Anterior scleral canal geometry in pressurised (IOP 10) and non-pressurised (IOP 0) normal monkey eyes

Abstract

Aims: To characterise lamina cribrosa and anterior scleral canal wall architecture in pressurised (IOP 10 mm Hg) and non-pressurised (IOP 0 mm Hg) normal monkey eyes.

Methods: Eight normal eyes from eight monkeys were enucleated before sacrifice and the optic nerve heads (ONH) trephined and immersion fixed in glutaraldehyde (IOP 0). Nine normal eyes from nine monkeys were perfusion fixed in situ with paraformaldehyde at IOP 10 mm Hg (IOP 10), and the ONHs trephined and stored in glutaraldehyde. Each ONH specimen was embedded in glycol methacrylate and cut into vertical or horizontal, 4 micro m thick, serial sections. Within digitised images of every sixth section, anterior laminar position and laminar thickness were measured at nine evenly spaced locations across the scleral canal opening. Additionally, scleral canal diameters at Bruch's membrane (SCD-B) and at the anterior laminar insertion (SCD-ALI) were measured within the 15 middle section images of each vertically sectioned ONH.

Results: Anterior laminar position was significantly more anterior (nearer Bruch's membrane) in the IOP 10 eyes, compared with the IOP 0 eyes (116 (+/-95% CI; 2) micro m v 184 (2) micro m, respectively). Also in the IOP 10 eyes, the lamina cribrosa was thinner (195 (2) micro m v 264 (2) micro m) and the scleral canal diameter was larger (SCD-B: 1751 (23) micro m v 1591 (19) micro m; SCD-ALI: 1961 (21) micro m v 1717 (17) micro m), compared with the IOP 0 eyes.

Conclusion: The anterior scleral canal wall is expanded and the lamina cribrosa is thinned and more tautly stretched within pressurised (perfusion fixed at IOP 10) young monkey eyes, compared with non-pressurised (immersion fixed at IOP 0) young monkey eyes. The constricted scleral canal and the relaxed and thickened lamina in the non-pressurised eyes may represent phenomena that contribute to optic disc swelling in hypotonous eyes.

Figure 1

Generation and measurement of composite section images. Each composite section image consists of four to six individual overlapping images taken at a resolution of approximately 2 μm/pixel. On each section image, an operator placed marks denoting the termination of Bruch’s membrane, the anterior insertion of the lamina into the sclera, and the anterior and posterior border of the lamina cribrosa. A custom image analysis software then connected the two Bruch’s membrane termination points, divided the distance into nine measurement points, and dropped perpendicular lines to the anterior and posterior lamina cribrosa as best determined by the image analysis software based on the operator’s landmarks. Next, nine automatic measures of anterior laminar position (ALP) and laminar thickness (LT) across the scleral canal opening were made, as well as one measurement of the diameter of the scleral canal opening at Bruch’s membrane (SCD-B) and at the anterior laminar insertion (SCD-ALI). (Adapted from Bellezza et al.)

Figure 2

Example of pooling the data for an individual optic nerve head. (A) Measurements were made on the set of images taken of the sections cut from between the termination points of Bruch’s membrane. (B) The nine measurement points for anterior laminar position and thickness (see Fig 1) from each section image were projected onto a confocal scanning laser tomographic image of the measured optic nerve head. (C) The measurement points were subdivided into central, superior, inferior, nasal, and temporal regions (see methods for details). (D) Vertical scleral canal diameter and anterior scleral canal wall geometry measurements were made only within the middle 15 section images. (Adapted from Bellezza et al.)

Figure 3

Representative middle sagittal section images from four representative IOP 0 and four representative IOP 10 ONHs. Note that within the IOP 0 eyes (left), the lamina cribrosa appears “sagged” posteriorly relative to the “taut,” more anteriorly positioned lamina in the IOP 10 eyes (right).

Figure 4

Schematic representation of the lamina cribrosa and scleral canal in a non-pressurised (IOP 0) and pressurised (IOP 10) eye. Left: Thickness (T) of the lamina cribrosa and diameter (D) of the scleral canal opening in an unpressurised (IOP 0) eye. Right: Pressure within the globe generates an expansion of the scleral shell which, in turn, generates (and is resisted by) tensile forces within the sclera. These forces (F) act on the scleral canal wall, causing the scleral canal opening to expand (Δd), which in turn stretches the lamina within the canal. Thus, the lamina is taut (more anteriorly positioned) and thinned (Δt) in the IOP 10 eye, compared with the IOP 0 eye.