Relating Retinal Ganglion Cell Function and Retinal Nerve Fiber Layer (RNFL) Retardance to Progressive Loss of RNFL Thickness and Optic Nerve Axons in Experimental Glaucoma

Current Status and Future Perspectives of Optic Nerve Imaging in Glaucoma

Being the primary site of degeneration, the optic nerve has always been the focus of structural glaucoma assessment. The technical advancements, mainly of optical coherence tomography (OCT), now allow for a very precise quantification of the optic nerve head and peripapillary retina morphology. By far the most commonly used structural optic nerve parameter is the thickness of the parapapillary retinal nerve fiber, which has great clinical utility but also suffers from significant limitations, mainly in advanced glaucoma. Emerging novel imaging technologies, such as OCT angiography, polarization-sensitive or visible-light OCT and adaptive optics, offer new biomarkers that have the potential to significantly improve structural glaucoma diagnostics. Another great potential lies in the processing of the data already available. Artificial intelligence does not only help increase the reliability of current biomarkers but can also integrate data from various imaging modalities and other clinical measures to increase diagnostic accuracy. And it can, in a more efficient way, draw information from available datasets, such as an OCT scan, compared to the current concept of biomarkers, which only use a fraction of the whole dataset.

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

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

Retrolaminar Demyelination of Structurally Intact Axons in Nonhuman Primate Experimental Glaucoma

Purpose

To determine if structurally intact, retrolaminar optic nerve (RON) axons are demyelinated in nonhuman primate (NHP) experimental glaucoma (EG).

Methods

Unilateral EG NHPs (n = 3) were perfusion fixed, EG and control eyes were enucleated, and foveal Bruch's membrane opening (FoBMO) 30° sectoral axon counts were estimated. Optic nerve heads were trephined; serial vibratome sections (VSs) were imaged and colocalized to a fundus photograph establishing their FoBMO location. The peripheral neural canal region within n = 5 EG versus control eye VS comparisons was targeted for scanning block-face electron microscopic reconstruction (SBEMR) using micro-computed tomographic reconstructions (µCTRs) of each VS. Posterior laminar beams within each µCTR were segmented, allowing a best-fit posterior laminar surface (PLS) to be colocalized into its respective SBEMR. Within each SBEMR, up to 300 axons were randomly traced until they ended (nonintact) or left the block (intact). For each intact axon, myelin onset was identified and myelin onset distance (MOD) was measured relative to the PLS. For each EG versus control SBEMR comparison, survival analyses compared EG and control MOD.

Results

MOD calculations were successful in three EG and five control eye SBEMRs. Within each SBEMR comparison, EG versus control eye axon loss was -32.9%, -8.3%, and -15.2% (respectively), and MOD was increased in the EG versus control SBEMR (P < 0.0001 for each EG versus control SBEMR comparison). When data from all three EG eye SBEMRs were compared to all five control eye SBEMRs, MOD was increased within the EG eyes.

Conclusions

Structurally intact, RON axons are demyelinated in NHP early to moderate EG. Studies to determine their functional status are indicated.

Multifunctional adaptive optics optical coherence tomography allows cellular scale reflectometry, polarimetry, and angiography in the living human eye

Clinicians are unable to detect glaucoma until substantial loss or dysfunction of retinal ganglion cells occurs. To this end, novel measures are needed. We have developed an optical imaging solution based on adaptive optics optical coherence tomography (AO-OCT) to discern key clinical features of glaucoma and other neurodegenerative diseases at the cellular scale in the living eye. Here, we test the feasibility of measuring AO-OCT-based reflectance, retardance, optic axis orientation, and angiogram at specifically targeted locations in the living human retina and optic nerve head. Multifunctional imaging, combined with focus stacking and global image registration algorithms, allows us to visualize cellular details of retinal nerve fiber bundles, ganglion cell layer somas, glial septa, superior vascular complex capillaries, and connective tissues. These are key histologic features of neurodegenerative diseases, including glaucoma, that are now measurable in vivo with excellent repeatability and reproducibility. Incorporating this noninvasive cellular-scale imaging with objective measurements will significantly enhance existing clinical assessments, which is pivotal in facilitating the early detection of eye disease and understanding the mechanisms of neurodegeneration.

Real-Time Imaging of Single Retinal Cell Apoptosis in a Non-Human Primate Ocular Hypertension Model

Purpose

To evaluate the feasibility of using DARC (detection of apoptosing retinal cells) technology as a biomarker for preclinical assessment of glaucomatous damage in a non-human primate (NHP) model of ocular hypertension (OHT).

Methods

Elevated intraocular pressure (IOP) was induced by applying a laser to the trabecular meshwork in each eye of NHPs. Changes in DARC counts in the retina, identified as fluorescent-tagged annexin V (ANX776)-positive cells, were evaluated together with optic nerve damage, assessed using spectral domain-optical coherence tomography. The pharmacokinetic properties of ANX776 in both healthy and OHT model monkeys were also examined.

Results

Sustained elevation of IOP and subsequent thinning of the retinal nerve fiber layer thickness (RNFLT) around the optic nerve head were confirmed in the OHT model. Increases in DARC counts were also detected after IOP elevation. We identified a statistically significant relationship between cumulative DARC counts and reductions in RNFLT both globally and in each peripapillary sector. Intravenous administration of ANX776 increased blood annexin V in a dose-dependent manner, which was subsequently eliminated.

Conclusions

This study revealed that DARC technology can effectively assess glaucomatous damage in an NHP OHT model. We obtained the fundamental data that could serve as a reference for developing preclinical models to evaluate the pharmacodynamics of neuroprotective agents using DARC technology in NHP OHT models.

Translational Relevance

Our basic data in a monkey OHT model could be useful for future preclinical studies using DARC technology to estimate the pharmacodynamic response of neuroprotective agents.

Utility of Light-Adapted Full-Field Electroretinogram ON and OFF Responses for Detecting Glaucomatous Functional Damage

Purpose

To compare parameters of electroretinogram (ERG) responses for their ability to detect functional loss in early stages of nonhuman primate (NHP) experimental glaucoma (EG), including photopic negative responses (PhNR) to a standard brief red flash on a blue background (R/B) and 200-ms-long R/B and white-on-white (W/W) flashes, to W/W flicker stimuli (5-50 Hz), and to a dark-adapted intensity series.

Methods

Light-adapted ERGs were recorded in 12 anesthetized monkeys with unilateral EG. Amplitudes and implicit times of the a-wave, b-wave, and d-wave were measured, as well as amplitudes of PhNRs and oscillatory potentials for flash onset and offset. Flicker ERGs were measured using peak-trough and fundamental frequency analyses. Dark-adapted ERG parameters were modeled by Naka-Rushton relationships.

Results

Only PhNR amplitudes were significantly reduced in EG eyes compared to fellow control (FC) eyes. The d-wave implicit time was delayed in EG versus FC eyes only for the W/W long flash, but in all eyes it was 10 to 20 ms slower for R/B versus the W/W condition. Flicker ERGs were <0.5 ms delayed in EG versus FC overall, but amplitudes were affected only at 5 Hz. The brief R/B PhNR amplitude had the highest sensitivity to detect EG and strongest correlation to parameters of structural damage.

Conclusions

The PhNR to the standard brief R/B stimulus was best for detecting and following early-stage functional loss in NHP EG.

Translational Relevance

These results suggest that there would be no benefit in using longer duration flashes to separate onset and offset responses for clinical management of glaucoma.

Optic Nerve Head Myelin-Related Protein, GFAP, and Iba1 Alterations in Non-Human Primates With Early to Moderate Experimental Glaucoma

Purpose

The purpose of this study was to test if optic nerve head (ONH) myelin basic protein (MBP), 2′,3′-cyclic nucleotide 3′-phosphodiesterase (CNPase), glial fibrillary acidic protein (GFAP), and ionized calcium binding adaptor molecule 1 (Iba1) proteins are altered in non-human primate (NHP) early/moderate experimental glaucoma (EG).

Methods

Following paraformaldehyde perfusion, control and EG eye ONH tissues from four NHPs were paraffin embedded and serially (5 µm) vertically sectioned. Anti-MBP, CNPase, GFAP, Iba1, and nuclear dye-stained sections were imaged using sub-saturating light intensities. Whole-section images were segmented creating anatomically consistent laminar (L) and retrolaminar (RL) regions/sub-regions. EG versus control eye intensity/pixel-cluster density data within L and two RL regions (RL1 [1-250 µm]/RL2 [251-500 µm] from L) were compared using random effects models within the statistical program “R.”

Results

EG eye retinal nerve fiber loss ranged from 0% to 20%. EG eyes’ MBP and CNPase intensity were decreased within the RL1 (MBP = 31.4%, P < 0.001; CNPase =62.3%, P < 0.001) and RL2 (MBP = 19.6%, P < 0.001; CNPase = 56.1%, P = 0.0004) regions. EG eye GFAP intensity was decreased in the L (41.6%, P < 0.001) and RL regions (26.7% for RL1, and 28.4% for RL2, both P < 0.001). Iba1+ and NucBlue pixel-cluster density were increased in the laminar (28.2%, P = 0.03 and 16.6%, P = 0.008) and both RL regions (RL1 = 37.3%, P = 0.01 and 23.7%, P = 0.0002; RL2 = 53.7%, P = 0.002 and 33.2%, P < 0.001).

Conclusions

Retrolaminar myelin disruption occurs early in NHP EG and may be accompanied by laminar and retrolaminar decreases in astrocyte process labeling and increases in microglial/ macrophage density. The mechanistic and therapeutic implications of these findings warrant further study.

Application of Whole Exome Sequencing and Functional Annotations to Identify Genetic Variants Associated with Marfan Syndrome

Marfan syndrome (MFS) is a rare disease that affects connective tissue, which causes abnormalities in several organ systems including the heart, eyes, bones, and joints. The autosomal dominant disorder was found to be strongly associated with FBN1, TGFBR1, and TGFBR2 mutations. Although multiple genetic mutations have been reported, data from Asian populations are still limited. As a result, we utilized the whole exome sequencing (WES) technique to identify potential pathogenic variants of MFS in a Taiwan cohort. In addition, a variety of annotation databases were applied to identify the biological functions as well as the potential mechanisms of candidate genes. In this study, we confirmed the pathogenicity of FBN1 to MFS. Our results indicated that TTN and POMT1 may be likely related to MFS phenotypes. Furthermore, we found nine unique variants highly shared in a MFS family cohort, of which eight are novel variants worthy of further investigation.

Lamina cribrosa vessel and collagen beam networks are distinct

Our goal was to analyze the spatial interrelation between vascular and collagen networks in the lamina cribrosa (LC). Specifically, we quantified the percentages of collagen beams with/without vessels and of vessels inside/outside of collagen beams. To do this, the vasculature of six normal monkey eyes was labeled by perfusion post-mortem. After enucleation, coronal cryosections through the LC were imaged using fluorescence and polarized light microscopy to visualize the blood vessels and collagen beams, respectively. The images were registered to form 3D volumes. Beams and vessels were segmented, and their spatial interrelationship was quantified in 3D. We found that 22% of the beams contained a vessel (range 14%-32%), and 21% of vessels were outside beams (13%-36%). Stated differently, 78% of beams did not contain a vessel (68%-86%), and 79% of vessels were inside a beam (64%-87%). Individual monkeys differed significantly in the fraction of vessels outside beams (p < 0.01 by linear mixed effect analysis), but not in the fraction of beams with vessels (p > 0.05). There were no significant differences between contralateral eyes in the percent of beams with vessels and of vessels outside beams (p > 0.05). Our results show that the vascular and collagenous networks of the LC in monkey are clearly distinct, and the historical notions that each LC beam contains a vessel and all vessels are within beams are inaccurate. We postulate that vessels outside beams may be relatively more vulnerable to mechanical compression by elevated IOP than are vessels shielded inside of beams.

Association of SIX1-SIX6 polymorphisms with peripapillary retinal nerve fibre layer thickness in children

PURPOSE

Association of SIX1-SIX6 variants with peripapillary retinal nerve fibre layer (p-RNFL) thickness had been reported in adults. This study aimed to investigate these associations in children, with further explorations by spatial, age and sex stratifications.

METHODS

2878 school children aged between 6 and 9 years were enrolled from the Hong Kong Children Eye Study. Three single-nucleotide polymorphisms (SNPs) at the SIX1-SIX6 locus were genotyped. The association of each SNP with p-RNFL thickness (including global and sectoral thickness) were evaluated using multiple linear regression.

RESULTS

SNPs rs33912345 (p=7.7×10^-4) and rs10483727 (p=0.0013) showed significant associations with temporal-inferior p-RNFL thickness. The C allele of rs33912345 was associated with a thinner temporal-inferior p-RNFL by an average of 2.44 µm, while rs10483727-T was associated with a thinner temporal-inferior p-RNFL by 2.32 µm. The association with temporal-inferior p-RNFL was the strongest in the 8-9 year-old group for rs33912345 (p=5.2×10^-4) and rs10483727 (p=3.3×10^-4). Both SNPs were significantly associated with temporal-inferior p-RNFL thickness in boys (p<0.0017), but not in girls (p>0.05). In contrast, rs12436579-C (β=1.66; p=0.0059), but not rs33912345-C (β=1.31; p=0.052) or rs10483727-T (β=1.19; p=0.078), was nominally associated with a thicker nasal-inferior p-RNFL.

CONCLUSIONS

Both rs33912345 and rs10483727 at SIX1-SIX6 were associated with p-RNFL thickness in children, especially at the temporal-inferior sector, with age-dependent and sex-specific effects. SNP rs12436579 was associated with nasal-inferior p-RNFL thickness. Our findings suggested a role of SIX1-SIX6 in RNFL variation during neural retina development in childhood.

Longitudinal In Vivo Changes in Radial Peripapillary Capillaries and Optic Nerve Head Structure in Non-Human Primates With Early Experimental Glaucoma

Purpose

There is conflicting evidence regarding whether a loss of radial peripapillary capillaries (RPCs) precedes neuronal loss in glaucoma. We examined the time course of in vivo changes in RPCs, optic nerve head (ONH) structure, and retinal nerve fiber layer thickness (RNFLT) in experimental glaucoma (EG).

Methods

Spectral domain optical coherence tomography images were acquired before and approximately every two weeks after inducing unilateral EG in nine rhesus monkeys to quantify mean anterior lamina cribrosa surface depth (ALCSD), minimum rim width (MRW), and RNFLT. Perfused RPC density was measured from adaptive optics scanning laser ophthalmoscope images acquired on the temporal half of the ONH. The time of first significant change was quantified as when values fell and remained outside of the 95% confidence interval established from control eyes.

Results

Mean ALCSD and/or MRW were the first parameters to change in eight EG eyes. RPC density changed first in the ninth. At their first points of change, mean ALCSD posteriorly deformed by 100.2 ± 101.2 µm, MRW thinned by 82.3 ± 65.9 µm, RNFLT decreased by 25 ± 14 µm, and RPC density decreased by 4.5 ± 2.1%. RPC density decreased before RNFL thinning in 5 EG eyes. RNFLT decreased before RPC density decreased in two EG eyes, whereas two EG eyes had simultaneous changes.

Conclusions

In most EG eyes, RPC density decreased before (or simultaneous with) a change in RNFLT, suggesting that vascular factors may play a role in axonal loss in some eyes in early glaucoma.

Age and intraocular pressure in murine experimental glaucoma

Age and intraocular pressure (IOP) are the two most important risk factors for the development and progression of open-angle glaucoma. While IOP is commonly considered in models of experimental glaucoma (EG), most studies use juvenile or adult animals and seldom older animals which are representative of the human disease. This paper provides a concise review of how retinal ganglion cell (RGC) loss, the hallmark of glaucoma, can be evaluated in EG with a special emphasis on serial in vivo imaging, a parallel approach used in clinical practice. It appraises the suitability of EG models for the purpose of in vivo imaging and argues for the use of models that provide a sustained elevation of IOP, without compromise of the ocular media. In a study with parallel cohorts of adult (3-month-old, equivalent to 20 human years) and old (2-year-old, equivalent to 70 human years) mice, we compare the effects of elevated IOP on serial ganglion cell complex thickness and individual RGC dendritic morphology changes obtained in vivo. We also evaluate how age modulates the impact of elevated IOP on RGC somal and axonal density in histological analysis as well the density of melanopsin RGCs. We discuss the challenges of using old animals and emphasize the potential of single RGC imaging for understanding the pathobiology of RGC loss and evaluating new therapeutic avenues.

Loss of P2Y1 receptors triggers glaucoma-like pathology in mice

BACKGROUND AND PURPOSE

Glaucoma, the leading cause of blindness, damages the retinal ganglion cells. Elevated intraocular pressure (IOP) is a high-risk factor for glaucoma, so topical hypotensive drugs are usually used for treatment. Because not all patients do not respond adequately to current treatments, there is a need to identify a new molecular target to reduce IOP. Here, we have assessed the role of P2Y1 receptors in mediating elevated IOP.

EXPERIMENTAL APPROACH

P2Y₁ receptor agonist was instilled into the eyes of mice, and the IOP changes were measured by a rebound-type tonometer. Expression of P2Y₁ receptors was estimated by immunohistochemistry. Ocular function was measured by a multifocal electroretinogram.

KEY RESULTS

A single dose of the P2Y₁ receptor agonist transiently reduced IOP and such effects were absent in P2Y₁ receptor-deficient (P2Y₁ KO) mice. P2Y₁ receptors were functionally expressed in the ciliary body, trabecular meshwork and Schlemm's canal. Activation of P2Y₁ receptors negatively regulated aquaporin 4 (AQP4) function but up-regulated endothelial NOS (eNOS). P2Y₁ KO mice showed chronic ocular hypertension regardless of age. P2Y₁ KO mice at 3 months old showed no damage to retinal ganglion cells, whereas 12-month-old mice showed a significant loss of these cells and impairment of ocular functions. Damage to retinal ganglion cells was attenuated by chronic administration of an IOP-reducing agent.

CONCLUSION AND IMPLICATIONS

Activation of P2Y₁ receptors reduced IOP via dual pathways including AQP4 and eNOS. Loss of P2Y₁ receptors resulted in glaucomatous optic neuropathy, suggesting that P2Y₁ receptors might provide an effective target in the treatment of glaucoma.

Determinants of macular ganglion cell-inner plexiform layer thickness in normal Chinese adults

Background

Demographic, systemic and ocular factors may impact macular ganglion cell–inner plexiform layer (GCIPL) thickness measurements. This study aimed to investigate the influences of multiple potential determinants of macular GCIPL thickness in normal Chinese adults.

Methods

This was a retrospective study conducted on 225 normal eyes from 225 healthy Chinese adults. GCIPL thickness were obtained using Cirrus high-definition optical coherence tomography (OCT). The age, gender, laterality, spherical equivalent (SE) refractive error, intraocular pressure (IOP), axial length (AL), central cornea thickness (CCT), circumpapillary retinal nerve fibre layer (pRNFL) thickness and OCT signal strength were recorded and their respective effect on GCIPL thickness parameters were evaluated.

Results

The mean (± SD) average, minimum, superotemporal, superior, superonasal, inferonasal, inferior, and inferotemporal GCIPL thickness was 84.56 ± 5.36, 81.32 ± 5.58, 83.08 ± 5.37, 85.70 ± 5.95, 87.15 ± 6.26, 85.07 ± 6.11, 82.46 ± 5.76, and 83.88 ± 5.59 μm, respectively. Determinants of thinner GCIPL thickness were older age (P = 0.001–0.117; effects enhanced if age over 40 years), thinner pRNFL (all P < 0.001), and weaker signal strength (all P < 0.001). No significant difference was found between males and females (P = 0.069–0.842), and between right eyes and the left eyes (P = 0.160–0.875) except that of superonasal GCIPL thickness (P < 0.001). There was no significant correlation between GCIPL thickness and SE, IOP, CCT, and AL (P = 0.135–0.968).

Conclusions

Individual determinants associated with thinner GCIPL thickness were older age (particularly over 40 years of age), thinner pRNFL, and weaker OCT signal strength. This is relevant in comprehensively understanding the normative data and differentiating normal aging from abnormalities.

A Simple Subjective Evaluation of Enface OCT Reflectance Images Distinguishes Glaucoma From Healthy Eyes

Purpose

We present a subjective approach to detecting glaucomatous defects in enface images and assess its diagnostic performance. We also test the hypothesis that if reflectivity changes precede thickness changes in glaucoma there should be reduced correlation between the modalities in glaucoma compared to controls.

Methods

Twenty glaucoma participants and 20 age-matched controls underwent high-resolution OCT scans of one eye. 4 µm-thick enface slabs were constructed through the retina. Enface indices were depths of first gap in visible retinal nerve fiber bundles (RNFBs) and last visible bundle, subjectively evaluated in six sectors of a 3.5 mm circle around the optic disc. Retinal nerve fiber layer thickness (RNFLT) along the same circle was extracted at angles corresponding to enface indices. Between-group differences were tested by linear mixed models. Diagnostic performance was measured by partial receiver operating characteristic area (pAUC).

Results

First gap and last visible bundle were closer to the inner limiting membrane in glaucoma eyes (both P < 0.0001). Enface indices showed excellent diagnostic performance (pAUCs 0.63-1.00), similar to RNFLT (pAUCs 0.63-0.95). Correlation between enface and RNFLT parameters was strong in healthy (r = 0.81-0.92) and glaucoma eyes (r = 0.73-0.80).

Conclusions

This simple subjective method reliably identifies glaucomatous defects in enface images with diagnostic performance at least as good as existing thickness indices. Thickness and reflectivity were similarly related in healthy and glaucoma eyes, providing no strong evidence of reflectivity loss preceding thinning. Objective analyses may realize further potential of enface OCT images in glaucoma.

Translational Relevance

Novel enface OCT indices may aid glaucoma diagnosis.

The Effect of the Idiopathic Epiretinal Membrane and Surgically Induced Posterior Vitreous Detachment on the Retinal Nerve Fiber Layer

Aim

To investigate the changes in the retinal nerve fiber layer (RNFL) following pars plana vitrectomy (PPV) with surgically induced posterior vitreous detachment (PVD) and idiopathic epiretinal membrane (ERM) and internal limiting membrane (ILM) peeling.

Methods

Patients with unilateral ERM with vitreomacular traction were included in this prospective, randomized, and controlled clinical trial. The control group (Group 1) was formed with the nonoperated fellow eyes of the patients, and the study group (Group 2) was formed with the eyes that underwent PPV including idiopathic ERM and ILM peeling. In the preoperative and postoperative periods (1^st, 2^nd, 3^rd, 6^th, and 12^th months), complete ophthalmological examination of the eyes was performed and RNFL measurements were examined in 4 different quadrants (superior, temporal, inferior, and nasal) with the help of spectral domain optical coherence tomography (OCT).

Results

There was no statistically significant change in Group 1 during the follow-up period in all quadrants (p > 0.05). The mean RNFL thickness in Group 2 was statistically significantly higher than in Group 1 in superior, inferior, and temporal quadrants (p < 0.01), preoperatively. The mean RNFL in Group 2 was higher in the 1^st, 2^nd, 3^rd, and 6^th months and lower in the 12^th month in superior, inferior, and temporal quadrants (p < 0.01) when compared to the preoperative period. The mean RNFL thickness in the nasal quadrant in Group 2 was higher in the 1^st, 2^nd, and 3^rd (p < 0.01) months, same in the 6^th month (p > 0.05), and lower in the 12^th (p < 0.01) month when compared to the preoperative period.

Conclusion

Idiopathic ERM may cause an increase in RNFL thickness in superior, inferior, and temporal quadrants with possible tractional effect. PPV with PVD induction and ERM and ILM peeling may cause these RNFL changes.

Visible-Light Optical Coherence Tomography Fibergraphy for Quantitative Imaging of Retinal Ganglion Cell Axon Bundles

Purpose

To develop a practical technique for visualizing and quantifying retinal ganglion cell (RGC) axon bundles in vivo.

Methods

We applied visible-light optical coherence tomography (vis-OCT) to image the RGC axon bundles, referred to as vis-OCT fibergraphy, of healthy wild-type C57BL/6 mice. After vis-OCT imaging, retinas were flat-mounted, immunostained with anti-beta-III tubulin (Tuj1) antibody for RGC axons, and imaged with confocal microscopy. We quantitatively compared the RGC axon bundle networks imaged by in vivo vis-OCT and ex vivo confocal microscopy using semi-log Sholl analysis.

Results

Side-by-side comparison of ex vivo confocal microscopy and in vivo vis-OCT confirmed that vis-OCT fibergraphy captures true RGC axon bundle networks. The semi-log Sholl regression coefficients extracted from vis-OCT fibergrams (3.7 ± 0.8 mm^–1) and confocal microscopy (3.6 ± 0.3 mm^–1) images also showed good agreement with each other (n = 6).

Conclusions

We demonstrated the feasibility of using vis-OCT fibergraphy to visualize RGC axon bundles. Further applying Sholl analysis has the potential to identify biomarkers for non-invasively assessing RGC health.

Translational Relevance

Our novel technique for visualizing and quantifying RGC axon bundles in vivo provides a potential measurement tool for diagnosing and tracking the progression of optic neuropathies.

Analysis of retinal nerve fiber layer birefringence in patients with glaucoma and diabetic retinopathy by polarization sensitive OCT

The retinal nerve fiber layer (RNFL) is a fibrous tissue that shows form birefringence. This optical tissue property is related to the microstructure of the nerve fiber axons that carry electrical signals from the retina to the brain. Ocular diseases that are known to cause neurologic changes, like glaucoma or diabetic retinopathy (DR), might alter the birefringence of the RNFL, which could be used for diagnostic purposes. In this pilot study, we used a state-of-the-art polarization sensitive optical coherence tomography (PS-OCT) system with an integrated retinal tracker to analyze the RNFL birefringence in patients with glaucoma, DR, and in age-matched healthy controls. We recorded 3D PS-OCT raster scans of the optic nerve head area and high-quality averaged circumpapillary PS-OCT scans, from which RNFL thickness, retardation and birefringence were derived. The precision of birefringence measurements was 0.005°/µm. As compared to healthy controls, glaucoma patients showed a slightly reduced birefringence (0.129 vs. 0.135°/µm), although not statistically significant. The DR patients, however, showed a stronger reduction of RNFL birefringence (0.103 vs. 0.135°/µm) which was highly significant. This result might open new avenues into early diagnosis of DR and related neurologic changes.

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.

Longitudinal changes in the peripapillary retinal nerve fiber layer thickness in the fellow eyes of unilateral retinal vein occlusion

To analyze longitudinal changes in peripapillary retinal nerve fiber layer (pRNFL) thicknesses over time in the fellow eyes of patients with unilateral retinal vein occlusion (RVO). A total of 47 patients with unilateral RVO and 47 healthy controls were enrolled. The mean and sectoral pRNFL thicknesses were measured using spectral domain-optical coherence tomography at 1 year intervals, and followed for 3 years. Linear mixed models were performed to calculate and compare the reduction rates of pRNFL thicknesses over time. The mean pRNFL thickness decreased significantly during the 3-year follow-up, with a significant decrease over time in both groups. The reduction rate in mean pRNFL thicknesses was -0.41 μm/year in the control group and -0.68 μm/year in the fellow eyes of RVO group, and the decrease was significantly higher in the fellow eyes of RVO group than in the control group (p < 0.001). Using a multivariate linear mixed model, age (estimate: -0.41, p = 0.011) and hypertension (HTN) (estimate: -6.51, p = 0.014) were significantly associated with the reduction in mean pRNFL thicknesses in fellow eyes of RVO group. The fellow eyes of RVO patients showed a greater reduction in pRNFL thickness over time than normal controls. Age and HTN should be considered as factors to decrease the pRNFL thickness over time in fellow eyes of RVO group.

In Vivo Imaging of the Retina, Choroid, and Optic Nerve Head in Guinea Pigs

PURPOSE

Guinea pigs are increasingly being used as a model of myopia, and may also represent a novel model of glaucoma. Here, optical coherence tomography (OCT) imaging was performed in guinea pigs. In vivo measurements of retinal, choroidal, and optic nerve head parameters were compared with histology, and repeatability and interocular variations were assessed.

METHODS

OCT imaging and histology were performed on adult guinea pigs (n = 9). Using a custom program in MATLAB, total retina, ganglion cell/nerve fiber layer (GC/NFL), outer retina, and choroid thicknesses were determined. Additionally, Bruch's membrane opening (BMO) area and diameter, and minimum rim width were calculated. Intraobserver, interocular, and intersession coefficients of variation (CV) and intraclass correlation coefficients (ICC) were assessed.

RESULTS

Retina, GC/NFL, outer retina and choroid thicknesses from in vivo OCT imaging were 147.7 ± 5.8 μm, 59.2 ± 4.5 μm, 72.4 ± 2.4 μm, and 64.8 ± 11.6 μm, respectively. Interocular CV ranged from 1.8% to 11% (paired t-test, p = 0.16 to 0.81), and intersession CV ranged from 1.1% to 5.6% (p = 0.12 to 0.82), with the choroid showing the greatest variability. BMO area was 0.192 ± 0.023 mm², and diameter was 493.79 ± 31.89 μm, with intersession CV of 3.3% and 1.7%, respectively. Hyper reflective retinal layers in OCT correlated with plexiform and RPE layers in histology.

CONCLUSION

In vivo OCT imaging and quantification of guinea pig retina and optic nerve head parameters were repeatable and similar between eyes of the same animal. In vivo visibility of retinal cell layers correlated well with histological images.

ABBREVIATIONS

optic nerve head (ONH), retinal ganglion cell (RGC), spectral domain optical coherence tomography (SD-OCT), enhanced depth imaging (EDI), minimum rim width (MRW), hematoxylin and eosin (H & E).

Change in retinal structural anatomy during the preclinical stage of Alzheimer's disease

INTRODUCTION

We conducted a 27-month longitudinal study of mid-life adults with preclinical Alzheimer's disease (AD), using spectral domain optical coherence tomography to compare changes in volume and thickness in all retinal neuronal layers to those of age-matched healthy control subjects.

METHODS

Fifty-six older adults (mean age = 65.36 years) with multiple risk factors for AD completed spectral domain optical coherence tomography retinal imaging and cognitive testing at baseline. Twenty-seven months later, they completed the same examinations and an 18F-florbetapir positron emission tomography imaging study.

RESULTS

Compared to healthy control subjects, those in the preclinical stage of AD showed a significant decrease in macular retinal nerve fiber layer (mRNFL) volume, over a 27-month follow-up interval period, as well as a decrease in outer nuclear layer and inner plexiform layer volumes and thickness in the inferior quadrant. However, only the mRNFL volume was linearly related to neocortical positron emission tomography amyloid standardized uptake value ratio after controlling for any main effects of age (R2 = 0.103; ρ = 0.017). Furthermore, the magnitude of mRNFL volume reduction was significantly correlated with performance on a task of participants' abilities to efficiently integrate visual and auditory speech information (McGurk effect).

DISCUSSION

We observed a decrease in mRNFL, outer nuclear layer, and inner plexiform layer volumes, in preclinical AD relative to controls. Moreover, the largely myelinated axonal loss in the RNFL is related to increased neocortical amyloid-β accumulation after controlling for age. Volume loss in the RNFL, during the preclinical stage, is not related to performance on measures of episodic memory or problem solving. However, this retinal change does appear to be modestly related to relative decrements in performance on a measure of audiovisual integration efficiency that has been recently advanced as a possible early cognitive marker of mild cognitive impairment.

Retinal changes following rapid ascent to a high-altitude environment

PurposeTo determine what impact rapid ascension to a high-altitude environment has on the retina with the aim of preventing and treating high-altitude oculopathy.Patients and methodsParticipants in the study were members of the Chinese military assigned to the high-altitude environment of the Tibetan plateau. Ninety-one participants were enrolled in the study. Optical coherence tomography was used to measure the thickness of retina-related indicators. Measurements were taken before and after exposure to the high-altitude environment and upon return to the baseline altitude.ResultsFollowing exposure to the high-altitude environment in Tibet, there was a significant increase in retinal nerve fiber layer (RNFL) thickness in the temporal and nasal quadrants of the optic disc, whilst a significant decrease in RNFL thickness in the inferior optic disc was also observed. A significant increase in RNFL thickness in the superior and inferior macula was also evident, along with a significant increase in the ganglion cell layer thickness in the superior macula. Upon return to the baseline altitude, all measurements returned to baseline levels except for the RNFL of the inferior macula, which was significantly thicker. Pathological changes were also documented in the eyes of nine participants upon returning to baseline altitude, including ischemic optic neuropathy, myopia, and cortical amaurosis.ConclusionsThe high-altitude environment can have a negative impact on the health of the retina and may contribute to the incidence of various eye diseases. This study deepens the understanding of what impact a high-altitude environment has on retina and provides reliable data for blindness prevention and treatment.

The ganglion cell complex as an useful tool in glaucoma assessment

Glaucoma is known as an optic neuropathy prone to progression that determines characteristic not only structural (loss of the ganglion cells as well as their axons) but also functional defects (visual field loss). Objective: To evaluate the possibility of applying ganglion cell complex analysis (GCC) in patients who associate ocular hypertension with tilted disc and marked peripapillary atrophy. Methods: In order to evaluate its components, GCC can be investigated using the Optical Coherence Tomography (OCT) revealing: ganglion cell layer (cells bodies), inner plexiform layer (dendrites and synapses), and nerve fiber layer (axons). Our study included 196 eyes divided into 3 groups: 52 diagnosed with primary open angle glaucoma (POAG), 63 with ocular hypertension (OH), and 81 healthy (normal) eyes (NE). All eyes were submitted to a complete ophthalmologic checkup that involved advanced optic nerve and GCC evaluation. Results: A positive statistically significant correlation was identified between the GCC thickness and the RNFL in all three categories taken into account: R=0,6, p<0,0001 for glaucoma group, R=0,66, p<0,0001 for OH group and R=0,46, p<0,0001 for normal group. Conclusions: GCC has been proved useful for the assessment of the retinal nerve fiber layer (RNFL) in eyes with OH that associate tilted disc or peripapillary atrophy where the optic disc edges might not be certainly determined by the OCT.

Attenuation Coefficients From SD-OCT Data: Structural Information Beyond Morphology on RNFL Integrity in Glaucoma

PURPOSE

The purpose of this study is to explore the attenuation coefficient (AC) of the retinal nerve fiber layer (RNFL) in spectral domain optical coherence tomography (OCT) images, in healthy eyes and eyes affected by glaucoma. To assess the relation between RNLF AC, disease severity, RNFL thickness, visual field sensitivity threshold, spatial location and age.

PATIENTS AND METHODS

We analyzed peripapillary circle scans of a clinical OCT device (Spectralis OCT, Heidelberg Engineering, Heidelberg, Germany) in 102 glaucoma patients and 90 healthy controls. The images were fully automatically converted into depth-resolved AC images. Next, the median AC within the RNFL was calculated based on the Spectralis segmentation. We compared the RNFL AC between healthy, mild, moderate and advanced glaucomatous eyes and assessed the correlation with patient characteristics such as age and visual field sensitivity threshold (HFA, Carl Zeiss Meditec, Dublin, CA) in a generalized estimating equations (GEE) model. Finally, we explored the ability to discriminate between glaucomatous and healthy eyes by RNFL AC.

RESULTS

Median RNFL AC decreased with increasing disease severity up to moderate glaucoma (P<0.001) in all 4 sectors around the optic nerve head. The largest relative decrease occurred in the nasal sector. The RNFL AC (AUC, 0.834±0.028) effectively discriminated healthy from glaucomatous eyes, although RNFL thickness (AUC, 0.975±0.013) performed even better (P<0.001). Prediction of visual field sensitivity improved significantly when RNFL thickness was augmented with RNFL AC as covariates (P<0.001).

CONCLUSIONS

This study demonstrated that RNFL AC provides complementary information on the RNFL's health compared with RNFL thickness measurements alone.

Purinergic dysregulation causes hypertensive glaucoma-like optic neuropathy

Glaucoma is an optic neuropathy characterized by progressive degeneration of retinal ganglion cells (RGCs) and visual loss. Although one of the highest risk factors for glaucoma is elevated intraocular pressure (IOP) and reduction in IOP is the only proven treatment, the mechanism of IOP regulation is poorly understood. We report that the P2Y6 receptor is critical for lowering IOP and that ablation of the P2Y6 gene in mice (P2Y6KO) results in hypertensive glaucoma-like optic neuropathy. Topically applied uridine diphosphate, an endogenous selective agonist for the P2Y6 receptor, decreases IOP. The P2Y6 receptor was expressed in nonpigmented epithelial cells of the ciliary body and controlled aqueous humor dynamics. P2Y6KO mice exhibited sustained elevation of IOP, age-dependent damage to the optic nerve, thinning of ganglion cell plus inner plexiform layers, and a reduction of RGC numbers. These changes in P2Y6KO mice were attenuated by an IOP lowering agent. Consistent with RGC damage, visual functions were impaired in middle-aged P2Y6KO mice. We also found that expression and function of P2Y6 receptors in WT mice were significantly reduced by aging, another important risk factor for glaucoma. In summary, our data show that dysfunctional purinergic signaling causes IOP dysregulation, resulting in glaucomatous optic neuropathy.

Parameters of ocular fundus on spectral-domain optical coherence tomography for glaucoma diagnosis

In this review, we summarize the progression of several parameters assessed by spectral-domain optical coherence tomography (SD-OCT) in recent years for the detection of glaucoma. Monitoring the progression of defects in the retinal nerve fiber layer (RNFL) thickness is essential. Imaging and analysis of retinal ganglion cells (RGCs) and inner plexiform layer (IPL), respectively, have been of great importance. Optic nerve head (ONH) topography obtained from 3D SD-OCT images is another crucial step. Other important assessments involve locating the Bruch's membrane opening (BMO), estimating the optic disc size and rim area, and measuring the lamina cribrosa displacement. Still other parameters found in the past three years for glaucoma diagnosis comprise central retinal artery resistive index, optic disc perfusion in optical coherence tomography angiography (OCTA) study, peripapillary choroidal thickness, and choroidal area in SD-OCT. Recently, several more ocular fundus parameters have been found, and compared with the earlier parameters to judge the accuracy of diagnosis. While a few of these parameters have been widely used in clinical practice, a fair number are still in the experimental stage.

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.

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.

Comparing Optic Nerve Head Rim Width, Rim Area, and Peripapillary Retinal Nerve Fiber Layer Thickness to Axon Count in Experimental Glaucoma

Purpose

We compare spectral-domain optical coherence tomography (SDOCT) measurements of minimum rim width (MRW), minimum rim area (MRA), and peripapillary retinal nerve fiber layer thickness (RNFLT) to complete orbital optic nerve axon counts in nonhuman primates (NHP) with unilateral experimental glaucoma (EG).

Methods

Biweekly SDOCT measurements of MRW, MRA, and RNFLT were acquired under manometric IOP control (10 mm Hg) in 51 NHP during baseline (mean ± SD, 5.0 ± 1.6 sessions) and after laser photocoagulation was applied to the trabecular meshwork of one eye to induce chronic IOP elevation. At the study endpoint (predefined for each NHP), 100% axon counts were obtained from each optic nerve.

Results

For SDOCT parameters at baseline, the correlation between the two eyes of each animal was strongest for RNFLT (R = 0.97) and MRW (R = 0.97), but lower for MRA (R = 0.85). At the final time point, average values in EG eyes relative to control eyes were: −22% for RNFLT, −38% for MRW, −36% for MRA, and −36% for optic nerve axons. The correlation with axon counts was strongest for RNFLT (R = 0.81), compared to MRW (R = 0.72, P = 0.001) or MRA (R = 0.70, P = 0.001). Diagnostic sensitivity was 75% for RNFLT, 90% for MRW, and 88% for MRA; all had 100% specificity.

Conclusions

Peripapillary RNFLT was correlated more closely with total orbital optic nerve axon count than were the ONH parameters MRW or MRA. This is likely because glaucomatous deformation (beyond axon loss alone) has a greater influence on the ONH parameters MRW and MRA than on RNFLT.

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

Purpose

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

Methods

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

Results

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

Conclusions

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

Experimental Glaucoma Causes Optic Nerve Head Neural Rim Tissue Compression: A Potentially Important Mechanism of Axon Injury

Purpose

We tested the hypothesis that experimental glaucoma (EG) results in greater thinning of the optic nerve head (ONH) neural rim tissue than the peripapillary retinal nerve fiber layer (RNFL) tissue.

Methods

Longitudinal spectral-domain optical coherence tomography (SDOCT) imaging of the ONH and peripapillary RNFL was performed every other week under manometric IOP control (10 mm Hg) in 51 nonhuman primates (NHP) during baseline and after induction of unilateral EG. The ONH parameter minimum rim area (MRA) was derived from 80 radial B-scans centered on the ONH; RNFL cross-sectional area (RNFLA) from a peripapillary circular B-scan with 12° diameter.

Results

In control eyes, MRA was 1.00 ± 0.19 mm² at baseline and 1.00 ± 0.19 mm² at the final session (P = 0.77), while RNFLA was 0.95 ± 0.09 and 0.95 ± 0.10 mm², respectively (P = 0.96). In EG eyes, MRA decreased from 1.00 ± 0.19 mm² at baseline to 0.63 ± 0.21 mm² at the final session (P < 0.0001), while RNFLA decreased from 0.95 ± 0.09 to 0.74 ± 0.19 mm², respectively (P < 0.0001). Thus, MRA decreased by 36.4 ± 20.6% in EG eyes, significantly more than the decrease in RNFLA (21.7 ± 19.4%, P < 0.0001). Other significant changes in EG eyes included increased Bruch's membrane opening (BMO) nonplanarity (P < 0.05), decreased BMO aspect ratio (P < 0.0001), and decreased MRA angle (P < 0.001). Bruch's membrane opening area did not change from baseline in either control or EG eyes (P = 0.27, P = 0.15, respectively).

Conclusions

Optic nerve head neural rim tissue thinning exceeded peripapillary RNFL thinning in NHP EG. These results support the hypothesis that axon bundles are compressed transversely within the ONH rim along with glaucomatous deformation of connective tissues.

Comparing three different modes of electroretinography in experimental glaucoma: diagnostic performance and correlation to structure

PURPOSE

To compare diagnostic performance and structure-function correlations of multifocal electroretinogram (mfERG), full-field flash ERG (ff-ERG) photopic negative response (PhNR) and transient pattern-reversal ERG (PERG) in a non-human primate (NHP) model of experimental glaucoma (EG).

METHODS

At baseline and after induction of chronic unilateral IOP elevation, 43 NHP had alternating weekly recordings of retinal nerve fiber layer thickness (RNFLT) by spectral domain OCT (Spectralis) and retinal function by mfERG (7F slow-sequence stimulus, VERIS), ff-ERG (red 0.42 log cd-s/m² flashes on blue 30 scotopic cd/m² background, LKC UTAS-E3000), and PERG (0.8° checks, 99% contrast, 100 cd/m² mean, 5 reversals/s, VERIS). All NHP were followed at least until HRT-confirmed optic nerve head posterior deformation, most to later stages. mfERG responses were filtered into low- and high-frequency components (LFC, HFC, >75 Hz). Peak-to-trough amplitudes of LFC features (N1, P1, N2) and HFC RMS amplitudes were measured and ratios calculated for HFC:P1 and N2:P1. ff-ERG parameters included A-wave (at 10 ms), B-wave (trough-to-peak) and PhNR (baseline-to-trough) amplitudes as well as PhNR:B-wave ratio. PERG parameters included P50 and N95 amplitudes as well as N95:P50 ratio and N95 slope. Diagnostic performance of retinal function parameters was compared using the area under the receiver operating characteristic curve (A-ROC) to discriminate between EG and control eyes. Correlations to RNFLT were compared using Steiger's test.

RESULTS

Study duration was 15 ± 8 months. At final follow-up, structural damage in EG eyes measured by RNFLT ranged from 9% above baseline (BL) to 58% below BL; 29/43 EG eyes (67%) and 0/43 of the fellow control eyes exhibited significant (>7%) loss of RNFLT from BL. Using raw parameter values, the largest A-ROC findings for mfERG were: HFC (0.82) and HFC:P1 (0.90); for ff-ERG: PhNR (0.90) and PhNR:B-wave (0.88) and for PERG: P50 (0.64) and N95 (0.61). A-ROC increased when data were expressed as % change from BL, but the pattern of results persisted. At 95% specificity, the diagnostic sensitivity of mfERG HFC:P1 ratio was best, followed by PhNR and PERG. The correlation to RNFLT was stronger for mfERG HFC (R = 0.65) than for PhNR (R = 0.59) or PERG N95 (R = 0.36), (p = 0.20, p = 0.0006, respectively). The PhNR flagged a few EG eyes at the final time point that had not been flagged by mfERG HFC or PERG.

CONCLUSIONS

Diagnostic performance and structure-function correlation were strongest for mfERG HFC as compared with ff-ERG PhNR or PERG in NHP EG.

Modeling the Chronic Loss of Optic Nerve Axons and the Effects on the Retinal Nerve Fiber Layer Structure in Primary Disorder of Myelin

Purpose

We determined whether the chronic lack of optic nerve myelination and subsequent axon loss is associated with optical coherence tomography (OCT) changes in the retinal nerve fiber layer (RNFL), and whether this models what occurs in multiple sclerosis (MS) and confers its use as a surrogate marker for axon degeneration.

Methods

Using an animal model of Pelizaeus-Merzbacher disease (shp) bilateral longitudinal measurements of the peripapillary RNFL (spectral-domain OCT), electroretinograms (ERG), and visual evoked potentials (VEP) were performed in affected and control animals from 5 months to 2 years and in individual animals at single time points. Light and electron microscopy of the optic nerve and retina and histomorphometric measurements of the RNFL were compared to OCT data.

Results

Of the shp animals, 17% had an average reduction of OCT RNFL thickness on the superior retinal quadrant compared to controls (P < 0.05). Electroretinograms showed normal photopic A- and B-waves but flash VEPs were disorganized in shp animals. Morphologically, the shp retinas and optic nerves revealed significant RNFL thinning (P < 0.001) without retinal ganglion cell (RGC) loss, decrease total and relative retinal axonal area, and loss of optic nerve axons. There was strong positive correlation between OCT and morphometric RNFL thickness measurements (r = 0.878, P = 0.004).

Conclusion

The loss of optic nerve axons demonstrated in the shp model resulted in moderate thinning of the RNFL confirmed by OCT and histology. These results indicate that OCT-derived RNFL measurement can be a useful surrogate biomarker of optic nerve axon loss and potentially disease progression in demyelinating diseases.

Macular Structure and Function in Nonhuman Primate Experimental Glaucoma

Purpose

To evaluate structure and function of macular retinal layers in nonhuman primate (NHP) experimental glaucoma (EG).

Methods

Twenty-one NHP had longitudinal imaging of macular structure by SDOCT, 16 also had recordings of function by multifocal ERG. The average thickness over 15° was derived for seven individual SDOCT layers: macular nerve fiber layer (m-NFL), retinal ganglion cell layer (RGCL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer+inner segments combined (ONL+IS), and outer segments (OS). Peripapillary RNFL thickness (ppRNFLT) was measured from a single circular B-scan with 12° diameter. Responses to a slow-sequence multifocal ERG (mfERG) stimulus (7F) were filtered (at 75 Hz) into low- and high-frequency components (LFC, HFC).

Results

At final follow-up, significant structural loss occurred only in EG eyes and only for ppRNFLT (−29 ± 23%), m-NFL (−17 ± 16%), RGCL (−22 ± 15%), and IPL (−19 ± 14%); though there was also a small increase in OPL (+6 ± 7%) and ONL+IS (4 ± 4%) and a similar tendency for INL. Macular structural loss was correlated with ppRNFLT only for the NFL, RGCL and IPL (R = 0.95, 0.93 and 0.95, respectively, P < 0.0001). Significant functional loss occurred only for HFC and N2 in EG eyes. Significant longitudinal structure–function correlations (P < 0.01) were observed only in EG eyes and only for mfERG HFC and N2: HFC was correlated with ppRNFLT (R = 0.69), macular NFL (R = 0.67), RGCL (R = 0.74), and IPL (R = 0.72); N2 was correlated with RGCL (R = 0.54) and IPL (R = 0.48). High-frequency components amplitude change was inversely correlated with outer retinal thickness change (= −0.66).

Conclusions

Macular structural and functional losses are correlated and specific to ganglion cells over a wide range of EG severity. Outer retinal changes are likely due to inner retinal loss.

Electroretinography in glaucoma diagnosis

PURPOSE OF REVIEW

Electrophysiological measures of vision function have for decades generated interest among glaucoma researchers and clinicians alike because of their potential to help elucidate pathophysiological processes and sequence of glaucomatous damage, as well as to offer a potential complementary metric of function that might be more sensitive than standard automated perimetry. The purpose of this article is to review the recent literature to provide an update on the role of the electroretinogram (ERG) in glaucoma diagnosis.

RECENT FINDINGS

The pattern reversal ERG (PERG) and the photopic negative response (PhNR) of the cone-driven full-field, focal or multifocal ERG provide objective measures of retinal ganglion cell function and are all sensitive to glaucomatous damage. Recent studies demonstrate that a reduced PERG amplitude is predictive of subsequent visual field conversion (from normal to glaucomatous) and an increased rate of progressive retinal nerve fiber layer thinning in suspect eyes, indicating a potential role for PERG in risk stratification. Converging evidence indicates that some portion of PERG and PhNR abnormality represents a reversible aspect of dysfunction in glaucoma.

SUMMARY

PERG and PhNR responses obtained from the central macula are capable of detecting early-stage, reversible glaucomatous dysfunction.

The non-human primate experimental glaucoma model

The purpose of this report is to summarize the current strengths and weaknesses of the non-human primate (NHP) experimental glaucoma (EG) model through sections devoted to its history, methods, important findings, alternative optic neuropathy models and future directions. NHP EG has become well established for studying human glaucoma in part because the NHP optic nerve head (ONH) shares a close anatomic association with the human ONH and because it provides the only means of systematically studying the very earliest visual system responses to chronic intraocular pressure (IOP) elevation, i.e. the conversion from ocular hypertension to glaucomatous damage. However, NHPs are impractical for studies that require large animal numbers, demonstrate spontaneous glaucoma only rarely, do not currently provide a model of the neuropathy at normal levels of IOP, and cannot easily be genetically manipulated, except through tissue-specific, viral vectors. The goal of this summary is to direct NHP EG and non-NHP EG investigators to the previous, current and future accomplishment of clinically relevant knowledge in this model.

In vivo imaging methods to assess glaucomatous optic neuropathy

The goal of this review is to summarize the most common imaging methods currently applied for in vivo assessment of ocular structure in animal models of experimental glaucoma with an emphasis on translational relevance to clinical studies of the human disease. The most common techniques in current use include optical coherence tomography and scanning laser ophthalmoscopy. In reviewing the application of these and other imaging modalities to study glaucomatous optic neuropathy, this article is organized into three major sections: 1) imaging the optic nerve head, 2) imaging the retinal nerve fiber layer and 3) imaging retinal ganglion cell soma and dendrites. The article concludes with a brief section on possible future directions.