What factors influence axial length growth in ortho-k wear?

Paper title: Factors associated with faster axial elongation after orthokeratology treatment

Authors: Ya, Qi (1,2); Liu, Lizhou (1); Li, Y (1); Zhang, Fengju (3)

1. Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Science Key Lab, #1 Dong Jiao Min Xiang, Dong Cheng District, Beijing, 100730, China.
2. Department of Ophthalmology, Beijing Children's Hospital, Capital Medical University, National Center for Children's Health, Beijing, China.
3. Beijing Tongren Eye Center, Beijing Tongren Hospital, Capital Medical University, Beijing Ophthalmology and Visual Science Key Lab, #1 Dong Jiao Min Xiang, Dong Cheng District, Beijing, 100730, China

Date: Feb 2022

Reference: Qi Y, Liu L, Li Y, Zhang F. Factors associated with faster axial elongation after orthokeratology treatment. BMC Ophthalmol. 2022 Feb 8;22(1):62

[Link to open access paper]

Orthokeratology (ortho-k) has been shown to be effective in slowing axial length growth, and therefore myopia progression.^1-11 However, there is a lack of uniform response to Ortho-K treatment and individual treatment outcomes can vary.

This retrospective study investigated baseline factors that related to faster axial growth after one year of ortho-k and the possible causes for axial length decrease in some individuals. The clinical record data for 73 myopic children fit with DreamLite ortho-k lenses in the Beijing Tongren Hospital was collected. Baseline measurements included parental myopia, axial length (AL), cycloplegic auto-refraction, central corneal thickness (CCT) and anterior chamber depth (ACD). The children wore the lenses for a year. They were seen for follow-ups after one day, one week, one month and every 3mths after. The measurements were repeated after 1-yr for comparison. Only values for the right eyes was included in the study (64.4% female).

At baseline, the data showed the following characteristics: average spherical equivalent refractive error (SER) of -3.22D, average age of 10.32yrs old when fit with ortho-k lenses and parental myopia of either Grade 1 (no myopia, 65.8%), Grade 2 (one or both parents with mild to moderate myopia up to -6.00D, 13.7%) or Grade 3 (-6.00D or stronger, 20.5%). 

After 1 year of ortho-k wear, corneal power, CCT, and ACD were found to have changed by an average of -1.99 ± 0.89D, −5.95 ± 14.23 μm, and -0.034 ± 0.075 mm, respectively.

The average axial length increase was 0.18 ± 0.17mm, other than for 9 right eyes which experienced negative growth of −0.06 ± 0.04 mm. There were no statistical differences between the AL of the increased and decreased groups at baseline, despite statistically significant changes in AL for each group.

Correlation analysis found that factors found to be associated with increased axial lengths after 1-yr of wear were:

• Younger baseline age
• A positive correlation with parental myopia and
• Lower baseline myopia

 Associated factors for decreased axial length after 1-yr of wear were via correlation analysis were:

• Older initial age when fit with ortho-k
• Larger baseline corneal power and
• Higher baseline myopia

This study found faster axial growth after 1 year of ortho-k wear was associated with having lower initial myopia, being younger when fitted with ortho-k and having higher parental myopia.  It also found that the younger the children were when fitted, the faster their axial elongation was. This would suggest that a harder-hitting approach to myopia management could be needed for this patient category.

• Eyecare practitioners could consider combining ortho-k with atropine, especially if the child has low myopia before commencing fitting.^12,13
• From the results of their 2-yr study, Tan et al found using 0.01% atropine alongside ortho-k gave a 50% slowing effect of axial elongation for 6-11yr olds.¹⁴ Read more about this study here.

Those children with slower axial growth were typically older when fitted with ortho-k, had a higher initial myopia and higher baseline corneal power.

• The results from this study confirm those found from previous research where older children (10—11yrs) or young adults (25-26yrs) had slower, or stopped, progression with Ortho-K. ^11,15

Faster myopia progression with orthokeratology may be related to genetic influences from parental myopia:

• One study found little correlation with children’s axial length changes and parent’s myopia, although lower levels of myopia were associated with smaller axial changes¹⁶
• Combining orthokeratology with atropine may enhance the effect of slowing progression for children with low myopia and parents with high degrees of myopia.
• Younger children are often fast progressors and Ortho-K may play a protective role in reducing rapid axial growth. Cho et al found younger children had faster progression after wearing lenses for 2yrs, but the myopia control effect was more pronounced for them.¹⁸

Further studies are needed to maximize the effects of Ortho-K, with or without combination atropine treatment.

This study found decreased axial length values for some children.

• Decreased AL has previously been assumed to be due to thinner central corneas and thicker choroid layer only
• Central cornea thickness (CCT) was seen to be thinner by a mean 5.95 ± 14.23µm and where AL was seen to decrease, the CCT was thinner by 9.44 ± 13.28µm. However, the AL decreased by a larger opposing amount (0.06 ± 0.04mm).
• Choroidal thickness was not measured in this study, but the value for the reduction in AL did not correspond to values for choroid thickening and corneal thinning from other studies

So far, no studies have been able to show the choroid has thickened to the degree that AL can be meaningfully reduced. Further research is needed to investigate other possible causes for axial length decreases other than choroidal thickening and corneal thinning.

 Changes in anterior chamber depth (ACD) were also seen in this study (0.034 ± 0.075 mm decrease). This has been observed in other studies after Ortho-K wear ^15,16,17

• The authors suggest the anterior segment after Ortho-K wear may become more prolate than oblate-shaped, giving a shorter sagittal axis to the eye. However, any axis changes may be less apparent than the faster eye growth as a whole and this may be why decreased axial changes are seen for older children with slow progression or adults who have ceased growing.

Future research can shed light on the underlying mechanism for decreased axial length.

 Being a retrospective study gave some limitations:

• The data could only be reviewed for correlations, rather than directly analyzed. This gives a lack of evidence to support hypotheses of underlying action for reduced axial lengths
• The sample size was relatively small

Future studies could include a larger cohort, confirm the effects found over longer periods of wear and if different lens designs were able to provide the same benefit.

Title: Factors associated with faster axial elongation after orthokeratology treatment

Authors: Ya, Qi; Liu, Lizhou; Li, Y; Zhang, Fengju

Purpose: To study the baseline factors that related to faster axial elongation after orthokeratology (OK) treatment and the characteristics of cases with axial length decrease in a group of myopia children.

Methods: This is a retrospective study. The records of 73 children who had wear OK lens for at least one year were reviewed. Only the data of right eyes were included. Baseline data included: age, gender, parental myopia, refractive error, corneal power, central corneal thickness, axial length and anterior chamber depth. Corneal power, central corneal thickness, anterior chamber depth and axial length after one-year of OK lens wear were also collected. The related factors affecting axial length change were analyzed. A comparison was made on the cases of axial length increase and axial length decrease.

Results: Of the 73 eyes, axial length increased by 0.18 ± 0.17 mm (P < 0.001) after one year of OK lens wear. Correlation analysis showed that one-year axial length change was negatively correlated with age and positively correlated with the parental myopia and baseline myopia. Stepwise multiple linear regression analysis showed that the factors associated with faster axial elongation were lower baseline myopic spherical equivalent (P = 0.018), higher parental myopia degree (P = 0.026), and younger age at the onset of lens wear. (P = 0.039). Nine eyes showed negative axial growth (−0.06 ± 0.04 mm), and had older initial age of lens wear, higher baseline myopic spherical equivalent, and lager baseline corneal power, when compared with cases of axial length increase.

Conclusions: Myopia children with lower baseline myopic spherical equivalent, younger initial age and higher parental myopia had faster axial elongation after orthokeratology treatment. More aggressive treatment should be considered. In children with slow axial elongation, OK lens wear may lead to negative axial growth. Whether there are reasons other than central corneal thinning and choroidal thickening needs further study.

[Link to open access paper]