Purpose: To provide a quantitative comparison of retardation values obtained with the scanning laser polarimeter (Nerve Fiber Layer Analyzer; Laser Dignostic Technologies, San Diego, California) in normal subjects, with the known histologic properties of the human retinal nerve fiber layer.
Methods: Scanning laser polarimetry was performed on 48 normal subjects. The retardation values obtained from each eye were processed to remove reflections from the optic nerve head vasculature and to derive peripapillary polar retardation profiles. The location of the maximum and minimum retardation values superior and inferior to the disk was determined. Retardation values were also determined along lines running radially from the center of the optic disk to through the points of maximal retardation.
Results: Retardation values were greater in the superior and inferior quadrants of the peripapillary retinal nerve fiber layer compared with the nasal and temporal quadrants. Superiorly, peak retardation values were nasal to the vertical meridian by 22.9 degrees (SD, 17.5) in right eyes and by 15.7 degrees (SD, 17.7) in left eyes. Inferiorly, the peak showed a closer correspondence with the vertical meridian, lying nasal to this by 0.8 degrees (SD, 14.9) in right eyes and temporal to this by 2.8 degrees (SD, 15.4) in left eyes. Radial analysis showed that the peak retardation occurred approximately (0.5 to 0.7 disk diameters from the optic disk margin) at the superior and inferior aspect of the disk. The degree of modulation of retardation around the optic disk was less than would be expected from the variation in peripapillary nerve fiber layer thickness seen histologically.
Conclusion: The location of the peak retardation values shows reasonable agreement with the angular location of the peak values of nerve fiber layer thickness published for humans. By contrast, the modulation of retardation values around the disk and the change with eccentricity from the optic disk center differ from the anatomic data. The Nerve Fiber Layer Analyzer appears to measure a correlate of nerve fiber layer thickness, but it is important that any discrepancies between the retardation and anatomic data are recognized in the clinical interpretation of polarimetric data.