Optic nerve head biomechanics in aging and disease

The action of 7,8-dihydroxyflavone preserves retinal ganglion cell survival and visual function via the TrkB pathway in NMDA-induced retinal excitotoxicity

PURPOSE

To analyze the response of different retinal ganglion cell (RGC) populations to NMDA-induced retinal excitotoxicity and the effect of an intraperitoneal treatment with 7,8-Dihydroxyflavone (DHF), a potent selective TrkB agonist.

METHODS

Adult albino rats were treated the day prior to NMDA injection and the three following days with intraperitoneal vehicle (1 %DMSO in 0.09 %NaCl) or DHF (5 mg/kg in vehicle) injections. DHF-afforded protection was studied in the population of Brn3a+RGCs, OPN+RGCs (α-RGCs), OPN+ Tbr2+RGCs (αONs-RGCs), OPN+ Tbr2-Brn3a-RGCs (αONt-RGCs) and OPN+Brn3a+RGCs (αOFF-RGCs) at 3,7,14, or 21 days. The functional response was analyzed longitudinally with full-field electroretinograms. The mechanisms underlying DHF-afforded neuroprotection were assessed by western blot (WB) analysis of the levels of phosphorylated and total TrkB, phosphatidylinositol 3 kinase (PIK3/AKT) and mitogen-activated protein kinase (MAPK).

RESULTS

NMDA intravitreal injection resulted in a significant diminution of the mean amplitudes of the pSTR and b-waves, as well as in severe depletion of all RGCs studied except αONt-RGCs. DHF treatment resulted in rescued mean amplitudes of the pSTR and b-waves up to 21 days after NMDA. WB analysis revealed an increase in p-TrkB which correlates to the increase of TRKB protein and an increase in normalized pAKT/AKT. pMAPK/MAPK was upregulated earlier and significantly higher in DHF-treated retinas. DHF afforded survival of up to 49 % of the Brn3a+RGCs versus 25 % of the vehicle group at 21 days after NMDA, and improved survival of the α-RGC and αONs-RGCs but did not rescue the αOFF-RGCs.

CONCLUSION

Different RGC types exhibit variable susceptibilities to NMDA injury, and DHF-mediated activation of TrkB affords neuroprotection.

The association between asymmetric stress distribution on the lamina cribrosa and glaucoma progression

PURPOSE

The purpose of this study was to assess the effect of ocular movements on the progression of glaucoma.

METHODS

A total of 118 primary open-angle glaucoma patients were enrolled, comprising 71 patients in the progression group and 47 patients in the non-progression group. Utilizing three geometric parameters-axial length, optic disc radius, and optic cup deepening-a personalized virtual optic nerve head (ONH) model was designed. ONH biomechanical changes during ocular movement were simulated using a finite element analysis. Simulation results were analyzed and compared between the progression and non-progression groups.

RESULTS

In both progression and non-progression groups, ONH strains significantly increased with increasing rotation angle. When the eye rotated by 10°, the stress on the anterior surface of the lamina cribrosa on the temporal side was significantly higher in the progression group compared to the non-progression group (16.19 ± 0.90 kPa vs. 13.24 ± 3.00 kPa, P < 0.001). The stress ratio, indicating asymmetric stress distribution, was higher in the progression group than in the non-progression group (0.56 ± 0.13 vs. 0.49 ± 0.19, P = 0.018). Stress ratio significantly increased with increasing optic disc radius (standardized β = 0.303, P < 0.001) and optic cup deepening (standardized β = 0.538, P < 0.001).

CONCLUSIONS

Asymmetric stress distribution with ocular movement was higher in the progression group. This asymmetry was associated with optic disc radius and optic cup deepening. Therefore, ocular movement may contribute to the progression of glaucoma, with ONH geometry playing a role.

KEY MESSAGES

WHAT IS KNOWN : Ocular movement is considered one of the physical stress factors affecting the optic nerve head.

WHAT IS NEW

Ocular movement increased the strain on the optic nerve head and resulted in an asymmetric stress distribution on the lamina cribrosa surface. Asymmetric stress distribution on lamina cribrosa with ocular movement was higher in the glaucoma progression group and associated with optic disc radius and optic cup deepening.

Comparison of retinal and choroidal vascular changes via optical coherence tomography angiography between pseudoexfoliative glaucoma and pseudoexfoliation syndrome and control group

BACKGROUND

This study aims to evaluate the differences in retinal and choroidal vasculature using optical coherence tomography angiography (OCTA) in patients with pseudoexfoliative glaucoma (PEX-G) and patients with pseudoexfoliation syndrome without glaucoma (PEX-S) and the control group.

MATERIALS AND METHODS

In this cross-sectional study, 25 patients with PEX-G, 27 PEX-S cases, and 25 patients as the control group (C) were evaluated. All patients and the control group underwent macular and optic nerve head (ONH) OCTA, performed using the Optovue device. The foveal and parafoveal vascular density (VD) in superficial capillary plexus (SCP) and deep capillary plexus (DCP), foveal avascular zone (FAZ) area, outer retina flow area, choriocapillaris flow area, peripapillary VD, and retinal nerve fiber layer (RNFL) thickness in the macula and peripapillary region were assessed.

RESULTS

77 eyes of 77 patients were included. The mean foveal SCP VD in the control group was 9.62 % and 8.92 % higher than in the PEX-S and PEX-G groups; the median foveal DCP VD had 11.3 % and 13.2 % higher values in the controls. The PEX-S and PEX-G groups showed no differences regarding the macular microvasculature. The intradisc VD and the peripapillary VD were 7.5 % and 17.9 % lower in the PEX-G group compared to the controls. Besides, the peripapillary VD was significantly correlated with the RNFL thickness (R2:0.789, p:<0.0001).

CONCLUSION

In patients with PEX, regardless of glaucoma, the retinal VD showed deficiencies compared to the controls. Also, the peripapillary VD was strongly correlated with the RNFL thickness. These findings mean that retinal vascular changes might precede the development of glaucoma in PEX syndrome and can even be its cause.

Horizontal Gaze Tolerance and Its Effects on Visual Sensitivity in Glaucoma

Purpose

This study evaluates the effect of 6° horizontal gaze tolerance on visual field mean sensitivity (MS) in patients with glaucoma using a binocular head-mounted automated perimeter, following findings of structural changes in the posterior globe from magnetic resonance imaging and optical coherence tomography.

Methods

In this cross-sectional study, a total of 161 eyes (85 primary open-angle glaucoma [POAG] and 76 healthy) from 117 participants were included. Logistic regression and 1:1 matched analysis assessed the propensity score for glaucoma and healthy eyes, considering age, sex, and axial length as confounders. Visual field tests were performed with the imo perimeter (CREWT Medical Systems, Inc., Tokyo, Japan) at central gaze, 6° abduction, and 6° adduction positions as fixation points. A mixed-effects model was used to compare MS under all conditions.

Results

The analysis included a total of 82 eyes, with 41 POAG and 41 healthy after matching. The mean (standard deviation) age was 68.0 (11.0) years, with a mean deviation of -9.9 (6.6) dB for POAG and -1.0 (1.9) dB for healthy eyes using Humphrey field analysis 24-2. MS did not significantly differ among central gaze (27.0 [1.8] dB), abduction (27.1 [1.9] dB), and adduction (26.9 [2.2] dB) in healthy eyes (P = 0.650). However, MS was significantly lower for adduction (17.2 [5.9] dB) compared to central gaze (18.1 [5.9] dB) and abduction (17.9 [5.9] dB) in glaucoma eyes (P = 0.001 and P = 0.022, respectively).

Conclusions

Horizontal gaze, especially in adduction, significantly reduces visual sensitivity in glaucoma, suggesting a specific vulnerability associated with eye movement. This finding highlights the importance of eye positioning in glaucoma, warranting further investigation of its clinical significance.

Continuous Wireless Telemetric Measurement of Intraocular Pressure (IOP), Ocular Perfusion Pressure (OPP), and Cerebrospinal Fluid Pressure (CSFP) in Nonhuman Primates (NHPs)

Intraocular pressure (IOP) and cerebrospinal fluid pressure (CSFP) telemetry in large animal models can be used to determine the exact IOP, CSFP, translaminar pressure, and translaminar pressure gradient exposure that each normal and treated eye is subjected to relative to its fellow eye. In this way, it is possible to determine the independent contributions of each of these parameters (mean and/or transient fluctuations) to the risk of both the onset and rate of progression of glaucoma. Importantly, we have shown that IOP and CSFP fluctuate continuously by up to 100% over the course of the day, so snapshot cage-side IOP measurements are unable to adequately capture the pressure in the eye; CSFP is not measurable noninvasively at all. Implementation of IOP and CSFP telemetry will allow us to precisely determine the pressure insult in each eye of each animal and thereby unravel the true mechanisms underlying pressure-induced damage to the retinal ganglion cells in glaucoma.

Effects of Stress and Strain on the Optic Nerve Head on the Progression of Glaucoma

PRCIS

In primary open angle glaucoma, the rate of retinal nerve fiber layer thickness decrease was negatively correlated with lamina cribrosa strain, which was associated with intraocular pressure and optic nerve head geometric factors.

PURPOSE

We hypothesized that the biomechanical deformation of the optic nerve head (ONH) contributes to the progression of primary open angle glaucoma (POAG). This study investigated the biomechanical stress and strain on the ONH in patients with POAG using computer simulations based on finite element analysis and analyzed its association with disease progression.

METHODS

We conducted a retrospective analysis that included patients diagnosed with early-to-moderate stage POAG. The strains and stresses on the retinal nerve fiber layer (RNFL) surface, prelaminar region, and lamina cribrosa (LC) were calculated using computer simulations based on finite element analysis. The correlations between the rate of RNFL thickness decrease and biomechanical stress and strain were investigated in both the progression and nonprogression groups.

RESULTS

The study included 71 and 47 patients in the progression and nonprogression groups, respectively. In the progression group, the factors exhibiting negative correlations with the RNFL thickness decrease rate included the maximum and mean strain on the LC. In multivariate analysis, the mean strain on the LC was associated with optic disc radius, optic cup deepening, axial length, and mean intraocular pressure (IOP), whereas the maximum strain was only associated with mean IOP.

CONCLUSIONS

In early-to-moderate stage POAG, the rate of RNFL thickness decrease was influenced by both the mean and maximum strain on the LC. Strains on the LC were associated with mean IOP, optic disc radius, axial length, and optic cup deepening. These results suggest that not only IOP but also ONH geometric factors are important in the progression of glaucoma.

Tensile properties of glaucomatous human sclera, optic nerve, and optic nerve sheath

We characterized the tensile behavior of sclera, optic nerve (ON), and ON sheath in eyes from donors with glaucoma, for comparison with published data without glaucoma. Twelve freshly harvested eyes were obtained from donors with history of glaucoma, of average age 86 ± 7 (standard deviation) years. Rectangular samples were taken from anterior, equatorial, posterior, and peripapillary sclera, and ON sheath, while ON was in native form and measured using calipers. Under physiological temperature and humidity, tissues were preconditioned at 5% strain before loading at 0.1 mm/s. Force-displacement data were converted into engineering stress-strain curves fit by reduced polynomial hyperelastic models and analyzed by tangent moduli at 3% and 7% strain. Data were compared with an age-matched sample of 7 published control eyes. Optic atrophy was supported by significant reduction in ON cross section to 73% of normal in glaucomatous eyes. Glaucomatous was significantly stiffer than control in equatorial and peripapillary regions (P < 0.001). However, glaucomatous ON and sheath were significantly less stiff than control, particularly at low strain (P < 0.001). Hyperelastic models were well fit to stress-strain data (R2 > 0.997). Tangent moduli had variability similar to control in most regions, but was abnormally large in peripapillary sclera. Tensile properties were varied independently among various regions of the same eyes. Glaucomatous sclera is abnormally stiff, but the ON and sheath are abnormally compliant. These abnormalities correspond to properties predicted by finite element analysis to transfer potentially pathologic stress to the vulnerable disk and lamina cribrosa region during adduction eye movement.

Corneal Confocal and Specular Microscopic Characteristics in Primary Open-Angle Glaucoma: An Optical Coherence Tomography (OCT)-Based Case-Control Study

BACKGROUND

Glaucoma, particularly open-angle glaucoma (OAG), is a leading cause of irreversible blindness, associated with optic nerve damage, retinal ganglion cell death, and visual field defects. Corneal biomechanical properties and cellular components, such as corneal nerve and keratocyte densities assessed by in vivo confocal microscopy (IVCM), may serve as biomarkers for glaucoma progression. This study aimed to explore the relationship between corneal nerve parameters, keratocyte density, and optical coherence tomography (OCT)-derived retinal nerve fiber layer (RNFL) thickness in primary open-angle glaucoma (POAG) patients and controls.

METHODS

This case-control study was conducted at Al-Shifa Trust Eye Hospital, Rawalpindi, Pakistan, from January 2023 to October 2024. It included 26 eyes of 17 glaucoma patients and 28 eyes of 18 age-matched controls. POAG was diagnosed based on elevated intraocular pressure (IOP), optic disc changes, RNFL defects, and visual field abnormalities. Participants underwent full ophthalmic evaluation, including OCT for RNFL thickness, specular microscopy, and corneal confocal microscopy (CCM). Data were analyzed using IBM SPSS Statistics for Windows, Version 26.0 (Released 2019; IBM Corp., Armonk, New York, United States), with Spearman's correlation and linear regression for association analysis.

RESULTS

Anterior stromal keratocyte density was significantly lower in glaucoma patients (436.63±145.44 cells/mm²) compared to controls (546.54±141.20 cells/mm²; p=0.007). No significant difference was found in posterior stromal keratocyte density (p=0.788). Corneal nerve parameters showed a higher nerve fiber length in glaucoma patients (19.25±5.74 mm/mm²), but the difference was not significant (p=0.143). Nerve branch density was significantly higher in glaucoma patients (40.22±23.44 branches/mm²) compared to controls (26.12±10.17 branches/mm²; p=0.054). Specular microscopy revealed significantly lower endothelial cell density in glaucoma patients (2159.8±393.39 cells/mm²) compared to controls (2474.15±272.59 cells/mm²; p=0.002). OCT measurements showed a significantly thinner global RNFL in glaucoma patients (69.79±24.79 µm) compared to controls (98.86±9.04 µm; p<0.001). Spearman's correlation analysis showed that anterior keratocyte density was positively correlated with global (r=0.294; p=0.045), superior (r=0.312; p=0.031), and inferior quadrant RNFL thickness (r=0.285; p=0.049). It was also negatively correlated with central corneal thickness (CCT) (r=-0.367; p=0.039). In multivariate analysis, the duration of glaucoma was significantly associated with RNFL thickness (p=0.011).

CONCLUSION

This study found that anterior stromal keratocyte density was positively correlated with RNFL thickness, suggesting a potential link between corneal cellular changes and glaucoma severity. Endothelial cell density was significantly lower in glaucoma patients, which may reflect the disease's impact or the effect of medications on corneal health. While corneal nerve parameters did not show significant differences, these findings highlight the importance of corneal biomechanical properties in glaucoma pathophysiology. Further studies with larger sample sizes and longer follow-up periods are needed to confirm these findings and explore the role of corneal parameters in glaucoma progression.

Peripapillary vessels density is closely related to cerebral white matter hyperintensities: An OCTA study

BACKGROUND

Chronic cerebral hypoperfusion triggers the development of white matter hyperintensities (WMHs), common in cerebral small vessel disease (CSVD). However, conventional imaging techniques cannot visualize cerebral small vessels. The retina, a direct extension of the central nervous system, has an unclear correlation with WMHs. This study employs Optical coherence tomographic angiography (OCTA) to investigate vascular changes in the retina and explore its correlation with WMHs, aiming to provide a new method for assessing perfusion in early ischemic brain WMHs.

METHODS

Forty-nine patients with WMHs were stratified into mild and moderate/severe WMHs groups based on MRI findings, utilizing the Fazekas and Scheltens scales. OCTA assessed fundus vessel microcirculation. Logistic regression analyzed the correlation between ocular fundus microcirculation and WMH severity and location. Additionally, ROC curves evaluated the diagnostic efficacy of each fundus vascular microcirculation index in determining WMH severity.

RESULTS

After adjusting for multiple confounders, finding consistently indicated that the moderate/ severe WMHs group exhibited lower vessel density (VD) in the superior quadrant of the inner peripapillary region compared to the mild group [OR = 0.487, CI (0.255,0.929), p < 0.05]. ROC curves revealed that when combined with age, diabetes, and superior quadrant VD of the inner peripapillary region, specificity could be increased to 94.1%.

CONCLUSION

Peripapillary vessel density correlates closely with the severity of cerebral WMHs. Early morphological changes due to chronic hypoperfusion may initiate from the inner layer of the optic disc, and OCTA could offer a novel method for evaluating blood perfusion in ischemic WMHs.

Optic Nerve Head Morphology and Macula Ganglion Cell Inner Plexiform Layer Thickness in Axially Anisometropic Rhesus Monkeys

Purpose

The purpose of this study was to determine the effects of axial elongation on optic nerve head morphology and macula inner retinal thickness in young rhesus monkeys.

Methods

Both eyes of 26 anisometropic, 1-year-old rhesus monkeys were imaged using optical coherence tomography (OCT). Before imaging, the animals were sedated, their eyes were dilated, and axial length was measured using an optical biometer. OCT imaging included a 20 degrees, 24-line radial scan centered on the optic nerve head (ONH) and two 20 degrees × 20 degrees raster scans, one centered on the ONH and the other on the macula. Radial scans were analyzed using programs written in MATLAB to quantify the Bruch's membrane opening (BMO) area and position, minimum rim width (MRW), anterior lamina cribrosa surface (ALCS) position, size of any scleral crescent, circumpapillary retinal nerve fiber layer (RNFL), and choroid thickness (pCh). Macula total retinal thickness (mTRT) and ganglion cell inner plexiform layer (GCIPL) thicknesses were quantified from macula scans. Linear least square regression was determined for OCT measures and axial length of the right eye, and for inter-eye differences.

Results

Animals were 341 ± 18 days old at the time of imaging. BMO area (R2 = 0.38), ALCS position (R2 = 0.45), scleral crescent area (R2 = 0.35), pCh thickness (R2 = 0.21), mTRT (R2 = 0.24), and GCIPL thickness (R2 = 0.16) were correlated with the axial length (all P < 0.05). For each of these parameters, the right-eye regression slope did not differ from the slope of the interocular difference (P > 0.57).

Conclusions

There are posterior segment morphological differences in anisometropic rhesus monkeys related to axial length. Whether these differences increase the risk of pathology remains to be investigated.

The Temporal Relation Between Rates of Retinal Nerve Fiber Layer and Minimum Rim Width Changes in Glaucoma

Purpose

This study aims to determine whether OCT-derived rates of change in minimum rim width (MRW) are associated with and can potentially predict corresponding alterations in retinal nerve fiber layer thickness (RNFLT) in people with glaucoma.

Methods

The rates of change between six-monthly visits were taken from 568 eyes of 278 participants in the P3 Study. Structural equation models (SEM) assessed whether one parameter was predicted by the concurrent or previous rate of the other parameter, after adjusting for its own rate in the previous time interval. Root mean square error of approximation (RMSEA, with 90% confidence intervals [CI]), Tucker Lewis index (TLI) and the comparative fit index (CFI) assessed goodness of fit.

Results

Models without a time lag provided a better fit for the data (RMSEA = 0.101 [CI, 0.089, 0.113]), compared to a model featuring a time lag in RNFLT (RMSEA = 0.114 [CI, 0.102, 0.126]) or MRW (RMSEA = 0.114 [CI, 0.102, 0.127]). The SEMs indicated that rates for both MRW and RNFLT were predicted by their own rate in the previous time interval and by the other measure's change in the concurrent time interval (P > 0.001 for all). No evidence of a clinically significant time lag for either parameter was determined.

Conclusions

MRW and RNFLT exhibit concurrent changes over time in patients with glaucoma, with no clinically significant time lag determined.

Translational Relevance

RNFLT may be more useful than MRW in early glaucoma assessment because of its previously reported lower variability and reduced sensitivity to intraocular pressure changes.

Adduction induces large optic nerve head deformations in subjects with normal-tension glaucoma

PURPOSE

To assess intraocular pressure (IOP)-induced and gaze-induced optic nerve head (ONH) strains in subjects with high-tension glaucoma (HTG) and normal-tension glaucoma (NTG).

DESIGN

Clinic-based cross-sectional study.

METHODS

The ONH from one eye of 228 subjects (114 subjects with HTG (pre-treatment IOP≥21 mm Hg) and 114 with NTG (pre-treatment IOP<21 mm Hg)) was imaged with optical coherence tomography (OCT) under the following conditions: (1) OCT primary gaze, (2) 20° adduction from OCT primary gaze, (3) 20° abduction from OCT primary gaze and (4) OCT primary gaze with acute IOP elevation (to approximately 33 mm Hg). We then performed digital volume correlation analysis to quantify IOP-induced and gaze-induced ONH tissue deformations and strains.

RESULTS

Across all subjects, adduction generated high effective strain (4.4%±2.3%) in the LC tissue with no significant difference (p>0.05) with those induced by IOP elevation (4.5%±2.4%); while abduction generated significantly lower (p=0.01) effective strain (3.1%±1.9%). The lamina cribrosa (LC) of HTG subjects exhibited significantly higher effective strain than those of NTG subjects under IOP elevation (HTG: 4.6%±1.7% vs NTG: 4.1%±1.5%, p<0.05). Conversely, the LC of NTG subjects exhibited significantly higher effective strain than those of HTG subjects under adduction (NTG: 4.9%±1.9% vs HTG: 4.0%±1.4%, p<0.05).

CONCLUSION

We found that NTG subjects experienced higher strains due to adduction than HTG subjects, while HTG subjects experienced higher strain due to IOP elevation than NTG subjects-and that these differences were most pronounced in the LC tissue.

Fibrous finite element modeling of the optic nerve head region

The optic nerve head (ONH) region at the posterior pole of the eye is supported by a fibrous structure of collagen fiber bundles. Discerning how the fibrous structure determines the region biomechanics is crucial to understand normal physiology, and the roles of biomechanics on vision loss. The fiber bundles within the ONH structure exhibit complex three-dimensional (3D) organization and continuity across the various tissue components. Computational models of the ONH, however, usually represent collagen fibers in a homogenized fashion without accounting for their continuity across tissues, fibers interacting with each other and other fiber-specific effects in a fibrous structure. We present a fibrous finite element (FFE) model of the ONH that incorporates discrete collagen fiber bundles and their histology-based 3D organization to study ONH biomechanics as a fibrous structure. The FFE model was constructed using polarized light microscopy data of porcine ONH cryosections, representing individual fiber bundles in the sclera, dura and pia maters with beam elements and canal tissues as continuum structures. The FFE model mimics the histological in-plane orientation and width distributions of collagen bundles as well as their continuity across different tissues. Modeling the fiber bundles as linear materials, the FFE model predicts the nonlinear ONH response observed in an inflation experiment from the literature. The model also captures important microstructural mechanisms including fiber interactions and long-range strain transmission among bundles that have not been considered before. The FFE model presented here advances our understanding of the role of fibrous collagen structure in the ONH biomechanics. STATEMENT OF SIGNIFICANCE: The microstructure and mechanics of the optic nerve head (ONH) are central to ocular physiology. Histologically, the ONH region exhibits a complex continuous fibrous structure of collagen bundles. Understanding the role of the fibrous collagen structure on ONH biomechanics requires high-fidelity computational models previously unavailable. We present a computational model of the ONH that incorporates histology-based fibrous collagen structure derived from polarized light microscopy images. The model predictions agree with experiments in the literature, and provide insight into important microstructural mechanisms of fibrous tissue biomechanics, such as long-range strain transmission along fiber bundles. Our model can be used to study the microstructural basis of biomechanical damage and the effects of collagen remodeling in glaucoma.

Parapapillary gamma zone associated with increased peripapillary scleral bowing: the Beijing Eye Study 2011

AIMS

To investigate the association between the backward configuration of the peripapillary sclera (PPS), measured as PPS angle (PPSA), and presence and extent of parapapillary gamma zone.

METHODS

Out of the population-based Beijing Eye Study 2011, we randomly selected individuals free of optic nerve and retinal diseases. With Spectralis optical coherence tomography, we measured gamma zone (zone free of Bruch's membrane (BM)) and determined the PPSA, defined as the angle between the anterior scleral surface lines from both sides of the optic nerve head (ONH).

RESULTS

The study included 678 individuals with age of 59.5±7.6 years (range: 50-90) and axial length of 23.5±1.3 mm (20.9-29.2). Gamma zone was more prevalent in eyes with larger PPSA (p=0.006) after adjustment for axial length (p<0.001) and BM opening area (p<0.001). Gamma zone width was positively associated with PPSA, axial length and BM opening area (all p<0.001) in multivariable analysis. Circular gamma zone was accompanied with larger PPSA as compared with focal gamma zone (19.9°±7.2° vs 6.3°±5.3°, p<0.001). Focal temporal gamma and focal inferior gamma had similar mean PPSA (p=0.69). However, the horizontal PPSA was significantly larger than the vertical PPSA in inferior gamma (6.9°±6.3° vs 4.7°±6.6°; p=0.005), while they were comparable in temporal gamma (6.1°±5.8° vs 6.3°±6.4°; p=0.073).

CONCLUSIONS

A more backward bowing of the PPS was linearly and spatially associated with the presence, size and extent of gamma zone. It suggested that the BM and the sclera were closely related in participating the biomechanical behaviour of the ONH.

Neurovascular dysfunction in glaucoma

Retinal ganglion cells, the neurons that die in glaucoma, are endowed with a high metabolism requiring optimal provision of oxygen and nutrients to sustain their activity. The timely regulation of blood flow is, therefore, essential to supply firing neurons in active areas with the oxygen and glucose they need for energy. Many glaucoma patients suffer from vascular deficits including reduced blood flow, impaired autoregulation, neurovascular coupling dysfunction, and blood-retina/brain-barrier breakdown. These processes are tightly regulated by a community of cells known as the neurovascular unit comprising neurons, endothelial cells, pericytes, Müller cells, astrocytes, and microglia. In this review, the neurovascular unit takes center stage as we examine the ability of its members to regulate neurovascular interactions and how their function might be altered during glaucomatous stress. Pericytes receive special attention based on recent data demonstrating their key role in the regulation of neurovascular coupling in physiological and pathological conditions. Of particular interest is the discovery and characterization of tunneling nanotubes, thin actin-based conduits that connect distal pericytes, which play essential roles in the complex spatial and temporal distribution of blood within the retinal capillary network. We discuss cellular and molecular mechanisms of neurovascular interactions and their pathophysiological implications, while highlighting opportunities to develop strategies for vascular protection and regeneration to improve functional outcomes in glaucoma.

Neuroretinal Rim Response to Transient Intraocular Pressure Challenge Predicts the Extent of Retinal Ganglion Cell Loss in Experimental Glaucoma

Purpose

To determine if the optic nerve head (ONH) response to transient elevated intraocular pressure (IOP) can predict the extent of neural loss in the nonhuman primate experimental glaucoma model.

Methods

The anterior chamber pressure of 21 healthy animals (5.4 ± 1.2 years, 8 female) was adjusted to 25 mm Hg for two hours followed by 10 mm Hg for an additional two hours. For the duration of IOP challenge the ONH was imaged using radial optical coherence tomography (OCT) scans at five-minute intervals. Afterward, a randomized sample of 14 of these subjects had unilateral experimental glaucoma induced and were monitored with OCT imaging, tonometry, and ocular biometry at two-week intervals.

Results

With pressure challenge, the maximum decrease in ONH minimum rim width (MRW) was 40 ± 10.5 µm at 25 mm Hg and was correlated with the precannulation MRW, Bruch's membrane opening (BMO) position, and the anterior lamina cribrosa surface position (P = 0.01). The maximum return of MRW at 10 mm Hg was 16.1 ± 5.0 µm and was not associated with any precannulation ONH feature (P = 0.24). However, healthy eyes with greater thickness return at 10 mm Hg had greater loss of MRW and retinal nerve fiber layer (RNFL) at a cumulative IOP of 1000 mm Hg · days after induction of experimental glaucoma. In addition, MRW and RNFL thinning was correlated with an increase in axial length (P < 0.01).

Conclusion

This study's findings suggest that the ONH's response to transient changes in IOP are associated with features of the ONH and surrounding tissues. The neural rim properties at baseline and the extent of axial elongation are associated with the severity of glaucomatous loss in the nonhuman primate model.

Glaucoma - 'A Stiff Eye in a Stiff Body'

Glaucoma is a progressive, age-related optic neuropathy, whereby the prevalence increases sharply over the age of 60 and is associated with increased systemic tissue stiffness. On a molecular basis, this is associated with increased deposition of collagen and loss of elastin structure, resulting in aberrant biomechanical compliance and reduced tissue elasticity. Increased tissue stiffness is a known driver of myofibroblast activation and persistence, especially in chronic cellular injuries via mechanotransduction pathways mediated by integrins and focal adhesion kinases. Evidence from histological and imaging studies plus force measurements of glaucomatous eyes show that several ocular tissues are stiffer than normal, healthy age-matched controls including the trabecular meshwork, Schlemm's canal, cornea, sclera and the lamina cribrosa. This is associated with increased extracellular matrix deposition and fibrosis. This review reports on the evidence to support the concept that glaucoma represents 'a stiff eye in a stiff body' and addresses potential mechanisms to attenuate this.

Ocular Rigidity and Current Therapy

Purpose: Ocular rigidity (OR) is an important biomechanical parameter of the eye accounting for the material and geometrical properties of the corneoscleral shell.Methods: This study used a literature search to review the role of ocular rigidity and the application of potential therapies targeting this parameter in glaucoma and myopia.Conclusion: Biomechanical modeling and improved understanding of the biochemistry, and molecular arrangement of sclera and its constituents have yielded important insights. Recent developments, including that of a non-invasive and direct OR measurement method and improved ocular imaging techniques are helping to elucidate the role of OR in healthy and diseased eyes by facilitating large scale and longitudinal clinical studies. Improved understanding of OR at the initial stages of disease processes and its alterations with disease progression will undoubtedly propel research in the field. Furthermore, a better understanding of the determinants of OR is helping to refine novel therapeutic approaches which target and alter the biomechanical properties of the sclera in sight-threatening conditions such as glaucoma and myopia.

Aging Effects on Optic Nerve Neurodegeneration

Common risk factors for many ocular pathologies involve non-pathologic, age-related damage to the optic nerve. Understanding the mechanisms of age-related changes can facilitate targeted treatments for ocular pathologies that arise at any point in life. In this review, we examine these age-related, neurodegenerative changes in the optic nerve, contextualize these changes from the anatomic to the molecular level, and appreciate their relationship with ocular pathophysiology. From simple structural and mechanical changes at the optic nerve head (ONH), to epigenetic and biochemical alterations of tissue and the environment, multiple age-dependent mechanisms drive extracellular matrix (ECM) remodeling, retinal ganglion cell (RGC) loss, and lowered regenerative ability of respective axons. In conjunction, aging decreases the ability of myelin to preserve maximal conductivity, even with "successfully" regenerated axons. Glial cells, however, regeneratively overcompensate and result in a microenvironment that promotes RGC axonal death. Better elucidating optic nerve neurodegeneration remains of interest, specifically investigating human ECM, RGCs, axons, oligodendrocytes, and astrocytes; clarifying the exact processes of aged ocular connective tissue alterations and their ultrastructural impacts; and developing novel technologies and pharmacotherapies that target known genetic, biochemical, matrisome, and neuroinflammatory markers. Management models should account for age-related changes when addressing glaucoma, diabetic retinopathy, and other blinding diseases.

Loss of epigenetic information as a cause of mammalian aging

All living things experience an increase in entropy, manifested as a loss of genetic and epigenetic information. In yeast, epigenetic information is lost over time due to the relocalization of chromatin-modifying proteins to DNA breaks, causing cells to lose their identity, a hallmark of yeast aging. Using a system called "ICE" (inducible changes to the epigenome), we find that the act of faithful DNA repair advances aging at physiological, cognitive, and molecular levels, including erosion of the epigenetic landscape, cellular exdifferentiation, senescence, and advancement of the DNA methylation clock, which can be reversed by OSK-mediated rejuvenation. These data are consistent with the information theory of aging, which states that a loss of epigenetic information is a reversible cause of aging.

Identification of Factors Associated With the Development of Optic Disc Edema During Spaceflight

Importance

Approximately 70% of crew members who complete long-duration missions to the International Space Station develop signs of optic disc edema, a hallmark finding of spaceflight-associated neuro-ocular syndrome. The onset and magnitude of edema differ across individuals, and the reason for this variability remains unknown. Identifying risk factors for spaceflight-induced disc edema is important because this condition may become more severe during extended-duration missions to the moon and Mars and could be associated with irreversible vision loss.

Objective

To assess whether preflight indicators of crowded optic nerve head morphology, other ocular measures (such as choroid thickness and axial length), body weight, body mass index, sex, age, and previous flight experience are associated with optic disc edema development.

Design, Setting, and Participants

This cohort study analyzed ocular, body weight, and demographic data collected from 31 US and international crew members before, during, and after spaceflight at the NASA Johnson Space Center and International Space Station. Ocular factors assessed included preflight and in-flight peripapillary total retinal thickness, minimum rim width, optic cup volume, mean cup depth, mean cup width, cup-disc ratio, Bruch membrane opening area, retinal nerve fiber layer thickness, choroid thickness, axial length, and refractive error. In addition, body weight, body mass index, sex, age, and previous spaceflight experience were assessed for associations with optic disc edema development. The data were analyzed from August 2021 to June 2022.

Exposure

Approximately 6 to 12 months of spaceflight.

Main Outcomes and Measures

In-flight increases in peripapillary total retinal thickness. Linear mixed models were used to assess for associations between a wide range of risk factors and in-flight increases in peripapillary total retinal thickness, which is a sensitive objective measure for detecting optic disc edema.

Results

This study included 31 International Space Station crew members with a mean (SD) age of 46.9 (6.0) years (25 men [80.6%]). During spaceflight, mean (SE) peripapillary total retinal thickness increased from 392.0 (5.8) μm to 430.2 (9.6) μm (P < .001), and greater individual changes were associated with smaller preflight cup volume (slope [SE], -62.8 [18.9]; P = .002), shallower preflight cup depth (slope [SE], -0.11 [0.03]; P < .001), and narrower preflight cup width (slope [SE], -0.03 [0.01]; P = .03). No associations were observed between changes in peripapillary total retinal thickness and any other variable evaluated.

Conclusions and Relevance

Findings of this cohort study suggest that smaller optic cup morphology may be associated with optic disc edema development during spaceflight. Crew members with this cup profile may benefit from enhanced ophthalmic monitoring during spaceflight and use of countermeasures against spaceflight-associated neuro-ocular syndrome.

Relative Contributions of Intraocular and Cerebrospinal Fluid Pressures to the Biomechanics of the Lamina Cribrosa and Laminar Neural Tissues

Purpose

The laminar region of the optic nerve head (ONH), thought to be the site of damage to the retinal ganglion cell axons in glaucoma, is continuously loaded on its anterior and posterior surfaces by dynamic intraocular pressure (IOP) and orbital cerebrospinal fluid pressure (CSFP), respectively. Thus, translaminar pressure (TLP; TLP = IOP-CSFP) has been proposed as a glaucoma risk factor.

Methods

Three eye-specific finite element models of the posterior human eye were constructed, including full 3D microstructures of the load-bearing lamina cribrosa (LC) with interspersed laminar neural tissues (NTs), and heterogeneous, anisotropic, hyperelastic material formulations for the surrounding peripapillary sclera and adjacent pia. ONH biomechanical responses were simulated using three combinations of IOP and CSFP loadings consistent with posture change from sitting to supine.

Results

Results show that tensile, compressive, and shear stresses and strains in the ONH were higher in the supine position compared to the sitting position (P < 0.05). In addition, LC beams bear three to five times more TLP-driven stress than interspersed laminar NT, whereas laminar NT exhibit three to five times greater strain than supporting LC (P < 0.05). Compared with CSFP, IOP drove approximately four times greater stress and strain in the LC, NT, and peripapillary sclera, normalized per mm Hg pressure change. In addition, IOP drove approximately three-fold greater scleral canal expansion and anterior-posterior laminar deformation than CSFP per mm Hg (P < 0.05).

Conclusions

Whereas TLP has been hypothesized to play a prominent role in ONH biomechanics, the IOP and CSFP effects are not equivalent, as IOP-driven stress, strain, and deformation play a more dominant role than CSFP effects.

Ocular Pulse Amplitude Correlates With Ocular Rigidity at Native IOP Despite the Variability in Intraocular Pulse Volume With Each Heartbeat

Purpose

The purpose of this study was to assess ocular coat mechanical behavior using controlled ocular microvolumetric injections (MVI) of 15 µL of balanced salt solution (BSS) infused over 1 second into the anterior chamber (AC) via a syringe pump.

Methods

Intraocular pressure (IOP) was continuously recorded at 200 Hz with a validated implantable IOP telemetry system in 7 eyes of 7 male rhesus macaques (nonhuman primates [NHPs]) during 5 MVIs in a series at native (3 trials), 15 and 20 mm Hg baseline IOPs, repeated in 2 to 5 sessions at least 2 weeks apart. Ocular rigidity coefficients (K) and ocular pulse volume (PV) were calculated for each trial. Data were averaged across sessions within eyes; PV was analyzed with a three-level nested ANOVA, and parameter relationships were analyzed with Pearson Correlation and linear regression.

Results

After MVI at native baseline IOP of 10.4 ± 1.6 mm Hg, IOP increased by 9.1 ± 2.8 mm Hg (∆IOP) at a 9.6 ± 2.7 mm Hg/s slope, ocular pulse amplitude (OPA) was 0.70 ± 0.13 mm Hg on average; the average K was 0.042 ± 0.010 µL-1 and average PV was 1.16 ± 0.43 µL. PV varied significantly between trials, days, and eyes (P ≤ 0.05). OPA was significantly correlated with K at native IOP: Pearson coefficients ranged from 0.71 to 0.83 (P ≤ 0.05) and R2 ranged from 0.50 to 0.69 (P ≤ 0.05) during the first trial.

Conclusions

The MVI-driven ∆IOP and slope can be used to assess ocular coat mechanical behavior and measure ocular rigidity.

Translational Relevance

Importantly, OPA at native IOP is correlated with ocular rigidity despite the significant variability in PV between heartbeats.

Evidence of an Annexin A4 mediated plasma membrane repair response to biomechanical strain associated with glaucoma pathogenesis

Glaucoma is a common neurodegenerative blinding disease that is closely associated with chronic biomechanical strain at the optic nerve head (ONH). Yet, the cellular injury and mechanosensing mechanisms underlying the resulting damage have remained critically unclear. We previously identified Annexin A4 (ANXA4) from a proteomic analyses of human ONH astrocytes undergoing pathological biomechanical strain that mimics glaucomatous conditions. Annexins are a family of calcium-dependent phospholipid binding proteins with key functions in plasma membrane repair (PMR); an active mechanism to limit and mend cellular injury that involves membrane and cytoskeletal reorganizations. However, a role for direct membrane damage and PMR has not been well studied in the context of biomechanical strain, such as that associated with glaucoma. Here we report that this moderate strain surprisingly damages cell membranes to increase permeability in a calcium-dependent manner, and induces rapid aggregation of ANXA4 at injury sites. ANXA4 loss-of-function increases permeability, while exogenous ANXA4 reduces it. Furthermore, ANXA4 aggregation is associated with F-actin dynamics in vitro, and remarkably this interaction and aggregation signature is also observed in the glaucomatous ONH in patient samples. Together these studies link moderate biomechanical strain with direct membrane damage and actin dynamics, and identify an active PMR role for ANXA4 in new model of cell injury associated with glaucoma pathogenesis.

Elevated IOP Alters the Material Properties of Sclera and Lamina Cribrosa in Monkeys

Objective

Elevated intraocular pressure (IOP) has significant impacts on different stages in the progression of chronic glaucoma. In this study, we investigated changes in the material properties of sclera and lamina cribrosa (LC) in a nonhuman primate model with elevated IOP.

Methods

Normal adult Tibetan macaques were selected for the construction of elevated IOP model. After 40 days of stable maintenance on the ocular hypertension, the binocular eyeballs were obtained for the measurement of macroscopic parameters of the eyeballs. Posterior scleral tissue strips were obtained in circumferential and axial directions, and thickness was measured, respectively. Biomechanical parameters were obtained with stress relaxation, creep, and tensile test. The nanoindentation test was performed on the LC and scleral tissue around optic nerve head (ONH) to obtain compressive modulus.

Results

In the presence of elevated IOP, variations of the axial diameter of the eyeball were greater than those of the transverse diameter, and the mean scleral thickness around ONH was smaller in the experimental group than control group. The elastic modulus and stress relaxation modulus of sclera were larger, and the creep rate was lower in the experimental group than control group. In the control group, the elastic modulus and stress relaxation modulus of the circumferential sclera were larger in the axial direction, and creep rate was smaller. In the experimental group, there was no significant difference in biomechanical characteristics between the two directions. Compared to the control group, the compression modulus of the LC was smaller, and the compression modulus of sclera around ONH was larger in the experimental group.

Conclusion

Elevated IOP alters the viscoelasticity and anisotropy of sclera and LC. These may contribute to reduction of the organizational resistance to external forces and decline in the ability of self-recovery.

Widefield OCT Imaging for Quantifying Inner Retinal Thickness in the Nonhuman Primate

Purpose

To determine the agreement and repeatability of inner retinal thickness measures from widefield imaging compared to standard scans in healthy nonhuman primates.

Methods

Optical coherence tomography (OCT) scans were acquired from 30 healthy rhesus monkeys, with 11 animals scanned at multiple visits. The scan protocol included 20° × 20° raster scans centered on the macula and optic nerve head (ONH), a 12° diameter circular scan centered on the ONH, and a 55 × 45° widefield raster scan. Each scan was segmented using custom neural network–based algorithms. Bland–Altman analysis were used for comparing average circumpapillary retinal nerve fiber layer (RNFL) thickness and ganglion cell inner plexiform layer (GCIPL) thickness for a 16° diameter region. Comparisons were also made for similar 1° × 1° superpixels from the raster scans.

Results

Average circumpapillary RNFL thickness from the circular scan was 114.2 ± 5.8 µm, and 113.2 ± 7.3 µm for an interpolated scan path from widefield imaging (bias = −1.03 µm, 95% limits of agreement [LOA] −8.6 to 6.5 µm). GCIPL thickness from standard raster scans was 72.7 ± 4.3 µm, and 73.7 ± 3.7 µm from widefield images (bias = 1.0 µm, 95% LOA −2.4 to 4.4 µm). Repeatability for both RNFL and GCIPL standard analysis was less than 5.2 µm. For 1° × 1° superpixels, the 95% limits of agreement were between −13.9 µm and 13.7 µm for RNFL thickness and −2.5 µm and 2.5 µm for GCIPL thickness.

Conclusions

Inner retinal thickness measures from widefield imaging have good repeatability and are comparable to those measured using standard scans.

Translational Relevance

Monitoring retinal ganglion cell loss in the non-human primate experimental glaucoma model could be enhanced using widefield imaging.

Remodeling of the Lamina Cribrosa: Mechanisms and Potential Therapeutic Approaches for Glaucoma

Glaucomatous optic neuropathy is the leading cause of irreversible blindness in the world. The chronic disease is characterized by optic nerve degeneration and vision field loss. The reduction of intraocular pressure remains the only proven glaucoma treatment, but it does not prevent further neurodegeneration. There are three major classes of cells in the human optic nerve head (ONH): lamina cribrosa (LC) cells, glial cells, and scleral fibroblasts. These cells provide support for the LC which is essential to maintain healthy retinal ganglion cell (RGC) axons. All these cells demonstrate responses to glaucomatous conditions through extracellular matrix remodeling. Therefore, investigations into alternative therapies that alter the characteristic remodeling response of the ONH to enhance the survival of RGC axons are prevalent. Understanding major remodeling pathways in the ONH may be key to developing targeted therapies that reduce deleterious remodeling.

Retinal Glutamate Neurotransmission: From Physiology to Pathophysiological Mechanisms of Retinal Ganglion Cell Degeneration

Glutamate neurotransmission and metabolism are finely modulated by the retinal network, where the efficient processing of visual information is shaped by the differential distribution and composition of glutamate receptors and transporters. However, disturbances in glutamate homeostasis can result in glutamate excitotoxicity, a major initiating factor of common neurodegenerative diseases. Within the retina, glutamate excitotoxicity can impair visual transmission by initiating degeneration of neuronal populations, including retinal ganglion cells (RGCs). The vulnerability of RGCs is observed not just as a result of retinal diseases but has also been ascribed to other common neurodegenerative and peripheral diseases. In this review, we describe the vulnerability of RGCs to glutamate excitotoxicity and the contribution of different glutamate receptors and transporters to this. In particular, we focus on the N-methyl-d-aspartate (NMDA) receptor as the major effector of glutamate-induced mechanisms of neurodegeneration, including impairment of calcium homeostasis, changes in gene expression and signalling, and mitochondrial dysfunction, as well as the role of endoplasmic reticular stress. Due to recent developments in the search for modulators of NMDA receptor signalling, novel neuroprotective strategies may be on the horizon.

Solving neurodegeneration: common mechanisms and strategies for new treatments

Across neurodegenerative diseases, common mechanisms may reveal novel therapeutic targets based on neuronal protection, repair, or regeneration, independent of etiology or site of disease pathology. To address these mechanisms and discuss emerging treatments, in April, 2021, Glaucoma Research Foundation, BrightFocus Foundation, and the Melza M. and Frank Theodore Barr Foundation collaborated to bring together key opinion leaders and experts in the field of neurodegenerative disease for a virtual meeting titled "Solving Neurodegeneration". This "think-tank" style meeting focused on uncovering common mechanistic roots of neurodegenerative disease and promising targets for new treatments, catalyzed by the goal of finding new treatments for glaucoma, the world's leading cause of irreversible blindness and the common interest of the three hosting foundations. Glaucoma, which causes vision loss through degeneration of the optic nerve, likely shares early cellular and molecular events with other neurodegenerative diseases of the central nervous system. Here we discuss major areas of mechanistic overlap between neurodegenerative diseases of the central nervous system: neuroinflammation, bioenergetics and metabolism, genetic contributions, and neurovascular interactions. We summarize important discussion points with emphasis on the research areas that are most innovative and promising in the treatment of neurodegeneration yet require further development. The research that is highlighted provides unique opportunities for collaboration that will lead to efforts in preventing neurodegeneration and ultimately vision loss.

Finite element modeling of the complex anisotropic mechanical behavior of the human sclera and pia mater

BACKGROUND AND OBJECTIVE

Accurate finite element (FE) simulation of the optic nerve head (ONH) depends on accurate mechanical properties of the load-bearing tissues. The peripapillary sclera in the ONH exhibits a depth-dependent, anisotropic, heterogeneous collagen fiber distribution. This study proposes a novel cable-in-solid modeling approach that mimics heterogeneous anisotropic collagen fiber distribution, validates the approach against published experimental biaxial tensile tests of scleral patches, and demonstrates its effectiveness in a complex model of the posterior human eye and ONH.

METHODS

A computational pipeline was developed that defines control points in the sclera and pia mater, distributes the depth-dependent circumferential, radial, and isotropic cable elements in the sclera and pia in a pattern that mimics collagen fiber orientation, and couples the cable elements and solid matrix using a mesh-free penalty-based cable-in-solid algorithm. A parameter study was performed on a model of a human scleral patch subjected to biaxial deformation, and computational results were matched to published experimental data. The new approach was incorporated into a previously published eye-specific model to test the method; results were then interpreted in relation to the collagen fibers' (cable elements) role in the resultant ONH deformations, stresses, and strains.

RESULTS

Results show that the cable-in-solid approach can mimic the full range of scleral mechanical behavior measured experimentally. Disregarding the collagen fibers/cable elements in the posterior eye model resulted in ∼20-60% greater tensile and shear stresses and strains, and ∼30% larger posterior deformations in the lamina cribrosa and peripapillary sclera.

CONCLUSIONS

The cable-in-solid approach can easily be implemented into commercial FE packages to simulate the heterogeneous and anisotropic mechanical properties of collagenous biological tissues.

Matrix Mechanotransduction via Yes-Associated Protein in Human Lamina Cribrosa Cells in Glaucoma

Purpose

Extracellular matrix stiffening is characteristic of both aging and glaucoma, and acts as a promoter and perpetuator of pathological fibrotic remodeling. Here, we investigate the role of a mechanosensitive transcriptional coactivator, Yes-associated protein (YAP), a downstream effector of multiple signaling pathways, in lamina cribrosa (LC) cell activation to a profibrotic, glaucomatous state.

Methods

LC cells isolated from glaucomatous human donor eyes (GLC; n = 3) were compared to LC cells from age-matched nonglaucomatous controls (NLC; n = 3) to determine differential YAP expression, protein levels, and proliferation rates. NLC cells were then cultured on soft (4 kPa), and stiff (100 kPa), collagen-1 coated polyacrylamide hydrogel substrates. Quantitative real-time RT-PCR, immunoblotting, and immunofluorescence microscopy were used to measure the expression, activity, and subcellular location of YAP and its downstream targets, respectively. Proliferation rates were examined in NLC and GLC cells by methyl thiazolyl tetrazolium salt assays, across a range of incrementally increased substrate stiffness. Endpoints were examined in the presence or absence of a YAP inhibitor, verteporfin (2 µM).

Results

GLC cells show significantly (P < 0.05) increased YAP gene expression and total-YAP protein compared to NLC cells, with significantly increased proliferation. YAP regulation is mechanosensitive, because NLC cells cultured on pathomimetic, stiff substrates (100 kPa) show significantly upregulated YAP gene and protein expression, increased YAP phosphorylation at tyrosine 357, reduced YAP phosphorylation at serine 127, increased nuclear pooling, and increased transcriptional target, connective tissue growth factor. Accordingly, myofibroblastic markers, α-smooth muscle actin (α-SMA) and collagen type I, alpha 1 (Col1A1) are increased. Proliferation rates are elevated on 50 kPa substrates and tissue culture plastic. Verteporfin treatment significantly inhibits YAP-mediated cellular activation and proliferation despite a stiffened microenvironment.

Conclusions

These data demonstrate how YAP plays a pivotal role in LC cells adopting a profibrotic and proliferative phenotype in response to the stiffened LC present in aging and glaucoma. YAP provides an attractive and novel therapeutic target, and its inhibition via verteporfin warrants further clinical investigation.

Is the human sclera a tendon-like tissue? A structural and functional comparison

Collagen rich connective tissues fulfill a variety of important functions throughout the human body, most of which having to resist mechanical challenges. This review aims to compare structural and functional aspects of tendons and sclera, two tissues with distinct location and function, but with striking similarities regarding their cellular content, their extracellular matrix and their low degree of vascularization. The description of these similarities meant to provide potential novel insight for both the fields of orthopedic research and ophthalmology.

An Eye on Movement Disorders

Eye disorders spanning a range of ocular tissue are common in patients with movement disorders. Highlighting these ocular manifestations will benefit patients and may even aid in diagnosis. In this educational review we outline the anatomy and function of the ocular tissues with a focus on the tissues most affected in movement disorders. We review the movement disorders associated with ocular pathology and where possible explore the underlying cellular basis thought to be driving the pathology and provide a brief overview of ophthalmic investigations available to the neurologist. This review does not cover intracranial primary visual pathways, higher visual function, or the ocular motor system.

Comparison of Corneal Wave Speed and Ocular Rigidity in Normal and Glaucomatous Eyes

PRECIS

Ocular biomechanics were compared between treated glaucoma patients and healthy subjects matched for age, intraocular pressure (IOP), and axial length. There was no difference in corneal wave propagation speed, but ocular rigidity was lower in glaucomatous eyes.

PURPOSE

Ocular biomechanical properties are important in understanding glaucoma pathogenesis but the affected tissues are unclear. In this study, we compared corneal wave speed (a measure of corneal elasticity) and ocular rigidity coefficient between glaucomatous and normal eyes.

MATERIALS AND METHODS

Twenty glaucomatous eyes from 10 patients and 20 normal eyes from 13 controls, matched for age, IOP, and axial length were included. Ocular rigidity was calculated based on the difference in supine IOP by pneumatonometry with and without a 10-g weight. Corneal wave speed was determined by ultrasound surface wave elastography. A small, 0.1-second harmonic vibration at 100 Hz was generated through the closed eyelids. Wave propagation was captured by an ultrasound transducer, and wave speed was determined from the phase change with distance. Comparisons were performed using generalized estimating equation models.

RESULTS

There were no significant differences in corneal wave speed between glaucomatous and normal eyes (2.16±0.25 vs. 2.07±0.16 m/s, P=0.17). However, ocular rigidity was significantly lower in glaucomatous eyes (0.0218±0.0033 vs. 0.0252±0.0050/μL, P=0.01). Corneal wave speed was not correlated with age and IOP in either group (P≥0.23) but was correlated with ocular rigidity (R=0.48, P=0.02) and inversely correlated with axial length (R=-0.53, P=0.01) in glaucomatous eyes.

CONCLUSION

Glaucomatous eyes tend to have lower ocular rigidity than healthy eyes with similar age, IOP, and axial length. However, the lack of a difference in corneal wave speed suggests that corneal tissue may not be significantly affected, and scleral changes likely play a more important role in glaucoma.

Influence of Chronic Ocular Hypertension on Emmetropia: Refractive, Structural and Functional Study in Two Rat Models

Chronic ocular hypertension (OHT) influences on refraction in youth and causes glaucoma in adulthood. However, the origin of the responsible mechanism is unclear. This study analyzes the effect of mild-moderate chronic OHT on refraction and neuroretina (structure and function) in young-adult Long-Evans rats using optical coherence tomography and electroretinography over 24 weeks. Data from 260 eyes were retrospectively analyzed in two cohorts: an ocular normotension (ONT) cohort (<20 mmHg) and an OHT cohort (>20 mmHg), in which OHT was induced either by sclerosing the episcleral veins (ES group) or by injecting microspheres into the anterior chamber. A trend toward emmetropia was found in both cohorts over time, though it was more pronounced in the OHT cohort (p < 0.001), especially in the ES group (p = 0.001) and males. IOP and refraction were negatively correlated at week 24 (p = 0.010). The OHT cohort showed early thickening in outer retinal sectors (p < 0.050) and the retinal nerve fiber layer, which later thinned. Electroretinography demonstrated early supranormal amplitudes and faster latencies that later declined. Chronic OHT accelerates emmetropia in Long–Evans rat eyes towards slowly progressive myopia, with an initial increase in structure and function that reversed over time.

Tensile Viscoelastic Properties of the Sclera after Glycosaminoglycan Depletion

PURPOSE

Fibrillar collagen network and glycosaminoglycans (GAGs) are the primary components of extracellular matrix (ECM) of the sclera. The main goal of this study was to investigate the possible structural roles of GAGs in the scleral tensile properties as a function of preconditioning and displacement rate.

METHODS

Four-step uniaxial stress-relaxation tests were used for characterizing the viscoelastic tensile response of the posterior porcine sclera with and without enzymatic GAG removal. The scleral strips were divided into different groups based on the displacement rate and the presence or absence of a preconditioning step in the loading protocol. The groups were (1) displacement rate of 0.2 mm/min without preconditioning, (2) displacement rate of 1 mm/min without preconditioning, (3) displacement rate of 0.2 mm/min with preconditioning, and (4) displacement rate of 1 mm/min with preconditioning. The peak stress, equilibrium stress, and the equilibrium elastic modulus were calculated for all specimens and compared against each other.

RESULTS

Increasing the displacement rate from 0.2 mm/min to 1.0 mm/min was found to cause an insignificant change in the equilibrium stress and equilibrium elastic modulus of porcine scleral strips. Removal of GAGs resulted in an overall stiffer tensile behavior independent of the displacement rate in samples that were not preconditioned (P < .05). The behavior of preconditioned samples with and without GAG removal was not significantly different from each other.

CONCLUSIONS

The experimental measurements of the present study showed that GAGs play an important role in the mechanical properties of the posterior porcine sclera. Furthermore, using a preconditioning step in the uniaxial testing protocol resulted in not being able to identify any significant difference in the tensile behavior of GAG depleted and normal scleral strips.

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.

Chronic Glaucoma Using Biodegradable Microspheres to Induce Intraocular Pressure Elevation. Six-Month Follow-Up

Background: To compare two prolonged animal models of glaucoma over 24 weeks of follow-up. A novel pre-trabecular model of chronic glaucoma was achieved by injection of biodegradable poly lactic-co-glycolic acid (PLGA) microspheres (10–20 µm) (Ms20/10) into the ocular anterior chamber to progressively increase ocular hypertension (OHT). Methods: Rat right eyes were injected to induce OHT: 50% received a suspension of Ms20/10 in the anterior chamber at 0, 2, 4, 8, 12, 16 and 20 weeks, and the other 50% received a sclerosing episcleral vein injection biweekly (EPIm). Ophthalmological clinical signs, intraocular pressure (IOP), neuroretinal functionality measured by electroretinography (ERG), and structural analysis of the retina, retinal nerve fiber layer (RNFL) and ganglion cell layer (GCL) protocols using optical coherence tomography (OCT) and histological exams were performed. Results: Both models showed progressive neuroretinal degeneration (p < 0.05), and contralateral eye affectation. The Ms20/10 model showed a more progressive increase in IOP and better preservation of ocular surface. Although no statistical differences were found between models, the EPIm showed a tendency to produce thicker retinal and thinner GCL thicknesses, slower latency and smaller amplitude as measured using ERG, and more aggressive disturbances in retinal histology. In both models, while the GCL showed the greatest percentage loss of thickness, the RNFL showed the greatest and earliest rate of thickness loss. Conclusions: The intracameral model with biodegradable microspheres resulted more like the conditions observed in humans. It was obtained by a less-aggressive mechanism, which allows for adequate study of the pathology over longer periods.

Neuroprotection in Glaucoma: NAD+/NADH Redox State as a Potential Biomarker and Therapeutic Target

Glaucoma is the leading cause of irreversible blindness worldwide. Its prevalence and incidence increase exponentially with age and the level of intraocular pressure (IOP). IOP reduction is currently the only therapeutic modality shown to slow glaucoma progression. However, patients still lose vision despite best treatment, suggesting that other factors confer susceptibility. Several studies indicate that mitochondrial function may underlie both susceptibility and resistance to developing glaucoma. Mitochondria meet high energy demand, in the form of ATP, that is required for the maintenance of optimum retinal ganglion cell (RGC) function. Reduced nicotinamide adenine dinucleotide (NAD⁺) levels have been closely correlated to mitochondrial dysfunction and have been implicated in several neurodegenerative diseases including glaucoma. NAD⁺ is at the centre of various metabolic reactions culminating in ATP production—essential for RGC function. In this review we present various pathways that influence the NAD⁺(H) redox state, affecting mitochondrial function and making RGCs susceptible to degeneration. Such disruptions of the NAD⁺(H) redox state are generalised and not solely induced in RGCs because of high IOP. This places the NAD⁺(H) redox state as a potential systemic biomarker for glaucoma susceptibility and progression; a hypothesis which may be tested in clinical trials and then translated to clinical practice.

Effect of Body Position on Intraocular Pressure (IOP), Intracranial Pressure (ICP), and Translaminar Pressure (TLP) Via Continuous Wireless Telemetry in Nonhuman Primates (NHPs)

Purpose

Recent retrospective clinical studies and animal experiments have suggested that cerebrospinal fluid pressure (CSFP) is important in glaucoma, acting through the translaminar pressure (TLP = IOP − CSFP), which directly affects the optic nerve head. In this study, IOP and intracranial pressure (ICP; a surrogate of CSFP) were measured at various body positions to quantify the determinants of TLP.

Methods

We have developed an implantable wireless pressure telemetry system based on a small piezoelectric sensor with low temporal drift. Telemetry transducers were placed in the anterior chamber to measure IOP and in the brain parenchyma at eye height to measure ICP. IOP was calibrated against anterior cannulation manometry, and ICP/CSFP was calibrated against an intraparenchymal Codman ICP Express microsensor. We measured IOP, ICP, and TLP = IOP − ICP continuously at 200 Hz in three male nonhuman primates (NHPs) in three trials; pressures were then averaged for 30 seconds per body position. Relative change of IOP, ICP, and TLP from the supine (baseline) position to the seated, standing, and inverted positions were quantified.

Results

TLP changed significantly and instantaneously from the supine to seated (+14 mm Hg), supine to standing (+13 mm Hg) and supine to inverted (−12 mm Hg) positions (P < 0.05). There was no significant TLP change for supine to prone. ICP showed greater relative change than IOP.

Conclusions

TLP change due to body position change is driven more by ICP/CSFP than IOP. IOP, ICP, and TLP variability, coupled with telemetry, should allow us to test the hypotheses that IOP, ICP, or TLP fluctuations contribute independently to glaucoma onset or progression.

Biomechanical properties of retina and choroid: a comprehensive review of techniques and translational relevance

Studying the biomechanical properties of biological tissue is crucial to improve our understanding of disease pathogenesis. The biomechanical characteristics of the cornea, sclera and the optic nerve head have been well addressed with an extensive literature and an in-depth understanding of their significance whilst, in comparison, knowledge of the retina and choroid is relatively limited. Knowledge of these tissues is important not only to clarify the underlying pathogenesis of a wide variety of retinal and vitreoretinal diseases, including age-related macular degeneration, hereditary retinal dystrophies and vitreoretinal interface diseases but also to optimise the surgical handling of retinal tissues and, potentially, the design and properties of implantable retinal prostheses and subretinal therapies. Our aim with this article is to comprehensively review existing knowledge of the biomechanical properties of retina, internal limiting membrane (ILM) and the Bruch’s membrane–choroidal complex (BMCC), highlighting the potential implications for clinical and surgical practice. Prior to this we review the testing methodologies that have been used both in vitro, and those starting to be used in vivo to aid understanding of their results and significance.

Adaptive responses to neurodegenerative stress in glaucoma

Glaucoma causes loss of vision through degeneration of the retinal ganglion cell (RGC) projection to the brain. The disease is characterized by sensitivity to intraocular pressure (IOP) conveyed at the optic nerve head, through which RGC axons pass unmyelinated to form the optic nerve. From this point, a pathogenic triumvirate comprising inflammatory, oxidative, and metabolic stress influence both proximal structures in the retina and distal structures in the optic projection. This review focuses on metabolic stress and how the optic projection may compensate through novel adaptive mechanisms to protect excitatory signaling to the brain. In the retina and proximal nerve head, the unmyelinated RGC axon segment is energy-inefficient, which leads to increased demand for adenosine-5'-triphosphate (ATP) at the risk of vulnerability to Ca²⁺-related metabolic and oxidative pressure. This vulnerability may underlie the bidirectional nature of progression. However, recent evidence highlights that the optic projection in glaucoma is not passive but rather demonstrates adaptive processes that may push back against neurodegeneration. In the retina, even as synaptic and dendritic pruning ensues, early progression involves enhanced excitability of RGCs. Enhancement involves depolarization of the resting membrane potential and increased response to light, independent of RGC morphological type. This response is axogenic, arising from increased levels and translocation of voltage-gated sodium channels (NaV) in the unmyelinated segment. During this same early period, large-scale networks of gap-junction coupled astrocytes redistribute metabolic resources to the optic projection stressed by elevated IOP to slow loss of axon function. This redistribution may reflect more local remodeling, as astrocyte processes respond to focal metabolic duress by boosting glycogen turnover in response to axonal activity in an effort to promote survival of the healthiest axons. Both enhanced excitability and metabolic redistribution are transient, indicating that the same adaptive mechanisms that apparently serve to slow progression ultimately may be too expensive for the system to sustain over longer periods.

Alteration in Lysophospholipids and Converting Enzymes in Glaucomatous Optic Nerves

Purpose

To determine whether lysophospholipid (LPL) profiles and corresponding conversion enzymes in the LPL pathways are altered in the optic nerve (ON) between human control and glaucoma samples.

Methods

Lipids extracted from control (n = 11) and glaucomatous (n = 12) ON samples using the Bligh and Dyer method were subjected to high-resolution mass spectrometry on a Q-exactive mass spectrometer coupled with a high-performance liquid chromatography (Accela 600) system. Analysis was performed for LPLs (lysophosphatidylcholines, lysophosphatidylserines, lysophosphatidylethanolamines, lysophosphatidylinositols, and lysosphingomyelines) using LipidSearch v.4.1, MZmine v.2.0, and MetaboAnalyst v.4.0. LPL synthesis and degradation pathway maps, utilizing UniProt and BRENDA database entries as needed, were created using Kyoto Encyclopedia of Genes and Genomes (KEGG)-based tools. The mRNA expression level in normal and glaucomatous human ON were analyzed using Gene Expression Omnibus (GEO) entry GSE45570. Protein amounts were determined using PHAST gel and dot blot and were used for normalization of protein amounts across samples. Western blot, ELISA, and protein quantification were performed using established protocols.

Results

Principal component analysis of ON LPL profile placed control and glaucomatous ONs in two distinct separate groups. Mass spectrometric analysis of ON revealed decrease in lysophosphatidic acid, lysophosphatidylethanolamine, lysophosphatidylcholine, and significant increase in diacylglycerol in glaucomatous ON. Statistical analysis of LPL conversion enzymes revealed significant overexpression of phosphatidate phosphatase LPIN2, phospholipid phosphatase 3, phosphatidylcholine-sterol acyltransferase, and calcium-dependent phospholipase 2, and significant downregulation of glycerol-3-phosphate acyltransferase 4 at mRNA level in glaucomatous ON. Western blot and ELISA confirmed proteomic differences between normal and diseased ON.

Conclusions

Our analysis revealed alterations in specific LPL levels and corresponding select enzyme-level changes in glaucomatous ON.

[Electroretinography in evaluation of neuroprotective effect in an experimental model of glaucoma]

For drugs that have a therapeutic effect on glaucoma through mechanisms not associated with decreasing intraocular pressure (IOP), special attention is paid to the choice of effectiveness criteria. The article examines the possibility of using a- and waves of electroretinography (ERG) in preclinical studies to predict the effectiveness of glaucoma drug candidates.

PURPOSE

To examine the possibility of reliably associating changes in the amplitude of a- and ERG waves with functional changes in the retina of experimental glaucoma rats with morphological evidence of loss of functional integrity of the retina.

MATERIAL AND METHODS

The study was carried out in the laboratory of the Research Institute of Pharmacology of Living Systems of the Belgorod State University. Adult outbred rats were used as a test system. Experimental glaucoma was modelled by multiple injections of hyaluronic acid into the anterior chamber of the eye; they were examined by recording the time history of intraocular pressure changes, and performing ERG, ophthalmoscopy, and histological examination of the retina and subcortical centers of vision. The following groups were formed: intact, pathology control, positive control.

RESULTS

The development of glaucoma in experimental rats was accompanied by neuronal death in the ganglionic layer of the retina; at the same time, characteristic changes were observed in the subcortical visual centers. A change in the ERG was recorded: for thewave, there was a dependence on the degree of changes in the ganglionic layer of the retina, change in the wave can also indicate the involvement of amacrine and horizontal cells in the process; for the a-wave, a correlation with the results of photoreceptor layer histology was noted, which was characterized as a deviation from the norm developing against the background of hydrodynamic load in the eye chambers.

CONCLUSION

ERG is suitable for use in preclinical studies of glaucoma drugs as an indicative in vivo method for diagnosing the state of the retina in animals. The use of this method is especially valuable for conducting preclinical studies of drugs that involve long-term use when ophthalmoscopy and intraocular pressure alone cannot fully characterize the course of glaucoma, and animal euthanasia seems unnecessary and inhumane.

Fluctuations in Macular Thickness in Patients with Retinal Vein Occlusion Treated with Anti-Vascular Endothelial Growth Factor Agents

PURPOSE

To evaluate macular thickness fluctuations in patients with retinal vein occlusions (RVOs) treated with anti-vascular endothelial growth factor (VEGF) agents and to assess whether patients with larger fluctuations have poorer visual outcomes.

DESIGN

Retrospective cohort study.

PARTICIPANTS

Treatment-naive patients with RVO.

METHODS

Central subfield thickness (CST), cube volume (CV), and cube average thickness (CAT) were collected from OCT images obtained at baseline and 3, 6, 9, and 12 months, and standard deviations (SDs) across 12 months were calculated. Mixed-effects regression was performed to examine the relationship between macular thickness SD and 12-month visual acuity (VA). Standard multiple regression was performed to identify predictors of macular thickness SD.

MAIN OUTCOME MEASURES

Standard deviations across 12 months for CST, CV, and CAT and VA at 12 months.

RESULTS

One hundred thirty-four eyes, including 71 with branch RVO (BRVO) and 63 with central RVO (CRVO), were evaluated. Mean baseline and 12-month CST were 488.6 ± 165.0 μm and 334.3 ± 131.9 μm (change, -154.3 ± 210.2 μm; P < 0.001), with CST SD of 114.1 ± 77.0 μm. Baseline and 12-month VA were 52.8 ± 20.9 letters and 65.9 ± 17.3 letters (change, +13.1 ± 20.3 letters; P < 0.001). Central subfield thickness SD was a significant negative predictor of 12-month VA (-5.21 letters/100 μm; 95% confidence interval [CI], -10.21 to -0.22 letters/100 μm; P = 0.041) when adjusting for baseline factors and injections. Baseline CST and number of injections were not predictive (P ≥ 0.101). Stratification by CST SD demonstrated a 10-letter difference in 12-month VA between the first and fourth quartiles. Baseline CST and RVO diagnosis were the only significant predictors of CST SD (CRVO vs. BRVO: +34.64 μm/100 μm [95% CI, 29.33-39.94 μm/100 μm; P < 0.001] and +22.13 μm/100 μm [95% CI, 4.81-39.44 μm/100 μm; P = 0.013]). Associations using CV and CAT were similar.

CONCLUSIONS

Larger macular thickness fluctuations are associated with poorer visual outcomes in patients with RVO treated with anti-VEGF agents. Macular thickness fluctuations, in addition to absolute macular thickness, may be an important prognostic biomarker in these patients.

Disc Hemorrhages Are Associated With the Presence and Progression of Glaucomatous Central Visual Field Defects

PRECIS

In this prospective cohort study, disc hemorrhages were associated with more severe central damage on 24-2 and 10-2 visual fields (VFs), and faster progression globally on 24-2 VFs and centrally on 10-2 VFs.

PURPOSE

To study the relationship between disc hemorrhage (DH) and the presence and progression of glaucomatous central VF damage.

METHODS

Cross-sectional and longitudinal analyses were performed on data from the African Descent and Glaucoma Evaluation Study (ADAGES) cohort. Two masked investigators reviewed disc photographs for the presence and location of DH. 24-2 central VF damage was based on the number of test locations within the central 10 degrees of the 24-2 field pattern deviation and their mean total deviation (MTD). 10-2 central VF damage was based on pattern deviation and MTD. Main outcome measures were the association between DH and presence of central VF damage and between DH and worsening of VF.

RESULTS

DH was detected in 21 of 335 eyes (6.2%). In the cross-sectional analysis, DH was significantly associated with more severe central damage on 24-2 [incidence rate ratio=1.47; 95% confidence interval (CI)=1.02-2.12; P=0.035] and 10-2 VFs (incidence rate ratio=1.81; 95% CI=1.26-2.60; P=0.001). In the longitudinal analysis, DH eyes progressed faster than non-DH eyes based on 24-2 global MTD rates (difference in slopes, β=-0.06; 95% CI=-0.11 to -0.01; P=0.009) and 10-2 MTD rates (β=-0.10; 95% CI=-0.14 to -0.06; P< 0.001), but not 24-2 central MTD rates (β=-0.02; 95% CI=-0.078 to 0.026; P=0.338).

CONCLUSION

DH was associated with the presence and progression of central VF defects. DH identification should prompt intensive central VF monitoring and surveillance with 10-2 fields to detect progression.

Quantification of Translaminar Pressure Gradient (TLPG) With Continuous Wireless Telemetry in Nonhuman Primates (NHPs)

Purpose

Recent retrospective clinical and animal studies suggest that cerebrospinal fluid pressure (CSFP) is important in glaucoma pathogenesis. Intraocular pressure (IOP) and CSFP are the driving components of translaminar pressure (TLP = IOP - CSFP), which acts across the lamina cribrosa (LC) thickness to create the translaminar pressure gradient (TLPG = TLP/LC thickness).

Methods

We developed an implantable wireless telemetry system based on a small piezoelectric sensor with low temporal drift. IOP, measured in the anterior chamber, and intracranial pressure (ICP), measured in the brain parenchyma (as a surrogate for CSFP) were measured at 200 Hz in three male rhesus macaques (nonhuman primates, NHPs) on a 10% duty cycle (15 seconds of every 150-second period). Three-dimensional LC thickness was autosegmented as the mean thickness of the visible hyperreflective band in 48 radial spectral-domain optical coherence tomography b-scans centered on the optic nerve head.

Results

Results indicated the rank order of IOP, ICP, TLP, and TLPG for waking, sleeping, and 24-hour periods averaged across all days. NHP 150110 had the highest IOP and ICP in all periods; however, it had the lowest TLPG in all periods due to its relatively thick LC. The other two NHPs showed similar shifts in the rank order of possible glaucoma risk factors.

Conclusions

IOP is the only modifiable and readily measurable pressure-based risk factor for glaucoma. However, other potential risk factors such as ICP, TLP, and TLPG, as well as their rank-order patterns, differed compared to IOP across subjects, demonstrating that a comprehensive view of relevant risk factors is warranted.

Translational Relevance

Future studies should consider including CSFP, TLP, and TLPG in addition to IOP as potential risk factors when assessing eye-specific glaucoma susceptibility.

Effect of age and sex on neurodevelopment and neurodegeneration in the healthy eye: Longitudinal functional and structural study in the Long-Evans rat

The processes involved in neurodevelopment and aging have not yet been fully discovered. This is especially challenging in premorbid or borderline situations of neurodegenerative diseases such as Alzheimer's or glaucoma. The retina, as part of the central nervous system, can be considered the easiest and most accessible neural structure that can be analyzed using non-invasive methods. Animal studies of neuroretinal tissue in situations of health and under controlled conditions allow the earliest sex- and aging-induced changes to be analyzed so as to differentiate them from the first signs occurring in manifested disease. This study evaluates differences by age and sex based on intraocular pressure (IOP) and neuroretinal function and structure in healthy young and adult rats before decline due to senescence. For this purpose, eighty-five healthy Long-Evans rats (31 males and 54 females) were analyzed in this 6-month longitudinal study running from childhood to adulthood. IOP was measured by tonometer (Tonolab; Tiolat Oy Helsinki, Finland), neuroretinal function was recorded by flash scotopic and light-adapted photopic negative response electroretinography (ERG) (Roland consult® RETIanimal ERG, Germany) at 4, 16 and 28 weeks of age; and structure was evaluated by in vivo optical coherence tomography (OCT) (Spectralis, Heidelberg® Engineering, Germany). Analyzing both sexes together, IOP was below 20 mmHg throughout the study; retina (R), retinal nerve fiber layer (RNFL) and ganglion cell layer (GCL) thicknesses measured by OCT decreased over time; an increase in ERG signal was recorded at week 16; and no differences were found between right and left eyes. However, analyzing differences by sex revealed that males had higher IOP (even reaching ocular hypertension [>20 mmHg] by the end of the study [7 months of age]), exhibited greater neuroretinal thickness but higher structural percentage loss, and had worse dark- and light-adapted function as measured by ERG than females. This study concludes that age and sex influenced neurodevelopment and neurodegeneration. Different structural and functional degenerative patterns were observed by sex; these occurred earlier and more intensely in males than in age-matched females.

Effect of Changing Heart Rate on the Ocular Pulse and Dynamic Biomechanical Behavior of the Optic Nerve Head

Purpose

To study the effect of changing heart rate on the ocular pulse and the dynamic biomechanical behavior of the optic nerve head (ONH) using a comprehensive mathematical model.

Methods

In a finite element model of a healthy eye, a biphasic choroid consisted of a solid phase with connective tissues and a fluid phase with blood, and the lamina cribrosa (LC) was viscoelastic as characterized by a stress-relaxation test. We applied arterial pressures at 18 ocular entry sites (posterior ciliary arteries), and venous pressures at four exit sites (vortex veins). In the model, the heart rate was varied from 60 to 120 bpm (increment: 20 bpm). We assessed the ocular pulse amplitude (OPA), pulse volume, ONH deformations, and the dynamic modulus of the LC at different heart rates.

Results

With an increasing heart rate, the OPA decreased by 0.04 mm Hg for every 10 bpm increase in heart rate. The ocular pulse volume decreased linearly by 0.13 µL for every 10 bpm increase in heart rate. The storage modulus and the loss modulus of the LC increased by 0.014 and 0.04 MPa, respectively, for every 10 bpm increase in heart rate.

Conclusions

In our model, the OPA, pulse volume, and ONH deformations decreased with an increasing heart rate, whereas the LC became stiffer. The effects of blood pressure/heart rate changes on ONH stiffening may be of interest for glaucoma pathology.