Abstract
Recent studies reveal that measurements of retinal nerve fiber layer (RNFL) reflectance provide more sensitive detection of glaucomatous damage than RNFL thickness, but most do not consider directional reflectance of the RNFL, an important source of variability. This study quantitatively compared RNFL directional reflectance, represented by an angular spread function (ASF), measured at different scattering angles, different wavelengths and different distances from the optic nerve head (ONH) and for bundles with different thicknesses (T). An ASF was characterized by its amplitude (A) and width (W). Internal reflectance of a bundle was expressed as A/T. The study found that A varied significantly with scattering angle and wavelength and that A/T was different among bundles but constant along the same bundle, indicating that the internal structure of axons may vary among bundles but does not change with distance. This study also found that W was larger near the ONH and at longer wavelengths, but did not depend on scattering angle or T. Because a 4.3° change in incident angle can change reflected intensity by a factor of 2.7, accounting for directional reflectance should improve the accuracy and reproducibility of RNFL reflectance measurements.
Highlights
The retinal nerve fiber layer (RNFL), lying just under the retinal surface, consists of the axons of retinal ganglion cells
Optical Coherence Tomography (OCT), which is widely used in clinical diagnosis of glaucoma, measures RNFL thickness by identifying the RNFL as a highly reflective layer under the retinal surface, with change of thickness used as a biomarker for axonal damage [1,2,3,4]
Recent studies of glaucomatous retinas reveal that a decrease of RNFL reflectance occurs prior to thinning of the RNFL [5,6,7,8,9,10,11]
Summary
The retinal nerve fiber layer (RNFL), lying just under the retinal surface, consists of the axons of retinal ganglion cells. Optical Coherence Tomography (OCT), which is widely used in clinical diagnosis of glaucoma, measures RNFL thickness by identifying the RNFL as a highly reflective layer under the retinal surface, with change of thickness used as a biomarker for axonal damage [1,2,3,4]. It seems likely that RNFL reflectance depends directly on the ultrastructure of the ganglion cell axons. Supporting this idea, depolymerization of axonal microtubules (MTs) decreases RNFL reflectance [21,22,23]. Our recent in vitro study in hypertensive retinas shows that change of RNFL reflectance associates with axonal cytoskeletal alteration [7]. Because RNFL reflectance appears to be more sensitive to glaucomatous damage than change of RNFL thickness, direct measurements of RNFL reflectance hold promise as a method for the early detection of glaucoma
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