Update on the Epidemiology and Genetics of Myopic Refractive Error

Justin C Sherwin; David A Mackey

Disclosures

Expert Rev Ophthalmol. 2013;8(1):63-87. 

In This Article

Refraction & Its Components

Four ocular structures contribute to the refractive apparatus of the human eye: cornea, lens, and aqueous and vitreous humors. Incoming light rays are refracted onto the retina, which then transmits an impulse along the optic nerve to the brain for processing. Refractive error arises when the eye is unable to perform this accurately. Most commonly, refractive error arises due to excessive axial length (AL), and less frequently, because the main refractive structure of the eye – the cornea and lens – lack the required refractive power. Blurred vision ensues. Myopia ('short-sightedness') results when light is focused in front of the retina rather than on the retina. Astigmatism may occur in conjunction with myopia. Without correction, myopia causes distant objects to appear blurred but near objects are viewed clearly. Blurred vision may be associated with squinting and eye rubbing, which may prompt a vision assessment.

Using a country-level analysis, refractive error was responsible for 27.7 million disability-adjusted life years worldwide in 2004, which was the highest of any eye disease and remained the highest of any eye disease separately across low/medium- and high-income countries.[1] The burden of refractive errors predominantly affects the World Bank regions of east Asia and Pacific, and south Asia, as well as high-income economies.[1] Myopia is the most common refractive error globally, and it is estimated that there are 1.44 billion people affected, equal to 22.6% of the world's population.[2] The prevalence of correction for myopia is lowest in areas of sub-Saharan Africa and south Asia (including India, Pakistan and Bangladesh).[2] Myopia prevalence has been increasing worldwide throughout the 20th century, especially in some populations in east Asia, where it is also associated with an increased prevalence of high myopia.[3,4] The reasons underlying this increase are not entirely understood.

Uncorrected refractive error (URE) represents the most common cause of visual impairment worldwide and the second leading cause of blindness.[5] URE does not always correlate positively with myopia severity,[6] as it is a reflection of the available health system's capacity to provide refractive services to a population. Therefore, in low- and middle-income countries, the proportion of individuals with refractive error who are uncorrected may be higher than in high-income countries. URE is a leading cause of visual impairment in urban China, where over 70% of 15-year-olds have myopia,[7] and in adults in sub-Saharan Africa, where the prevalence of myopia and other refractive error is far less.[8] A systematic review identified social factors (including socioeconomic status, isolation and education), treatment/service factors (rural domicile, access among minority groups and access to health insurance) and individual factors (including psychological factors) as being associated with URE.[9] It has been shown that uncorrected myopia is associated with poorer overall visual function and having difficulty with specific activities including reading street signs, recognizing friends and watching television.[10] However, separate research found that lower quality-of-life (QoL) scores were associated with myopia irrespective of correction status.[11] In children and younger adults, myopia has little or no impact on general health, but a positive relationship exists between increasing severity of myopia and poorer vision-specific functioning; myopia may cause visual deficits that transcend decreased visual acuity.[12]

Although mild myopia has a relatively uncomplicated and benign natural history, the potential for sight loss in patients with severe (high) myopia is substantial, with the risk increasing with worsening degree of myopic refractive error. High myopia may be associated with sight-threatening cataract, open-angle glaucoma, maculopathy and choroidal neovascularization, peripheral retinal changes (such as lattice degeneration), retinal holes and tears, as well as retinal detachment.[13–15] In adults, severe myopia may affect more adversely on QoL than myopia of lesser severity.[16] Pathology associated with higher degrees of myopia is chronic, progressive and more prevalent with increasing age; this may account for the minimal impact on QoL of myopia in children. As myopia is often present during the majority of one's lifespan, this may explain why refractive error is a greater contributor to the overall global burden of eye disease than chronic, age-related eye diseases such as cataract and macular degeneration.[1]

The economic consequences of myopia at a population level are extensive. Assuming myopic individuals receive appropriate refraction and correction, the global burden of myopia has been estimated at approximately US$45 billion.[2] However, this estimate does not take into account the difference in spectacle price according to region, intercountry differences in health systems, the cost of alternative forms of myopic correction (e.g., refractive surgery), or the direct and indirect cost of treatments for consequences of pathological (high) myopia. At a country level, the mean annual direct cost of myopia for Singapore school children was US$148 per child, with greater costs associated with the use of contact lenses, higher family income and paternal education.[17]

processing....