The population-based, cross-sectional Beijing Eye Study 2011 was carried out in 5 communities in the urban district of Haidian in the North of Central Beijing and in 3 communities in the village area of Yufa of the Daxing District in the South of Beijing.16,17 The only eligibility criteria for inclusion into the study were an age of 50+ years and living in the study region18. The study population consisted of 3468 (78.8%) individuals out of 4403 eligible individuals. The mean age was 64.6±9.8 years (median, 64 years; range, 50-93 years). Among the study participants, 1633 individuals (47.1%; 943 [57.7%] women) were from the rural region. The Medical Ethics Committee of the Beijing Tongren Hospital approved the study protocol, and all participants gave informed written consent.
Criteria for inclusion into the present study were the absence of systematic diseases such as arterial hypertension, hyperlipidemia, diabetes mellitus, manifest cardiovascular diseases, previous myocardial infarction, cerebral trauma and stroke, and the absence of ocular diseases such as glaucoma, diabetic retinopathy, status after cataract surgery, ocular trauma, retinal vascular occlusions, age-related macular degeneration, pigment epithelium detachment, retinal detachment, polypoidal choroidal vasculopathy and central serous chorioretinopathy. Myopia and incipient cataract not affecting the quality of the OCT images were no reason for exclusion of the individual.
The study participants underwent a series of examinations starting with an interview with standardized questions on the family status, level of education, income, depression, known major systemic diseases and quality of vision. The examination also included blood tests to measure the fasting blood concentrations of lipids, glucose and glycosylated hemoglobin A1c. Blood pressure, body height and weight and the circumference of the waist and hip were recorded.
The ophthalmic examination included measurement of best-correcting refractive error, pneumatonometry, slit-lamp examination of the anterior segment and biometry of the right eye using optical low-coherence reflectometry (Lenstar 900 Optical Biometer, Haag-Streit, 3098 Koeniz, Switzerland). A slit-lamp examination performed by an ophthalmologist assessed lid abnormalities, corneal disorders, and peripheral anterior chamber depth. After medical dilatation of the pupil, photographs of the cornea and lens (slit-lamp digital photography, camera type BG-4, Topcon Medical Systems, Inc, Tokyo, Japan), and of the macula and optic disc (fundus camera type CR6-45NM, Canon Inc, Tokyo, Japan) were taken.
Retinal imaging was performed with spectral-domain OCT (SD-OCT) (Spectralis®, wavelength: 870nm; Heidelberg Engineering Co, Heidelberg, Germany) measuring a macular volume scan (25×30° field, 25 B-scans). Segmentation of the retinal layers was performed automatically in each of the horizontal scans (Segmentation Technology; Heidelberg Engineering, Inc., Heidelberg, Germany). We used the 9 segmentation lines: 1= inner limiting membrane; 2= posterior border of the retinal nerve fiber layer (RNFL); 3= posterior border of the retinal ganglion cell layer; 4= posterior border of the inner plexiform layer; 5= posterior border of the inner nuclear layer; 6= posterior border of the outer plexiform layer; 7= outer limiting membrane; 8= retinal pigment epithelium; 9= Bruch’s membrane. Using these nine segmentation lines, we measured the thickness of the retinal layers located between neighboring segmentation lines (Fig.1). These layers were the retinal nerve fiber layer (RNFL), ganglion cell layer (GCL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer (ONL), retinal pigment epithelium (RPE), the inner retinal layer (IRL) as region between the inner limiting membrane and the outer limiting membrane, and the photoreceptor layer as the region between the outer limiting membrane and Bruch´s membrane. The automatically drawn segmentation lines were checked, and if needed interactively corrected by a trained examiner (QW).
Using the Early Treatment of Diabetic Retinopathy Study (ETDRS) map, we measured the thickness of the retinal layers in 9 regions (Fig. 2). The macular area was divided into three concentric rings measuring 1 mm, 3 mm and 6 mm in diameter and which were centered on the fovea. The two outer rings with a diameter of 3 mm and 6 mm, respectively, were further divided into 4 equal regions (superior, inferior, nasal and temporal). The innermost ring with a diameter of 1 mm included the fovea (central subfield), while the 3 mm inner ring included the parafovea and 6 mm outer ring included the perifovea. The subfoveal choroidal thickness was additionally measured using the enhanced depth imaging modality of the OCT device. Only the right eye of each study participant was assessed. To assess the reproducibility of the measurement, we randomly selected 30 eyes of 30 participants and each parameter was measured three times in a masked manner with intervals of 2 weeks.
Statistical analysis was performed using a commercially available statistical software package (SPSS for Windows, V. 25.0, IBM-SPSS, Chicago, IL). We first calculated the mean and standard deviation of the main outcome parameters. i.e. the thickness of the retinal layers in the fovea, parafovea and perifovea areas. Secondly, we compared the thickness of the 10 retinal layers in each sector using the Student t-test for paired samples. We assessed associations between the main outcome parameters and the other ocular or systemic variables in a univariate analysis. Finally, we carried out a multivariable analysis, with the retinal thickness parameters as dependent variable and with all those parameters as independent variables, which were significantly (P<0.05) associated with the retinal thickness parameters in the univariate analysis. We then dropped step by step those variables from the list of independent parameters, which either showed a high collinearity or which were no longer significantly associated with the outcome parameters. We additionally included the parameters of age, gender and axial length into the list of independent variables into the multivariate analysis, independently whether or not they were associated with the outcome parameters in the univariate analysis. The reason was that age, gender and axial length were associated with the retinal thickness in some previous studies.19 Since the inner retinal layers were mostly absent in the fovea, only the thickness of the outer retinal layers was analyzed in the central subfield. We presented the standardized regression coefficient beta and the 95% confidence intervals (CIs) of the non-standardized regression coefficient B. To assess the reproducibility of the measurement, the intra-class correlation coefficient (ICC) and coefficient of variation were calculated. All P-values were two-sided and were considered statistically significant when the values were <0.05.