Seeing the Hidden Lamina: Effects of Exsanguination on the Optic Nerve Head

Abstract

PurposeTo introduce an experimental approach for direct comparison of the primate optic nerve head (ONH) before and after death by exsanguination.MethodThe ONHs of four eyes from three monkeys were imaged with spectral-domain optical coherence tomography (OCT) before and after exsanguination under controlled IOP. ONH structures, including the Bruch membrane (BM), BM opening, inner limiting membrane (ILM), and anterior lamina cribrosa (ALC) were delineated on 18 virtual radial sections per OCT scan. Thirteen parameters were analyzed: scleral canal at BM opening (area, planarity, and aspect ratio), ILM depth, BM depth; ALC (depth, shape index, and curvedness), and ALC visibility (globally, superior, inferior, nasal, and temporal quadrants).ResultsAll four ALC quadrants had a statistically significant improvement in visibility after exsanguination (overall P < 0.001). ALC visibility increased by 35% globally and by 36%, 37%, 14%, and 4% in the superior, inferior, nasal, and temporal quadrants, respectively. ALC increased 4.1%, 1.9%, and 0.1% in curvedness, shape index, and depth, respectively. Scleral canals increased 7.2%, 25.2%, and 1.1% in area, planarity, and aspect ratio, respectively. ILM and BM depths averaged −7.5% and −55.2% decreases in depth, respectively. Most, but not all, changes were beyond the repeatability range.ConclusionsExsanguination allows for improved lamina characterization, especially in regions typically blocked by shadowing in OCT. The results also demonstrate changes in ONH morphology due to the loss of blood pressure. Future research will be needed to determine whether there are differences in ONH biomechanics before and after exsanguination and what those differences would imply.

So-Called Lamina Cribrosa Defects May Mitigate IOP-Induced Neural Tissue Insult

“…First, optical coherence tomography scans show that LC defects are often located at the periphery of the lamina. 14,18,40 Second, evidence from ex vivo 56,[67][68][69][70] and in vivo 44,[71][72][73][74] studies suggest that neural tissues at the periphery of the lamina often experience the largest IOPinduced strains. Modeling defects in this region can provide an upper-bound estimation of the influence of LC defects on IOP-induced mechanical insult to neural tissues.…”

Morphometric, Hemodynamic, and Biomechanical Factors Influencing Blood Flow and Oxygen Concentration in the Human Lamina Cribrosa

“…Considering a macroscale model of the ONH (a homogeneous LC with uniform biomechanical properties), a change in microcapillary blood pressure in the LC tissue may alter its stiffness (e.g., the LC will become stiffer with an increase in microcapillary pressure) and the magnitude of microcapillary compression. 60 An eye model with solid-fluid interaction that includes a feedback loop will be the target for our next work, and it will be especially useful when we want to look at dynamic models accounting for variations in IOP and blood pressure.…”

Interplay between intraocular and intracranial pressure effects on the optic nerve head in vivo

“…Another strength of this work is that we studied monkeys, the animal model that most resembles humans (Burgoyne, 2015 have multiple risks, and therefore cannot be conducted in human subjects. Our in vivo measurements also avoid postmortem effects, such as tissue processing, histology and the absence of blood pressure (Tran et al, 2018).…”

“…The dense markings allowed us to reconstruct detailed BMO planes and ALC surfaces, which were then used to quantify their changes and deformations in 3D. A major concern of ONH studies based on OCT data is the variable visibility, particularly that of the deep structures like the LC (Girard et al, 2015b; Loureiro et al, 2017; Lucy et al, 2017; Tran et al, 2018). Inconsistent visibility means that a simple comparison of parameters across conditions has a substantial risk of being biased by region visibility.…”

“…In this study, ICP was estimated based on donor body position and not probed or controlled directly as done here. Our in vivo measurements also avoid postmortem effects, such as tissue processing, histology and the absence of blood pressure 26 .…”