Deformation of the early glaucomatous monkey optic nerve head connective tissue after acute IOP elevation in 3-D histomorphometric reconstructions

Analysis of deep optic nerve head structures with spectral domain and swept-source optical coherence tomography

PURPOSE

Histological evidence indicates that the earliest structural changes to the optic nerve head (ONH) in glaucoma occur in the lamina cribrosa (LC) and its interface with the sclera. However, clinical imaging of these structures remains challenging. We compared the visibility of deep ONH structures with newer iterations of spectral-domain (SD) and swept-source (SS) optical coherence tomography (OCT).

METHODS

Twelve patients with open-angle glaucoma were imaged with SD-OCT with 24 radial B-scans centred on Bruch's membrane opening (BMO) and SS-OCT with a horizontal and vertical raster scan pattern containing five lines each, centred on the ONH. Single best-matched horizontal and vertical scans from the two modalities were selected and exported. Three independent observers masked to modality determined if BMO, posterior choroid surface, anterior scleral canal opening and anterior and posterior LC insertions into the sclera were detectable in the matched B-scan images. We determined the interobserver agreement and concordance in detecting each structure with the two OCT imaging modalities.

RESULTS

There was a high interobserver agreement with both SS-OCT and SD-OCT (inter-item correlations: 0.81-0.93 and 0.77-0.82, respectively). There was a consistent tendency for higher overall detection rates with SS-OCT, however, the differences failed to reach statistical significance. With respect to individual structures, only the posterior LC insertion in the nasal quadrant was statistically different, with a detection rate of 13 and 6 (pooled out of a total of 36 across the three observers) with SS-OCT and SD-OCT, respectively (p=0.04).

CONCLUSION

Overall, both SS-OCT and SD-OCT showed statistically equivalent visualisation of ONH structures, however, SS-OCT tended to have higher visualisation rates.

Modeling complex age-related eye disease

Modeling complex eye diseases like age-related macular degeneration (AMD) and glaucoma poses significant challenges, since these conditions depend highly on age-related changes that occur over several decades, with many contributing factors remaining unknown. Although both diseases exhibit a relatively high heritability of >50%, a large proportion of individuals carrying AMD- or glaucoma-associated genetic risk variants will never develop these diseases. Furthermore, several environmental and lifestyle factors contribute to and modulate the pathogenesis and progression of AMD and glaucoma. Several strategies replicate the impact of genetic risk variants, pathobiological pathways and environmental and lifestyle factors in AMD and glaucoma in mice and other species. In this review we will primarily discuss the most commonly available mouse models, which have and will likely continue to improve our understanding of the pathobiology of age-related eye diseases. Uncertainties persist whether small animal models can truly recapitulate disease progression and vision loss in patients, raising doubts regarding their usefulness when testing novel gene or drug therapies. We will elaborate on concerns that relate to shorter lifespan, body size and allometries, lack of macula and a true lamina cribrosa, as well as absence and sequence disparities of certain genes and differences in their chromosomal location in mice. Since biological, rather than chronological, age likely predisposes an organism for both glaucoma and AMD, more rapidly aging organisms like small rodents may open up possibilities that will make research of these diseases more timely and financially feasible. On the other hand, due to the above-mentioned anatomical and physiological features, as well as pharmacokinetic and -dynamic differences small animal models are not ideal to study the natural progression of vision loss or the efficacy and safety of novel therapies. In this context, we will also discuss the advantages and pitfalls of alternative models that include larger species, such as non-human primates and rabbits, patient-derived retinal organoids, and human organ donor eyes.

Denoising OCT videos based on temporal redundancy

The identification of eye diseases and their progression often relies on a clear visualization of the anatomy and on different metrics extracted from Optical Coherence Tomography (OCT) B-scans. However, speckle noise hinders the quality of rapid OCT imaging, hampering the extraction and reliability of biomarkers that require time series. By synchronizing the acquisition of OCT images with the timing of the cardiac pulse, we transform a low-quality OCT video into a clear version by phase-wrapping each frame to the heart pulsation and averaging frames that correspond to the same instant in the cardiac cycle. Here, we compare the performance of our one-cycle denoising strategy with a deep-learning architecture, Noise2Noise, as well as classical denoising methods such as BM3D and Non-Local Means (NLM). We systematically analyze different image quality descriptors as well as region-specific metrics to assess the denoising performance based on the anatomy of the eye. The one-cycle method achieves the highest denoising performance, increases image quality and preserves the high-resolution structures within the eye tissues. The proposed workflow can be readily implemented in a clinical setting.

Heterogenous thinning of peripapillary tissues occurs early during high myopia development in juvenile tree shrews

Myopia is an independent risk factor for glaucoma, but the link between both conditions remains unknown. Both conditions induce connective tissue remodeling at the optic nerve head (ONH), including the peripapillary tissues. The purpose of this study was to investigate the thickness changes of the peripapillary tissues during experimental high myopia development in juvenile tree shrews. Six juvenile tree shrews experienced binocular normal vision, while nine received monocular -10D lens treatment starting at 24 days of visual experience (DVE) to induce high myopia in one eye and the other eye served as control. Daily refractive and biometric measurements and weekly optical coherence tomography scans of the ONH were obtained for five weeks. Peripapillary sclera (Scl), choroid-retinal pigment epithelium complex (Ch-RPE), retinal nerve fiber layer (RNFL), and remaining retinal layers (RRL) were auto-segmented using a deep learning algorithm after nonlinear distortion correction. Peripapillary thickness values were quantified from 3D reconstructed segmentations. All lens-treated eyes developed high myopia (-9.8 ± 1.5 D), significantly different (P < 0.001) from normal (0.69 ± 0.45 D) and control eyes (0.76 ± 1.44 D). Myopic eyes showed significant thinning of all peripapillary tissues compared to both, normal and control eyes (P < 0.001). At the experimental end point, the relative thinning from baseline was heterogeneous across tissues and significantly more pronounced in the Scl (-8.95 ± 3.1%) and Ch-RPE (-16.8 ± 5.8%) when compared to the RNFL (-5.5 ± 1.6%) and RRL (-6.7 ± 1.8%). Furthermore, while axial length increased significantly throughout the five weeks of lens wear, significant peripapillary tissue thinning occurred only during the first week of the experiment (until a refraction of -2.5 ± 1.9 D was reached) and ceased thereafter. A sectorial analysis revealed no clear pattern. In conclusion, our data show that in juvenile tree shrews, experimental high myopia induces significant and heterogeneous thinning of the peripapillary tissues, where the retina seems to be protected from profound thickness changes as seen in Ch-RPE and Scl. Peripapillary tissue thinning occurs early during high myopia development despite continued progression of axial elongation. The observed heterogeneous thinning may contribute to the increased risk for pathological optic nerve head remodeling and glaucoma later in life.

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.

p53 and Myofibroblast Apoptosis in Organ Fibrosis

Organ fibrosis represents a dysregulated, maladaptive wound repair response that results in progressive disruption of normal tissue architecture leading to detrimental deterioration in physiological function, and significant morbidity/mortality. Fibrosis is thought to contribute to nearly 50% of all deaths in the Western world with current treatment modalities effective in slowing disease progression but not effective in restoring organ function or reversing fibrotic changes. When physiological wound repair is complete, myofibroblasts are programmed to undergo cell death and self-clearance, however, in fibrosis there is a characteristic absence of myofibroblast apoptosis. It has been shown that in fibrosis, myofibroblasts adopt an apoptotic-resistant, highly proliferative phenotype leading to persistent myofibroblast activation and perpetuation of the fibrotic disease process. Recently, this pathological adaptation has been linked to dysregulated expression of tumour suppressor gene p53. In this review, we discuss p53 dysregulation and apoptotic failure in myofibroblasts and demonstrate its consistent link to fibrotic disease development in all types of organ fibrosis. An enhanced understanding of the role of p53 dysregulation and myofibroblast apoptosis may aid in future novel therapeutic and/or diagnostic strategies in organ fibrosis.

AxoNet 2.0: A Deep Learning-Based Tool for Morphometric Analysis of Retinal Ganglion Cell Axons

Purpose

Assessment of glaucomatous damage in animal models is facilitated by rapid and accurate quantification of retinal ganglion cell (RGC) axonal loss and morphologic change. However, manual assessment is extremely time- and labor-intensive. Here, we developed AxoNet 2.0, an automated deep learning (DL) tool that (i) counts normal-appearing RGC axons and (ii) quantifies their morphometry from light micrographs.

Methods

A DL algorithm was trained to segment the axoplasm and myelin sheath of normal-appearing axons using manually-annotated rat optic nerve (ON) cross-sectional micrographs. Performance was quantified by various metrics (e.g., soft-Dice coefficient between predicted and ground-truth segmentations). We also quantified axon counts, axon density, and axon size distributions between hypertensive and control eyes and compared to literature reports.

Results

AxoNet 2.0 performed very well when compared to manual annotations of rat ON (R2 = 0.92 for automated vs. manual counts, soft-Dice coefficient = 0.81 ± 0.02, mean absolute percentage error in axonal morphometric outcomes < 15%). AxoNet 2.0 also showed promise for generalization, performing well on other animal models (R2 = 0.97 between automated versus manual counts for mice and 0.98 for non-human primates). As expected, the algorithm detected decreased in axon density in hypertensive rat eyes (P ≪ 0.001) with preferential loss of large axons (P < 0.001).

Conclusions

AxoNet 2.0 provides a fast and nonsubjective tool to quantify both RGC axon counts and morphological features, thus assisting with assessing axonal damage in animal models of glaucomatous optic neuropathy.

Translational Relevance

This deep learning approach will increase rigor of basic science studies designed to investigate RGC axon protection and regeneration.

[The value of lamina cribrosa in the diagnosis and treatment of glaucoma. Remodeling of lamina cribrosa collagen and approaches to its therapeutic treatment]

Among the first structures suffering damage with an increase in intraocular pressure (IOP) and in early stage of glaucoma are the lamina cribrosa (LC) and peripapillary sclera (ppScl). Changes in these structures occur at the molecular and cellular level. Extracellular matrix (ECM) is the basis of connective tissue, provides mechanical support for the cells, facilitates intercellular interactions and transport of chemicals, including in LC and ppScl. Mechanical stress causes remodeling and disorganization of the ECM, which leads to changes in the structure of the tissue itself, an increase in its rigidity and a decrease in elasticity. Taking into account the molecular and cellular mechanisms of damage to LC and ppScl, various researchers have developed strategies and tactics for therapeutic intervention on these structures, contributing to a decrease in ECM secretion and, as a consequence, suspension of their remodeling. These approaches may in the future form the basis for the treatment of glaucomatous optic neuropathy.

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.

High-quality, large-scale, semi-thin, & ultra-thin sections of the optic nerve in large animals: An optimized procedure

Large animal model of optic nerve (ON) injury is an essential tool for translational medicine. Perfusion fixation with paraformaldehyde is mainly used for preparing the semi-thin (1-2 μm thick) and ultra-thin (<0.5 μm thick) sections of the ON tissues. However, this conventional fixation technique in large animals needs a large volume of fixatives, which increases the risk of toxic exposure and is environmentally unfriendly. Additionally, fixed residual ON cannot be used for other tests that require fresh tissue samples. Although conventional immersion fixation is feasible for preparing a semi-thin section of the ON in small animals (0.2-0.6 mm in diameter), it faces technical challenges when fixing the ON of large animals (3 mm in diameters), as increased diameter limits the permeability of the fixatives into deeper tissue. Therefore, we optimized the immersion-fixation method to obtain high-quality, large-scale, semi-thin, and ultra-thin sections for the ON of goat and rhesus macaques. Using this optimized technique, the ON microstructure was well preserved throughout the entire area of 1.5*1.5 square millimeters, allowing confident quantification of axon density/diameter on semi-thin section and identification of specific organelles and glial cells on ultra-thin sections. Furthermore, the optimized technique is a quick, simple, and environmentally friendly fixation method. Notably, the ON regions of large animals with or without an intact neurovascular system can be prepared for light and electron microscopy. In contrast, the residual unfixed ON from the same animal can be further utilized for experiments such as tissue culture and biomolecular tests.

Effect of pseudoexfoliation syndrome on lamina cribrosa morphology after uneventful phacoemulsification

AIM

To evaluate the behavior of the lamina cribrosa (LC) following successful phacoemulsification (PE) surgery in non-glaucomatous PXS cases with cataract.

METHODS

A total of 25 bilateral PXS and 56 non-PXS cataract patients were included in this prospective, interventional, and controlled study. The lamina cribrosa thickness (LCt) and anterior laminar depth (ALD) were measured horizontally and vertically from two sections using EDI-OCT. The measurements were taken preoperatively, and the LCt and ALD measurements were repeated at week one (W1), month one (M1), month two (M2), and month three (M3) postoperatively.

RESULTS

The baseline horizontal and vertical LCt values were found to be significantly thinner in the PXS group (p = 0.001, p = 0.006 respectively). The horizontal section ALD values were 463.32 ± 98.82 µm and 383.88 ± 85.8 µm (p < 0.001) for the PXS and non-PXS groups, respectively, and this significance continued during follow-up. The vertical ALD values were 469.64 ± 117.83 µm and 390.3 ± 98.1 µm (p = 0.002) in the PXS and non-PXS groups, respectively. There was no significant change in the PXS group for the depth following PE, but a statistically significant depth increase response was observed in the non-PXS group during follow-up.

CONCLUSION

It is possible to conclude that the bending capacity of the lamina cribrosa may be lower than seen in the normal population, but we do not know how this lack of flexibility will affect the eye.

Strain by virtual extensometers and video-imaging optical coherence tomography as a repeatable metric for IOP-Induced optic nerve head deformations

PURPOSE

To determine if in vivo strain response of the Optic Nerve Head (ONH) to IOP elevation visualized using Optical Coherence Tomography (OCT) video imaging and quantified using novel virtual extensometers was able to be provided repeatable measurements of tissue specific deformations.

METHODS

The ONHs of 5 eyes from 5 non-human primates (NHPs) were imaged by Spectralis OCT. A vertical and a horizontal B-scan of the ONH were continuously recorded for 60 s at 6 Hz (video imaging mode) during IOP elevation from 10 to 30 mmHg. Imaging was repeated over three imaging sessions. The 2D normal strain was computed by template-matching digital image correlation using virtual extensometers. ANOVA F-test (F) was used to compare inter-eye, inter-session, and inter-tissue variability for the prelaminar, Bruch's membrane opening (BMO), lamina cribrosa (LC) and choroidal regions (against variance the error term). F-test of the ratio between inter-eye to inter-session variability was used to test for strain repeatability across imaging sessions (FIS).

RESULTS

Variability of strain across imaging session (F = 0.7263, p = 0.4855) and scan orientation was not significant (F = 1.053, p = 0.3066). Inter session variability of strain was significantly lower than inter-eye variability (FIS = 22.63, p = 0.0428) and inter-tissue variability (FIS = 99.33 p = 0.00998). After IOP elevation, strain was highest in the choroid (-18.11%, p < 0.001), followed by prelaminar tissue (-11.0%, p < 0.001), LC (-3.79%, p < 0.001), and relative change in BMO diameter (-0.57%, p = 0.704).

CONCLUSIONS

Virtual extensometers applied to video-OCT were sensitive to the eye-specific and tissue-specific mechanical response of the ONH to IOP and were repeatable across imaging sessions.

The Relationship Between Macula Retinal Ganglion Cell Density and Visual Function in the Nonhuman Primate

Purpose

Loss of ganglion cell inner plexiform layer (GCIPL) and visual sensitivity in the macula region are known to occur at all stages of glaucoma. While both are dependent on the underlying retinal ganglion cells (RGCs), the relationship between structure and function is modest. We hypothesize that the imprecise relationship is due to a lack of direct correspondence between in vivo measures and RGC counts, as well as the relatively large stimulus size used by standard perimetry, which exceeds spatial summation.

Methods

The relationship between optical coherence tomography (OCT)–derived GCIPL thickness and corresponding inner cell density from retinal flat mounts was determined for four nonhuman primates with varying stages of neuropathy. Normative data for 10-2 threshold using Goldman size I to V stimuli were established for 10 animals, 4 of which were then followed longitudinally with OCT and perimetry. The relationship between GCIPL volume, which incorporated stimulus size after removal of residual thickness, and differential light sensitivity was determined for both experimental glaucoma and healthy eyes.

Results

Peak inner retinal cell density was 63,052 ± 9238 cells/mm² in the healthy eye. Cell density was related to both GCIPL thickness and eccentricity (R² = 0.74, P < .01). For all 10-2 eccentricities, size III stimuli were greater than the critical area (P < 0.01). Based on the structural and histologic relationship, the critical area corresponds to approximately 156 RGCs.

Conclusions

The relationship between cell density and GCIPL thickness is dependent on retinal eccentricity. For 10-2 perimetry, perimetric loss, especially at earlier stages of neuropathy, may best be detected using size II or smaller stimuli.

Racial Differences in the Rate of Change in Anterior Lamina Cribrosa Surface Depth in the African Descent and Glaucoma Evaluation Study

Purpose

The purpose of this study was to determine if the rate of change in the depth of the surface of the lamina cribrosa due to glaucomatous remodeling differs between glaucoma patients of African descent (AD) and European descent (ED).

Methods

There were 1122 images taken longitudinally over an average of 3 years (range = 0.9-4.1 years) from 122 patients with glaucoma followed in the African Descent and Glaucoma Evaluation Study (ADAGES) and Diagnostic Intervention and Glaucoma Study (DIGS) were automatically segmented to compute anterior lamina cribrosa surface depth (ALCSD). The rate of ALCSD change was compared across racial groups after adjusting for baseline characteristics known to be associated with ALCSD or disease progression (visual field, ALCSD, corneal thickness, optic disk size, and age).

Results

After adjusting for all other covariates, the ED group had significantly greater ALCSD posterior migration (deepening) than the AD group (difference = 2.57 µm/year, P = 0.035). There was a wider range of ALCSD change in the ED compared with the AD group, and more individuals had greater magnitude of both deepening and shallowing. No other covariates measured at baseline had independent effects on the longitudinal changes in ALCSD (baseline visual field severity, baseline ALCSD, corneal thickness, Bruch's membrane opening [BMO] area, or age).

Conclusions

Glaucomatous remodeling of the lamina cribrosa differs between AD and ED patients with glaucoma. Unlike the cross-sectional associations seen with aging, in which a deeper ALCSD was seen with age in the ED group, glaucomatous remodeling in this longitudinal study resulted in more posterior migration of ALCSD in ED compared to AD patients.

The Shape of Posterior Sclera as a Biometric Signature in Open-angle Glaucoma: An Intereye Comparison Study

Supplemental Digital Content is available in the text.

Purpose:

To characterize intereye differences in posterior segment parameters and determine their significance in open-angle glaucoma patients with unilateral damage.

Methods:

Both eyes from 65 subjects without any nerve damage and 43 patients undergoing treatment for unilateral open-angle glaucoma were included in this study. A 12.0×9.0×2.6 mm volume of the posterior segment in each eye was scanned with swept-source optical coherence tomography. Coronally reconstructed optical coherence tomography images were analyzed to determine the deepest point of the eye (DPE), which we then calculated the distance (Disc-DPE distance), depth (Disc-DPE depth), angle (Disc-DPE angle) from the optic disc center to the DPE. Posterior pole shape was analyzed measuring the posterior pole-cross-sectional area, posterior pole-horizontal width (PP-HW), and posterior pole-vertical width) of the posterior pole. These measurements and their intereye absolute difference (IAD; absolute difference in measurements between the right and left eyes) values were compared between the healthy and unilateral glaucomatous patients.

Results:

The posterior sclera measurements, including the Disc-DPE distance, Disc-DPE depth, and posterior pole-cross-sectional area, were significantly different between the unilateral glaucoma eyes and contralateral healthy eyes (P=0.043, P=0.035, and P=0.049, respectively). By contrast, none of the intereye differences in optic nerve head parameters were significant in the unilateral glaucoma patients. In comparison with the IAD values, the baseline intraocular pressure and PP-HW of the posterior segment showed significant differences between the healthy and the unilateral glaucoma patients (P=0.019 and P=0.036, respectively). A multivariate analysis showed that a larger baseline intraocular pressure IAD [odds ratio (OR), 1.381; P=0.009)] and larger PP-HW IAD (OR, 1.324; P=0.032) were significantly associated with the presence of glaucoma.

Conclusions:

Compared with the fellow healthy eyes, glaucomatous eyes had larger and more steeply curved posterior poles, which represent a structural variation of the posterior sclera that might be associated with glaucomatous optic neuropathy.

Longitudinal assessment of optic nerve head changes using optical coherence tomography in a primate microbead model of ocular hypertension

In humans, the longitudinal characterisation of early optic nerve head (ONH) damage in ocular hypertension (OHT) is difficult as patients with glaucoma usually have structural ONH damage at the time of diagnosis. Previous studies assessed glaucomatous ONH cupping by measuring the anterior lamina cribrosa depth (LCD) and minimal rim width (MRW) using optical coherence tomography (OCT). In this study, we induced OHT by repeated intracameral microbead injections in 16 cynomolgus primates (10 unilateral; 6 bilateral) and assessed the structural changes of the ONH longitudinally to observe early changes. Elevated intraocular pressure (IOP) in OHT eyes was maintained for 7 months and serial OCT measurements were performed during this period. The mean IOP was significantly elevated in OHT eyes when compared to baseline and compared to the control eyes. Thinner MRW and deeper LCD values from baseline were observed in OHT eyes with the greatest changes seen between month 1 and month 2 of OHT. Both the mean and maximum IOP values were significant predictors of MRW and LCD changes, although the maximum IOP was a slightly better predictor. We believe that this model could be useful to study IOP-induced early ONH structural damage which is important for understanding glaucoma pathogenesis.

Laminar and Prelaminar Tissue Characteristics of Glaucomatous Eyes Using Enhanced Depth Imaging OCT

PURPOSE

To investigate characteristics of the lamina cribrosa (LC) and prelaminar tissue (PLT) in eyes of patients with primary open-angle glaucoma (POAG), primary angle-closure glaucoma (PACG), and pseudoexfoliation glaucoma (PXG) using enhanced depth imaging (EDI) OCT.

DESIGN

Observational case-control study.

PARTICIPANTS

A total of 116 eyes from 116 participants with POAG (n = 30 eyes), PACG (n = 29 eyes), or PXG (n = 29 eyes) and 28 healthy eyes.

METHODS

Participants underwent a complete ophthalmologic examination in addition to corneal pachymetry, standard achromatic perimetry, peripapillary retinal nerve fiber layer (RNFL) OCT, and EDI OCT, as well as measurement of LC, PLT thickness, and LC depth.

MAIN OUTCOME MEASURES

Lamina cribrosa measurement, PLT thickness, and LC thickness.

RESULTS

Average LC thickness was significantly different among the 4 groups with the thinnest values in the PXG group. Prelaminar tissue also was significantly thinner in PXG eyes. The POAG eyes demonstrated the largest LC depths (P < 0.05), whereas PACG and healthy eyes demonstrated the smallest LC depths (P < 0.05). Mean deviation on standard achromatic perimetry and average thickness on peripapillary RNFL OCT were associated significantly with LC and PLT thickness.

CONCLUSIONS

Pseudoexfoliation glaucoma eyes demonstrate reduced LC and PLT thickness compared with POAG, PACG, and healthy eyes. Lamina cribrosa depth was larger in POAG eyes as compared with PXG, PACG, and healthy eyes. These findings may account for the peculiar appearance of the optic nerve head in PXG eyes.

Astrocyte responses to experimental glaucoma in mouse optic nerve head

PURPOSE

To delineate responses of optic nerve head astrocytes to sustained intraocular pressure (IOP) elevation in mice.

METHODS

We elevated IOP for 1 day to 6 weeks by intracameral microbead injection in 4 strains of mice. Astrocyte alterations were studied by transmission electron microscopy (TEM) including immunogold molecular localization, and by laser scanning microscopy (LSM) with immunofluorescence for integrin β1, α-dystroglycan, and glial fibrillary acidic protein (GFAP). Astrocyte proliferation and apoptosis were quantified by Ki67 and TUNEL labeling, respectively.

RESULTS

Astrocytes in normal optic nerve head expressed integrin β1 and α-dystroglycan by LSM and TEM immunogold labeling at electron dense junctional complexes that were found only on cell membrane zones bordering their basement membranes (BM) at the peripapillary sclera (PPS) and optic nerve head capillaries. At 1-3 days after IOP elevation, abnormal extracellular spaces appeared between astrocytes near PPS, and axonal vesical and mitochondrial accumulation indicated axonal transport blockade. By 1 week, abnormal spaces increased, new collagen formation occurred, and astrocytes separated from their BM, leaving cell membrane fragments. Electron dense junctional complexes separated or were absent at the BM. Astrocyte proliferation was modest during the first week, while only occasional apoptotic astrocytes were observed by TEM and TUNEL.

CONCLUSIONS

Astrocytes normally exhibit junctions with their BM which are disrupted by extended IOP elevation. Responses include reorientation of cell processes, new collagen formation, and cell proliferation.

Factors affecting optic nerve head biomechanics in a rat model of glaucoma

Glaucoma is the leading cause of irreversible blindness and is characterized by the death of retinal ganglion cells, which carry vision information from the retina to the brain. Although it is well accepted that biomechanics is an important part of the glaucomatous disease process, the mechanisms by which biomechanical insult, usually due to elevated intraocular pressure (IOP), leads to retinal ganglion cell death are not understood. Rat models of glaucoma afford an opportunity for learning more about these mechanisms, but the biomechanics of the rat optic nerve head (ONH), a primary region of damage in glaucoma, are only just beginning to be characterized. In a previous study, we built finite-element models with individual-specific rat ONH geometries. Here, we developed a parametrized model of the rat ONH and used it to perform a sensitivity study to determine the influence that six geometric parameters and 13 tissue material properties have on rat optic nerve biomechanical strains due to IOP elevation. Strain magnitudes and patterns in the parametrized model generally matched those from individual-specific models, suggesting that the parametrized model sufficiently approximated rat ONH anatomy. Similar to previous studies in human eyes, we found that scleral properties were highly influential: the six parameters with highest influence on optic nerve strains were optic nerve stiffness, IOP, scleral thickness, the degree of alignment of scleral collagen fibres, scleral ground substance stiffness and the scleral collagen fibre uncrimping coefficient. We conclude that a parametrized modelling strategy is an efficient approach that allows insight into rat ONH biomechanics. Further, scleral properties are important influences on rat ONH biomechanics, and additional efforts should be made to better characterize rat scleral collagen fibre organization.

In Vivo Measurements of Prelamina and Lamina Cribrosa Biomechanical Properties in Humans

Purpose

To develop and use a custom virtual fields method (VFM) to assess the biomechanical properties of human prelamina and lamina cribrosa (LC) in vivo.

Methods

Clinical data of 20 healthy, 20 ocular hypertensive (OHT), 20 primary open-angle glaucoma, and 16 primary angle-closure glaucoma eyes were analyzed. For each eye, the intraocular pressure (IOP) and optical coherence tomography (OCT) images of the optic nerve head (ONH) were acquired at the normal state and after acute IOP elevation. The IOP-induced deformation of the ONH was obtained from the OCT volumes using a three-dimensional tracking algorithm and fed into the VFM to extract the biomechanical properties of the prelamina and the LC in vivo. Statistical measurements and P values from the Mann-Whitney-Wilcoxon tests were reported.

Results

The average shear moduli of the prelamina and the LC were 64.2 ± 36.1 kPa and 73.1 ± 46.9 kPa, respectively. The shear moduli of the prelamina of healthy subjects were significantly lower than those of the OHT subjects. Comparisons between healthy and glaucoma subjects could not be made robustly due to a small sample size.

Conclusions

We have developed a methodology to assess the biomechanical properties of human ONH tissues in vivo and provide preliminary comparisons in healthy and OHT subjects. Our proposed methodology may be of interest for glaucoma management.

Racial Differences in the Association of Anterior Lamina Cribrosa Surface Depth and Glaucoma Severity in the African Descent and Glaucoma Evaluation Study (ADAGES)

Purpose

To determine if variation in anterior lamina cribrosa surface depth (ALCSD) differs between glaucoma patients of African (AD) and European descent (ED).

Methods

A total of 178 eyes from 123 glaucoma patients in the African Descent and Glaucoma Evaluation Study (ADAGES) and Diagnostic Innovations in Glaucoma Study (DIGS) were included. ALCSD and choroidal thickness were measured using the San Diego Automated Layer Segmentation Algorithm (SALSA). ALCSD was defined by both Bruch's membrane opening (BMO)-based (ALCSD-BMO) and scleral-based (ALCSD-Scl) reference planes. Racial differences in ALCSD were evaluated using cross-sectional univariate and multivariable models.

Results

A deeper ALCSD-Scl was found in males (52.4 μm, P = 0.0401), AD individuals (78.6 μm, P = 0.0004), younger individuals (−3.1 μm/year, P < 0.0213), and eyes with larger discs (81.0 μm/mm², P = 0.024), increased visual field loss (mean defect, MD: −6.4 μm/dB [decibel], P = 0.0106), and higher intraocular pressure (IOP: 14.1 μm/mm Hg, P = 0.0256). Significant deepening of ALSCD was observed with increasing IOP and visual field severity only in the AD group. Race modified the relationship between ALCSD-Scl and age (P = 0.0145) with ALCSD-Scl in AD individuals becoming more shallow with increasing age (−3.1 μm/year, P = 0.0213), while there was no significant association in the ED group (2.1 μm/mm Hg, P < 0.2026).

Conclusions

This study demonstrates that a deeper ALCSD, regardless of the ALCSD reference plane used, is associated with more severe glaucoma and higher IOP in the ADAGES cohort, particularly in individuals of AD. These results suggest that characterizing ALCSD morphology and its relationships to IOP, aging, and glaucoma progression may help explain racial differences in disease susceptibility.

A neuroglia-based interpretation of glaucomatous neuroretinal rim thinning in the optic nerve head

Neuroretinal rim thinning (NRR) is a characteristic glaucomatous optic disc change. However, the precise mechanism of the rim thinning has not been completely elucidated. This review focuses on the structural role of the glioarchitecture in the formation of the glaucomatous NRR thinning. The NRR is a glia-framed structure, with honeycomb geometry and mechanically reinforced astrocyte processes along the transverse plane. When neural damage selectively involves the neuron and spares the glia, the gross structure of the tissue is preserved. The disorganization and loss of the glioarchitecture are the two hallmarks of optic nerve head (ONH) remodeling in glaucoma that leads to the thinning of NRR tissue upon axonal loss. This is in contrast to most non-glaucomatous optic neuropathies with optic disc pallor where hypertrophy of the glioarchitecture is associated with the seemingly absent optic disc cupping. Arteritic anterior ischemic optic neuropathy is an exception where pan-necrosis of ONH tissue leads to NRR thinning. Milder ischemia indicates selective neuronal loss that spares glia in non-arteritic anterior ischemic optic neuropathy. The biological reason is the heterogeneous glial response determined by the site, type, and severity of the injury. The neuroglial interpretation explains how the cellular changes underlie the clinical findings. Updated understandings on glial responses illustrate the mechanical, microenvironmental, and microglial modulation of activated astrocytes in glaucoma. Findings relevant to the possible mechanism of the astrocyte death in advanced glaucoma are also emerging. Ultimately, a better understanding of glaucomatous glial response may lead to glia-targeting neuroprotection in the future.

The Relationship Between Scleral Strain Change and Differential Cumulative Intraocular Pressure Exposure in the Nonhuman Primate Chronic Ocular Hypertension Model

Purpose

To determine the relationship between peripapillary scleral strain change and cumulative differential IOP exposure in nonhuman primates (NHPs) with unilateral chronic ocular hypertension.

Methods

Posterior scleral shells from 6 bilaterally normal and 10 unilateral chronic ocular hypertension NHPs were pressurized from 5 to 45 mm Hg, and the resulting full-field, three-dimensional, scleral surface deformations were acquired using laser speckle interferometry. Scleral tensile strain (local tissue deformation) was calculated by analytical differentiation of the displacement field; zero strain was assumed at 5 mm Hg. Maximum principal strain was used to represent the scleral strain, and strains were averaged over a 15°-wide (∼3.6-mm) circumpapillary region adjacent to the ONH. The relative difference in mean strain was calculated between fellow eyes and compared with the differential cumulative IOP exposure within NHPs during the study period. The relationship between the relative difference in scleral strain and the differential cumulative IOP exposure in fellow eyes was assessed using an F test and quadratic regression model.

Results

Relative differential scleral tensile strain was significantly associated with differential cumulative IOP exposure in contralateral eyes in the chronic ocular hypertension NHPs, with the bilaterally normal NHPs showing no significant strain difference between fellow eyes. The sclera in the chronic ocular hypertension eyes was more compliant than in their fellow eyes at low levels of differential cumulative IOP exposure, but stiffer at larger differential IOPs (P < 0.0001).

Conclusions

These cross-sectional findings suggest that longitudinal IOP-induced changes in scleral mechanical behavior are dependent on the magnitude of differential cumulative IOP exposure.

Is "normal tension glaucoma" glaucoma?

Primary open angle glaucoma (POAG) represents a distinct disease entity with elevated intraocular pressure (IOP) as the main risk factor, even though the reasons for why the IOP is elevated remains to be elucidated. It is considered that normal tension glaucoma (NTG) is a subtype of POAG, comprising a special form of glaucomatous neurodegeneration or glaucomatous optic neuropathy (GON) almost exactly the same as that seen in POAG, but the IOP, as named, remains in the statistically normal range. Actually the disease entity of NTG has been a profound confusion and it is difficult to be accurately conceptualized. One of the reasons is that the IOP is closely linked to the occurrence of GON in POAG but not in NTG, and for the latter, it seems that GON is secondary to a number of local or systemic disorders. In recent years, increasing evidences suggest that NTG or IOP independent GON is a non-glaucomatous disease with different disease entities from POAG and with more diverse and complex etiologies. Here we hypothesized that NTG, at least for those with recognizable primary diseases, is not a glaucomatous disease; instead, it represents a group of disorders with GON as a characteristic clinical feature or phenotype.

Lamina cribrosa pore movement during acute intraocular pressure rise

Purpose

To assess changes in the position of lamina cribrosa pores (LCPs) induced by acute intraocular pressure (IOP) elevation.

Methods

A prospective observational study. Acute angle-closure suspects who underwent the 2-hour dark room prone provocative test (DRPPT) were included. At baseline and within 5 min after the DRPPT end, tonometry, fundus photography and optical coherence tomography were performed. Optic disc photos taken before and after the DRPPT were aligned and moving distance of each visible LCP was measured (LCPMD).

Results

38 eyes from 27 participants (age: 52.5±10.8 years) were included. The IOP rose from 16.7±3.2 mm Hg at baseline to 23.9±4.3 mm Hg at the DRPPT end. The mean lateral LCPMD was 28.1±14.6 µm (range: 5.0–77.2 µm), which increased with higher IOP rise (p=0.01) and deeper optic cup (p=0.02) in multivariate analysis. The intralamina range and SD of the LCPMD increased with younger age (p=0.01 and p=0.02, respectively) and with wider optic cup (p=0.01 and p=0.02, respectively). The LCP movements were headed to the superior direction in 12 (33%) eyes, inferior direction in 10 (28%) eyes, temporal direction in 9 (25%) eyes, and nasal direction in 5 (14%) eyes.

Conclusions

IOP rise is associated with LCP movements in the frontal plane, which are more pronounced with higher IOP rise and deeper optic cup. The intralamina variability in the IOP rise-associated LCPMD increased with younger age and wider optic cup. IOP variation-associated lateral LCP movements may be of interest to elucidate glaucomatous optic nerve damage.

Scleral structure and biomechanics

As the eye's main load-bearing connective tissue, the sclera is centrally important to vision. In addition to cooperatively maintaining refractive status with the cornea, the sclera must also provide stable mechanical support to vulnerable internal ocular structures such as the retina and optic nerve head. Moreover, it must achieve this under complex, dynamic loading conditions imposed by eye movements and fluid pressures. Recent years have seen significant advances in our knowledge of scleral biomechanics, its modulation with ageing and disease, and their relationship to the hierarchical structure of the collagen-rich scleral extracellular matrix (ECM) and its resident cells. This review focuses on notable recent structural and biomechanical studies, setting their findings in the context of the wider scleral literature. It reviews recent progress in the development of scattering and bioimaging methods to resolve scleral ECM structure at multiple scales. In vivo and ex vivo experimental methods to characterise scleral biomechanics are explored, along with computational techniques that combine structural and biomechanical data to simulate ocular behaviour and extract tissue material properties. Studies into alterations of scleral structure and biomechanics in myopia and glaucoma are presented, and their results reconciled with associated findings on changes in the ageing eye. Finally, new developments in scleral surgery and emerging minimally invasive therapies are highlighted that could offer new hope in the fight against escalating scleral-related vision disorder worldwide.

Optic disk hemorrhage and vitreous hemorrhage after phacoemulsification in a normal tension glaucoma patient: A case report

RATIONALE

Optic disk hemorrhage has been closely correlated with glaucoma for its development and progression. Phacoemulsification surgery results in large intraocular pressure (IOP) fluctuation. We report a case of optic disk hemorrhage and consequently progressive vitreous hemorrhage after an unsuccessful phacoemulsification surgery in an advanced normal tension glaucoma (NTG) patient.

PATIENT CONCERNS

An advanced NTG patient of 82 years old with chronic hypertension underwent an unsuccessful phacoemulsification surgery complicated by posterior capsule rupture. During the postoperative 2 weeks, recurrent episodes of fresh hyphema occurred and B ultrasonography scan revealed the progressive vitreous hemorrhage. The IOP went out of control under the maximum tolerable IOP-lowering medications.

DIAGNOSIS

Vitreous hemorrhage after phacoemulsification in an advanced NTG patient.

INTERVENTIONS

Vitrectomy was performed to search for the cause of the progressive vitreous hemorrhage. After removal of the thick vitreous hemorrhage, a fresh spot of optic disk hemorrhage was detected at the nasal margin of the significantly-cupping disk.

OUTCOMES

Postoperatively, the hemorrhagic vitreous opacity gradually resolved and the IOP remained stable at 10 ∼13 mmHg with topical prostaglandin analogue drops. Fundus examination revealed the dilated disk vessel with localized angiomatous change at the nasal disk margin.

LESSONS

Severe optic disk hemorrhage may occur after phacoemulsification in advanced glaucoma patients. Systemic vascular factors, such as chronic hypertension and old age, and surgical complications, such as posterior capsule rupture and postoperative IOP elevation, would further increase the risk. For phacoemulsification in advanced glaucoma cases, extra care should be taken to control intraoperative IOP fluctuations and monitor postoperative IOP.

Comparison of the Lamina Cribrosa Measurements Obtained by Spectral-Domain and Swept-Source Optical Coherence Tomography

Purpose/Aim of the study: The aim of this study was to compare the lamina cribrosa (LC) measurements obtained by Spectral-Domain Optical Coherence Tomography (SD-OCT) and Swept-Source Optical Coherence Tomography (SS-OCT) in the same eye and we also investigate how the differences in measurement will change in the presence of pseudoexfoliation glaucoma (PEG). Materials and Methods: A total of 30 patients from one institution have accepted. Detailed medical case histories and clinical examination, optic nerve head imaging using the SD-OCT (Heidelberg Engineering, Heidelberg, Germany) and SS-OCT (Triton; Topcon Medical Systems, Tokyo, Japan) for patients with pseudoexfoliation glaucoma and healthy volunteer. Results: Fifty-three eyes of the 30 patients (13 female, 17 male) were included in the study. Twenty-nine of eyes were healthy and 24 of eyes had pseudoexfoliation glaucoma. The mean age was 62 ± 7.3 years (range, 50-86 years). The difference between the mean Bruch's membrane opening distance measured by SD-OCT (1504.7 ± 154.2 µm) and by SS-OCT (1568.6 ± 193.3 µm) was statistically significant (p = .009). The difference between LC depth and LC thickness measurements between two OCT devices were not statistically significant. There was a negative correlation between the difference of two devices Bruch's membrane opening distance measurements and the cup/disc ratio (p = .007). Conclusion: Generally accepted belief is that lamina cribrosa visibility would be worse in healthy subjects who have thick prelaminar tissues, and better in individuals with glaucoma who had suffered the loss of prelaminar tissue. The difference between Bruch's membrane opening distance measurements taken with two devices was greater in normal eyes, and this difference was decreasing in glaucomatous eyes in our study. The Lamina Cribrosa measurement values shall not be directly compared between SD-OCT and SS-OCT.

Assessing the True Intraocular Pressure in the Non-human Primate

SIGNIFICANCE

For glaucoma patients, high intraocular pressure (IOP) is a risk factor for progressive neuropathy. Similarly, animal models used to study the disease are based on an experimental elevation of IOP. Thus, accurate IOP measurements are important in characterizing experimental models and resulting effects.

PURPOSE

The purpose of the present study was to investigate IOP measurements in a non-human primate model of experimental glaucoma by comparing clinical tonometry (Tono-Pen and TonoVet) to the true IOP from intracameral manometry.

METHODS

A total of 17 rhesus macaque eyes from 12 animals were used for this study. Eleven eyes had no previous experimental intervention, whereas six eyes were at varying stages of laser-induced experimental glaucoma. IOPs were adjusted by inserting a needle in the anterior chamber that was attached to a pressure transducer and syringe pump system. The anterior chamber IOP was adjusted to values between 10 and 50 mmHg and corresponding measures with Tono-Pen and TonoVet were taken.

RESULTS

The IOPs by TonoVet and Tono-Pen were linearly related over the range of pressures tested (slope = 0.68 normal/healthy and 0.72 experimental glaucoma). For the most, TonoVet measures overestimated IOP at all anterior chamber pressure settings (mean difference of 3.17 mmHg, 95% CI 12.53 to -4.74 normal and 3.90 mmHg, 95% CI 12.90 to -6.53 experimental glaucoma). In contrast, Tono-Pen measures overestimated IOP at lower IOPs and underestimated at higher IOP (slope = -0.26 normal and -0.21 experimental glaucoma).

CONCLUSIONS

The TonoVet and Tono-Pen tonometers that are often used to assess IOP in both clinical and experimental settings generally reflect the status of IOP, but the results from this study suggest that the instruments need calibration with true anterior chamber pressure for accurate measures in experimental models of glaucoma.

Peripapillary sclera architecture revisited: A tangential fiber model and its biomechanical implications

The collagen fiber architecture of the peripapillary sclera (PPS), which surrounds the scleral canal, is a critical factor in determining the mechanical response of the optic nerve head (ONH) to variations in intraocular pressure (IOP). Experimental and clinical evidence point to IOP-induced deformations within the scleral canal as important contributing factors of glaucomatous neural tissue damage and consequent vision loss. Hence, it is imperative to understand PPS architecture and biomechanics. Current consensus is that the fibers of the PPS form a closed ring around the canal to support the delicate neural tissues within. We propose an alternative fiber architecture for the PPS, in which the scleral canal is supported primarily by long-running fibers oriented tangentially to the canal. We present evidence that this tangential model is consistent with histological observations in multiple species, and with quantitative measurements of fiber orientation obtained from small angle light scattering and wide-angle X-ray scattering. Using finite element models, we investigated the biomechanical implications of a tangential fiber PPS architecture. We found that the tangential arrangement of fibers afforded better mechanical support to the tissues within the scleral canal as compared to a simple circumferential ring of fibers or a combination of fibers oriented radially and circumferentially. We also found that subtle variations from a tangential orientation could reproduce clinically observed ONH behavior which has yet to be explained using current theories of PPS architecture and simulation, namely, the contraction of the scleral canal under elevated IOP.

STATEMENT OF SIGNIFICANCE

It is hypothesized that vision loss in glaucoma is due to excessive mechanical deformation within the neural tissue inside the scleral canal. This study proposes a new model for how the collagen of the peripapillary sclera surrounding the canal is organized to support the delicate neural tissue inside. Previous low-resolution studies of the peripapillary sclera suggested that the collagen fibers are arranged in a ring around the canal. Instead, we provide microscopic evidence suggesting that the canal is also supported by long-running interwoven fibers oriented tangentially to the canal. We demonstrate that this arrangement has multiple biomechanical advantages over a circular collagen arrangement and can explain previously unexplained experimental findings including contraction of the scleral canal under elevated intraocular pressure.

Polarized light microscopy for 3-dimensional mapping of collagen fiber architecture in ocular tissues

Collagen fibers play a central role in normal eye mechanics and pathology. In ocular tissues, collagen fibers exhibit a complex 3-dimensional (3D) fiber orientation, with both in-plane (IP) and out-of-plane (OP) orientations. Imaging techniques traditionally applied to the study of ocular tissues only quantify IP fiber orientation, providing little information on OP fiber orientation. Accurate description of the complex 3D fiber microstructures of the eye requires quantifying full 3D fiber orientation. Herein, we present 3dPLM, a technique based on polarized light microscopy developed to quantify both IP and OP collagen fiber orientations of ocular tissues. The performance of 3dPLM was examined by simulation and experimental verification and validation. The experiments demonstrated an excellent agreement between extracted and true 3D fiber orientation. Both IP and OP fiber orientations can be extracted from the sclera and the cornea, providing previously unavailable quantitative 3D measures and insight into the tissue microarchitecture. Together, the results demonstrate that 3dPLM is a powerful imaging technique for the analysis of ocular tissues.

Polarized light microscopy for 3-dimensional mapping of collagen fiber architecture in ocular tissues

Collagen fibers play a central role in normal eye mechanics and pathology. In ocular tissues, collagen fibers exhibit a complex 3-dimensional (3D) fiber orientation, with both in-plane (IP) and out-of-plane (OP) orientations. Imaging techniques traditionally applied to the study of ocular tissues only quantify IP fiber orientation, providing little information on OP fiber orientation. Accurate description of the complex 3D fiber microstructures of the eye requires quantifying full 3D fiber orientation. Herein, we present 3dPLM, a technique based on polarized light microscopy developed to quantify both IP and OP collagen fiber orientations of ocular tissues. The performance of 3dPLM was examined by simulation and experimental verification and validation. The experiments demonstrated an excellent agreement between extracted and true 3D fiber orientation. Both IP and OP fiber orientations can be extracted from the sclera and the cornea, providing previously unavailable quantitative 3D measures and insight into the tissue microarchitecture. Together, the results demonstrate that 3dPLM is a powerful imaging technique for the analysis of ocular tissues.

Protruded retinal layers within the optic nerve head neuroretinal rim

PURPOSE

To determine the frequency with which retinal tissues other than the nerve fibre layer, hereafter referred to as protruded retinal layers (PRL), are a component of optical coherence tomography (OCT) neuroretinal rim measurements.

METHODS

Ninety healthy (30 White, Black and Japanese, respectively) subjects were included in the study. A radial scan pattern (24 B-scans centred on Bruch's membrane opening [BMO]) was used. For each of the 48 minimum rim width (MRW) measurement points, we determined whether PRL were present, absent or indeterminate. When present, the proportion of PRL within the MRW was quantified.

RESULTS

Protruded retinal layers were present in 503 (11.6%), absent in 3805 (88.1%) and indeterminate in 12 (0.3%) measurement points. Overall, 69 (76.6%) subjects had ≥1 points with PRL, with White subjects having the highest frequency and Japanese the lowest (29 [97%] and 18 [60%], respectively; p < 0.01). PRL were present in one-third of points in the temporal sector, but ≤5% in other sectors. When present, the median PRL thickness was 53.0 (interquartile range [IQR]: 33.0 to 78.5) μm, representing 20.6 (IQR: 13.0 to 28.5)% of MRW. Globally, the median PRL thickness comprised 1.3 (IQR: 0.2 to 3.5)% of the MRW; however, in the temporal sector, it exceeded 30% of MRW in some subjects.

CONCLUSIONS

Protruded retinal layers are a component of MRW measurements in most normal subjects, occurring in almost 12% of all measurement points analysed. There were racial variations in the presence of PRL and a significantly higher frequency of PRL in the temporal sector.

Morphological and functional changes in the rat retina associated with 2 months of intermittent moderate intraocular pressure elevation

Morphological and functional changes in the rat retina and optic nerve head (ONH), associated with 8 weeks of intermittent moderately elevated intraocular pressure (IOP) were measured with a combined ultrahigh resolution optical coherence tomography (UHR-OCT) and electroretinography (ERG) system. The IOP of male Sprague-Dawley rats was raised in one eye to ~35 mmHg for 1 hour/day on 6 days each week using vascular loops. Single-flash ERG traces and volumetric UHR-OCT images of the ONH were acquired from both eyes before, during and after IOP elevations at weeks 1, 5 and 9 of the study. The UHR-OCT images showed depression of the posterior eye around the ONH during the IOP elevations, the magnitude of which increased significantly from week 1 to week 9 (p = 0.01). The ERG a-wave and b-wave amplitudes increased temporarily during IOP elevations and returned to normal ~30 minutes after loop removal. Recurrent intermittent IOP spikes caused > 30% decrease in the ERG a-wave and b-wave amplitudes measured during the IOP elevations over the course of 2 months. This study suggests that recurrent, relatively short-duration IOP spikes for extended period of time are associated with peri-ONH tissue hypercompliance and reduced retinal functional response to visual stimulation during acute IOP elevation.

Lamina Cribrosa Changes after Laser In Situ Keratomileusis in Myopic Eyes

PURPOSE

To determine deep optic nerve head structure changes after transient intraocular pressure elevation during laser in situ keratomileusis (LASIK) for myopia.

METHODS

Enhanced depth imaging-optical coherence tomography was performed in each myopic eye that underwent LASIK surgery. Enhanced depth imaging-optical coherence tomography images were created at postoperative 1 day, 1 week, 2 weeks, and 1 month. Lamina cribrosa (LC) thickness, LC depth and prelaminar thickness at the superior, middle and inferior portions of the optic nerve head were measured by two investigators.

RESULTS

Forty eyes in 40 patients were included in the present study. During follow-up, there were no significant differences in prelaminar thickness or LC depth. The LC demonstrated increased thickness at postoperative 1 day at all three locations (superior, middle, and inferior) (p < 0.001, p < 0.001, p < 0.001, respectively). However, no significant changes were observed at postoperative 1 week, 2 weeks, and 1 month.

CONCLUSIONS

The LC thickness could increase at 1 day after LASIK surgery. However, the thickness will gradually return to baseline morphology. Temporary intraocular pressure increase during LASIK does not appear to induce irreversible LC thickness changes.

Structural, functional and blood perfusion changes in the rat retina associated with elevated intraocular pressure, measured simultaneously with a combined OCT+ERG system

Acute elevation of intraocular pressure (IOP) to ischemic and non-ischemic levels can cause temporary or permanent changes in the retinal morphology, function and blood flow/blood perfusion. Previously, such changes in the retina were assessed separately with different methods in clinical studies and animal models. In this study, we used a combined OCT+ ERG system in combination with Doppler OCT and OCT angiography (OCTA) imaging protocols, in order to evaluate simultaneously and correlate changes in the retinal morphology, the retinal functional response to visual stimulation, and the retinal blood flow/blood perfusion, associated with IOP elevation to ischemic and non-ischemic levels in rats. Results from this study suggest that the inner retina responds faster to IOP elevation to levels greater than 30 mmHg with significant reduction of the total retinal blood flow (TRBF), decrease of the capillaries’ perfusion and reduction of the ON bipolar cells contribution to the ERG traces. Furthermore, this study showed that ischemic levels of IOP elevation cause an additional significant decrease in the ERG photoreceptor response in the posterior retina. Thirty minutes after IOP normalization, retinal morphology, blood flow and blood perfusion recovered to baseline values, while retinal function did not recover completely.

Measurement of the hypotenuse of the vertical optic nerve head cup with spectral-domain optical coherence tomography for the structural diagnosis of glaucoma

Purpose

To evaluate the hypotenuse of the vertical optic nerve head cup (HVOC), measured using the length and depth of the cup obtained with enhanced depth imaging spectral-domain optic coherence tomography (SD-OCT), as a biomarker for glaucoma diagnosis.

Methods

This was a prospective cross-sectional study of patients with glaucoma and controls. SD-OCT was performed in all participants to assess average circumpapillary retinal nerve fiber layer (RNFL) thickness. A vertical B-scan of the optic nerve head (ONH) was obtained for HVOC measurement. The length and depth of the optic nerve cup formed the sides of a right triangle that were used to calculate the HVOC. Participants also underwent standard automated perimetry.

Results

One hundred and fifty-six eyes were divided into three groups: mean deviation (MD) <−7 dB (60 eyes); MD ≥−7 dB (74 eyes); and healthy subjects (22 eyes). The mean (SD) HVOC in these groups was 1,419.8 (347.2) µm, 1,234.6 (258.8) µm, and 685.79 (315.4) µm (P<0.01), respectively. In the secondary structure–function analysis, only discs with a vertical diameter of 1.51–2.00 mm were included (120 eyes). The HVOCs were divided into four percentile groups, with the following means: 940, 1,128, 1,390, and 1,662 µm. There was a significant difference in MD between percentile groups 1 and 3 (P<0.03), 1 and 4 (P<0.001), 2 and 3 (P<0.02), and 2 and 4 (P<0.001). RNFL thickness differed among all percentile groups (P<0.001).

Conclusion

HVOC may provide an additional morphometric biomarker for the structural evaluation of ONH remodeling in glaucoma.

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.

Clinical Implications of In Vivo Lamina Cribrosa Imaging in Glaucoma

The lamina cribrosa (LC) is a multilayered, collagenous, sieve-like structure at the deep optic nerve head, and is presumed to be the primary site of axonal injury. According to biomechanical theory, intraocular pressure-induced posterior deformation of the LC causes blockage of axonal transport and alters the ocular blood flow, so that the axons of the retinal ganglion cells lead to apoptosis, which results in glaucomatous optic disc change. Although most of the research on the LC to date has been limited to experimental animal or histologic studies, the recent advances in optical coherence tomography devices and image processing techniques have made possible the visualization of the LC structure in vivo. LC deformation in glaucoma typically has been evaluated in terms of its position from a structural reference plane (LC depth), entire curvature or shape, thickness, or localized structural change (focal LC defects or LC pore change). In this review, we highlight the methods of assessing LC deformation from in vivo optical coherence tomography scans, and we discuss the clinical implications of the recent investigations of the in vivo structure of LC in glaucoma.

Racioethnic Differences in Human Posterior Scleral and Optic Nerve Stump Deformation

Purpose

The purpose of this study was to quantify the biomechanical response of human posterior ocular tissues from donors of various racioethnic groups to better understand how differences in these properties may play a role in the racioethnic health disparities known to exist in glaucoma.

Methods

Sequential digital image correlation (S-DIC) was used to measure the pressure-induced surface deformations of 23 normal human posterior poles from three racioethnic groups: African descent (AD), European descent (ED), and Hispanic ethnicity (HIS). Regional in-plane principal strains were compared across three zones: the optic nerve stump (ONS), the peripapillary (PP) sclera, and non-PP sclera.

Results

The PP scleral tensile strains were found to be lower for ED eyes compared with AD and HIS eyes at 15 mm Hg (P = 0.024 and 0.039, respectively). The mean compressive strains were significantly higher for AD eyes compared with ED eyes at 15 mm Hg (P = 0.018). We also found that the relationship between tensile strain and pressure was significant for those of ED and HIS eyes (P < 0.001 and P = 0.004, respectively), whereas it was not significant for those of AD (P = 0.392).

Conclusions

Our results suggest that, assuming glaucomatous nerve loss is caused by mechanical strains in the vicinity of the optic nerve head, the mechanism of increased glaucoma prevalence may be different in those of AD versus HIS. Our ONS strain analysis also suggested that it may be important to account for ONS geometry and material properties in future scleral biomechanical analysis.

Association of intraocular pressure-related factors and retinal vessel diameter with optic disc rim area in subjects with and without primary open angle glaucoma

IMPORTANCE

The data may support the notion that the intra-ocular pressure (IOP)-related factors and vascular factors were implicated concurrently in glaucomatous optic nerve damage.

BACKGROUND

To study the association of intraocular pressure (IOP)-related factors, IOP, trans-lamina cribrosa pressure difference (TLCPD), cerebrospinal fluid pressure (CSFP) and retinal vessel diameters (RVD), central retinal arteriolar equivalent (CRAE), central retinal venular equivalent (CRVE) with neuro-retinal rim area (RA).

DESIGN

A population-based, cross-sectional study.

PARTICIPANTS

A total of 6830 people aged 30 years and over.

METHODS

All participants underwent a comprehensive eye examination, fundus photograph-based measurements of RVD and Heidelberg retinal tomogram (HRT) measurement of optic disc.

MAIN OUTCOME MEASURES

RA, CRAE, CRVE, IOP, body mass index (BMI), CSFP and TLCPD.

RESULTS

Primary open-angle glaucoma (POAG) was diagnosed using two separate methods: 67 from expert consensus, and 125 from the International Society of Geographical and Epidemiologic Ophthalmology (ISGEO) classification. After excluding of those with high myopia or without gradable HRT images, 4194 non-glaucoma and 40 POAG were analysed for determinants of RA. On multivariable analysis determinants of reduced RA were POAG (P < 0.001), higher IOP (P = 0.03), higher refractive error (P < 0.01), longer axial length (P = 0.01), CRVE (P < 0.001), lower BMI (P = 0.015), older age (P < 0.001) smaller disc area (P < 0.001) and higher TLCPD (P = 0.03). When age and/or BMI were omitted from the model, reduced RA was also associated with lower CSFP (P < 0.001).

CONCLUSIONS AND RELEVANCE

Reduced RA is associated with narrow CRVE and higher IOP or lower CSFP. The data supports the concurrent role of IOP-related and vascular factors in glaucomatous optic nerve damage.

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.

Roles of exosomes in the normal and diseased eye

Exosomes are nanometer-sized vesicles that are released by cells in a controlled fashion and mediate a plethora of extra- and intercellular activities. Some key functions of exosomes include cell-cell communication, immune modulation, extracellular matrix turnover, stem cell division/differentiation, neovascularization and cellular waste removal. While much is known about their role in cancer, exosome function in the many specialized tissues of the eye is just beginning to undergo rigorous study. Here we review current knowledge of exosome function in the visual system in the context of larger bodies of data from other fields, in both health and disease. Additionally, we discuss recent advances in the exosome field including use of exosomes as a therapeutic vehicle, exosomes as a source of biomarkers for disease, plus current standards for isolation and validation of exosome populations. Finally, we use this foundational information about exosomes in the eye as a platform to identify areas of opportunity for future research studies.

Aging and ocular tissue stiffness in glaucoma

Glaucoma is a progressive and chronic neurodegenerative disorder characterized by damage to the inner layers of the retina and deformation of the optic nerve head. The degeneration of retinal ganglion cells and their axons results in an irreversible loss of vision and is correlated with increasing age. Extracellular matrix changes related to natural aging generate a stiffer extracellular environment throughout the body. Altered age-associated ocular tissue stiffening plays a major role in a significant number of ophthalmic pathologies. In glaucoma, both the trabecular meshwork and the optic nerve head undergo extensive extracellular matrix remodeling, characterized by fibrotic changes associated with cellular and molecular events (including myofibroblast activation) that drive further tissue fibrosis and stiffening. Here, we review the literature concerning the role of age-related ocular stiffening in the trabecular meshwork, lamina cribrosa, sclera, cornea, retina, and Bruch membrane/choroid and discuss their potential role in glaucoma progression. Because both trabecular meshwork and lamina cribrosa cells are mechanosensitive, we then describe molecular mechanisms underlying tissue stiffening and cell mechanotransduction and how these cellular activities can drive further fibrotic changes within ocular tissues. An improved understanding of the interplay between age-related tissue stiffening and biological responses in the trabecular meshwork and optic nerve head could potentially lead to novel therapeutic strategies for glaucoma treatment.

Deformation of the Lamina Cribrosa and Optic Nerve Due to Changes in Cerebrospinal Fluid Pressure

Purpose

Cerebrospinal fluid pressure (CSFp) changes are involved or implicated in various ocular conditions including glaucoma, idiopathic intracranial hypertension, and visual impairment and intracranial pressure syndrome. However, little is known about the effects of CSFp on lamina cribrosa and retrolaminar neural tissue (RLNT) biomechanics, potentially important in these conditions. Our goal was to use an experimental approach to visualize and quantify the deformation of these tissues as CSFp increased.

Methods

The posterior eye and RLNT of porcine eyes (n = 3) were imaged using synchrotron radiation phase-contrast micro-computed tomography (PC μCT) at an intraocular pressure of 15 mm Hg and CSFps of 4, 10, 20, and 30 mm Hg. Scans of each tissue region were acquired at each CSFp step and analyzed using digital volume correlation to determine 3-dimensional tissue deformations.

Results

Elevating CSFp increased the strain in the lamina cribrosa and RLNT of all three specimens, with the largest strains occurring in the RLNT. Relative to the baseline CSFp of 4 mm Hg, at 30 mm Hg, the lamina cribrosa experienced a mean first and third principal strain of 4.4% and −3.5%, respectively. The corresponding values for the RLNT were 9.5% and −9.1%.

Conclusions

CSFp has a significant impact on the strain distributions within the lamina cribrosa and, more prominently, within the RLNT. Elevations in CSFp were positively correlated with increasing deformations in each region and may play a role in ocular pathologies linked to changes in CSFp.

In Vivo Detection of Laminar and Peripapillary Scleral Hypercompliance in Early Monkey Experimental Glaucoma

Purpose

To compare optical coherence tomography (OCT) detected, optic nerve head (ONH) compliance within control and experimental glaucoma (EG) eyes of 15 monkeys at EG onset.

Methods

Intraocular pressure (IOP) was chronically elevated in one eye of each animal using a laser. Experimental glaucoma onset was identified using confocal scanning laser tomography (CSLT). Optical coherence tomography ONH imaging (40 radial B-scans) was performed at 10 mm Hg before and after laser. At EG onset, OCT scans were obtained at IOP 10 and 30 mm Hg. Optical coherence tomography landmarks within the IOP 10/30 images were delineated to quantify IOP 10/30 differences (compliance) for anterior lamina cribrosa surface depth (ALCSD) relative to Bruch's membrane opening (BMO) (ALCSD-BMO), ALCSD relative to peripheral BM (ALCSD-BM), and BMO depth relative to peripheral BM (BMOD-BM). A linear mixed effects model assessed for acute IOP elevation effects, control versus EG eye effects, and their interaction

Results

Effects of IOP elevation were greater in EG versus control eyes for ALCSD-BMO (−46 ± 45 vs. −8 ± 13 μm, P = 0.0042) and ALCSD-BM (−92 ± 64 vs. −42 ± 22 μm, P = 0.0075). Experimental glaucoma eye-specific ALCSD-BMO and ALCSD-BM compliance exceeded the range of control eye compliance in 9 and 8 of the 15 EG eyes, respectively. Post-laser peak IOP (R² = 0.798, P < 0.0001) and post-laser mean IOP (R² = 0.634, P < 0.0004) most strongly correlated to EG versus control eye differences in ALCSD-BMO compliance.

Conclusions

Laminar (ALCSD-BMO) and peripapillary scleral (ALCSD-BM) hypercompliance are present in most monkey eyes at the onset of EG.

Lamina Cribrosa Microarchitecture in Monkey Early Experimental Glaucoma: Global Change

Purpose

The purpose of this study was to characterize experimental glaucoma (EG) versus control eye differences in lamina cribrosa (LC), beam diameter (BD), pore diameter (PD), connective tissue volume fraction (CTVF), connective tissue volume (CTV), and LC volume (LV) in monkey early EG.

Methods

Optic nerve heads (ONHs) of 14 unilateral EG and 6 bilateral normal (BN) monkeys underwent three-dimensional reconstruction and LC beam segmentation. Each beam and pore voxel was assigned a diameter based on the largest sphere that contained it before transformation to a common cylinder with inner, middle, and outer layers. Full-thickness and layer averages for BD, PD, CTVF, CTV, and LV were calculated for each ONH. Beam diameter and PD distributions for each ONH were fit to a gamma distribution and summarized by scale and shape parameters. Experimental glaucoma and depth effects were assessed for each parameter by linear mixed-effects (LME) modeling. Animal-specific EG versus control eye differences that exceeded the maximum intereye difference among the six BN animals were considered significant.

Results

Overall EG eye mean PD was 12.8% larger (28.2 ± 5.6 vs. 25.0 ± 3.3 μm), CTV was 26.5% larger (100.06 ± 47.98 vs. 79.12 ± 28.35 × 10⁶ μm³), and LV was 40% larger (229.29 ± 98.19 vs. 163.63 ± 39.87 × 10⁶ μm³) than control eyes (P ≤ 0.05, LME). Experimental glaucoma effects were significantly different by layer for PD (P = 0.0097) and CTVF (P < 0.0001). Pore diameter expanded consistently across all PDs. Experimental glaucoma eye-specific parameter change was variable in magnitude and direction.

Conclusions

Pore diameter, CTV, and LV increase in monkey early EG; however, EG eye-specific change is variable and includes both increases and decreases in BD and CTVF.

Displacement of the Lamina Cribrosa in Response to Acute Intraocular Pressure Elevation in Normal Individuals of African and European Descent

Purpose

To assess if the in vivo mechanical displacement of the anterior laminar cribrosa surface (ALCS) as a response of an acute elevation in intraocular pressure (IOP) differs in individuals of European (ED) and African descent (AD).

Methods

Spectral-domain optical coherence tomography (SDOCT) scans were obtained from 24 eyes of 12 individuals of AD and 18 eyes of 9 individuals of ED at their normal baseline IOP and after 60 seconds IOP elevation using ophthalmodynamometry. Change in depth (displacement) of the LC and to the prelaminar tissue (PLT) were computed in association with the change (delta) in IOP (Δ IOP), race, age, corneal thickness, corneal rigidity (ocular response analyzer [ORA]), and axial.

Results

In the ED group for small IOP elevations (Δ IOP < 12 mm Hg), the ALCS initially displaced posteriorly but for larger increase of IOP an anterior displacement of the lamina followed. Inversely, in the AD group the ALCS did not show a significant posterior displacement for small Δ IOP, while for larger IOP increases the ALCS significantly displaced posteriorly. Posterior displacement of the lamina cribrosa (LC) was also significantly correlated with longer axial length, higher corneal thickness, and ORA parameters. Prelaminar tissue posteriorly displaced for any magnitude of Δ IOP, in both groups.

Conclusions

The African descent group demonstrated a greater acute posterior bowing of the LC after adjustment for age, axial length, Bruch's membrane opening (BMO) area, and ORA parameters. Greater PLT posterior displacement was also seen in the AD group with increasing IOP, which was tightly correlated with the displacement of the LC.