Abstract
Purpose: :
Retinal nerve fiber layer (RNFL) consists of axons of retinal ganglion cells. Because glaucoma damages these axons, RNFL thickness, such as measured with OCT, is often used in clinic assessment of this damage. RNFL reflectance arises from light scattering by cylindrical structures in axons. We have shown that glaucomatous damage alters these cylindrical structures before changes of RNFL thickness and, therefore, we hypothesize that reflectance also will change. To test this hypothesis, RNFL reflectance was studied in both normal and glaucomatous retinas.
Methods: :
A rat model of glaucoma was used. High intraocular pressure (IOP) was induced unilaterally in Wistar rats by laser photocoagulation of trabecular meshwork. To measure reflectance of RNFL, a retina isolated free from the pigment epithelium was measured by means of imaging microreflectometry. The RNFL reflectance was measured at wavelengths of 400 - 830 nm across the retina. After reflectance measurements, the retina was stained with antibodies to label the axonal cytoskeleton; RNFL thickness was measured by confocal fluorescence imaging. Reflectance relative to a diffuse white surface was calculated for bundle areas located at a radius of approximately 500 µm from the optic disc center. Because RNFL reflectance is very directional and wavelength dependent, bundles with peak reflectance and similar scattering geometry were selected and the mean reflectance at 500 - 580 µm was used for comparison. Corresponding thickness of the bundles was measured from the confocal images. For glaucomatous eyes, only those retinas with no apparent structural damage were used in the study.
Results: :
Twenty-seven bundles of nine control retinas and fifteen bundles of six treated retinas were examined. The thickness of both groups ranged from 7 µm to 26 µm with mean thicknesses of 14 ± 4 µm and 14 ± 6 µm for the control and treated, respectively. The reflectance of the RNFL in treated retinas (0.13 ± 0.08 %), however, was significantly lower than control (0.18 ± 0.06%) (p = 0.04). Reflectance per thickness was also lower in the treated group (0.009 ± 0.004 %/µm), compared with control (0.013 ± 0.005 %/µm) (p = 0.02).
Conclusions: :
Elevation of IOP causes decrease of RNFL reflectance that precedes change of RNFL thickness. The result suggests that decrease of RNFL reflectance is an early sign of glaucomatous damage.
Keywords: nerve fiber layer • optical properties • imaging/image analysis: non-clinical