Effects of brief periods of clear vision on the defocus-mediated changes in axial length and choroidal thickness of human eyes

Purpose To investigate the influence of brief, repeated periods of clear vision on the changes in axial length and choroidal thickness in response to short-term imposed defocus. Methods The right eye of 16 young adults was exposed to 60 min episodes of continuous and interrupted defocus conditions (+3 DS and -3 DS) over five separate sessions, with the left eye optimally corrected for distance. For interrupted defocus, 2 min episodes of clear vision were imposed before each 15 min episode of myopic or hyperopic defocus (2/15 min). For hyperopic defocus, the effect of frequency of clear vision exposure was also assessed by imposing 1 min of clear vision before each 7.5 min of defocus (1/7.5 min). The right eye axial length and choroidal thickness were measured before, during and after each defocus condition. Results After 60 min of continuous hyperopic defocus the eye elongated significantly by +9 +/- 9 mu m (p = 0.02). When exposed to interrupted (2/15 min) hyperopic defocus, axial elongation was significantly reduced by 77% compared to continuous hyperopic defocus (p = 0.03), with a final change of only +2 +/- 10 mu m relative to baseline. During interrupted (1/7.5 min) hyperopic defocus, axial elongation reduced slightly compared to continuous hyperopic defocus (+6 +/- 8 mu m relative to baseline, p = 0.12). For continuous myopic defocus, a reduction in axial length occurred but was not statistically significant (p > 0.05). A similar pattern of response was observed for choroidal thickness changes with continuous and interrupted (1/7.5 min) hyperopic defocus conditions. Conclusions Brief periods of clear vision can diminish axial elongation and choroidal thinning induced by hyperopic defocus exposure in human eyes. If hyperopic defocus contributes to myopia progression in humans, then interruption with brief periods of clear vision could reduce its myopiagenic effects.