In this large, countrywide study of refractive errors in primary school students, 31.2% of students who had vision loss identified from school vision screenings received a confirmatory diagnosis of refractive error (Fig. 1), and prescription eyeglasses corrected vision loss in 85.6% of the affected population (Table 3). The prevalence of severe vision impairment and blindness among the student population before ametropic correction with eyeglasses was 2.4% and after eyeglasses prescription was 0.3% (Table 3). Only 16.3% of students with refractive errors were already wearing eyeglasses at school at the time of this study, indicating that URE is a major issue in Mexican and schools, with a troubling gap in eyeglass usage among children, and suggesting that many public primary school students may benefit from vision screening and eyeglass donation.
More than half of the refractive errors in our study was caused by astigmatism (Table 1). Simple myopic astigmatism was the most frequent refractive error found in 25.7% of our study population and was more common in southern Mexico (Tables 1 and 2), whereas myopia was only found in 11.9% of the study population, which was nearly equivalent to the 11.7% rate reported in the 2017 global systematic review and meta-analysis [2], but a much lower rate than those reported in other Mexican studies. A 2005 study of 1,136 students aged 6 to 15 years from the central state of Mexico reported a myopia prevalence rate of 33% [23, 24]. In the southern state of Oaxaca in 2006, among 493 students aged 5 to 18 years old, a staggering 74.5% had myopia [17]. Much lower prevalence rates were reported from the most recent Mexican studies. Among 2,647 students aged 5 to 14 years in Quintana Roo in 2014, only 4.6 had myopia [25]. Among 722 pediatric patients of the public health system of Aguascalientes in 2018, 7.0% had myopia [26]. The great differences in prevalence rates of myopia could have resulted from different definitions of myopia used in the Mexican studies [27]. Multiple studies have demonstrated that a slight change in the threshold definition by 0.25 D can significantly affect the prevalence rates [27–30]. Furthermore, unlike in the other smaller Mexican studies, in our study, vision screenings were held at nearly 20% of all public primary schools in Mexico, which were attended by 90% of primary school-aged children during the 2013–2014 academic year [31]. Thus, our findings were more generalizable and highly representative of the target population of Mexico.
The overall anisometropia ≥ 1.00 D rate was 7.9% in our study; 3.9% of students had > 1.5 D (Table 4). A more recent Portuguese study of 749 students aged 3 to 16 years during the 2018–2019 academic year reported a similar prevalence rate of 6.1% [32]. Anisometropia at a child's first clinical examination has been associated with a high risk of amblyopia [33], and so, these combined findings highlight the importance of preschool vision screening programs to identify and treat early cases of amblyopia.
Globally, we should be concerned over the increasing rates of refractive errors among children, largely fueled by the SARS-CoV-2 (COVID-19) pandemic lockdown measures. In 2020, school closures and remote learning affected an estimated 1.37 billion students [34]. Multiple systematic reviews and meta-analyses, albeit of predominantly Asian studies, have reported a rapid increase and/or progression in myopia among school-aged children since the pandemic onset, largely due to the increased use of digital devices during remote learning and decreased outdoor activity [35–38]. Studies have similarly reported a 1.5-fold increase in astigmatism among children following school closures [39, 40]. Unlike in our prepandemic study, COVID-19 lockdown studies have reported gender inequities in refractive errors among students. Among 3,850 public school students in Southern India, there was a 3- to 6-fold increase in myopia after the lockdown, which was more predominant in girls than boys, likely due to traditional gender roles resulting in girls being even less likely than boys to spend time outdoors and more likely to be perform household chores and have increased screen time [41]. There are no pre- vs postpandemic refractive error data for students in Mexico, and thus, an immediate follow-up study is warranted to see how remote learning, increased use of digital devices, and decreased outdoor activities have affected the refractive error status of Mexican students.
The major limitation of this study is that the data are from the 2013 and 2014 and therefore may not reflect the current situation of refractive error in primary school students, especially after prolific reports from all over the world cite the increase in refractive error among children following the COVID-19 pandemic. We received delayed authorization to publish the study data, and then, the manuscript was further delayed by the COVID-19 pandemic. Nonetheless, it is important to publish these older data to provide a historic understanding of the state of refractive error in children before the pandemic.
Another major limitation of this study was that the gold standard cycloplegic refraction was not performed [27]. Thus, the prevalence of high hyperopia (only 4.5% in 17,865 students) may have been underestimated (Table 5), although it was very similar to the global prevalence rate of 4.6% reported in the prepandemic, global systematic review and meta-analysis [2]. Similarly, the prevalence of myopia (although quite low at 11.9%) may have been overestimated in this study due to the use of noncycloplegic refraction (Table 1). Noncycloplegic refraction is known to have no significant effect on identifying astigmatism [39, 42, 43].
Other study limitations include the lack of a follow-up period; we do not know if students continued to wear their eyeglasses after the study ended, nor do we know the long-term impact of eyeglass provision on students’ activities of daily living, academic performance, or refractive status. While data on effective refractive error coverage are now being collected and reported for adults aged 50 years and older, with global coverage reported to be 20.5% (95% CI: 17.8–24.4) and Latin American coverage reported to be 34.5% (95% CI: 29.4–40.0) [44], an effective coverage indicator is not reported for children [45]. However our finding that only approximately 16.3% of children wore eyeglasses at the time of examination aligns with the 2006 Oaxaca study, which reported 13% of students with refractive errors wearing eyeglasses as the time of examination [17], as well as a more recent Latin American study from Chile, where 14% (144/1,017) of the students with refractive error in at least 1 eye wore eyeglasses at the time of examination [46]. More research on eyeglass usage and compliance among children is urgently needed from the region.
The causes of vision loss that were not corrected with eyeglasses in 10.4% of participants in this study were not identified; thus, we could not confirm or manage (for example) definitive amblyopia diagnoses.
This study provides historic data confirming that the prevalence of refractive errors and their related vision loss has been high among primary school students in Mexico. The provision of free eyeglasses to affected students improved vision in most children, highlighting the importance of free school vision screenings and eyeglass provision to manage URE. Given the toll of the COVID-19 pandemic, including school closures, remote learning measures, and a general widespread increase in dependence and usage of digital devices among Mexican students, a 10-year follow-up study is urgently needed to assess the evolving trends and current burden of refractive errors among primary school students in Mexico. Future investigations should also analyze eyeglass usage and compliance, as well as changes in academic performance among students with refractive errors in Mexico to understand the long-term benefit of school vision programs.