Study sites and populations
This study was based on the program “Surveillance for common disease and health risk factors among students, sub-program: ophthalmological investigation” during the 2018-2019 academic year in Jiangsu Province. We enrolled 26,461 students aged 6-17 years from 12 regions in Jiangsu Province (Supplement figure1) in this program, and A total of 6,363 primary school students were aged from 6 to 12.5, participated in the sub-program: ophthalmological physical examination.
The inclusion criteria for our subjects were(1)non-myopic children and lack of other serious eye diseases;(2) Chinese Han Nationality Students;(3) age between 6 to 12;(4) Ability of parents/guardians to provide informed consent.
Data collection and ethics statement
Myopia was defined as -0.50 diopters(D) in the worse eye and the worse eye was defined as the eye with the greater absolute value of refractive error (spherical equivalent). All students took part in this sub-program, and they were required to provide basic demographic information including name, sex, regional, and some of their parents filled in questionnaires concerning myopic related questions. Detailed information could be listed as follows: Electronic questionnaire was used in this study. The child’s parents filled the form by logging into wechat app. The establishment of the questionnaire was based on the The SCORM cohort study[15, 16]. It would take parents 30-40min to finish this questionnaire. The main contents include the following three aspects. Firstly, basic information including parental myopia and Whether the child had brothers/sisters. Secondly, learning-related issues including the near work activities. Thirdly, work and rest related issues including outdoor activities, the duration of sleep.
An autorefractor (Topcon RM-8900 or KR-800; Topcon Co., Tokyo, Japan) was applied with cycloplegia. The cycloplegic refraction is measured using tropicamide phenylephrine eye drops every 5 min,3 times, and then the refractive error is measured 30 min after the first drop of tropicamide by autorefractor with five repeated measurements. The spherical equivalent of the refractive error was calculated as the spherical value of refractive error plus one half of the cylindrical value.
The study protocol was approved by the Institutional Review Board of Ethics committee of Jiangsu Province, and detailed information can be found in the previous article[10]. We used an autorefractor with cycloplegia under parents’ informed consent.
Statistical analysis
The age-BMI-specific spherical equivalent values were calculated for the percentiles of 5th, 10th, 25th,50th,75th,90th, 95th for both boys and girls. We used 5th, 10th, 25th,50th,75th,90th and 95th percentiles spherical equivalent values as potential age-specific alert spherical equivalent values.(Supplement table2) According to previous studies[17], Jiangsu Province had the top prevalence of reduced visual acuity (76.2%), and then we set the upper limit value (UPPER) for the non-myopic population as 76.2%. Then the alarming threshold value percentile for non-myopic students can be calculated as follows:
Alert Value Percentiles
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=
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UPPER(0.762)×Total Number(6362)-Number of myopic students (2462)
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Number of non-myopic students (3900)
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Therefore, we selected 50th spherical equivalent values as alert values based on the calculated Alert Value Percentiles.
Then we verified the correctness of the alarming threshold value by calculating OR values with proven myopia related factors, such as outdoor activity time and parental myopia.
We then performed a logistic model to select variables fit for the nomogram model. Nomograph was drawn by R software with rms packages. Statistical analyses were performed with R software (www.R-project. Org, version 3.5.3) with additional rms package[18] Detailed calculation principle can be found in previous studies[19].Besides, continuous variables were presented as the mean with standard deviation (SD).