Preventing the epidemic of myopia in China : an integrated epidemiological and biological approach

Myopia is the most common refractive error. It is generally accepted that there is an epidemic of myopia in urban areas of East Asia, as more than 80% of the children who complete secondary school are shorted-sight, with around 20% highly myopic. Importantly, this problem is not unique to East Asia, and prevalence of myopia is increasing in other parts of the world. The high prevalence of myopia imposes costs for both optical correction and visual impairment. Furthermore, the high prevalence of severe myopia increases the risk of uncorrectable visual impairment and even blindness. Myopia has emerged as major heath issue in the world, and been included as one of the priority areas of Vision 2020. The key findings of this thesis are: 1. The prevalence of myopia has increased progressively from 20-30% to over 80% over the past 100 years in East Asia, and it is plausible that a secular shift in life-style has taken place, resulting in more intensive education and less time spent outside by most children; 2. Environmental influences played an important role during the process, as in Guangzhou, most children now become myopic, and some become highly myopic, irrespective of the refractive status of their parents. There is an additional risk of myopia in children with myopic parents, and an increased risk of developing more severe myopia for those who had severe myopic parents. Exposure to the environment in younger generations did not simply shift the population mean of the refraction; 3. Increased rates of progression in a myopic direction and increased rates of axial elongation occur prior to the onset of clinical myopia. It might therefore be possible to initiate myopia prevention before the onset of myopia. Once a myopic refraction is established, the rates of axial elongation and myopic progression are reduced. The rapid slowing of axial elongation after the onset may suggest that myopic defocus is able to inhibit myopic progression to some extent. 4. The first two years of follow up of the Guangzhou Outdoor Activity Longitudinal Study showed promising results, with a consistent pattern of reduced axial elongation and myopia development in children with outdoor intervention, although greater exposure is needed to obtain clinically significant effects