Defining the critical axial length threshold for transition from physiological growth to pathological fundus structural remodeling in adolescent myopia.

The dynamic evolution of fundus tessellation density (FTD) and peripapillary atrophy (PPA) during childhood myopia progression-and their threshold-associated transitions relative to axial length (AL)-remain incompletely characterized. To address this gap, we prospectively quantified longitudinal FTD and PPA changes in a school-aged cohort with progressive myopia, and modeled their nonlinear dose-response relationships with AL to identify the biomechanical threshold at which fundus remodeling shifts from physiological adaptation to early pathological tissue deformation. A cohort of 371 children aged 8-11 years was prospectively followed for 2 years. FTD and PPA were measured via a validated deep learning system; myopia progression subtypes were stratified via k-means clustering by annual AL elongation rate; longitudinal trajectories and inflection points were estimated via linear mixed-effects models and quadratic regression. Three progression phenotypes emerged: stable (0.147 mm/y), moderate (0.443 mm/y), and rapid (0.827 mm/y). Each 1 mm AL increase correlated with 0.127 mm2 (95% CI 0.106-0.148) PPA expansion (p < 0.001) and 0.011 FTD elevation (p < 0.001). PPA showed J-shaped acceleration with inflection at 23.99 mm; FTD showed U-shaped trajectory, declining before 23.66 mm then rising exponentially. This is the first longitudinal study to identify a ∼24 mm (23.66-23.99 mm) biomechanical threshold for early fundus pathology in children-substantially below the adult high-myopia criterion (>26 mm)-supporting a paradigm shift toward earlier AL-driven risk stratification based on tissue biomechanical susceptibility rather than conventional static refractive cutoffs.