Can baseline axial length measurements be predictive of OrthoK efficacy?

Paper title: The relationship between baseline axial length and axial elongation in myopic children undergoing orthokeratology

Authors: Weiping Lin (1), Na Li (1), Kunpeng Lu (1), Zhaochun Li (2), Xiaohua Zhuo (2), Ruihua Wei (1)

1. Tianjin Key Laboratory of Retinal Functions and Diseases, Tianjin Branch of National Clinical Research Center for Ocular Disease, Eye Institute and School of Optometry, Tianjin Medical University Eye Hospital, Tianjin, China
2. Euclid China (Euclid Trading (Shanghai) Ltd.), Shanghai, China

Date: Nov 2022

Reference:  Lin W, Li N, Lu K, Li Z, Zhuo X, Wei R. The relationship between baseline axial length and axial elongation in myopic children undergoing orthokeratology. Ophthalmic Physiol Opt. 2023 Jan;43(1):122-131.

[Link to open access paper]

Orthokeratology (OK) is a widely used method of myopia control and studies have shown axial elongation could be slowed by up to 63% over 2 years compared to spectacle wear.^1-7 However, myopia progression has also been found to be influenced by individual variability and some baseline ocular parameters.

This retrospective study assessed the effect of different baseline axial lengths on the efficacy of OK treatment over 3 years.

• Data on children aged 8-14yrs with refractive error between -0.75 to -6.00D and astigmatism no more than 1.50D were accessed through the medical records database of Tianjin Medical University Eye Hospital Optometric Centre.
• There were 1176 children in the 1-year follow-up (the first cohort), wearing their personal choice of either Ok (n = 588) or spectacles (n = 588). All had a comprehensive eye examination at baseline to include axial length (AL) measurement.
• Those aged 8-11yrs who completed the 3-yr follow up made up the second cohort (n = 150).

The OK wearers from this group were further divided according to their baseline axial length values: sub-group 1 (AL lower than 24.5mm), sub-group 2 (AL between 24.5 and 26mm), sub-group 3 (AL of 26mm or more). AL was measured during annual visits and changes in the AL in these sub-groups were compared.

Although both groups of children showed axial length increases in the 1-year follow-up, those in the OK group showed 39% slower axial elongation (0.19 ± 0.21mm) in the 1^st year than the spectacle-wearing group (0.31 ± 0.19mm) and the axial elongation was found to be negatively correlated with initial age for both groups (more axial elongation in younger children, as expected).

After adjusting for age, axial elongation in the 1^st year was found to have a negative association with baseline AL for the OK, but only for 8-11yr olds within the OK group. There was no association seen for any age in the spectacle wearing group. This means more axial elongation was found in children wearing OK who had shorter initial axial lengths; less in children with initially longer axial lengths.

When the sub-groups were examined at the 3-yr follow-up, sub-group 1 showed significantly higher annual axial elongation compared to sub-groups 2 and 3 in the 1^st and 2^nd years, but not in the 3^rd year. Axial changes for sub-group 1 appeared to stabilise in the final year.

Baseline spherical equivalent error (SER) was also found to have a significant association with axial elongation for the OK group only and the higher myopes at baseline showed less change during the 1-yr follow-up.

The children in this study aged 8-11yrs who wore orthokeratology lenses and had baseline axial lengths of less than 24.5mm had similar axial growth to spectacle wearers in the 1^st year.

However, the children who had longer baseline axial lengths (of 24.5mm or more) and who wore orthokeratology showed slower axial growth during the first 2 years of treatment than children with baseline axial lengths of less than 24.5mm.

• This suggests that baseline AL may be predictive of myopia progression for 8-11yr olds wearing OK.
• The annual AL change for those with baseline AL values between 24.5 and 26mm was consistent in the first 2yrs before stabilising in the 3^rd year, meaning the OK lenses had demonstrated efficacy.

Knowing this could help eyecare practitioners identify children who would most benefit from OK treatment and achieve the optimal results for their myopia progression

This was a retrospective study, meaning there may be other factors such as pupil size relative to the OK treatment zone and rate of myopia progression before baseline which were not investigated in the original data source and therefore don't appear in this study.

Other limitations to this study include small cohort sizes which may mean the findings cannot be extrapolated to larger populations and where orthokeratology efficacy may be based on more than one element, considering baseline AL only may oversimplify the picture.

Further research with longer studies and larger cohorts could further explore the impact of modifying OK lens parameters and the extent that optical changes such as higher order aberrations influence OK efficacy.

Title: The relationship between baseline axial length and axial elongation in myopic children undergoing orthokeratology

Authors: Weiping Lin, Na Li, Kunpeng Lu, Zhaochun Li, Xiaohua Zhuo, Ruihua Wei

Purpose: To investigate the correlation between the baseline axial length (AL) and axial elongation in myopes undergoing orthokeratology (ortho-k).

Methods: This was a retrospective study. During the 1-year follow-up, 1176 children (aged 8-14 years) were included and divided into an ortho-k group (n = 588) and a single-vision spectacle group (n = 588). The ortho-k group participants (8-11 years of age) who completed the 3-year follow-up (n = 150) were further divided into three subgroups stratified by their baseline AL: subgroup 1 (AL < 24.5 mm), subgroup 2 (24.5 ≤ AL < 26 mm) and subgroup 3 (AL ≥ 26 mm). AL was measured at baseline and during the annual visit.

Results: The ortho-k group exhibited slower 1-year axial elongation (39% reduction) than the spectacle group. The 1-year axial elongation was negatively correlated with initial age in both groups. A negative association between 1-year axial elongation and baseline AL was observed in the ortho-k group but not in the spectacle group. However, this relationship only existed in ortho-k participants 8-11 years of age. For the younger ortho-k participants who completed the 3-year follow-up, the annual axial elongation was significantly higher in subgroup 1 for the first and second years but not in the third year compared with subgroups 2 and 3.

Conclusions: Axial elongation was negatively correlated with baseline AL in the ortho-k group. Children aged 8-11 years with longer baseline AL (≥24.5 mm) demonstrated slower annual axial elongation during the first 2 years of ortho-k treatment, which may provide insight into establishing individual guidelines for controlling myopia using ortho-k in children with different baseline characteristics.

[Link to open access paper]