Although many previous studies have reported on the prevalence and risk factors of myopia and VI in school-aged children1,3,4,9,10,15,16,17,18. There remains a significant gap in research regarding the epidemiology of AL-related VI in children. Early myopia control clinical studies did not measure and monitor axial elongation, making refractive error the preferred outcome measure. More recently, the measurement of ocular biometric components of the eye has become more widely included and is a key measurement in myopia control clinics. The advantages of using axial elongation versus refractive error as the primary outcome measure relates to the direct relationship between the excessive growth of the myopic eye and the associated risk for posterior pole complications, although the two are strongly correlated19. Current evidence primarily focuses on refractive error as the primary metric for VI, with few population-based studies examining the burden and risk factors of AL-related VI20,21. Myopia-related VI is associated with structural changes of the retina and the globe because of elongation of the eye axis. Previous study demonstrated that VI is associated with AL and spherical equivalent (SE)22. This gap is critical given that excessive AL elongation precipitates vision-threatening complications (e.g., myopic maculopathy, retinal detachment) and is more strongly associated with irreversible vision loss than refractive error alone. The AL-VI might provide a missing link between anatomical change and functional vision loss, enabling targeted screening prior to refractive shift. The aims of this study were to assess the common causes of myopia and AL-related VI, and examine variations in prevalence rates by sex and age groups. The findings might inform the development of targeted ocular health care services for children in specific geographic regions.
This cross-sectional observational study constituted a sub-study of Beijing Children and Adolescents Health Cohort (BCHC) from 2022 to 2023. The parent BCHC is a prospective cohort study that enrolled 5579 children and adolescents from a demographically representative district of Beijing, characterized by urban-suburban integration and moderate socioeconomic status. At baseline, all participants underwent comprehensive assessments of exposure factors related to obesity, myopia, growth and development, physical activity, dietary patterns, sleep behaviors, and other behavioral determinants. For this sub-study, we employed a stratified cluster random sampling approach to select 2742 children and adolescents aged 4-18 years from nine educational institutions in Beijing.
All procedures were approved by the Institutional Review Board of the Capital Institute of Pediatrics prior to study commencement. We conducted a vision survey in which all students from nine educational institutions in Beijing (186 students from one kindergarten, 1452 students from five primary schools, 1394 students from two middle schools, and 664 students from one high school) who met the inclusion criteria were enrolled as study participants in 2022. The inclusion criteria were: (1) children and adolescents aged 4-18 years who were permanent residents of Beijing; (2) no history of ocular diseases, eye injuries, or ocular surgeries(except for refractive errors); (3) in good physical and mental health without congenital developmental abnormalities or systemic conditions that might affect vision or refractive examinations; (4) no wearing of corneal gas-permeable contact lenses within the previous month; and (5) willingness to cooperate by both children and their parents or guardians. In total, 2742 children and adolescents aged 4-18 years underwent comprehensive screening, including non-cycloplegic refraction, binocular uncorrected visual acuity testing, AL measurement. Standardized testing conditions were maintained by optimizing ambient illumination and minimizing potential visual distractions. A questionnaire survey was administered (completed by their parents for kindergarten children, and jointly by participants and their parents for school-age children). Following data quality control procedures involving logical verification and exclusion of inconsistent records, complete datasets were obtained for 2595 participants (effective response rate: 94.6%), as detailed in Fig. 1. The study participants were stratified into four age groups: 4-6 years (children), 7-9 years (children), 10-12 years (adolescents), and 13-18 years (adolescents).
Based on the reported myopia prevalence rate of 51.9% among children and adolescents in 2022, we estimated a similar rate of myopia in our target population. Allowing for an error of 2% level of significance (type 1 error), a 95% confidence interval, and a loss to follow-up rate less than 8%, a sample size of 2582 children and adolescents would be needed to achieve our study objectives.
We followed all Helsinki declaration and national ethical standards. All participants were ensured about the matter of confidentiality, written consent of the participants who was witnessed and formally recorded, was obtained before data collection. Identifiable information was removed from the collected data to ensure confidentiality, and access to these digital files was restricted to members of the research team. This study was approved by the Medical Ethics Committee of the Capital Institute of Pediatrics (SHERLL2022043). The guardian and/or their children provided informed consent and were willing to participate, and the investigation was approved by the local education bureau and school. To express our gratitude for their contributions, participants were given a Health check-up report which was promptly emailed to them following their interview.
This study evaluated visual acuity, refractive status, and AL in children and adolescents following standardized protocols. Visual acuity testing was performed using logarithmic visual acuity charts compliant with the GB/T 11,533-2011 standard for, which is a 5-mark record. Refractive status was assessed using a desktop autorefractor (KR-800; Topcon Corporation, Tokyo, Japan) that meets ISO 10,342 ophthalmic instrument standards for non-cycloplegic measurements. AL was measured using an optical biometer (IOL Master; ZEISS Group, Oberkochen, Germany). All instruments were validated and approved by regulatory authorities. Both autorefractors and biometers underwent simulated human eye calibration prior to use. Participant screening followed the Guidelines for Appropriate Techniques for Myopia Prevention and Control in Children and Adolescents (Updated Version) issued by China's National Health Commission.
Myopia in children aged 4-6 years: According to the Guidelines for Pediatric Eye Evaluations Preferred Practice Pattern (Updated Version) issued by the American Academy of Ophthalmology, the criteria for determining myopia were that children aged 4-6 years with the sphere power(S) of <- 2.00 diopters(D) (detected using a computer refractometer in a noncycloplegic refraction state).
Myopia in children and adolescents aged ≥ 7 years: According to the International Myopia Institute (IMI) and the National Health Commission's "Guidelines for Appropriate Techniques for Prevention and Control of Myopia in Children and Adolescents (Updated Edition)", the criteria for determining myopia eye the standard logarithmic visual acuity of the naked eye < 5.0 and SE < -0.50D.
While non-cycloplegic refraction is a practical approach, particularly in large-scale studies, it is predisposed to overestimating refractive error in younger children (versus cycloplegic gold-standard).The myopia prevalence in pre-school children and school-age children were defined respectively, and should be interpreted as screening indicators.
According to the Expert Consensus on AL Reference Ranges for School-Aged children in 2022, and the average normal AL is 23.5 mm (range: 22-24.5mm). Clinically, AL is shorter than average in hyperopic eyes and longer than average in myopic eyes. AL > 23.5 mm is generally regarded as indicative of abnormal axial elongation. In this study, we defined AL-related VI as AL measurement > 23.5 mm and uncorrected visual acuity < 5.0. Any participant meeting the definition in at least one eye was included in the total count of AL-related VI cases.
We designed a self-administered questionnaire based on "the work plan of monitoring and intervention of common diseases and health effects among students" normative guidelines, and similar previous studies. The questionnaire including demographic and behavioral data (e.g., sex, age, parental myopia, visual behaviors and environment, screen time, reading habits) were collected and administered by trained investigators. Both participants and their parents or legal guardians were imformed ahout the study, provided their consent and filled out questionnaires. Before completing the questionnaire, the survey or explained the significance of the survey, emphasized the confidentiality of the questionnaire, and any questions that were not understood would be explained by the investigator until the participants or their parents could understand the questions correctly in order to guarantee the credibility of the results. This approach aimed to enhance response reliability,though some degree of recall bias may still be present in the questionnaires.
The on-site investigation was conducted by trained and qualified medical personnel using standardized vision examination methods and survey questionnaires. The original data were collected and organized by the school as a unit.Designated personnel verified and entered the data. Health records were created for follow-up care and to notify participant of the results. Vision examinations adhered to national standards (using a standard light box), adhering to prescribed methodology, testing environment and viewing distance. In a semi-dark room, refractive testing of children in the natural state was performed to ensure that the system error was within a reasonable range. All examinations took place in a controlled environment with fixed conditions and appropriate lighting. The child's seat was fixed within a radius of 50 cm, the head was kept upright, and the eyes were level with the instrument. The interocular distance between the right and left eyes was 35 cm. The average value of at least three consecutive readings from an autorefractor was used for analysis, and the average value of at least five consecutive readings from an axial biometric instrument was used for analysis.
The data were entered into an Excel 2007 spreadsheet. All statistical analyses were performed with R software 4.1.3. The Shapiro Wilk test was used to determine the normality of the data. Continuous variables were presented as mean ± standard deviation (mean ± SD). For skewed variables, non-parametric tests (Mann-Whitney U) were employed. Descriptive data are presented as frequency and percentage or mean with Wald 95% confidence interval (CI). Categorical variables are presented as frequency and percentage, and continuous variables are expressed as mean with standard deviation. These associations were tested using chi-square analyses or the correlation index, depending on the type of variables involved. Variables, which have a p-value of less than 0.05 in the univariate analysis of myopia, were selected and evaluated by multivariable logistic regression models. Our models demonstrated excellent discriminative power (AUC ≥ 0.70). The low VIF values (< 1.12) indicates no multicollinearity. Hosmer-Lemeshow P > 0.05 (adequate fit). The results of multivariable logistic regression are presented with odds ratio (OR) and 95%CI. Statistical significance was defined as P < 0.05.