An optical model of the human retinal nerve fiber layer: implications of directional reflectance for variability of clinical measurements.

PURPOSE: The reflectance of the retinal nerve fiber layer is highly directional--that is, it depends strongly on the angles of illumination and viewing. This study explored and illustrated the implications of this directional reflectance for nerve fiber layer measurements in the human eye.
METHODS: The retina was modeled as a sphere centered on the optic axis of a schematic eye. Nerve fiber ribbons were projected onto the retina and cylindrical light scattering was calculated along each ribbon. The reflectance along the ribbon was then determined for the illuminating and viewing apertures of two hypothetical optical instruments, a fundus camera and a confocal scanning laser ophthalmoscope. Results were displayed as reflectance maps.
RESULTS: Uniformly illuminated nerve fiber ribbons exhibited a nonuniform reflectance pattern that was very sensitive to the location in the pupil of the instrument apertures. Ribbon reflectance at the superior and inferior disc margins varied with ribbon orientation, being higher with temporal tilt and lower with nasal tilt. Ribbons nasal to the disk could be quite dim.
CONCLUSIONS: In quantitative nerve fiber layer assessment technologies, the observed reflectance depends on the configuration of the illuminating and viewing apertures of the measuring instrument and on the retinal position and orientation of each nerve fiber bundle. In clinical practice, this dependence may cause significant measurement variability that can be reduced by specific measurement maneuvers.