Expansions of the neurovascular scleral canal and contained optic nerve occur early in the hypertonic saline rat experimental glaucoma model

Enhanced Optic Nerve Expansion and Altered Ultrastructure of Elastic Fibers Induced by Lysyl Oxidase Inhibition in a Mouse Model of Marfan Syndrome

Two major constituents of exfoliation material, fibrillin-1 and lysyl oxidase-like 1 (encoded by FBN1 and LOXL1), are implicated in exfoliation glaucoma, yet their individual contributions to ocular phenotype are minor. To test the hypothesis that a combination of FBN1 mutation and LOXL1 deficiency exacerbates ocular phenotypes, the pan-lysyl oxidase inhibitor β-aminopropionitrile (BAPN) was used to treat adult wild-type (WT) mice and mice heterozygous for a missense mutation in Fbn1 (Fbn1C1041G/+) for 8 weeks and their eyes were examined. Although intraocular pressure did not change and exfoliation material was not detected in the eyes, BAPN treatment worsened optic nerve and axon expansion in Fbn1C1041G/+ mice, an early sign of axonal damage in rodent models of glaucoma. Disruption of elastic fibers was detected only in Fbn1C1041G/+ mice, which increased with BAPN treatment, as shown by histologic and immunohistochemical staining of the optic nerve pia mater. Transmission electron microscopy showed that Fbn1C1041G/+ mice had fewer microfibrils, smaller elastin cores, and a lower density of elastic fibers compared with WT mice in control groups. BAPN treatment led to elastin core expansion in both WT and Fbn1C1041G/+ mice, but an increase in the density of elastic fiber was confined to Fbn1C1041G/+ mice. LOX inhibition had a stronger effect on optic nerve and elastic fiber parameters in the context of Fbn1 mutation, indicating the Marfan mouse model with LOX inhibition warrants further investigation for exfoliation glaucoma pathogenesis.

IOP and glaucoma damage: The essential role of optic nerve head and retinal mechanosensors

There are many unanswered questions on the relation of intraocular pressure to glaucoma development and progression. IOP itself cannot be distilled to a single, unifying value, because IOP level varies over time, differs depending on ocular location, and can be affected by method of measurement. Ultimately, IOP level creates mechanical strain that affects axonal function at the optic nerve head which causes local extracellular matrix remodeling and retinal ganglion cell death - hallmarks of glaucoma and the cause of glaucomatous vision loss. Extracellular tissue strain at the ONH and lamina cribrosa is regionally variable and differs in magnitude and location between healthy and glaucomatous eyes. The ultimate targets of IOP-induced tissue strain in glaucoma are retinal ganglion cell axons at the optic nerve head and the cells that support axonal function (astrocytes, the neurovascular unit, microglia, and fibroblasts). These cells sense tissue strain through a series of signals that originate at the cell membrane and alter cytoskeletal organization, migration, differentiation, gene transcription, and proliferation. The proteins that translate mechanical stimuli into molecular signals act as band-pass filters - sensing some stimuli while ignoring others - and cellular responses to stimuli can differ based on cell type and differentiation state. Therefore, to fully understand the IOP signals that are relevant to glaucoma, it is necessary to understand the ultimate cellular targets of IOP-induced mechanical stimuli and their ability to sense, ignore, and translate these signals into cellular actions.

Mechanical strain in the mouse astrocytic lamina increases after exposure to recombinant trypsin

The responses of astrocytes in the optic nerve head (ONH) to mechanical and biochemical stimuli are important to understanding the degeneration of retinal ganglion cell axons in glaucoma. The ONH in glaucoma is vulnerable to stress produced by the intraocular pressure (IOP). Notably, after three days of elevated IOP in a mouse model, the junctions between the astrocytic processes and the peripapillary sclera were altered and the structural compliance of the ONH increased. In order to simulate this aspect of glaucomatous remodeling, explanted mouse eyes were treated with TrypLE, a recombinant trypsin enzyme. Treatment with TrypLE caused the periphery of the astrocytic lamina to contract radially by 0.044 ± 0.038. Transmission electron microscopy showed that TrypLE caused a separation of the end-feet of the astrocyte processes from the basement membrane at the junction with the sclera. Inflation testing after treatment with TrypLE caused an increased strain response in the astrocytic lamina compared to the strain response before treatment. The greatest increase was in the radial Green-Lagrange strain, Err = 0.028 ± 0.009, which increased by 340%. The alterations in the microstructure and in the strain response of the astrocytic lamina reported in mouse experimental glaucoma were partially reproduced by experimental treatment of mouse eyes with TrypLE. The results herein suggest that separation of junctions between the astrocyte processes and the sclera may be instrumental in increasing the structural compliance of the ONH after a period of elevated IOP. STATEMENT OF SIGNIFICANCE: Astrocytes of the optic nerve of the eye spread out from edge to edge across the optic nerve in a region referred to as the astrocytic lamina. In an experimental model of glaucoma caused by elevated eye-pressure, there is disruption of the connections between astrocytes and the edge of the astrocytic lamina. We caused a similar event in the lamina by incubating explanted mouse eyes with an enzyme. Disruption of the astrocyte connections to the edge of their tissue caused the tissue to stretch more when we increased the eye-pressure, compared to the control tissue. This work is the first on the tissue of the optic nerve to demonstrate the importance of cell connections in preventing the over-stretching of the astrocytic lamina.

The Influence of Mitochondrial Dynamics and Function on Retinal Ganglion Cell Susceptibility in Optic Nerve Disease

The important roles of mitochondrial function and dysfunction in the process of neurodegeneration are widely acknowledged. Retinal ganglion cells (RGCs) appear to be a highly vulnerable neuronal cell type in the central nervous system with respect to mitochondrial dysfunction but the actual reasons for this are still incompletely understood. These cells have a unique circumstance where unmyelinated axons must bend nearly 90° to exit the eye and then cross a translaminar pressure gradient before becoming myelinated in the optic nerve. This region, the optic nerve head, contains some of the highest density of mitochondria present in these cells. Glaucoma represents a perfect storm of events occurring at this location, with a combination of changes in the translaminar pressure gradient and reassignment of the metabolic support functions of supporting glia, which appears to apply increased metabolic stress to the RGC axons leading to a failure of axonal transport mechanisms. However, RGCs themselves are also extremely sensitive to genetic mutations, particularly in genes affecting mitochondrial dynamics and mitochondrial clearance. These mutations, which systemically affect the mitochondria in every cell, often lead to an optic neuropathy as the sole pathologic defect in affected patients. This review summarizes knowledge of mitochondrial structure and function, the known energy demands of neurons in general, and places these in the context of normal and pathological characteristics of mitochondria attributed to RGCs.

Individual-Specific Modeling of Rat Optic Nerve Head Biomechanics in Glaucoma

Glaucoma is the second leading cause of blindness worldwide and is characterized by the death of retinal ganglion cells (RGCs), the cells that send vision information to the brain. Their axons exit the eye at the optic nerve head (ONH), the main site of damage in glaucoma. The importance of biomechanics in glaucoma is indicated by the fact that elevated intraocular pressure (IOP) is a causative risk factor for the disease. However, exactly how biomechanical insult leads to RGC death is not understood. Although rat models are widely used to study glaucoma, their ONH biomechanics have not been characterized in depth. Therefore, we aimed to do so through finite element (FE) modeling. Utilizing our previously described method, we constructed and analyzed ONH models with individual-specific geometry in which the sclera was modeled as a matrix reinforced with collagen fibers. We developed eight sets of scleral material parameters based on results from our previous inverse FE study and used them to simulate the effects of elevated IOP in eight model variants of each of seven rat ONHs. Within the optic nerve, highest strains were seen inferiorly, a pattern that was consistent across model geometries and model variants. In addition, changing the collagen fiber direction to be circumferential within the peripapillary sclera resulted in more pronounced decreases in strain than changing scleral stiffness. The results from this study can be used to interpret data from rat glaucoma studies to learn more about how biomechanics affects RGC pathogenesis in glaucoma.

Redistribution of metabolic resources through astrocyte networks mitigates neurodegenerative stress

In the central nervous system, glycogen-derived bioenergetic resources in astrocytes help promote tissue survival in response to focal neuronal stress. However, our understanding of the extent to which these resources are mobilized and utilized during neurodegeneration, especially in nearby regions that are not actively degenerating, remains incomplete. Here we modeled neurodegeneration in glaucoma, the world's leading cause of irreversible blindness, and measured how metabolites mobilize through astrocyte gap junctions composed of connexin 43 (Cx43). We elevated intraocular pressure in one eye and determined how astrocyte-derived metabolites in the contralateral optic projection responded. Remarkably, astrocyte networks expand and redistribute metabolites along distances even 10 mm in length, donating resources from the unstressed to the stressed projection in response to intraocular pressure elevation. While resource donation improves axon function and visual acuity in the directly stressed region, it renders the donating tissue susceptible to bioenergetic, structural, and physiological degradation. Intriguingly, when both projections are stressed in a WT animal, axon function and visual acuity equilibrate between the two projections even when each projection is stressed for a different length of time. This equilibration does not occur when Cx43 is not present. Thus, Cx43-mediated astrocyte metabolic networks serve as an endogenous mechanism used to mitigate bioenergetic stress and distribute the impact of neurodegenerative disease processes. Redistribution ultimately renders the donating optic nerve vulnerable to further metabolic stress, which could explain why local neurodegeneration does not remain confined, but eventually impacts healthy regions of the brain more broadly.

Optical Coherence Tomography May Help Distinguish Glaucoma from Suprasellar Tumor-Associated Optic Disc

Purpose

This study aimed to differentiate patients with bilateral disc cupping associated with suprasellar tumor from patients with open-angle glaucoma by analyzing differences in optical coherence tomography (OCT) of the optic nerve.

Methods

In this retrospective cross-sectional study, we collected data from the eyes of 25 patients with suprasellar craniopharyngioma or pituitary macroadenomas (group 1) and 35 patients with primary open-angle glaucoma (POAG) (group 2), seen between 2001 and 2015, all with a visual acuity of ≥20/40, for whom Stratus Time-Domain (TD) optic nerve OCT scans were available. The main outcome measures were the retinal nerve fiber layer (RNFL) thickness, disc area, cup volume, cup/disc ratio, and rim area.

Results

A total of 31 patients met the inclusion criteria and were included in the study: 16 with suprasellar tumors and 15 with POAG. Both groups were similar in terms of gender and age (P > 0.05). The glaucoma group had a borderline greater total RNFL thickness (74.2 μm versus 62.8 μm, P=0.07), disc area (2.70 mm² versus 2.16 mm², P=0.004), and cup volume (0.20 mm³ versus 0.08 mm³, P=0.02). In multivariate, glaucoma was associated with increased total RNFL thickness (OR = 1.116 per μm, P=0.008), increased disc area (OR = 2.402 per 100 μm², P=0.002), and decreased rim area (OR = 0.272 per 100 μm², P=0.011). Of these, the parameter with the greatest AUC was the disc area (AUC = 0.79). Using the Youden index, the optimal cut-off point identified for stratification was a disc area greater than 2.33 μm².

Conclusions

In patients with bilateral disc cupping, a decreased total RNFL thickness and smaller disc area seem to be associated with suprasellar tumors (when compared with open-angle glaucoma). These findings may aid in early diagnosis of cupping from suprasellar tumors, before compressive visual loss occurs.

A Methodology for Individual-Specific Modeling of Rat Optic Nerve Head Biomechanics in Glaucoma

Glaucoma is the leading cause of irreversible blindness and involves the death of retinal ganglion cells (RGCs). Although biomechanics likely contributes to axonal injury within the optic nerve head (ONH), leading to RGC death, the pathways by which this occurs are not well understood. While rat models of glaucoma are well-suited for mechanistic studies, the anatomy of the rat ONH is different from the human, and the resulting differences in biomechanics have not been characterized. The aim of this study is to describe a methodology for building individual-specific finite element (FE) models of rat ONHs. This method was used to build three rat ONH FE models and compute the biomechanical environment within these ONHs. Initial results show that rat ONH strains are larger and more asymmetric than those seen in human ONH modeling studies. This method provides a framework for building additional models of normotensive and glaucomatous rat ONHs. Comparing model strain patterns with patterns of cellular response seen in studies using rat glaucoma models will help us to learn more about the link between biomechanics and glaucomatous cell death, which in turn may drive the development of novel therapies for glaucoma.

Early Optic Nerve Head Glial Proliferation and Jak-Stat Pathway Activation in Chronic Experimental Glaucoma

PURPOSE

We previously reported increased expression of cell proliferation and Jak-Stat pathway-related genes in chronic experimental glaucoma model optic nerve heads (ONH) with early, mild injury. Here, we confirm these observations by localizing, identifying, and quantifying ONH cellular proliferation and Jak-Stat pathway activation in this model.

METHODS

Chronic intraocular pressure (IOP) elevation was achieved via outflow pathway sclerosis. After 5 weeks, ONH longitudinal sections were immunolabeled with proliferation and cell-type markers to determine nuclear densities in the anterior (unmyelinated) and transition (partially myelinated) ONH. Nuclear pStat3 labeling was used to detect Jak-Stat pathway activation. Nuclear density differences between control ONH (uninjected) and ONH with either early or advanced injury (determined by optic nerve injury grading) were identified by ANOVA.

RESULTS

Advanced injury ONH had twice the nuclear density (P < 0.0001) of controls and significantly greater astrocyte density in anterior (P = 0.0001) and transition (P = 0.006) ONH regions. An increased optic nerve injury grade positively correlated with increased microglia/macrophage density in anterior and transition ONH (P < 0.0001, both). Oligodendroglial density was unaffected. In glaucoma model ONH, 80% of anterior and 66% of transition region proliferating cells were astrocytes. Nuclear pStat3 labeling significantly increased in early injury anterior ONH, and 95% colocalized with astrocytes.

CONCLUSIONS

Astrocytes account for the majority of proliferating cells, contributing to a doubled nuclear density in advanced injury ONH. Jak-Stat pathway activation is apparent in the early injury glaucoma model ONH. These data confirm dramatic astrocyte cell proliferation and early Jak-Stat pathway activation in ONH injured by elevated IOP.

EFFECT OF GEOMETRICAL PARAMETERS ON THE DEFORMATIONS OF THE HUMAN OPTIC NERVE HEAD BASED ON INDIVIDUAL-SPECIFIC MODELS

The elevated intraocular pressure (IOP) is one of the important risk factors of glaucomatous optic nerve damage. The deformations of glaucomatous optic nerve head (ONH) due to the elevated IOP might lead to further damage to the optic nerve. The ONH deformations due to the elevated IOP depend on their geometry. Therefore, it is meaningful to investigate the relationship between geometrical parameters and the deformation of ONH. Firstly, the geometrical parameters of ONH were obtained by in vivo using spectral domain optical coherence tomography. Then, individual-specific three-dimensional (3D) models of ONH for glaucoma and normal subjects were established and the deformations of ONH were acquired by using finite element method. Furthermore, the thickness changes of retina and lamina cribrosa were calculated under different IOPs. Finally, correlations between geometrical parameters and the relative deformations of the human ONH were researched by using Spearman correlation. It was found that the relative thickness change of patient’s retina was larger than that of normal subjects under the same condition. The correlation of several parameters, including cup area, and lamina cribrosa depth, with relative deformations of ONH had statistical significance.

Astrocyte remodeling without gliosis precedes optic nerve Axonopathy

Astroyctes serve myriad functions but are especially critical in white matter tracts, where energy-demanding axons propagate action potentials great distances between neurons. Axonal dependence on astrocytes for even normal function accentuates the critical role astrocytes serve during disease. In glaucoma, the most common optic neuropathy, sensitivity to intraocular pressure (IOP) challenges RGC axons early, including degradation of anterograde transport to the superior colliculus (SC). Astrocyte remodeling presages overt axon degeneration in glaucoma and thus may present a therapeutic opportunity. Here we developed a novel metric to quantify organization of astrocyte processes in the optic nerve relative to axon degeneration in the DBA/2 J hereditary mouse model of glaucoma. In early progression, as axons expand prior to loss, astrocyte processes become more parallel with migration to the nerve’s edge without a change in overall coverage of the nerve. As axons degenerate, astrocyte parallelism diminishes with increased glial coverage and reinvasion of the nerve. In longitudinal sections through aged DBA/2 J nerve, increased astrocyte parallelism reflected elevated levels of the astrocyte gap-junction protein connexin 43 (Cx43). In the distal nerve, increased Cx43 also indicated with a higher level of intact anterograde transport from retina to SC. Our results suggest that progression of axonopathy in the optic nerve involves astrocyte remodeling in two phases. In an early phase, astrocyte processes organize in parallel, likely through gap-junction coupling, while a later phase involves deterioration of organization as glial coverage increases and axons are lost.

3D Histomorphometric Reconstruction and Quantification of the Optic Nerve Head Connective Tissues

Accurately characterizing the 3D geometry of the optic nerve head neural and connective tissues has been the goal of a large and important body of scientific work. In the present report, we summarize our methods for the high-resolution, digital, 3D histomorphometric reconstruction of the optic nerve head tissues, including their visualization, parameterization, and quantification. In addition, we present our methods for between-eye comparisons of this anatomy, and their use to determine animal-specific and experiment-wide experimental glaucoma versus Control eye differences in the unilateral, monkey experimental glaucoma model. Finally, we demonstrate its application to finite element modeling, 3D optic nerve head reconstruction of other species, and 3D optic nerve head reconstructions using other imaging modalities.

The connective tissue phenotype of glaucomatous cupping in the monkey eye - Clinical and research implications

In a series of previous publications we have proposed a framework for conceptualizing the optic nerve head (ONH) as a biomechanical structure. That framework proposes important roles for intraocular pressure (IOP), IOP-related stress and strain, cerebrospinal fluid pressure (CSFp), systemic and ocular determinants of blood flow, inflammation, auto-immunity, genetics, and other non-IOP related risk factors in the physiology of ONH aging and the pathophysiology of glaucomatous damage to the ONH. The present report summarizes 20 years of technique development and study results pertinent to the characterization of ONH connective tissue deformation and remodeling in the unilateral monkey experimental glaucoma (EG) model. In it we propose that the defining pathophysiology of a glaucomatous optic neuropathy involves deformation, remodeling, and mechanical failure of the ONH connective tissues. We view this as an active process, driven by astrocyte, microglial, fibroblast and oligodendrocyte mechanobiology. These cells, and the connective tissue phenomena they propagate, have primary and secondary effects on retinal ganglion cell (RGC) axon, laminar beam and retrolaminar capillary homeostasis that may initially be "protective" but eventually lead to RGC axonal injury, repair and/or cell death. The primary goal of this report is to summarize our 3D histomorphometric and optical coherence tomography (OCT)-based evidence for the early onset and progression of ONH connective tissue deformation and remodeling in monkey EG. A second goal is to explain the importance of including ONH connective tissue processes in characterizing the phenotype of a glaucomatous optic neuropathy in all species. A third goal is to summarize our current efforts to move from ONH morphology to the cell biology of connective tissue remodeling and axonal insult early in the disease. A final goal is to facilitate the translation of our findings and ideas into neuroprotective interventions that target these ONH phenomena for therapeutic effect.

Cupping in the Monkey Optic Nerve Transection Model Consists of Prelaminar Tissue Thinning in the Absence of Posterior Laminar Deformation

Purpose

To use optical coherence tomography (OCT) to test the hypothesis that optic nerve head (ONH) “cupping” in the monkey optic nerve transection (ONT) model does not include posterior laminar deformation.

Methods

Five monkeys (aged 5.5–7.8 years) underwent ONH and retinal nerve fiber layer (RNFL) OCT imaging five times at baseline and biweekly following unilateral ONT until euthanization at ∼40% RNFL loss. Retinal nerve fiber layer thickness (RNFLT) and minimum rim width (MRW) were calculated from each pre- and post-ONT imaging session. The anterior lamina cribrosa surface (ALCS) was delineated within baseline and pre-euthanasia data sets. Significant ONT versus control eye pre-euthanasia change in prelaminar tissue thickness (PLTT), MRW, RNFLT, and ALCS depth (ALCSD) was determined using a linear mixed-effects model. Eye-specific change in each parameter exceeded the 95% confidence interval constructed from baseline measurements.

Results

Animals were euthanized 49 to 51 days post ONT. Overall ONT eye change from baseline was significant for MRW (−26.2%, P = 0.0011), RNFLT (−43.8%, P < 0.0001), PLTT (−23.8%, P = 0.0013), and ALCSD (−20.8%, P = 0.033). All five ONT eyes demonstrated significant eye-specific decreases in MRW (−23.7% to −31.8%) and RNFLT (−39.6% to −49.7%). Four ONT eyes showed significant PLTT thinning (−23.0% to −28.2%). The ALCS was anteriorly displaced in three of the ONT eyes (−25.7% to −39.2%). No ONT eye demonstrated posterior laminar displacement.

Conclusions

Seven weeks following surgical ONT in the monkey eye, ONH cupping involves prelaminar and rim tissue thinning without posterior deformation of the lamina cribrosa.

Biological aspects of axonal damage in glaucoma: A brief review

Intraocular pressure (IOP) is a critical risk factor in glaucoma, and the available evidence derived from experimental studies in primates and rodents strongly indicates that the site of IOP-induced axonal damage in glaucoma is at the optic nerve head (ONH). However, the mechanisms that cause IOP-induced damage at the ONH are far from understood. A possible sequence of events could originate with IOP-induced stress in the ONH connective tissue elements (peripapillary sclera, scleral canal and lamina cribrosa) that leads to an increase in biomechanical strain. In consequence, molecular signaling cascades might be activated that result in extracellular matrix turnover of the peripapillary sclera, changing its biomechanical properties. Peripapillary sclera strain might induce reactive changes in ONH astrocytes and cause astrogliosis. The biological changes that are associated with ONH astrocyte reactivity could lead to withdrawal of trophic or metabolic support for optic nerve axons and cause their degeneration. Alternatively, the expression of neurotoxic molecules might be induced. Unfortunately, direct experimental in vivo evidence for these or other scenarios is currently lacking. The pathogenic processes that cause axonal degeneration at the ONH in glaucoma need to be identified before any regenerative therapy is likely to succeed. Several topics and emerging techniques should be pursued to enhance our understanding of the mechanisms that are behind axonal degeneration. Among them are: Advanced imaging techniques, the development of in vivo markers to identify axonal injury, the generation of molecular approaches for in vivo detection of mechanosensitivity and for molecular manipulation of the ONH, a more complete characterization of retinal ganglion cells, the use of organ cultures, 3D-bioprinting, and the engineering of microdevices that can measure pressure. Questions that need to be answered relate to the specific roles of astrogliosis, neuroinflammation, blood flow and intracranial pressure in axonal degeneration at the ONH.

Biomechanical aspects of axonal damage in glaucoma: A brief review

The biomechanical environment within the optic nerve head (ONH) is complex and is likely directly involved in the loss of retinal ganglion cells (RGCs) in glaucoma. Unfortunately, our understanding of this process is poor. Here we describe factors that influence ONH biomechanics, including ONH connective tissue microarchitecture and anatomy; intraocular pressure (IOP); and cerebrospinal fluid pressure (CSFp). We note that connective tissue factors can vary significantly from one individual to the next, as well as regionally within an eye, and that the understanding of ONH biomechanics is hindered by anatomical differences between small-animal models of glaucoma (rats and mice) and humans. Other challenges of using animal models of glaucoma to study the role of biomechanics include the complexity of assessing the degree of glaucomatous progression; and inadequate tools for monitoring and consistently elevating IOP in animal models. We conclude with a consideration of important open research questions/challenges in this area, including: (i) Creating a systems biology description of the ONH; (ii) addressing the role of astrocyte connective tissue remodeling and reactivity in glaucoma; (iii) providing a better characterization of ONH astrocytes and non-astrocytic constituent cells; (iv) better understanding the role of ONH astrocyte phagocytosis, proliferation and death; (v) collecting gene expression and phenotype data on a larger, more coordinated scale; and (vi) developing an implantable IOP sensor.

Posterior rat eye during acute intraocular pressure elevation studied using polarization sensitive optical coherence tomography

Polarization sensitive optical coherence tomography (PS-OCT) operating at 840 nm with axial resolution of 3.8 µm in tissue was used for investigating the posterior rat eye during an acute intraocular pressure (IOP) increase experiment. IOP was elevated in the eyes of anesthetized Sprague Dawley rats by cannulation of the anterior chamber. Three dimensional PS-OCT data sets were acquired at IOP levels between 14 mmHg and 105 mmHg. Maps of scleral birefringence, retinal nerve fiber layer (RNFL) retardation and relative RNFL/retina reflectivity were generated in the peripapillary area and quantitatively analyzed. All investigated parameters showed a substantial correlation with IOP. In the low IOP range of 14-45 mmHg only scleral birefringence showed statistically significant correlation. The polarization changes observed in the PS-OCT imaging study presented in this work suggest that birefringence of the sclera may be a promising IOP-related parameter to investigate.