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Refer this article as: Kanxing, Z. et al., Pathological and risk factors of myopia in Chinese Population, Points de Vue, International Review of Ophthalmic Optics, N63, Autumn 2010

Pathological and risk factors of myopia in Chinese Population

Date of publication :
10/2010

Content

Myopia, or nearsightedness, which has reached epidemic proportions and is already a large public health problem in certain parts of the world, including East Asia ,affects 25% of the adult population in the United States and 75% or more of the adult population in Asian countries such as China [1] (Fig1). China also has more than 300 million people wearing glasses to correct nearsightedness. This accounts for nearly a quarter of the total population, ten percent higher than the world average, according to Ge Jian, director of the renowned Ophthalmic Center at Sun Yat-sen University in Guangzhou. Recent study showed Chinese prevalence rates for low, moderate, and high myopia that were similar to those reported in North America and South India, were higher than those in epidemiological studies in Australia, and they were lower than those in Singaporean population-based investigations [2, 3].


Fig. 1: The prevalence of myopia between Chinese and American.

It appears that Chinese ethnicity is more prone to the development of myopia. The incidence of myopia in school children has been reported to be 70% in Taiwan [4]. Myopia is also common in Hong Kong, where about 98% of the population is ethnic Chinese [5, 6]. A similar study in Malaysia, where approximately one third of the population is Chinese, showed that 37% of school children have myopia [7] (Tab.1) [17]. Respective studies of Americans in Los Angeles showed corresponding frequencies of 25% were myopic [8]. The high prevalence of myopia in Chinese ethnic groups may have something to do with the genetic factors, and the observation had evidenced that myopic parents are more likely to give rise to offspring with myopia (Tab.2) [18]. Autosomal dominant, autosomal recessive and X-linked inheritance has been observed. In 1998, Young et al. has reported linkage of autosomal dominant high myopia to a 7.6-cM interval between markers D18S59 and D18S1138 on 18p11.31. It was a linkage study on 8 multigenerational pedigrees with high myopia, i.e. more than 2 or more family members with myopia >=-6.00 diopters (D). One family in that study was Chinese [9].


Tab. 1: Comparative distribution of ocular refraction in the three ethnic groups (Chinese, Malays, Indians).

In fact, the etiology of myopia involves a multitude of factors although heredity definitely plays a role. It also occurs as a result of environmentally induced conditions. As in previous studies, myopia was more prevalent in younger subjects and was associated with region, educational background, reading habits, use of computers, gender, and nuclear cataract etc. That is to say, higher educational levels, higher individual income, and professional or office-related occupations were significantly associated with higher rates of myopia. The reasons for developing myopic may also due to stretched reading hours, detailed work and highly competitive lifestyle. Recent data shows the degeneration of eye sights of many Chinese students is mainly due to competitive education. The association between the onset and development of myopia and long time near work has been demonstrated in the research for many years.

Furthermore, a number of factors may combine and determine an individual’s refractive state, including genetic, nutritional and environmental factors. Myopia can also be due to functional focusing problems - such as difficulty in maintaining near focus and other visual stressors. However, it is well known that myopia is a manifestation of the combination between the optical components of the eye (i.e., the curvatures, refractive indices, and distances between the cornea, aqueous, lens, and vitreous) and the axial length of the eye, work carried out over the last century has demonstrated that, while the components of refraction are distributed normally, the distribution of refractive errors is not normal and shows an excess of values near emmetropia [10]. The recent animal and some human studies have shown that visual experience after birth, such as complete form deprivation, dark rearing or imposed defocus influences the ocular refraction, implying that an active “emmetropization” (or perhaps more accurately, adaptation) process which depends on the characteristics of the retinal image may be in operation. As a development of these ideas, blur of the retinal image might have a causative role in at least some types of myopia [11].


Tab. 2: Proportion of children with and children without myopia as a function of number of parents with myopia (none, one parent, two parents).

In addition, strong evidence from clinical and experimental studies indicates that the biochemical and biomechanical properties of the sclera determine the shape and size of the globe and therefore play a major role in affecting the refractive state of the eye. The evidences have been noted by many observers supplement the earlier suggestion that the posterior sclera of the myopic eye gets thinning and localized ectasia as the characteristic changes, which is associated with thinning of collagen fiber bundles as well as with a reduction in the size of the individual collagen fibrils with a preponderance of unusually small diameter fibrils averaging below 60–70 nm [12]. Remarkably, studies using animal models have demonstrated that young eyes can recover from induced myopia following removal of the diffuser or negative lens [13]. Additionally, it has been found that choroidal thickness changes occur in the same direction as the related axial length changes [14]. The importance of elastic fibers in the sclera is evidenced by the scleral pathology and high myopia associated with Marfan syndrome1, an autosomal dominant disorder due to mutations in the fibrillin gene [15]. Moreover, based on results of linkage analysis, it has been suggested that certain forms of inherited high myopia are the result of defects in scleral extracellular matrix components that localize at the chromosomal loci 12q21–23 which includes the extracellular matrix genes, lumican, decorin, and dermatan sulfate proteoglycan (DSPG-3) [9].

Generally the treatments of myopia include the use of prescription eyeglasses or contact lenses to restore clear distance vision. The prescription of spectacles lenses involves not merely the refractive power nowadays, but also the prismatic power, base curve, panoscopic tilt, face form, horizontal and vertical PRP location, vertex distance, lens material, thickness, tint, and coating, providing for the desired optical functions of the lenses. In some cases - those individuals at risk of being myopic - perhaps it is possible to train the eye to influence refractive development, for example, vision training to improve the accuracy of accommodation, otherwise, to use a bifocal or progressive addition lens [16]. What’s more, the availability of contact lens materials and designs has grown extensively over time as contact lens wear expanded into the population. Other modes of correction are now available (e.g. various forms of refractive surgery). Recommended for people who find glasses and contact lenses inconvenient and uncomfortable, refractive surgery performed on an outpatient basis and generally take 10-20 minutes. There are many types of corrective surgeries available in china now, such as PRK (Photorefractive Keratectomy), LASIK (Laser-assisted in situ keratomileusis), LASEK (Laser epithelial keratomileusis), Epi-LASIK (Epipolis laser in situ keratomileusis), and SBK (Sub-Bowmans Keratectomy) etc.

The impact of myopia, an apparently benign ocular disease, may be larger than it seems. Thus, The World Health Organization has nam ed refraction as one of the five priorities for Vision 2020 - the Right to Sight. At this point in time, there are many challenges for myopia research with respect to preventative and therapeutic aspects of myopia.

Other reference: [19].

References

01. Sperduto RD, Seigel D, Roberts J, et al. Prevalence of myopia in the United States. Arch. Ophthalmol. 1983;101:405–7.
02. Wong TY, Foster PJ, Hee J, et al. Prevalence and risk factors for refractive errors in adult Chinese in Singapore. Invest Ophthalmol Vis Sci 2000;41:2486–94.
03. Wu HM, Seet B, Yap EP, et al. Does education explain ethnic differences in myopia prevalence? A populationbased study of young adult males in Singapore. Optom Vis Sci 2001;78 : 234–9.
04. Lin LL, Chen CJ, Hung PT, Ko LS. Nation-wide survey of myopia among schoolchildren in Taiwan, 1986. Acta Ophthalmol Suppl 1988;185:29-33.
05. Goh WS, Lam CS. Changes in refractive trends and optical components of Hong Kong Chinese aged 19-39 years. Ophthalmic Physiol Opt 1994;14:378-82.
06. Wu MM, Edwards MH. The effect of having myopic parents: An analysis of myopia in three generations. Optom Vis Sci 1999;76:387-92.
07. Wilson A, Woo G. A review of the prevalence and causes of myopia. Singapore Med J 1989;30:479-484.
08. Sperduto RD, Siegel D, Roberts J, et al. Prevalence of myopia in the United States. Arch Ophthalmol 1983;101:405-7.
09. Young TL, Ronan SM, Drahozal LA, et al. Evidence that a locus for familial high myopia maps to chromosome 18p. Am J Hum Genet 1998;63:109-119.
10. Smith EL, Hung LF. The role of optical defocus in regulating refractive development in infant monkeys. Vis. Res 1999;39:1415–35.

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