This cross-sectional study was conducted from July 2020 to September 2021 at the First Affiliated Hospital with Nanjing Medical University. In concordance with the tenets of the Declaration of Helsinki, informed consent was obtained from all parents or legal guardians. This study was approved by the Ethics Committee of The First Affiliated Hospital with Nanjing Medical University.

Forty newly developed obese children aged 84–96 months and an equal number of age- and sex-matched healthy control subjects were recruited. Obesity was defined as the body mass index (BMI) greater than or equal to the 95th percentile [14], according to Chinese age- and sex- specific BMI reference values [15]. Children, who were diagnosed obesity in the last three years, were classified as newly developed obesity. All included subjects were required to have 20/20 or better best-corrected visual acuity (BCVA).

The exclusion criteria were the presence of amblyopia, nystagmus, retinal disease, hypermetropia exceeding 2 D, myopia and astigmatism exceeding 1 D, glaucoma, intraocular inflammation, strabismus, elevated intraocular pressure > 21 mmHg and media opacity, such as corneal disease or cataract. Patients with a history of prematurity, neurologic disease, ocular trauma, ocular surgery or systemic conditions that could alter the microvasculature, such as diabetes, hypertension, cardiovascular disease, renal disease, and autoimmune disease, were excluded. Additionally, patients with head, neck or other injuries preventing proper positioning or unable to maintain retinal fixation on a specified target (owing to poor cooperation) were excluded.

Body weight was measured the children having fasted for 12 h, having emptied the bladder, and standing in light clothing and without shoes and height was measured using a standard scale weighing machine. Height and weight were measured to the nearest 0.1 cm and 0.1 kg, respectively. BMI was calculated as weight divided by height squared (in kg/m2). We collected detailed information regarding previous height and weight through medical records for each child.

Ophthalmological examinations were performed, including distance visual acuity (VA), refraction diopter after cycloplegia, axial length (AL) measurements, OCTA (AngioVue; Optovue, Inc., Fremont, California USA), etc.

Distance VA was measured with the Early Treatment Diabetic Retinopathy Study (ETDRS) visual acuity chart (Precision, Vision, LaSalle, IL, USA) at a distance of 4 m. BCVA was recorded with full correction under cycloplegia. The refractive status of each participant was measured after cycloplegia using table-mounted autorefraction (Cannon R-F10, Tokyo, Japan). AL measurement was performed with an IOL Master (Carl Zeiss Meditec, Jena, Germany; V5.5.0.0062).

OCTA was used to capture the OCTA scanning for microvascular in the macular retina and choriocapillaris (CC). The AngioVue OCTA has an A-scan rate of 70,000 scans per second, and 2 successive B-scans were taken at the same location.

Each imaging cube consisted of 2 repeated volumes (304 B-scans × 304 A-scans). 6 mm × 6 mm macular scans centered on the fovea were performed. Retinal layers were automatically segmented by the AngioVue software. Superficial vascular complex (SVC) extends from the internal limiting membrane to 10 μm above the lower boundary of the inner plexiform layer (IPL). Deep vascular complex (DVC) is measured from 10 μm above the lower boundary of the inner plexiform layer (IPL) to 10 μm below the lower boundary of the outer plexiform layer (OPL). The vessel density (VD) was defined as the proportion of vessel area showing blood flow relative to the total image area. Following the ETDRS, the images were made in the nine subfields: foveal, parafoveal and perifoveal quadrants (superior, inferior, nasal and temporal) [15]. The AngioVue software automatically analyzed the VD and retinal thickness in each quadrant.

As described before [16, 17], we imported the OCTA images into MATLAB (MathWorks, Inc., Natick, Massachusetts) and segmented the choriocapillaris slabs (31 μm to 59 μm below the retinal pigment epithelium). Then the retinal vessel projection artifacts were removed from the CC. The area of CC flow voids was defined as a percentage between the region absent from flow and the total scanned region. Finally, the binarized image of CC was imported into Image J software (National Institutes of Health, Bethesda, MD) for calculating the size and number of flow voids. The subfoveal choroidal thickness (SFCT) extends from the outer border of the retinal pigment epithelium to the inner border of the sclera running through the center of the fovea. And it was measured by a trained examiner using a manual caliper of the device software. All scans were performed in dim light by the same technician trained in the use of the equipment. Poor-quality scans with a signal strength lower than 7 were excluded from the analysis.

Retinal parameters (retinal thickness, VD and foveal avascular zone), SFCT and CC flow void from right eyes were included for statistical analysis. The Kolmogorov–Smirnov test was used to evaluate the normal distribution. According to normality tests, either independent samples t-test or Mann–Whitney U-test was used to compare differences between obese children and control. The Chi-square test was used to compare percentages between groups. All statistical tests were performed using the Statistical Package for the Social Sciences (SPSS) statistical software (V.13.0, IBM, USA), and two-sided P < 0.05 was considered statistically significant. Regarding refraction, the spherical equivalent was calculated as a spherical diopter plus half of the diopter of cylindrical power using cycloplegic refraction.