Abstract:
Many more children suffer from myopia (short-sight) than was the case 100 years ago; in parts of Asia, over 70% of children are now myopic and will require some form of vision correction for life. Although genetics plays a role in determining whether a child will develop a refractive error like myopia, the environment – and in particular the light falling on the retina – is equally important. Although myopia was identified more than 2000 years ago by Aristotle (384-321 BC), there is no cure, and its prevalence is increasing. The prevalence in New Zealand is estimated at about 20%. The symptoms of myopia include blurred distance vision, while near objects are seen clearly. What causes myopia? Why does it often get worse (progress) but never get better? Does wearing spectacles make it worse? Can myopia progression be inhibited? Providing evidencebased answers to such questions is difficult. A vast literature on myopia has built up over the years and distinguishing between myopia-myth and real evidence poses a significant challenge. An eye with myopia has grown abnormally large (long). Because the myopic eye is too long, the retina is located behind the focal plane of the optics (Figure 1b), with the result that distant objects are seen as blurred. In contrast, an eye with hyperopia (long-sight) is ‘too short’ (Figure 1c). It is only when the eye grows so that its length accurately matches the power of the optics that the eye has no refractive error and is said to be ‘emmetropic’. Most cases of myopia involve only a moderate enlargement of the eye, and good vision can be obtained by correcting the refractive error with spectacles or contact lenses. This ‘common’ or school myopia is an expensive inconvenience which can limit career and lifestyle choices. However, about 2% of the population suffer from ‘high’ myopia, in which the abnormal expansion of the eye is so great that the retina and its blood supply become stretched and can be damaged sufficiently to threaten vision (Figure 2). In fact, the abnormal eye enlargement in high myopia is a significant cause of registered blindness. An understanding of how normal eye growth is controlled makes an obvious starting point for understanding what goes wrong in eyes which develop myopia.