Among the main conclusions from the statistical analyses is the finding that as much as 28.39%, 95% CI: (22.82%, 34.67%) of all myopic children did not have previous examinations, which demonstrates a considerable lack in the health care coverage. Even more alarming is the discovery that as much as 56.78%, 95% CI: (50.19%, 63.15%) of all myopic children don’t have glasses although they need them. Only 65 myopic children (out of 236) wear glasses all the time (point estimate: 27.54%; 95% confidence interval: (22.04%, 33.79%)) and 83 wear glasses all the time or sometimes (point estimate: 35.17%; 95% confidence interval: (29.16%, 41.67%)). The results of the logistic regression (Table 5) highlights that girls (compared to boys), children that have at least one parent wearing glasses (compared to children whose parents don’t wear glasses), children from schools Gorubliane (Sofia) and Veliko Tarnovo (compared to those from Devnya) have increased odds of myopia. Concerning age, children below 10 years have lower risk of myopia compared to the children above 10. The children from the Sport school (Sofia) have lower risk of myopia compared to the children from Devnya but the odds ratio is not statistically different from 1.
Considering the outcomes of the presented cross-sectional study, the obtained results are comparable to other recently published epidemiological studies in Europe, reporting myopia prevalence as follows: Poland (7 years 4.0%, 12 years 14.4%), Ireland (6–7 years 3.3%, 12–13 years 19.9%), the UK Northern Ireland Childhood Errors of Refraction (NICER) study (6–7 years 2.8%, 12–13 years 17.7%), Aston Eye Study (AES) (6–7 years 5.7%, 12–13 years 18.6%).17, 18, 20, 21 The results are slightly lower in Australia (6 years 1.6%, 12 years 12.8%).21, 22 A direct comparison of the prevalence of myopia should not be done, due to the different methods and criteria used to measure it: with or without cycloplegia, auto-refractometer, retinoscopy, subjective refraction, and a cut-off value ranging from -0.25 to -1.0 D in different studies.23
Regarding Bulgarian children, a small number of epidemiological studies on myopia are available. For example, a comparable study of Plainis et al. (2009), finds out that the percentage of myopic children is 14.1% at primary school level and 13.0% at secondary school level for children in Stara Zagora (Bulgaria). Among the myopic pupils, only 35.8% of Bulgarian children are reported to wear corrective spectacles.23 The study of Plainis at al. use a similar methodology and myopia is defined on refractive error recordings taken with an auto-refractor in the absence of cycloplegia. The data on 310 Bulgarian children in the study of Plainis et al. are collected in Stara Zagora, a Bulgarian town similar to Veliko Tarnovo in terms of population and socio-economic development. Our results show a higher prevalence of myopia among children of the older age group and also a similar percentage of children with optical correction of their myopia.
The study by Slaveykov and Trifonova (2020)25 publishes data about examination of 596 children aged 3-6 years in Kazanlak, a town with a population of 44 760 residents. Myopia is defined as SE ≤ −0.50 D.25 The children underwent non-mydriatic refraction screening using the Plus-optix S12с Mobile camera. The study reports a myopia rate of 6.8–9.3% for different age groups, highlighting that 33% of the children with myopia have never visited ophthalmologist.
Another trend, concerning the age distribution of myopia prevalence is also observed in our study. This confirms the finding that age is the most critical parameter for epidemiological analysis of myopia worldwide, and the prevalence rate of myopia increases significantly with age.14, 22 Therefore, preventing early onset of myopia requires a collaborative effort among professionals, health care institutions, schools, parents, and the entire society.
In the first place, it should be noted the parents’ role for the prevention and early treatment of myopia. Usually, the impaired vision of the parents is associated with higher odds of myopia10,12 (of which our data show evidence), but this can be linked not only to genetic inheritance but also to the inherited family lifestyle and habits, such as time spent near work, daily sport trainings and outdoor activities. Parental attitudes can affect the health coverage of the child's myopia: paying attention to diagnose myopia on time, providing glasses for optical correction of the child's vision and motivating regular wearing of the glasses.
In our study, surprisingly, the screen time is not associated with different odds of myopia, given that all other risk factors considered are included in the logistic model (p-value = 0.124) and when this is the only risk factor considered (p-value = 0.226, Table 2). One of the explanations could be that the data are not reported correctly, relying on self-reports of the children or parents, biasing intentionally or not the time spent in front of a smartphone or a computer. In the literature, when using an objective method to measure the time of smartphone use (as the amount of data used), the correlation with myopia is clearly observed. According to MacCrann et al., myopic students use almost double the amount of smartphone data per day compared to non-myopic, while the smartphone time use does not significantly differ.24 More interestingly, the screen time and its role for myopia onset should be more carefully examined following the COVID-19 pandemic measures in 2020 and 2021 and the imposed online learning alternatives. Further studies need to investigate the effects of COVID-19 on the epidemiology of myopia, as it changed profoundly the daily routine of the school-age children, reducing the time for sport and outdoor activities.
Limitations of this study: In the study design, it was chosen to measure refractive error using an autorefractor in the absence of cycloplegia. This decision was made based on the following considerations: to not cause discomfort to the children, to obtain parental consent to participate. To avoid overestimation of the prevalence of myopia, the condition uncorrected visual acuity <0.8 has been added to the definition.
The present study has terminated earlier than planned due to the outbreak of Covid 19 pandemic. During the 2020 lockdowns, serious changes are imposed to the daily life of all school children: longer time in front of screens, limited time outside and almost lack of outdoor sports activities. It would be interesting to conduct a similar study after the pandemic ends and compare the new data for myopia prevalence.
To some questions from the questionnaire the children/parents did not give an answer. We imputed answer “No” to the missing values in the variable if the children wear glasses. We examined more closely the children with missing values for this variable (141 in total) and 134 of them don’t have glasses (67 of them did not have even an examination before and 67 had an examination). Only 7 children have an examination before and have prescribed glasses. We believe that such manipulation of the data is justifiable. Regarding the missing values in the other variables, we only did pairwise deletion for all analyses.