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Xiang-Run Huang, Ye Z Spector, XiaoPeng Zhao, Qi Zhao; Birefringence of Retinal Nerve Fiber Layer: Correlation with Degrees of Glaucomatous Damage. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4774. doi: https://doi.org/.
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The retinal nerve fiber layer (RNFL) exhibits linear birefringence due to the oriented cylindrical structure of retinal ganglion cell axons. Glaucoma damages axonal ultrastructure. Retardance of polarized light by RNFL birefringence has been measured by optical methods to assess the RNFL for clinical diagnosis of glaucoma. The purpose of this study was to investigate the relationships between changes of RNFL retardance, birefringence and axonal ultrastructure of retinas with early glaucomatous damage.
Both normal and glaucomatous retinas were studied. Experimental glaucoma in rats was induced unilaterally by laser photocoagulation of trabecular meshwork. To measure retardance, retinas were isolated free from the pigment epithelium. Images were taken in transmission at 500 nm by means of imaging polarimetry. The Stokes vectors of the exiting light beam from the retinas were derived from the images. Retardance of nerve fiber bundles was calculated from the Stokes vectors. After retardance measurements, retinas were processed for immunohistological staining of axonal cytoskeleton, F-actin, microtubules (MTs) and neurofilaments (NFs), and then imaged by confocal laser scanning microscopy. Confocal images provided examination of axonal distribution across the retina and measurement of RNFL thickness. Birefringence was calculated as the ratio of retardance and thickness. Degrees of damage were assessed from the appearance of cytoskeletal staining. Grade 1 had normal looking bundles, Grades 2, 3, and 4 showed different degrees of F-actin change and Grade 5 had mild distortion of all cytoskeletal components within bundles.
RNFL retardance and birefringence in normal rats varied along a circular path around the ONH. Birefringence did not change along bundles, just as in human eyes. In glaucomatous eyes with Grades 1 - 4 damage, RNFL thickness and retardance did not change significantly compared with the normal retinas (p > 0.4). However, retinas with Grades 3 - 5 damage showed significant decrease of birefringence (p< 0.001, Fig.).
Glaucomatous damage causes decrease of RNFL birefringence. Change of RNFL birefringence is associated with distortion of axonal cytoskeleton. RNFL birefringence may provide a useful and early means to monitor subcellular damage in glaucoma.
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