Two-year outcomes of repeated red light therapy in premyopic children: sustained efficacy and rebound effects

Background: To evaluate the long-term prevention effectiveness and rebound effect of repeated red light therapy (RRLT) in children with premyopia over 2 years. Methods: A total of 108 premyopic children (cycloplegia spherical equivalent refraction (SER): -0.50 to +0.75 D) were enrolled and followed for 24 months. Participants were randomly assigned to the RRLT or control groups. The RRLT was administered two times per day for 3 min per session, with at least 4-hour interval. At the beginning of the second year, participants receiving RRLT were further randomised into continued treatment and washout subgroups. Axial length (AL), SER and subfoveal choroidal thickness (SChT) were measured. Results: Over 2 years, the RRLT group showed significantly smaller AL elongation (0.26 mm; 95% CI 0.18 to 0.35 mm) and SER progression (-0.21 D; 95% CI -0.35 to -0.08 D) compared with the controls (AL: 0.43 mm; 95% CI 0.36 to 0.49 mm; SER: -0.66 D; 95% CI -0.79 to -0.52 D). The RRLT group also demonstrated significantly less SChT thinning (-2.44 µm; 95% CI -16.11 to 11.23 µm) than the controls (-44.12 µm; 95% CI -53.05 to -35.19 µm). After RRLT cessation in the second year, the washout subgroup exhibited significantly faster AL elongation and more SChT thinning than the controls, with no significant difference in SER progression. Conclusions: The 2-year RRLT intervention effectively retarded AL elongation and SER progression in premyopic children by 0.17 mm and -0.45 D, respectively. Notably, a significant rebound effect was observed in AL growth following 1-year RRLT cessation.