|Year : 2018 | Volume
| Issue : 1 | Page : 1-7
Myopia: Etiology, epidemiology, and management strategy
Yazan Sultan Gammoh
Department of Optometry, Faculty of Pharmacy and Medical Sciences, Al-Ahliyya Amman University, Amman, Jordan
|Date of Web Publication||4-Sep-2018|
Yazan Sultan Gammoh
Faculty of Pharmacy and Medical Sciences, Al-Ahliyya Amman University, P. O. Box 121, Amman
Myopia, once seen as a simple refractive error where the image is focused in front of the retina, is now considered as an epidemic with projected estimates of half the world population becoming myopic by the year 2050. It has been shown that the prevalence of myopia is highest in Asian countries, while evidence in the Western world shows an increase in the prevalence of myopia, with a financial burden leading to uncorrected refractive errors including myopia being a major cause of avoidable vision impairment. Although no definite single cause of myopia has been proven, both hereditary factors and near work play a role in the development and progression of myopia. Inaccuracy of accommodation at near and hyperopic defocus at the periphery of the retina have been investigated as possible causes of myopia development, with research on animals providing an insight into the development of myopia in humans. Optical, pharmacological, and corneal reshaping strategies have been developed to control myopia progression with variable results. While undercorrection of myopes and correction with single vision lenses have not been proven to be effective in myopia management, other methods such as orthokeratology and use of atropine are showing evidence of controlling the progression of myopia.
Keywords: Epidemiology, myopia, myopia control, near work, outdoor play
|How to cite this article:|
Gammoh YS. Myopia: Etiology, epidemiology, and management strategy. Sudanese J Ophthalmol 2018;10:1-7
| Introduction|| |
Myopia or nearsightedness is the most common ocular anomaly, being responsible for more than two-thirds of ocular complications related to refractive errors. Myopia is a public health issue and can be considered as a worldwide threat to visual health. Uncorrected refractive errors, mainly myopia, are considered the leading cause of vision impairment. While visual impairment resulting from uncorrected myopia can be remedied by the use of negative correcting lenses, myopia is still considered a public health issue as a result of the economic burden driven by the cost of refractive correction. The effect of myopia is not limited to its economic burden; it extends to the social aspects of the uncorrected individuals where depression, risk of falls, and loss of social independence are prevalent.,,,,,,,,,,,,,,,,
| The Epidemic of Myopia|| |
Comparison of studies conducted on the prevalence of refractive errors is complicated by the differing methodologies adopted in the studies including cutoff point of myopia and whether cycloplegic refraction was used to obtain data on the refractive status of the eye. There is an evidence of increase in the prevalence of myopia in countries such as the United Kingdom and the United States of America, where the prevalence of myopia was between 11% and 25% in the 1960s and 1970s increasing to around 40% in recent years. Similar estimates were observed in other Western countries.,,,,,, This has also been observed in other parts of the world where doubling in the prevalence of myopia in Singaporean cadets has been observed between the 1970s (prevalence of 23%) and the 1990s (prevalence of 43.3%).
Myopia in certain Asian countries has been considered to have reached epidemic levels with more than 70% of young adults in Hong Kong being myopic. It has been shown that myopia is found in more than 80% of Singapore Chinese, with a reported prevalence of 80%–90% in children from certain Asian countries.,, While there is an abundance of studies conducted on the prevalence of refractive error in the world, there is a shortage in studies curried in the Eastern Mediterranean region. Data obtained from studies conducted on Middle Eastern adults have shown the prevalence of myopia similar to that reported in Western countries ranging from 46.71% in Saudi adults to 53.71% in Jordanian adults although more recent data are needed from various countries in the region.,
| Autonomic and Biometric Correlates of Myopia|| |
The biometric components of the eye which mainly contribute to the refractive status of the eye are the corneal curvature and axial length. Various studies have investigated the structural correlates of refractive error to understand the mechanism of development of emmetropia and ametropia. In addition, several investigators attempted to investigate the correlation between the eye's accommodative system and refractive errors to ascertain if refractive errors occur due to irregularities in accommodation, or abnormalities of the accommodative system are developed as a result of change in the refractive status of the eye.
The corneal radius of curvature shows a normal distribution in the population, with insignificant changes after early childhood, and a weak correlation between the refractive power of the eye and the radius of curvature. It has been shown that the axial length exhibits the strongest correlation with the refractive power of the eye, with myopes exhibiting longer axial lengths compared to adults with other refractive errors.,,,,,
Lag of accommodation during near work has been studied in relation to myopia development and progression. It has been shown that a high lag of accommodation is present before the onset of myopia. In addition, emmetropes who became myopic exhibited a higher of accommodation in comparison to those who remained emmetropic with an evidence of higher accommodative lag in children with progressive myopia. The lag of accommodation during near work has been shown to cause hyperopic defocus which is implicated for axial elongation and myopia progression.,, Vergence at near, mainly esophoria, has been investigated in relation to myopia. Results from several studies including the Correction of Myopia Evaluation Trial (COMET) study have shown that esophoria at near, in addition to higher lag in accommodation, was associated with myopia in both children and adults.,,
| Nature Versus Nurture|| |
Studies conducted on twins provide a strong evidence for the role of genetics in the development of myopia as twins share between 50% of the genes for dizygotic and 100% for monozygotic twins in comparison to the large variability in genes found in nontwin siblings. All studies conducted on twins show evidence of correlation between refraction, ocular components, and heredity with correlation being stronger in monozygotic twin compared to dizygotic twins. Hammond et al. conducted one of the largest studies on twins where 506 female twin pairs showed that the correlation for refraction was greater in monozygotic twin pairs (>0.8) compared to dizygotic twin pairs (<0.5). Lyhne et al. conducted a study on 114 twin pairs of whom 53 were monozygotic twin pairs and 61 were dizygotic twin pairs. A high heritability for refractive error and ocular biometric parameters was observed among these twins.,,,
Advances in genetic sequencing are allowing researchers to investigate the genes related to the development of myopia, rather than relying on observational studies on twins. The Human Genome Organization Gene Nomenclature Committee has approved twenty gene loci to be associated with myopia; MYP1 to MYP18, OPEM while MYP6 has been divided into two loci: SCO2 and mitochondrial respiratory chain complex assembly factors. Most of the genes are associated with high myopia with few being related to moderate myopia.,,,,
Near work and education
Several studies have been conducted to investigate the relationship between near work, levels of education, and myopia. Cross-sectional and longitudinal studies from the 1970s and 1980s reveal a significant correlation between amount of near work and myopia.,, Cross-sectional and population-based studies have shown a correlation between myopia and occupations with near work demand. In addition, longitudinal studies conducted on specific occupation groups such as textile weavers, quality control workers, and microscopists, all known for demanding near work have shown a progression of myopia in these occupational groups in comparison with occupations that had little or no near work demand.,,,,,, To further investigate the relation between near work and myopia, the concept of “diopter-hours” was adopted by some studies conducted on children which weighs time spent on different near work-related activities. The most important find was that time spent on reading from printed material can be considered as a risk factor for myopia, independently of parental myopia, and time spent on other near work activities.,
There is a strong evidence for the relationship between education and myopia, where university students exhibiting higher prevalence of myopia compared to the general population. Furthermore, while progressing with their study years, it has been shown that there is an increase in myopia, with some emmetropic students developing myopia after spending few years at the university.,,,
Both hereditary factors and near work activities have been implicated as possible risk factors for myopia development and progression. However, new evidence has shown that outdoor play has a protective role in myopia development. Myopia has shown to be inversely related to time spent playing sports in children, with few studies providing evidence that time spent outdoors is associated with a reduced risk of myopia development.,,, It has been shown that the effect of time spent outdoors on myopia is independent of the associated reduced time spent indoors on near work. Extended depth of focus through pupil restriction, exposure to ultraviolet light, and increase in dopamine release have been suggested as mechanisms of the protective action of outdoor play, with dopamine release causing reduced axial elongation being the most probable mechanism. However, there is no concrete evidence of the mechanism of action of outdoor play in reducing the onset of myopia, and if outdoor activities play a role in stopping the progression of myopia in myopic children.
| Animal Models of Myopia: an Insight Into Humans|| |
Animal models of myopia have been investigated extensively to study the role of environment in the development of myopia. Myopia was induced in animals by artificially manipulating the animal's environment, either by optical defocus through convex or concave lenses or form deprivation using lid suturing. Hyperopic defocus in humans is thought to be due to the lag of accommodation during near work. It has been shown that optical defocus imposed on rhesus monkeys' eyes altered the shape and refraction of the monkey's peripheral retina. Defocus of the retinal image has been proposed as a stimulus to myopia development and progression.,
Unilateral lid suturing of newborn macaque monkeys produced high degrees of myopia through axial elongation and increase in equatorial diameter of the monkeys' eyes due to visual input distortion. The macaque monkey model of myopia is of importance to humans as it is considered the closest to humans' model of myopia. Form deprivation in monkeys of 3.7 and 5 years of age caused axial elongation of the monkeys' eyes which resulted in the development of myopia. This age range studied in the macaque monkeys is equivalent to 15–20 years of age in humans, which indicates that retinal imagery distortion may induce myopia even after the age limit where the eye is considered to have reached full development.,
| Myopia Management Strategies: Are We There Yet?|| |
With the increase in the prevalence of myopia, reaching to epidemic levels in some Asian countries, efforts have been done to control the progression and perhaps the development of myopia. Outdoor play and reducing time spent indoors on near work activities have shown promising results, yet more research needs to be done as the exact mechanism of controlling myopia by spending more time outdoors still needs to be investigated. Several optical and nonoptical methods have been employed to control myopia progression in children with various success rates.
Undercorrection of myopia and use of single vision lenses
It has been proposed that undercorrection of myopia may inhibit the progression of myopia due to the reduced accommodative demand at near or to myopia defocus caused by undercorrection which may cause reduction in axial length elongation. However, studies related to undercorrection of myopic children are conflicting with results showing increased progression of myopia instead of halting the progression. Further studies are required to fully investigate the effect of undercorrection on the progression of myopia in children. Single vision lenses, either in the form of ophthalmic lenses or contact lenses, are the lenses of choice for correcting myopia in clinical practice as they are readily available and affordable. It has been suggested that the use of single vision rigid gas permeable contact lenses may reduce the progression of myopia; however, it has not been proven that single vision lenses can reduce myopia progression or limit the elongation of axial length. Currently, single vision lenses are not used for the control of myopia.,
Progressive addition lenses
It has been noted earlier that myopia was found to be associated with inaccuracies in accommodation (mainly lag of accommodation) which are associated with esophoria. This esophoria causes a deficiency in accommodation to relieve the stress on binocular vision system. The combination of relaxation of accommodation and accommodation lag further increases the peripheral hyperopic defocus experienced by myopes. It was suggested that instead of wearing single vision lenses which cause hyperopic defocus in the periphery, children can wear progressive addition lenses (PALs) which theoretically would reduce the accommodative demand and limit the accommodative lag., The COMET study investigated myopia progression and related it to lag of accommodation and heterophoria at near in children while wearing PALs and compared the progression in a group of children wearing single vision lenses. Esophoria at near was found to be higher in children with myopia which would have caused a reduction in accommodation to relieve the accommodative convergence. The most significant effect of PALs was found in myopic children who exhibited the largest accommodative lag and esophoria at near. The effect of PALs was on reducing myopia progression, was not clinically significant, and did not show any effect after the 1st year of treatment. The COMET II study investigated the use of PALs in children with esophoria (≥2Δ) and lag of accommodation (≥0.50D) for 3 years. Children with accommodative lag of more than 1.50 D showed a higher reduction in myopia progression (0.41 D/3 years) compared to children with less accommodative lag (0.28 D/3 years). However, these changes were not clinically significant, and this shows that the use of PALs in myopia control is controversial.
Orthokeratology or corneal refractive therapy is a method of temporarily correcting myopia by wearing rigid contact lenses overnight which causes a temporary reshaping of the cornea, mainly flattening the center of the cornea. Few researchers have noted that wearing orthokeratology lenses provides a myopia control effect, perhaps due to the myopic defocus in the periphery created by the transient reshaping of the cornea. This induced myopic defocus in the periphery reduces the hyperopic defocus experienced by myopes which in animals have been shown to reduce the progression of myopia. The Children's Overnight Orthokeratology Investigation pilot study was published in 2004 and investigated the use of orthokeratology lenses in children. The study showed that it was safe to use orthokeratology lenses in children to attempt the control of myopia progression. The same conclusions were drawn from the results of the Longitudinal Orthokeratology Research in Children study. Two further studies are of interest when assessing the use of orthokeratology in children; the Stabilizing Myopia by Accelerated Reshaping Technique (SMART) study and the Corneal Reshaping and Yearly Observation of Nearsightedness (CRAYON) study. SMART has shown a reduction in myopia progression in children when wearing orthokeratology lenses, while the CRAYON study has shown a reduction in axial length elongation in children wearing orthokeratology lenses compared to those wearing soft contact lenses. Although studies conducted on children exhibit promising results, it is to be noted that most of the studies conducted are limited in terms of study design and high dropout rates and do not exceed the 2-year period limit. Further studies are needed with more rigid selection criteria, better control of participants' dropout, and longitudinal studies of more than 2-year period to provide a solid evidence of the role of orthokeratology in myopia control.,,
Atropine is an antimuscarinic agent, which inhibits the muscarinic action of acetylcholine, resulting in mydriasis and cycloplegia. It is mainly used as a diagnostic drug in children when conducting cycloplegic refraction. Animal studies have shown that the use of atropine on animals reduced axial elongation, hence myopia in animals such as chicks. Since chicks do not have muscarinic receptors, it is thought that the atropine-induced reduction in myopia progression maybe nonaccommodative. Several studies conducted on children have shown that the use of atropine causes a significant reduction in myopia progression. However, this effect seems temporarily with a rebound effect of axial elongation and continued myopia progression after cessation of atropine, especially when using 1% dose of atropine. Atropine, especially in high doses such as 1%, may have unpleasant side effects such as light sensitivity, mydriasis, rashes, fever, bradycardia, and tachycardia. The side effects caused by high concentrations of atropine have encouraged researchers to investigate the use of atropine in various doses and to investigate its role in the reduction of myopia. Despite showing that higher doses provide better results in terms of myopia control, a low dose of 0.01% was clinically effective, and the differences in efficacy were not statistically significant. Further studies are required to investigate the long-term effect of using low-dose atropine in the control of myopia and to understand the mechanism by which atropine acts to control myopia progression.,,
| Conclusion|| |
It is evident from the abundance of studies on the prevalence of refractive errors that myopia is reaching epidemic levels in certain regions, especially Asia. Recent studies from Europe and North America are showing increase in the prevalence of myopia in comparison to studies conducted earlier in these regions. Scarcity of studies in the Eastern Mediterranean region is of concern as myopia is public health burden, and without data on myopia and refractive errors' distribution, in general, it would be difficult for the public health system to have an informed decision on strategies related to the eye health.
Although it is still difficult to identify the exact mechanism of myopia development and progression, several theories have been proposed with evidence from animal models of myopia and longitudinal studies on children favoring the hyperopic retinal defocus which leads to axial elongation and shift toward myopia in both children and adults. Several strategies have been proposed and employed to control myopia progression and perhaps to prevent the development of myopia with various success rates. Advances in technology, especially in gene sequencing, may lay the foundation for “genetic myopia therapy” in the future, meanwhile research in contact lens technology is providing practical and clinically proven optical methods of controlling myopia progression in children.
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Conflicts of interest
There are no conflicts of interest.
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