The visual demand of concentrating on close-up tasks such as reading and studying are thought to be a driving force for increased myopia in children.
To better understand this relationship the authors consolidated data from several studies to quantify the effect of near work activities on myopia in children and discover any association there may be between them. They chose studies which concentrated on near activities as a co-variate of the incidence, prevalence or progression of myopia.
This systematic review showed that near work activities were associated with myopia and that increased ‘dioptre-hours’ of near work might increase myopia prevalence.
It was found that more time spent on near work activities was associated with higher odds of myopia (OR = 1.14 or an 14% increased chance) and that the odds of myopia increased by 2% for every one dioptre-hour more of near work per week.
The authors suggest that the impact of near work could be cumulative over time and that other factors such as lighting conditions and break times from near work could influence results.
- It is important for eye care practitioners to ask their young patients specific questions about what their near work activities are and how long they spend on each one daily.
- It is important that the patient and parent is encouraged to be truthful and not downplay the time spent on near work - we shouldn’t suggest that near work is bad, more that it is important to fully understand behavior to help us advise appropriate management strategies.
- Some children may be having extra after-school tuition which extends their daily near work time.
- This meta-analysis study showed the increased odds of myopia with every extra dioptre-hour of near work time to be 2%.
- We can encourage more ‘balance’ in children’s lifestyles by considering how much time they spend on schoolwork, reading and studying compared to how much time is spent playing sports and being outdoors.
- Recommendations can be made to limit the hours spent on close tasks and screen time where feasible without impacting schooling, and encourage regular breaks. This will be relevant to all of our young patients, but especially those children who:
- might already be at an increased risk of myopia from having myopic parents
- are susceptible to myopia due to their ethnicity
- are in an age range for potential onset of myopia
Limitations and future research
- Due to the nature of combining studies for meta-analysis, differences in study designs and data collection can be a cause of inconsistent results:
- Several studies had differing definitions of myopia and what constituted near work.
- Patient/parent answers to the questions regarding time spent doing near work may not have been accurate or truthful.
- The results from this study reinforce the association between near work activities (such as reading, studying, playing video games, etc) which has been found elsewhere, but it doesn’t show us conclusively how much near work is too much.
- Further research may give us a definite time limit as to how long children should spend concentrating at a given distance each day to either avoid myopic progression, or even avoid onset in the first place.
- When the time that myopes spent reading per week was compared to the time non-myopes spent, it was shown that on average, those who were myopic spent more time reading. However:
- There was no significant difference found associating myopia with other near past-times such as playing video games, watching TV or studying.
- We can’t be sure if the reading had made them myopic, or if they happened to be myopic and like reading more than the other activities.
- The authors of this study suggested that the impact of near work could be cumulative over time and that other factors such as lighting conditions and break times from near work could influence results.
- More longitudinal research is needed to better quantify the risk factor of near work on myopia development over time.
- The authors compared outcomes from this meta-analysis which revealed 2% increased odds for myopia progression for each hour spent indoors to the results of a meta-analysis conducted by Sherwin et al in 2012¹ that showed 2% reduced odds for myopia progression for each hour spent outdoors:
- The relationship between time children spend indoors vs time spent outdoors warrants further research into the prevention of myopia.
Childhood refractive error is dictated by several factors including parental myopia and how much time they may spend outdoors each day. The shape and size of the eye constantly changes with growth in response to these factors with axial length most influenced as myopia increases.
This longitudinal study sought to produce a model for predicting myopia development for Chinese children based on axial length percentile curves. The authors collected data from 12,780 children and were able to show a growth trend in axial length elongation in line with increasing myopia in 75% of the children between the ages of 6yrs and 15yrs old. The authors found that across all percentiles above the first quartile, the axial length increased with age, had already progressed in the 6yr old children and continued to progress in those up to 15yrs.
They authors concluded that the information gathered from study could be used to predict a likely progression of axial length for school-age Chinese children.
- Percentile growth charts are useful to show patients and their parents where a child currently sits on a growth curve and how this can be used to predict future likelihood or level of myopia without any intervention to limit myopic progress.
- Practitioners need to be mindful that growth curves are based on averages and an individual child may not follow their centile group perfectly.
- Age seems to be a large risk factor in myopic progression with large jumps in AL for both girls and boys at the same age with a key age of 6-7yrs where fast progression takes place, and has been found in other studies.
- In their study of European children and adults, Tideman at el (1) found that axial length increased more for myopic children than those who were hyperopic.
- Further support can be found from the Northern Ireland Childhood Errors of Refraction (NICER) study by Breslin et al (2)
- This research was carried out in China where there is higher prevalence of myopia compared to many European countries and compared their findings to those from a European cohort study by Tideman et al (1):
- At age 6yrs percentile values for axial length were similar.
- By age 9yrs and 15yrs AL in the Chinese children was higher than for the equivalent aged European children.
- These comparisons made between the two studies suggest that in practice ethnicity needs to be considered when using percentile charts to establish potential for future myopic progression.
- To achieve maximum effect some children may benefit from having their myopia managed at an earlier stage if their percentile curve indicates likelihood of eye growth is most apparent.
Limitations and future research
- There are many variables to myopic progression including parental myopia and individual variation between time spent outdoors and indoors, and time spent on close tasks like reading. The axial length growth curves established from this research do not take these factors into consideration like so can only be used to illuminate expected future progression as an average..
- To improve accuracy, this may mean that separate growth curves may need to be established that take potentially influential factors into consideration.
- Differences in myopia prevalence within the population may indicate that percentile charts are not interchangeable between populations leading to the need for development of different percentile charts across different geographical regions.
Education and myopia have had a long association, both anecdotally and from research findings with studies in general finding strong correlations with education and prolonged close work rather than a definite causative link. This study investigated the impact of education on refractive error by examining the relationship between increasing the school leaving age and myopia.
Plotnikov et al analysed data gathered from UK Biobank participants born in a nine-year interval centered on 1957 who were directly impacted by the Raising of School Leaving Age (ROSLA) policy implemented in England and Wales in 1972. ROSLA was found to have a causal effect on refractive error of -0.77D, indicating higher myopia from an extra year of schooling.
This study revealed that an extra year of schooling led to higher adulthood myopia with greater effect on those with genetic predisposition to higher myopia (-1.47D) rather than lower myopia (-0.50D). The suggested explanation is that those in the higher risk group were destined to be myopic anyway and that the extra year at school was to blame for increased myopia in those who were otherwise not expected to become myopic. Consequently we need to be aware that it isn’t just the myopic children that need monitoring for myopic progression. Any child, regardless of their refractive error can potentially be at risk of becoming myopic. While this study does not investigate indoor vs outdoor time, it highlights the potential benefit from spending time outdoors to help offset this studies demonstrated impact on myopia from increased time spent at school, following the implication that extra schooling will likely result in more time spent indoors.
Limitations and future research
This analysis was limited to participants from England and Wales and suffered from potential for selection bias and a modest sample size, however its contribution is invaluable as there are limited opportunities to assess the true impact of direct alteration of schooling years on refractive error. The authors were cognisant of the many errors that could be introduced from their analysis and detailed the steps they took to analyse and eliminate errors. By example the found a difference in education attainment between the UK biobank population and the general population - they applied an inverse probability weighting function to account for the higher proportion of educated individuals in the UK biobank dataset resulting in a shift from -0.55D causal effect of ROSLA on adulthood myopia to the reported -0.77D.
Much of the published research reports association between a predictor and outcome variable rather than direct causal effect of an interaction. E.g. in this study the direct effect of 1 year less schooling on myopia could be established, which is a different measure to reporting an association between time spent in school and myopia and using this to infer the effect of longer time spent at school. The difference between the -0.29D effect from the standard linear regression analysis and the -0.77D effect from the more advanced RD analysis used to estimate the causal effect highlights how the way this research question is posed influences the outcomes. Ultimately it is measurement of outcomes from direct interactions that provide the most value which should be kept in mind when interpreting outcomes from published research. In this regard it requires drastic action like the ROSLA reform to create the test from control conditions in research on how the number of years spent at school influences adulthood myopia, limiting the future research on this topic.