The prevalence of myopia reported for year old

The prevalence of myopia reported for 6year old children varies from 0.6% in Oman to 29% in Singapore. The prevalence in Oman for 6year old children was 0.6%, but the definition of myopia was more than −1.0 diopter when most studies use −0.5 diopter. The prevalence of myopia among pre-school children at King Abdulaziz Medical City, Riyadh, Saudi Arabia is 2.5%.
The visual system has an active process of emmetropization that involves defocus detection and a coordinated growth of the refractive components toward emmetropia with active structural changes. It is amazing how well emmetropization works and understanding what occurs when this process fails is the target of the research. In the first three years of life the cornea and lens change to counterbalance an approximately 20 diopter increase in axial length of the growing eye. Between ages 3 and 13 the lens and or cornea need to adjust about 3 diopters to maintain emmetropia.
As the human eye grows the lens adds layers of tissue yet thins by stretching in the equatorial plane so that it flattens, thins and loses power to compensate for the increasing axial length and maintains emmetropia. When the lens fails to stretch and thin the eye becomes myopic and the eyeball shape becomes more prolate or less oblate. The source of this interruption of equatorial expansion is unknown with one meclizine hcl being the thickening of the ciliary muscle which is found in myopic children and adults.
When myopia develops the eye is longer than it is wider (greater anteroposterior length than lateral transverse dimensions). This prolate shape of the eyeball will create a relative hyperopic defocus in the peripheral vision, along the lateral dimensions away from the macula. This peripheral vision refraction is another hypothesis as a potential impact or trigger on the active emmetropization process. Peripheral refraction in the myopic eye becomes relatively more hyperopic (Fig. 1). Local retinal regions can control local eye growth and myopia. The peripheral refractive state of the eye can affect eye development especially the progression of myopia. An interesting study found 77% of young entering emmetropic pilots with relative hyperopic defocus in their peripheral refraction developed myopia during their training. Hyperopic eyes are usually myopic in the periphery adding to the hypothesis that the periphery focus could be a trigger in eye growth. Also being investigated is the increase in the lag of accommodation during near work and the increase in myopia.
A 2010 search in PubMed yields over 14,000 citations with many research disciplines working to identify risk factors and potential interventions to help control myopia. Understanding, controlling and treating myopia are also a goal of the World Health Organization, Vision 2020 project.

Genetic factors
High heritability in myopia suggests that there is a significant genetic component to explain the variance in the population. A high heritability index is found in twin studies varying from 75% to 94%. A recent large sample study of monozygotic and dizygotic twins estimates a heritability index of 77%. However, this high index does not preclude an environmental precursor, and has some contestable assumptions (Morgan and Rose). Other genetic evidence pointed to is the prevalence of myopia in children increased with the number of myopic parents from 7.6, 14.9, to 43.6 percent for no, one or two myopic parents. However, it is an interesting observation of low heritability values in parent-offspring correlations when there has been rapid environmental change between generations. The Genes in Myopia (GEM) family study calculated the heritability index between 27% and 55%. In a non twin study heritable factors accounted for 80% of juvenile myopia.
Multiple myopia genetic loci have been identified establishing myopia as a common complex disorder. A recent review of data from the past decade in searching for myopia genes points to axial length and refraction sharing common genes and states that the majority of myopia cases are not likely caused by defects in structural proteins, but in defects involving the control of structural proteins. They conclude in discussing genes and their effects on myopia, “it is hard to show anything but a modest effect on their etiologies. Thus we are still left with the impression that the influence of environment exerts a greater effect than does the concerted action of several genes”.