopiaProfile.com. Connie also manages our Myopia Profile and My Kids Vision Instagram and My Kids Vision Facebook platforms.

About Kimberley Ngu

Kimberley is a clinical optometrist from Perth, Australia, with experience in patient education programs, having practiced in both Australia and Singapore.

Read Kimberley's work in many of the case studies published on MyopiaProfile.com. Kimberley also manages our Myopia Profile and My Kids Vision Instagram and My Kids Vision Facebook platforms.

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