Overnight orthokeratology (ortho-k) provides refractive error correction, with the added “side benefit” of likely slowing myopia progression. With a relatively high success rate in eliminating refractive error for low myopes, how significant is the side benefit in this group?

The Theory

Prescribing full myopia correction provides an excellent foveal retinal image, resulting in high-quality visual acuity. Full central correction of myopia with most traditional methods (glasses, contact lenses) also results in a peripheral hyperopic defocus. Because more than 90% of retinal ganglion cells are located outside of the fovea, it is postulated that this peripheral image may be instrumental in driving axial growth, resulting in increasing myopia (Smith et al, 2009). Any refractive correction that provides a good central focus, while limiting or eliminating the peripheral hyperopic defocus, tends to be effective in lessening the rate of myopia progression while providing good central acuity.

Ortho-k reshapes the corneal refractive power, resulting in a less myopic central zone surrounded by a more myopic midperipheral region. The relative power differences between these two areas are related to the amount of myopia being corrected. In the case of a 3.00D to 4.00D myope, this midperipheral power would assist in significantly reducing or eliminating the peripheral hyperopic defocus.

A central corneal reduction of 3.00D to 4.00D would produce a relative effect of being 3.00D to 4.00D more myopic in the midperiphery. In fact, the midperiphery also demonstrates epithelial thickening, resulting in even more plus power delivered to the retina in this area. But what is the impact with lower levels of myopia?

The Case of Low Myopes

Consider children who have myopia of –0.75D to –1.25D. They require better distance acuity, and it is desirable to provide some amount of myopia progression control as well. Ortho-k can very often correct these low myopic errors, but the relative difference between the central and midperipheral corneal refractive power induced will be fairly low and not very effective for controlling myopia progression.

With custom optical designs, it might be possible to increase the myopia control effectiveness. Decreasing the optic zone places more of the annular peripheral optics in the pupil area and closer to the fovea (Harthan, 2017). Manipulating the landing angle on a reverse geometry ortho-k lens may also increase the myopia control effect.

While this can result in some degree of success (Chow et al, 2018), others have found it difficult to produce a significant effect with these parameter changes (Kang et al, 2013; Michaud and Simard, 2017).

So Where Do We Stand?

We need to learn a good bit more about the use and effectiveness of overnight ortho-k for myopia control in the case of low myopes. Rather than grouping all subjects together, it would be useful if future investigations into myopia control with ortho-k would separate out the low myopes, as they seem to represent a distinct case. Re-examining past reports may reveal whether myopia control is less effective when ortho-k is used in low myopes.

Additional studies looking at design changes in ortho-k lenses to maximize the midperipheral refractive effect in low myopes would be welcome. Would slightly under-correcting the central refraction (e.g., aim for plano as a target refraction versus the typical +0.50D or +0.75D), thus moving the entire retinal focus shell forward and somewhat reducing the peripheral hyperopic defocus, be beneficial in terms of myopia progression in low myopes? For most effective myopia control, are these low myopes better suited for soft multifocal lenses, with which higher adds can be used compared to overnight ortho-k?

Hopefully, answers to these and other questions will come in the not-too-distant future as we all increase our awareness and abilities in the myopia control arena. CLS

For references, please visit www.clspectrum.com/references and click on document #274.