Comparison of clinical and spectral domain optical coherence tomography optic disc margin anatomy.

An ability to identify, understand and appreciate the morphological changes occurring at the optic nerve head (ONH) is vital in the assessment of patients with, or at risk of, glaucoma. The ONH is a complex three-dimensional (3-D) structure and interpretation of its underlying anatomy can be a challenge for clinicians. The optic disc margin is a fundamental clinical landmark in the assessment of the ONH as it marks the peripheral boundary of neural tissue within the disc. As such the disc margin is an essential component of both the assessment of neuroretinal rim loss and of disc size. The ring of Elschnig is widely accepted as the structure underpinning the fundoscopic appearance of the optic disc margin in human eyes.1 The ring of Elschnig comprises the Border Tissue of Elschnig (hereby referred to as Border Tissue) which is collagenous tissue, arising from the sclera to join Bruch’s Membrane, thereby enclosing the choroid.2 It is this scleral ‘cuff’ or ‘lip’ between the ONH and the choroid which has been thought to account for the white ‘rim’ perceived fundoscopically as the disc margin in the human eye.3, 4 The appearance of the disc margin is therefore that of a whitish halo (if present for 360°) or crescent (if present for less than 360°) at the edge of the disc. However, these landmarks may not be detectable in some regions or not at all in certain discs. In this context, the subjective location of the disc margin can be highly variable. In a series of recent publications we have proposed three important concepts that are necessary to understand optic disc margin anatomy within 3-D histomorphometric and spectral domain optical coherence tomography (SD-OCT) reconstructions of the monkey ONH.5-7 The first concept is that of a ‘neural canal opening’ (NCO) (Figure 1A - arrows), which is the anatomic opening in Bruch’s Membrane (also referred to as Bruch’s Membrane Opening – BMO) through which retinal ganglion cell axons must pass to enter the choroidal and scleral portions of the neural canal.6-8 The second concept is that of a ‘neural canal’ which is the axonal pathway through the eye wall.5 The neural canal begins at the NCO, and then extends through a choroidal component, bound on either side by the Border Tissue (Figure 1A - arrowheads). Beyond the choroidal component, the neural canal has a scleral component, bound on either side by the internal wall of the sclera. The neural canal terminates at the point at which the optic nerve leaves the globe. The third concept is that what the clinician perceives to be the disc margin is not a single anatomic structure but is instead variable with some portions being the NCO, and others the Border Tissue (Figure 1A - arrowheads) or the anterior scleral canal opening, depending upon the 3-D architecture of these structures.7 Figure 1 Neural canal architecture, demonstrating the neural canal opening (NCO) and Border Tissue within a histologic section (upper) and a co-localized spectral domain OCT B-scan The NCO is an important anatomic landmark because it can be identified within SD-OCT images and has been proposed as the basis for a reference plane for SD-OCT imaging.8 Within our 3-D histomorphometric reconstructions we define the NCO as the termination of Bruch’s Membrane (BMO) because Bruch’s Membrane is within the resolution of our reconstruction technique.6 Within SD-OCT reconstructions we define the NCO as the termination of the Bruch’s Membrane/retinal pigment epithelium (BM/RPE) complex because it is uncertain whether Bruch’s Membrane in isolation can be resolved by SD-OCT.9 We recently co-localized post mortem 3-D ONH histomorphometric reconstructions to stereophotographs acquired in vivo in twenty-eight normal monkey eyes.7 In that study we found that the NCO constituted the disc margin throughout its full extent in the majority of monkey eyes. However in some eyes, Border Tissue was detectable in disc photographs and it co-localized to the disc margin in regions where it demonstrated an ‘externally oblique’ configuration (described below) and where its junction with the anterior scleral surface was internal to the termination of Bruch’s Membrane. From these 3-D histomorphometric reconstructions, we have proposed that there are two principal Border Tissue orientations (both illustrated in Figure 2) and that Border Tissue orientation determines what the clinician identifies as the disc margin. The Border Tissue orientation is most commonly ‘internally oblique’ whereby Border Tissue extends from the sclera into the neural canal to meet Bruch’s Membrane (Figure 2A). Border Tissue is less commonly ‘externally oblique’ whereby Border Tissue extends from the sclera away from the neural canal to meet Bruch’s Membrane (Figure 2B). Where an overhang of Bruch’s Membrane extends beyond the innermost termination of Border Tissue (whether internally or externally oblique), the termination of Bruch’s Membrane (BMO) is detected as the disc margin. Where the innermost termination of an externally oblique Border Tissue is internal to the termination of Bruch’s Membrane, the Border Tissue is visible clinically and comprises the disc margin.7 We now propose that the anatomical basis of the disc margin can also be identified and delineated within SD-OCT volumes. The purpose of this study was to examine whether the NCO and Border Tissue identified in SD-OCT volumes define the optic disc margin in the normal monkey eye. Figure 2 Two principal Border Tissue configurations, their relationship to a pigmented or unpigmented extension of Bruch’s Membrane and the resultant clinical disc margin anatomy