A MODEL OF THE CONTRIBUTION OF OCULOMOTOR AND OPTICAL FACTORS TO EMMETROPIZATION AND MYOPIA

The purpose of this work was to investigate quantitatively the interac tions between accommodation, vergence and a mechanism of emmetropizati on driven by optical blur within the retinal image with a view to deve loping a model that provides an explanation of both normal emmetropiza tion 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 emmetrop ization, i.e. a progressive reduction in refractive errors over time l eading towards emmetropia. This occurs when viewing conditions involve a preponderance of distance work. With increasing near work, the mode l predicts that the refraction of the eves will converge towards myopi a. In keeping with the previously reported associations of myopia with esophoria, poor accommodation function and high AC/A ratios, these co nditions increase the amount of myopia produced under intensive near v iewing conditions but do not lead to myopia during mainly distance vie wing. This model provides quantitative validation of the hypothesis th at the epidemiological association between myopia and increased nearwo rk may be caused by a disturbance of normal emmetropization by steady state errors of accommodation. The same model can explain normal emmet ropization, increasing myopia with increasing nearwork demands and the currently recognised oculomotor associations that have been reported to precede the development of myopia. (C) 1998 Elsevier Science Ltd. A ll rights reserved.