Effects of different powers of repeated low-level red light on form-deprivation myopia inhibition in guinea pigs.

This study aims to determine whether 1.2 mW repeated low-level red light (RLRL) is as effective as 2.0 mW RLRL in inhibiting the development of myopia in the guinea pig model of form-deprivation myopia (FDM), while providing greater safety. Forty-eight guinea pigs were randomized into four groups: normal control (NC), FDM, 1.2 mW RLRL, and 2.0 mW RLRL. FDM was induced in the right eye by fitting it with an opaque mask for 4 weeks. Two RLRL groups underwent the same FDM and received twice-daily RLRL therapy (1.2 mW or 2.0 mW, 3 min each) to the masked eye. Parameters included refraction, axial length (AL), choroidal thickness (ChT), vessel density of the choriocapillaris (VDCC), vessel density of the choroidal layer (VDCL), and the ultrastructure of scleral collagen. TUNEL staining was used to assess retinal cell apoptosis. The average difference of parameters was compared by repeated-measures analysis of variance. 1.2 mW and 2.0 mW RLRL significantly weakened FDM-induced axial elongation (0.217 ± 0.11 mm and 0.233 ± 0.07 mm vs. FDM: 0.553 ± 0.088 mm; P < 0.001) and myopic shift (− 3.458 ± 0.958 D and − 2.792 ± 0.958 D vs. FDM: −6.25 ± 1.422 D; P < 0.001) compared with the FDM group at 4 weeks. Both powers restored ChT and VDCC, preserved scleral collagen structure, and prevented retinal cell apoptosis. No significant inter-dose differences were observed (P > 0.05). Both 1.2 mW and 2.0 mW RLRL effectively slowed down the progression of FDM in the guinea pig model by regulating choroidal thickness, hemodynamics, and scleral remodeling, and were safe. This supports the possibility of a lower-power protocol for clinical translation.