Refractive Axial Sensitivity Index: A Novel Metric for Quantifying the Optical Efficiency of Axial Elongation in Myopia Progression

Purpose To introduce and validate the Refractive Axial Sensitivity Index (RASI), a novel metric quantifying the refractive change per unit of axial elongation, and to investigate factors influencing the optical efficiency of ocular growth in children and adolescents. Methods This longitudinal cohort study enrolled 13,860 children aged 6–18 years with complete biometric and cycloplegic refraction data at baseline and one-year follow-up. RASI was defined as |ΔSE/ΔAL|, where SE denotes spherical equivalent and AL denotes axial length. Multivariable linear regression and restricted cubic spline (RCS) analyses were employed to identify factors associated with RASI and the ratio of anterior chamber depth change to axial length change (ΔACD/ΔAL). Results The mean RASI was 2.05 D/mm. ΔACD/ΔAL emerged as the strongest independent predictor of RASI (β = 2.575, 95% CI: 2.469–2.681, P < 0.001). RASI exhibited a nonlinear relationship with axial elongation rate, peaking at 2.28 D/mm at an elongation rate of 0.27 mm/year before declining to a nadir of 1.76 D/mm at 0.59 mm/year, then rising again at rates exceeding 0.60 mm/year. Female participants demonstrated significantly higher RASI compared with males (2.17 vs. 1.94 D/mm, P = 0.033). Conclusions RASI provides a clinically meaningful metric for characterizing individual differences in the refractive consequences of axial elongation. The nonlinear relationship between axial elongation rate and RASI likely reflects dynamic crystalline lens compensation mechanisms, with distinct phases corresponding to physiological growth, active compensation, and compensatory exhaustion. These findings offer novel insights into the heterogeneous mechanisms underlying myopia progression.