This prospective cross-sectional research was in accordance with the principles of the Declaration of Helsinki and was approved by the local ethics committee. Signed consent forms were obtained from the subjects. Only one eye of each participant was included according to the criteria described below. If both eyes met the criteria, a random eye was selected.
Patients with myopia less than -6.00 D and astigmatism less than -2.00 D were included. Patients with an axial length over 26 mm, irregular corneal astigmatism, ocular surface inflammation, previous ocular intervention, history or signs of glaucoma or ocular hypertension, optic nerve disease or neurodegenerative disease, and media opacity affecting OCT image quality were excluded from the study.
All patients underwent a full ophthalmologic evaluation including best-corrected visual acuity measurement, automated kerato-refractometer (Topcon Co., Tokyo, Japan), biomicroscopy, intraocular pressure measurement, and fundoscopy. The CCT measurement was performed with an optical biometry device (Lenstar LS 900, Hagg-Streit AG, Koeniz, Switzerland)). Based on the evidence from the Ocular Hypertension Treatment Study (OHTS), the participants were divided into 2 groups; CCT over 555 µm (Group 1) and below 555 µm (Group 2) .
Heidelberg spectralis OCT device (software version 22.214.171.124, Heidelberg Engineering Inc., Germany) was used for the GCC and RNFL measurements in the patient groups. Before capturing the images, keratorefractive values of the subjects were entered into the software of the OCT device to estimate optical magnification. Heidelberg spectral domain-OCT applies an automatic modification process to reverse the ocular magnification effect, developing individual scan lengths based on 3 parameters (refraction, keratometry and axial length). All measurements were made by an observer masked to the study groups. The OCT images were recorded under dim light conditions between 9:00 am and 12:00 pm in the same room.
A scan circle with a diameter of 3.45 mm was centered at the optic disc. Nine B-scan images were captured and automatically averaged. More attention was paid to obtain good-quality scans with focused images, proper adjustment of the disc margins, and signal strength ≥20. The RNFL thickness parameters measured were; average RNFL thickness (a-RNFL thickness), superior-temporal (ST), temporal (T), inferior-temporal (IT), inferior-nasal (IN), nasal (N), and superior-nasal (SN) quadrant RNFL thicknesses (Figure 1).
The GCC protocol was used to measure the macular GCC thickness from the inner limiting membrane (ILM) to the inner plexiform layer (IPL). Layer segmentation was executed automatically using the new software for the Spectralis OCT, and it was checked to be adequate in the 61 B-scans of each imaged eye using the criteria of Ishikawa et al. (Figure 1).
The statistical analyses were done using SPSS (Statistical Package for Social Sciences; version 15.0). A Shapiro-Wilk test was used to detect normal distribution. The differences between the groups were evaluated by Student T and Mann-Whitney U tests. Spearman correlation coefficients for the results were calculated. The data adjusted for age and corrected means were estimated with standard error and 95% confidence interval. Multiple linear regression models were created to evaluate the relationship among the RNFL, GCC, GC-IPL thicknesses and age, CCT, axial length (AL), and spherical refraction values. "Enter method" was used as the variable selection method. The standardized and non-standardized regression coefficients of the models were presented together with the p values, and the risk factors affecting these variables were investigated. The statistical significance was set at a level of 5% (p<0.05).