The position of the ONH relative to the fovea, measured by distance and angle, can be captured clinically using readily available imaging devices. In this study, we used an optical coherence tomographer (Spectralis; Heidelberg Engineering GmbH, Heidelberg, Germany). Following RNFL circle scans, the instrument provides the angle (degrees) between the fixation point and the center of the user-placed scan circle around the ONH, relative to horizontal. The distance (μm) between these two locations can then be measured using the caliper tool in the instrument software provided.
To determine the precision of fovea–ONH distance and angle measurements made this way, we studied one eye each of 10 healthy, young adults who were imaged with the RNFL circle scan protocol of the optical coherence tomographer 10 times in a single session. Participants were aged 23 to 43 years, had Snellen visual acuity 6/6 or better with spectacle correction, refractive error less than 5.00 diopters (D) spherical equivalent and 1.50 D cylinder, clear ocular media, and no current or previous ocular disease. In between image captures, the participants moved away from the instrument and the instrument was repositioned into a random position in order to simulate the clinical scenario, whereby patients are imaged once only. There are three main sources of variation in these measurements: variations in the positioning of the scan circle by the operator, variations in head position by the subject, and variations in fixation that affect the accuracy of the registration with the fovea. The same experienced operator captured all the images, and took care to center the scan circle on the ONH as accurately as possible. The operator also monitored image quality, ensuring that the captured fundus image was well focused and evenly illuminated, and the optical coherence tomography (OCT) image quality was greater than 20 dB. Any poor images were recaptured. Subjects were asked to position their head straight on the chin/forehead rest as would be done in clinic, and no further adjustment was made. Registration of previous images was not used. Fixation variation was minimized by recapturing images where there was a clear misregistration between the fixation location and the fovea in the infrared retinal image. Within-subject variability in fovea–ONH distance and angle measurements was assessed for use as input to the structure–function model (see the Structure–Function Mapping Simulations section). Ethics approval for collection of these data was given by the University of Melbourne Human Research ethics committee (Melbourne, Australia). The study protocol adhered to the tenets of the Declaration of Helsinki, and all subjects provided written informed consent to participate.