Effects of transient myopic/hyperopic defocus on myopia development in guinea pigs

Exposure to transient myopic defocus in the evening is known to counteract the myopia-inducing effects of negative lenses and form deprivation, but its influence on eyes that are already myopic remains unclear. We examined how brief periods of myopic or hyperopic defocus affect ocular growth in fully compensated myopic eyes using young guinea pigs. Two-week-old animals (n = 19) wore -10D rigid gas-permeable lenses for 3 weeks to induce myopia and were then randomly assigned to one of three groups: (1) Stable Myopia (SM; -10D), (2) Transient Myopic Defocus (TMD; -10D for 10 h/-5D for 2 h), and (3) Transient Hyperopic Defocus (THD; -10D for 10 h/-15D for 2 h). Transient defocus was applied for 2 h before lights off, and fellow eyes served as controls. Refractive error, Axial Length(AL), Vitreous Chamber Depth(VCD), and choroidal thickness(ChT) were measured at baseline, after 3 weeks of lens wear, and weekly for 6 weeks. After 3 weeks, all groups exhibited similar myopia, with corresponding increases in AL and VCD, and choroidal thinning. During the subsequent 3-week intervention period, the TMD group showed stable refractive error and AL but significantly reduced vitreous chamber elongation at weeks 4 and 6, along with increased ChT by week 6. In contrast, the THD group did not differ from the SM group for any parameter. These findings demonstrate that myopic guinea pig eyes remain responsive to the inhibitory effects of brief daily myopic defocus, whereas transient hyperopic defocus has minimal impact. If applicable to humans, this highlights the importance of monitoring compliance with optical myopia control interventions.