 Welcome to this session on the epidemiology of refractive errors and specifically myopia. At the end of this session you should know the definition of refractive errors and myopia in children, be able to describe the distribution of refractive errors and myopia, be aware of the geographic variation in refractive errors and myopia as a cause of visual loss and possible reasons for this variation. You should be able to list the potential risk factors for the development of refractive errors and understand the main hypotheses for the development of refractive errors. You will remember from last term and other sessions in this module that epidemiology is the study of the distribution and determinants of disease or health related states to understand and allow for its control. So in this case of refractive errors and myopia we want to understand what is refractive error, how many people are blind and who is blind from refractive errors and myopia and why are children blind from refractive errors and myopia and then finally how can blindness from refractive error and myopia be reduced. We will cover this in a separate session later today. I want you to pause for a minute and define what is a refractive error. You pause the video now and make a note of your definition. It is common to have different ways to define a refractive error such as based on vision, anatomical or physiological condition, the type of correction that is required, the symptoms someone has or even just the type of refractive error. None of these are incorrect and they're used dependent on the context. So we'll define refractive errors as a disorder in which parallel light entering the eye from a distant object is not focused on the retina. The retina converts the light rays into signals that are sent to the brain via the optic nerve. The brain then interprets these signals into the images that we see. Refractive errors can cause blurred vision in the affected eye or eyes and can vary in severity. The refractive state of the eye is determined by the cornea, the lens, anterior chamber depth and axial length of the globe. The three most common types of refractive errors are myopia, hyperopia and astigmatism. Presbyopia typically affects the adult as part of the aging process and we won't consider this further in this session. We'll start with hyperopia and astigmatism and then go into a bit more detail about myopia. Hyperopia or hypermatropia is caused by a focusing problem. If you think about it like this, when you watch a movie at the cinema the film projector has to be focused to get a nice sharp picture on the screen. The light passes through your eye's natural lens like it passes through the projector's lens. Light then focuses on the retina at the back of your eye the way it focuses on the cinema screen. Imagine if the distance between the projector and the screen was too short. The picture on the screen then would be blurred. In hyperopia the length of the eyeball is too short for the strength of the eye's lens. So the light is focused too far back behind the retina and so things appear blurred. Is hyperopia common in young children? Yes it's perfectly normal for most children to have mild hyperopia or farsightedness early in life. In many cases no treatment is necessary because a child can accommodate by using their own eye muscles to properly focus. Astigmatism. Astigmatism causes distorted vision as the refracted power of the cornea or the lens are not consistent in all the meridians. So parallel light rays enter the eye and are refracted unequally in different directions. This leads to a lack of sharp point of focus on the retina and again causes blurred or distorted vision. Astigmatism can be classified in three ways. With the rule when the refracted power of the vertical meridian is the greatest. Against the rule when the refracted power of the horizontal meridian is the greatest. And then oblique when the two primary meridians are neither horizontal nor vertical. Astigmatism can affect all age groups and doesn't significantly change with age. Myopia or short sightedness is when objects up close appear clear but there is difficulty in seeing distant objects. So parallel light rays from the distance objects enter the eye and focus in front of the retina instead of on the retina because the eye is too long. Sometimes myopia can also be the result of the cornea being too curved for the length of the eye or the lens being too thick. Myopia is the most common from a refractive error in children and usually it starts around the age of eight to 10 years and it progresses in severity throughout the childhood and adolescence and into adulthood. However in China and other East Asian and Southeast Asian countries myopia begins earlier and is prevalent in children as young as five years old. The prevalence of refractive errors in children can vary in different ethnicities and cultural settings and comparing regional prevalences are very difficult because different definitions of myopia, hyperopia and astigmatism are used. These studies are done in different age groups and also the different procedures that are used to measure visual acuity and refractive error status. However uncorrected refractive errors are the most common cause of visual impairment in children. Global estimators indicated that there were about 12.8 million children visually impaired from uncorrected refractive errors. This translates into approximately one percent of all children and given the current global trends in myopia this is set to rise. What I'd like to talk about now is something called the refractive error studies in children. The risk studies are a set of eight population based studies not school based and they were conducted in children aged five or seven up to about 15 years old using a very standardized methodology to estimate the prevalence of refractive errors in different ethnic groups and cultural settings. They used consistent definitions and common methods in all of the eight locations. These locations were Nepal, India where they did two studies urban and rural settings, Chile, Malaysia, South Africa and then again China where they also lived at the urban and rural difference. The results from these studies confirmed that there is a significant variation in the prevalence of refractive errors in the different regions and also that there is a difference within countries and the variation is between urban and rural settings and this was seen in India and China. On this graph you can see that the difference between presenting visual acuity and best corrected visual acuity is the unmet need for spectacle correction. The prevalence of hyperopia. Again it's difficult to compare studies that report on the prevalence of hyperopia due to the different definitions that are used. The risk studies attempted to use the same definitions throughout. Hyperopia is a known risk factor for the development of strabismus and ambliopia and there is also a growing body of evidence that suggests uncorrected hyperopia may be linked to poor academic performance and deficits in the visual cognitive and visual motor motor measures. Privilege of myopia in teenagers and adults. This map is a compilation of various sources to illustrate the range of myopia prevalence in different regions. In 2016 Rednicka et al published a review to quantify the global variation in childhood myopia and myopia prevalence taking into account ethnicity age and the study design. In this paper it stated that there are noticeable differences in prevalence by ethnicity and these differences increase with age. So East Asians have the highest prevalence of myopia reaching to about 80% by the age of 18 years of age compared to Black children in Africa in late adolescence where 5.5% of 15 year olds are affected. Also children who predominantly live in urban areas are 2.6 times more likely to be myopic than children living in rural environments. This graph shows the distribution of people with myopia and the prevalence across all age groups. In 2000 the greatest number of people with myopia were between the age of 10 to 39 years. However the projections suggest that throughout both cohort and age effects this distribution will spread by 2015 with large numbers of people with myopia from 10 years of age all the way through to the age of 79. Looking at the numbers of myopia in the year 2000 and looking at those values you can see that the bulk of myopia in age groups younger than 40 years. This reflects the significant lifestyle changes for children and young people over the past 10 to 25 years especially in large population centres of Asia. Looking at this graph this is from a recent systematic review and meta analysis by Holden et al. This suggests that between 2020 and 2050 the number of people with myopia globally will increase from 1.9 billion to 5 billion and 1 billion of those will have high myopia. Now these projections were based on existing data and assumed that current lifestyle patterns would continue so things like the amount of time that's spent indoors, the amount of time that's spent doing near work and that lifestyle changes will continue to spread with increasing urbanisation and development. There is a jury out there at the moment whether the COVID-19 pandemic will have an impact on these figures given the increased screen time and time spent indoors changing drastically for all age groups especially children with homeschooling. With this increasing prevalence of myopia, vision impairment from uncorrected myopia is also increasing. So what we've understood from this is that there's an increasing prevalence of myopia globally. Myopia affects children from a young age which means that the vision impairment from uncorrected myopia is also increasing. Given this epidemic in myopia there is insufficient attention from a public health perspective in terms of assessment of prevalence, preventive and possible treatments. There is also a lack of consensus on the threshold for classifying myopia and high myopia and the terminology classification and methods used in epidemiological studies is not always defined in order to assess the extent and contribution of myopia to vision impairment. These findings together with the lack of consensus on standard definitions for myopia and high myopia and insufficient attention from a public health perspective led to a global scientific meeting on myopia in March 2015 and this was led by the WHO and the Brian Holden Vision Institute. Participants at this meeting were scientific and clinical experts in myopia from six different WHO regions and they published a report called the impact of myopia and high myopia. This report led to a great definition of myopia and high myopia. Myopia was defined as a condition in which the spherical equivalent objective refractive error is greater than or equal to minus 0.5 diopter in either eye. High myopia defined as a condition in which the spherical equivalent objective refractive error is greater than or equal to minus five diopters in either eye. There was also a clinical definition of myopic macular degeneration and the need for it to be used clinically and in research agreed. There was a call for clinical and epidemiological research in myopia to use standardized methodologies and then also the inclusion of myopia and high myopia and myopic macular degeneration as attributable causes of vision impairment in epidemiological surveys. There were calls for evaluation of the evidence and myopia control strategies such as increasing time spent outdoors, the use of atropine or spectacles of contact lenses. There was also a call for evaluation of the evidence on environmental, optical and therapeutic factors on the progression of myopia. So what causes myopia and why does it progress in some children versus other and why are some children more at risk of becoming myopic? These are all really valid questions however they're also very complex with a number of factors making it very difficult to disentangle the effect of nature versus nurture that is genetics versus the environment. The next few slides will briefly go through the different determinants of myopia and I'll highlight the different conclusions from various studies. So the four things that we will look at in terms of etiology and risk factors for myopia are genetics, emitropization, near work and outdoor time. So we'll start with genetics. Now it's now sort of generally accepted that genetic change is too slow to explain the rapid increase in the prevalence of myopia seen over a short period of time in places in East Asia, thus indicating that environmental factors associated with more widespread education and urbanization may be playing a larger role in this. Furthermore, the higher prevalence of myopia is not limited to a single race. It is higher in people of Chinese, Indian and Malay origin in Singapore than in Indians in India and rural Chinese in China. For the purpose of completeness it's good to note that genetic factors impose a level of baseline risk of myopia, but the association of myopia with environmental risk factors has led to the epidemic of myopia. So twin studies have showed that genetics can explain 60 to 80 percent of variants in spherical equivalent refractive error and axial length. Other studies however have indicated that the genetic contribution is far less and does not explain the majority of cases of high myopia. Studies of the genetic contribution should include evaluation of environmental aspects as factors such as time spent outdoors and time spent doing new work vary by families. This other studies have found that the children of myopic parents have higher prevalence of myopia, but the relative risk varies and is lower where the prevalence of myopia is high as in East Asia. What we do know is that family history and environment increases the risk of developing myopia and so this is an opportunity for clinicians to manage myopia in school age children. Emitropization. The development of an eye towards emitropia is known as emitropization and this process is guided by visual input and the mechanisms that coordinate this process are not fully understood. What we do know is that the fovea alone is not responsible for the development of vision. Eye growth is responsible too and that peripheral retina signals can dominate eye growth regulation. So based on this there are certain optical strategies that bring the peripheral image onto the retina therefore removing the stimulus for elongation. I won't dwell on this too much but there is thought by some that relative peripheral hyperopsy focus encourages the growth of the eyeball and as a consequence increased axial length in myopia. This also forms the basis of some myopia management interventions that we will cover later on. So basically hyperopsy focus which is common in corrected myopic eyes can increase ocular growth whereas myopic de-focus especially when it is imposed of a large area of retina can slow axial elongation. Environmental factors. The projected increase in myopia and high myopia are widely considered to be driven by environmental factors so nurture. Principally lifestyle changes resulting from a combination of decreased time outdoors and increased near work. Long environmental factors so-called high pressure educational systems especially at a very young age in countries such as Singapore, Korea, Taiwan and China may be a causative lifestyle change as may the excess use of near electronic devices. Other proposed causes include light levels which may directly be related to time outdoors with peripheral hyperopia in the myopic eye encouraging axial growth and diet. There's also some observed seasonal variation in the progression of myopia and that adds weight to the argument that time spent outdoors slows down the progression of myopia. Near work. The next two slides summarize the different pieces of evidence linked to near work and outdoor time and the theories for myopia onset and progression. There is some evidence that does a higher prevalence of myopia with increased near work and for example students, tailors etc. There is also accommodative lag and this has been studied extensively. Accommodative lag contributes to the hyperopic de-focus on the retina and that promotes eye growth. Myopes have a greater accommodative lag than emetropes. It's very unclear and it's a bit like chicken and egg unclear whether myopia comes first or the accommodative lag. Outdoor time as a strategy to prevent the onset of myopia. As a strategy to prevent the onset of myopia outdoor time is now recommended and is also built into some school curriculums. Time outdoors is protective against the development of myopia. There's also evidence that higher levels and spectral composition of the natural light act as a protective factor. Then there's individual factors such as smaller pupils, a relaxed accommodation, reduced dioptic value variation that also play a role. The high light levels increase dopamine and slow eye growth in animals however this has not been validated in human myopia. Myopia progresses faster in winter compared to the summer suggesting that time outdoors or light plays a role in myopia development. There is a hypothesis that myopia myopes have a lower level of vitamin D however evidence of vitamin D playing a role in myopia is not very strong. There are theories about why time outdoors reduces myopia risk. It's more likely that it's because of the hyperopathy focus and the increased light levels outdoors rather than it being because of the spectral composition of light. Advocating for children to spend more time outdoors generally has a positive impact on their well-being and their development so regardless of how and why it reduces the risk of developing myopia it's strongly encouraged that children spend more time outdoors. So in summary, refractive errors are a common cause of vision impairment. Myopia is the most common type of refractive error in children and usually starts from a young age. There are geographic variations in the distribution of refractive errors in specifically myopia and this is now being known as the epidemic of myopia. Both genetics and environmental factors play a role in the development of myopia however it is widely accepted that genetic contribution is considered small and there is a consensus that genes may determine susceptibility to environmental factors. Also the genetic contribution does not explain the majority of cases of high myopia.