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R.W. Knighton, D.R. Anderson, G. Gregori, J.E. Legarreta, R. Lee, S. Gojraty; Structure of the Optic Nerve Head as Assessed by Spectral–Domain Optical Coherence Tomography . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3345.
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© ARVO (1962-2015); The Authors (2016-present)
To understand the anatomic features seen in optical coherence tomography (OCT) images of the optic nerve head. Particular emphasis was directed toward the peripapillary tissue layers through which axons exit the eye (disc margin).
Spectral–domain OCT (6–8 µm axial resolution) was used to obtain three–dimensional (3–D) data sets of a 6×6 mm region centered on the optic nerve head in normal subjects and patients with glaucoma. A custom image viewer produced a near–infrared (NIR) fundus image and cross–sectional OCT images (B–scans) from the 3–D data. The vessel pattern in the fundus image allowed alignment of the OCT data with color stereoscopic optic disc photographs. Corresponding peripapillary and papillary features seen in the B–scans and disc photographs were compared in the context of known optic nerve head anatomy.
In the simplest disc configuration the retinal layers, the retinal pigment epithelium (RPE), and the choroid all ended together and axons curved over the disc border thus formed to exit the eye. More interesting, however, were eyes with disc crescents or halos, which occurred when the RPE ended before the choroid and sclera. The crescent or halo appeared where deeper layers emerged from the shadow of the RPE. Often an extension of the line formed by the RPE layer, perhaps Bruch’s membrane, continued toward the disc margin, but did not cast a noticeable shadow. A clinically–observed "scleral lip" appeared in B–scans as a thick, hyper–reflective region that extended from the edge of the choroid toward the disc margin. The NIR and visible light fundus images provided different, complementary views of crescents and halos. Exposed sclera appeared bright in both, but choroidal melanin was more obvious in visible light.
In most discs the axons of the RNFL, which reflect light when perpendicular to the axis of view, became dim or disappeared as they entered the disc and turned parallel to the axis of view. In tilted discs, however, axons remained visible as they entered the sloping edge of the oblique optic nerve exit canal. In addition, the scleral wall of the exit canal could be seen under the axon bundles.
This type of correlative analysis will lead to a better understanding of the normal variations in disc anatomy and how it changes with glaucomatous damage. In particular, OCT may help in defining the "disc margin" as the narrowest passage through which axons must pass and thus aid in quantifying the number of remaining axons.
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