Myopia has become one of the fastest-growing eye health challenges of the 21st century (1). Over the past 50 years, the worldwide prevalence of myopia has increased dramatically (2). Myopia stands as the predominant cause of correctable visual impairment across both adults and children in developed nations. Moreover, it emerges as a primary contributor to avoidable blindness in developing regions, underscoring its significant impact on global eye health (3). According to the World Health Organization’s findings in 2019, over 2.2 billion individuals are affected by vision impairments, with a staggering 1 billion suffering from preventable visual challenges (4). Increasing degrees of myopia put people at greater risk of sight-threatening complications, such as myopic maculopathy, retinal detachment, glaucoma, and cataracts (5). High myopia can cause pathological changes in the retina, choroid, and vitreous, leading to serious complications such as retinal detachment and neovascularization, macular hemorrhaging, and macular degeneration (6). The higher the degree of myopia, the greater the possibility of complications (blindness can result in severe cases) (5). Retinoscopy under cycloplegia is widely regarded as the gold standard for refractive assessment in children, owing to its ability to minimize the influence of accommodation, thus providing more accurate results (7). Considering the significant burden of the disease and its complications, tackling myopia becomes imperative (1). Timely identification of myopia progression patterns is the leading requisite for myopia practice as it aids in the decision of appropriate management (8). International Myopia Institute (IMI) introduced the concept of premyopia, which is defined as a refractive state of an eye of ≤ + 0.75 D and > −0.50 D in children where a combination of baseline refraction, age, and other quantifiable risk factors provide a sufficient likelihood of the future development of myopia to merit preventative interventions (9). Myopia progression has been analyzed in various studies (10–13), but only a few studies have been conducted in Europe (14).

The aims of this study were to analyze demographic and refractive characteristics, as well as progression patterns, of premyopia, low myopia, and high myopia among children and adolescents diagnosed and followed up in Croatia.

To the best of our knowledge, this is the first organized database on premyopia, low myopia, and high myopia in the Republic of Croatia. Analyzing data from individuals aged 4–18 years at baseline and followed in the period from 2008 to 2023 at the University Eye Department of the University Hospital “Sveti Duh,” Zagreb, the CroMyop™ registry for the first time described the proportion patterns and progression rates of premyopia, low myopia, and high myopia throughout years of follow-up and different ages of myopia diagnosis.

High prevalence and incidence of myopia in Asian children and adolescents have been reported in the past few decades (16), resulting in a large number of data regarding myopia progression (17–19). Recently, there has been an increased number of published data on myopia progression in European children (14).

In accordance with the Studies on Health of Children and Adolescents (KiGGS studies) (20), our data confirmed that the total number of myopic children and adolescents did not meaningfully change between 2008 and 2019. However, an increased number of myopic patients during and after the COVID-19 pandemic was reported. Several myopia studies have demonstrated an increase in the incidence of myopia in connection with the COVID-19 pandemic (21–31). Data on European children demonstrated that in England, after the pandemic, a reduction of visual acuity in children was recorded, suggesting an increase in myopia progression (25). Slovakian, Russian, and Spanish authors demonstrated a decrease in spherical equivalent and an increase in the myopia prevalence and axial length of eyes in children following the COVID-19 pandemic compared to the period preceding the pandemic (25–31). During the COVID-19 pandemic, habitual changes affecting myopia progression in children were studied by Xu et al. (24). The progression of myopia over a period of 6 months worsened from −0.23 D to −0.34 D, and the incidence of myopia doubled, from 8.5 to 13.6% (24). In our study, myopia progression rates in the years 2017 and 2019 varied from −0.1 D/y to −0.21 D/y; however, in 2021 and 2022, an increase in myopia progression was observed, −0.45 D/y and − 0.43 D/y. The number of premyopic and low myopic children and adolescents increased significantly. Heatmaps of average first-year progression rates indicate faster progression rates in all age groups during COVID-19 and post-COVID-19 years. This increase in myopia prevalence and progression could be explained by the reduction in outdoor activities combined with an increase in close-range work activities, digital screen time, and homeschooling, which are proven risk factors for myopia onset and progression (32).

Together with environmental factors, described risk factors for faster myopia progression are myopic parents (33), higher myopia at baseline, age between 7 and 12 years, and female gender (14, 34). Parental myopia significantly increases the risk of high myopia (33), myopia onset, and progression (35, 36). Myopia prevalence was significantly linked with paternal history of myopia (OR = 2.4). The Finnish study noted that both parents were myopic in 54% of cases, and the OR of developing high myopia was 3.96 compared to non-myopic parents (33). In Bulgaria, 23.46% of myopic children had at least one myopic parent, leading to a significant risk factor for myopia onset (37). In Croatia, parents of myopic children and adolescents were mostly non-myopic, but a high degree of individuals had one myopic parent (35%). If both parents were myopic, hazard ratios for developing low myopia from premyopia and high myopia from low myopia were 920.27 and 199.23 compared to individuals without myopic parents. Progression rates between 11 and 24 months after the first visit were fastest among children and adolescents with both myopic parents (−0.69 D/y). However, such observations should not be associated with just genetic inheritance but also with the family lifestyle and habits, such as outdoor activity, use of digital screens, time spent near work, daily sports training, and time spent with children in general (37). Genetic variations cannot fully explain the majority of variations in myopia risk, hence the known genetic loci from genetic exploration in the recent 20 years only explained less than 20% of the heritability for myopia in European children (38, 39).

Our study demonstrated that the mean age of diagnosis for premyopia, low myopia, and high myopia was between 11 and 12 years of age, which is similar to findings from French and Irish studies (14, 36). Regarding progression rates, age was identified as the most important risk factor for faster myopia progression (10, 40, 41). Children from 7–9 years demonstrated the highest progression rates (−0.36 D/y). Heatmaps of low myopia individuals’ odds ratio to progress faster or equal −0.5 D/y in the period between 11 and 24 months showed that the 7–9 age group with baseline SE from −4 D to −6 D demonstrated the strongest association with faster progression (OR = 2.0, 95% CI −1.00 to 2.39). Furthermore, younger individuals had a higher risk of developing low myopia from premyopia and high myopia from low myopia. High myopia progression rates illustrated statistically significant differences between younger and older individuals, where 8-year-old children progressed more than 0.5 D/y compared to 12-year-old individuals with high myopia. School starting age in Croatia is around 7 years, which is in accordance with the fastest progression rate of premyopia and low myopia at the age of 8 years (−1.19 D/y and − 0.46 D/y). Our data are comparable to other studies that included children from 4 to 18 years of age. The French study revealed that the same age group from 7 to 9 years progressed the fastest (−0.43 D) (14), while progression reported in the control arm of children aged 6–15 years of the Houston Myopia Control Study in the youngest age group was −0.34 D/year (42). Differences were noted in other age groups, especially low myopic individuals aged 4–6 years, who progressed second fastest (average progression rate – 0.29 D/y). Progression in myopic individuals in France for the 4–6 years age group was −0.18 D (14). Myopic children and adolescents in the UK, with an average age of 5.6 years, progressed at a rate of −0.30 D/y (43). Different progression rates can be explained by differences in lifestyle and educational practices and socioeconomic factors, including access to healthcare, awareness of eye health, public health initiatives, availability of corrective measures such as spectacles or contact lenses, cultural attitudes toward outdoor activities, near-work habits, and the emphasis on academic achievements can vary from one country to another resulting in different myopia progression rates. Furthermore, the age groups from 13 to 18 years showed the slowest progression rates. This phenomenon might be attributed to the observation that the average age of myopia stabilization typically falls within this age bracket (11).

An important factor in myopia progression rate is ethnicity, demonstrating faster progression rates (−0.63 D/y to −2.00 D/y) in children of East Asia (12). Our population of children and individuals was ethnically homogeneous, which helps in a better understanding of myopia progression in Caucasian European children.

The ratio of individuals progressing from low to high myopia was around 3%, emphasizing the risk assessment of developing high myopia in different baseline SE groups. Lower SE groups at baseline proved to have higher hazard ratios than higher baseline SE groups. Our finding was in concordance with observations from other European studies (14, 33), suggesting refractive characteristics of individuals that should be more often followed up due to a higher risk of developing high myopia (14).

Although girls’ progression rates are faster than in boys (−0.21 D/y vs. −0.18 D/y), it is not clinically meaningful. In China, United Kingdom, and France, progression rates in girls were faster than in boys, but these observations were equally clinically unmeaningful (14, 41, 44) and usually a common finding (44).

This is the first Croatian study on myopia that included a large sample of individuals and respective records of visits. The strength of this study is the clinical setup where all examiners were experienced pediatric ophthalmologists performing a complete ophthalmological examination, including retinoscopy in cycloplegia, excluding individuals with secondary myopia. Children aged 4–18 years, in the period from 2008 to 2023, were diagnosed with myopia and followed up. The study included Caucasian children and adolescents from Croatia and surrounding countries, featuring an examination of both eyes, with the worst eye refraction status analysis. Individuals were prescribed spectacle correction without any other interventions affecting the sight. Regarding the size of the population and number of children in the Republic of Croatia, we analyzed data from 3% of children and adolescents, filtering data to approximately 0.3% of children and adolescents with primary myopia who underwent cycloplegia on the first examination and followed up longer than 6 months. In comparison, the number of children and adolescents included in the study regarding the population of children and adolescents in a country was France 2.3% (14), United Kingdom 0.3% (43), Germany 0.5% (20), Spain 0.1% (45), United States 0.01% (44), India 0.1% (46), and China 0.2% (23). By performing a complete ophthalmic examination and retinoscopy in cycloplegia, we managed to exclude secondary myopia and other causes that can lead to misinterpretation of the results. We also acknowledge several limitations in this study. Children with constant myopia progression could be over-represented, given that they would require more frequent changes of optical correction than those with more stable vision. This may explain a paradox in the data: while it is clear that older children progress more slowly, older age groups do not appear to slow over time. Retrospective data collection limited the analysis of biometrical eye data, which would help in better understanding of progression rates in some groups. A standardized classification of myopia defining premyopia was proposed in 2018 (9), and excluded children with cycloplegic SE ≥ 0 D examined in the years before 2018. Progression rates of children diagnosed with myopia, in 2021 and 2022 and followed till June 2023, could be affected by a shorter follow-up period. Furthermore, the retrospective design of the study resulted in a loss of follow-up over the years. Additionally, analyzing cycloplegic SE in children who were examined and followed up for spectacle prescription may have influenced progression rates, while small refractive changes that did not express a need for new spectacle prescription, did not include retinoscopy in cycloplegia, or possibly were not reported at the end leading to steep changes in cycloplegic SE.

This is the first Croatian database analyzing premyopic, low myopic, and high myopic Caucasian children and adolescents aged 4–18 years and followed in the period from 2008 to 2023, providing data on myopia progression according to age, gender, parental myopia, and initial degree of myopia at first correction, based on cycloplegic refraction.

Anonymized data is available upon reasonable request. Please direct any requests to access the dataset to the corresponding author.

The studies involving humans were approved by Ethics committee of University Hospital “Sveti Duh”, Zagreb, Ethics committee of the School of Medicine of the University of Zagreb. The studies were conducted in accordance with the local legislation and institutional requirements. Written informed consent for participation was not required from the participants or the participants’ legal guardians/next of kin in accordance with the national legislation and institutional requirements.

AV: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Validation, Visualization, Writing – original draft. LM: Conceptualization, Data curation, Formal analysis, Investigation, Software, Validation, Writing – review & editing. ZS: Funding acquisition, Project administration, Resources, Software, Supervision, Writing – review & editing.