Comparison of clinical and three-dimensional histomorphometric optic disc margin anatomy

Abstract

Purpose: To investigate the anatomic basis of the optic disc margin in the normal monkey eye by colocalizing optic disc photographs to three-dimensional (3D) histomorphometric reconstructions of the same optic nerve head.

Methods: Optic disc photographs from 28 normal monkey eyes were overlaid onto 3D central retinal vessel reconstructions generated as part of postmortem optic nerve histomorphometric reconstructions for each eye. Within each reconstruction, the Bruch's membrane opening (BMO) was delineated. Alignment was achieved by matching the clinical vessel outline to the vessel reconstruction with parallel viewing software. An experienced observer viewed stereophotographs and marked the disc margin onto clinical photographs with custom software. Alignment of the delineated disc margin to the histomorphometrically defined BMO was qualitatively assessed within each image.

Results: In 20 eyes, BMO aligned well to the disc margin delineation. In four eyes, alignment improved after repeated colocalization. Careful review of the histomorphometric reconstructions identified that in most cases Bruch's membrane extended beyond the termination of the border tissue of Elschnig, most substantially in the superior and nasal sectors. Misalignments could be explained by inaccurate BMO marking or where Bruch's membrane terminated externally to the inferior edge of the border tissue; this latter structure aligned to the disc margin.

Conclusions: BMO was a clinically detectable entity and represented the disc margin in most eyes in this study. The 3D architecture of the border tissue combined with the presence of an overhang of Bruch's membrane makes an important contribution to disc margin anatomy.

Figures

Figure 1. Method for co-localizing the clinical disc photograph to the three-dimensional vessel reconstruction using parallel viewing software

1. The clinical photograph is viewed prior to co-localization at a distance from the vessel reconstruction

2. X- and Y- axis shifts allow an approximate co-localization in the horizontal plane

3. The magnification of the clinical image has been increased to match the dimensions of the vessel reconstruction

4. The clinical photograph and vessel reconstruction are viewed in the coronal plane, with the BMO point cloud visible (red glyphs). This maneuver is performed to facilitate Z-axis adjustment. In this panel, the clinical photograph can be seen to be tilted in a different orientation to the long axis of the BMO point cloud

5. The Z-axis tilt has been adjusted so that the clinical image (seen in coronal profile, highlighted by arrows) is now orientated in the same plane as the long axis of the BMO point cloud

6. The clinical image has been moved vertically in the Z-axis so that the coronal image profile (highlighted by arrows) coincides with the BMO point cloud. Once this adjustment has been completed, the BMO points are switched off.

7. The clinical image is now viewed in the en-face orientation following z-axis adjustment. A rotation about the image centroid has been performed to accurately align the clinical vessel outline to the vessel reconstruction

8. The final location of the BMO points in relation to the clinical disc photograph is displayed

Figure 2. The identification of Bruch’s Membrane and Bruch’s Membrane Opening in a histomorphometric section

Top left panel: The clinical disc photograph (OD) prior to disc margin delineation Top right panel: The clinical disc photograph displaying both the co-localized histomorphometric BMO points (red glyphs) and the clinical disc margin points (blue and green glyphs). The black line is the approximate location of the vertical histomorphometric section shown in the middle panel. The area circled in black is the approximate location of the histomorphometric region (white box in the middle panel) magnified in the two bottom panels. Middle panel: Central vertical histomorphometric section shown as a black line in the top right panel. Superior is left and inferior is right. The area within the white box is magnified in the bottom panels. Note that because the tissues are sectioned from the vitreous (top) to the orbital optic nerve (bottom) a dark shadow is present until the serial sectioning plane passes through the dense pigment of the retinal pigment epithelium, choroid and Bruch’s Membrane Bottom left panel: Magnified view of the highlighted white box in the middle panel which demonstrates the superior disc margin anatomy Bottom right panel: The same region, labeled as follows:

1. A = Sclera

2. B = Choroid

3. C = Bruch’s Membrane

4. D = Commencement of pigmented Bruch’s Membrane which in this section appears to co-localize to the termination of the choroid (B)

5. E = Termination of pigmented Bruch’s Membrane and the commencement of unpigmented Bruch’s Membrane. Note the presence of pigment ‘shadows’ of variable density cast vertically along the course of the pigmented Bruch’s Membrane which are absent in the regions where Bruch’s Membrane is unpigmented

6. F = Termination of unpigmented Bruch’s Membrane, which would be delineated as Bruch’s Membrane Opening in this section

7. G = Border Tissue of Elschnig. In this eye, Bruch’s Membrane fuses with the superior edge of the Border Tissue and extends slightly beyond its termination

Figure 3. Pigment on the lamina surface causing obfuscation of Bruch’s Membrane Opening

Top left panel: Clinical disc photograph (OS) Bottom left panel: En face view of the histomorphometric reconstruction of the same eye. Black line shows the orientation of a histomorphometric section image, a portion of which (the superior disc margin) is magnified in the right panels. The black circle highlights the region viewed in the right panels; note the presence of pigment on the lamina surface Top right panel: Histomorphometric view of the superior part of the neural canal Bottom right panel: Bruch’s Membrane delineated (orange glyphs). The white rectangle highlights an area where the view of Bruch’s Membrane is obscured by a dark shadow cast from the lamina pigment below. In this circumstance, accurate delineation of Bruch’s Membrane Opening can be difficult. The white arrowheads highlight the extent of an artifactual choroidal detachment, most likely caused by the perfusion fixation process

Figure 4. Two Principal Border Tissue Configurations with Variations

PANEL A. Internally oblique Left panel: Disc photograph (OS) showing the co-localized BMO (red glyphs) and disc margin delineations (blue and green glyphs). The black line shows the approximate orientation of the histomorphometric section from which the circular region is magnified in the middle panel Middle panel: Histomorphometric disc margin region. Bruch’s Membrane (orange glyphs), Bruch’s Membrane Opening (red glyph) and the Border Tissue (green glyphs) are each delineated. The inferior edge of the Border Tissue communicates with sclera (the Border Tissue/scleral junction) and the superior edge extends into the neural canal fusing with Bruch’s Membrane (the Border Tissue termination), which extends beyond this point and includes an unpigmented portion Right panel: Representative histologic section taken from a different, normal monkey eye (perfusion fixed at IOP 10 mmHg; mid-horizontal sagittal section, hematoxylin and eosin stain) demonstrating an internally oblique configuration. White arrows highlight Border Tissue, black arrows highlight an extension of unpigmented Bruch’s Membrane. In this case there is no clear extension of Bruch’s Membrane beyond the termination of the Border Tissues PANEL B. Externally oblique Left panel: Disc photograph (OD), demarcated as in panel A. Note that within and around the circled region the clinical disc margin has been marked internal to the histomorphometric BMO points Middle panel: Histomorphometric section showing the Border Tissue configuration, demarcated as in panel A. Note that the inferior edge of the Border Tissue is internal to its termination at Bruch’s Membrane. Bruch’s Membrane does not extend beyond the Border Tissue’s termination. In this instance, dense pigment within the sclera immediately adjacent to the neural canal casts a shadow upwards that probably explains the lack of a highly reflective “scleral lip” (white arrows) within this region of the clinical photograph Right panel: representative histologic section taken from a different, normal monkey eye (perfusion fixed at IOP 10 mmHg; mid-horizontal sagittal section, hematoxylin and eosin stain) demonstrating the externally oblique configuration. White arrows highlight the Border Tissue, black arrows highlight Bruch’s Membrane (pigmented) PANEL C. Vertical configuration The Border Tissue extends vertically from the sclera to meet Bruch’s Membrane. Bruch’s Membrane extends beyond this point with both pigmented (outer) and unpigmented (inner) portions PANEL D. Horizontal configuration The Border Tissue extends horizontally to meet Bruch’s Membrane Opening. This configuration is an extreme form of an internally oblique Border Tissue configuration

Figure 5. Two principal Border Tissue configurations, their relationship to a pigmented or unpigmented extension of Bruch’s Membrane and the resultant clinical disc margin anatomy

1. Internally Oblique

   The diagram shows the clinical optic disc appearance (above) and a cross section of the optic nerve head (below). Labeling is as follows:

   1. 1 = Sclera

   2. 2 = Choriocapillaris

   3. 3 = Retinal pigment epithelium with Bruch’s Membrane

   4. 4 = Border Tissue

   5. 5 = Neural canal boundary

   6. 6 = Pigment on the surface of Bruch’s Membrane

   7. 7 = Bruch’s Membrane

   Magnified Inset Left - pigmented Bruch’s Membrane corresponds to the halo of pigment on the left side of the disc margin

   Magnified Inset Right - a region of unpigmented Bruch’s Membrane is shown; this corresponds to a white crescent internal to the pigment halo at the disc margin (which corresponds to a portion of pigmented Bruch’s Membrane)

2. Externally Oblique

   Labeling is as per the schematic in panel A

   Magnified Inset Left - Bruch’s Membrane is pigmented to its end and does not extend beyond the termination of the Border Tissue. This Bruch’s Membrane extension corresponds to an external crescent of pigment at the disc margin that is internal to the termination of the retinal pigment epithelium. The portion of the Border Tissue that is internal to the end of Bruch’s Membrane (BMO) may be clinically recognizable as an inner reflective (if there is no pigment on the Border Tissue surface) or a pigmented crescent (if there is pigment on the Border Tissue surface) that is posterior to the plane of the retinal pigment epithelium. An inner pigmented halo (lighter grey and stippled) is shown on both sides of the disc diagram

   Magnified Inset Right-unpigmented Bruch’s Membrane extends internally to the Border Tissue termination, corresponding to a reflective crescent internal to the pigment crescent. Again, pigmented Border Tissue (lighter grey and stippled) extends internal to the reflective crescent. In both the left and right insets the Border Tissue/scleral junction is depicted without a true scleral lip which when present and visible, appears internal and deep to the other structures

Figure 6. Five examples (A-E) of good alignment between BMO points and the clinical disc margin, following ‘first pass’ co-localization

Clinical images are shown in the left panels. The co-localizations of the vessel reconstructions to the clinical images are shown in the middle panels. The co-localized BMO points (red glyphs) and the clinical disc margin delineations (blue and green glyphs) are shown in the right panels

Figure 7. The termination of unpigmented Bruch’s Membrane is clinically visible and aligns to the disc margin

1. A histomorphometric section taken at 67.5° from a left eye. A white box highlights the area of interest in the nasal region of the optic nerve head

2. The area within the white box has been magnified to highlight the structures comprising the disc margin. In this section, there is a substantial overhang of Bruch’s Membrane (both pigmented and unpigmented) beyond the termination of the Border Tissue of Elschnig

3. The structures comprising the disc margin have been delineated within Multiview; termination of unpigmented Bruch’s Membrane, in other words Bruch’s Membrane Opening (red glyph), termination of pigmented Bruch’s Membrane (light blue glyph), junction of Border Tissue of Elschnig with Bruch’s Membrane (scleral ring of Elschnig - white glyph) and the anterior scleral canal opening (dark blue glyph)

4. The in vivo disc photograph of the eye from which the histomorphometric reconstruction was obtained

5. Following co-localization of the three-dimensional vessel reconstruction to the disc photograph, the termination of unpigmented Bruch’s Membrane (red glyph) coincides with the innermost white reflective halo at the disc margin. The termination of pigmented Bruch’s Membrane (light blue glyph) coincides with the inner edge of the pigment at the disc margin. The Border Tissue/Bruch’s Membrane junction (scleral ring - white glyph) coincides with a white reflective stripe within the mottled disc margin pigment; the anterior scleral canal opening (dark blue glyph) is external to this.

   It is apparent in this eye that the innermost white reflective structure at the disc margin is not the scleral ring of Elschnig but the edge of unpigmented Bruch’s Membrane. In this region of the disc, the scleral ring is considerably external to what the clinician perceives as the disc margin