Study population

This large population cross-sectional study was approved by the research ethics committees of Central South University Xiangya School of Medicine, Affiliated Haikou Hospital and adhered to the tenets of the Declaration of Helsinki. Written informed consent and assent were obtained from their parent or legal guardian of all participants. Prior to enrolling school children and adolescents, the purpose and methods of this study were explicated by the investigators. This was a cross-sectional study conducted from May 2018 to July 2018 in Haikou, a provincial capital of about 2.2 million people in Hainan province, P. R. China, and it stretches from 19.32N-20.05N latitude with the long daylight exposure in China.

Using randomized method, MeiLan District was randomly selected from 4 districts in the Haikou city, 12 primary schools were selected from 30 primary schools and 7 junior middle schools were selected from 16 junior middle schools. All students from the selected schools were invited to participate. In this study children and adolescents stands for boys and girls aged between 6 and 15years old. The inclusion criteria for our study were as follows: (1) age between 6 and 15years; (2) with no congenital abnormalities; (3) Chinese Han Nationality Students; (4) an informed consent was obtained by the parents or legal guardian. The reasons for exclusion from the analysis of refraction were failure to complete the examination, previous ophthalmic surgery, wearing intraocular lenses, ocular diseases (ocular trauma, cataracts, glaucoma, optic neuropathy) or ocular injuries caused the significant refractive error. Of 31,780 eligible students, 31,524 students took part in this study (99.20% participation rate).

All the participants accepted the UCVA testing, refractive error measurement, slit lamp microscope examination, cover-uncover test, alternate cover test and pupil fundus examination. Before test of visual acuity, students were asked to take off glasses or contact lens. Uncorrected visual acuity (UCVA) was tested for each eye by trained doctors under bright daylight with a measured distance of five meters, by using the standard Logarithmic visual acuity E chart. The values were converted to Snellen for subsequent analyses. The children and adolescents underwent all examinations by two trained ophthalmologists and auto-refractometry was conducted by two experienced senior optometrists. Refractive error (RE) of each eye was measured without cycloplegia at least three times via performing an automatic refractometer (NIDEK, ARK-1, Japan). Three readings of refractive error were taken from each eye and their average was entered for analysis. Repeated measurements were performed if one measured outcome deviated from the other two by more than 0.50 D.

In addition, for better quality control, the professional engineers calibrated the autorefractors every day to minimize the measuring refractive error. About 5% of the students were randomly selected to perform the repeated test of refraction. Data Recording were double entered into Microsoft Excel spreadsheets, meanwhile, by checking the original paper records against the database was conducted to resolve the differences.