Variation of peripapillary retinal nerve fiber layer birefringence in normal human subjects

PURPOSE. The retinal nerve fiber layer (RNFL) exhibits linear birefringence due to the oriented cylindrical structure of ganglion cell axons. The birefringence (Deltan) depends on the density and composition of axonal organelles. The purpose of this study was to evaluate the distribution of birefringence around the optic nerve head (ONH) in normal subjects. METHODS. Birefringence was calculated along circular scan paths around the ONH as Deltan = R/T, where R is RNFL retardance measured by scanning laser polarimetry (SLP) and T is RNFL thickness measured by optical coherence tomography (OCT). OCT scans on a 3.4 mm diameter circle were obtained from 26 normal subjects aged 18 to 53 years. Scans on circles with various diameters were obtained from 17 of these subjects. RESULTS. The average reproducibility of Deltan measured on three separate days in four subjects was +/- 0.05 nm/mum. In most subjects Deltan varied significantly along a circular path around the ONH, with maxima in superior and inferior bundles, minima temporally and nasally, and a mean of 0.32 +/- 0.03 nm/mum. Deltan profiles on circles of different diameter were similar, suggesting that Deltan did not vary along nerve fiber bundles. CONCLUSIONS. RNFL birefringence varies with position around the ONH. This variation may result from known structural differences among nerve fiber bundles that serve different retinal regions. Constant Deltan along bundles is consistent with this hypothesis. Measurements of RNFL birefringence may provide a means to detect early subcellular changes in glaucoma.