The Science

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Myopia can sometimes appear in children before they start school. It is uncommon, likely to be genetic in origin, and can be severe. Far more common is the myopia that develops during the school years, often referred to as simple myopia. In this section, we explore deeper into the natural process by which baby’s eyes grow, what can cause the eyes to deviate from this path, and what actual changes occur in the eye when simple myopia develops. (For a guide to some terms used in explaining the way the eye focuses, please refer to this article.)

How do Normal Infants’ Eyes Grow?

The newborn’s eye is around 17mm in length and grows to about 20mm in the first year. They are moderately hyperopic to begin with, averaging around +3.00 D with a wide range initially, and gradually reduces to around +1.00 D by the age of 5 years. This highly coordinated process of eyeball growth, precisely matched by changes in its focusing power, leading to normal refraction of the eye, is called emmetropization.

Emmetropization is believed to be controlled by visual feedback from the infant’s eyes to their environment and is usually completed by 6 years of age. A mismatch between the length to which the eye grows and the focusing power of the eye results in refractive errors. Simple (school-age) myopia typically develops between 6 and 14 years of age, progresses during the school years, and stabilizes in late adolescence or early adulthood (Mutti, 2024).

What Happens to the Eye In Myopia?

‍ ‍Using a projector and screen analogy, in simple myopia, the projector is stationary, but the screen is set too far away from the projector to produce a clear image. The distance from projector to screen is analogous to the axial length of the eyeball (front-to-back distance of the eye). The increasing distance between the projector and the screen, rather than the focusing ability of the projector, is the key feature of simple myopia.

There are other changes within the structure of the eye that are worthy of note, as they lead us to understand more precisely the mechanism by which risk factors lead to myopia progression. For example, the sclera, which is likened to the wall of the eyeball, is stretched because of the elongation of the eyeball in high myopia. This occurs mainly at the back of the eye, causing it to thin and lose its tissue strength. This renders the area around the optic nerve more susceptible to damage from glaucoma, one of the more important complications of high myopia.         

There has also been a lot of research interest in the choroid, which comprises a layer of blood vessels and pigment sandwiched between the sclera on the outside and the retina on the inside. Its major functions are to provide oxygen and nutrients to the retina and to absorb excessive light through its pigmented cells.

Changes in this layer can also be a marker for some changes that occur during eye growth. For example, the choroid layer appears to thin during myopia development, which is now measurable with high-resolution optical coherence tomography (OCT). But it thickens when interventions that slow down myopia progression, like eyedrops or light exposure, are applied. These changes happen quickly, unlike axial length, which may take weeks or months to see meaningful change. Understanding its function, therefore, can help to develop new strategies in prevention and slow down myopia progression.

Risk Factors for Simple Myopia

Before moving to specifics, we should establish what we mean by the terms cause, risk factor, and association, which will help when interpreting results from research studies on myopia.

Cause is defined as something that directly leads to the development of myopia. So, A makes B happen, and removing A prevents B.

Risk factors are defined as something that increases the chance of developing myopia; it doesn’t guarantee it, but it makes it more likely. So, if you are exposed to A, you are more likely to get B without necessarily knowing how A leads to B.

Associations are defined as something that occurs at the same time as developing myopia. They could influence each other, or they could be influenced by another factor entirely. So, when A happens, B happens, but we don’t know if A makes B happen, or if B makes A happen, or if something else (C) is making both happen.

In simple (school) myopia, there are no direct causes that we know of. However, many risk factors and associations have been proposed, some with stronger evidence than others. In the previous section, a non-modifiable risk factor in parental myopia was discussed. What follows are modifiable risk factors and associations, where opportunities for intervention might be discovered. They are placed in order based on the strength of evidence supporting their role in myopia development in childhood.

Protection by Time Outdoors

‍ ‍Strong evidence has emerged in the past decade linking time spent outdoors with a protective effect on myopia progression (Xiong, 2017). Randomised controlled trials conducted on children in China have shown that an addition of 40-80 minutes per day of outdoor activity at school led to lower rates of myopia (He, 2015). Furthermore, increased time outdoors can compensate, to a certain extent, for the effects of prolonged near work and the impact of parental myopia as risk factors. In other words, a lack of time spent outdoors is a powerful driver of myopia development in children.

Various theories have been put forward to explain the protective effect of being outdoors, including ultraviolet light exposure, outdoor objects appearing more uniformly in focus, and the absence of near work when outside. However, researchers now believe that exposure to bright light appears to be the major factor, and this has been reproduced in animal studies. The mechanism appears to be an effect on dopamine release within the eye, a chemical that slows down axial elongation of the eye and hence myopia development. The recommendation is therefore for children to spend a minimum of 2 hours per day outdoors to deter the onset of myopia.

Education

‍ ‍Epidemiological studies show quite a striking difference in the prevalence rates of myopia in young adults, ranging from 70 to 90% in parts of East and Southeast Asia, down to less than 10% in societies where education opportunities are still limited. Where western style education shares the number of years of schooling with its East Asian counterparts, the prevalence is only between 10-60%. The more moderate rates in Western countries have been thought to relate to less intense education, lesser homework loads, and more time outdoors (Morgan, 2018).

A frequently cited study highlighting education as a risk factor was conducted on Orthodox Jewish students in Israel (Zylbermann, 1993). They found that over 80% of male Orthodox students, who received an intensive religious education, were myopic. This was compared to less than 50% prevalence among their sisters who attended the same schools, and boys and girls who attended general schools. The time spent reading and writing was as high as 16 hours per day for Orthodox males compared to 2-3 hours per day for the other students in their study. Intense near-work activity has therefore often been suggested as one factor by which education influences myopia development.

Near Work

‍ ‍The evidence that the time spent on near-work tasks influences myopia development has been mixed. The problem may lie in the surveys that are often used in these studies. Having parents recall the amount of time their children spent on an activity over a period of months and years may not be reliable. Randomised controlled trials have ethical concerns, as subjecting a child to more close-up activity is not workable.

There have been various theories put forward to explain the mechanism by which near work might influence myopia development. Examples include the excessive use of the focusing power of the eye, different types of defocus (blur) that near work brings to the eye, or the way black text on a white background stimulates different cells within the eye. None have emerged yet as the dominant theory.

However, one aspect that might have escaped attention in these studies is the timing of such exposures in early childhood. The Orthodox children of Israel learned to read 1-2 years earlier than their less myopic pupils who attended secular schools, while spending similar amounts of time outdoors and doing near work when not in school (Gordon-Shaag, 2021). Similarly, in China, children who start school a year earlier, because they were assigned to a higher grade having been born before the 1st of September cut-off date, had a higher prevalence of myopia, an effect that carries through the primary school years (He, 2021).

During the COVID-19 pandemic, another study from China found an association with a substantial myopic shift in younger school-aged children (6-8 years old) compared to older children (9-13 years) despite receiving less online instruction (Wang, 2021). Investigating the effects of near work earlier in childhood may therefore give us a better understanding of its effect on the onset of myopia.

Screens

‍ ‍Over the past two decades, the use of digital devices has become an integral part of schooling and children’s daily lives. While researchers have suggested a link between myopia progression and screen use (Cyril Kurupp, 2022), it has been difficult to isolate this as an independent risk factor over increased near-work time and reduced outdoor time. This has led leading authorities to conclude that, “There is currently no evidence that time using digital devices is more dangerous than a similar amount of time reading, but more work in this area is clearly required.” (Morgan, 2021). There are, however, differences between screens and conventional printed books, and these are highlighted below.

First, not all screens are the same. It is important to distinguish between smaller screens that are held in the hands (for example, smartphones and gaming devices) which are viewed up close and therefore require focusing power, versus larger screens (such as televisions) which are typically viewed much further away. A study on Taiwanese young adults found a correlation between time spent reading and myopia, but not time spent watching TV (Lee, 2013).

Second, electronic devices differ from books in the light emitted by their screens. The spectrum of light, brightness, contrast, and screen flicker are all properties that are under investigation in relation to myopia (Wagner, 2023).

Third, children may use screen devices differently from other near-work tasks. Bao (2015) investigated 120 schoolchildren aged 6-13 years, comparing their posture whilst performing three near tasks: playing a video game on a hand-held device, reading at a desk, and writing on paper. They found that the viewing distance was significantly closer when playing the video game (21.3 cm) than when reading (27.2 cm) and writing (24.9 cm). While this difference seems small, the focusing effort needed to look at an object rises disproportionately higher the closer the viewing distance.

The lack of consistency in definitions and measures of digital device usage makes it challenging to confirm any causal link between screens and myopia. Emerging technologies that measure device usage and screen distance more accurately will help further research that can provide more definitive recommendations.

Ethnicity and Urban Living

‍ ‍Ethnicity has often been thought to be a determinant of myopia. While studies show differences in myopia prevalence among different racial groups, deeper analysis shows that these are probably because of environmental factors. For example, the high prevalence of myopia in Singapore among the Chinese, Indian, and Malay population, contrasts with the much lower prevalences in India and Malaysia, implying that it is the education system in Singapore and perhaps the limited time outdoors that are responsible. The difference in prevalence rates seen in urban living versus rural living can likewise be explained by environmental factors.

Other Factors

There have been many other associations that have been investigated as possible independent risk factors for myopia. They include gender, pollution, housing, height, diet, sleep, birth order, season of birth, and others. These will not be described in detail as research shows most of these are probably mediated by education and time spent outdoors in one way or another. There are also some common misconceptions about the causes of myopia that have arisen and taken hold over the years. Examples such as reading in poor light, in a moving car, or bad posture while reading and writing, have never been proven.

Summary

The strongest evidence-backed risk factors for the development of myopia are education and limited time spent outdoors. These two factors come together and can modulate eye growth during childhood. Strategies aimed at increasing time outdoors and reducing near work time will go some way towards slowing down the current myopia epidemic.

Next: The Context‍ ‍

References

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Wagner, S., & Strasser, T. (2023). Impact of text contrast polarity on the retinal activity in myopes and emmetropes using modified pattern ERG. Scientific Reports, 13(1), 11101.

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Xiong, S., Sankaridurg, P., Naduvilath, T., Zang, J., Zou, H., Zhu, J., Lv, M., He, X., & Xu, X. (2017). Time spent in outdoor activities in relation to myopia prevention and control: A meta-analysis and systematic review. Acta Ophthalmologica, 95(6), 551–566.

Zylbermann R, Landau D, Berson D. The influence of study habits on myopia in Jewish teenagers. J Pediatr Ophthalmol Strabismus. 1993 Sep-Oct;30(5):319-22.