Myopia, or near-sightedness, has been steadily on the rise over the last half-century. A recent systematic review updates the literature on the extent and nature of this epidemic. Let’s get straight to the findings and then discuss what this means.
The reviews includes: “276 studies, involving a total of 5,410,945 participants from 50 countries across all six continents.” The researchers find a steady increase in the pooled prevalence of myopia, increasing from 24.32% in 1990 to 35.81% in 2023. They further extrapolate that myopia prevalence may hit 40% by 2050.
These results are in line with other published studies, and are even conservative. Other estimates are closer to 50%. Going back further, although the data is not as good, in the 1950s the prevalence of myopia was in the 1-3% range. Prevalence also differs dramatically by region of the world, being much higher in Asia, especially China, with prevalence as high as 80% in some areas. What is the cause of this dramatic, long term public health outcome?
Researchers have long suspected the answer, but a big confirming clue came during the COVID 19 pandemic. There was a significant spike in myopia following the shutdown period.
Myopia is a structural problem of the eye, resulting from the globe of the eye being too elongated. It can also result from too steeped a curve of the cornea. This causes the image to be focused too far in front of the retina, making it difficult to focus in the distance. What factors can affect the shape of the eye as it develops?
Whenever we detect an increase in the incidence or prevalence of a diagnostic entity, we have to ask some basic questions. Is the incidence really increasing, or is is just an artifact of how we are gathering data? Are we looking for it more, or perhaps just better at diagnosis the condition? Has the diagnostic criteria changed? Is there diagnostic substitution – other diagnoses now being labeled with the one that appears to be rising? For prevalence, earlier diagnosis and longer survival would increase the number of people with the condition at any one time, even without an increase in incidence over time.
Or is the increase a genuine increase in the true incidence/prevalence. For myopia it seems that the increase is genuine. None of the other explanations are very plausible and there is no evidence they are playing a significant role. We often turn to demographics as a clue to causation. According to the new review:
“Notably, individuals residing in East Asia (35.22%) or in urban areas (28.55%), female gender (33.57%), adolescents (47.00%), and high school students (45.71%) exhibit a higher proportion of myopia prevalence.”
We also have the COVID spike as more evidence. What the evidence points to is that reduced exposure to sunlight contributes to myopia risk. Kids are spending less time outdoors, and more time in front of screens. This may have nothing to do with screen time itself, as the effect predates extensive screen time by children, and my be largely or entirely due to reduced exposure to sunlight. Similarly, more time spent outdoors by children reduces their myopia risk.
Why is the prevalence so much higher in China? This has been linked to urbanization, and cultural changes that put high pressure on students to spend a lot of time studying indoors.
What is the mechanism by which lack of time outdoors causes myopia? We don’t know, but there are theories. One theory is that sunlight triggers the release of dopamine which is important for eye growth. Lack of sunlight leads to lack of dopamine leads to abnormal eye growth. Another theory is that spending too much time focusing close up, and not enough time focusing in the distance, causes the abnormal growth of the eye.
Whatever the mechanism, the fix is simple (if not easy). Increased time outdoors reduces myopia risk, at least during development (probably not as adults). This means we have to get kids outdoors – as a public health measure. This may be challenging, especially in highly urbanized areas, but it is important. There are other benefits to being outside as well, including reduced risk of obesity and improved psychological health.
School schedules need to build in outdoor time. Schools themselves should be designed to provide outdoor space for students. An urban areas need to maximize parks and green outdoor spaces for children.
We have known about the connection between lack of sunlight an myopia for years, and yet prevalence of myopia continues. If we do not take action as a public health concern, it’s possible that by 2050 or soon after half the world will be myopic.