A Noninvasive Clinical Method to Measure in Vivo Mechanical Strains of the Lamina Cribrosa by OCT

Objective

To measure mechanical strain of the lamina cribrosa (LC) after intraocular pressure (IOP) change produced 1 week after a change in glaucoma medication.

Design

Cohort study.

Participants

Adult glaucoma patients (23 eyes, 15 patients) prescribed a change in IOP-lowering medication.

Intervention

Noninvasive OCT imaging of the eye.

Main Outcome Measures

Deformation calculated by digital volume correlation of OCT scans of the LC before and after IOP lowering by medication.

Results

Among 23 eyes, 17 eyes of 12 persons had IOP lowering ≥ 3 mmHg (reduced IOP group) with tensile anterior-posterior Ezz strain = 1.0% ± 1.1% (P = 0.003) and compressive radial strain (Err) = -0.3% ± 0.5% (P = 0.012; random effects models accounting inclusion of both eyes in some persons). Maximum in-plane principal (tensile) strain and maximum shear strain in the reduced-IOP group were as follows: Emax = 1.7% ± 1.0% and Γmax = 1.4% ± 0.7%, respectively (both P < 0.0001 vs. zero). Reduced-IOP group strains Emax and Γmax were significantly larger with greater % IOP decrease (P < 0.0001 and P < 0.0001, respectively). The compliances of the Ezz, Emax, and Γmax strain responses, defined as strain normalized by the IOP decrease, were larger with more abnormal perimetric mean deviation or visual field index values (all P ≤ 0.02). Strains were unrelated to age (all P ≥ 0.088). In reduced-IOP eyes, mean LC anterior border posterior movement was only 2.05 μm posteriorly (P = 0.052) and not related to % IOP change (P = 0.94, random effects models). Only Err was significantly related to anterior lamina depth change, becoming more negative with greater posterior LC border change (P = 0.015).

Conclusions

Lamina cribrosa mechanical strains can be effectively measured by changes in eye drop medication using OCT and are related to degree of visual function loss in glaucoma.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Long-term Remodeling Response in the Lamina Cribrosa Years after Intraocular Pressure Lowering by Suturelysis after Trabeculectomy

OBJECTIVE

To measure the remodeling of the lamina cribrosa (LC) years after intraocular pressure (IOP) lowering by suturelysis.

DESIGN

Cohort study.

PARTICIPANTS

Glaucoma patients were imaged 20 minutes after laser suturelysis after trabeculectomy surgery and at their follow-up appointment 1 to 4 years later (16 image pairs; 15 persons).

INTERVENTION

Noninvasive OCT imaging of the eye.

MAIN OUTCOME MEASURES

Deformation calculated by correlating OCT scans of the LC immediately after IOP lowering by suturelysis and those acquired years later (defined as remodeling strain).

RESULTS

The LC anterior border moved 60.9 ± 54.6 μm into the eye (P = 0.0007), and the LC exhibited regions of large local stretch in the anterior-posterior direction on long-term, maintained IOP lowering, resulting in a mean anterior-posterior remodeling strain of 14.0% ± 21.3% (P = 0.02). This strain and the LC border movement was 14 times and 124 times larger, respectively, than the direct response to IOP lowering by suturelysis. A larger anterior LC border movement was associated with greater mean anterior-posterior remodeling strain (P = 0.004). A thinner retinal nerve fiber layer at suturelysis was also associated with greater mean anterior-posterior remodeling strain at follow-up (P = 0.05). Worsening visual field indexes during follow-up were associated with a greater mean circumferential remodeling strain (P = 0.02), due to regions of large local circumferential stretch of the LC. Eyes with a more compliant LC torsional shear strain response at lysis were associated with worse mean deviation at follow-up (P = 0.03).

CONCLUSIONS

Strains and LC border position changes measured years after IOP lowering are far larger than the immediate response to IOP lowering and indicate dramatic remodeling of the LC anatomical structure caused by IOP lowering and glaucoma progression. The remodeling strains indicate substantial local stretch in the anterior-posterior direction and are associated with movement of the LC anterior border into the eye. Eyes with greater direct strain response to IOP lowering, greater glaucoma damage at suturelysis, and greater worsening of visual field at follow-up experienced greater remodeling.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT03267849.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Optical Coherence Tomographic Optic Nerve Head Morphology in Myopia III: The Exposed Neural Canal Region in Healthy Eyes-Implications for High Myopia

PURPOSE

To determine the prevalence and magnitude of optical coherence tomography (OCT) exposed neural canal (ENC), externally oblique choroidal border tissue (EOCBT), and exposed scleral flange (ESF) regions in 362 non-highly myopic (spherical equivalent -6.00 to 5.75 diopters) eyes of 362 healthy subjects.

DESIGN

Cross-sectional study.

METHODS

After OCT optic nerve head (ONH) imaging, Bruch membrane opening (BMO), the anterior scleral canal opening (ASCO), and the scleral flange opening (SFO) were manually segmented. BMO, ASCO, and SFO points were projected to the BMO reference plane. The direction and magnitude of BMO/ASCO offset as well as the magnitude of ENC, EOCBT, and ESF was calculated within 30° sectors relative to the foveal-BMO axis. Hi-ESF eyes demonstrated an ESF ≥100 µm in at least 1 sector. Sectoral peri-neural canal choroidal thickness (pNC-CT) was measured and correlations between the magnitude of sectoral ESF and proportional pNC-CT were assessed.

RESULTS

Seventy-three Hi-ESF (20.2%) and 289 non-Hi-ESF eyes (79.8%) were identified. BMO/ASCO offset as well as ENC, EOCBT, and ESF prevalence and magnitude were greatest inferior temporally where the pNC-CT was thinnest. Among Hi-ESF eyes, the magnitude of each ENC region correlated with the BMO/ASCO offset magnitude, and the sectors with the longest ESF correlated with the sectors with proportionally thinnest pNC-CT.

CONCLUSIONS

ONH BMO/ASCO offset, either as a cause or result of ONH neural canal remodeling, corresponds with the sectoral location of maximum ESF and minimum pNC-CT in non-highly myopic eyes. Longitudinal studies to characterize the development and clinical implications of ENC Hi-ESF regions in non-highly myopic and highly myopic eyes are indicated.

Comparison of the Biomechanics of the Mouse Astrocytic Lamina Cribrosa Between Glaucoma and Optic Nerve Crush Models

Purpose

The strain response of the mouse astrocytic lamina (AL) to an ex vivo mechanical test was compared between two protocols: eyes that underwent sustained intraocular pressure (IOP) increase and eyes after optic nerve crush.

Methods

Chronic IOP elevation was induced by microbead injection or the optic nerve was crushed in mice with widespread green fluorescence. After 3 days or 6 weeks, eyes were inflation tested by a published method of two-photon fluorescence to image the AL. Digital volume correlation was used to calculate strains. Optic nerve axon damage was also evaluated.

Results

In the central AL but not the peripheral AL, four strains were greater in eyes at the 3-day glaucoma time point than control (P from 0.029 to 0.049, n = 8 eyes per group). Also, at this time point, five strains were greater in the central AL compared to the peripheral AL (P from 0.041 to 0.00003). At the 6-week glaucoma time point, the strains averaged across the specimen, in the central AL, and the peripheral AL were indistinguishable from the respective controls. Strains were not significantly different between controls and eyes 3 days or 6 weeks after crush (n = 8 and 16).

Conclusions

We found alterations in the ex vivo mechanical behavior in eyes from mice with experimental glaucoma but not in those with crushed optic nerves. The results of this study demonstrate that significant axon injury does not directly affect mechanical behavior of the astrocytic lamina.

Rare intercellular material transfer as a confound to interpreting inner retinal neuronal transplantation following internal limiting membrane disruption

Intercellular cytoplasmic material transfer (MT) occurs between transplanted and developing photoreceptors and ambiguates cell origin identification in developmental, transdifferentiation, and transplantation experiments. Whether MT is a photoreceptor-specific phenomenon is unclear. Retinal ganglion cell (RGC) replacement, through transdifferentiation or transplantation, holds potential for restoring vision in optic neuropathies. During careful assessment for MT following human stem cell-derived RGC transplantation into mice, we identified RGC xenografts occasionally giving rise to labeling of donor-derived cytoplasmic, nuclear, and mitochondrial proteins within recipient Müller glia. Critically, nuclear organization is distinct between human and murine retinal neurons, which enables unequivocal discrimination of donor from host cells. MT was greatly facilitated by internal limiting membrane disruption, which also augments retinal engraftment following transplantation. Our findings demonstrate that retinal MT is not unique to photoreceptors and challenge the isolated use of species-specific immunofluorescent markers for xenotransplant identification. Assessment for MT is critical when analyzing neuronal replacement interventions.

The Curvature, Collagen Network Structure, and Their Relationship to the Pressure-Induced Strain Response of the Human Lamina Cribrosa in Normal and Glaucoma Eyes

The lamina cribrosa (LC) is a connective tissue in the optic nerve head (ONH). The objective of this study was to measure the curvature and collagen microstructure of the human LC, compare the effects of glaucoma and glaucoma optic nerve damage, and investigate the relationship between the structure and pressure-induced strain response of the LC in glaucoma eyes. Previously, the posterior scleral cups of 10 normal eyes and 16 diagnosed glaucoma eyes were subjected to inflation testing with second harmonic generation (SHG) imaging of the LC and digital volume correlation (DVC) to calculate the strain field. In this study, we applied a custom microstructural analysis algorithm to the maximum intensity projection of SHG images to measure features of the LC beam and pore network. We also estimated the LC curvatures from the anterior surface of the DVC-correlated LC volume. Results showed that the LC in glaucoma eyes had larger curvatures p≤0.03), a smaller average pore area (p = 0.001), greater beam tortuosity (p < 0.0001), and more isotropic beam structure (p = 0.01) than in normal eyes. The difference measured between glaucoma and normal eyes may indicate remodeling of the LC with glaucoma or baseline differences that contribute to the development of glaucomatous axonal damage.

Computational study of the mechanical behavior of the astrocyte network and axonal compartments in the mouse optic nerve head

Glaucoma is a blinding disease characterized by the degeneration of the retinal ganglion cell (RGC) axons at the optic nerve head (ONH). A major risk factor for glaucoma is the intraocular pressure (IOP). However, it is currently impossible to measure the IOP-induced mechanical response of the axons of the ONH. The objective of this study was to develop a computational modeling method to estimate the IOP-induced strains and stresses in the axonal compartments in the mouse astrocytic lamina (AL) of the ONH, and to investigate the effect of the structural features on the mechanical behavior. We developed experimentally informed finite element (FE) models of six mouse ALs to investigate the effect of structure on the strain responses of the astrocyte network and axonal compartments to pressure elevation. The specimen-specific geometries of the FE models were reconstructed from confocal fluorescent images of cryosections of the mouse AL acquired in a previous study that measured the structural features of the astrocytic processes and axonal compartments. The displacement fields obtained from digital volume correlation in prior inflation tests of the mouse AL were used to determine the displacement boundary conditions of the FE models. We then applied Gaussian process regression to analyze the effects of the structural features on the strain outcomes simulated for the axonal compartments. The axonal compartments experienced, on average, 6 times higher maximum principal strain but 1800 times lower maximum principal stress compared to those experienced by the astrocyte processes. The strains experienced by the axonal compartments were most sensitive to variations in the area of the axonal compartments. Larger axonal compartments that were more vertically aligned, closer to the AL center, and with lower local actin area fraction had higher strains. Understanding the factors affecting the deformation in the axonal compartments will provide insights into mechanisms of glaucomatous axonal damage.

Regional Gene Expression in the Retina, Optic Nerve Head, and Optic Nerve of Mice with Optic Nerve Crush and Experimental Glaucoma

A major risk factor for glaucomatous optic neuropathy is the level of intraocular pressure (IOP), which can lead to retinal ganglion cell axon injury and cell death. The optic nerve has a rostral unmyelinated portion at the optic nerve head followed by a caudal myelinated region. The unmyelinated region is differentially susceptible to IOP-induced damage in rodent models and human glaucoma. While several studies have analyzed gene expression changes in the mouse optic nerve following optic nerve injury, few were designed to consider the regional gene expression differences that exist between these distinct areas. We performed bulk RNA-sequencing on the retina and separately micro-dissected unmyelinated and myelinated optic nerve regions from naïve C57BL/6 mice, mice after optic nerve crush, and mice with microbead-induced experimental glaucoma (total = 36). Gene expression patterns in the naïve unmyelinated optic nerve showed significant enrichment of the Wnt, Hippo, PI3K-Akt, and transforming growth factor β pathways, as well as extracellular matrix-receptor and cell membrane signaling pathways, compared to the myelinated optic nerve and retina. Gene expression changes induced by both injuries were more extensive in the myelinated optic nerve than the unmyelinated region, and greater after nerve crush than glaucoma. Changes present three and fourteen days after injury largely subsided by six weeks. Gene markers of reactive astrocytes did not consistently differ between injury states. Overall, the transcriptomic phenotype of the mouse unmyelinated optic nerve was significantly different from immediately adjacent tissues, likely dominated by expression in astrocytes, whose junctional complexes are inherently important in responding to IOP elevation.

A noninvasive clinical method to measure in vivo mechanical strains of the lamina cribrosa by optical coherence tomography

Objective

To measure mechanical strain of the lamina cribrosa (LC) after intraocular pressure (IOP) change produced one week after a change in glaucoma medication.

Design

Cohort study.

Participants

Adult glaucoma patients (23 eyes, 15 patients) prescribed a change in IOP-lowering medication.

Intervention

Non-invasive optical coherence tomography (OCT) imaging of the eye.

Main Outcomes

Deformation calculated by digital volume correlation of OCT scans of the LC before and after IOP lowering by medication.

Results

Among 23 eyes, 17 eyes of 12 persons had IOP lowering ≥ 3 mmHg (reduced IOP group) with tensile anterior-posterior E zz strain = 1.0% ± 1.1% (p = 0.003) and compressive radial strain ( E rr ) = -0.3% ± 0.5% (p=0.012; random effects models accounting inclusion of both eyes in some persons). Maximum in-plane principal (tensile) strain and maximum shear strain in the reduced IOP group were: E max = 1.7% ± 1.0% and Γ max = 1.4% ± 0.7%, respectively (both p<0.0001 versus zero). Reduced IOP group strains E max and Γ max were significantly larger with greater %IOP decrease (<0.0001, <0.0001). The compliance of the E zz , E max , and Γ max strain response, defined as strain normalized by the IOP decrease, were larger with more abnormal perimetric mean deviation or visual field index values (all p≥0.02). Strains were unrelated to age (all p≥0.088). In reduced IOP eyes, mean LC anterior border posterior movement was only 2.05 μm posteriorly (p=0.052) and not related to % IOP change (p=0.94, random effects models). Only E rr was significantly related to ALD change, becoming more negative with greater posterior LC border change (p=0.015).

Conclusion

LC mechanical strains can be effectively measured by changes in eye drop medication using OCT and are related to degree of visual function loss in glaucoma.

Trial Registration

ClinicalTrials.gov Identifier: NCT03267849.

Regional Gene Expression in the Retina, Optic Nerve Head, and Optic Nerve of Mice with Experimental Glaucoma and Optic Nerve Crush

A major risk factor for glaucomatous optic neuropathy is the level of intraocular pressure (IOP), which can lead to retinal ganglion cell axon injury and cell death. The optic nerve has a rostral unmyelinated portion at the optic nerve head followed by a caudal myelinated region. The unmyelinated region is differentially susceptible to IOP-induced damage in rodent models and in human glaucoma. While several studies have analyzed gene expression changes in the mouse optic nerve following optic nerve injury, few were designed to consider the regional gene expression differences that exist between these distinct areas. We performed bulk RNA-sequencing on the retina and on separately micro-dissected unmyelinated and myelinated optic nerve regions from naïve C57BL/6 mice, mice after optic nerve crush, and mice with microbead-induced experimental glaucoma (total = 36). Gene expression patterns in the naïve unmyelinated optic nerve showed significant enrichment of the Wnt, Hippo, PI3K-Akt, and transforming growth factor β pathways, as well as extracellular matrix-receptor and cell membrane signaling pathways, compared to the myelinated optic nerve and retina. Gene expression changes induced by both injuries were more extensive in the myelinated optic nerve than the unmyelinated region, and greater after nerve crush than glaucoma. Changes three and fourteen days after injury largely subsided by six weeks. Gene markers of reactive astrocytes did not consistently differ between injury states. Overall, the transcriptomic phenotype of the mouse unmyelinated optic nerve was significantly different from immediately adjacent tissues, likely dominated by expression in astrocytes, whose junctional complexes are inherently important in responding to IOP elevation.

The Strain Response to Intraocular Pressure Decrease in the Lamina Cribrosa of Patients with Glaucoma

OBJECTIVE

To measure biomechanical strains in the lamina cribrosa (LC) of living human eyes with intraocular pressure (IOP) lowering.

DESIGN

Cohort study.

PARTICIPANTS

Patients with glaucoma underwent imaging before and after laser suturelysis after trabeculectomy surgery (29 image pairs; 26 persons).

INTERVENTION

Noninvasive imaging of the eye.

MAIN OUTCOME MEASURES

Strains in optic nerve head tissue and changes in depths of the anterior border of the LC.

RESULTS

Intraocular pressure decreases caused the LC to expand in thickness in the anterior-posterior strain (Ezz = 0.94 ± 1.2%; P = 0.00020) and contract in radius in the radial strain (Err = - 0.19 ± 0.33%; P = 0.0043). The mean LC depth did not significantly change with IOP lowering (1.33 ± 6.26 μm; P = 0.26). A larger IOP decrease produced a larger, more tensile Ezz (P < 0.0001), greater maximum principal strain (Emax; P < 0.0001), and greater maximum shear strain (Γmax; P < 0.0001). The average LC depth change was associated with the Γmax and radial-circumferential shear strain (Erθ; P < 0.02) but was not significantly related to tensile or compressive strains. An analysis by clock hour showed that in temporal clock hours 3 to 6, a more anterior LC movement was associated with a more positive Emax, and in clock hours 3, 5, and 6, it was associated with a more positive Γmax. At 10 o'clock, a more posterior LC movement was related to a more positive Emax (P < 0.004). Greater compliance (strain/ΔIOP) of Emax (P = 0.044), Γmax (P = 0.052), and Erθ (P = 0.018) was associated with a thinner retinal nerve fiber layer. Greater compliance of Emax (P = 0.041), Γmax (P = 0.021), Erθ (P = 0.024), and in-plane shear strain (Erz; P = 0.0069) was associated with more negative mean deviations. Greater compliance of Γmax (P = 0.055), Erθ (P = 0.040), and Erz (P = 0.015) was associated with lower visual field indices.

CONCLUSIONS

With IOP lowering, the LC moves either into or out of the eye but, on average, expands in thickness and contracts in radius. Shear strains are nearly as substantial as in-plane strains. Biomechanical strains are more compliant in eyes with greater glaucoma damage. This work was registered at ClinicalTrials.gov as NCT03267849.

Quantitative Microstructural Analysis of Cellular and Tissue Remodeling in Human Glaucoma Optic Nerve Head

Purpose

To measure quantitatively changes in lamina cribrosa (LC) cell and connective tissue structure in human glaucoma eyes.

Methods

We studied 27 glaucoma and 19 age-matched non-glaucoma postmortem eyes. In 25 eyes, LC cross-sections were examined by confocal and multiphoton microscopy to quantify structures identified by anti-glial fibrillary acidic protein (GFAP), phalloidin-labeled F-actin, nuclear 4',6-diamidino-2-phenylindole (DAPI), and by second harmonic generation imaging of LC beams. Additional light and transmission electron microscopy were performed in 21 eyes to confirm features of LC remodeling, including immunolabeling by anti-SOX9 and anti-collagen IV. All glaucoma eyes had detailed clinical histories of open-angle glaucoma status, and degree of axon loss was quantified in retrolaminar optic nerve cross-sections.

Results

Within LC pores, the proportionate area of both GFAP and F-actin processes was significantly lower in glaucoma eyes than in controls (P = 0.01). Nuclei were rounder (lower median aspect ratio) in glaucoma specimens (P = 0.02). In models assessing degree of glaucoma damage, F-actin process width was significantly wider in glaucoma eyes with more damage (P = 0.024), average LC beam width decreased with worse glaucoma damage (P = 0.042), and nuclear count per square millimeter rose with worse damage (P = 0.019). The greater cell count in LC pores represented 92.3% astrocytes by SOX9 labeling. The results are consistent with replacement of axons in LC pores by basement membrane labeled by anti-collagen IV and in-migrating astrocytes.

Conclusions

Alteration in LC structure in glaucoma involves migration of astrocytes into axonal bundles, change in astrocyte orientation and processes, production of basement membrane material, and thinning of connective tissue beams.

Aquaporin 4 is not present in normal porcine and human lamina cribrosa

Aquaporin 4 is absent from astrocytes in the rodent optic nerve head, despite high expression in the retina and myelinated optic nerve. The purpose of this study was to quantify regional aquaporin channel expression in astrocytes of the porcine and human mouse optic nerve (ON). Ocular tissue sections were immunolabeled for aquaporins 1(AQP1), 4(AQP4), and 9(AQP9), myelin basic protein (MBP), glial fibrillary acidic protein (GFAP) and alpha-dystroglycan (αDG) for their presence in retina, lamina, myelin transition zone (MTZ, region just posterior to lamina) and myelinated ON (MON). Semi- quantification of AQP4 labeling & real-time quantitative PCR (qPCR) data were analyzed in retina and ON tissue. Porcine and control human eyes had abundant AQP4 in Müller cells, retinal astrocytes, and myelinated ON (MON), but minimal expression in the lamina cribrosa. AQP1 and AQP9 were present in retina, but not in the lamina. Immunolabeling of GFAP and αDG was similar in lamina, myelin transition zone (MTZ) and MON regions. Semi-quantitative AQP4 labeling was at background level in lamina, increasing in the MTZ, and highest in the MON (lamina vs MTZ, MON; p≤0.05, p≤0.01, respectively). Expression of AQP4 mRNA was minimal in lamina and substantial in MTZ and MON, while GFAP mRNA expression was uniform among the lamina, MTZ, and MON regions. Western blot assay showed AQP4 protein expression in the MON samples, but none was detected in the lamina tissue. The minimal presence of AQP4 in the lamina is a specific regional phenotype of astrocytes in the mammalian optic nerve head.

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

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

Electronically Monitored Corticosteroid Eye Drop Adherence after Trabeculectomy Compared to Surgical Success

OBJECTIVE

To compare electronically measured adherence with topical corticosteroid drops to outcomes of glaucoma surgery.

DESIGN

This prospective cohort study included eyes undergoing surgery from August 2019 to January 2021 and followed for up to 1 year.

PARTICIPANTS

All patients were recruited from the Glaucoma Center of Excellence at the Wilmer Eye Institute, Johns Hopkins. Eligible patients had either primary open angle or angle closure glaucoma, were ≥18 years old, and underwent either trabeculectomy (with or without cataract surgery) or tube-shunt implantation.

INTERVENTION

Patients were instructed on use of an eyedrop monitoring device (Kali Drop®), with knowledge that it would be used to record postoperative corticosteroid instillation in real time.

MAIN OUTCOMES

Adherence to a regimen of corticosteroid eye drops during the first 5 postoperative weeks; achievement of target intraocular pressure (IOP) at 6 weeks, 6 months to 1 year after surgery; and bleb morphology at 1 year.

RESULTS

Among 90 patients, adherence was 90 ± 14% overall and 81 ± 16% during q2h dosing. Target IOP was achieved at final visit (6 months or 1 year) in 81% (59/73) without reoperation. Eyes with a higher ratio of drops taken vs prescribed were significantly more likely to achieve target IOP at 6 months/1 year (p=0.05). Total adherence was better in younger persons, eyes with less field loss, and patients of one particular surgeon (p<0.03). Percent adherence during every 2 hours (q2h) dosing was higher in eyes with higher target IOP (p=0.01). No adherence outcome was significantly related to race, sex, bleb morphology, postoperative pain, or post-operative anterior chamber inflammation. Adherence values did not significantly correlate with adherence questionnaire data (predicted mean = 70 ± 17%, actual mean = 91 ± 13% adherent, p < 0.001).

CONCLUSION

Adherence with frequent postoperative eye drops was high and can be successfully monitored remotely. Surgical success was greater among eyes with nearly ideal adherence and was poorer in older persons and those with more advanced glaucoma.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT03402802.

Improving the Power of Glaucoma Neuroprotection Trials Using Existing Visual Field Data

PURPOSE

Selecting reliable visual field (VF) test takers could improve the power of randomized clinical trials in glaucoma. We test this hypothesis via simulations using a large real world data set.

DESIGN

Methodology analysis: assessment of how improving reliability affects sample size estimates.

METHODS

A variability index (VI) estimating intertest variability was calculated for each subject using the residuals of the regression of the mean deviation over time for the first 6 tests in a series of at least 10 examinations for 2,804 patients. Using data from the rest of the series, we simulate VFs at regular intervals for 2 years. To simulate the neuroprotective effect (NE), we reduced the observed progression rate by 20%, 30%, or 50%. The main outcome measure was the sample size to detect a significant difference (P < .05) at 80% power.

RESULTS

In the first experiment, we simulated a trial including one eye per subject, either selecting randomly from the database or prioritizing patients with low VI. We could not reach 80% power for the low NE with the available patients, but the sample size was reduced by 38% and 49% for the 30% and 50% NE, respectively. In the second experiment, we simulated 2 eyes per subject, one of which was the control eye. The sample size (smaller overall) was reduced by 26% and 38% for the 30% and 50% NE by prioritizing patients with low VI.

CONCLUSIONS

Selecting patients with low intertest variability can significantly improve the power and reduce the sample size needed in a trial.

Assessment of Cumulative Incidence and Severity of Primary Open-Angle Glaucoma Among Participants in the Ocular Hypertension Treatment Study After 20 Years of Follow-up

Importance

Ocular hypertension is an important risk factor for the development of primary open-angle glaucoma (POAG). Data from long-term follow-up can be used to inform the management of patients with ocular hypertension.

Objective

To determine the cumulative incidence and severity of POAG after 20 years of follow-up among participants in the Ocular Hypertension Treatment Study.

Design, Setting, and Participants

Participants in the Ocular Hypertension Treatment Study were followed up from February 1994 to December 2008 in 22 clinics. Data were collected after 20 years of follow-up (from January 2016 to April 2019) or within 2 years of death. Analyses were performed from July 2019 to December 2020.

Interventions

From February 28, 1994, to June 2, 2002 (phase 1), participants were randomized to receive either topical ocular hypotensive medication (medication group) or close observation (observation group). From June 3, 2002, to December 30, 2008 (phase 2), both randomization groups received medication. Beginning in 2009, treatment was no longer determined by study protocol. From January 7, 2016, to April 15, 2019 (phase 3), participants received ophthalmic examinations and visual function assessments.

Main Outcomes and Measures

Twenty-year cumulative incidence and severity of POAG in 1 or both eyes after adjustment for exposure time.

Results

A total of 1636 individuals (mean [SD] age, 55.4 [9.6] years; 931 women [56.9%]; 1138 White participants [69.6%]; 407 Black/African American participants [24.9%]) were randomized in phase 1 of the clinical trial. Of those, 483 participants (29.5%) developed POAG in 1 or both eyes (unadjusted incidence). After adjusting for exposure time, the 20-year cumulative incidence of POAG in 1 or both eyes was 45.6% (95% CI, 42.3%-48.8%) among all participants, 49.3% (95% CI, 44.5%-53.8%) among participants in the observation group, and 41.9% (95% CI, 37.2%-46.3%) among participants in the medication group. The 20-year cumulative incidence of POAG was 55.2% (95% CI, 47.9%-61.5%) among Black/African American participants and 42.7% (95% CI, 38.9%-46.3%) among participants of other races. The 20-year cumulative incidence for visual field loss was 25.2% (95% CI, 22.5%-27.8%). Using a 5-factor baseline model, the cumulative incidence of POAG among participants in the low-, medium-, and high-risk tertiles was 31.7% (95% CI, 26.4%-36.6%), 47.6% (95% CI, 41.6%-53.0%), and 59.8% (95% CI, 53.1%-65.5%), respectively.

Conclusions and Relevance

In this study, only one-fourth of participants in the Ocular Hypertension Treatment Study developed visual field loss in either eye over long-term follow-up. This information, together with a prediction model, may help clinicians and patients make informed personalized decisions about the management of ocular hypertension.

Trial Registration

ClinicalTrials.gov Identifier: NCT00000125.

A novel ocular function for decorin in the aqueous humor outflow

Primary open-angle glaucoma, a neurodegenerative disorder characterized by degeneration of optic nerve axons, is a frequent cause of vision loss and blindness worldwide. Several randomized multicenter studies have identified intraocular pressure as the major risk factor for its development, caused by an increased outflow resistance to the aqueous humor within the trabecular meshwork. However, the molecular mechanism for increased outflow resistance in POAG has not been fully established. One of the proposed players is the pro-fibrotic transforming growth factor (TGF)-β2, which is found in higher amounts in the aqueous humor of patients with POAG. In this study we elucidated the role of decorin, a small leucine-rich proteoglycan and known antagonist of TGF-β, in the region of aqueous humor outflow tissue. Utilizing decorin deficient mice, we discovered that decorin modulated TGF-β signaling in the canonical outflow pathways and the lack of decorin in vivo caused an increase in intraocular pressure. Additionally, the Dcn^-/- mice showed significant loss of optic nerve axons and morphological changes in the glial lamina, typical features of glaucoma. Moreover, using human trabecular meshwork cells we discovered that soluble decorin attenuated TGF-β2 mediated synthesis and expression of typical downstream target genes including CCN2/CTGF, FN and COL IV.  Finally, we found a negative reciprocal regulation of decorin and TGF-β, with a dramatic downregulation of decorin in the canonical outflow pathways of patients with primary open-angle glaucoma. Collectively, our results indicate that decorin plays an important role in the pathogenesis of primary open-angle glaucoma and offers novel perspectives in the treatment of this serious disease.

Regional Differences and Physiologic Behaviors in Peripapillary Scleral Fibroblasts

Purpose

The purpose of this study was to describe the cellular architecture of normal human peripapillary sclera (PPS) and evaluate surface topography's role in fibroblast behavior.

Methods

PPS cryosections from nonglaucomatous eyes were labelled for nuclei, fibrillar actin (FA), and alpha smooth muscle actin (αSMA) and imaged. Collagen fibrils were imaged using second harmonic generation. Nuclear density and aspect ratio of the internal PPS (iPPS), outer PPS (oPPS), and peripheral sclera were determined. FA and αSMA fibril alignment with collagen extracellular matrix (ECM) was determined. PPS fibroblasts were cultured on smooth or patterned membranes under mechanical strain and in the presence of TGFβ1 and 2.

Results

The iPPS (7.1 ± 2.0 × 10⁻⁴, P < 0.0001) and oPPS (5.3 ± 1.4 × 10⁻⁴, P = 0.0013) had greater nuclei density (nuclei/µm²) than peripheral sclera (2.5 ± 0.8 × 10⁻⁴). The iPPS (2.0 ± 0.3, P = 0.002) but not oPPS (2.4 ± 0.4, P = 0.45) nuclei had smaller aspect ratios than peripheral (2.7 ± 0.5) nuclei. FA was present throughout the scleral stroma and was more aligned with oPPS collagen (9.6 ± 1.9 degrees) than in the peripheral sclera (15.9 ± 3.9 degrees, P =0.002). The αSMA fibers in the peripheral sclera were less aligned with collagen fibrils (26.4 ± 4.8 degrees) than were FA (15.9 ± 3.9 degrees, P = 0.0002). PPS fibroblasts cultured on smooth membranes shifted to an orientation perpendicular to the direction of cyclic uniaxial strain (1 Hz, 5% strain, 42.2 ± 7.1 degrees versus 62.0 ± 8.5 degrees, P < 0.0001), whereas aligned fibroblasts on patterned membranes were resistant to strain-induced reorientation (5.9 ± 1.4 degrees versus 10 ± 3.3 degrees, P = 0.21). Resistance to re-orientation was reduced by TGFβ treatment (10 ± 3.3 degrees without TGFβ1 compared to 23.1 ± 4.5 degrees with TGFβ1, P < 0.0001).

Conclusions

Regions of the posterior sclera differ in cellular density and nuclear morphology. Topography alters the cellular response to mechanical strain.

Biomechanics of the optic nerve head and peripapillary sclera in a mouse model of glaucoma

The deformation of the mouse astrocytic lamina (AL) and adjacent peripapillary sclera (PPS) was measured in response to elevated intraocular pressure. We subjected explanted mouse eyes to inflation testing, comparing control eyes to those 3 days and 6 weeks after induction of ocular hypertension (OHT) via ocular microbead injection. Laser scanning microscopy was used with second harmonic generation to image the collagenous PPS and two-photon fluorescence to image transgenic fluorescent astrocytes in the AL. Digital volume correlation was applied to calculate strains in the PPS and AL. The specimen-averaged strains were biaxial in the AL and PPS, with greater strain overall in the x- than y-direction in the AL and greater strain in the θ- than the r-direction in the PPS. Strains increased after 3-day OHT, with greater strain overall in the 3-day AL than control AL, and greater circumferential strain in the 3-day PPS than control PPS. In the 6-week OHT eyes, AL and PPS strains were similar overall to controls. This experimental glaucoma model demonstrated a dynamic change in the mechanical behaviour of the AL and PPS over time at the site of neuronal injury and remodelling in glaucoma.

Pressure-Induced Changes in Astrocyte GFAP, Actin, and Nuclear Morphology in Mouse Optic Nerve

Purpose

To conduct quantitative analysis of astrocytic glial fibrillary acidic protein (GFAP), actin and nuclei distribution in mouse optic nerve (ON) and investigate changes in the measured features after 3 days of ocular hypertension (OHT).

Method

Serial cross-sections of 3-day microbead-induced OHT and control ONs were fluorescently labelled and imaged using confocal microscope. Eighteen structural features were measured from the acquired images, including GFAP coverage, actin area fraction, process thickness, and aspect ratio of cell nucleus. The measured features were analyzed for variations with axial locations along ON and radial zones transverse to ON, as well as for the correlations with degree of intraocular pressure (IOP) change.

Results

The most significant changes in structural features after 3-day OHT occurred in the unmyelinated ON region (R1), and the changes were greater with greater IOP elevation. Although the GFAP, actin, axonal, and ON areas all increased in 3-day OHT ONs in R1 (P ≤ 0.004 for all), the area fraction of GFAP actually decreased (P = 0.02), the actin area fraction was stable and individual axon compartments were unchanged in size. Within R1, the number of nuclear clusters increased (P < 0.001), but the mean size of nuclear clusters was smaller (P = 0.02) and the clusters became rounder (P < 0.001). In all cross-sections of control ONs, astrocytic processes were thickest in the rim zone compared with the central and peripheral zones (P ≤ 0.002 for both), whereas the overall process width in R1 decreased after 3 days of OHT (P < 0.001).

Conclusions

The changes in structure elucidated IOP-generated alterations that underlie astrocyte mechanotranslational responses relevant to glaucoma.

Peripapillary Scleral Bowing Increases with Age and Is Inversely Associated with Peripapillary Choroidal Thickness in Healthy Eyes

PURPOSE

To use optical coherence tomography (OCT) to 3-dimensionally characterize the optic nerve head (ONH) in peripapillary scleral bowing in non-highly myopic healthy eyes.

DESIGN

Cross-sectional, multicenter study.

METHODS

A total of 362 non-highly myopic (+6 diopters [D] > spherical equivalent > -6D) eyes of 362 healthy subjects from 20-90 years old underwent OCT ONH radial B-scan imaging. Bruch's membrane (BM), BM opening (BMO), anterior scleral canal opening (ASCO), and the peripapillary scleral surface were segmented. BMO and ASCO planes were fit, and their centroids, major axes, ovality, areas and offsets were determined. Peripapillary scleral bowing was characterized by 2 parameters: peripapillary scleral slope (ppSS) of 3 anterior peripapillary scleral segments (0-300, 300-700, and 700-1,000 μm from the ASCO centroid); and ASCO depth relative to a peripapillary scleral reference plane (ASCOD-ppScleral). Peripapillary choroidal thickness (ppCT) was calculated relative to the ASCO as the minimum distance between the anterior scleral surface and BM.

RESULTS

Both ppSS and ASCOD-ppScleral ranged from slightly inward through profoundly outward in direction. Both parameters increased with age and were independently associated with decreased ppCT.

CONCLUSIONS

In non-highly myopic healthy eyes, outward peripapillary scleral bowing achieved substantial levels, was markedly increased with age, and was independently associated with decreased peripapillary choroidal thickness. These findings provide a normative foundation for characterizing this anatomy in cases of high myopia and glaucoma and in eyes with optic disc tilt, torsion, and peripapillary atrophy.

The Effects of Glaucoma on the Pressure-Induced Strain Response of the Human Lamina Cribrosa

Purpose

To measure the ex vivo pressure-induced strain response of the human optic nerve head and analyze for variations with glaucoma diagnosis and optic nerve axon damage.

Methods

The posterior sclera of 16 eyes from 8 diagnosed glaucoma donors and 10 eyes from 6 donors with no history of glaucoma were inflation tested between 5 and 45 mm Hg. The optic nerve from each donor was examined for degree of axon loss. The posterior volume of the lamina cribrosa (LC) was imaged with second harmonic generation and analyzed using volume correlation to calculate LC strains between 5 and 10 and 5 and 45 mm Hg.

Results

Eye length and LC area were larger in eyes diagnosed with glaucoma (P≤ 0.03). Nasal-temporal E_XX and circumferential E_θθ strains were lower in the LC of diagnosed glaucoma eyes at 10 mm Hg (P≤ 0.05) and 45 mm Hg (P≤ 0.07). E_XX was smaller in the LC of glaucoma eyes with <25% axon loss compared with undamaged normal eyes (P = 0.01, 45 mm Hg). In general, the strains were larger in the peripheral than central LC. The ratio of the maximum principal strain E_max in the peripheral to central LC was larger in glaucoma eyes with >25% axon loss than in glaucoma eyes with milder damage (P = 0.004, 10 mm Hg).

Conclusions

The stiffness of the LC pressure-strain response was greater in diagnosed glaucoma eyes and varied with glaucomatous axon damage. Lower LC strains in glaucoma eyes with milder damage may represent baseline biomechanical behavior that contributes to axon loss, whereas greater LC strain and altered radial LC strain variation in glaucoma eyes with more severe damage may be caused by glaucoma-related remodeling.

The inflation response of the human lamina cribrosa and sclera: Analysis of deformation and interaction

This study investigated the inflation response of the lamina cribrosa (LC) and adjacent peripapillary sclera (PPS) in post-mortem human eyes with no history of glaucoma. The posterior sclera of 13 human eyes from 7 donors was subjected to controlled pressurization between 5-45 mmHg. A laser-scanning microscope (LSM) was used to image the second harmonic generation (SHG) response of collagen and the two-photon fluorescent (TPF) response of elastin within the volume of the LC and PPS at each pressure. Image volumes were analyzed using digital volume correlation (DVC) to calculate the three-dimensional (3D) deformation field between pressures. The LC exhibited larger radial strain, Err, and maximum principal strain, Emax, (p < 0.0001) and greater posterior displacement (p=0.0007) compared to the PPS between 5-45 mmHg, but had similar average circumferential strain, Eθθ, and maximum shear strain, Γmax. The Emax and Γmax were highest near the LC-PPS interface and lowest in the nasal quadrant of both tissues. Larger LC area was associated with smaller Emax in the peripheral LC and larger Emax in the central LC (p ≤ 0.01). The Emax, Γmax, and Eθθ in the inner PPS increased with increasing strain in adjacent LC regions (p ≤ 0.001). Smaller strains in the PPS were associated with a larger difference in the posterior displacement between the PPS and central LC (p < 0.0001 for Emax and Err), indicating that a stiffer pressure-strain response of the PPS is associated with greater posterior bowing of the LC. STATEMENT OF SIGNIFICANCE: Glaucoma causes vision loss through progressive damage of the retinal ganglion axons at the lamina cribrosa (LC), a connective tissue structure that supports the axons as they pass through the eye wall. It is hypothesized that strains caused by intraocular pressure may initiate this damage and that these strains are modulated by the combined deformation of the LC and adjacent peripapillary sclera (PPS). In this study we present a method to measure the pressure-induced 3D displacement and strain field in the LC and PPS simultaneously. Regional strain variation in the LC and PPS was investigated and compared and strains were analyzed for associations with age, LC area, LC strain magnitude, and LC posterior motion relative to the PPS.

Characterizing the Collagen Network Structure and Pressure-Induced Strains of the Human Lamina Cribrosa

Purpose

The purpose of this study was to measure the 2D collagen network structure of the human lamina cribrosa (LC), analyze for the correlations with age, region, and LC size, as well as the correlations with pressure-induced strains.

Methods

The posterior scleral cups of 10 enucleated human eyes with no known ocular disease were subjected to ex vivo inflation testing from 5 to 45 mm Hg. The optic nerve head was imaged by using second harmonic generation imaging (SHG) to identify the LC collagen structure at both pressures. Displacements and strains were calculated by using digital volume correlation of the SHG volumes. Nine structural features were measured by using a custom Matlab image analysis program, including the pore area fraction, node density, and beam connectivity, tortuosity, and anisotropy.

Results

All strain measures increased significantly with higher pore area fraction, and all but the radial-circumferential shear strain (E_rθ) decreased with higher node density. The maximum principal strain (E_max) and maximum shear strain (Γ_max) also increased with larger beam aspect ratio and tortuosity, respectively, and decreased with higher connectivity. The peripheral regions had lower node density and connectivity, and higher pore area fraction, tortuosity, and strains (except for E_rθ) than the central regions. The peripheral nasal region had the lowest E_max, Γ_max, radial strain, and pore area fraction.

Conclusions

Features of LC beam network microstructure that are indicative of greater collagen density and connectivity are associated with lower pressure-induced LC strain, potentially contributing to resistance to glaucomatous damage.

OCT-Detected Optic Nerve Head Neural Canal Direction, Obliqueness, and Minimum Cross-Sectional Area in Healthy Eyes

PURPOSE

To assess anterior scleral canal opening (ASCO) offset relative to Bruch's membrane opening (BMO) (ASCO/BMO offset) so as to determine neural canal direction, obliqueness, and minimum cross-sectional area (NCMCA) in 362 healthy eyes.

DESIGN

Cross-sectional study.

METHODS

After optical coherence tomography optic nerve head and retinal nerve fiber layer thickness (RNFLT) imaging, BMO and ASCO were manually segmented. Planes, centroids, size, and shape were calculated. Neural canal direction was defined by projecting the neural canal axis vector (connecting BMO and ASCO centroids) onto the BMO plane. Neural canal obliqueness was defined by the angle between the neural canal axis and the BMO plane perpendicular vector. NCMCA was defined by projecting BMO and ASCO points onto a neural canal axis perpendicular plane and measuring the area of overlap. The angular distance between superior and inferior peak RNFLT was measured, and correlations between RFNLT, BMO, ASCO, ASCO/BMO offset, and NCMCA were assessed.

RESULTS

Mean (SD) NCMCA was significantly smaller than either the BMO or ASCO area (1.33 (0.42), 1.82 (0.38), 2.22 (0.43) mm², respectively), and most closely correlated to RNFLT (P < .001, R² = 0.158). Neural canal direction was most commonly superior-nasal (55%). Mean neural canal obliqueness was 39.4° (17.3°). The angular distance between superior and inferior peak RNFLT correlated to neural canal direction (P ≤ .008, R² = 0.093).

CONCLUSIONS

ASCO/BMO offset underlies neural canal direction, obliqueness, and NCMCA. RNFLT is more strongly correlated to NCMCA than to BMO or ASCO, and its peripapillary distribution is influenced by neural canal direction.

In vivo characterization of the deformation of the human optic nerve head using optical coherence tomography and digital volume correlation

We developed a method to measure the 3-dimensional (3D) strain field in the optic nerve head (ONH) in vivo between two intraocular pressures (IOP). Radial optical coherence tomography (OCT) scans were taken of the ONH of 5 eyes from 5 glaucoma patients before and after IOP-lowering surgery and from 5 eyes from 3 glaucoma suspect patients before and after raising IOP by wearing tight-fitting swimming goggles. Scans taken at higher and lower IOP were compared using a custom digital volume correlation (DVC) algorithm to calculate strains in the anterior lamina cribrosa (ALC), retina, and choroid. Changes in anterior lamina depth (ALD) relative to Bruch's membrane were also analyzed. Average displacement error was estimated to be subpixel and strain errors were smaller than 0.37%. Suturelysis decreased IOP by 9-20 mmHg and decreased compressive anterior-posterior strain Ezz in the ALC by 0.76% (p=0.002,n=5). Goggle-wearing increased IOP by 3-4 mmHg and produced compressive Ezz in the ALC (-0.32%,p=0.001,n=5). Greater IOP decrease was associated with greater ALD change (p=0.047,n=10) and greater strains in the ALC (Ezz:p=0.002,n=10). A deepening of ALD was associated with lower IOP and greater ALC strains (p⩽0.045,n=10). A DVC-based method to measure strains from OCT images caused by IOP changes as small as 2.3 mmHg provides preliminary evidence that ALD is shallower and ALC strains are less compressive at higher IOP and that ALD change is associated with ALC strains. STATEMENT OF SIGNIFICANCE: Glaucoma causes vision loss through progressive damage of the retinal ganglion axons at the lamina cribrosa, a connective tissue structure in the optic nerve head that supports the axons as they pass through the eye wall. It is hypothesized that strains caused by intraocular pressure (IOP) may initiate this damage, but few studies have measured the strain response to pressure of the optic nerve head in patients. We present a method to measure the 3D displacement and strain field in the optic nerve head caused by IOP alteration in glaucoma patients using clinically available images. We used this method to measure strain within the optic nerve head from IOP changes caused by glaucoma surgery and wearing tight-fitting swimming goggles.

Factors Influencing Optical Coherence Tomography Peripapillary Choroidal Thickness: A Multicenter Study

Purpose

To quantify peripapillary choroidal thickness (PCT) and the factors that influence it in healthy participants who represent the racial and ethnic composition of the U.S. population.

Methods

A total of 362 healthy participants underwent optical coherence tomography (OCT) enhanced depth imaging of the optic nerve head with a 24 radial B-scan pattern aligned to the fovea to Bruch's membrane opening axis. Bruch's membrane, anterior scleral canal opening (ASCO), and the anterior scleral surface were manually segmented. PCT was measured at 100, 300, 500, 700, 900, and 1100 μm from the ASCO globally and within 12 clock-hour sectors. The effects of age, axial length, intraocular pressure, ethnicity, sex, sector, and ASCO area on PCT were assessed by ANOVA and univariable and multivariable regressions.

Results

Globally, PCT was thicker further from the ASCO border and thinner with older age, longer axial length, larger ASCO area, European descent, and female sex. Among these effectors, age and axial length explained the greatest proportion of variance. The rate of age-related decline increased further from the ASCO border. Sectorally, the inferior-temporal sectors were thinnest (10.7%-20.0% thinner than the thickest sector) and demonstrated a higher rate of age-related loss (from 15.6% to 20.7% faster) at each ASCO distance.

Conclusions

In healthy eyes, PCT was thinnest in the inferior temporal sectors and thinner PCT was associated with older age, European descent, longer axial length, larger ASCO area, and female sex. Among these associations, age had the strongest influence, and its effect was greatest within the inferior temporal sectors.

Quantification of collagen fiber structure using second harmonic generation imaging and two-dimensional discrete Fourier transform analysis: Application to the human optic nerve head

Second harmonic generation (SHG) microscopy is widely used to image collagen fiber microarchitecture due to its high spatial resolution, optical sectioning capabilities and relatively nondestructive sample preparation. Quantification of SHG images requires sensitive methods to capture fiber alignment. This article presents a two-dimensional discrete Fourier transform (DFT)-based method for collagen fiber structure analysis from SHG images. The method includes integrated periodicity plus smooth image decomposition for correction of DFT edge discontinuity artefact, avoiding the loss of peripheral image data encountered with more commonly used windowing methods. Outputted parameters are as follows: the collagen fiber orientation distribution, aligned collagen content and the degree of collagen fiber dispersion along the principal orientation. We demonstrate its application to determine collagen microstructure in the human optic nerve head, showing its capability to accurately capture characteristic structural features including radial fiber alignment in the innermost layers of the bounding sclera and a circumferential collagen ring in the mid-stromal tissue. Higher spatial resolution rendering of individual lamina cribrosa beams within the nerve head is also demonstrated. Validation of the method is provided in the form of correlative results from wide-angle X-ray scattering and application of the presented method to other fibrous tissues.

Evaluation of Automated Segmentation Algorithms for Optic Nerve Head Structures in Optical Coherence Tomography Images

Purpose

To compare the identification of optic nerve head (ONH) structures in optical coherence tomography images by observers and automated algorithms.

Methods

ONH images in 24 radial scan sets by optical coherence tomography were obtained in 51 eyes of 29 glaucoma patients and suspects. Masked intraobserver and interobserver comparisons were made of marked endpoints of Bruch's membrane opening (BMO) and the anterior lamina cribrosa (LC). BMO and LC positional markings were compared between observer and automated algorithm. Repeated analysis on 20 eyes by the algorithm was compared. Regional ONH data were derived from the algorithms.

Results

Intraobserver difference in BMO width was not significantly different from zero (P ≥ 0.32) and the difference in LC position was less than 1% different (P = 0.04). Interobserver were slightly larger than intraobserver differences, but interobserver BMO width difference was 0.36% (P = 0.63). Mean interobserver difference in LC position was 14.74 μm (P = 0.004), 3% of the typical anterior lamina depth (ALD). Between observer and algorithm, BMO width differed by 1.85% (P = 0.23) and mean LC position was not significantly different (3.77 μm, P = 0.77). Repeat algorithmic analysis had a mean difference in BMO area of 0.38% (P = 0.47) and mean ALD difference of 0.54 ± 0.72%. Regional ALD had greater variability in the horizontal ONH regions. Some individual outlier images were not validly marked by either observers or algorithm.

Conclusions

Automated identification of ONH structures is comparable to observer markings for BMO and anterior LC position, making BMO a practical reference plane for algorithmic analysis.

The Contribution of Sulfated Glycosaminoglycans to the Inflation Response of the Human Optic Nerve Head

Purpose

In this study, we measured the effect of the removal of sulfated glycosaminoglycans (sGAGs) on the pressure-induced strains of the human lamina cribrosa (LC).

Methods

We applied an ex vivo inflation method to measure the three-dimensional (3D) deformation response of six human LCs to pressure, before and after the degradation of chondroitin and dermatan sulfates. The experiment used a laser-scanning microscope (LSM) to acquire the second harmonic generation (SHG) signal of the collagen structure in the LC. Digital volume correlation (DVC) was used to calculate the deformation in the LC after a change in pressure from 5 to 45 mm Hg.

Results

The average strains between 5 and 45 mm Hg in the LC decreased significantly after sGAG degradation (P ≤ 0.03), with the greatest change occurring in regions of previously high strain (P ≤ 0.003) and the peripheral regions of the LC (P ≤ 0.02). The stiffening effect was greater in the LC of middle-aged (42–49 years) donors compared with those of older (64–88 years) donors (P < 0.0001).

Conclusions

The LC experienced less strain at the same pressures after most sGAGs were removed. These results suggest that the natural decrease in sGAGs within the LC with age may contribute to the stiffer inflation response of older LC to IOP. Likewise, the increase in the amount of sGAGs observed in the LC of glaucomatous eyes, may contribute to a more compliant LC, which may affect the susceptibility and progression of axon damage.

Factors Influencing Central Lamina Cribrosa Depth: A Multicenter Study

Purpose

To quantify the influence of ocular and demographic factors on central laminar depth (LD) in healthy participants.

Methods

A total of 362 normal subjects underwent optical coherence tomography (OCT) enhanced depth imaging of the optic nerve head (ONH) with a 24 radial B-scan pattern aligned to the fovea–to–Bruch's membrane opening (BMO) axis. BMO, anterior lamina, anterior scleral canal opening (ASCO), Bruch's membrane (BM), and the peripapillary scleral surface were manually segmented. The extent of laminar segmentation was quantified within 72 ASCO subsectors. Central LD was quantified relative to four reference planes: BMO, ASCO, BM, and scleral. The effects of age, sex, ethnicity, IOP, BMO area, ASCO area, and axial length on LD were assessed.

Results

Laminar visibility was most consistent within the central ASCO (median 89%, range, 69%–95%). LD_BMO and LD_BM were significantly shallower in eyes with greater age, BMO area, and axial length and in females. LD_ASCO was shallower in eyes with greater ASCO area and axial length and in European and Hispanic descent compared to African descent eyes. LD_Sclera behaved similarly, but was not associated with axial length. BMO and ASCO area were not different between African descent and European descent eyes.

Conclusions

Central LD was deeper in African descent eyes and influenced least by age, axial length, and sex, but more by ASCO area, when measured relative to the ASCO and sclera. However, the magnitude of these effects for all four reference planes was small, and their clinical importance in the detection of glaucoma and its progression remains to be determined.

Detection of Age-Related Macular Degeneration by Portable Optical Coherence Tomography Operated by Nonexpert Personnel: Potential Use for Screenings

Purpose

The purpose of this study is to determine the sensitivity and specificity of detecting age-related macular degeneration (AMD) using portable optical coherence tomography (OCT) operated by nonexpert photographers on undilated patients.

Methods

In this case-control study, 92 individuals were recruited from the glaucoma and retina clinics at the Wilmer Eye Institute (Johns Hopkins University, Baltimore, MD). Using the portable iVue (Optovue, Inc, Fremont, CA) spectral-domain OCT (SD-OCT), 2 nonexpert photographers acquired retina map scans on undilated eyes of all participants. In total, 33 AMD eyes and 105 control eyes were evaluated and graded by ophthalmologists masked to the diagnoses.

Results

Detection of AMD on the portable OCT by ophthalmologists exhibited sensitivities of 0.91 and 0.94 and specificities of 0.88 and 0.89, for graders 1 and 2, respectively. A strong intergrader agreement was observed (κ = 0.87).

Conclusions

Nonexpert photographers can use a portable OCT on undilated eyes to acquire images for the detection of AMD. These findings present the potential utility of implementing a portable OCT in community screenings for earlier detection and treatment of disease.

21st century glaucoma care

Glaucoma care has evolved dramatically over the past generation, with changes that have incorporated new technology and improved understanding of the disease process. A major need is to construct a useful definition of glaucomatous optic neuropathy that can be used to compare data across clinical research studies. The treatment of glaucoma should now be based on achievement of a goal target for intraocular pressure, unique to each patient. Adherence with eye drop treatment is far from ideal and can be improved using reminder systems. Sustained delivery of glaucoma medication is on the horizon. New surgical approaches to glaucoma are being actively studied but have not as yet found their place in its care, with rigorous testing against present treatments needed.

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.

The effects of age on mitochondria, axonal transport, and axonal degeneration after chronic IOP elevation using a murine ocular explant model

The purpose of this study was to compare younger and older mice after chronic intraocular pressure (IOP) elevation lasting up to 4 days with respect to mitochondrial density, structure, and movement, as well as axonal integrity, in an ex vivo explant model. We studied 2 transgenic mouse strains, both on a C57BL/6J background, one expressing yellow fluorescent protein (YFP) in selected axons and one expressing cyan fluorescent protein (CFP) in all mitochondria. Mice of 4 months or 14 months of age were exposed to chronic IOP by anterior chamber microbead injection for 14 h, 1, 3, or 4 days. The optic nerve head of globe--optic nerve explants were examined by laser scanning microscopy. Mitochondrial density, structure, and movement were quantified in the CFP explants, and axonal integrity was quantified in YFP explants. In control mice, there was a trend towards decreased mitochondrial density (# per mm2) with age when comparing younger to older, control mice, but this was not significant (1947 ± 653 vs 1412 ± 356; p = 0.19). Mitochondrial density decreased after IOP elevation, significantly, by 31%, in younger mice (p = 0.04) but trending towards a decrease, by 22%, in older mice (p = 0.82) compared to age matched controls. Mitochondrial mean size was not altered after chronic IOP elevation for 14 h or more (p ≥ 0.16). When assessing mitochondrial movement, in younger mice, 5% were mobile at any given time; 4% in the anterograde direction and 1% retrograde. In younger untreated tissue, only 75% of explants had moving mitochondria (mean = 15.8 moving/explant), while after glaucoma induction only 24% of explants had moving mitochondria (mean = 4.2 moving/explant; difference from control, p = 0.03). The distance mitochondria traveled in younger mice was unchanged after glaucoma exposure, but in older glaucoma explants the distance traveled was less than half of older controls (p < 0.0003). In younger mice, mitochondrial speed increased after 14 h of elevated IOP (p = 0.006); however, in older glaucoma explants, movement was actually slower than controls (p = 0.02). In RGC-YFP explants, axonal integrity declined significantly after 4 days of IOP elevation to a similar degree in both younger and older mice. Older mice underwent greater loss of mitochondrial movement with chronic IOP elevation than younger mice, but suffered similar short-term axonal fragmentation in C57BL/6J mice. These transgenic strains, studied in explants, permit observations of alterations in intracellular structure and organelle activity in experimental glaucoma.

Sustained Dorzolamide Release Prevents Axonal and Retinal Ganglion Cell Loss in a Rat Model of IOP-Glaucoma

Purpose

To determine if one injection of a sustained release formulation of dorzolamide in biodegradable microparticles (DPP) reduces retinal ganglion cell (RGC) loss in a rat model of glaucoma.

Methods

We injected either DPP or control microparticles intravitreally in rats. Two days later, unilateral ocular hypertension was induced by translimbal, diode laser treatment by a surgeon masked to treatment group. IOP and clinical exams were performed until sacrifice 6 weeks after laser treatment. RGC loss was measured by masked observers in both optic nerve cross-sections and RGC layer counts from retinal whole mounts.

Results

Cumulative IOP exposure was significantly reduced by DPP injection (49 ± 48 mm Hg × days in treated versus 227 ± 191 mm Hg × days in control microparticle eyes; P = 0.012, t-test). While control-injected eyes increased in axial length by 2.4 ± 1.7%, DPP eyes did not significantly enlarge (0.3 ± 2.2%, difference from control, P = 0.03, t-test). RGC loss was significantly less in DPP eyes compared with control microparticle injection alone (RGC axon count reduction: 21% vs. 52%; RGC body reduction: 25% vs. 50% [beta tubulin labeling]; P = 0.02, t-test).

Conclusions

A single injection of sustained release DPP protected against RGC loss and axial elongation in a rat model of IOP glaucoma.

Translational Relevance

Sustained release IOP-lowering medications have the potential to stop glaucoma progression.

Use of Animal Models and Techniques in Glaucoma Research: Introduction

This chapter provides an overview over the use of animal models in glaucoma research from a historical perspective. Strengths and limitations of various models are considered, and the challenges of translating results in animal experiments to successful clinical trials in humans are discussed.

Regional Retinal Ganglion Cell Axon Loss in a Murine Glaucoma Model

Purpose

To determine if retinal ganglion cell (RGC) axon loss in experimental mouse glaucoma is uniform in the optic nerve.

Methods

Experimental glaucoma was induced for 6 weeks with a microbead injection model in CD1 (n = 78) and C57BL/6 (B6, n = 68) mice. From epoxy-embedded sections of optic nerve 1 to 2 mm posterior to the globe, total nerve area and regional axon density (axons/1600 μm²) were measured in superior, inferior, nasal, and temporal zones.

Results

Control eyes of CD1 mice have higher axon density and more total RGCs than control B6 mice eyes. There were no significant differences in control regional axon density in all mice or by strain (all P > 0.2, mixed model). Exposure to elevated IOP caused loss of RGC in both strains. In CD1 mice, axon density declined without significant loss of nerve area, while B6 mice had less density loss, but greater decrease in nerve area. Axon density loss in glaucoma eyes was not significantly greater in any region in either mouse strain (both P > 0.2, mixed model). In moderately damaged CD1 glaucoma eyes, and CD1 eyes with the greatest IOP elevation exposure, density loss differed by region (P = 0.05, P = 0.03, mixed model) with the greatest loss in the temporal and superior regions, while in severely injured B6 nerves superior loss was greater than inferior loss (P = 0.01, mixed model, Bonferroni corrected).

Conclusions

There was selectively greater loss of superior and temporal optic nerve axons of RGCs in mouse glaucoma at certain stages of damage. Differences in nerve area change suggest non-RGC responses differ between mouse strains.

Measuring Deformation in the Mouse Optic Nerve Head and Peripapillary Sclera

Purpose

To develop an ex vivo explant system using multiphoton microscopy and digital volume correlation to measure the full-field deformation response to intraocular pressure (IOP) change in the peripapillary sclera (PPS) and in the optic nerve head (ONH) astrocytic structure.

Methods

Green fluorescent protein (GFP)-glutamate transporter-GLT1 (GLT1/GFP) mouse eyes were explanted and imaged with a laser-scanning microscope under controlled inflation. Images were analyzed for regional strains and changes in astrocytic lamina and PPS shape. Astrocyte volume fraction in seven control GLT1/GFP mice was measured. The level of fluorescence of GFP fluorescent astrocytes was compared with glial fibrillary acidic protein (GFAP) labeled astrocytes using immunohistochemistry.

Results

The ONH astrocytic structure remained stable during 3 hours in explants. Control strain-globally, in the central one-half or two-thirds of the astrocytic lamina-was significantly greater in the nasal-temporal direction than in the inferior-superior or anterior-posterior directions (each P≤ 0.03, mixed models). The PPS opening (perimeter) in normal eye explants also became wider nasal-temporally than superior-inferiorly during inflation from 10 to 30 mm Hg (P = 0.0005). After 1 to 3 days of chronic IOP elevation, PPS area was larger than in control eyes (P = 0.035), perimeter elongation was 37% less than controls, and global nasal-temporal strain was significantly less than controls (P = 0.007). Astrocyte orientation was altered by chronic IOP elevation, with processes redirected toward the longitudinal axis of the optic nerve.

Conclusions

The explant inflation test measures the strain response of the mouse ONH to applied IOP. Initial studies indicate regional differences in response to both acute and chronic IOP elevation within the ONH region.

The pressure-induced deformation response of the human lamina cribrosa: Analysis of regional variations

The objective of this study was to measure the pressure-induced deformation response of the human lamina cribrosa (LC) and analyze for variations with age and anatomical region. The posterior scleral cup of 8 eyes from 6 human donors was mounted onto a custom inflation chamber. A laser-scanning microscope was used for second harmonic generation (SHG) imaging of the collagen structure in the posterior volume of the LC at pressures from 5mmHg to 45mmHg. The SHG volumes were analyzed by the Fast-Fourier Iterative Digital Volume Correlation (DVC) algorithm for the three dimensional (3D) displacement field. The components of the Green-Lagrange strain tensor and the in-plane principal and maximum shear strains were evaluated from the DVC displacement field for the central and peripheral regions of the LC and the nasal, temporal, inferior, and superior quadrants surrounding the central retinal artery and vein. Among the major findings were that older age was associated with lower strains, the maximum shear strain was larger in the peripheral than central region, and the maximum principal strain was lower in the nasal quadrant. The elliptical shape of the LC was also predictive of the biaxial strain ratio. Age-related and structure-related variations in the pressure-induced strains of the LC may contribute to the susceptibility and severity of optic nerve damage in glaucoma, and regional variations may explain the progression of axonal damage and tissue remodeling observed in the LC in glaucoma.

STATEMENT OF SIGNIFICANCE

Glaucoma causes vision loss through progressive damage of the retinal ganglion axons at the lamina cribrosa (LC), the connective tissue structure that supports the axons as they leave the eye. Mechanical characterization of the LC is challenging because of the complex 3D shape and inaccessibility of the tissue. We present a new method using digital volume correlation to map the 3D displacement and strain fields in the LC under inflation. We report for the first time significant regional variations in the strains that are consistent with the pattern of optic nerve damage in early glaucoma. Thus regional strain variations may be predictive of the progression of axonal damage in glaucoma.

A mouse ocular explant model that enables the study of living optic nerve head events after acute and chronic intraocular pressure elevation: Focusing on retinal ganglion cell axons and mitochondria

We developed an explant model of the mouse eye and optic nerve that facilitates the study of retinal ganglion cell axons and mitochondria in the living optic nerve head (ONH) in an ex vivo environment. Two transgenic mouse strains were used, one expressing yellow fluorescent protein in selected axons and a second strain expressing cyan fluorescent protein in all mitochondria. We viewed an explanted mouse eye and optic nerve by laser scanning microscopy at and behind the ONH, the site of glaucoma injury. Explants from previously untreated mice were studied with the intraocular pressure (IOP) set artificially at normal or elevated levels for several hours. Explants were also studied from eyes that had undergone chronic IOP elevation from 14 h to 6 weeks prior to ex vivo study. Image analysis in static images and video of individual mitochondria or axonal structure determined effects of acute and chronic IOP elevation. At normal IOP, fluorescent axonal structure was stable for up to 3 h under ex vivo conditions. After chronic IOP elevation, axonal integrity index values indicated fragmentation of axon structure in the ONH. In mice with fluorescent mitochondria, the normal density decreased with distance behind the ONH by 45% (p = 0.002, t-test). Density increased with prior chronic IOP elevation to 21,300 ± 4176 mitochondria/mm² compared to control 16,110 ± 3159 mitochondria/mm² (p = 0.025, t-test), but did not increase significantly after 4 h, acute IOP elevation (1.5% decrease in density, p = 0.83, t-test). Mean normal mitochondrial length of 2.3 ± 1.4 μm became 13% smaller after 4 h of IOP elevation ex vivo compared to baseline (p = 0.015, t-test, N-10). Normal mitochondrial speed of movement was significantly slower in the anterograde direction (towards the brain) than retrograde, but there were more mitochondria in motion and traveling longer lengths in anterograde direction. The percent of mitochondria in motion decreased by >50% with acute IOP increase to 30 mm Hg after 60 min. A new ocular explant model implemented with eyes from transgenic mice with fluorescent cellular components provided real time measurement of the early events in experimental glaucoma and quantitative outcomes for neuroprotection therapy experiments.

Neuroprotection for glaucoma: Requirements for clinical translation

Within the field of glaucoma research, neuroprotection is defined as slowing the functional loss in glaucoma by a mechanism independent of lowering of intraocular pressure. There is currently a great potential for research surrounding neuroprotection as it relates to glaucoma. Anatomical targets for neuroprotection should focus on upstream rather than downstream factors, and could include any part of the retinal ganglion cell, the glia, especially astrocytes or Muller cells, and vasculature. The great number of anatomical targets is exceeded only by the number of possible biochemical pathways and potential treatments. Successful treatment may be accomplished through the targeting of one or even a combination of multiple pathways. Once a treatment is shown effective in vitro, it should be evaluated in vivo with carefully chosen animal models and studied in sufficient numbers to detect statistically and clinically significant effects. Such a drug should have few systemic side effects and its delivery should be optimized so as to encourage compliance. There are still a multitude of possible screens available to test the efficacy of a neuroprotective drug and a single gold standard is ideal for the accurate assessment and comparison of new drugs. Future studies in neuroprotection should investigate the genetic component of the disease, novel pharmaceutical agents for new or known pathways, modulations of scleral biomechanics, and relation to research of other complex disorders of the central nervous system.

Changes in Anterior Segment Morphology and Predictors of Angle Widening after Laser Iridotomy in South Indian Eyes

PURPOSE

To compare anterior segment optical coherence tomography (ASOCT) angle morphology before and after laser peripheral iridotomy (LPI) in a cohort of South Indian subjects with primary angle-closure suspect (PACS) or primary angle-closure/primary angle-closure glaucoma (PAC/PACG) and to examine baseline parameters associated with angle widening.

DESIGN

Prospective observational study.

PARTICIPANTS

A total of 244 subjects aged ≥30 years with PACS or PAC/PACG in at least 1 eye.

METHODS

The ASOCT images and angle gonioscopic grades were analyzed for all subjects at baseline and 2 weeks after LPI. Multivariable linear and logistic regression models were used to determine predictors of angle widening (change in mean angle opening distance [AOD750]) and angle opening (all 4 quadrants with trabecular meshwork [TM] visible on gonioscopy after LPI).

MAIN OUTCOME MEASURES

Change in ASOCT parameters with LPI and baseline predictors of angle widening.

RESULTS

Laser peripheral iridotomy resulted in angle widening on ASOCT with significant increases in AOD750, angle recess area, and trabecular iris surface area (P < 0.05 for all). Gonioscopically, 44.7% of all subjects had open angles in all 4 quadrants after LPI, with a greater percentage of angles open in the PACS group compared with the PAC/PACG group (52.4% vs. 36.4%; P = 0.01). In multivariable regression analyses, greater postoperative angle widening as defined by change in AOD750 was associated with shorter baseline AOD750 and axial length, and greater baseline anterior chamber depth, iris curvature, and lens vault (P ≤ 0.002 for all). Gonioscopic angle opening after LPI was more common with wider baseline angle width (modified Shaffer grade) and lower cup-to-disc ratio (P < 0.001 for both).

CONCLUSIONS

In a South Indian population with PACS or PAC/PACG, LPI results in significant anterior chamber angle widening seen on both ASOCT and gonioscopy, although some degree of persistent iridotrabecular contact was present in approximately half of PACS eyes and approximately two thirds of PAC/PACG eyes on gonioscopy. The greatest widening by ASOCT was observed in eyes with features most consistent with greater baseline pupillary block.

Unilateral Glaucoma Associated with Conjunctival Angioma and Choroidal Thickening without Facial Angioma

We report a case of a suspected Sturge-Weber syndrome variant diagnosed at the age of 58 with the help of enhanced depth imaging spectral-domain ocular coherence tomography (EDI-SDOCT). A 58-year-old female with unilateral glaucoma was suspected to have choroidal vascular lesion, conjunctival angioma, and no facial port-wine stain who presented to the clinic with bleb dysesthesia many years after trabeculectomy. EDI-SDOCT was performed and revealed markedly increased choroidal thickness in the affected eye. EDI-SDOCT may be helpful in diagnosing Sturge-Weber variants without facial involvement and may aid in the investigation of the pathogenesis of this disease.