Prevention of Myopia Onset
Given the prevalence of myopia has been increasing significantly across the world, especially in East Asia, great challenges have been raised for myopia correction and the management of pathological myopia. Delaying the onset of myopia will likely reduce the prevalence of myopia as well as high myopia in school-aged children. Prevention of the onset of myopia is therefore an important priority. This chapter focuses on the clinical strategy to prevent or delay the onset of myopia among school-aged children, summarizing the interventions currently available, including increased time outdoors, reduced near work, latest optical interventions, eye exercises of acupoints, and takes a glance at future perspectives.
KeywordsMyopia onset Prevention Children Time outdoors
Time spent outdoors is well recognized as a factor preventing the development of myopia onset, and measures including adding an outdoor class, locking classroom doors during class recess, or glassed roof and walls incorporated in a classroom have been developed. But promotion to increase time spent outdoors as a school-based intervention program remains challenging especially in East Asia.
Near work activity has been suggested as a risk factor for myopia although the evidence is not entirely consistent. The total duration of near work activity may not be as important as the type of near work activity. Core techniques to implementing interventions of near work activities include effective measures of near work-related parameters, real-time data analyses, and alert systems.
School children growing up in myopigenic environments are likely to benefit from optical interventions designed to induce myopic defocus. The lens with multiple segments of defocus showed promising effect on slowing progression of myopia.
The Chinese eye exercises of acupoints advocated in mainland China and Taiwan were designed to relieve ocular fatigue and reduce the development and progression of myopia. But clinical significance of its efficacy has not been established according to available data.
Maximizing the utility of time outdoors is still the priority in the prevention of myopia onset. The role of screen time in myopia development is still ambiguous, the restriction may enable more time for children to go outdoors. Other approaches to prevent myopia onset still require further investigations, such as imposed myopic defocus, low-dose atropine, or some novel pharmacological agents for non-myopes.
Over recent decades, the prevalence of myopia in school-aged children has been increasing significantly in East Asia. Up to 80% of junior high school students have myopia, of which 20% have high myopia in mainland China [1, 2], Hong Kong , Taiwan , South Korea , Japan , and Singapore . Longitudinal data suggests that the incidence of myopia is around 10–20% per year among school-aged children [8, 9, 10, 11]. If the onset of myopia can be delayed, the prevalence of myopia as well as high myopia in school-aged children will likely reduce. This chapter focuses on the clinical strategy to prevent or delay the onset of myopia among school-aged children, summarizing the interventions currently available, and takes a glance at future perspectives.
7.2 Onset of Myopia
Several factors have been found to be associated with the development of incident myopia in school. Asian ethnicity [19, 24], parental history of myopia [25, 26], reduced time outdoors , and level of near work activity [27, 28] are risk factors for incident myopia, although the evidence can be seen as controversial in some instances. Some studies have also suggested that non-myopic children with less hyperopic refractions and greater axial length/corneal radius of curvature ratios are more predisposed to develop myopia [18, 19]. The impact of gender to the development of myopia varies among populations. In Chinese school children, females have a greater chance of progressing to myopia ; while in multiethnic populations, gender seems to be less impactful [19, 24, 27].
7.3 Increased Time Outdoors as an Intervention
Time spent outdoors is well recognized as a factor preventing the development of myopia onset. Evidence of this was first presented in a three-year follow-up study of myopia in school children, showing that those who spent more time outdoors were less likely to progress . Consistent results were reported in various studies, such as the Sydney Myopia Study, Orinda study as well as the Singapore Cohort Study of Risk Factors for Myopia [34, 35, 36]. This led to the commencement of several clinical trials, which confirmed the protective effect against myopia and indicated a dose-dependent effect, among them is the randomized clinical trial in Guangzhou which reported that an additional 40 min of outdoor activity can reduce the incidence of myopia by 23%. Additionally, the trial in Taiwan suggested that an extra 80 min may further reduce incidence by 50% [10, 37].
The mechanism of increased outdoor time as an intervention is not completely clear. Spending time outdoors itself, instead of physical activities outdoors, has been suggested to be the major protective factor . Results from animal experiments indicated that protection due to bright light may be mediated by dopamine . Some believe that ultraviolet light also plays an important role , with evidence suggesting that there is an association between vitamin D level and myopia [41, 42, 43], but data from a population-based cohort did not support this idea . Alternatively, patterns of defocus on the retina by three-dimensional structures of the environment have also been proposed as a possible mechanism of protection from outdoor activities .
Another concern is how do we measure outdoor activities? Traditionally, questionnaires have been commonly used to measure time outdoors but are less accurate due to recall bias. Objective measures can provide more precise real-time data. Wearable detectors have been developed to record light intensity and time outdoors, and even track the patterns of outdoor activities, including HOBO light meters, Nike+ Fuel Band, and others. Other measures have been used to estimate exposure to natural light such as conjunctival ultraviolet autofluorescence and skin photodamage, but these are more appropriate when measuring the cumulative light dosage or UV exposure [50, 51].
If increased time spent outdoors can prevent or delay the onset of myopia, it would ultimately reduce the prevalence of myopia or even high myopia among school-aged children. Documentation on the impact of time spent outdoors on the prevalence of myopia is of public health importance.
7.4 Reduced Near Work Intensity as an Intervention
Near work activity as a risk factor for myopia has been documented in some studies, although the evidence is not entirely consistent. A recent meta-analysis has reported a modest, but statistically significant, association between time spent performing near work and myopia (odds ratio, 1.14) .
It has been argued that the total duration of near work activity may not be as important as the type of near work activity. Studies have found that continuous reading of more than 30–45 min is associated with the presence of myopia and greater myopic refractive errors [52, 53]. Interestingly, after prolonged continuous reading, children are more likely to take up their preferred relaxed postures , such as close reading distance  and head tilt , both of which have been reported to be associated with the presence of myopia. Other factors that have been proposed to potentially contribute to the development of myopia include close nib-to-finger distance , downward angle of gaze [55, 56] and inadequate desk , or classroom lighting .
Efforts have been dedicated to developing novel devices to detect and correct inadequate near work behaviors. In mainland China, pens for myopia prevention has been invented. The pens are capable of detecting close reading distances (China Invention Patent, 200620010200.6) or nib-to-finger distances (China Invention Patent, 201020640746.6). Real-time retraction of the nib will occur when the eyes are too close to the reading materials or the nib-to-finger distance is inadequate, compelling children to adopt correct postures. The effect of the pens on preventing myopia onset is to be examined by future clinical trials.
7.5 Optical Interventions
While these optical interventions are an attractive idea, they have not been fully explored in human subjects—this will be elaborated further in Chap. 13 (Optical interventions for Prevention of Myopia Progression). A recent study has prescribed plus lenses to non-myopic children aged 5–8 years who are at risk of developing myopia . The plus lenses imposed a 1.0 D myopic defocus and the children wore the correction the entire day. No cases of myopia onset have been observed in this group of children during the follow-up period (ranging from 3 to 9 years) . The robustness of these study findings needs to be examined by future investigations.
The Defocus Incorporated Multiple Segments Lens (DIMS Lens) is a novel spectacle lens primarily designed for use in myopic children. The lens is composed of a central zone for optical correction of refractive error and an annular peripheral zone to induce myopic defocus. Interestingly, the lens peripheral zone contains numerous well-arranged small plus lenses, separated by small non-defocus areas. By using this design, myopic defocus is induced and visual quality is well reserved at the same time (presentation at the 16th International Myopia Conference [IMC]). In a pilot study conducted among school children, the DIMS lens slowed progression of myopia by 59% (presentation at the 16th IMC). The lens may also be promising for preventing myopia onset if adaptation can be successfully made for non-myopic children.
7.6 Eye Exercises of Acupoints
The efficacy of the Chinese eye exercises is believed to be from the theory of Traditional Chinese Medicine. By massaging the acupoints, Chi can be achieved and help relieve eye strain and recover ocular functions. Peak systolic velocity in the central retinal and ophthalmic arteries is observed after the eye exercises , which might provide some evidences to support this theory. Accommodative lag decreases significantly by 0.1 D after 5 min of performing the eye exercises .
Clinical significance has never been established in the published literature thus far. Cross-sectional studies have assessed the association between Chinese eye exercises and myopia with varied results [66, 67, 68, 69]. The inconsistency can be explained by the different settings studied (rural vs. urban), failure to adjust for potential confounders (including parental myopia, time outdoors, and near work) and the lack of representative populations in some studies. In a recent study, the impact of Chinese eye exercises on the development of myopia has been examined using a longitudinal design . No association between eye exercises and myopia onset has been found . However, due to the limited sample size, low level of intervention time, and performance qualities of the exercises in the study, the actual impact is still not conclusive and needs to be justified by further studies.
7.7 Future Prospects
It has been estimated that without any effective controls or interventions the proportion of myopes in the population will reach up to 50% and 10% for high myopes by 2050 . Approaches that have produced a reduction of at least 50% in incidence, such as time outdoors, have the potential to make a significant difference on the impending myopia epidemic. But the level of impact of the full utility of available interventions needs to be evaluated by further studies.
Another critical issue is how to implement both education intensity and outdoor time interventions in East Asia. There needs to be a balance between educational achievement and interventions delivered, which don’t exacerbate the prevalence of myopia in East Asia. This balance can be seen in Australia , with some of the highest educational ranks in the world (PISA, https://www.oecd.org/pisa/data/) but also high levels of outdoor activity and light intensity. Preventing the onset of myopia is certainly challenging in the East Asian population and requires a collaborative effort among clinics, schools, parents, and the entire society.
Another behavior control method focuses on limiting the screen time on computers, tablets, and smart phones. Though the role of screen time in myopia development is still ambiguous, the restriction may enable more time for children to go outdoors. Applications to provide screen distance and time monitoring, alerts to rest eyes, blue light filters, and remote locking capability for parents are available in some countries (plano, https://www.plano.co/). The effect of this technology on myopia prevention remains to be seen.
Other approaches to prevent myopia onset still require further investigations, such as imposed myopic defocus, low-dose atropine, or some novel pharmacological agents for non-myopes. They may help to prevent myopia in those who are rapidly progressing or have high-risk genetic forms. Thus, a question is raised: how do we identify children that have an increased risk of becoming myopic or highly myopic? Risk estimation is therefore critical to achieve personalized treatment for individuals. For children who are non-myopic but at an increased risk of developing high myopia in the future, additional outdoor activities and aggressive approaches should be introduced with frequent follow-up visits. Tools of myopia prediction have been developed as mentioned before, and we expect to see the outcomes of the integration of risk prediction and clinical practice in the near future.
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