Table of Contents Author Guidelines Submit a Manuscript
Journal of Ophthalmology
Volume 2019, Article ID 3419576, 7 pages
https://doi.org/10.1155/2019/3419576
Research Article

Prevalence and Risk Factors of Myopia in Spain

Faculty of Biomedical and Health Sciences, Universidad Europea de Madrid, Madrid 20822, Spain

Correspondence should be addressed to Miguel Angel M. A. Sanchez-Tena; se.aeporuedadisrevinu@zehcnas.legnaleugim

Received 15 February 2019; Revised 28 May 2019; Accepted 1 July 2019; Published 18 August 2019

Guest Editor: Malgorzata Mrugacz

Copyright © 2019 Cristina C. Alvarez-Peregrina et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Objective. To analyse the prevalence of myopia among a sample of more than 6000 children in Spain as well as to determine the impact of risk factors in its progression. Methodology. A total of 6,152 children aged from 5 to 7 were examined. The participants underwent an eye examination that included visual acuity, refraction without cycloplegia, and tests of accommodative and binocular function. In addition, a questionnaire regarding their lifestyle, family history, and geographical data was carried out. Finally, data were analysed using the SPSS version 25 program. Results. The prevalence of myopia in the sample of children studied has increased from 17% in 2016 to 20% in 2017. Likewise, the number of children with high myopia has also increased, from 1.7% in 2016 to 3.6% in 2017. 43.3% of the participants spent more than 3 hours a day doing near activities, and 48.9% of this group spent more than 50% of this time using electronic devices. In addition, only 9.7% spent more than 2.5 hours outdoors each day. Conclusion. Myopia prevalence appears to be increasing in Spain. Lifestyle factors appear to be increasing the risk of myopia.

1. Introduction

Uncorrected refractive errors are one of the main public health problems throughout the world, regardless of age, sex, and race [1]. As a result, it is expected that by 2060, there will have been a 26% increase in the number of children with visual disability, which will have a negative effect on their educational and psychosocial development [2, 3].

In recent years, there has been a significant increase in the number of cases of myopia globally, and it has become an epidemiological problem [4]. Between 1993 and 2016, the prevalence rate increased from 10.4% to 34.2%, respectively [5]. Short-term estimates indicate that in 2050, 49.8% of all people will be myopic [6].

The prevalence of myopia varies on a geographical basis; it is more prevalent in Asia (70–90%) [7], whereas the figures appear to be lower than in Europe, Australia, and USA [8]. Regarding this, recent studies have determined a higher myopia rate among the children examined in Singapore (62%) and China (49.7%) in comparison with those examined in the USA (20%) and Australia (11.9%) [6, 9]. Additionally, high myopia could be associated with multiple pathologies including retinal detachment, macular degeneration, cataracts, or glaucoma [10]. However, there are no current data about the myopia prevalence in Spain since 2000, when myopia incidence in children from 3 to 8 years old was 2.5% [11].

Nowadays, there is enough evidence on the influence of near activities (reading, writing, watching TV, etc.), in the development of myopia. The hypermetropic peripheral blur in the retina leads to an increase in the axial length of the eye, therefore accelerating its progression [12].

Genetics also plays an important role, so the risk of suffering myopia increases depending on the number of parents with myopia [13].

Recent studies suggest that time outdoors has a protective effect on the appearance of myopia, but it does not stop its progression [14].

An important point when we look at prevalence figures is to know the procedure to measure myopia. The recent report published by the IMI group of experts—Defining and Classifying Myopia Report—defines myopia by refraction “when ocular accommodation is relaxed. These definitions avoid the requirement for objective refraction so as to be independent of technique, but by making reference to relaxation of accommodation are compatible with both cycloplegic and standard clinical subjective techniques” [15]. Although cycloplegic refraction is the gold standard, limitations in the use of some drugs in some countries make important having other alternatives to measure myopia, like objective refraction by noncycloplegic retinoscopy.

If we assess the economic impact which is associated with myopia, a study carried out in 2013 estimated a total cost in the whole population of Singapore of 755 million US dollars per year [16].

Therefore, due to the lack of studies of myopia prevalence in Spain and the need to know which associate factors can help to prevent this epidemiologic problem, the authors carried out this study. We analysed myopia prevalence among children from 5 to 7 years old and the influence of lifestyle and genetics in the figures.

2. Methods

2.1. Data Collection and Inclusion Criteria

A cross-sectional study to estimate myopia prevalence in a sample of children in Spain has been carried out.

Data were collected by convenience sampling from the 2016 and 2017 “School campaign in favor of children's visual health” that is taken every year in Spain. The school campaign is targeted to all schools, so all participants between 5 and 7 years of age that participated were included in the study. The school campaign supplies a free spectacle to those who need them, funded by the Fundación Alain Afflelou.

2.2. Examination

Parents of all of the children that participate in this research signed the informed consent form and underwent an optometric test, which consisted of a questionnaire and an assessment of the refractive and binocular conditions:(i)Questionnaire: it was divided into several sections and included questions about their demographic data (city of residence, age, sex, and nationality), their lifestyle and family ocular history (extracurricular activities and number of hours/weeks spent doing these activities, time spent using electronic devices, and genetics), and anamnesis (symptoms, main complaint, diagnosis or previous ocular treatment, medication and systemic diseases, and date of last checkup).(ii)Optometric test: the standard procedure was as follows:(1)Best-corrected and uncorrected visual acuity.(2)Objective refraction: non cycloplegic retinoscopy. The authors have estimated differences of ±0.5D in the SE when comparing noncycloplegic retinoscopy versus cycloplegic refraction [17].(3)Subjective refraction.(4)Binocular vision and accommodative tests: cover-uncover, alternating cover test, ocular motility, Hirschberg test, Worth test, near point of convergence, accommodation range, stereopsis, and colour vision.(5)Finally, the anterior segment was checked (eyelid, eyelashes, palpebral margin, corneal, conjunctive, and crystalline) using a slit lamp.

2.3. Variable Description

In order to determine the refractive status of the children, and in accordance with other research, the criteria for the spherical equivalent (SE) were as follows: hyperopia (S.E. > +0.50), myopia (S.E. < −0.50), or emmetropia (−0.50 < S.E. > +0.50) [2, 15]. SE was defined as sphere + cylinder/2.

Within the myopic group, a subdivision of myopia was carried out, based on the American Academy of Optometry’s classifications [18] as low (−0.50 < S.E. > −3), medium (−3 < S.E. > −6), and high (S.E. > −6).

To calculate the number of hours that children spend in near activities, using electronic devices and outdoors, and to get the genetic risks, several variables were taken based on the Clinical Myopia Profile [19]. Therefore, according to this study, we estimated the risk of suffering myopia in high, medium, or low, taking into consideration the criteria shown in Table 1.

Table 1: Factors that affect the risk of suffering from myopia.
2.4. Statistical Analysis

The data analysis was carried out using the SPSS 25.0 program (SPSS Inc., Chicago, Illinois). To establish the parametric distribution of the variables, the Kolmogorov–Smirnov test was used, resulting in a nonparametric distribution. Therefore, the variables were analysed using the Kruskal–Wallis test. The prevalence was calculated with 95% confidence interval. To assess the statistical significance, we considered a cutoff point .

3. Results

The checkouts were carried out in September 2016 and September 2017. A total of 6152 children were examined (4159 in 2016 and 1993 in 2017). A total of 711 children were excluded: 210 participants did not fulfill the inclusion criteria (younger than 5 or older than 7 years old) and 501 forms were incomplete as the optometrist didn’t follow method properly. The average age was 6.17 ± 0.77 years (2016: 6.16 ± 0.77 years old; 2017: 6.19 ± 0.78 years old). In terms of gender, 55% were male and 45% were female (2016: 56.3% male; 43.7% female; 2017: 52.5% male; 47.5% female). Table 2 shows the percentage of participants from the different autonomous communities across Spain by age and sex.

Table 2: Participants from the different autonomous community by age and gender.

Figures of myopia prevalence in children aged between 5 and 7 years increased from 16.8% in 2016 to 19.1% in 2017 (OR: 1.19; IC: 1.16–1.22; ). Likewise, the percentage of cases of myopia in female increased by 1.6% (16.5% in 2016, ; 18.1% in 2017, ; average: 17.25 ± 1.2%) and 3% in male (17% in 2016, ; 20% in 2017, ; average = 18.55 ± 2.05%). Therefore, no statistically significant differences were found between the risk of suffering from myopia and gender (). With regards to age, Figure 1 shows how the prevalence of myopia increases progressively with age ().

Figure 1: Myopia prevalence according to age.

Table 3 shows myopia prevalence by gender and place in 2016 and 2017.

Table 3: Myopia prevalence by gender and place in 2016 and 2017.

Out of all of the participants with myopia, in 2016, 90.1% had low myopia, 8.2% had medium myopia, and 1.7% had high myopia. On the other hand, in 2017, the percentage of children with low myopia was 89.1%, with a slight increase in the moderate myopia rates (9%) and the high myopia rates (1.9%). Likewise, there was an increase in the number of individuals who used glasses, from 70.6% in 2016 to 81.5% in 2017. In relation to this and with regards to the prevalence of myopia in the different autonomous communities, statistically significant differences have been found ().

The spheric myopic equivalence values according to age, sex, and autonomous community in 2016 and 2017 can be observed in Table 4.

Table 4: Myopic spherical equivalence according to age, sex, and autonomous community.
3.1. Risk Factors

To assess the number of hours in which participants perform near activities, three groups were established: low (between 0 and 2 hours), moderate (between 2 and 3 hours), and high (more than 3 hours). To determine the time spent using electronic devices, three subgroups were established, according to whether they spend <25%, between 25% and 50%, or more than 50% of the time in near activities.

In both 2016 and 2017, 45.5% and 39.7% of the children, respectively, spent a lot of time carrying out near activities. However, 36.1% (35.9% in 2016 and 36.3% in 2017) spent few hours and 21.2% (19.3% in 2016 and 24.1% in 2017) spent a moderate amount of time.

With regards to the use of electronic devices, 48.3% of the children (57.9% in 2016 and 33.1% in 2017) used them >50% of the time in near activities. Only 26.2% (21.9% in 2016 and 32.9% in 2017) used them <25% of the time and 25.6% (20.2% in 2016 and 34% in 2017) between 25% and 50%.

Figure 2 shows that the more time spent performing near activities and using a phone, tablet, or videogames, the higher the prevalence of myopia ().

Figure 2: Prevalence of myopia according to (a) the use of electronic devices and (b) the time spent performing activities in near vision.

On the other hand, a moderate correlation was found between the spherical equivalent value with regards to the time spent in near activities and using electronic devices ().

With regards to the predisposition, as shown in Figure 3, a significant association has been found between the presence of myopia in one or both parents and the refractive condition of the children (). Therefore, the risk of having myopia increases from 9.7% if neither parent is myopic to 28.3%, if both are, respectively.

Figure 3: Percentage of refractive condition of children in relation with their family history.
3.2. Prevention Factors

Each child was allocated to a group depending on the hours he spent outdoors each day: low (between 0 and 1.6 hours), moderate (between 1.6 and 2.7 hours), and high (>2.7 hours). 80.7% of the participants spent short time outdoors, while only a 9.9% of the children spent a moderate amount of time, and 9.4% of children spent long time outdoors, respectively.

However, in this study, we did not obtain statistically significant differences between the prevalence of myopia and the time they spend outdoors ().

4. Discussion

According to the WHO, myopia is considered as one of the main public health problems worldwide [20]. Our study included a group of children between 5 and 7 years of age, of which 18% were myopic in 2016 and 2017. Therefore, it has been concluded that figures of myopia prevalence in our sample of children in Spain are similar to that of Australia (14.02%) [21], Central Asia (17%), Andean Latin America (20.5%), and Tropical Latin America (14.5%) [6]. Contrasting, figures of prevalence are higher in Pakistan (36.5%) [22] and in Saudi Arabia (53.71%) [23].

Regarding gender, we did not find any significant differences in the prevalence of myopia. These results agree with those obtained by Uchenna et al. [24] and COMET [25], showing that there is no connection between sex and myopia and that figures can vary along time. However, there are studies, like the ones carried out in China [26, 27] and Saudi Arabia [28], that show higher figures of myopia prevalence in female than in male.

According to other studies, the prevalence of myopia increases with age. Thus, in 2016, Ma et al. [29] indicated an increase of 50.4% in children from 3 to 10 years old. When comparing the SE value of our research with the one carried out by Pi et al. [30] in 2010, a tendency of myopisation is observed, going from +1.25D in 2010 versus +0.78D, found in our study, in 2017. Likewise, similar studies show an increase in S.E. value of −0.27D per year, in 50% of the children [31].

With regards to lifestyle, the latest reviews indicate that children spend on average 4.8 ± 1.6 hours each day doing near activities. Likewise, it was shown that male spend more time doing near activities than female (4.9 ± 1.7 vs 4.6 ± 1.5) [32]. In 2006, Khader et al., proved that children with myopia spend around 0.95 hours/day in front of a computer, as opposed to the 0.69 hours/day spent by nonmyopic children [33]. These results agree with the ones obtained in our study in Spain. On the other hand, Lu et al. [34], Rose et al. [35], and Lin et al. [36] have pointed out that near activities are not a risk factor in the development of myopia.

With regards to the time spent outdoors, we found that most children spend between 0 and 1.6 hours outdoors. Similar results were obtained in Sydney in 2008, where children spend around 2.3 hours/day outdoors [37]. This difference could be due to the greater use of electronic devices nowadays and the geographical location.

There are a lot of studies that look for relations between spending outdoors time and myopia. Jin et al. [38] found the less figures of myopia, by means of pupil constriction and the release of dopamine, the greater the exposure to sunlight. However, we did not find a connection between the time spent outdoors and prevalence of myopia. This leads us to believe that in Spain, no association has been found due to the lack of children in our sample who spend more than 2.5 hours per day exposed to sunlight; therefore, it would be interesting to confirm these results through future research.

With regards to the limitations of our study, it is important to highlight the low number of participants aged 5 years (23%), in comparison with 37% of 6-year-old children and 40% of 7-year-old children, respectively. It is also important to say that centres from Balearic Islands, Melilla, and La Rioja did not participate in the 2017 collection, so comparison between 2016 and 2017 has not been included for these autonomous communities in Tables 3 and 4. In addition, only noncycloplegic refraction has been taken in this study, so it must be taken into consideration when compared to other studies. Similar studies have found that the difference between noncycloplegic and cycloplegic refraction is 0.95D in young children [39]. Finally, it should also be noted that the campaign offered a free spectacle to children that needed, so it could suppose a bias in the study.

5. Conclusion

Myopia prevalence appears to be increasing in Spain. Lifestyle factors appear to be increasing the risk of myopia.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

Publication of this article was funded by the Fundación Alain Afflelou (Spain) through a research agreement.

References

  1. D. Pascolini and S. P. Mariotti, “Global estimates of visual impairment: 2010,” British Journal of Ophthalmology, vol. 96, no. 5, pp. 614–618, 2012. View at Publisher · View at Google Scholar · View at Scopus
  2. R. Varma, K. Tarczy-Hornoch, and X. Jiang, “Visual impairment in preschool children in the United States,” JAMA Ophthalmology, vol. 135, no. 6, pp. 610–616, 2017. View at Publisher · View at Google Scholar · View at Scopus
  3. R. R. A. Bourne, B. P. Dineen, D. M. N. Huq, S. M. Ali, and G. J. Johnson, “Correction of refractive error in the adult population of Bangladesh: meeting the unmet need,” Investigative Opthalmology & Visual Science, vol. 45, no. 2, pp. 410–417, 2004. View at Publisher · View at Google Scholar · View at Scopus
  4. P. Mitchell, F. Hourihan, J. Sandbach, and J. J. Wang, “The relationship between glaucoma and myopia: the blue mountains eye study,” Ophthalmology, vol. 106, no. 10, pp. 2010–2015, 1999. View at Google Scholar
  5. H. Hashemi, A. Fotouhi, A. Yekta, R. Pakzad, H. Ostadimoghaddam, and M. Khabazkhoob, “Global and regional estimates of prevalence of refractive errors: systematic review and meta-analysis,” Journal of Current Ophthalmology, vol. 30, no. 1, pp. 3–22, 2018. View at Publisher · View at Google Scholar · View at Scopus
  6. B. A. Holden, T. R. Fricke, D. A. Wilson et al., “Global prevalence of myopia and high myopia and temporal trends from 2000 through 2050,” Ophthalmology, vol. 123, no. 5, pp. 1036–1042, 2016. View at Publisher · View at Google Scholar · View at Scopus
  7. D. R. Fredrick, “Myopia,” BMJ, vol. 324, no. 7347, pp. 1195–1199, 2002. View at Publisher · View at Google Scholar
  8. Group TEDPR, “The prevalence of refractive errors among adults in the United States, Western Europe, and Australia,” Archives of Ophthalmology, vol. 122, no. 4, pp. 495–505, 2004. View at Publisher · View at Google Scholar · View at Scopus
  9. P. J. Foster and Y. Jiang, “Epidemiology of myopia,” Eye, vol. 28, no. 2, pp. 202–208, 2014. View at Publisher · View at Google Scholar · View at Scopus
  10. S.-M. Saw, G. Gazzard, E. C. Shih-Yen, and W.-H. Chua, “Myopia and associated pathological complications,” Ophthalmic and Physiological Optics, vol. 25, no. 5, pp. 381–391, 2005. View at Publisher · View at Google Scholar · View at Scopus
  11. R. M. Mico and T. F. Blasco, “Distribution of refractive errors in Spain,” Documenta Ophtalmologica, vol. 101, no. 1, pp. 25–23, 2000. View at Google Scholar
  12. K. Rajendran, M. Haneef, K. Chandrabhanu, K. Moorthy, M. Muhammed, and R. T. Pillai, “A prevalence study on myopia among school going children in a rural area of south India,” Indian Journal of Clinical Practice, vol. 25, no. 4, pp. 374–380, 2014. View at Google Scholar
  13. M. J. Smith and J. J. Walline, “Controlling myopia progression in children and adolescents,” Adolescent Health, Medicine and Therapeutics, vol. 6, pp. 133–140, 2015. View at Publisher · View at Google Scholar
  14. A. N. French, R. S. Ashby, I. G. Morgan, and K. A. Rose, “Time outdoors and the prevention of myopia,” Experimental Eye Research, vol. 114, pp. 58–68, 2013. View at Publisher · View at Google Scholar · View at Scopus
  15. K. L. Gifford, K. Richdale, P. Kang et al., “IMI–clinical management guidelines report,” Investigative Ophthalmology & Visual Science, vol. 60, no. 3, pp. 184–203, 2019. View at Publisher · View at Google Scholar · View at Scopus
  16. S. M. Saw, Y. Zheng, J. Chay et al., “The economic cost of myopia in Singapore,” Investigative Ophthalmology & Visual Science, vol. 54, no. 13, p. 5711, 2013. View at Google Scholar
  17. P. Funarunart, S. Tengtrisorn, P. Sangsupawanich et al., “Accuracy of noncycloplegic refraction in primary school children in Southern Thailand,” Journal of the Medical Association of Thailand, vol. 92, no. 6, pp. 806–812, 2009. View at Google Scholar
  18. American Optometric Association, Care of the Patient with Myopia, American Optometric Association, St. Louis, MO, USA, 2018, http://www.myopiaprofile.com/.
  19. Gifford and Myopia Profile, Myopia Profile: An Information Resource for Optometrists, 2018, https://www.aoa.org/.
  20. World Health Organization, The Impact of Myopia and High Myopia, World Health Organization, Geneva, Switzerland, 2018, https://www.who.int/.
  21. B. M. Junghans and S. G. Crewther, “Little evidence for an epidemic of myopia in Australian primary school children over the last 30 years,” BMC Ophthalmology, vol. 5, no. 1, 2005. View at Publisher · View at Google Scholar · View at Scopus
  22. A. Das, H. Dutta, G. Bhaduri, A. De Sarkar, K. Sarkar, and M. Bannerjee, “A study on refractive errors among school children in Kolkata,” Journal of the Indian Medical Association, vol. 105, no. 4, pp. 169–172, 2017. View at Google Scholar
  23. M. A. AI Rowaily and B. M. Alanizi, “Prevalence of uncorrected refractive errors among adolescents at king Abdul-Aziz Medical City, Riyadh, Saudi Arabia,” Journal of Clinical & Experimental Ophthalmology, vol. 1, no. 3, 2010. View at Publisher · View at Google Scholar
  24. U. C. Otawa, A. J. Munsamy, and S. O. Wajuihian, “Prevalence and risk factors for myopia among school children in Aba, Nigeria,” African Vision and Eye Health, vol. 76, no. 1, 2017. View at Publisher · View at Google Scholar · View at Scopus
  25. COMET Group, “Myopia stabilization and associated factors among participants in the correction of myopia evaluation trial (COMET),” Investigative Ophthalmology & Visual Science, vol. 54, no. 13, pp. 7871–7884, 2013. View at Publisher · View at Google Scholar · View at Scopus
  26. C. Xu, C. Pan, M. Bi et al., “Prevalence and risk factors for myopia in older adult East Chinese population,” BMC Ophthalmology, vol. 17, no. 1, p. 191, 2017. View at Publisher · View at Google Scholar · View at Scopus
  27. M. Chen, A. Wu, L. Zhang et al., “The increasing prevalence of myopia and high myopia among high school students in Fenghua city, eastern China: a 15-year population-based survey,” BMC Ophtalmology, vol. 18, no. 1, p. 159, 2018. View at Publisher · View at Google Scholar · View at Scopus
  28. H. G. Ahmed, R. M. Algorinees, N. T. Alqatani et al., “Prevalence of myopia and its related risk factors among medical students in Saudi Arabia,” Advances in Ophthalmology & Visual System, vol. 6, no. 1, 2017. View at Publisher · View at Google Scholar
  29. Y. Ma, X. Qu, X. Zhu et al., “Age-specific prevalence of visual impairment and refractive error in children aged 3–10 years in Shanghai, China,” Investigative Opthalmology & Visual Science, vol. 57, no. 14, pp. 6188–6191, 2016. View at Publisher · View at Google Scholar · View at Scopus
  30. L. H. Pi, L. Chen, Q. Liu et al., “Refractive status and prevalence of refractive errors in suburban school-age children,” International Journal of Medical Sciences, vol. 7, no. 6, pp. 324–353, 2010. View at Publisher · View at Google Scholar · View at Scopus
  31. R. Saxena, P. Vashist, R. Tandon et al., “Incidence and progression of myopia and associated factors in urban school children in Delhi: the North India myopia study (NIM study),” PLoS One, vol. 12, no. 12, Article ID e0189774, 2017. View at Publisher · View at Google Scholar · View at Scopus
  32. L. Zhong, T. Y. Gao, B. Vasudevan et al., “Near work, outdoor activity, and myopia in children in rural China: the Handan offspring myopia study,” BMC Ophtalmology, vol. 17, no. 1, p. 203, 2017. View at Publisher · View at Google Scholar · View at Scopus
  33. Y. S. Khader, W. Q. Batayha, S. M. Abdul-Aziz, and M. I. Al-Shiekh-Khalil, “Prevalence and risk indicators of myopia among school children in Amman, Jordan,” Eastern Mediterranean Health Journal, vol. 12, no. 3-4, pp. 434–439, 2006. View at Google Scholar
  34. B. Lu, N. Congdon, X. Liu et al., “Associations between near work, outdoor activity, and myopia among adolescent students in rural China,” Archives of Ophthalmology, vol. 127, no. 6, pp. 769–775, 2009. View at Publisher · View at Google Scholar · View at Scopus
  35. K. A. Rose, I. G. Morgan, W. Smith, G. Burlutsky, P. Mitchell, and S. Saw, “Myopia, lifestyle, and schooling in students of Chinese ethnicity in Singapore and Sydney,” Archives of Ophthalmology, vol. 126, no. 4, pp. 527–530, 2008. View at Publisher · View at Google Scholar · View at Scopus
  36. Z. Lin, B. Vasudevan, V. Jhanji et al., “Near work, outdoor activity, and their association with refractive error,” Optometry and Vision Science, vol. 91, no. 4, pp. 376–382, 2014. View at Publisher · View at Google Scholar · View at Scopus
  37. K. A. Rose, I. G. Morgan, J. Ip et al., “Outdoor activity reduces the prevalence of myopia in children,” Ophthalmology, vol. 115, no. 8, pp. 1279–1285, 2008. View at Publisher · View at Google Scholar · View at Scopus
  38. J. X. Jin, W. J. Hua, X. Jiang et al., “Effect of outdoor activity on myopia onset and progression in school-aged children in northeast China: the Sujiatun eye care study,” BMC Ophtalmology, vol. 15, no. 1, p. 73, 2015. View at Publisher · View at Google Scholar · View at Scopus
  39. P. Sankaridurg, H. Xiangui, T. Naduvilath et al., “Comparison of noncycloplegic and cycloplegic autorefraction in categorizing refractive error data in children,” Acta Ophtalmologica, vol. 95, no. 7, pp. e633–e640, 2017. View at Publisher · View at Google Scholar · View at Scopus