Factors that Affect the Development and Progression of Myopia (Research Paper Sample)

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Abstract
Efforts to determine the cause or causes of myopia, as well as the means to prevent it, have been an important topic of scientific interest for at least 150 years. In spite of this long span of attention and a significant increase during the last two or three decades in the number of research studies addressing myopia, these efforts have been singularly unsuccessful. This paper aims to discuss the different causes of myopia. The definition of myopia will be covered by this paper. This paper will also discuss the prevalence of myopia and the factors affecting the development and progression of myopia.
Keywords: Myopia, etiology, environmental influence, genetics
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The Role of Genetic Factors
in the Development of Myopia
Myopia or nearsightedness is a refractive error in the eye wherein the light that enters your eye does not focus properly and results in a blurred vision when you look at distant objects. Figure 1 illustrates the difference between a normal and a myopic eye: a myopic eye focuses incorrectly because its shape is slightly abnormal. A nearsighted eyeball is usually a little too long, and sometimes its cornea is too rounded. The cornea is the clear covering on the front of your eye.
Figure 1.
The Difference between Normal and Nearsighted Vision
Source: Vision Health Webpage
People with myopia often complain of headaches, eyestrain, squinting or fatigue when driving, playing sports or looking more than a few feet away. Myopia can easily be diagnosed by a doctor using standard eye exams (e.g. Snellen’s Chart). A diopter is the power of the lens that is needed to correct your vision to normal (20/20 or metric 6/6) the higher the number the stronger the lens. If you are near sighted the notation will be minus – and is described in diopters. Eyes with high myopia (≤-6.0 diopters [D]) had a higher prevalence of coexisting disease and
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complications, such as retinal detachment. The development of high-grade myopia can lead to serious vision problems including retinal detachment, macular degeneration, glaucoma, cataract, and even blindness.
Prevalence and Demographic Patterns
The prevalence of myopia often varies with age, country, sex, race, ethnicity, occupation and environment. In general, myopia firstly occurs in school-age children, and typically progresses until about the age of 21, because the eye continues to grow during childhood. According to the National Eye Institute, myopia is often diagnosed between the ages of 8 and 12. Your eyes are growing at this age, so the shape of your eyes can change. Adults usually remain nearsighted if they have the condition as a child. However, myopia may also develop in adults due to visual stress or health conditions such as diabetes. Consequently, great efforts have been undertaken to identify and understand the mechanisms underlying the development and progression of myopia.
The estimated prevalence of high grade myopia is ~2.5 to 9.6% in the elderly world population. However, its highest prevalence rates are in Asians, in whom almost 50 to 80% of the adult populations are myopic. Recent population-based studies suggest that the prevalence is increasing specifically in Asian populations. (Kleinstein et al., 2003) states that there were significantly different prevalence rates among children of four different ethnic groups. The highest rates were found among Asian children with a prevalence of 18.5% and Hispanic children with 13.2%. Lower prevalence rates were found in African American children with
6.6% followed by Caucasian children with a prevalence rate of 4.4%. (Mrugacz, 2013) reports a large epidemiologic study in 2000 by Lin et al. showed that myopic prevalence rates for Taiwanese children were reaching 80%. It is reported that there are over 80 million myopic
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children worldwide. Though the exact cause for this increase in nearsightedness is unknown, many eye doctors feel it has something to do with eye fatigue from computer use and other extended near vision tasks, coupled with a genetic predisposition for myopia.
Risk Factors for Myopia
The theories abound about what causes myopia. The loss of distance vision has been known since ancient time. According to visiontraining.com, “the Greek concept of myopia was that too little spirit of vision poured out from the brain and hence was too feeble to extend to a distant object.” However, not much attention was paid to vision problems until mid-19th century.
In 1866, German ophthalmologist Dr. Hermann Cohn, published his study of the eyes of 10,000 children attending the schools of Breslau and proposed a reasonable conclusion: the functional myopia theory or, the use-abuse theory. The use-abuse theory states that the use (and particularly abuse) of the eyes causes myopia, as seen in the higher prevalence of myopia among persons who are more highly educated and are in white collar occupations. Essentially you are gradually training your eyes to focus only on near objects or neglecting to exercise your distance vision thereby increasing the risk of developing myopia.
The genetic theory, on the other hand, is based on the belief that natural individual variation in eye growth will produce myopia in certain individuals. Both environmental and genetic factors have been associated with the onset and progression of myopia but, the mechanisms underlying the environmental and genetic factors, and the nature of the interaction between the two factors, is not certain.
Visual Stress. Visual stress is eyestrain from doing detailed work, such as reading or using a computer. Environmental factors including near work, accommodation, accommodative lag, intelligence scores, urbanization and diet have been investigated for their role in myopia and
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myopic progression. A mounting body of evidence cites time spent outdoors as crucial to the healthy development of the eye.
Near work and Reading. Many studies have reported a correlation between near work and increased myopia, however the association is usually weak and the data tends to be confounded by other genetic and environmental factors. A large study in 2008 by Ip et al. suggested a correlation between high intensity near work and increased rates of myopia. The authors showed a minimal correlation between hours spent engaged in continuous reading and parental reports of close reading distance with increased myopia. Time spent doing homework or time spent doing near activities did not show a correlation, in fact time spent playing hand held console games was associated with more farsightedness in crude analysis. Still, eye strain and eye fatigue has become a common problem in the digital age as people spend hours looking at screens. When the eye is focused on something, the muscles are flexed, putting it under tension and stress. Looking at or focusing on something for an extended period of time - such as hours spent behind a computer screen at work or flicking through social media on a smartphone - can lead to the strain. This data suggests a link between near work and myopia but does not conclude a causative relationship. A number of studies have looked at computer use and myopia and found no significant correlation.
Accommodation and Accommodative Lag. Accommodation, as defined by medicinenet.com, the ability of the eye to change its focus from distant to near objects (and vice versa). This process is achieved by the lens changing its shape. On other hand, accommodative lag or lag of accommodation is the extent to which the eyes fail to focus accurately. In other words, the cornea and lens fail to compensate for axial length (AL) elongation (myopia) or shortening (hyperopia). Accommodation is a significant covariant of reading and near work and as a result,
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several studies have focused on the association between accommodation, accommodative lag and myopic progression. A study in 2004 by Gwiazda et al. demonstrated that children with higher degrees of myopia also had increased accommodative lag. The children enrolled in the study were already myopic and although a correlation between myopia and accommodative lag can be assumed, it does not represent a causative relationship. More recent studies have shown opposite results to those of Gwianda et al. In 2006 Mutti et al. showed no changes in accommodative lag prior to the onset of myopia, moreover accommodative lag was only documented in children after the onset of myopia. The authors speculate that accommodative lag may be a consequence rather than a cause of myopia and is unlikely a predictive factor. This was further supported in 2008 by Weizhong et al who showed no significant relationship between accommodative lag and myopic progression.
Higher Intelligence. There are numerous multi-cultural studies that have shown higher intelligence quotient (IQ) scores in children with myopia. Although the causal relationship has yet to be determined, it is hypothesized that ocular axial length is directly related to cerebral development. It has also been speculated that similar genes influence both IQ and myopia. Seang-Mei et al studied 1204 children in Singapore to determine the association between intelligence and myopia. After controlling for age, gender, parental myopia and education, and books read per week they showed that children with higher IQ scores had significantly more myopia than those with lower IQ scores. The authors suggest that non-verbal IQ may be an independent and stronger risk factor than increase...