Schmid, Katrina L.*; Leyden, Karla†; Chiu, Yu-huan†; Lind, Sarah-Rose†; Vos, Dow James†; Kimlin, Michael*; Wood, Joanne M.*
The prevalence of myopia in children and young adults varies greatly around the world.1 The Refractive Error Study in Children Survey Group reports five sites where myopia prevalence is less than 15% at 17 years of age (eastern Nepal, rural and urban southern India, South Africa, and Chile) and three sites with a much higher myopia prevalence (urban Malaysia and urban and semirural China) (reviewed in Morgan et al.2). The prevalence of myopia is extremely high in East Asia.1 For example, 85% of Taiwanese 17-year-olds, at the end of schooling and about to commence university, require an optical correction for myopia.3 There is a related high prevalence of severe myopia (more than −6 diopters [D])3 and associated ocular pathologies, including chorio-retinal degeneration and retinal detachment.4 A vastly different scenario is evident in Australia,5,6 where only 12 to 14% of 12-year-olds have a myopic refractive error. The possible cause(s) of this large geographical variation in myopia prevalence is the focus of much current discussion.7
Although it is well known that there is a strong genetic predisposition underlying myopia susceptibility,6,8,9 new findings suggest that environmental factors, such as leading an outdoor lifestyle, may also have a strong impact on refractive error development.10–13 The Sydney Myopia Study10 found that the group of children who self-reported spending more than 2.8 hours per day performing outdoor activities had a more hyperopic mean spherical equivalent refraction (SER) than children who reported that they participated in lower amounts of outdoor activity. Furthermore, a comparative study found that Chinese children living in Australia had a lower myopia prevalence compared with children of the same age and ethnicity residing in Singapore.11 Although this difference could result from the earlier and more intense education requirements of children in Asia, the authors observed that Australian children spent more time reading and writing and less time watching television than their Singaporean counterparts. Another major difference was that the children living in Australia spent greater amounts of time outdoors.11
There are a number of possible mechanisms by which outdoor activity could protect against the development of myopia.10,11 One possibility is that distance viewing when outdoors encourages relaxation of the accommodation system that negates accommodative adaptations associated with performing prolonged near tasks.10 Another hypothesis is that pupil constriction caused by the high outdoor illumination results in a reduction in retinal image blur, and this mitigates the error signal driving the emmetropization process.14,15 Physical activity has also been suggested as a means for myopia prevention rather than simply being outdoors per se10; however, performing large amounts of sport indoors does not seem to impact on myopia risk.10,12 Animal models of myopia, however, point to the greater intensity of illumination that is experienced outdoors being the likely critical factor.14,15
Bright light inhibits the development of both deprivation14 and lens-induced myopia15 in the chick model. Bright light, such as natural daylight, is known to stimulate the release of retinal dopamine, which is an important neurotransmitter in the control of axial eye growth.16,17,18 Dopamine agonists inhibit the development of myopia in animal models,17 and dopamine antagonists block the ability of brief periods of normal vision to prevent form-deprivation myopia.16 Related to these findings, Fulk et al.19 reported that the rate of myopia progression in children wearing single-vision and bifocal spectacles was three times greater in winter than in summer. Higher myopia prevalence has also been found in Eskimos20 and Finnish army conscripts21 living near the Arctic Circle, where sunrise is nonexistent during the winter months.
The broad spectrum of wavelengths in sunlight, including the emission of rays in the UV range (10 to 400 nm), has also been suggested to be of significance in myopia prevention.22 Here, it is suggested that the important factors are that sunlight contains UV light whereas indoor lighting consists of a more limited spectrum of wavelengths. However, no conclusive evidence is currently available to confirm that UV light exposure is required to prevent myopia development.
The aim of this study was to measure the daily light levels and UV exposure that university students experience and determine whether this had an impact on their refractive status recorded retrospectively during the previous 2 to 3 years. We hypothesized that emmetropic students and those with stable myopia would spend more time outdoors and be exposed to higher ambient levels of illumination and UV than students with progressing myopia. We also sought to ascertain whether young adults in Brisbane were exposed to safe or unsafe levels of UV.
Participants were third- and fourth-year university students, aged 17 to 25 years, studying optometry at the Queensland University of Technology. This sample was recruited to ensure that participants had similar levels of education, were completing the same course, and individual variations in the amount of nearwork performed would be restricted. Participants were classified as being emmetropic (n = 13) or having stable (n = 12) or progressing myopia (n = 10). Myopia was defined as 0.50 D or more of myopia and emmetropia from −0.25 to +1.00 D. This classification was based on the SER determined from a noncycloplegic subjective refraction. Progression status was determined retrospectively by analysis of past optometry clinic record data; all participants had subjective refraction conducted in the first semester of university 2 to 3 years previously but usually not since. The emmetropic group members were emmetropic both at commencement of university and at the time of the study. The stable myopic group members were initially myopic, and myopia progressed by 0.25 D or less during the 2- to 3-year period. Progressing myopes were initially myopic and progressed by 0.50 D or more during the same period. All experiments were conducted with ethics approval in accordance with the Declaration of Helsinki and the requirements of the Queensland University of Technology Human Research Ethics Committee. Written consent was obtained from participants before commencing experimental work.
Subjective refraction was performed using the maximum plus for best visual acuity methodology and blur back techniques to minimize accommodative demand influencing the refractive error results.23 Students with hyperopia (+1.50 D or higher), anisometropia (≥1.50 D), astigmatism (≥1.50 D), amblyopia, or keratoconus were excluded. Visual acuity was measured using a Bailey-Lovie distance chart at 6 m; inclusion criteria included best corrected distance acuities of 6/6 or better in each eye. Cover tests at distance and near were performed to exclude participants with a strabismus. All participants reported good general and ocular health; ophthalmoscopy and slit lamp biomicroscopy were conducted to screen for ocular abnormalities. Individuals who had received past treatment for myopia progression in the form of therapeutic agents, bifocal or progressive addition lenses, orthokeratology, or Lasik eye surgery were excluded. Axial length and corneal power were measured using an optical biometer (IOL Master; Carl Zeis Meditec Inc, Jena, Germany).
Participants completed a 57-item questionnaire pertaining to age, ethnicity, nearwork, family ocular history, outdoor activities (sport and recreation), and country of birth and upbringing. Questions regarding sun protection and lifestyle during the past 3 years were also asked. The questionnaire was derived from surveys used in published research projects on factors influencing myopia in children.11,24
Light Intensity Measurements
HOBO light sensor data loggers (model UA-002-08; Onset Computer Corporation) were used to measure daily illuminance levels for each participant individually. All devices were programmed to record light intensity at 5-minute intervals. A clip was attached to the back surface of the HOBO to allow participants to secure the light logger to their clothing (either on a shirt pocket, collar, or midline) in a stable upright position. A chain was threaded through the 3-mm eyelet at the top of the device and was worn around the neck to ensure that the light logger was not lost if the clip became detached. The HOBO was worn for 3 days (Wednesday, Friday, and Saturday) and then returned to the experimenters. The device was then plugged into an optical USB base station, and data points were downloaded and transferred to the HOBO software program for analyses. The HOBO light loggers have been used previously to monitor seasonal light exposure in plants25 and circadian light rhythms in elderly patients.26
UV Radiation Measurements
Daily UV exposure was measured using polysulfone film (PSF) dosimeters; one dosimeter was used each day. The PSF dosimeters were manufactured at the Sun and Health Research Laboratory (Institute of Health and Biomedical Innovation, Queensland University of Technology) and consisted of a clear central film surrounded by a gray plastic support. A clip was attached to the back surface to allow attachment to clothing at a similar location to the HOBO light logger. When polysulfone is exposed to UV radiation, it undergoes photodegradation and a change in optical absorbency,27 which is a very similar process to the cellular damage and cutaneous reddening that occurs during sunburn and is therefore an effective means of measuring accumulative UV exposure.27 The optical absorbency of dosimeters at 330 nm was measured both before exposure and after exposure, and the spectrophotometer values were determined by calculating the difference between these two measures to derive the change in optical absorbency, which corresponded to the daily UV exposure.28 Measurements of UV using PSF dosimeters have been conducted previously in Queensland schoolchildren and in Danish adults.24,27
Recording Days and Diary
Two weekdays (Wednesday and Friday) and one weekend day (Saturday) were chosen as the designated experimental days to represent “typical” activity days of students with respect to their university schedule. In addition to wearing the HOBO and UV dosimeter, participants also completed a 24-hour light exposure diary for each of the recording days. The start and finishing times of indoor and outdoor activities and the type of sun protection used were documented. The daily activity log was based on that used to assess agreement between diary reports of time spent outdoors and UV dosimetry.24
Refractive error and ocular biometry measurements of the right eye of each of the participants were used for data analyses. Astigmatic corrections were converted to SER using the formula sphere +0.5× cylinder. The HOBO data collected for daily illuminance levels were then categorized into hours spent in sunlight (≥30,000 lux), outdoor shade (10,000 to 30,000 lux), bright indoor/dim outdoor light (500 to 10,000 lux), and dim room illumination (<500 lux). We selected these categories of illuminance based on our own measurements using a HOBO light logger under a series of different representative lighting conditions at the location and time of year that this study was conducted, including direct sunlight, office room illumination, a lecture theater, and within close proximity to a window. Nevertheless, we acknowledge that there will be a range of light levels that occur both indoors and outdoors, and thus the HOBO data can only provide an approximation of outdoor activity, regardless of the illuminance cutoff levels selected. We thus calculated hours of outdoor activity based on more than 10,000 lux (most definitively outdoors) and more than 500 lux (where some bright indoor activity may be included). The daily UV exposure was calculated from the change in optical absorbency of the UV dosimeters, as described previously.28 The units of erythemal exposure are in terms of minimal erythemal dose (MED), where 1 MED is taken as 20 mJ · cm−2.29
Statistical analyses were performed using the software program SPSS (Statistical Package for the Social Sciences). A one-way analysis of variance was performed to assess whether there was a significant (p < 0.05) refractive error group effect with respect to continuous variables. If the data were significant, then an LSD (least significant difference) post hoc comparison was performed. For noncontinuous variables (sex, ethnicity, history of parental myopia, and iris color), the nonparametric Kruskal-Wallis test was used. A simple two-tailed Pearson correlation was used to determine if UV dosimetry and HOBO illuminance levels were correlated and whether objective HOBO measures of time spent under different illuminations were correlated with self-reports in the daily activity log.
Refraction and Biometry Data
Emmetropes had a mean SER of +0.11 ± 0.39 D, and the stable and progressing myopes had a mean SER of -2.48 ± 1.74 D and −3.61 ± 1.47 D, respectively (myopes vs emmetropes significant, F34,2 = 25.001, p = 0.001) (Table 1). Progressing myopic students experienced an average change in refraction of −1.01 ± 0.40 D during the previous 2 to 3 years. Axial length was negatively correlated with refractive error; the greater the myopia, the greater the axial length (R = 0.81, p = 0.001). Axial length was significantly longer in the myopic groups (p = 0.001). There was no correlation between corneal power and refractive error (R = 0.20, p = 0.251) and no difference among the three groups.
There were more female (77.1%) than male participants. Students with Asian ethnicity accounted for almost one-half of the participants (48.6%), the remaining students had a European/white (40%) or Indian (11.4%) background. Age (F2,34 = 0.249, p = 0.781), sex (p = 0.124), ethnicity (p = 0.799), history of parental myopia (p = 0.187), and iris color (p = 0.58) were not significantly different between the groups. There was a clear trend for students with two myopic parents to have a more myopic SER; 80% of progressing myopic students had at least one myopic parent. Seventy-five percent of participants had brown irides.
HOBO Data, Daily Activity Log, and UV Dosimetry
There were no significant differences in the daily illuminance experienced by the three refractive groups; for example, emmetropes, 252 ± 192 × 103 lux, versus stable myopes, 221 ± 127 × 103 lux, versus progressing myopes, 202 ± 117 × 103 lux (F2,34 = 0.316, p = 0.73) (Table 2). There was also no significant correlation between average daily illuminance and refractive error (R = 0.153, p = 0.438). The three groups spent a similar amount of time each day in sunlight and shade conditions outdoors (sun + shade = total outdoors), for example, the emmetropes spent 0.38 ± 0.23 hours per day, the stable myopes spent 0.34 ± 0.20 hours per day, and the progressing myopes spent 0.27 ± 0.19 hours per day (F2,34 = 0.714, p = 0.50). Times spent indoors under either bright (F2,34 = 0.565, p = 0.574) or dim (F2,34 = 0.742, p = 0.484) conditions were also not significantly different between groups. The number of daily alternations from indoors to outdoors per day, a measure of the frequency of large changes in light levels, was not significantly different between groups: emmetropes, 5.2 ± 2.6 per day; stable myopes, 4.5 ± 2.6; progressing myopes, 3.6 ± 1.2 (F2,34 = 1.340, p = 0.276).
As for the HOBO data, there was no significant difference between groups based on self-reports of daily activities recorded in the participant log (Table 2). The emmetropic participants reported spending 2.67 ± 1.06 hours per day, the stable myopic students reported spending 1.85 ± 1.04 hours per day, and progressing myopic students reported spending 2.51 ± 1.49 hours per day outdoors; these durations were not different (F2,34 = 1.614, p = 0.215). Similarly, time spent indoors, time spent on nearwork, and time spent sleeping each day were the same across the three groups. Self-reported measures of time spent outdoors were much greater than the time spent outdoors (based on >10,000 lux) calculated from the HOBO data (Fig. 1) (p = 0.001); the two values were not significantly correlated (R = 0.27, p = 0.104). This correlation was, however, improved for total bright light (based on >500 lux) HOBO data against reports of time spent outdoors (R = 0.31, p = 0.067) but still was not significant. In contrast, the times spent under low illumination calculated from the HOBO data were correlated to the reported hours spent indoors based on the activity log (R = 0.521, p = 0.001).
The UV dosimetry data showed significant differences across the three refractive groups (Table 2; Fig. 2). On average, the stable myopes had the greatest UV exposure (0.32 ± 0.12 MED) followed by the progressing myopes (0.17 ± 0.11 MED) and the emmetropes (0.17 ± 0.09 MED) (F2,34 = 7.041, p = 0.003). Subsequent post hoc testing showed that the significant differences were between the emmetropes and stable myopes (p = 0.002) as well as the stable myopes and the progressing myopes (p = 0.004); in both cases, the average MEDs of these groups differed by 90%. The daily MEDs ranged from 0.07 to 0.52 across all participants and all test days; 1 MED is considered the maximum daily safe level. The average daily UV exposure of participants and the daily hours spent in sunlight based on the HOBO illuminance measures were significantly correlated (R = 0.384, p = 0.023). The relatively low R value of the correlation indicated that UV dose varies with other factors and not just the total illumination exposed to. Chodick et al.24 report a similar correlation coefficient (R = 0.33) between total daily time outdoors and UV dose.
Relationships and Interactions
Illuminance and UV measurements were significantly correlated in the positive direction (Fig. 3). This finding was consistent across all testing days and was used as a means for checking data reliability as greater UV exposure should equate to more time spent outdoors under higher light levels (Wednesday, R = 0.404, p = 0.016; Friday, R = 0.572, p < 0.001; Saturday, R = 0.381, p = 0.024). Although we hypothesized that there may be differences across the three measurement days, as 1 day was a weekend and participants went to university the other two days, no difference was observed. In addition, there were two participants for whom the daily illuminance values measured with the HOBO were high and the UV exposure was very low. We can only assume that, on these days, the participants concerned spent a large portion of time under bright indoor illuminations and little time outdoors; this would give high illuminance and low UV exposure data.
This study investigated the relationship between refractive errors, the duration of time spent outdoors, and the light levels and UV radiation that young-adult university students were exposed to. No significant differences were observed between refractive error groups with respect to average daily illuminance or time (hours per day) spent under the different lighting conditions (sunlight, outdoor shade, indoor bright light, and indoor dim light). However, there were significant between group differences for the daily UV dose, with stable myopes having the highest daily UV exposure. Although much of the data were not significant, this may have been because of the relatively small sample size and lack of power of the study. We have performed an a priori power analysis, which suggests that, for any given predictor (e.g., daily UV exposure) if a cutoff score could be identified, which was associated with a 6 to 7% increased risk of progressing myopia, a sample of 963 participants would be sufficient to capture this effect with a power of 0.95. Thus, a sample of 1000 participants allows for up to 5% dropout and would provide sufficient power to detect a clinically significant effect.
A possible reason daily UV exposure, but not daily illumination differences across the groups, reached significance is that the UV measure is cumulative across the day, whereas illumination measures were taken at 5-minute intervals throughout the day; the HOBO device records light intensity as a discrete/stepped amount rather than a continuous variable.30 The UV measure represents the more accurate measure of sunlight exposure as the HOBO logger measures across a wide spectrum of wavelengths, including indoor lighting; although it is stated in the instrument catalog that the HOBO devices are designed to sense outdoor rather than indoor illumination.30 The HOBO devices cannot be calibrated, which means that each individual HOBO measures a slightly different illuminance for the same incident light intensity,30 and this would add to the variability of the data. Importantly, we cannot rule out the possibility that UV exposure data are simply a surrogate measure for outdoor activity and exposure to bright light levels.
Based on our measures with the HOBO device, which were undertaken at representative locations and the same time of year that this study was undertaken, we used a cutoff of more than 10,000 lux to represent outdoor illuminations. Scheuermaier et al.26 used a cutoff of 1000 lux as did Dharani et al.31 in their studies involving children. However, as reported by Dharani et al.,31 there is an overlap between the light levels measured outdoors on a dark cloudy day (∼3500 lux) and those measured near a window indoors on a sunny day (up to ∼4000 lux). Thus, the criteria that we used, which were based on a level more than 10,000 lux, could have underestimated outdoor activity, whereas that adopted by Scheuermaier et al.26 and Dharani et al.31 and our own bright light condition (>500 lux) may overestimate it; this would also contribute to the lack of concordance between the objective and self-reported measures of outdoor activity found in these studies. When we added bright light measures (>500 lux) to calculate the hours of outdoor activity performed, the correlation between HOBO data and self-reported measures of outdoor activity was improved but was still not significant. If the lower illumination cutoff was used, the young-adult students performed much more weekly activity under bright light than children in a study undertaken in Singapore31 (17.57 ± 9.78 hours versus 7.08 hours); however, if the brighter illumination cutoff was used, the young adults performed much less outdoor activity (2.35 ± 1.47 hours).
Our findings, to some extent, relate to those of other studies that have observed a link between increased light intensity and myopia protection. For example, Cohen et al.32 found that after 90 days, chicks exposed to a constant illuminance of 10,000 lux developed a mean hyperopic SER of +1.10 D, whereas chicks exposed to lower intensities of 500 lux and 50 lux developed a mean myopic SER of −1.20 and −2.00 D, respectively.32 Similarly, Ashby et al.14 report that intense lighting of 15,000 lux inhibits the development of form-deprivation myopia in chicks. The progressing myopes in our study experienced very high average daily illuminance values, which, based on the animal model data, should be sufficient to prevent myopia if light levels were the key factor controlling eye growth. Although data from chick models provide valuable insights in eye growth control processes, the findings may not be necessarily translatable to human myopia in this instance.
There were large differences between objective measurements of light intensity based on the HOBO data and UV dosimetry and self-reported estimates of outdoor exposure based on the daily activity log. A likely explanation is that outdoor time is not always spent in the sun but is also spent in shade or in the car/bus under a lower illuminance. Another possibility is that participants overestimated their outdoor activity when completing the activity log. Indeed, although it is usually reported that diary data of this kind are reliable,24 this does not necessarily mean that they are valid. Chodick et al.24 report a significant correlation (0.57, p < 0.001) between UV dosimetry and recorded time spent outdoors, but this was for a greater number of participants (n = 124) and more measurements (7 days). They also report that the correlation was greatest when outdoor activities were performed in the middle of the day and least when outdoor activities were performed in the early morning or late afternoon. Our findings highlight the importance of including objective measures in studies of activity and lighting and suggest the possibility that measures derived from self-reported activities may not accurately represent actual light exposures.
A significant difference was observed between groups with respect to daily UV exposure; stable myopes had the highest MED levels, and the progressing myopes and the emmetropes had similar exposure levels. Given the limitation that myopia progression was determined retrospectively, we suggest that the finding that stable myopes have higher UV exposures and progressing myopes and emmetropes less may indicate that sunlight plays a fundamental role in preventing myopia progression in individuals who have a strong myopic tendency (through genetics and ethnicity) but has little effect on the refraction of those who are destined to be emmetropic. Data in children support this hypothesis; the effect of outdoor activity on myopia is greatest in children with two myopic parents and high genetic myopia risk.12 In addition, children who combined low levels of nearwork with high levels of outdoor activity had the most hyperopic refractions, and those who combined high levels of nearwork with low levels of outdoor activity had relatively more myopic refraction.10 As participants were selected based on their participation in the same university course, all were likely to have had similar high levels of near activity, and thus, our data cannot inform the debate on the possible interaction between nearwork, outdoor activity, and myopia. It remains to be seen whether these measured differences between refractive groups are clinically relevant, but they are consistent with data from animal models.15,33,34 Although it is possible that time spent outdoors during childhood influences the refractive status of young adults and that this effect has the potential to override any observed differences because of current light exposure behaviors, the fact that many young adults have progressive myopia indicates that current behaviors are also important.
Although current literature indicates that there may be a role for UV in the prevention of myopia, this is yet to be confirmed. In a study by Ashby et al.,14 form-deprived chicks given 15 minutes of normal vision under bright natural daylight (30,000 lux) had shorter axial lengths and less form-deprivation myopia compared with those of chicks exposed to intense (15,000 lux) and normal (500 lux) laboratory light during the period of diffuser removal. Second, chicks deprived indoors under low light levels developed greater myopia than those deprived under much bright illumination. The authors suggested that high illuminance was the important determinant of their findings as the halogen lamps of laboratory lighting had a spectral distribution of 400 to 1000 nm (visible and infrared) and no output in the UV part of the spectrum (10 to 400 nm) and yet inhibited myopia. More recently, Smith et al.33 report that high ambient indoor lighting (∼25,000 lux) also retards the development of form-deprivation myopia in monkeys.
Human data demonstrate that there is a lower prevalence of pterygium and pinguecula, which are generally a result of chronic UV exposure, in myopic subjects.34 The authors suggest that these data support, but do not prove, the hypothesis that childhood sun exposure is associated with a decreased risk of myopia. This finding is, however, confounded by the UV-blocking effects of spectacle lens wear, which might potentially alter the association between time spent outdoors and the presence of UV damage. However, Sherwin et al.35 reported no effect of sunglass use on signs of ocular surface UV damage (as measured using conjunctival UV autofluorescence) and an inverse relationship between the level of UV damage and the presence of myopia. Sun exposure is required to generate vitamin D. A multivariate linear model, adjusted for age and dietary nutrients, revealed higher blood levels of vitamin D (16.9 ng/mL) in nonmyopic subjects compared with those in myopic subjects (13.5 ng/mL), although the amount of time spent outdoors was similar for both groups.36
We found higher UV exposure in stable myopes versus progressing myopes; however, this does not prove whether it is the UV light or the high light levels of sunlight more generally that are important. A further aim of this experiment was to determine whether the levels of UV exposure in young adults with different refractive errors fell within the range of safe exposure levels. The average daily UV dose for stable myopes was 0.32 ± 0.12 MED and ranged from 0.07 to 0.52 MED across all subjects and experimental days. This is well below 1 MED, which is the maximum safe level of daily occupational UV exposure set by the National Medical Health Research Council of Australia.37 It may be that the high illuminations required to inhibit myopia can be produced by increasing indoor lighting rather than increasing outdoor activity; however, whether this is economically feasible is an issue that would need to be determined in future studies.
The effect of outdoor illumination and UV exposure on progressing myopia was investigated. Daily UV exposure was greatest in stable myopes and lower in emmetropic and progressing myopic students. Therefore, this experiment provides some preliminary evidence to support the hypothesis that sunlight and/or UV may offer some protection against myopia. Further larger scale prospective studies are required to fully explore the relationship between light levels and impact on myopia progression.
School of Optometry and Vision Science
Faculty of Health and Institute of Health & Biomedical Innovation
Queensland University of Technology
60 Musk Ave
Kelvin Grove Queensland 4059
The UV dosimeters were provided by AusSun and the HOBO loggers by grant funding to Katrina L. Schmid.
No conflicting relationship exists.
Received December 14, 2011; accepted October 9, 2012.
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