Tractional Abnormalities of the Central Foveal Bouquet in Epiretinal Membranes: Clinical Spectrum and Pathophysiological Perspectives

PURPOSE

To investigate the tractional alterations of the central bouquet (CB) in idiopathic epiretinal membranes (ERMs).

DESIGN

Retrospective, consecutive, observational case series.

METHODS

ERMs were classified according to a 4-stage grading system. The CB was defined as a circular area of approximately 100 μm composed of densely packed cones (and Müller cells) in the central fovea. Tractional abnormalities of the CB were identified with spectral-domain optical coherence tomography. Ex vivo histopathologic analysis was performed.

RESULTS

In this study 263 eyes with ERMs were included. Mean follow-up was 21.2 ± 16.7 months. At baseline, tractional abnormalities of the CB were diagnosed in 58 out of 263 eyes (22%) and divided into 3 categories: cotton ball sign (defined as a fuzzy hyperreflective area between the ellipsoid zone and the interdigitation zone in the central fovea), foveolar detachment, and acquired vitelliform lesion. The presence of ectopic inner foveal layers was negatively correlated with the presence of CB tractional abnormalities (P = .002). Visual acuity was highest in association with the cotton ball sign and lowest in the acquired vitelliform lesion group. Sequential morphologic progression was identified in 7 eyes. Ex vivo histopathologic analysis illustrated characteristic staining patterns supporting a potential mechanism of traction by Müller cells in the CB.

CONCLUSIONS

The cotton ball sign, foveolar detachment, and acquired vitelliform lesion may comprise a continuum in the same clinical spectrum and may represent subsequent stages of CB abnormalities. Foveal Müller cells may play an integral role in the transmission of mechanical forces to the central foveal cones.

Determinants and Characteristics of Bruch's Membrane Opening and Bruch's Membrane Opening-Minimum Rim Width in a Normal Japanese Population

Purpose

To identify determinants of Bruch's membrane opening (BMO), and BMO-minimum rim width (BMO-MRW) and circumpapillary retinal nerve fiber layer thickness (RNFLT) centered on BMO center and characterize these parameters in a normal Japanese population.

Methods

Spectral-domain optical coherence tomography images of optic nerve head and circumpapillary and macular retina were obtained in 258 eyes of 258 normal Japanese with mean (standard deviation) age of 51.7 (18.2) years. BMO area, BMO-MRW, RNFLT (measured with a 3.5-mm-diameter circle scan) were all acquired and analyzed relative to the eye-specific fovea to BMO (FoBMO) axis. One randomly selected eye of each subject was analyzed. Multiple regression analysis was used to identify determinants to the parameters.

Results

BMO area, global BMO-MRW, RNFLT, and FoBMO angle averaged 2.06 (0.45) mm2, 305.5 (50.0) μm, 101.8 (9.6) μm, and -7.8° (3.8°), respectively. There was a modest correlation between global BMO-MRW and RNFLT (r = 0.337; P < 0.001), while the sectorwise correlations were highest in the superior-temporal sector (r = 0.500; P < 0.001) and lowest in the nasal sector (r = 0.117; P = 0.063). Global BMO-MRW and RNFLT declined with age at -1.04 μm/y (P < 0.001) and -0.12 μm/y (P = 0.001), and the former correlated negatively (P = 0.001) and the latter positively (P < 0.001) with BMO area after adjustment for other factors (R2 = 0.191 and 0.272, respectively). BMO area correlated positively with axial length (P = 0.023) and negatively with age (P < 0.001) (R2 = 0.157).

Conclusions

BMO-MRW and RNFLT declined with age with a difference between them in their relationship to BMO area. BMO area positively correlated with axial length and negatively with age.

Variation in the Three-Dimensional Histomorphometry of the Normal Human Optic Nerve Head With Age and Race: Lamina Cribrosa and Peripapillary Scleral Thickness and Position

Purpose

This study quantified the thickness and depth of the lamina cribrosa (LC) and peripapillary scleral thickness in high-resolution three-dimensional (3D) fluorescent reconstructions of the optic nerve head (ONH) in eyes from donors of African (AD) and European descent (ED).

Methods

A total of 64 eyes (45 ED, 19 AD) from 51 normal donors were obtained within 6 hours of death and fixed at 10 mm Hg of pressure. The optic nerve head was trephined from the globe and digitally reconstructed at 1.5 × 1.5 × 1.5 μm voxel resolution with an automated episcopic fluorescence technique. The load-bearing ONH connective tissue surfaces were manually delineated in 3D using custom software.

Results

The lamina cribrosa and peripapillary sclera were significantly thinner in AD eyes adjusting for age and sex (LC was 24 ± 11 μm thinner; P = 0.0350; scleral was 56 ± 22 μm thinner; P = 0.0097). The lamina cribrosa was significantly thinner in females (23 ± 11 μm thinner; P = 0.0425). Age was not significantly associated with any morphologic parameter in the ED group. However, increasing age was associated with an increase in scleral thickness (1.3 μm/year, P = 0.0499) and an increase in LC depth (2.3 μm/year, P = 0.0035) in the AD group. The sclera was thickest in the superior and temporal regions while the LC was thinnest superiorly.

Conclusions

Substantial sectorial and racial differences in LC and scleral morphology were observed, as well as increasing LC depth and scleral thickness with age in the AD group. Results suggest greater age-related remodeling of the load-bearing ONH connective tissues in eyes from AD individuals that could explain, in part, the greater predilection to glaucomatous injury seen in aged AD populations.

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.

Lamina Cribrosa Microarchitecture in Monkey Early Experimental Glaucoma: Global Change

Purpose

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

Methods

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

Results

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

Conclusions

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

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.

Biomechanical aspects of axonal damage in glaucoma: A brief review

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

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.

Total Retinal Blood Flow in a Nonhuman Primate Optic Nerve Transection Model Using Dual-Beam Bidirectional Doppler FD-OCT and Microsphere Method

PURPOSE

We validated noninvasive Doppler-optical coherence tomography (OCT) blood flow measurements against the terminal microsphere method in a surgical induced optic nerve transection nonhuman primate model.

METHODS

In 6 nonhuman primates, total retinal blood flow (TRBF) was measured with a custom-built dual-beam bidirectional Doppler Fourier Domain (FD)-OCT. Peripapillary retinal nerve fiber layer thickness (RNFLT) was measured by Spectralis spectral-domain (SD)-OCT. Measurements were performed every 10 to 15 days before and after unilateral optic nerve transection (ONT) until RNFLT was reduced by more than 40% from baseline. Before the animals were killed, TRBF was measured using the microsphere technique.

RESULTS

A significant correlation between all arterial and venous Doppler OCT TRBF measurements was found in ONT and contralateral control eyes (both P < 0.01, n = 6). The Bland-Altman analysis showed a bias of 0.57 in the ONT group and 0.02 in the contralateral control group. Also, excellent agreement was observed between Doppler OCT and microsphere measurements (P < 0.01, r = 0.976, bias = 0.54). After ONT, TRBF and RNFLT decreased by -51% ± 42% and -44% ± 2% (n = 5), respectively. In the contralateral control eyes, TRBF and RNFLT were unchanged.

CONCLUSIONS

Very good accordance was found between TRBF measurements, obtained with dual-beam bidirectional Doppler FD-OCT and the microsphere method. It also was possible to monitor changes over time in TRBF after ONT with Doppler OCT. These findings highlight the accuracy and potential of noninvasive Doppler OCT to provide valuable information for detecting early changes in ocular disease in future.

The Connective Tissue Components of Optic Nerve Head Cupping in Monkey Experimental Glaucoma Part 1: Global Change

PURPOSE

To characterize optic nerve head (ONH) connective tissue change within 21 monkey experimental glaucoma (EG) eyes, so as to identify its principal components.

METHODS

Animals were imaged three to five times at baseline then every 2 weeks following chronic unilateral IOP elevation, and euthanized early through end-stage confocal scanning laser tomographic change. Optic nerve heads were serial-sectioned, three-dimensionally (3D) reconstructed, delineated, and quantified. Overall EG versus control eye differences were assessed by general estimating equations (GEE). Significant, animal-specific, EG eye change was required to exceed the maximum physiologic intereye differences in six healthy animals.

RESULTS

Overall EG eye change was significant (P < 0.0026) and animal-specific EG eye change most frequent, for five phenomena (number of EG eyes and range of animal-specific change): posterior laminar deformation (21, -29 to -437 μm), laminar thickening (11, 20-73 μm) and thinning (3, -23 to -31 μm), scleral canal expansion (17, 20-139 μm), outward anterior (16, -16 to -124 μm) and posterior (17, -22 to -279 μm) laminar insertion migration, and peripapillary scleral bowing (11, 21-77 μm). Experimental glaucoma versus control eye laminar thickness differences were bimodal in behavior, being thickened in most EG eyes demonstrating the least deformation and less thickened or thinned in most EG eyes demonstrating the greatest deformation.

CONCLUSIONS

Our postmortem studies retrospectively identify five connective tissue components of ONH "cupping" in monkey EG which serve as targets for longitudinally staging and phenotyping ONH connective tissue alteration within all forms of monkey and human optic neuropathy.

Compromised Optic Nerve Blood Flow and Autoregulation Secondary to Neural Degeneration

PURPOSE

To test the hypothesis that optic nerve head (ONH) blood flow (BF) and autoregulation compromise are consequences of optic nerve degeneration induced by surgical optic nerve transection (ONT).

METHODS

In both eyes of five nonhuman primates, peripapillary retinal nerve fiber layer thickness (RNFLT) was measured by spectral-domain optical coherence tomography. Optic nerve head BF and dynamic autoregulation responses to a rapid manometric IOP increase (from 10-40 mm Hg) were measured by Laser Speckle Flowgraphy. The measurements were conducted every 10 to 15 days before and after unilateral ONT. Post-ONT measurements were repeated until RNFLT in the ONT eye was reduced by more than 40% of baseline value.

RESULTS

After ONT, RNFLT, and ONH BF progressively declined over time (P < 0.0001 and P = 0.02, respectively). Longitudinal changes between the two were highly correlated (P < 0.0001). When data was grouped by test session, the first significant decreases for RNFLT and BF were found 13 ± 0.8 and 24 ± 3.2 days post ONT, respectively (P < 0.05, both). At the final time point (55 ± 0.5 days post ONT), RNFLT, and BF were reduced by 44% ± 2.0% and 38 ± 5.0% from baseline, respectively. Dynamic autoregulation analysis showed marginal increased response time in post-ONT eyes (P = 0.05). Control eyes showed no longitudinal changes for any parameter.

CONCLUSIONS

The close association between RNFLT loss and ONH BF decrease following optic nerve degeneration demonstrated a clear cause and effect relationship. Increased BF response time appears to be a sign of dynamic autoregulation dysfunction in this ONT model.

Structural Measurements for Monitoring Change in Glaucoma: Comparing Retinal Nerve Fiber Layer Thickness With Minimum Rim Width and Area

PURPOSE

Minimum rim width (MRW) and area (MRA) have been introduced as anatomically defensible measures of neuroretinal rim tissue observable using spectral-domain optical coherence tomography (SDOCT). They have been reported to change earlier than retinal nerve fiber layer thickness (RNFLT) in glaucoma. This study sought to determine which is better to distinguish subsequent change from variability, using the previously described longitudinal signal-to-noise ratio (LSNR).

METHODS

Data from 157 eyes of 157 participants with high-risk ocular hypertension or non-end-stage glaucoma (mean deviation [MD] from -22 to +3 dB) were used. Participants were tested approximately every 6 months for at least six visits. For each eye, MRW, MRA, and RNFLT were regressed linearly against time. Longitudinal signal-to-noise ratio for each eye was defined as the rate of change over time (signal) divided by the standard deviation of residuals from this trend (noise). These were compared between parameters using a Wilcoxon signed rank test.

RESULTS

The median LSNRs were -0.58y⁻¹ for RNFLT (bootstrapped 95% confidence interval -0.69 to -0.48y⁻¹); -0.44y⁻¹ (-0.59 to -0.32y⁻¹) for MRW; and -0.23y⁻¹ (-0.32 to -0.08y⁻¹) for MRA. Longitudinal signal-to-noise ratios were significantly more negative for RNFLT than for MRW (P = 0.025) or for MRA (P < 0.001).

CONCLUSIONS

Retinal nerve fiber layer thickness measured by SDOCT had a better LSNR than MRW or MRA. Although MRW and MRA may be more sensitive for early detection of glaucomatous damage, these data suggest that RNFLT may be preferable for monitoring change.

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

PURPOSE

To characterize early optic nerve head (ONH) structural change in rat experimental glaucoma (EG).

METHODS

Unilateral intraocular pressure (IOP) elevation was induced in Brown Norway rats by hypertonic saline injection into the episcleral veins and animals were sacrificed 4 weeks later by perfusion fixation. Optic nerve cross-sections were graded from 1 (normal) to 5 (extensive injury) by 5 masked observers. ONHs with peripapillary retina and sclera were embedded, serial sectioned, 3-D reconstructed, delineated, and quantified. Overall and animal-specific EG versus Control eye ONH parameter differences were assessed globally and regionally by linear mixed effect models with significance criteria adjusted for multiple comparisons.

RESULTS

Expansions of the optic nerve and surrounding anterior scleral canal opening achieved statistical significance overall (p < 0.0022), and in 7 of 8 EG eyes (p < 0.005). In at least 5 EG eyes, significant expansions (p < 0.005) in Bruch's membrane opening (BMO) (range 3-10%), the anterior and posterior scleral canal openings (8-21% and 5-21%, respectively), and the optic nerve at the anterior and posterior scleral canal openings (11-30% and 8-41%, respectively) were detected. Optic nerve expansion was greatest within the superior and inferior quadrants. Optic nerve expansion at the posterior scleral canal opening was significantly correlated to optic nerve damage (R = 0.768, p = 0.042).

CONCLUSION

In the rat ONH, the optic nerve and surrounding BMO and neurovascular scleral canal expand early in their response to chronic experimental IOP elevation. These findings provide phenotypic landmarks and imaging targets for detecting the development of experimental glaucomatous optic neuropathy in the rat eye.

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

PURPOSE

To relate changes in retinal function and retinal nerve fiber layer (RNFL) retardance to loss of RNFL thickness and optic nerve axon counts in a nonhuman primate (NHP) model of experimental glaucoma (EG).

METHODS

Bilateral longitudinal measurements of peripapillary RNFL thickness (spectral-domain optical coherence tomography, SDOCT; Spectralis), retardance (GDxVCC), and multifocal electroretinography (mfERG; VERIS) were performed in 39 NHP at baseline (BL; median, 5 recordings; range, 3-10) and weekly after induction of unilateral EG by laser photocoagulation of the trabecular meshwork. Multifocal ERG responses were high-pass filtered (>75 Hz) to measure high- and low-frequency component (HFC and LFC) amplitudes, including LFC features N1, P1, and N2. High-frequency component amplitudes are known to specifically reflect retinal ganglion cell (RGC) function. Complete (100%) axon counts of orbital optic nerves were obtained in 31/39 NHP.

RESULTS

Postlaser follow-up was 10.4 ± 7.9 months; mean and peak IOP were 18 ± 5 and 41 ± 11 mm Hg in EG eyes, 11 ± 2 and 18 ± 6 mm Hg in control (CTL) eyes. At the final available time point, RNFL thickness had decreased from BL by 14 ± 14%, retardance by 20 ± 11%, and the mfERG HFC by 30 ± 17% (P < 0.0001 each). Longitudinal changes in retardance and HFC were linearly related to RNFL thickness change (R2 = 0.51, P < 0.0001 and R2 = 0.22, P = 0.002, respectively); LFC N2 was weakly related but N1 or P2 (N1: R2 = 0.07, P = 0.11; P1: R2 = 0.04, P = 0.24; N2: R2 = 0.13, P = 0.02). At zero change from BL for RNFL thickness (Y-intercept), retardance was reduced by 11% (95% confidence interval [CI]: -15.3% to -6.8%) and HFC by 21.5% (95% CI: -28.7% to -14.3%). Relative loss of RNFL thickness, retardance, and HFC (EG:CTL) were each related to axon loss (R2 = 0.66, P < 0.0001; R2 = 0.42, P < 0.0001; R2 = 0.42, P < 0.0001, respectively), but only retardance and HFC were significantly reduced at zero relative axon loss (Y-intercept; retardance: -9.4%, 95% CI: -15.5% to -3.4%; HFC: -10.9%, 95% CI: -18.6% to -3.2%; RNFL thickness: +1.8%, 95% CI: -4.9% to +5.4%).

CONCLUSIONS

Retinal nerve fiber layer retardance and RGC function exhibit progressive loss from baseline before any loss of RNFL thickness or orbital optic nerve axons occurs in NHP EG. These in vivo measures might serve as potential biomarkers of early-stage glaucomatous damage preceding axon loss and RGC death.

Bruch's Membrane Opening Minimum Rim Width and Retinal Nerve Fiber Layer Thickness in a Normal White Population: A Multicenter Study

PURPOSE

Conventional optic disc margin-based neuroretinal rim measurements lack a solid anatomic and geometrical basis. An optical coherence tomography (OCT) index, Bruch's membrane opening minimum rim width (BMO-MRW), addresses these deficiencies and has higher diagnostic accuracy for glaucoma. We characterized BMO-MRW and peripapillary retinal nerve fiber layer thickness (RNFLT) in a normal population.

DESIGN

Multicenter cross-sectional study.

PARTICIPANTS

Normal white subjects.

METHODS

An approximately equal number of subjects in each decade group (20-90 years of age) was enrolled in 5 centers. Subjects had normal ocular and visual field examination results. We obtained OCT images of the optic nerve head (24 radial scans) and peripapillary retina (1 circular scan). The angle between the fovea and BMO center (FoBMO angle), relative to the horizontal axis of the image frame, was first determined and all scans were acquired and analyzed relative to this eye-specific FoBMO axis. Variation in BMO-MRW and RNFLT was analyzed with respect to age, sector, and BMO shape.

MAIN OUTCOME MEASURES

Age-related decline and between-subject variability in BMO-MRW and RNFLT.

RESULTS

There were 246 eyes of 246 subjects with a median age of 52.9 years (range, 19.8-87.3 years). The median FoBMO angle was -6.7° (range, 2.5° to -17.5°). The BMO was predominantly vertically oval with a median area of 1.74 mm(2) (range, 1.05-3.40 mm(2)). Neither FoBMO angle nor BMO area was associated with age or axial length. Both global mean BMO-MRW and RNFLT declined with age at a rate of -1.34 μm/year and -0.21 μm/year, equivalent to 4.0% and 2.1% loss per decade of life, respectively. Sectorially, the most rapid decrease occurred inferiorly and the least temporally; however, the age association was always stronger with BMO-MRW than with RNFLT. There was a modest relationship between mean global BMO-MRW and RNFLT (r = 0.35), whereas sectorially the relationship ranged from moderate (r = 0.45, inferotemporal) to nonexistent (r = 0.01, temporal).

CONCLUSIONS

There was significant age-related loss of BMO-MRW in healthy subjects and notable differences between BMO-MRW and RNFLT in their relationship with age and between each other. Adjusting BMO-MRW and RNFLT for age and sector is important in ensuring optimal diagnostics for glaucoma.

Lamina cribrosa microarchitecture in normal monkey eyes part 1: methods and initial results

PURPOSE

To introduce quantitative postmortem lamina cribrosa (LC) microarchitecture (LMA) assessment and characterize beam diameter (BD), pore diameter (PD), and connective tissue volume fraction (CTVF) in 21 normal monkey eyes.

METHODS

Optic nerve heads (ONHs) underwent digital three-dimensional (3D) reconstruction and LC beam segmentation. Each beam and pore voxel was assigned a diameter based on the largest sphere that contained it before transformation to one of twelve 30° sectors in a common cylinder. Mean BD, PD, and CTVF within 12 central and 12 peripheral subsectors and within inner, middle, and outer LC depths were assessed for sector, subsector, and depth effects by analysis of variance using general estimating equations. Eye-specific LMA discordance (the pattern of lowest connective tissue density) was plotted for each parameter.

RESULTS

The ranges of mean BD, PD, and CTVF were 14.0 to 23.1 μm, 20.0 to 35.6 μm, and 0.247 to 0.638, respectively. Sector, subsector, and depth effects were significant (P < 0.01) for all parameters except subsector on CTVF. Beam diameter and CTVF were smaller and PD was larger within the superior-temporal (ST) and inferior-temporal (IT) sectors (P < 0.05). These differences were enhanced within the central versus peripheral subsectors. Beam diameter and CTVF were larger and PD was smaller (P < 0.05) within the middle LC layer. Lamina cribrosa microarchitecture discordance most commonly occurred within the ST and IT sectors, varied by eye, and generally diminished as CTVF increased.

CONCLUSIONS

Our data support previous characterizations of diminished connective tissue density within the ST and IT ONH regions. The clinical importance of eye-specific LMA discordance warrants further study.

Comparison of retinal nerve fiber layer thickness in vivo and axonal transport after chronic intraocular pressure elevation in young versus older rats

PURPOSE

To compare in young and old rats longitudinal measurements of retinal nerve fiber layer thickness (RNFLT) and axonal transport 3-weeks after chronic IOP elevation.

METHOD

IOP was elevated unilaterally in 2- and 9.5-month-old Brown-Norway rats by intracameral injections of magnetic microbeads. RNFLT was measured by spectral domain optical coherence tomography. Anterograde axonal transport was assessed from confocal scanning laser ophthalmolscopy of superior colliculi (SC) after bilateral intravitreal injections of cholera toxin-B-488. Optic nerve sections were graded for damage.

RESULTS

Mean IOP was elevated in both groups (young 37, old 38 mmHg, p = 0.95). RNFL in young rats exhibited 10% thickening at 1-week (50.9±8.1 µm, p<0.05) vs. baseline (46.4±2.4 µm), then 7% thinning at 2-weeks (43.0±7.2 µm, p>0.05) and 3-weeks (43.5±4.4 µm, p>0.05), representing 20% loss of dynamic range. RNFLT in old rats showed no significant change at 1-week (44.9±4.1 µm) vs. baseline (49.2±5.3 µm), but progression to 22% thinning at 2-weeks (38.0±3.7 µm, p<0.01) and 3-weeks (40.0±6.6 µm, p<0.05), representing 59% loss of dynamic range. Relative SC fluorescence intensity was reduced in both groups (p<0.001), representing 77-80% loss of dynamic range and a severe transport deficit. Optic nerves showed 75-95% damage (p<0.001). There was greater RNFL thinning in old rats (p<0.05), despite equivalent IOP insult, transport deficit and nerve damage between age groups (all p>0.05).

CONCLUSION

Chronic IOP elevation resulted in severely disrupted axonal transport and optic nerve axon damage in all rats, associated with mild RNFL loss in young rats but a moderate RNFL loss in old rats despite the similar IOP insult. Hence, the glaucomatous injury response within the RNFL depends on age.

Age-related differences in longitudinal structural change by spectral-domain optical coherence tomography in early experimental glaucoma

PURPOSE

To characterize age-related differences in the magnitude of spectral-domain optical coherence tomography (SD-OCT) structural change in early experimental glaucoma (EG).

METHODS

Both eyes from four young (1.4-2.6 years) and four old (18.6-21.9 years) rhesus monkeys were imaged at least three times at baseline, and then every 2 weeks after laser-induced, chronic, unilateral IOP elevation until the onset of EG (confocal scanning laser tomographic surface change confirmed twice). Two to 20 weeks after EG onset, animals were euthanized and optic nerve axon counts for all eyes were performed. Masked operators delineated retinal and ONH landmarks in 40 radial B-scans from each eye and imaging session to quantify change from baseline in five SD-OCT neural and connective tissue parameters. The effects of EG, age, and EG × age interactions on the magnitude, rate (magnitude per postlaser time), and IOP responsiveness (magnitude per cumulative IOP insult) of postlaser parameter change were individually assessed using general estimating equation models.

RESULTS

Presac SD-OCT RNFLT and minimum rim width change and postmortem axon loss was not significantly different in old compared with young EG eyes. The rate of change and IOP responsiveness of the parameters anterior lamina cribrosa surface depth relative to Bruch's membrane opening (BMO) and BMO depth relative to peripheral Bruch's membrane were significantly lower (P < 0.05) in the old compared with the young EG eyes.

CONCLUSIONS

At similar postlaser times, levels of cumulative IOP insult and axonal damage, SD-OCT-detected ONH connective tissue structural change is greater in young compared with old monkey EG eyes.

The Effect of Age on Optic Nerve Axon Counts, SDOCT Scan Quality, and Peripapillary Retinal Nerve Fiber Layer Thickness Measurements in Rhesus Monkeys

PURPOSE

To evaluate the effect of age on optic nerve axon counts, spectral-domain optical coherence tomography (SDOCT) scan quality, and peripapillary retinal nerve fiber layer thickness (RNFLT) measurements in healthy monkey eyes.

METHODS

In total, 83 healthy rhesus monkeys were included in this study (age range: 1.2-26.7 years). Peripapillary RNFLT was measured by SDOCT. An automated algorithm was used to count 100% of the axons and measure their cross-sectional area in postmortem optic nerve tissue samples (N = 46). Simulation experiments were done to determine the effects of optical changes on measurements of RNFLT. An objective, fully-automated method was used to measure the diameter of the major blood vessel profiles within each SDOCT B-scan.

RESULTS

Peripapillary RNFLT was negatively correlated with age in cross-sectional analysis (P < 0.01). The best-fitting linear model was RNFLT_(μm) = -0.40 × age_(years) + 104.5 μm (R² = 0.1, P < 0.01). Age had very little influence on optic nerve axon count; the result of the best-fit linear model was axon count = -1364 × Age_(years) + 1,210,284 (R² < 0.01, P = 0.74). Older eyes lost the smallest diameter axons and/or axons had an increased diameter in the optic nerve of older animals. There was an inverse correlation between age and SDOCT scan quality (R = -0.65, P < 0.0001). Simulation experiments revealed that approximately 17% of the apparent cross-sectional rate of RNFLT loss is due to reduced scan quality associated with optical changes of the aging eye. Another 12% was due to thinning of the major blood vessels.

CONCLUSIONS

RNFLT declines by 4 μm per decade in healthy rhesus monkey eyes. This rate is approximately three times faster than loss of optic nerve axons. Approximately one-half of this difference is explained by optical degradation of the aging eye reducing SDOCT scan quality and thinning of the major blood vessels.

TRANSLATIONAL RELEVANCE

Current models used to predict retinal ganglion cell losses should be reconsidered.

Longitudinal alterations in the dynamic autoregulation of optic nerve head blood flow revealed in experimental glaucoma

PURPOSE

To use a novel dynamic autoregulation analysis (dAR) to test the hypothesis that the optic nerve head (ONH) blood flow (BF) autoregulation is disrupted during early stages of experimental glaucoma (EG) in nonhuman primates.

METHODS

Retinal nerve fiber layer thickness (RNFLT, assessed by optical coherence tomography) and ONH BF (assessed by laser speckle imaging technique) were measured biweekly before and after unilateral laser treatment to the trabecular meshwork. Each nonhuman primate was followed until reaching either an early stage of damage (RNFLT loss < 20%, n = 6) or moderate to advanced stages of damage (RNFLT loss > 20%, n = 9). At each test, dAR was assessed by characterizing ONH BF changes during the first minute of rapid manometrical intraocular pressure (IOP) elevation from 10 to 40 mm Hg. The dAR analysis extracted the following parameters: baseline BF, average BF 10 seconds before IOP elevation; BFΔmax, maximum BF change from baseline BF; Tr, time from baseline BF to the BFΔmax; Kr, average descending BF rate.

RESULTS

Mean postlaser IOP was 20.2 ± 5.9 and 12.3 ± 2.6 mm Hg in EG and control eyes, respectively (P < 0.0001). Compared with prelaser values, baseline BF was higher in early EG, but lower in moderate to advanced EG (P = 0.01). Tr was increased and Kr was reduced in both stages (P < 0.01). BFΔmax was smaller in the early EG (P = 0.05) and remained low in the moderate to advanced EG (P = 0.15). No changes in the parameters were observed in control eyes.

CONCLUSIONS

Chronic IOP elevation causes ONH autoregulation dysfunction in the early stage of EG, characterized by a disrupted BF response and delayed Tr, revealed by dAR analysis.

Anatomic vs. acquired image frame discordance in spectral domain optical coherence tomography minimum rim measurements

Purpose

To quantify the effects of using the fovea to Bruch's membrane opening (FoBMO) axis as the nasal-temporal midline for 30° sectoral (clock-hour) spectral domain optical coherence tomography (SDOCT) optic nerve head (ONH) minimum rim width (MRW) and area (MRA) calculations.

Methods

The internal limiting membrane and BMO were delineated within 24 radial ONH B-scans in 222 eyes of 222 participants with ocular hypertension and glaucoma. For each eye the fovea was marked within the infrared reflectance image, the FoBMO angle (θ) relative to the acquired image frame (AIF) horizontal was calculated, the ONH was divided into 30°sectors using a FoBMO or AIF nasal/temporal axis, and SDOCT MRW and MRA were quantified within each FoBMO vs. AIF sector. For each sector, focal rim loss was calculated as the MRW and MRA gradients (i.e. the difference between the value for that sector and the one clockwise to it divided by 30°). Sectoral FoBMO vs. AIF discordance was calculated as the difference between the FoBMO and AIF values for each sector. Generalized estimating equations were used to predict the eyes and sectors of maximum FoBMO vs. AIF discordance.

Results

The mean FoBMO angle was −6.6±4.2° (range: −17° to +7°). FoBMO vs. AIF discordance in sectoral mean MRW and MRA was significant for 7 of 12 and 6 of 12 sectors, respectively (p<0.05, Wilcoxon test, Bonferroni correction). Eye-specific, FoBMO vs. AIF sectoral discordance was predicted by sectoral rim gradient (p<0.001) and FoBMO angle (p<0.001) and achieved maximum values of 83% for MRW and 101% for MRA.

Conclusions

Using the FoBMO axis as the nasal-temporal axis to regionalize the ONH rather than a line parallel to the AIF horizontal axis significantly influences clock-hour SDOCT rim values. This effect is greatest in eyes with large FoBMO angles and sectors with focal rim loss.

Anterior lamina cribrosa surface depth, age, and visual field sensitivity in the Portland Progression Project

PURPOSE

To assess the effect of age on spectral-domain optical coherence tomography (SDOCT)-detected lamina cribrosa depth while controlling for visual field (VF) status and retinal nerve fiber layer thickness (RNFLT) in 221 high-risk ocular hypertension and glaucoma patients enrolled in the Portland Progression Project.

METHODS

In this cross-sectional study, each participant underwent 870-nm SDOCT to obtain high-resolution radial B-scans centered on the optic nerve head (ONH) and a standardized ophthalmologic examination, including automated perimetry, on the same day. For each ONH, an anterior lamina cribrosa surface depth (ALCSD) parameter was generated as the average perpendicular distance from each anterior lamina cribrosa surface point relative to Bruch's membrane opening (BMO) reference plane within all 24 delineated B-scans. The relative effects of age, age-corrected VF status (mean deviation [MD]), and RNFLT on ALCSD were analyzed.

RESULTS

The mean age ± SD of participants was 64 ± 11 years (range, 33-90 years). The relationship between ALCSD and MD was age-dependent. ALCSD = 407.68 - 67.13 × MD - 0.08 × Age + 0.89 × MD × Age (MD, P = 0.001; MD × Age, P = 0.004). The relationship between ALCSD and RNFLT may also be age-dependent but did not achieve significance (interaction term, P = 0.067). ALCSD increased with worse VF status in younger eyes but not in older eyes. In older eyes, the anterior lamina was shallower than in younger eyes for the same VF status and RNFLT.

CONCLUSIONS

These data are consistent with the concept that structure/structure and structure/function relationships change with age.