Ocular biometric characteristics of young Chinese people with axial lengths greater than 26.00 mm

To the Editor: The prevalence of high myopia (HM) has been increasing globally, particularly in the young Chinese population.[1] Most early investigations on ocular biometric parameters in myopic individuals focused primarily on children, adolescents, and elderly individuals with cataracts, leaving a dearth of research on ocular biometric parameters among young adults with HM. Additionally, the associations between ocular biometric parameters and axial length (AL) in individuals with HM are unclear.[2,3] Therefore, understanding the ocular biometric characteristics of young Chinese individuals with HM and their correlations with AL is crucial. Ocular biometric parameters serve as a fundamental basis for preoperative surgical planning, assessing surgical safety, determining the appropriate size of implantable collamer lenses (ICLs), calculating intraocular lens (IOL) power, and predicting postoperative refractive outcomes. Our cross-sectional study was conducted across seven hospitals within the Aier Eye Group (Beijing Aier Intech Eye Hospital, Shenyang Aier Eye & Vision Hospital, Shanghai Aier Eye Hospital, Shenzhen Aier Xiyu Eye Hospital, Guangzhou Aier Eye Hospital, Wuhan Aier Eye Hospital, and Lanzhou Aier Eye Hospital). Individuals seeking advice regarding refractive errors or considering surgery at these hospitals were consequently enrolled from September 2021 to February 2023. Individuals with an AL ≥26.00 mm were included in the HM group, whereas those with an AL ≥23.00 mm and <26.00 mm were included in the control group. The exclusion criteria were as follows: (1) age ≥45 years; (2) eye diseases such as keratoconus, corneal scars, cataracts, glaucoma, lens dislocation, and retinal detachment; (3) previous eye surgery; (4) inability to cooperate with the examination or poor-quality images, such as images with severe artifacts; and (5) failure to complete the whole examination. Experienced technicians performed the examinations. This study followed the principles of the Declaration of Helsinki, and the Ethics Committee of Beijing Aier-Intech Eye Hospital reviewed the study protocol and provided ethical approval (No. BJAIER2021IRB24). All participants were examined with an IOLMaster 700 (Carl Zeiss Meditec AG, Jena, Germany) to obtain measurements of AL, anterior chamber depth (ACD), lens thickness (LT), central corneal thickness (CCT), and white-to-white distance (WTW). A Pentacam (Pentacam HR, Oculus GmbH, Wetzlar, Germany) was used to measure corneal refractive power, including flat, steep, and mean anterior corneal curvature (K1_a, K2_a, and Km_a, respectively), as well as posterior corneal curvature (K1_p, K2_p, and Km_p), anterior corneal astigmatism (ACA), the astigmatic axis, posterior corneal astigmatism (PCA), and the astigmatic axis. The astigmatic axis was defined as the angle of the steep meridian. The anterior segment (AS) was defined as the sum of the ACD and LT. Only the right eye was used in all the patients. The Shapiro–Wilk test (S–W test) was used to assess the normality of the data. The median (Q1, Q3) was used to express skewed data, and other variables were expressed as the mean ± standard deviation (SD). Two-sample independent t-tests were used to compare sex differences, the Mann–Whitney test for skewed data, and the chi-squared test for count data. Multiple linear regression analyses were used to explore the relationships among the AS, AS/AL ratio, and AL. The changes in each parameter with increasing AL were explored via age- and sex-adjusted restricted cubic splines. A two-tailed P value <0.05 was considered statistically significant. Statistical analyses were performed via IBM SPSS Statistics 27.0 software (SPSS, Inc., Chicago, IL, USA) and R software (4.2.2, r-project.org, Vienna, Austria). The analysis included 1208 eyes from 1208 patients, with a mean age of 27.0 ± 6.5 years (range: 17.0–44.0 years). Among them, 698 (57.8%) were female patients. The AL ranged from 23.00 mm to 35.45 mm. The median (Q1, Q3) AL was 26.87 (25.21, 28.48) mm. The biometric parameters of the study population are detailed in Supplementary Table 1, https://links.lww.com/CM9/C172. We analyzed the differences in ocular biometric parameters between young Chinese individuals in the HM group and those in the control group. A total of 410 eyes were in the control group, with 798 eyes in the HM group. Comparative analysis revealed distinct ocular biometric characteristics between the HM group and the control group. Patients with HM presented a greater WTW (t = −8.677, P <0.001), flatter anterior and posterior corneal curvatures (t = 19.285, P <0.001 and t = −18.129, P <0.001, respectively), and larger ACA (Z = −2.361, P = 0.018). However, no significant differences were detected in the CCT (t = −1.492, P = 0.136) or PCA (Z = −1.269, P = 0.205). In terms of AS parameters, the HM group had a deeper ACD (t = −6.594, P <0.001), thinner LT (t = 2.976, P = 0.003), larger AS (t = −3.048, P = 0.002), and smaller AS/AL ratio (t = 40.788, P <0.001) than did the control group [Supplementary Table 2, https://links.lww.com/CM9/C172]. Considering these differences in ocular biometric parameters between the HM group and the control group, it is important to consider limitations when diagnosing and treating eye diseases using normal reference values for highly myopic patients. Therefore, the study population was stratified into eight groups according to AL. The ocular biometric parameters for each group are shown in Supplementary Tables 3 and 4, https://links.lww.com/CM9/C172. Age- and sex-adjusted multiple linear regression was used to explore the effects of AL on AS and AS/AL ratio. As shown in Supplementary Table 5, https://links.lww.com/CM9/C172, a longer AL was independently associated with a greater AS ( β = 0.020, P <0.001) and a smaller AS/AL ratio ( β = −0.934, P <0.001), with adjusted R2 values of 0.208 and 0.829, respectively. Multiple linear regression analysis revealed a negative correlation between the AS/AL ratio and AL, indicating that elongation of the posterior pole of the eye became the main contributing factor to increasing AL. Notably, patients with HM are at greater risk for ocular pathological changes.[4] Building upon the established correlations and investigating the associations between AL and various ocular parameters, we identified nonlinear relationships with ACD, LT, CCT, Km_a, Km_p, and WTW. The trends were modeled via age- and sex-adjusted restricted cubic splines with multivariable linear regression [Figure 1]. The knots were set at the 5th, 35th, 65th, and 95th centiles of the AL. We found nonlinear relationships of AL with ACD (F = 11.87, P <0.001), LT (F = 19.45, P <0.001), CCT (F = 12.48, P = 0.006), Km_a (F = 45.26, P <0.001), Km_p (F = 38.72, P <0.001), and WTW (F = 8.90, P <0.001). According to the restricted cubic splines, AL had an “inverse U-shaped” correlation with the ACD, WTW, and Km_p, and a “U-shaped” correlation with LT and Km_a. According to Figure 1, the ACD, Km_p, and WTW reached their maximum values when the AL reached 28.48 mm, 29.40 mm and 29.10 mm, respectively; LT and Km_a reached their minimum values when the AL reached 27.86 mm and 29.35 mm, respectively. The relationship between the CCT and AL was complex. The CCT decreased as the AL increased for AL values less than 25.84 mm and greater than 29.30 mm. Conversely, the CCT increased with increasing AL within the range of 26.87 mm to less than 29.60 mm [Figure 1]. This nonlinearity affects the accuracy of IOL formulas, particularly in patients with longer ALs.[5]Figure 1: Associations of AL with (A) ACD, (B) LT, (C) CCT, (D) Km_a, (E) Km_p, and (F) WTW. Adjusted for gender and age (modeled using restricted cubic splines). 95% CIs are indicated by gray shaded areas. The red dashed line indicates AL = 26.00 mm. The black dashed line indicates the predicted turning point. ACD: Anterior chamber depth; AL: Axial length; CCT: Central corneal thickness; CI: Confidence interval; Km_a: Anterior mean keratometry; Km_p: Posterior mean keratometry; LT: Lens thickness; WTW: White-to-white distance.Restricted cubic splines revealed that males had deeper ACD, thinner lens, larger WTW, thinner CCT, and flatter cornea after controlling for the effects of AL and age [Supplementary Figure 1, https://links.lww.com/CM9/C172]. Additionally, there were sex-specific differences in the trends of the associations of ocular biometric parameters with AL. These findings suggest potentially lower accommodation capacity during emmetropization in females than in males. Considering the differences in ocular biometric parameters between the sexes, sex should be considered in IOL calculations, refractive surgery selection, and postoperative safety prediction. In conclusion, this study provided a detailed characterization of ocular biometric parameters among young Chinese individuals with HM, contrasting these findings with those of control subjects and revealing alterations as AL increased. The results indicated that the CCT, WTW, and ACD initially increased before decreasing with increasing AL in patients with HM. Additionally, the LT and mean corneal curvature of the anterior/posterior region initially decreased, but then increased. Additionally, sex differences in ocular biometric parameters were observed. Moreover, the statistical descriptions of parameters across varying ALs may provide reference values for the diagnostic management of ocular pathologies, inform the planning of refractive surgical interventions, and facilitate the comparative analysis of ocular characteristics among diverse ethnic populations. Funding This work was supported by the Science and Technology Innovation Program of Socialization Investment of Hunan Province (No. 2020SKC2002). Conflicts of interest None. Data availability statement The data that support the findings of this study are available from the corresponding author, SWL, upon reasonable request.