As eyecare technology becomes more advanced with each decade, our approach as optometrists is beginning to shift from retroactive care to proactive care.
Whether it's identifying glaucoma before any vision loss occurs or using a swab to determine if a patient is at high risk for macular degeneration, optometrists are now focusing on genetic and environmental factors that may put a patient at higher risk for developing a disease.
Overview of myopia risk
In the past, myopia was mainly treated through spectacle or contact lens correction, which ultimately was used to “correct” or “band-aid” the problem.
However, it was not until statistics showed myopia was increasing at an alarming rate (nearly doubled in the last 30 years and expected to reach 3 billion worldwide in 2030) that researchers realized that myopia is a worldwide epidemic and we need to understand the risk factors associated in order to stop myopia in its tracks.1
This article will review the four key fundamentals for assessing the risk of myopia onset as well as five identifiers for myopia progression. We will then discuss how practitioners can apply the information available to them better to prevent and/or slow myopia in their patients.
4 key principles for assessing the risk of myopia onset
1. Family history
One of the most important risk factors for assessing the risk factors of children developing myopia is the number of myopic parents (>-0.50D). In a study by Lim et al., the authors found that the prevalence of myopia in offspring increased significantly for every additional myopic parent.2 However, no significant connection was found depending on which parent had myopia.3
In a separate study, Jones et al. sought to determine if there were any confounding factors between myopia onset and the number of myopic parents, hours of sports/outdoor activity, hours of reading, hours of television watched, hours of studying, and hours of computer games.
“The only interaction was the number of hours of sports/outdoor activity and myopic parents.”
The explanation for these findings may be due to the fact that having parents who are more myopic are likely to be more educated and have careers that involve more indoor work. This lifestyle is then passed along to their children, who spend more time indoors and focus on education. Furthermore, it was discovered that for children with one myopic parent, which parent had myopia had no significant effect.2
Interestingly, one study by Guggenheim et al. investigated the prevalence of myopia among twin and sibling studies. The authors found that there was a correlation in refractive error of approximately 45% after adjusting for age and sex.4
2. Visual environment
Many practitioners have debated whether being outdoors from an early age reduces the chance of onset of myopia due to fewer hours of near-work or the physical light properties only found in outdoor lighting.
A famous study by Rose et al. tried to uncover the answer to this ongoing discussion. Investigators found that less than 90 minutes a day spent outdoors increases the risk of developing myopia (not progression), especially if combined with more than 3 hours a day spent on near-work activities. Children with low outdoor and high near-work activity had 2- to 3-fold higher odds for myopia than the reference group (high outdoor and low near-work activity levels).5
The exact reason for these findings is still unclear. Some theories are that high levels of outdoor activities may mean less time for near-work activities; however, this is unlikely because near-work and mid-work distance activities appeared to have little effect on refraction. Perhaps being outdoors results in low accommodation demand.5
Another theory is that the light intensity only found outdoors triggers the release of retinal dopamine, which has been shown to slow myopia progression.6 This theory is still being tested by researchers and clinicians.
One important thing to note is that being outdoors has only been shown to prevent myopia onset and has not shown to be effective in reducing myopia progression.7 Therefore, it is important to have children exposed to the outdoor elements frequently and for long periods at a young age.
3. Binocular vision
Another area of our visual system that has been determined to be a possible risk factor for myopia onset is reduced accommodative system/lag. Studies have shown that children with higher accommodative convergence (AC/A) ratios, typically seen with esophoria, have an increased risk of myopia development within one year by 50%. Once myopia has stabilized, the AC/A ratio has been shown to normalize.8,9
“It is still unclear, however, if increased accommodative lag in myopes occurs before or after the onset of myopia.”
The latter is more likely the case, which was supported by a study by Mutti et al., who demonstrated that accommodative lag was not significantly elevated until the year after the onset of myopia.10
In the last decade, researchers have found that intermittent exotropia may also be a risk factor for myopia. Ekdawi et al. found that approximately 80% of children with intermittent exotropia had developed myopia by the age of 20. More studies are needed to be performed to determine the cause-effect relationship between exotropia and myopia.11
4. Current refraction
The current refraction of the patient has been found to be the most significant risk factor for the onset of myopia. A well-known study by Zadnik et al. focused on children ages 6 to 11 and the corresponding refractive error that would lead to myopia in the future. They determined that children who exhibited 0.50D or less of manifest hyperopia at ages 6 to 7 have a high risk of developing myopia. This risk is independent of family history and environment.12
A separate study by Mutti et al. determined that the fastest rate of refractive change in myopic children occurs the year prior to myopia onset. Therefore, children who are less hyperopic than age normal should be closely monitored, especially if other risk factors are present.13
5 key identifiers for myopia progression
1. Age
Many studies have supported the theory that the younger a child becomes myopic, the faster they will progress. Donovan et al. explored the estimated annual progression at specific baseline ages for Asian and European children. Children, 7 years of age were estimated to progress by at least 1D per year, while 11- to 12-year-olds were estimated to progress by 0.5D per year. Asians always showed a higher myopia baseline than Europeans of all ages, which is to be further discussed in the ethnicity section below.14
These findings are supported by Hyman et al., who found that subjects who developed myopia at a younger age (6 to 7 years old) resulted in a greater myopic change in both diopters and axial length measurements.15
2. Family history
Family history is not only a high-risk factor for myopia development but also myopia progression. Children with two myopic parents are the fastest progressors compared to children with one myopic parent, who progress at a rate between those that have zero and two myopic parents.
This statement is supported by Kurtz et al., who studied the effects of parental refractive error on both child refractive error progression and axial length change over a 5-year period. They found that for the children in the single vision lens group, there was a positive correlation between the number of myopic parents and increased progression of myopic refractive error and axial length.16
These conclusions are in agreement with another famous study by Saw et al., who also found dose-dependent changes in myopia with parental myopia. They also took it a step further and investigated the progression of myopia in children with two parents reporting high myopia (6.0D) and found that this group progressed faster than any other subgroup.17
“Many theories have been postulated as to how genes result in increased change of myopia development and progression.”
It is unlikely that genes produce myopia directly but instead affect the susceptibility of eyes to myogenic environmental factors. For example, genes may contribute to an increased amount of higher-order optical aberrations, which may induce more retinal blue during near work.18,19 Perhaps genes influence the efficiency of neural pathways that control the near triad, resulting in poor accommodative response when reading.20,21
3. Visual environment
The only relationship that has been found between near-work and increased myopia progression is when the near-work required less than 20cm working distance for durations of longer than 45 minutes, but not overall time spent on near-work.22
There is a question of how the increased use of digital screens, especially during the COVID-19 pandemic, impacted the risk of myopia progression. Mohan et al. found that 96.7% of children were using smartphones to attend online classes. However, it was not the smartphones themselves that increased the risk of myopia progression but more so the home confinement and decreased outdoor exposure.23
Most studies support these findings in that it is not the near-work activity itself that results in increased myopia but instead less time spent outdoors.
4. Ethnicity/gender
As mentioned above, studies have shown that Asian ethnicity has been linked to faster myopia progression, whereas African American/Native American people show the least. Furthermore, females have been shown to progress faster than males.24,25
5. Binocular vision
It was stated previously that possible risk factors for myopia onset include esophoria, accommodative lag, and intermittent exotropia. In myopia control studies of children in progressive lenses vs. single vision lenses, children with esophoria in single vision lenses progressed more rapidly.26
It was determined by Gwiazda et al. that myopic children accommodate significantly less than emmetropic children for real targets at near distances.27 A decade later, Gwiazda et al. tested the effects of progressive lenses on slowing myopia progression in children and found that children with larger baseline accommodative lag demonstrated a statistically greater treatment effect.28 This was also found to be true with children who were fitted for orthokeratology lenses.29
Lastly, although Ekdawi et al. found that 50% of children with intermittent exotropia were myopic by age 10 and 90% by age 20. The effect of treating intermittent exotropia on myopic progression has not been studied.11
Conclusion
Understanding the risk factors involved in myopia onset and progression can be essential to preserving the vision of patients who are at high risk of developing/progressing myopia.
Because of scientific studies such as the ones mentioned in this article, practitioners can now provide parents with evidence-based research on the likelihood of their child developing myopia and what they can do to prevent and/or slow progression. As medicine begins to understand better the risk factors and early signs of certain diseases, it is necessary to take the knowledge of our past and use it to protect our quality of life in the future.
Slowing myopia progression by even 50% can greatly reduce the risk of vision loss.
