The study protocol followed the principles of the Declaration of Helsinki and was approved by the Ethics Committee of Wenzhou Medical University (approval number: No. KYK2018-29). All individuals were informed about the content and purpose of the study, and their informed consent was obtained prior to the examination. Young myopic patients were prospectively recruited between September 2021 and February 2022 from the Affiliated Eye Hospital of Wenzhou Medical University in Hangzhou, Zhejiang. These patients sought consultation for myopic refractive surgery at our hospital’s Refractive Surgery Center.

The inclusion criteria were as follows: healthy individuals aged between 18 and 45 years, BCVA of 20/25 or better, astigmatism within ± 2.0 D, and IOP < 21 mm Hg. This age group was chosen because it represents a period when myopia progression has typically stabilized, minimizing confounding variables related to ongoing myopic changes during childhood or age-related degenerative changes in older adults. The exclusion criteria were as follows: presence of other intraocular diseases, such as keratoconus, glaucoma, uveitis, and cataract; various pathologic changes on the myopic fundus or OCT, such as staphyloma, lacquer cracks, and myopic macular degeneration; history of previous eye surgery; use of myopia control measures in the past 6 months (including progressive multifocal lens, orthokeratology lens, etc.) and recent history of wearing contact lenses within 2 weeks. Participants with pupil diameters less than 6 mm in a scotopic environment were also excluded.

Sample size calculations were performed using G*Power software, assuming a medium effect size (Cohen’s d = 0.5), 80% power, and an alpha value of 0.05. This indicated that a minimum of 95 participants per group was required to detect significant differences between groups. These calculations ensured that the study was adequately powered to support the reliability of the findings.

Each participant underwent a complete ocular examination, including best-corrected visual acuity (BCVA), autorefractometry, slit-lamp examination, intraocular pressure (IOP) using a non-contact tonometry (CANNON TX-F), axial length (AL) using an optical biometer (IOL Master 700; Carl Zeiss Meditec, Jena, Germany), Swept source optical coherence tomography (SS-OCT, VG200S; SVision Imaging, Henan, China) examination, and dilated fundus examination. Corneal tomography, total corneal HOAs, and pupil diameters of the participants were analyzed through Pentacam (Pentacam HR type 70900). Cycloplegia was achieved with three drops of Mydrin-P (tropicamide 0.5%, phenylephrine HCl 0.5%; Santen Pharmaceutical, Shiga, Japan) at 5 min intervals [26], and performed before autorefractometry and fundus examination.

The study population consisted of 321 individuals aged 18–45 years (mean age: 28.4 ± 6.25 years), of which 109 were male (33.9%) and 212 were female (66.1%). The participants were predominantly of Han Chinese ethnicity, and all had at least a secondary education level. The axial length thresholds were selected based on previous studies [24, 27, 28], which categorize mild (AL ≤ 25 mm), moderate (AL > 25 mm and < 26 mm), and high myopia (AL ≥ 26 mm) to investigate the effect of axial elongation on ocular structures, such as the choroid and HOAs. The right eye of each participant was selected for analysis.

Corneal HOAs for a 6-mm pupil were measured using a high-resolution rotating Scheimpflug camera (Pentacam HR type 70900) in a scotopic environment (0.1 cd/m2) through a natural pupil without dilation. The acquired data sets were expanded using a normalized sixth-order Zernike polynomial. The magnitudes of the coefficients of these Zernike polynomials are represented as the RMS and used to indicate wavefront aberrations. The HOAs included the following components: total HOA: Comprising third-to sixth-order terms; Spherical HOA: Represented by Z40, Comatic HOA RMS: Comprising Z3 − 1 and Z31 combined; and Trefoil HOA RMS: Comprising Z3 − 3 and Z33 combined (Fig. 1). The measurements were repeated at least five times for each eye, and the three best-focused images were selected and averaged.

Normalized sixth-order Zernike polynomials of aberrations based on corneal tomography examination (Pentacam HR type 70900). Black rectangle, higher-order aberrations; red rectangle, coma aberrations; blue rectangle, trefoil aberrations; green rectangle, spherical aberrations

To ensure consistency, SS-OCT (VG200S; SVision Imaging, Henan, China) measurements were taken between 13:30 to 17:00 to minimize the effects of diurnal variation on the measurements [29]. Eighteen radial scan lines, each measuring 12 mm in length, were employed for structural OCT scans centered on the fovea. A cutoff signal-strength index value ≥ 6 was used as the inclusion criterion. The product was equipped with an eye-tracking system based on an integrated confocal scanning superluminescent ophthalmoscope (cSSO) to eliminate eye-motion artifacts. In addition, the current study corrected for AL before data analysis to minimize the effect of ocular magnification on measurements.

To compare the macular choroidal structure, we calculated the choroidal vascularity index (CVI) and choroidal thickness (CT) using both the horizontal and vertical lines from the 18 radial B-scans. The region between the choroid-sclera interface and retinal pigment epithelium–Bruch’s membrane complex was defined as the choroid. Choroidal segmentation was performed semiautomatically using an algorithm implemented in MATLAB R2017 (MathWorks, Natick, MA, USA) and further refined manually by two trained examiners (KR and XZ). Any discrepancies between observers were resolved by a third adjudicator (DC). Following segmentation, each image was binarized to distinguish the luminal area (LA) and stromal region using Niblack’s autolocal threshold [30]. After image processing, the total choroidal area (TCA), LA, and stromal area were determined. CVI was calculated using the LA/TCA ratio. To evaluate the reproducibility of choroidal segmentation between two trained examiners, we calculated the intraclass correlation coefficient (ICC) for both CT and CVI. The ICC values were 0.962 for mean CT and 0.973 for mean CVI, indicating excellent agreement between the examiners, thus supporting the reliability of the SS-OCT imaging data. The macular zone was further divided into regions consisting of three concentric rings with diameters of 1, 3, and 6 mm according to the ETDRS grid. The CVI and CT were assessed within each grid of the choroidal region (Fig. 2).

Choroidal vascularity index (CVI) and choroidal thickness (CT) were measured in macular zone according to ETDRS grid (A). Both vertical (B) and horizontal (C) scans were measured using semi-automatic algorithms in MATLAB R2017a. T1, temporal parafovea; T2, temporal perifovea; N1, nasal parafovea; N2, nasal perifovea; I1, inferior parafovea; I2, inferior perifovea; S1, superior parafovea; S2, superior perifovea; C, center

SPSS statistics (IBM, Armonk, NY, USA) was used for statistical analysis. To compare differences among the three groups, we used the χ2 test and analysis of variance test, as appropriate. Linear regression analysis was used to identify the associations between choroidal parameters and HOA variables, as well as the relationship between spherical equivalent (SE), AL, and choroidal parameters. Multivariate linear regression models were used to explore the relationships among corneal HOAs, CVI, CT, SE, and AL. We ensured that the assumptions of linearity, normality, and homoscedasticity were met. Normality was assessed using the Shapiro-Wilk test, and homoscedasticity was checked with the Breusch-Pagan test. Parameters with P values < 0.05 in the univariate analysis were included in the multivariate models. The significance level was set to α = 0.05.