Hyperopic reserve linked to AL/CR ratio in early childhood

Paper title: Relationship between the axial length/corneal radius of curvature ratio and hyperopia reserve in preschool children aged 3 – 6 years

Authors: Wang J (1), Zhou J (2)

1. Department of Ophthalmology, Children’s Hospital of Fudan University, National Children’s Medical Center, Shanghai, China
2. Shanghai University of Medicine & Health Sciences, Shanghai, China

Date: Published online April 11, 2025

Reference: Wang J, Zhou J. Relationship between the axial length/corneal radius of curvature ratio and hyperopia reserve in preschool children aged 3 – 6 years. BMC Ophthalmol. 2025 Apr 11;25(1):198

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Hyperopic reserve refers to the physiological hyperopia present in early childhood that supports healthy emmetropization. This hyperopia gradually reduces with age, although children with insufficient reserve in pre-school years may be at risk of developing myopia once they are school-age. Using the axial length to corneal radius (AL/CR) ratio has been proposed as an alternative to cycloplegic refraction for evaluating hyperopia reserve. Previous studies have reported that AL/CR ratios exceeding 3.0 are risk marker for myopia onset in older children.1-3 However, axial length, corneal curvature, and hyperopia reserve change with age during preschool years. This variability may influence how reliable the AL/CR ratio may be as a risk marker for this age group.

The purpose of this study was to investigate the relationship between AL/CR ratio and hyperopia reserve in children aged 3–6 years. This cross-sectional study included 338 preschool children (mean age 4.8 years) in Shanghai, China, who underwent cycloplegic refraction and ocular biometry. Children were grouped by age and spherical equivalent refraction, and refractive status was compared with AL, CR, and AL/CR ratio. Regression equations and ROC analysis were used to assess the diagnostic value of AL/CR for hyperopia reserve insufficiency.

Key findings were as follows.

• Mean AL increased and hyperopia reserve decreased with age, from 0.15–0.64 mm/year and 0.20–0.76 D/year respectively.
• AL/CR ratio increased with age, while CR changed only slightly.
• An AL/CR ratio ≥2.91 was associated with hyperopia reserve insufficiency.
• At the 2.91 AL/CR threshold, the test correctly identified low hyperopia reserve in 84% of cases and correctly ruled it out in 88%
• No significant sex differences in AL/CR or hyperopia reserve were observed

While cycloplegic refraction remains the gold standard for assessing refractive error in young children, the AL/CR ratio offers a practical, objective alternative for identifying those at higher risk of myopia. This is especially relevant in preschool-aged children, where concerns over the sensation and side-effects of cycloplegia may limit its use.

The findings of this study confirm earlier research which reported that AL/CR values of 2.90–2.91 were linked to myopia risk in children aged 3–6 years.⁴  It also demonstrates that the AL/CR ratio increases with age, while hyperopic reserve decreases.

An AL/CR ratio of 2.91 or higher indicates insufficient hyperopia reserve and may help identify children needing closer monitoring, earlier cycloplegic evaluation, or proactive preventive advice. The more commonly used threshold of ≥3.0 may not be suitable for younger children still undergoing emmetropization.

For practices able to measure axial length, calculating the AL/CR ratio in routine preschool eye exams provides eyecare practitioners with an additional means of identifying children with reduced hyperopia reserve. 

This study had a small sample size and was conducted over a short period. Larger studies that follow children over time would provide stronger evidence to support the use of AL/CR in early risk assessment.

The same AL/CR cut-off value of ≥2.91 was applied to all children aged 3 to 6 years. However, hyperopia reserve changes with age during this period. Future research could look at whether age-specific thresholds offer more accurate identification of children at risk.

Prediction equations used to estimate hyperopia reserve were developed from the same group of children included in this study and haven’t been tested in other populations. They may be more useful as a screening measure, rather than as part of clinical decisions for individual children. Children identified as having low hyperopia reserve based on AL/CR should still be assessed with cycloplegic refraction when possible. 

Even with these limitations, this study supports previous research showing that an AL/CR ratio near 2.91 is linked to early myopia risk.⁶ Further studies should confirm this threshold in other populations and explore how biometric data can be combined with known myopia risk factors in early assessments.

 Purpose: This study aimed to investigate the association between the axial length (AL) to average corneal curvature (CR) ratio and hyperopia reserve in preschool children.

Methods: AL, CR, horizontal and vertical meridians of the corneal radius (CR1, CR2), and 1% atropine cycloplegic refraction were measured in preschool children aged 3 - 6 years. The corneal curvatures were then used to calculate the AL/CR1, AL/CR2, and AL/CR ratios.

Results: A total of 338 children were included, comprising 178 boys (52.7%) and 160 girls (47.3%). The mean values for AL, CR, AL/CR1, AL/CR2, AL/CR, and spherical equivalent refractive error (SER) were 22.11 ± 0.88 mm, 7.77 ± 0.26 mm, 2.80 ± 0.09, 2.90 ± 0.09, 2.85 ± 0.09, and + 2.13 ± 1.46 D, respectively. AL, AL/CR1, AL/CR2, and AL/CR increased with age, showing significant differences among age groups (P < 0.001). Conversely, SER moved from higher hyperopia toward lesser hyperopia with age, also showing significant differences among age groups (P < 0.001). Linear regression equations were established, with Y representing hyperopia reserve and X representing AL/CR: Age 3: Y = 44.67 - 15.02X; Age 4: Y = 33.96 - 11.19X; Age 5: Y = 42.11 - 13.98X; Age 6: Y = 44.94 - 15.00X. These results suggest that the AL/CR ratio could be used to assess hyperopia reserve insufficiency. The optimal cut-off point for the receiver operating characteristic (ROC) curve was ≥ 2.91, with a sensitivity, specificity, and Youden index of 0.84, 0.88, and 0.73, respectively. The critical value of the ROC curve increased with age in children aged 3 - 6 years.

Conclusion: This cross-sectional study found that the hyperopia reserve in children of different ages can be estimated using the hyperopia reserve equation. Additionally, the AL/CR ratio can serve as an effective index for detecting hyperopia reserve insufficiency, with an optimal ROC curve cut-off point of ≥ 2.91 in preschool children aged 3 - 6 years, and the critical value increasing with age.

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