Early Axial Length Response Predicts Long-Term Myopia Control in Children Undergoing Orthokeratology

To evaluate whether early axial length (AL) change after 6 months of orthokeratology (OK) wear serves as a clinically meaningful biomarker for predicting long-term myopia control after 2 years. This retrospective analysis included 101 children from a multicentre OK cohort who completed 24 months of follow‑up, with AL measured at baseline and at 6, 12, 18 and 24 months. Early AL shortening was defined as a ≥0.01 mm reduction after 6 months. Longitudinal AL change was assessed using linear mixed‑effects models. To evaluate predictive utility, separate logistic regression models were fitted using the 6-month AL change (ΔAL₆ m ) as a continuous predictor for two predefined AL outcome categories: favourable AL response (ΔAL₂₄ m ≤ 0.30 mm) and poor AL response (>0.59 mm). Receiver operating characteristic (ROC)‑derived probability thresholds were obtained using Youden’s J statistic. External validation was performed in an independent OK cohort of 69 Japanese children (138 eyes). Fourteen children (13.9%) showed early AL shortening. Mixed‑effects modelling showed a significant time × shortening interaction ( p < 0.001), with shorteners elongating by ∼0.006 mm/month versus ∼0.019 mm/month in non‑shorteners, yielding ∼0.33 mm less AL growth at 24 months. ΔAL₆ m strongly predicted a favourable AL response (area under the curve (AUC) = 0.90, p < 0.001), with an optimal threshold p *=0.33 corresponding to ΔAL₆ m ≈ +0.07 mm. ΔAL₆ m also predicted a poor AL response (AUC = 0.86, p < 0.001), with p *=0.11 (ΔAL₆ m ≈ +0.09 mm). External validation confirmed high AUCs (0.92 and 0.86) but revealed imperfect calibration, particularly for poor AL response. Early AL change after 6 months of OK wear is a useful biomarker for risk‑stratifying long‑term myopia control response. Children with ΔAL₆ m ≤ 0.07 mm were much more likely to achieve a favourable AL response, whereas those with ΔAL₆ m ≥ 0.09 mm had an elevated risk of poor AL response.