The cross-sectional clinical observational study included normal eyes of individuals who were consecutively examined. The study protocol was approved by the Institutional Review Board and written informed consent was obtained from all participants. All investigations adhered to the tenets of the Declaration of Helsinki. All study participants underwent a complete ophthalmological examination, including refractometry and assessment of visual acuity, slit-lamp assisted biomicroscopy of the anterior and posterior segment of the eye, gonioscopy, applanation tonometry, biometry with measurement of the axial length (IOL Master; Carl Zeiss Meditec, Jena, Germany), color photography of the optic nerve head (CR-DGI; Canon, Inc., Tokyo, Japan), perimetry (Humphrey Visual Field Analyzer; Zeiss, Inc., Oberkochen, Germany), and EDI-OCT imaging of the optic nerve head (Spectralis HRA-OCT; Heidelberg Engineering GmbH, Heidelberg, Germany).
The EDI-OCT scan was obtained using a 24 radial line B-scan centered on the optic disc, each at an angle of 7.5°. In a rectangle of 20° × 20°, we additionally obtained horizontal scans that included the parapapillary region and the optic disc. The number of these automatic real-time repeat scans was set to an average of 23 images. Additional single scans with 100 OCT frames averaged were performed if necessary. As imaged by the OCT, the parapapillary region was differentiated into α zone, β zone and γ zone.
5,6,13 The α zone was defined as presence of Bruch's membrane with irregular RPE, the β zone was characterized by the presence of Bruch's membrane without RPE, and the γ zone was defined by the absence of Bruch's membrane. The peripapillary region around optic disc was divided by clock hours and the location of the γ zone around optic disc was recorded.
On the EDI-OCT images and on the near-infrared reflectance fundus images, which were taken by the OCT device simultaneously with the EDI-OCT images, the borders of the optic disc were marked on the near-infrared reflectance fundus image so that the corresponding line could be seen on the B-scan OCT images, as described previously in detail (
Fig. 1).
6 The optic disc diameter was defined as the length between the two borders marked on Bruch's membrane in eyes without a parapapillary γ zone, that is, in eyes in which the peripapillary ring was covered by Bruch's membrane. In eyes with a γ zone (i.e., eyes in which Bruch's membrane did not extend to the border of the peripapillary ring), the peripapillary ring was defined to be the optic disc border. On the EDI-OCT images and on the near-infrared reflectance fundus images, we measured the horizontal, vertical, minimal, and maximal disc diameters using the built-in measurement tools. The ratio of the maximal-to-minimal disc diameter was calculated as the ovality index.
As described in detail recently, we differentiated between a rotation of the optic disc around the vertical axis (“vertical rotation”), the sagittal axis (“sagittal rotation”), and/or the horizontal axis (“horizontal rotation”).
14 Based on geometry, a rotation around the vertical disc axis leads to a horizontally shortened, or seemingly vertically elongated, image of the optic disc. If measured by a two-dimensional method, the horizontal disc diameter measurements get falsely low, while the vertical disc diameter measurements remain unaffected. Using trigonometrical calculations, a disc rotation around the vertical axis was calculated as the cosine of the ratio of the horizontal disc diameter measured on the two-dimensional reflectance fundus images divided by the horizontal disc diameter as measured on the EDI-OCT images. A disc rotation around the sagittal disc axis, usually moving the superior disc pole into the temporal direction, is not associated with a change in the perspective ophthalmoscopic image of the optic nerve head since the en face view remains unchanged. Since the ophthalmoscopic image is not affected, neither are the measurements of any disc diameter or the disc area. A disc rotation around the sagittal axis was measured on the near-infrared reflectance fundus images by the angle between the maximal disc diameter and the vertical axis. If the optic disc is rotated around the horizontal disc axis, a typically “tilted” disc would occur with a retinal vessel exit at the superior disc pole, a prominent and slightly unsharp disc border in the superior disc region, and often a so-called inferior scleral crescent. As a corollary to discs rotated around the vertical disc axis, discs rotated around the horizontal disc axis show a perspective shortening of the vertical disc diameter, while the image of the horizontal disc diameter remains unaffected. A disc rotation around the horizontal axis was calculated as the cosine of the ratio of the vertical disc diameter measured on the two-dimensional reflectance fundus images divided by the vertical disc diameter as measured on the EDI-OCT images.
The statistical analysis was performed using a commercially available statistical software package (SPSS for Windows, version 22.0; IBM-SPSS, Inc., Chicago, IL, USA). Means and standard, and medians and ranges were presented. The paired sample t-test and Pearson correlation analysis were used to investigate the difference between parameters measured on the EDI-OCT and near-infrared reflectance fundus images. All P values were 2-sided and considered statistically significant when they were less than 0.05.