A curved demarcation line visible with varying degrees of contrast on SW-AF inferior to the optic disc was observed in 31 out of 32 subjects (
Figs. 1,
2). The open side of the curved demarcation line was always directed toward the nasal side.
The fundus location of the demarcation line was similar for all eyes (
Fig. 3). Accuracy of this data was somewhat compromised by the fact that we did not monitor for head tilt during imaging (see Methods). The locations of the line between eyes of the same subject were symmetrical, although the shape of the line varied slightly; at 20° below the center of the disc the line was within ±2° horizontally (after vertically turning the images of left eyes and aligning the lines between the fovea and the disc center). The line could not be detected at distances shorter than 10° from the disc center, as its course was often masked by large vessels and by nerve fibers converging near the disc. For distances larger than 25° from the disc center, the line could be traced in 55° SW-AF images to approximately 35° inferiorly (data not shown). The position of the demarcation was always nasal to the radial area of thickest nerve fiber layer (NFL) as judged by a slight darkening in the SW-AF images (
Fig. 1; asterisk).
For the subgroup of 12 eyes that had other types of imaging, the demarcation line that was seen on SW-AF images in all eyes was observed at the same retinal location by the three observers for 10/12 eyes on 488-R images and for 6/12 eyes on color images. Interobserver agreement was substantial for 488-R images (kappa = 0.77,
P < 0.0001) and moderate for the color images (kappa = 0.52,
P = 0.0009).
11 The demarcation line was never detected on NIR-R or on NIR-AF images. In SD-OCT images, there were no apparent features that differentiated structures at or on either side of the demarcating line seen on SW-AF. The choroid on the nasal side was thinner, but the transition was more gradual, occurring over approximately 5 to 15°.
The nasal-temporal ratio of AF relative to the demarcation line was between 0.75 (highest contrast) and 1.0 (line not visible), the median ratios decreasing with increasing distance from the disc center (
Fig. 4). The ratios at 15, 20, and 25° from the disc center correlated significantly with each other (Spearman rho >= 0.36,
P < 0.04). The mean ratios for those three locations were not significantly different for left and right eyes of the same subject (Wilcoxon paired test,
Z = 0.6,
P = 0.5) but correlated significantly with each other (Spearman rho = 0.67,
P = 0.003). The ratios (averaged between eyes when both eyes were tested) were not significantly correlated with age (Spearman rho = 0.13,
P = 0.4) and refractive error (Spearman rho = −0.2,
P = 0.2). No significant differences were found between sexes (Mann-Whitney,
Z = 0.30,
P = 0.7), between whites and nonwhites (
Z = 0.9,
P = 0.3), or between whites with brown and blue iris (
Z = 0.7,
P = 0.4).
The median nasal-temporal ratio at the three locations for the initial group, 0.87 (interquartile range [IQR] = 0.04), was significantly lower than that for all other images, 0.91 (IQR = 0.05) (Z = 2.2, P = 0.02). Thus, the higher contrast of the demarcation line observed for this initial group may reflect the selection process used for that group. However, this did not affect the above statistical outcomes in regard to the correlation between eyes and correlations with age, sex, refraction, and iris color.
For the subgroup of 12 subjects, the nasal-temporal ratio of 488-R was significantly higher (lower contrast) than the SW-AF ratio at the same location (Wilcoxon paired test, Z > 2.5, P < 0.009 at 15 and 20° and Z = 1.7, P = 0.08 at 25°). The nasal-to-temporal ratio of 488-R correlated with the ratio of SW-AF when only the nine subjects with dark irides were considered (minimizing choroidal contributions, Spearman rho = 0.78, P = 0.01), but not for all 12 subjects (Spearman rho = 0.39, P = 0.2).