The cross-sectional school-based study enrolled a total of 765 children aged 6–16 years from two elementary schools (The Second Primary School of Menyuan County and Qingshizui Town Boarding School of Menyuan County) and one junior high school (The Third Junior High School of Menyuan County) in the Menyuan county in Qinghai province, China. The Ethics Committee of Shandong University of Traditional Chinese Medicine approved the study protocol, and all participants one parent or legal guardian signed a written informed consent. The study was conducted in accordance with the tenets of the Declaration of Helsinki.
The Menyuan Hui autonomous county in the Qinghai province is in the northeast of Qinghai province, with a total area of 6,896 square kilometers at an altitude of 2,388 to 5,254 meters above sea level. It has a continental plateau climate, with snow and wind in spring, cool and rainy summers, mild and short autumns, and cold and long winters. There are 22 ethnic groups in Menyuan county, including Hui, Han, Tibetan, Mongolian, and Tujia. Among these ethnicities, 62.4% belong to the so-called ethnic minorities and 45.8% of the people have the Hui nationality. Most of the population are farmers and herdsmen. The school-based study enrolled 596 children, of which 353 (59.2%) are Han Chinese, 183 (30.7%) were Hui, and 60 (10.1%) were Tibetan.
All participants underwent an ophthalmic examination including measurement of distance and near, uncorrected, and best corrected, visual acuity and cycloplegic refractometry. For cycloplegia, the children first received one drop of 0.4% oxybuprocaine (Santen Corp., Shiga, Japan) for topical anesthesia. Two minutes later, the first of three drops of 1% cyclopentolate (Alcon, Fort Worth, Tex., USA) were instilled at 5-minute intervals. If 30 minutes later the pupil diameter was at least 6 mm, refractometry was performed. If the pupil diameter was < 6 mm, a fourth drop of cyclopentolate was applied. The spherical equivalent of refraction (SE) was defined as the spherical refractive power plus half of the cylindrical refractive power.
Using optical coherence interferometry (LENSAR LS900; Haag-Streit Co., Koeniz, Switzerland), we measured the axial length, corneal curvature radius, central corneal thickness, anterior chamber depth, lens thickness, and the vitreous chamber depth. All biometric measurements were performed five times, and the mean value of these measurements was taken for statistical analysis. The mean corneal power was calculated based on the mean corneal curvature radius with a refractive index of the cornea of 1.3315 25. Applying the formula of Bennett, the RLP was calculated based on the cycloplegic refractive error and the biometric measurements. The A and B constants in this formula were calculated using Gullstrand’s reduced eye model 22, 23. With respect to the refractive status, we differentiated between high myopia (SE < − 6.0D), myopia (SE ≤ − 0.5D), emmetropia (SE > − 0.5D to ≤ + 0.5D), mild hyperopia (0.5D < SE ≤ 2.0D) and medium to marked hyperopia (SE > 2.0D) 26. As the spherical equivalents of right and left eyes were highly correlated with each other, the data are presented for right eyes only.
The statistical analysis was conducted using a commercially available software (SPSS for Windows, version 25.0, IBM-SPSS, Chicago, Ill., USA). The mean values were presented as mean and standard deviations. Comparisons were performed using the analysis of variance (ANOVA) and post hoc tests. Correlations of the ocular biometric parameters were analyzed using the Pearson´s correlation coefficient. We assessed the standardized regression coefficient β, the non-standardized regression coefficient B and its 95% confidence intervals (CIs). The multivariate regression analysis included the parameters of RLP (or of lens thickness) as the dependent variable and as independent variables all those parameters which were significantly associated with RLP (or lens thickness) in the univariate analyses. Statistical significance was considered with a P-value of less than 0.05.