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

• Question: Does brief end-of-day defocus affect myopic progression? • Test: Young myopic guinea pigs exposed to 2 h defocus just before lights–off. • Transient myopic defocus reduced vitreous chamber elongation and induced choroidal thickening. • Transient hyperopic defocus minimally affected ocular axial dimensions. • Translation: Timing −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.