To analyze the trends in refractive error and ocular biological parameters in elementary school students over 5 years, and to investigate the patterns of change before and after myopia onset.

A cohort study was adopted.A total of 1 986 first-grade students from the Anyang Childhood Eye Study were enrolled in this cohort study and their right eye data were taken for analysis, including 1 126 boys and 860 girls.Every year, cycloplegic autorefraction was performed with 1% cyclopentolate eyedrops to obtain the spherical equivalent (SE).The axial length (AL), anterior chamber depth, lens thickness, mean corneal curvature (Km) and other parameters were obtained by ocular biometry.The lens refractive power (LP) was calculated using the Bennett formula.The subjects were assigned to persistent myopia group, non-myopia group and new onset myopia group.According to the age of myopia onset, the new onset myopia group was subdivided into the 8-, 9-, 10-, 11- and 12-year-old myopia groups to compare the differences in refractive error and ocular bioparameters among groups at different time points of follow-up.This study adhered to the Declaration of Helsinki.The study protocol was approved by the Ethics Committee of Beijing Tongren Hospital, Capital Medical University (No.TRECKY2018-030).Written informed consent form was obtained from the guardians of each subject.

All children had a gradual SE drift toward myopia and a gradual increase in the AL with age, and there were significant differences in SE and AL between adjacent follow-up ages within the three groups (all at P<0.05).The earlier the onset of myopia, the higher the myopia SE and the longer the AL of the eye at the same follow-up age, the differences in SE between adjacent groups were statistically significant (all at P<0.05), and the differences in AL between adjacent groups at the follow-up age of 8 to 12 years were statistically significant (all at P<0.05).In the nonmyopia group, SE drifted toward emmetropia at a slow and steady rate of (-0.23±0.27)D/year, and AL also increased slowly and steadily at (0.18±0.13)mm/year.In the new onset myopia group, the changes in SE in the third, second, and first years before myopia onset were (-0.32±0.25), (-0.45±0.33), and (-0.98±0.44)D, and the increases in AL were (0.25±0.12), (0.32±0.15), and (0.48±0.19)mm, respectively.Both SE and AL change rates began to accelerate before myopia onset and slowed down after myopia onset, with statistically significant differences in the overall comparison of SE and AL change rates at different time intervals before and after myopia onset (all at P<0.001).The AL at myopia onset in boys was (24.11±0.70)mm, which was longer than (23.60±0.66)mm in girls (t=159.71, P<0.01).LP decreased with age in all groups, with a faster rate before the age of 9 years and a slower rate after the age of 9 years.The mean decrease rate in LP was (-0.48±0.19), (-0.44±0.20), (-0.49±0.16), (-0.51±0.18), and (-0.48±0.19)D/year in the persistent myopia group and 8~11-year-old myopia group, respectively, which were significantly faster than -0.42±0.17 D/year in 12-year-old myopia group and (0.37±0.15)D/year in nonmyopia group (all at P<0.05).There was no statistically significant difference in Km among groups at different follow-up ages (all at P>0.05).

The AL begins to grow at an accelerated rate 3 years before myopia onset, and the increase rate of the AL slows down after the onset of myopia, but it is still significantly faster than that of non-myopic children.In this process, the decrease in LP plays a compensatory role; there is no significant change in corneal curvature.The AL of males at the onset of myopia is longer than that of females at the same age.AL is an important indicator for the prevention and control of myopia.It is important to consider gender differences and to pay more attention to the growth rate when assessing AL.

Myopia; Refractive power; Biological parameters, ocular; Axial length; Lens power; Eye; Child; Longitudinal studies