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G.K. Hung, K.J. Ciuffreda; IRDT Model Predicts Effects of Constant Light and Constant Darkness on Myopic Progression . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1982.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Previously (ARVO, 2003; J. Behav. Optom, 2004), the Incremental Retinal–Defocus Theory (IRDT) predicted the effect of undercorrection of lens prescription on myopic progression. We now extend this to predict the effects of constant light and constant darkness on myopic progression, as recent controversy exists in the area related to infant visual development and myopia. Experimental results have shown that ocular growth under constant light as well as constant darkness produced axial enlargement and corneal flattening. These findings, however, cannot be explained if the illumination level alone was the sole factor. On the other hand, the two conditions do share a common aspect at night–time, which is an absence of form vision both for constant light with closed eyelids and for darkness. Nevertheless, there has not been a satisfactory explanation for the underlying mechanisms for these results related to daily cycles of light and dark. Methods: The IRDT was used to form the basis for a schematic model to ascertain the overall change in retinal defocus, and in turn axial growth rate, following repeated daily cycles under four conditions: normal, form–deprivation, constant light, and constant darkness. Furthermore, a qualitative biomechanical model of the eye was developed to investigate shape changes of the ocular tunic for these four conditions. Results: The schematic model analysis results showed that the average axial growth rate was slowest under the normal condition, faster for both constant light and darkness conditions, and fastest for the form deprived condition. The biomechanical modeling results demonstrated how form deprivation leads to axial elongation and a steeper cornea, whereas darkness leads to a rounder shape corresponding to ocular enlargement and a flatter cornea. Furthermore, constant light at night–time is similar to form deprivation, with the additional effect of closed eyelids, leading to axial elongation and corneal flattening. All of these results are consistent with experimental findings. Conclusions: The IRDT–based schematic analysis showed how both constant light and constant darkness could result in axial elongation and corneal flattening. And, the biomechanical model illustrated how local and global shape changes can occur under these conditions. Together, the schematic and biomechanical modeling analyses provide a clearer explanation of the underlying mechanisms for axial elongation and corneal curvature change, and in turn the development of refractive error in both animal and human studies.
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