Reproducibility of measuring lamina cribrosa pore geometry in human and nonhuman primates with in vivo adaptive optics imaging.

Glaucoma is a multifaceted group of eye diseases that results in the degeneration of retinal ganglion cell axons and the death of retinal ganglion cells (RGCs). The mechanisms of glaucomatous damage (e.g., mechanical, vascular, and glial) are not fully understood. Although several of these factors likely contribute to RGC death, substantial evidence suggests that the initial site of axonal injury likely occurs at the level of the lamina cribrosa in the optic nerve head (ONH).1–11 In humans and nonhuman primates, the lamina cribrosa is a three-dimensional porous structure consisting of flexible beams of collagenous tissue that support and nourish the RGC axons passing through it from the retina to the brain.12 Increases in intraocular pressure (IOP) impart stress and strain on the lamina,13–15 resulting in a posterior bowing and stretching of the load-bearing laminar beams12,16 and potential increases in laminar pore area and elongation (or more elliptically shaped pores). Stretching and deformation of the laminar beams (and associated pores) could sheer or damage encompassed axons and laminar capillaries, thereby hampering axonal transport, blood flow and the diffusion of nutrients,17 and/or the neurotrophic support provided to the axons by alterations in glial cells.18 ONH tissues have been examined in human glaucomatous eyes and nonhuman primate models of experimental glaucoma. Postmortem histologic studies have shown significant changes in prelaminar and laminar tissues within the ONH in early experimental glaucoma.16,19,20 Additional ex vivo studies in glaucomatous human and nonhuman primate eyes have described early changes in laminar morphology and position,16,21 laminar pore geometry,22 and the composition and architecture of laminar connective tissues.20,23 In light of this work, there is a growing consensus that in vivo studies are necessary to validate ex vivo results and examine longitudinal changes during glaucoma.24,25 The lamina cribrosa has been examined in vivo. Fontana et al.26 imaged anterior laminar pores in living human glaucomatous eyes using a modified confocal scanning laser ophthalmoscope (SLO) and showed that pore area and elongation were larger in eyes with more severe glaucoma. However, images were taken using a wide field of view (20°) and were not of high resolution because of the presence of ocular aberrations. Therefore, the size and total number of pores that could be resolved and the number of subjects in which laminar pores could be successfully imaged were limited. Vilupuru et al.27 used a confocal adaptive optics scanning laser ophthalmoscope (AOSLO), an instrument that noninvasively provides cellular-level images through a correction of the eye's aberrations, to improve the resolution and contrast of laminar images in vivo in nonhuman primates with experimental glaucoma. This study quantified laminar pore geometries in three diseased monkey eyes, each at a single time point. However, changes in pore geometry with disease were not investigated. Recent studies have combined adaptive optics with optical coherence tomography (OCT) to image the human lamina cribrosa in three spatial dimensions,28,29 but a quantitative analysis of laminar geometry and pore structure has not been reported. We have assessed the repeatability of imaging and quantifying laminar pores in vivo in two normal macaque and three normal human eyes using an AOSLO at different time points. Statistical tests were performed to investigate whether significant differences were present in any analyzed laminar pore parameters within and across imaging sessions. This study provides an important first step in understanding the feasibility of using in vivo AOSLO imaging to assess laminar structure in normal eyes, human glaucomatous eyes, and nonhuman primate eyes with experimentally induced glaucoma.