Abstract
Abstract: :
Purpose: A recent study (Chung et al., Vis. Res., 2002, 42: 2555-2559) found that myopic children who were purposely under-corrected by 0.75 D over a 2-year period showed a small but statistically significant increase in myopic progression that was 0.25 D greater than those who were fully corrected. This appears to contradict previous animal studies using high-powered plus lenses that produced relative hyperopic growth. The purpose of this analysis was to determine whether these findings could be accounted for by our recently proposed Incremental Retinal Defocus Theory (IRDT) (Hung and Ciuffreda, ARVO, 2001). Methods: The IRDT states that any time-integrated reduction in retinal-image defocus magnitude decreases the rate of retinal neuromodulator release, which in turn decreases the rate proteoglycan synthesis and adversely affects scleral structural integrity, thus resulting in axial elongation and myopia development. The theory was used to analyze the conditions above. Results: (1) For a large imposed plus lens, which precludes accommodative feedback, the retinal image defocus magnitude increases during an increment of normal genetically-programmed growth, leading to relative hyperopic growth. (2) With fully-corrected vision, looking from near to far can be represented on the accommodative stimulus/response (AS/R) plot as a change from a specific defocus magnitude (lag of accommodation) at the high near-stimulus level to a similar defocus magnitude (lead of accommodation) at the low (~0 D) far-stimulus level. (3) On the other hand, with a 0.75 D under-correction, looking from near to far represents a change from a specific near magnitude of defocus (lag of accommodation) to a significantly smaller defocus magnitude at the far (0.75 D) stimulus level because the accommodative response is now near the crossover point on the 1:1 line of the AS/R plot. Repeated near-to-far viewing cycles would result in a cumulative time-integrated decrease in retinal defocus magnitude that would lead to increased myopic progression. This is consistent with the experimental finding of a significantly greater rate of myopic growth in under-corrected than fully-corrected myopic children. Conclusions: Based on IRDT analysis, high-powered plus lens, full correction, and 0.75 D under-correction result in relative hyperopic, emmetropic, and myopic growth, respectively. Thus, the theory is able to explain these apparently contradictory findings. Moreover, the IRDT provides a consistent theoretical framework for understanding the development of myopia under a variety of experimental and clinical conditions.
Keywords: myopia • refractive error development • refraction