A model of the contribution of oculomotor and optical factors to emmetropization and myopia

The purpose of this work was to investigate quantitatively the interactions between accommodation, vergence and a mechanism of emmetropization driven by optical blur within the retinal image with a view to developing a model that provides an explanation of both normal emmetropization and near-work associated myopia. The simulations of the change in the refractive state of the eye over time that derive from this model indicate that optical regulation of eye growth can result in emmetropization, i.e. a progressive reduction in refractive errors over time leading towards emmetropia. This occurs when viewing conditions involve a preponderance of distance work. With increasing near work, the model predicts that the refraction of the eyes will converge towards myopia. In keeping with the previously reported associations of myopia with esophoria, poor accommodation function and high AC/A ratios, these conditions increase the amount of myopia produced under intensive near viewing conditions but do not lead to myopia during mainly distance viewing. This model provides quantitative validation of the hypothesis that the epidemiological association between myopia and increased nearwork may be caused by a disturbance of normal emmetropization by steady state errors of accommodation. The same model can explain normal emmetropization, increasing myopia with increasing nearwork demands and the currently recognised oculomotor associations that have been reported to precede the development of myopia.