Distance From the Foveal Center: A Method for the Calculation of Eccentric Fixation

The purpose of this study was to describe a method to determine the position of the preferred retinal locus (PRL).

Cross-sectional data were obtained prospectively. Microperimetry was completed with the Macular Integrity Assessment (MAIA) under mesopic testing conditions. Spectral domain optical coherence tomography (SD-OCT) was acquired with the Spectralis SD-OCT system. The printout of the MAIA containing the PRL and both the horizontal and vertical transfoveal scans were interpolated in Adobe Photoshop, which was used to calculate the foveal center (FC) and to create a custom ruler to measure the distance from the foveal center (DFC) and the distance between PRL (DPRL). The methodology was tested by two independent graders in participants of a natural history study for KCNV2-associated retinopathy.

Twenty-two eyes of 12 subjects were analyzed at a mean age of 31.9 years (range = 11-54 years, SD = ±14.3). The mean DFC and DPRL was 1398 µm (range = 182.8-2896, SD = ±755.4), and 751.4 µm (range = 144.5-1493.3, SD = ±458.3) in the right eyes, and 1104 µm (range = 341.8-2513, SD = ±653.78) and 742.5 µm (range = 120-1918, SD = ±586.5) in the left eyes, respectively. There was no significant interocular correlation of DFC (r = -0.036, P = 0.92) or DPRL (r = 0.41, P = 0.26), or between the two variables in the right (r = 0.519, P = 0.084) and left eyes (r = 0.014, P = 0.97). These suggest that DPRL may not be related to the location of eccentric fixation and that the values of either eye are independent of each other.

Our data suggest that these are reliable parameters, which could be of relevance in the context of clinical trials. The sample size is small and its correlation with other functional and structural outcomes remains to be explored, but these findings provide a framework for further development.

This work bridges the distance between basic science and clinical care by providing a reliable and replicable method to quantify the preferred retinal locus of patients with eccentric fixation.