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X. Huang, R.W. Knighton, R. Vessani, J.M. Liebmann, R. Ritch; Birefringence of Retinal Nerve Fiber Layer in Normal Human Subjects . Invest. Ophthalmol. Vis. Sci. 2003;44(13):3363.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The retinal nerve fiber layer (RNFL) exhibits linear birefringence due to the oriented cylindrical structure of ganglion cell axons. The birefringence value (Δn) depends on the density and/or composition of axonal organelles. Measurements of Δn for the RNFL, therefore, might provide a way to detect subcellular changes in optic nerve diseases such as glaucoma. It is generally assumed that all nerve fiber bundles have the same value of Δn, but this has not been shown. 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 as the ratio of retardation (R) and thickness (T), i.e., Δn = (½R) / T, where the factor 1/2 embodies the assumption that the measuring beam makes a double pass through the RNFL. The RNFL retardation of normal subjects was measured by scanning laser polarimetry (SLP) with a variable corneal compensator (GDx Access VCC). Optical coherence tomography (OCT II) provided T profiles of the RNFL along two circular scan paths around the ONH (diameters 2 and 2.5 times disc size). Each measurement was repeated three times. The scan circles in the OCT fundus image were transferred to the retardation image obtained in SLP by registering the SLP image onto the OCT fundus image. The R profiles were then extracted from the scan circles on the SLP image. Δn profiles were calculated from the corresponding pairs of R and T values. Results: Contrary to expectation, Δn was not constant, but rather varied along a circular path around the ONH. In these normal eyes, Δn was highest in superior and inferior bundles and declined temporally and nasally. The Δn profiles on circles of different diameter were well correlated, suggesting that Δn did not vary along nerve fiber bundles. The mean value for Δn was 0.43 ± 0.02 nm/µm. Conclusions: The apparent variation of Δn around the ONH could be due to underlying structural differences among nerve fiber bundles that serve different retinal regions. The more constant behavior of Δn along bundles is consistent with this hypothesis.
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