Three-Dimensional Histomorphometry of the Normal and Early Glaucomatous Monkey Optic Nerve Head: Neural Canal and Subarachnoid Space Architecture

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Abstract
Purpose. To delineate three dimensionally the neural canal landmarks—Bruch’s membrane opening (BMO), anterior sclera canal opening (ASCO), anterior laminar insertion (ALI), posterior laminar insertion (PLI), and posterior scleral canal opening (PSCO)—and the anterior-most aspect of the subarachnoid space (ASAS), within digital three-dimensional (3-D) reconstructions of the monkey optic nerve head (ONH). methods. The trephinated ONH and peripapillary sclera from both eyes of three early glaucoma (EG) monkeys (one eye normal, one eye with laser-induced EG) were serial sectioned at 3-μm thickness, with the embedded tissue block face stained and imaged after each cut. The images were aligned and stacked in a 3-D volume, within which the BMO, ASCO, ALI, PLI, PSCO, and ASAS were delineated in 40 digital, radial, and sagittal sections. An ellipse was fitted to the 80 BMO points to establish a BMO zero reference plane, on which all other points were projected. The distance from each projected point to the BMO centroid (offset) and BMO zero reference plane (depth) were calculated and compared regionally between normal and EG eyes, both overall and within each monkey, by analysis of variance. results. BMO was the clinically visible optic disc margin in all six eyes. The neural canal architecture was highly variable in the three normal eyes. Radial expansion of the neural canal was greatest posteriorly in the EG eyes. Axial elongation of the canal was less pronounced overall but was regionally present within all three EG eyes. ASAS was regionally radially expanded and anteriorly displaced within two of the three EG eyes. conclusions. Profound deformation of the neural canal and ASAS architecture are present in young adult monkey eyes at the onset of ONH surface change in early experimental glaucoma.