In vivo imaging methods to assess glaucomatous optic neuropathy

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.

Optical Coherence Tomography and Optical Coherence Tomography Angiography: Essential Tools for Detecting Glaucoma and Disease Progression

Early diagnosis and detection of disease progression are critical to successful therapeutic intervention in glaucoma, the leading cause of irreversible blindness worldwide. Optical coherence tomography (OCT) is a non-invasive imaging technique that allows objective quantification in vivo of key glaucomatous structural changes in the retina and the optic nerve head (ONH). Advances in OCT technology have increased the scan speed and enhanced image quality, contributing to early glaucoma diagnosis and monitoring, as well as the visualization of critically important structures deep within the ONH, such as the lamina cribrosa. OCT angiography (OCTA) is a dye-free technique for noninvasively assessing ocular microvasculature, including capillaries within each plexus serving the macula, peripapillary retina and ONH regions, as well as the deeper vessels of the choroid. This layer-specific assessment of the microvasculature has provided evidence that retinal and choroidal vascular impairments can occur during early stages of glaucoma, suggesting that OCTA-derived measurements could be used as biomarkers for enhancing detection of glaucoma and its progression, as well as to reveal novel insights about pathophysiology. Moreover, these innovations have demonstrated that damage to the macula, a critical region for the vision-related quality of life, can be observed in the early stages of glaucomatous eyes, leading to a paradigm shift in glaucoma monitoring. Other advances in software and hardware, such as artificial intelligence-based algorithms, adaptive optics, and visible-light OCT, may further benefit clinical management of glaucoma in the future. This article reviews the utility of OCT and OCTA for glaucoma diagnosis and disease progression detection, emphasizes the importance of detecting macula damage in glaucoma, and highlights the future perspective of OCT and OCTA. We conclude that the OCT and OCTA are essential glaucoma detection and monitoring tools, leading to clinical and economic benefits for patients and society.

Biological Correlations and Confounders for Quantification of Retinal Ganglion Cells by Optical Coherence Tomography Based on Studies of Outbred Mice

Purpose

Despite popularity of optical coherence tomography (OCT) in glaucoma studies, it's unclear how well OCT-derived metrics compare to traditional measures of retinal ganglion cell (RGC) abundance. Here, Diversity Outbred (J:DO) mice are used to directly compare ganglion cell complex (GCC) thickness measured by OCT to metrics of retinal anatomy measured ex vivo with retinal wholemounts and optic nerve histology.

Methods

J:DO mice (n = 48) underwent fundoscopic and OCT examinations, with automated segmentation of GCC thickness. RGC axons were quantified from para-phenylenediamine-stained optic nerve cross-sections and somas from BRN3A-immunolabeled retinal wholemounts, with total inner retinal cellularity assessed by TO-PRO and subsequent hematoxylin staining.

Results

J:DO tissues lacked overt disease. GCC thickness, RGC abundance, and total cell abundance varied broadly across individuals. GCC thickness correlated significantly to RGC somal density (r = 0.58) and axon number (r = 0.44), but not total cell density. Retinal area and nerve cross-sectional area varied widely. No metrics were significantly influenced by sex. In bilateral comparisons, GCC thickness (r = 0.95), axon (r = 0.72), and total cell density (r = 0.47) correlated significantly within individuals.

Conclusions

Amongst outbred mice, OCT-derived measurements of GCC thickness correlate significantly to RGC somal and axon abundance. Factors limiting correlation are likely both biological and methodological, including differences in retinal area that distort sampling-based estimates of RGC abundance.

Translational Relevance

There are significant-but imperfect-correlations between GCC thickness and RGC abundance across genetic contexts in mice, highlighting valid uses and ongoing challenges for meaningful use of OCT-derived metrics.

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.

Interpreting Retinal Nerve Fiber Layer Reflectance Defects Based on Presence of Retinal Nerve Fiber Bundles

SIGNIFICANCE

Adaptive-optics scanning-laser-ophthalmoscopy (AOSLO) retinal imaging of the retinal nerve fiber layer (RNFL) helps predict the severity of perimetric damage based on absence of fibers and projection of the defects in en face images of the RNFL from spectral-domain optical coherence tomography (SD-OCT).

PURPOSE

En face images of the RNFL reveal reflectance defects in patients with glaucoma and predict locations of perimetric defects. These defects could arise from either loss of retinal nerve fiber bundles or reduced bundle reflectance. This study used AOSLO to assess presence of bundles in areas with RNFL reflectance defects on SD-OCT.

METHODS

Adaptive-optics scanning laser ophthalmoscopy was used to image a vertical strip of RNFL measuring approximately 30 × 3° between the optic disc and the fovea. Fifteen patients with glaucoma who had SD-OCT reflectance defects that passed through this region were chosen. Four patients had reflectance defects in both superior and inferior hemifields, so presence of bundles on AOSLO was assessed for 19 hemifields. Where bundles were present, the hemifield was scored for whether bundles seemed unusual (low contrast and/or low density). Perimetric defects were considered deep when sensitivity was below 15 dB.

RESULTS

Ten hemifields had a region with no fibers present on AOSLO; all had a corresponding deep perimetric defect. The other nine hemifields had no region in the AOSLO image without fibers: four with normal fibers and five with unusual fibers. The only one of these nine hemifields with a deep perimetric defect was one with low-contrast fibers and overall thin RNFL.

CONCLUSIONS

Retinal nerve fiber layer reflectance defects, which were associated with deep perimetric defects, usually had a region with absence of fibers on AOSLO images of RNFL. Ability to predict severity of perimetric damage from en face SD-OCT RNFL reflectance images could benefit from quantification that differentiated between absence of fibers and unusual fibers.

Cellular-Scale Imaging of Transparent Retinal Structures and Processes Using Adaptive Optics Optical Coherence Tomography

High-resolution retinal imaging is revolutionizing how scientists and clinicians study the retina on the cellular scale. Its exquisite sensitivity enables time-lapse optical biopsies that capture minute changes in the structure and physiological processes of cells in the living eye. This information is increasingly used to detect disease onset and monitor disease progression during early stages, raising the possibility of personalized eye care. Powerful high-resolution imaging tools have been in development for more than two decades; one that has garnered considerable interest in recent years is optical coherence tomography enhanced with adaptive optics. State-of-the-art adaptive optics optical coherence tomography (AO-OCT) makes it possible to visualize even highly transparent cells and measure some of their internal processes at all depths within the retina, permitting reconstruction of a 3D view of the living microscopic retina. In this review, we report current AO-OCT performance and its success in visualizing and quantifying these once-invisible cells in human eyes.

A Common Glaucoma-risk Variant of SIX6 Alters Retinal Nerve Fiber Layer and Optic Disc Measures in a European Population: The EPIC-Norfolk Eye Study

PURPOSE

A common missense variant in the SIX6 gene (rs33912345) is strongly associated with primary open-angle glaucoma (POAG). We aimed to examine the association of rs33912345 with optic disc and retinal nerve fiber layer (RNFL) measures in a European population.

METHODS

We examined participants of the population-based EPIC-Norfolk Eye Study. Participants underwent confocal laser scanning tomography (Heidelberg Retina Tomograph II, HRT) to estimate optic disc rim area and vertical cup-disc ratio (VCDR). Scanning laser polarimetry (GDxVCC) was used to estimate average RNFL thickness. The mean of right and left eye values was considered for each participant. Genotyping was performed using the Affymetrix UK Biobank Axiom Array. Multivariable linear regression with the optic nerve head parameter as outcome variable and dosage of rs33912345 genotype as primary explanatory variable was used, adjusted for age, sex, disc area, axial length, and intraocular pressure. We further repeated analyses stratified into age tertiles.

RESULTS

In total, 5433 participants with HRT data and 3699 participants with GDxVCC data were included. Each "C" allele of rs33912345 was associated with a smaller rim area (-0.030 mm [95% CI -0.040, -0.020]; P=5.4×10), a larger VCDR (0.025 [95% CI 0.017, 0.033]; P=3.3×10) and a thinner RNFL (-0.39 μm [95% CI -0.62, -0.15]; P=0.001). The RNFL association was strongest in the oldest age tertile, whereas rim area and VCDR associations were strongest in the youngest and oldest age tertiles.

CONCLUSIONS

The protein-coding SIX6 variant rs33912345, previously associated with POAG, has a functional effect on glaucoma-associated optic nerve head traits in Europeans.

Temporal change of retinal nerve fiber layer reflectance speckle in normal and hypertensive retinas

This study investigated temporal change of retinal nerve fiber layer (RNFL) reflectance speckle in retinas with ocular hypertensive (OHT) damage and in control retinas from untreated eyes. Experimental OHT damage to rat retinas was induced by laser photocoagulation of the trabecular meshwork. A series of 660 nm reflectance images was collected from isolated retinas at 10-sec intervals. Areas containing speckled texture were selected on nerve fiber bundles. Correlation coefficients between images with different imaging delays were calculated and plotted as a function of delay. To evaluate the temporal change of speckles, decay of correlation coefficients with time was fitted with an exponential function characterized by a time constant τ. Reflectance per unit thickness (σ) of the areas was also measured and low σ was used as a surrogate of OHT damage. Speckle phenomena occurred in the control RNFL and the RNFL with reduced σ. In the control retinas, τ and σ were nearly constant along bundles but differed significantly among bundles in the same retinas. Among the control retinas, σ was similar, whereas τ varied significantly. In the retinas with OHT damage (low σ) τ could be within, greater or lower than the range in controls. The parameters τ and σ provide independent assessment of the RNFL with OHT damage. Measurements of temporal change of RNFL reflectance speckle may offer a method for detecting functional abnormality of the RNFL.

Peripapillary Retinoschisis in Glaucoma: Association With Progression and OCT Signs of Müller Cell Involvement

Purpose

To examine demographic and clinical factors associated with glaucomatous peripapillary retinoschisis (PPRS) and assess its association with glaucoma progression.

Methods

Using a case control study design and longitudinal data from a cohort of 166 subjects with a diagnosis of glaucoma or glaucoma suspect, we compared functional, structural, clinical, and demographic characteristics between PPRS cases and controls.

Results

The frequency of PPRS was 6.0% (12 eyes from 10/166 subjects) with two eyes having PPRS in different sectors for a total of 15 retinoschisis events. There were no significant differences (P > 0.05) in age, sex, visual acuity, central corneal thickness, intraocular pressure, or presence of vitreous adhesion between PPRS and controls. However, eyes with PPRS tended to have a higher cup-to-disc ratio (P = 0.06), thinner RNFL (P = 0.02), and worse visual field mean deviation (MD, P = 0.06) than controls. The rate of RNFL thinning was faster in PPRS (average: −2.8%/year; range: −7.4% to 0.0%/year) than controls (−1.3%/year; range: −4.4% to 0.6%/year; P = 0.021). The rate of visual field MD change was faster in PPRS (−0.49 dB/year; range: −2.0 to 0.9 dB/year) than controls (−0.06 dB/year; range: −0.8 to 0.3 dB/year; P = 0.030). OCT scans showed hyperreflective structures spanning the PPRS whose morphology and spacing (50 ± 7 μm) are consistent with Müller glia, causing signal attenuation casting “shadows” onto distal retina.

Conclusions

This is the first report showing that glaucomatous PPRS is associated with a faster overall rate of RNFL thinning and visual field deterioration and to specifically identify OCT signs of Müller cell involvement.

Evaluating glaucomatous abnormality in peripapillary optical coherence tomography enface visualisation of the retinal nerve fibre layer reflectance

Purpose

Optical coherence tomography (OCT) enface visualisation of the retinal nerve fibre layer (RNFL) reflectance has been found to have some advantages over retinal thickness measures. However, it is not yet clear how abnormalities on enface images relate to findings of abnormalities from other clinical measures such as the circumpapillary retinal nerve fibre layer thickness (cRNFLT). We developed a technique to analyse the RNFL reflectance on the OCT enface images, and to investigate its relation with the cRNFLT.

Methods

Spectralis (http://www.heidelbergengineering.com) OCT scans of the central retinal ±24° were analysed in the study eye of 31 controls and 33 patients, ages 61 (±9) and 69 (±8) years respectively. Enface slab‐images were extracted at 16–24, 24–36, and 24–52 μm from the inner limiting membrane in the temporal raphe, perifoveal and disc regions respectively. Reflectance probability maps were generated for the patients based on the control data. Glaucomatous abnormality was defined on the slab‐images when the slab‐area with reflectance abnormality was greater than the 95th percentile, and on the cRNFLT when the thickness measure was less than the fifth percentile, of that found in controls. The fraction of slab‐image showing reflectance abnormality was compared to cRNFLT in the patient group, using Spearman's rho. Agreement between the findings of abnormality based on cRNFLT and slab‐image reflectance was assessed using Cohen's kappa.

Results

Slab‐image and cRNFLT findings were in agreement for 26/33 eyes; four subjects showed cRNFLT abnormality but not slab‐image abnormality, and three subjects showed slab‐image abnormality but not cRNFLT abnormality. Spearman's rho found r_s(31) = −0.82. The reflectance findings and cRNFLT findings were consistent in 27/33 for both the superior temporal (ST) and inferior temporal (IT) sectors, and Cohen's kappa found 0.53 and 0.61 respectively.

Conclusion

The surface area of enface slab‐images showing RNFL reflectance were strongly related to the cRNFLT measures, and the classification of a subject with glaucoma based on enface reflectance findings and cRNFLT findings had a generally good agreement. The larger retinal area assessed by the enface method preserves the spatial location of the RNFL abnormalities, and makes the technique a useful approach for identifying regions of potential RNFL abnormality for targeted perimetry.

In Vivo Small Molecule Delivery to the Optic Nerve in a Rodent Model

Small molecule delivery to the optic nerve would allow for exploration of molecular and cellular pathways involved in normal physiology and optic neuropathies such as glaucoma, and provide a tool for screening therapeutics in animal models. We report a novel surgical method for small molecule drug delivery to the optic nerve head (ONH) in a rodent model. In proof-of-principle experiments, we delivered cytochalasin D (Cyt D; a filamentous actin inhibitor) to the junction of the superior optic nerve and globe in rats to target the actin-rich astrocytic cytoskeleton of the ONH. Cyt D delivery was quantified by liquid chromatography and mass spectrometry of isolated optic nerve tissue. One day after Cyt D delivery, anterior ONH filamentous actin bundle content was significantly reduced as assessed by fluorescent-tagged phalloidin labeling, relative to sham delivery. Anterior ONH nuclear counts and axon-specific beta-3 tubulin levels, as well as peripapillary retinal ganglion cell layer nuclear counts were not significantly altered after Cyt D delivery relative to sham delivery. Lastly, the surgical delivery technique caused minimal observable axon degeneration up to 10 days post-surgery. This small molecule delivery technique provides a new approach to studying optic neuropathies in in vivo rodent models.

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.

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.

Whole-globe biomechanics using high-field MRI

The eye is a complex structure composed of several interconnected tissues acting together, across the whole globe, to resist deformation due to intraocular pressure (IOP). However, most work in the ocular biomechanics field only examines the response to IOP over smaller regions of the eye. We used high-field MRI to measure IOP induced ocular displacements and deformations over the whole globe. Seven sheep eyes were obtained from a local abattoir and imaged within 48 h using MRI at multiple levels of IOP. IOP was controlled with a gravity perfusion system and a cannula inserted into the anterior chamber. T2-weighted imaging was performed to the eyes serially at 0 mmHg, 10 mmHg, 20 mmHg and 40 mmHg of IOP using a 9.4 T MRI scanner. Manual morphometry was conducted using 3D visualization software to quantify IOP-induced effects at the globe scale (e.g. axial length and equatorial diameters) or optic nerve head scale (e.g. canal diameter, peripapillary sclera bowing). Measurement sensitivity analysis was conducted to determine measurement precision. High-field MRI revealed an outward bowing of the posterior sclera and anterior bulging of the cornea due to IOP elevation. Increments in IOP from 10 to 40 mmHg caused measurable increases in axial length in 6 of 7 eyes of 7.9 ± 5.7% (mean ± SD). Changes in equatorial diameter were minimal, 0.4 ± 1.2% between 10 and 40 mmHg, and in all cases less than the measurement sensitivity. The effects were nonlinear, with larger deformations at normal IOPs (10-20 mmHg) than at elevated IOPs (20-40 mmHg). IOP also caused measurable increases in the nasal-temporal scleral canal diameter of 13.4 ± 9.7% between 0 and 20 mmHg, but not in the superior-inferior diameter. This study demonstrates that high-field MRI can be used to visualize and measure simultaneously the effects of IOP over the whole globe, including the effects on axial length and equatorial diameter, posterior sclera displacement and bowing, and even changes in scleral canal diameter. The fact that the equatorial diameter did not change with IOP, in agreement with previous studies, indicates that a fixed boundary condition is a reasonable assumption for half globe inflation tests and computational models. Our results demonstrate the potential of high-field MRI to contribute to understanding ocular biomechanics, and specifically of the effects of IOP in large animal models.

Assessment of retinal ganglion cell damage in glaucomatous optic neuropathy: Axon transport, injury and soma loss

Glaucoma is a disease characterized by progressive axonal pathology and death of retinal ganglion cells (RGCs), which causes structural changes in the optic nerve head and irreversible vision loss. Several experimental models of glaucomatous optic neuropathy (GON) have been developed, primarily in non-human primates and, more recently and commonly, in rodents. These models provide important research tools to study the mechanisms underlying glaucomatous damage. Moreover, experimental GON provides the ability to quantify and monitor risk factors leading to RGC loss such as the level of intraocular pressure, axonal health and the RGC population. Using these experimental models we are able to gain a better understanding of GON, which allows for the development of potential neuroprotective strategies. Here we review the advantages and disadvantages of the relevant and most often utilized methods for evaluating axonal degeneration and RGC loss in GON. Axonal pathology in GON includes functional disruption of axonal transport (AT) and structural degeneration. Horseradish peroxidase (HRP), rhodamine-B-isothiocyanate (RITC) and cholera toxin-B (CTB) fluorescent conjugates have proven to be effective reporters of AT. Also, immunohistochemistry (IHC) for endogenous AT-associated proteins is often used as an indicator of AT function. Similarly, structural degeneration of axons in GON can be investigated via changes in the activity and expression of key axonal enzymes and structural proteins. Assessment of axonal degeneration can be measured by direct quantification of axons, qualitative grading, or a combination of both methods. RGC loss is the most frequently quantified variable in studies of experimental GON. Retrograde tracers can be used to quantify RGC populations in rodents via application to the superior colliculus (SC). In addition, in situ IHC for RGC-specific proteins is a common method of RGC quantification used in many studies. Recently, transgenic mouse models that express fluorescent proteins under the Thy-1 promoter have been examined for their potential to provide specific and selective labeling of RGCs for the study of GON. While these methods represent important advances in assessing the structural and functional integrity of RGCs, each has its advantages and disadvantages; together they provide an extensive toolbox for the study of GON.