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M.V. Boland, R.H. Kardon; Changes in Nerve Fiber Layer Reflectivity using Optical Coherence Tomography in the Setting of Optic Neuropathy . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1605.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: It has been observed in some patients with severe visual field loss from glaucoma and other optic neuropathies that the optical coherence tomography (OCT) estimate of the retinal nerve fiber layer is thicker than expected. Patients with acute optic neuropathy have been followed over time and showed initial thinning of the retinal nerve fiber layer, followed by some degree of re–thickening. The purpose of this work is to determine whether there are features of retinal OCT that can be used to differentiate a healthy nerve fiber layer from the damaged and paradoxically thickened ones described above. Methods: Four patients with various optic neuropathies and subsequent nerve fiber layer thinning and re–thickening on longitudinal OCT data were identified. All OCT data were collected using a Zeiss Humphrey OCT2 and each patient had at least 12 months of follow up. Scans of the retina adjacent to the optic nerve in both the healthy and diseased eye were processed using third party software (OCTPro 2k1) to identify the nerve fiber layer. The raw reflectivity data from the nerve fiber layer were then exported and analyzed using simple statistics (e.g., mean, standard deviation) and histograms. Results: The mean reflectivity of acutely damaged nerve fiber layers decreased over time and then remained low even as the layer re–thickened. Histogram analysis of reflectivity data are consistent with a decrease in the average reflectivity and do not demonstrate a multi–modal distribution of reflectivity, at least for nerves with severe damage. Conclusions: The reflectivity of the nerve fiber layer is an important feature that is not currently used in analysis of OCT data. Furthermore, thickness of the nerve fiber layer alone is not a perfect predictor of visual field loss. We hypothesize that one reason for this discrepancy is the re–thickening phenomenon mentioned above. We also believe that the re–thickened nerve fiber layer may contain gliotic tissue that has replaced damaged nerve fibers. The observed change in the reflectivity of the nerve fiber layer, even after re–thickening, is consistent with the presence of non–nerve tissue. Finally, given the changes we have observed in the reflectivity of the nerve fiber layer, it may be possible to distinguish normal from abnormal tissue and better correlate thickness with function.
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