Quantifying interocular asymmetry of axial elongation in childhood myopia: metric framework, prevalence, and age-related dynamics

Background: Interocular asymmetry of axial elongation predicts accelerated myopia progression in adults, but childhood prevalence and dynamics are uncharacterised, with no standardised metric. A scale-invariant metric is proposed and characterised in a paediatric cohort with progressive myopia. Methods: A retrospective cohort of 267 children (5-16 years; 913 follow-up intervals) with progressive myopia and routine optical biometry collected during 2015-2026 was analysed. The Relative Asymmetry Index was defined per interval as the absolute interocular axial length difference normalised to the larger eye's change; its patient-level median defined the Cumulative Relative Asymmetry Index (CRAI). An annualised Absolute Asymmetry Index (AAI) was introduced as a complementary rate metric. Results: Median CRAI was 22.7 % [interquartile range 12.8-38.8]; median AAI 0.073 mm/year. Statistically detectable asymmetry (AAI > minimal detectable change at 95 % confidence) was present in 15 % of patients; 13.9 % showed pronounced asymmetry (CRAI > 50 %; AAI 0.218 mm/year). CRAI rose with age (Spearman rho = +0.28; partial rho = +0.25 after adjustment for net axial length change rate, both p < 0.001), and was independent of the overall rate after age adjustment (partial rho = -0.08, p = 0.20), consistent with scale invariance. The overall net axial length change rate decreased approximately three-fold across age strata (rho = -0.23, p < 0.001). Conclusions: CRAI and AAI provide complementary approximately scale-invariant and absolute metrics of interocular asymmetry. Interocular asymmetry appears common in this cohort and shows a marked age-related rise independent of overall progression rate.