IT WAS ALMOST two decades ago that the first controlled trial of orthokeratology (ortho-k) for myopia control in children was published (Cho et al, 2005). Since then, ortho-k has amassed arguably the largest volume of evidence for myopia control efficacy in children, with meta-analyses showing consistent and robust effects to slow myopia progression by around half across multiple studies (Sun et al, 2015).

The newest research in ortho-k for myopia control has explored various attempts to “boost” efficacy by altering lens design or lens optics. These include reducing the back optic zone diameter (BOZD) to achieve a smaller treatment zone diameter (TZD) on the eye.

A simple reduction of BOZD does not automatically translate to a similar topographical change without additional lens design modifications (Kang et al, 2013; Gifford et al, 2020). But the first randomized controlled trial (RCT) has shown that reducing BOZD from 6mm to 5mm achieves a 0.72mm smaller TZD on the eye at 12 months and increases the myopia control effect (Guo et al, 2021). The entirety of the “boost” occurred in the first six months, with subsequent data showing no differential eye growth between groups after this in up to 24 months of follow-up (Guo et al, 2023).

These results are mirrored by a similar RCT combining atropine 0.01% with ortho-k (Tan et al, 2023). Various reports of retrospective and clinical data had shown indication of a “boost” to ortho-k’s treatment efficacy through the addition of atropine (Gao et al, 2021), but this study found that the “boost” occurred again as differential rates of growth in the first six months, after which the ortho-k and atropine-plus-ortho-k groups showed a similar pace of eye growth (Tan et al, 2023).

Another RCT confirmed that the “boost” was only found in the first four months (Yu et al, 2022). Why is this occurring?

The key gain likely achieved with these “boost” efforts is increasing higher-order aberrations (HOAs). Indeed, the optic zone diameter RCT found that total HOAs, spherical aberration, and coma were increased in the 5mm group compared to the 6mm group (Tan et al, 2023).

Other studies have shown that increases in HOAs were associated with slower axial elongation in children wearing ortho-k (Hiraoka et al, 2015; Kim et al, 2019). This study found that only primary spherical aberration was correlated with axial elongation in the 5mm group, but not the 6mm group (Tan et al, 2023). Data on the six-month “boost” achieved by adding 0.01% atropine to ortho-k found the myopia control effect was correlated with larger pupil size and with some HOA metrics in the “boost” group, but not in the standard ortho-k-wearing comparison group (Vincent et al, 2021).

As a whole picture, what does this mean? 1) That increasing pupil size (with atropine) or decreasing TZD (with a smaller BOZD) in ortho-k increases HOAs; 2) that either of these treatment approaches provide a short-term “boost” in myopia control effect; and 3) that these optical changes then remain stable over time (Guo et al, 2023), so perhaps an adaptation process attenuates the short-term “boost” effect. The lack of clear association between HOAs and the myopia control effect in the conventional (non-boosted) ortho-k-wearing control groups indicates complexity; HOAs are perhaps not the entire story.

Adding to this fascinating picture are studies showing that 0.01% atropine does not “boost” the efficacy of center-distance multifocal (Jones et al, 2022) or dual-focus (Erdinest et al, 2022) soft contact lenses. Clinical data without matched groups indicate that there may be a boost achieved in defocus incorporated multiple segments spectacle lenses (Nucci, 2023). There is something special about ortho-k and much more to learn. CLS

References

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