Aqueous Angiography with Fluorescein and Indocyanine Green in Bovine Eyes

Aqueous Humor Angiography

Aqueous humor angiography, or aqueous angiography, is an anterior segment imaging technique capable of visualizing the conventional/trabecular aqueous humor outflow pathways. As a translational technique, it is applicable for in vivo imaging in living subjects and ex vivo imaging using whole-globe preparations or anterior segment perfusion setups. Excellent spatial and temporal resolution has enabled insights into the segmental distribution of aqueous humor outflow in physiological conditions as well as after trabecular bypass surgery. In this chapter, we thoroughly describe aqueous humor angiography in various experimental setups. The necessary materials for different settings and their application for in vivo and ex vivo experiments as well as notes on dye choice, dye sequences, and special considerations on performing aqueous humor angiography during surgery are presented.

Robotic Visible-Light Optical Coherence Tomography Visualizes Segmental Schlemm's Canal Anatomy and Segmental Pilocarpine Response

Purpose

To use robotic visible-light OCT (vis-OCT) to study circumferential segmental Schlemm's canal (SC) anatomy in mice after topical pilocarpine administration.

Methods

Anterior segment imaging was performed using a vis-OCT sample arm attached to a 6-degree-of-freedom robotic arm to maintain normal (perpendicular) laser illumination aimed at SC around the limbus. Sixteen mice were studied for repeatability testing and to study aqueous humor outflow (AHO) pathway response to topical drug. Pharmaceutical-grade pilocarpine (1%; n = 5) or control artificial tears (n = 9) were given, and vis-OCT imaging was performed before and 15 minutes after drug application. After SC segmentation, SC areas and volumes were measured circumferentially in control- and drug-treated eyes.

Results

Circumferential vis-OCT provided high-resolution imaging of the anterior segment and AHO pathways, including SC. Segmental SC anatomy was visualized with the average cross-sectional area greatest temporal (3971 ± 328 μm2) and the least nasal (2727 ± 218 μm2; p = 0.018). After pilocarpine administration, the iris became flatter, and SC became larger (pilocarpine: 26.8 ± 5.0% vs. control: 8.9 ± 4.6% volume increase; p = 0.030). However, the pilocarpine alteration was segmental as well, with a greater increase observed superior (pilocarpine: 31.6 ± 8.9% vs. control: 1.8 ± 5.7% volume increase; p = 0.023) and nasal (pilocarpine: 41.1 ± 15.3% vs. control: 13.9 ± 4.5% volume increase; p = 0.045).

Conclusion

High-resolution circumferential non-invasive imaging using AS-OCT of AHO pathways is possible in living animals with robotic control. Segmental SC anatomy was seen at baseline and was consistent with the known segmental nature of trabecular AHO. Segmental SC anatomical response to a muscarinic agonist was seen as well. Segmental glaucoma drug response around the circumference of AHO pathways is a novel observation that may explain the variable patient response to glaucoma treatments.

Clinical applications of aqueous angiography in glaucoma

Aqueous humor outflow (AHO) pathways are the main site of resistance causing elevated intraocular pressure in glaucoma, especially primary open-angle glaucoma patients. With the recently introduced technique of aqueous angiography (AA); functional, real time assessment of AHO from proximal (trabecuar meshwork) to distal pathways under physiological conditions has been made possible. AHO pathways are segmental, and AA can identify high-flow region (increased angiographic signals) and low flow region (decreased angiographic signals) in an individual. With the introduction of canal-based minimally invasive glaucoma surgeries (MIGS), the assessment of AHO can help guide the placement of stents/incisions during MIGS procedures. This can allow individualized and targeted MIGS procedures in glaucoma patients for better results. Based on the density of AHO pathways visualized on AA, surgeons can decide whether to perform MIGS or conventional glaucoma surgery for improved outcomes for the patient. Immediate intraoperative assessment for functionality of the MIGS procedure performed is possible with AA, allowing for surgical adjustments of MIGS procedure in the same sitting, if needed. This review provides a summary of the studies performed with AA to date, with a special focus on Indian patients. It covers the basics and clinical applications of AA for improving surgical outcomes in glaucoma patients.

Using a Novel, Subconjunctival, Sustained-Release Mitomycin C Formulation in a Rabbit Model of Filtration Surgery with Gel Stent Implantation

Purpose: To investigate gel stent implantation with and without intraoperative sustained-release mitomycin C (MMC SR) in a rabbit model for gel stent implantation, and to examine aqueous humor outflow (AHO) postimplantation. Methods: Four groups of rabbits were included. Group 1 was untreated (control). Groups 2, 3, and 4 received the gel stent without MMC, with MMC solution (subconjunctival injection), and with MMC SR (subconjunctival injection), respectively. Intraocular pressure (IOP) and AHO were assessed via tonometry and indocyanine green-based angiography, respectively. The main efficacy measure was change in IOP from baseline. Results: Following gel stent implantation, Groups 2, 3, and 4 maintained ≥20% IOP reduction (response) for a median duration of 1 week, 6.5 weeks, and 30 weeks, respectively. Angiography showed normal aqueous humor drainage (Group 1) beginning at the perilimbal trabecular plexus and continuing posteriorly to episcleral outflow vessels. Following implantation, drainage occurred preferentially and directly into the subconjunctival bleb. Conclusions: Gel stent implantation with MMC SR was most effective in achieving sustained, long-term IOP reduction in the rabbit model, compared with implantation with or without MMC solution. Bleb presence and the postimplantation aqueous angiography results indicated redirection of the AHO to the subconjunctival vasculature and presumed lymphatics, suggesting efficient glaucoma filtration to lower IOP in this model. This rabbit model and aqueous angiography may help refine understanding of the mechanism of action of minimally invasive glaucoma surgeries and ultimately translate to improved surgical devices and procedures for patients with glaucoma.

Reduced Aqueous Humor Outflow Pathway Arborization in Childhood Glaucoma Eyes

Purpose

To compare aqueous humor outflow (AHO) pathway patterns between eyes of childhood glaucoma patients and non-glaucomatous patients receiving cataract surgery.

Methods

Aqueous angiography was performed in childhood glaucoma eyes (n = 5) receiving glaucoma surgery and in pediatric (n = 1) and healthy adult (n = 5) eyes receiving cataract surgery. Indocyanine green (0.4%) was introduced into the anterior chamber, and AHO was imaged using an angiographic camera (SPECTRALIS HRA+OCT with Flex Module). Images were acquired and analyzed (ImageJ with Analyze Skeleton 2D/3D plugin) from the nasal sides of the eyes, the usual site of glaucoma angle procedures. Image analysis endpoints included AHO vessel length, maximum vessel length, number of branches, number of branch junctions, and vessel density.

Results

Qualitatively, childhood glaucoma eyes demonstrated lesser AHO pathway arborization compared to pediatric and adult eyes without glaucoma. Quantitatively, childhood glaucoma and healthy adult cataract eyes showed similar AHO pathway average branch lengths and maximum branch lengths (P = 0.49-0.99). However, childhood glaucoma eyes demonstrated fewer branches (childhood glaucoma, 198.2 ± 35.3; adult cataract, 506 ± 59.5; P = 0.002), fewer branch junctions (childhood glaucoma, 74.6 ± 13.9; adult cataract, 202 ± 41.2; P = 0.019), and lower vessel densities (childhood glaucoma, 8% ± 1.4%; adult cataract, 17% ± 2.5%; P = 0.01).

Conclusions

Childhood glaucoma patients demonstrated fewer distal AHO pathways and lesser AHO pathway arborization. These anatomical alternations may result in a new source of trabecular meshwork-independent AHO resistance in this disease cohort.

Translational Relevance

Elevated distal outflow pathway resistance due to decreased AHO pathway arborization may explain some cases of failed trabecular bypass surgery in childhood glaucoma.

Segmental outflow dynamics in the trabecular meshwork of living mice

Aqueous humour does not drain uniformly through the trabecular meshwork (TM), but rather follows non-uniform or "segmental" routes. In this study, we examined whether segmental outflow patterns in the TM change over time in living mice and whether such changes are affected by age. Segmental outflow patterns were labelled by constant-pressure infusion of fluorescent tracer microparticles into the anterior chamber of anesthetised C57BL/6J mice at 3 or 8 months of age. Two different tracer colours were infused at separate time points with an interval of Δt = 0, 2, 7 or 14 days. In a separate experiment, one tracer was infused in vivo while the second tracer was infused ex vivo after 2 days. The spatial relationship between the two tracer patterns was analysed using the Pearson's correlation coefficient, r. In 3-month-old mice, there was a time-dependent decay in r, which was near unity at Δt = 0 and near zero at Δt = 14 days. In 8-month-old mice, r remained elevated for 14 days. Segmental outflow patterns measured in young mice ex vivo were not significantly different from those measured in vivo after accounting for the expected changes over 2 days. Therefore, segmental outflow patterns are not static in the TM but redistribute over time, achieving near complete loss of correlation by 2 weeks in young healthy mice. There is an age-related decline in the rate at which segmental outflow patterns redistribute in the TM. Further research is needed to understand the dynamic factors controlling segmental outflow.

Subconjunctival Lymphatics Respond to VEGFC and Anti-Metabolites in Rabbit and Mouse Eyes

Purpose

To characterize and pharmacologically influence subconjunctival lymphatics in rabbit and mouse eyes.

Methods

Rabbits received subconjunctival injections of trypan blue or fixable fluorescent dextrans. Bleb-related outflow pathways were quantified. Immunofluorescence for vessel-specific markers (lymphatics [podoplanin and LYVE-1] and blood vessels [CD31]) were performed in native rabbit conjunctiva and after fixable fluorescent dextran injection. Vascular endothelial cell growth factor-C (VEGFC) was injected subconjunctivally in rabbits. mRNA and protein were assessed for the above markers using RT-PCR and Western blot. Alternatively, mouse studies used Prox1-tdTomato transgenic reporter mice. Subconjunctival injection conditions included: no injection, balanced salt solution (BSS), VEGFC, 5-fluorouracil (5FU) and two concentrations of mitomycin-C (MMC). Two mouse injection protocols (short and long) with different follow-up times and number of injections were performed. Mouse eyes were enucleated, flat mounts created, and subconjunctival branching and length assessed.

Results

Rabbit eyes demonstrated clear bleb-related subconjunctival outflow pathways that were distinct from blood vessels and were without nasal/temporal predilection. Immunofluorescence against vessel-specific markers showed lymphatics and blood vessels in rabbit conjunctiva, and these lymphatics overlapped with bleb-related subconjunctival outflow pathways. Subconjunctival VEGFC increased lymphatic (P = 0.004-0.04) but not blood vessel (P = 0.77-0.84) mRNA or protein in rabbits. Prox1-tdTomato transgenic reporter mice demonstrated natively fluorescent lymphatics. Subconjunctival VEGFC increased murine lymphatic branching and length (P ≤ 0.001-0.004) while antimetabolites (P ≤ 0.001-0.043) did the opposite for the long protocol.

Discussion

Subconjunctival lymphatics are pharmacologically responsive to both VEGFC and antimetabolites in two animal models studied using different methodologies. These results may be important for bleb-forming glaucoma surgeries or ocular drug delivery.

Schlemm’s Canal Endothelium Cellular Connectivity in Giant Vacuole and Pore Formation in Different Flow-type Areas: A Serial Block-Face Scanning Electron Microscopy Study

Glaucoma is associated with increased resistance in the conventional aqueous humor (AH) outflow pathway of the eye. The majority of resistance is thought to reside in the juxtacanalicular connective tissue (JCT) region of the trabecular meshwork and is modulated by the inner wall (IW) endothelial cells of Schlemm’s canal (SC). The IW cells form connections with the underlying JCT cells/matrix, and these connections are thought to modulate outflow resistance. Two ways by which AH crosses the IW endothelium are through: 1) the formation of outpouchings in IW cells called giant vacuoles (GVs) and their intracellular pores (I-pores), and 2) intercellular pores between two adjacent IW cells (B-pores). AH outflow is segmental with areas of high-, low-, and non-flow around the circumference of the eye. To investigate whether changes in cellular connectivity play a role in segmental outflow regulation, we used global imaging, serial block-face scanning electron microscopy (SBF-SEM), and 3D reconstruction to examine individual IW cells from different flow areas of ex vivo perfused normal human donor eyes. Specifically, we investigated the differences in cellular dimensions, connections with JCT cells/matrix, GVs, and pores in SC IW cells between high-, low-, and non-flow areas. Our data showed that: 1) IW cell-JCT cell/matrix connectivity was significantly decreased in the cells in high-flow areas compared to those in low- and non-flow areas; 2) GVs in the cells of high-flow areas had significantly fewer connections beneath them compared to GVs in the cells of low- and non-flow areas; 3) Type IV GVs (with I-pores and basal openings) had significantly fewer connections beneath them compared to Type I GVs (no I-pore or basal opening). Our results suggest that a decreased number of cellular connections between the IW and JCT in high-flow areas is associated with increased numbers of GVs with I-pores and larger Type IV GVs observed in previous studies. Therefore, modulating the number of cellular connections may affect the amount of high-flow area around the eye and thereby modulate AH outflow.

Aqueous angiography in pre-glaucomatous and glaucomatous ADAMTS10-mutant canine eyes: A pilot study

OBJECTIVE

To evaluate intravenous scleral and intracameral aqueous angiography in normotensive (n = 4) and hypertensive glaucomatous (n = 6) ADAMTS10-mutant canine eyes.

ANIMALS STUDIED

Ten ADAMTS10-mutant dogs were used in this study.

PROCEDURES

Dogs were sedated and one eye from each dog underwent scleral angiography following intravenous injection of 0.25% indocyanine green (ICG). After a 24-h recovery period, the same eye underwent aqueous angiography via intracameral administration of ICG. Imaging of identical scleral sectors from the same eye was performed using a Heidelberg Spectralis^® Confocal Scanning Laser Ophthalmoscope. Intrascleral vessel depth and lumen diameters were measured using Heidelberg Spectralis^® optical coherence tomography and computer software.

RESULTS

Scleral angiography permitted visualization of vascular components associated with conventional aqueous humor outflow pathways with an average time from injection to fluorescence of 35.8 ± 10.6 s (mean ± SD). Two normotensive eyes (2/10;20%) demonstrated turbulent dye movement, while 4 hypertensive eyes (4/10;40%) exhibited laminar flow. Aqueous angiography demonstrated dye fluorescence within the post-trabecular conventional aqueous humor outflow pathways in all 10 eyes at 34.3 ± 11.0 s post-injection. Sectoral and dynamic outflow patterns were observed primarily within the superotemporal sector in nine eyes (9/10; 90%). Seven eyes (7/10; 70%) demonstrated pulsatile dye movement and five eyes (5/10; 50%) exhibited laminar flow. The degree of laminar movement of dye was greatest in hypertensive eyes. Vessel lumen diameters measured 133.85 ± 28.36 µm and 161.18 ± 6.02 µm in hypertensive and normotensive eyes, respectively.

CONCLUSIONS

Aqueous angiography allowed for visualization of fluorescent dye in the superotemporal sclera. Laminar flow and smaller lumen vessels were observed mainly in hypertensive eyes.

Development and validation of methods to visualize conventional aqueous outflow pathways in canine primary angle closure glaucoma

PURPOSE

Angle closure glaucoma (PACG) is highly prevalent in dogs and is often refractory to medical therapy. We hypothesized that pathology affecting the post-trabecular conventional aqueous outflow pathway contributes to persistent intraocular pressure (IOP) elevation in dogs with PACG. The goal of this study was to determine the potential for aqueous angiography (AA) and optical coherence tomography (OCT) to identify abnormalities in post-trabecular aqueous outflow pathways in canine PACG.

METHODS

AA and anterior segment OCT (Spectralis HRA + OCT) were performed ex vivo in 19 enucleated canine eyes (10 normal eyes and 9 irreversibly blind eyes from canine patients enucleated for management of refractory PACG). Eyes were cannulated and maintained at physiologic IOP (10-20 mmHg) prior to intracameral infusion of fluorescent tracer. OCT scleral line scans were acquired in regions of high and low perilimbal AA signal. Eyes were then perfusion fixed and cryosections prepared from 10/10 normal and 7/9 PACG eyes and immunolabeled for a vascular endothelial marker.

RESULTS

Normal canine eyes showed segmental, circumferential limbal AA signal, whereas PACG eyes showed minimal or no AA signal. AA signal correlated with scleral lumens on OCT in normal dogs, but lumens were generally absent or flattened in PACG eyes. Collapsed vascular profiles were identified in tissue sections from PACG eyes, including those in which no lumens were identified on AA and OCT.

CONCLUSIONS

In canine eyes with PACG, distal aqueous outflow channels are not identifiable by AA, despite normalization of their IOP, and intra-scleral vascular profiles are collapsed on OCT and histopathology.

Aqueous outflow regulation - 21st century concepts

We propose an integrated model of aqueous outflow control that employs a pump-conduit system in this article. Our model exploits accepted physiologic regulatory mechanisms such as those of the arterial, venous, and lymphatic systems. Here, we also provide a framework for developing novel diagnostic and therapeutic strategies to improve glaucoma patient care. In the model, the trabecular meshwork distends and recoils in response to continuous physiologic IOP transients like the ocular pulse, blinking, and eye movement. The elasticity of the trabecular meshwork determines cyclic volume changes in Schlemm's canal (SC). Tube-like SC inlet valves provide aqueous entry into the canal, and outlet valve leaflets at collector channels control aqueous exit from SC. Connections between the pressure-sensing trabecular meshwork and the outlet valve leaflets dynamically control flow from SC. Normal function requires regulation of the trabecular meshwork properties that determine distention and recoil. The aqueous pump-conduit provides short-term pressure control by varying stroke volume in response to pressure changes. Modulating TM constituents that regulate stroke volume provides long-term control. The aqueous outflow pump fails in glaucoma due to the loss of trabecular tissue elastance, as well as alterations in ciliary body tension. These processes lead to SC wall apposition and loss of motion. Visible evidence of pump failure includes a lack of pulsatile aqueous discharge into aqueous veins and reduced ability to reflux blood into SC. These alterations in the functional properties are challenging to monitor clinically. Phase-sensitive OCT now permits noninvasive, quantitative measurement of pulse-dependent TM motion in humans. This proposed conceptual model and related techniques offer a novel framework for understanding mechanisms, improving management, and development of therapeutic options for glaucoma.

Incidence of steroid response in microinvasive glaucoma surgery with trabecular microbypass stent and ab interno trabeculectomy

OBJECTIVE

To determine the incidence and risk factors for steroid response in patients undergoing combined phacoemulsification cataract extraction (PCE) and microinvasive glaucoma surgery with either trabecular microbypass stent implantation (iStent) or ab interno trabeculectomy (Trabectome).

DESIGN

Retrospective, noncomparative, single-institutional observational chart review.

PARTICIPANTS

Consecutive patients with open-angle glaucoma who underwent PCE with iStent or Trabectome with 3 months of follow-up.

METHODS

Data were collected from patient charts, including pre- and postoperative intraocular pressure (IOP) following application of topical corticosteroid on postoperative visits for at least 3 months. A steroid response was defined as an IOP rise of greater than 5 mm Hg beginning at least 3 days after surgery with no other obvious explanation and with IOP < 20 mm Hg following rapid tapering or withdrawal of the steroid.

RESULTS

A total of 118 eyes from 89 patients, average age of 71.4 ±12.1 years, were included. Overall, a steroid response was seen in 12.7% of eyes (n = 15), and no difference was noted between Trabectome (11.8%) and iStent (13.6%, p = 0.782) eyes. Axial length (AL; p = 0.01), younger age (p = 0.009), traumatic glaucoma (p = 0.004), and normal-tension glaucoma (NTG; p = 0.0048) were significant predictors of steroid response in a multivariate analysis. In eyes with AL ≥ 25 mm, the steroid response rate was 40%, in contrast to eyes with AL < 25 mm, where it was 10.2%.

CONCLUSION

A steroid response develops in approximately 1 in 8 patients undergoing PCE with Trabectome or iStent. Young age, AL > 25 mm, traumatic glaucoma, and NTG were found to be significant predictors of steroid response.

Aqueous humour outflow imaging: seeing is believing

Elevated intraocular pressure (IOP) is the primary risk factor for blindness in glaucoma. IOP is determined by many factors including aqueous humour production and aqueous humour outflow (AHO), where AHO disturbance represents the primary cause of increased IOP. With the recent development of new IOP lowering drugs and Minimally Invasive Glaucoma Surgeries (MIGS), renewed interest has arisen in shedding light on not only how but where AHO is occurring for the trabecular/conventional, uveoscleral/unconventional, and subconjunctival outflow pathways. Historical studies critical to understanding outflow anatomy will be presented, leading to the development of modern imaging methods. New biological behaviours uncovered by modern imaging methods will be discussed with relevance to glaucoma therapies emphasized.

Aqueous outflow imaging techniques and what they tell us about intraocular pressure regulation

Recent advances in the medical and surgical management of open-angle glaucoma have increased the number of treatment options available. Several new intraocular pressure (IOP)-lowering treatments target the conventional aqueous outflow (AO) system. However, success rates are variable and outcomes in individual patients are often difficult to predict. Variable treatment responses remain unexplained and highlight deficiencies in our current understanding of AO regulation and IOP homeostasis. Imaging is often relied upon to confirm diagnoses and monitor treatment responses in other ocular and systemic pathologies. As yet no suitable AO imaging tool has been developed to fulfil this role in glaucoma. A variety of imaging techniques have been used to study the AO tracts of humans and animals in ex vivo and in vivo eyes. In this review, results from novel imaging techniques that assess aqueous drainage through the episcleral venous system are considered and we argue these provide new insights into AO regulation. We suggest that the ability to objectively measure AO responses to interventions would be a significant clinical advance, and we have demonstrated that this can be achieved with direct visualisation of aqueous drainage. We predict that the evolution of AO imaging technology will continue to reveal critical components of AO and IOP regulation, and that personalised IOP-lowering treatment in glaucoma care may well become a reality in the near future.

Aqueous Angiography in Normal Canine Eyes

Purpose

To conduct aqueous angiography (AA) using a clinically applicable technique in normal dogs and to compare findings to intravenous scleral angiography (SA).

Methods

We examined 10 canine cadaver eyes and 12 eyes from live normal dogs. A gravity-fed trocar system delivered 2% sodium fluorescein and 0.25% indocyanine green (ICG) intracamerally (IC) in cadaver eyes. In vivo AA was subsequently performed in one eye of each of the 12 dogs via IC bolus of ICG under sedation. The same 12 dogs received SA via intravenous ICG (mean ± SD) 10.7 ± 3.3 days later. Identical scleral sectors were imaged using a Spectralis confocal scanning laser ophthalmoscope.

Results

The gravity-fed trocar system permitted visualization of the conventional aqueous humor outflow (CAHO) pathways in cadaver eyes, but not in vivo. Fluorescence was observed superonasally in four of the 10 cadaver eyes within 24.0 ± 3.6 seconds. A single IC bolus of ICG showed CAHO pathways in vivo, demonstrating sectoral outflow patterns in the superotemporal sclera in 10 of the 12 eyes within 35.0 ± 4.3 seconds; four of the 12 eyes exhibited pulsatile aqueous movement. SA exhibited fluorescence patterns comparable to AA with weak pulsatile aqueous humor outflow.

Conclusions

Angiography (AA or SA) in dogs permits visualization of the CAHO pathway and its vascular components in vivo. AA may be a more useful modality to assess aqueous humor outflow.

Translational Relevance

Intracameral AA has potential utility for evaluating CAHO in vivo in dogs, an important animal model species.

Gravitational Influence on Intraocular Pressure: Implications for Spaceflight and Disease

Spaceflight-associated neuro-ocular syndrome (SANS) describes a series of morphologic and functional ocular changes in astronauts first reported by Mader and colleagues in 2011. SANS is currently clinically defined by the development of optic disc edema during prolonged exposure to the weightless (microgravity) environment, which currently occurs on International Space Station (ISS). However, as improvements in our understanding of the ocular changes emerge, the definition of SANS is expected to evolve. Other ocular SANS signs that arise during and after ISS missions include hyperopic shifts, globe flattening, choroidal/retinal folds, and cotton wool spots. Over the last 10 years, ~1 in 3 astronauts flying long-duration ISS missions have presented with ≥1 of these ocular findings. Commensurate with research that combines disparate specialties (vision biology and spaceflight medicine), lessons from SANS investigations may also yield insight into ground-based ocular disorders, such as glaucomatous optic neuropathy that may have the potential to lessen the burden of this irreversible cause of vision loss on Earth.

Functional, structural, and molecular identification of lymphatic outflow from subconjunctival blebs

The purpose of this study is to evaluate outflow pathways from subconjunctival blebs and to identify their identity. Post-mortem porcine (n = 20), human (n = 1), and bovine (n = 1) eyes were acquired, and tracers (fluorescein, indocyanine green, or fixable/fluorescent dextrans) were injected into the subconjunctival space to create raised blebs where outflow pathways were visualized qualitatively and quantitatively. Rodents with fluorescent reporter transgenes were imaged for structural comparison. Concurrent optical coherence tomography (OCT) was obtained to study the structural nature of these pathways. Using fixable/fluorescent dextrans, tracers were trapped to the bleb outflow pathway lumen walls for histological visualization and molecular identification using immunofluorescence against lymphatic and blood vessel markers. Bleb outflow pathways could be observed using all tracers in all species. Quantitative analysis showed that the nasal quadrant had more bleb-related outflow pathways compared to the temporal quadrant (nasal: 1.9±0.3 pathways vs. temporal: 0.7±0.2 pathways; p = 0.003). However, not all blebs resulted in an outflow pathway (0-pathways = 18.2%; 1-pathway = 36.4%; 2-pathways = 38.6%; and 3-pathways = 6.8%). Outflow signal was validated as true luminal pathways using optical coherence tomography and histology. Bicuspid valves were identified in the direction of flow in porcine eyes. Immunofluorescence of labeled pathways demonstrated a lymphatic (Prox-1 and podoplanin) but not a blood vessel (CD31) identity. Therefore, subconjunctival bleb outflow occurs in discrete luminal pathways. They are lymphatic as assessed by structural identification of valves and molecular identification of lymphatic markers. Better understanding of lymphatic outflow may lead to improved eye care for glaucoma surgery and ocular drug delivery.

Segmental differences found in aqueous angiographic-determined high - and low-flow regions of human trabecular meshwork

This work sought to compare aqueous angiographic segmental patterns with bead-based methods which directly visualize segmental trabecular meshwork (TM) tracer trapping. Additionally, segmental protein expression differences between aqueous angiographic-derived low- and high-outflow human TM regions were evaluated. Post-mortem human eyes (One Legacy and San Diego eye banks; n = 15) were perfused with fluorescent tracers (fluorescein [2.5%], indocyanine green [0.4%], and/or fluorescent microspheres). After angiographic imaging (Spectralis HRA+OCT; Heidelberg Engineering), peri-limbal low- and high-angiographic flow regions were marked. Aqueous angiographic segmental outflow patterns were similar to fluorescent microsphere TM trapping segmental patterns. TM was dissected from low- and high-flow areas and processed for immunofluorescence or Western blot and compared. Versican expression was relatively elevated in low-flow regions while MMP3 and collagen VI were relatively elevated in high-flow regions. TGF-β2, thrombospondin-1, TGF-β receptor1, and TGF-β downstream proteins such as α-smooth muscle actin were relatively elevated in low-flow regions. Additionally, fibronectin (FN) levels were unchanged, but the EDA isoform (FN-EDA) that is associated with fibrosis was relatively elevated in low-flow regions. These results show that segmental aqueous angiographic patterns are reflective of underlying TM molecular characteristics and demonstrate increased pro-fibrotic activation in low-flow regions. Thus, we provide evidence that aqueous angiography outflow visualization, the only tracer outflow imaging method available to clinicians, is in part representative of TM biology.

Minimally Invasive Glaucoma Surgery: What Do We Know? Where Should We Go?

With the arrival of a plethora of new and revolving minimally invasive glaucoma surgery techniques, glaucoma specialists currently are fortunate to have various surgical options that aim to recovery of the function of the aqueous outflow system in different ways. Meanwhile, the aqueous outflow system has become the hot point of researching. In ARVO 2019, a special interest group session was held on new perspectives on minimally invasive glaucoma surgery. Ten surgeons, clinical professors, and experimental scientists were invited to report their latest studies and discussed on five hot topics in this special interest group. This review summarizes the special interest group session and posts the issues of greatest concern, providing insight to the aqueous outflow system and areas that require further study.

Cellular and cytoskeletal alterations of scleral fibroblasts in response to glucocorticoid steroids

Steroid-induced ocular hypertension can be seen even after trabecular meshwork (TM) bypass/ablation. Thus, the purpose was to investigate steroid-response in cells distal to the TM by using primary scleral fibroblasts. Primary scleral cell cultures were generated using mid-depth scleral wedges from human donor corneo-scleral rims (n = 5) after corneal transplantation. Cells were treated with dexamethasone (DEX; 100 nM) and compared to media (MED)/vehicle (DMSO) controls. Cell size, shape, and migration were studied using the IncuCyte Live-Cell Analysis System. Cytoskeleton was compared using Alexa Fluor-568 Phalloidin and senescence tested by evaluating beta-galactosidase. Western blot comparison was performed for α-SMA, FKBP-51, fibronectin, phospho-myosin light chain, and myocilin. Scleral fibroblasts upregulated FKBP-51 in response to DEX indicating the existence of steroid-responsive pathways. Compared to controls, DEX-treated cells proliferated slower (~50%; p < 0.01-0.02), grew larger (~1.3-fold; p < 0.001), and migrated less (p = 0.01-0.006). Alexa Fluor 568 Phalloidin actin stress fiber labeling was more diffuse in DEX-treated cells (p = 0.001-0.004). DEX-treated cells showed more senescence compared to controls (~1.7-fold; p = 0.01-0.02). However, DEX-treated cells did not show increased cross-linked actin network formation or elevated myocilin/fibronectin/α-SMA/phospho-myosin light chain protein expression. For all parameters, MED- and DMSO-treated control cells were not significantly different. Primary scleral fibroblasts, grown from tissue collected immediately distal to the TM, demonstrated scleral-response behaviors that were similar to, but not identical with, classic TM steroid-response. Further study is needed to understand how these scleral cellular alterations may contribute to steroid-response IOP elevation after TM bypass/ablation surgery.

Imaging Distal Aqueous Outflow Pathways in a Spontaneous Model of Congenital Glaucoma

PURPOSE

To validate the use of aqueous angiography (AA) in characterizing distal aqueous outflow pathways in normal and glaucomatous cats.

METHODS

Ex vivo AA and optical coherence tomography (OCT) were performed in nine adult cat eyes (5 feline congenital glaucoma [FCG] and 4 normal), following intracameral infusion of 2.5% fluorescein and/or 0.4% indocyanine green (ICG) at physiologic intraocular pressure (IOP). Scleral OCT line scans were acquired in areas of high- and low-angiographic signal. Tissues dissected in regions of high- and low-AA signal, were sectioned and hematoxylin and eosin (H&E)-stained or immunolabeled (IF) for vascular endothelial and perivascular cell markers. Outflow vessel numbers and locations were compared between groups by Student's t-test.

RESULTS

AA yielded circumferential, high-quality images of distal aqueous outflow pathways in normal and FCG eyes. No AA signal or scleral lumens were appreciated in one buphthalmic FCG eye, though collapsed vascular profiles were identified on IF. The remaining eight of nine eyes all showed segmental AA signal, distinguished by differences in time of signal onset. AA signal always corresponded with lumens seen on OCT. Numbers of intrascleral vessels were not significantly different between groups, but scleral vessels were significantly more posteriorly located relative to the limbus in FCG.

CONCLUSIONS

A capacity for distal aqueous humor outflow was confirmed by AA in FCG eyes ex vivo but with significant posterior displacement of intrascleral vessels relative to the limbus in FCG compared with normal eyes.

TRANSLATIONAL RELEVANCE

This report provides histopathologic correlates of advanced diagnostic imaging findings in a spontaneous model of congenital glaucoma.

Precision medicine to prevent glaucoma-related blindness

PURPOSE OF REVIEW

Approximately 10% of patients become blind despite using evidence-based guidelines developed from clinical trials and epidemiology studies. Our purpose is to review opportunities to decrease glaucoma-related blindness using the emerging principles of precision medicine.

RECENT FINDINGS

The current review focuses on three topics: first, candidate biomarkers for angle-based surgeries, second, head-mounted display (HMD) technology for vision and testing, and third, glaucoma risk alleles discovered by genome-wide association studies. First, in angle-based surgeries, tracers injected into the anterior chamber or Schlemm's canal have allowed visualization of aqueous veins. We describe an innovative use of optical coherence tomography angiography to visualize aqueous veins in a case with 6-year successful outcome following catheter-based trabeculotomy. Second, HMD technology can augment perceived vision and can be used for perimetry testing. Third, developing genetic risk scores that characterize patients who are at highest risk for blindness is a priority. Such biomarker risk scores will integrate genome-wide association study-based risk alleles for glaucoma along with well known demographic and clinical risk factors.

SUMMARY

As we gain more knowledge, precision medicine will enable clinicians to decrease glaucoma-related blindness by providing more timely interventions to those patients who are at highest risk for progression to blindness. VIDEO ABSTRACT: http://links.lww.com/COOP/A29.

Visual Assessment of Aqueous Humor Outflow

In the past decade, many new pharmacological and surgical treatments have become available to lower intraocular pressure (IOP) for glaucoma. The majority of these options have targeted improving aqueous humor outflow (AHO). At the same time, in addition to new treatments, research advances in AHO assessment have led to the development of new tools to structurally assess AHO pathways and to visualize where aqueous is flowing in the eye. These new imaging modalities have uncovered novel AHO observations that challenge traditional AHO concepts. New behaviors including segmental, pulsatile, and dynamic AHO may have relevance to the disease and the level of therapeutic response for IOP-lowering treatments. By better understanding the regulation of segmental, pulsatile, and dynamic AHO, it may be possible to find new and innovative treatments for glaucoma aiming at these new AHO behaviors.

High-Resolution, Three-Dimensional Reconstruction of the Outflow Tract Demonstrates Segmental Differences in Cleared Eyes

Purpose

The rate of conventional aqueous humor outflow is the highest nasally. We hypothesized that this is reflected in regionally different outflow structures and analyzed the entire limbus by high-resolution, full-thickness ribbon-scanning confocal microscopy (RSCM).

Methods

We perfused pig eyes by anterior chamber cannulation with eight lectin-fluorophore conjugates, followed by optical clearance with benzyl alcohol benzyl benzoate (BABB). RSCM and advanced analysis software (Imaris) were used to reconstruct a three-dimensional (3D), whole-specimen rendering of the perilimbal outflow structures. We performed morphometric analyses of the outflow tract from the level of the trabecular meshwork (TM) to the scleral vascular plexus (SVP).

Results

Except for pigmented structures, BABB cleared the entire eye. Rhodamine-conjugated Glycine max agglutinin (soybean [SBA]) labeled the outflow tract evenly and retained fluorescence for months. RSCM produced terabyte-sized files allowing for in silico dissection of outflow tract vessels at a high resolution and in 3D. Networks of interconnected lumens were traced from the TM to downstream drainage structures. The collector channel (CC) volumes were 10 times smaller than the receiving SVP vessels, the largest of which were in the inferior limbus. Proximal CC diameters were up to four times the size of distal diameters and more elliptical at their proximal ends. The largest CCs were found in the superonasal and inferonasal quadrants where the highest outflow occurs.

Conclusion

RSCM of cleared eyes enabled high-resolution, volumetric analysis of the outflow tract. The proximal structures had greater diameters nasally, whereas the SVP was larger in the inferior limbus.

Aqueous Angiographic Outflow Improvement after Trabecular Microbypass in Glaucoma Patients

Purpose

To study changes in aqueous humor outflow (AHO) patterns after trabecular micro-bypass (TMB) in glaucoma patients using intraoperative sequential aqueous angiography.

Design

Prospective comparative case series.

Subjects

Fifteen subjects (14 with glaucoma and 1 normal).

Methods

Sequential aqueous angiography (Spectralis HRA+OCT; Heidelberg Engineering) was performed on fourteen glaucoma patients undergoing routine TMB (iStent Inject; Glaukos Corporation) and cataract surgery and one normal patient undergoing cataract surgery alone. Indocyanine green (ICG) aqueous angiography established initial baseline nasal angiographic AHO patterns. Two TMB stents were placed in regions of baseline low or high angiographic AHO in each eye (n = 2 eyes with enough space to place two stents in both low angiographic regions; n = 8 eyes with two stents both placed in high angiographic regions; n = 4 eyes with enough space to place one stent in a low angiographic region and the other stent in a high angiographic region). Subsequent fluorescein aqueous angiography was utilized to query alterations to angiographic AHO patterns.

Main Outcome Measure

Angiographic signal and patterns before and after TMB.

Results

At baseline, all eyes showed segmental angiographic AHO patterns. Focused on the nasal hemisphere of each eye, for each stent TMB in initially low ICG angiographic signal regions showed transient or persistently improved fluorescein angiographic signal (11.2-fold; p = 0.014). TMB in initially high ICG signal regions led to faster development of fluorescein angiographic patterns (3.1-fold; p = 0.02).

Conclusion

TMB resulted in different patterns of aqueous angiographic AHO improvement whose further understanding may advance basic knowledge of AHO and possibly enhance intraocular pressure reduction after glaucoma surgery in the future.

Fluorescein Aqueous Angiography in Live Normal Human Eyes

PURPOSE

To evaluate aqueous humor outflow (AHO) in intact eyes of live human subjects during cataract surgery using fluorescein aqueous angiography.

METHODS

Aqueous angiography was performed in 8 live human subjects (56 to 86 y old; 2 men and 6 women). After anesthesia, fluorescein (2%) was introduced into the eye [either alone or after indocyanine green (ICG; 0.4%)] from a sterile, gravity-driven constant-pressure reservoir. Aqueous angiographic images were obtained with a Spectralis HRA+OCT and FLEX module (Heidelberg Engineering). Using the same device, anterior-segment optical coherence tomography (OCT) and infrared images were also concurrently taken with aqueous angiography.

RESULTS

Fluorescein aqueous angiography in the live human eye showed segmental AHO patterns. Initial angiographic signal was seen on average by 14.0±3.0 seconds (mean±SE). Using multimodal imaging, angiographically positive signal colocalized with episcleral veins (infrared imaging) and intrascleral lumens (anterior-segment OCT). Sequential aqueous angiography with ICG followed by fluorescein showed similar segmental angiographic patterns.

DISCUSSION

Fluorescein aqueous angiography in live humans was similar to that reported in nonhuman primates and to ICG aqueous angiography in live humans. As segmental patterns with sequential angiography using ICG followed by fluorescein were similar, these tracers can now be used sequentially, before and after trabecular outflow interventions, to assess their effects on AHO in live human subjects.

Microcatheter-assisted Trabeculotomy for Primary Congenital Glaucoma After Failed Glaucoma Surgeries

PURPOSE

To evaluate the effectiveness of microcatheter-assisted trabeculotomy (MAT) to treat primary congenital glaucoma after failed previous glaucoma surgeries.

MATERIALS AND METHODS

Retrospective, noncomparative, interventional case series conducted at Beijing Tongren Eye Center, China. Outcome measures were compared between 3 groups: successful and complete (≥330 degrees) MAT; successful and partial (<330 degrees) MAT; or cases converted to traditional trabeculotomy when the Schlemm's canal could not be catheterized >180 degrees. Success was defined as final intraocular pressure ≤21 mm Hg, with (qualified success) or without (complete success) glaucoma medications.

RESULTS

In total, 74 eyes of 63 consecutive patients were included. MAT was performed in 50 eyes (67.6%). Postoperative intraocular pressure and number of glaucoma drops (17.7±8.6 mm Hg, 0.6±1.2 medications) was significantly less than the preoperative values (35.3±7.2 mm Hg, 2.7±0.8 medications; P<0.001). Cumulative probabilities of qualified and complete success were 84.0% and 80.0% at 3-year follow-up with no difference between complete and partial trabeculotomies. MAT was not successfully performed in 24 eyes (32.4%), requiring conversion to traditional trabeculotomy and associated with greater incidence of previous surgeries (P<0.001), earlier age of disease onset (P=0.024) and worse corneal transparency (P=0.010). Cumulative probabilities of qualified and complete success were 37.0% and 29.2% at 3-year follow-up.

CONCLUSIONS

Both complete and partial MAT achieved significant pressure reduction in cases of primary congenital glaucoma with previous failed glaucoma surgeries in intermediate term.

Structural and functional imaging of aqueous humour outflow: a review

Maintaining healthy aqueous humour outflow (AHO) is important for intraocular cellular health and stable vision. Impairment of AHO can lead to increased intraocular pressure, optic nerve damage and concomitant glaucoma. An improved understanding of AHO will lead to improved glaucoma surgeries that enhance native AHO as well as facilitate the development of AHO-targeted pharmaceuticals. Recent AHO imaging has evolved to live human assessment and has focused on the structural evaluation of AHO pathways and the functional documentation of fluid flow. Structural AHO evaluation is predominantly driven by optical coherence tomography, and functional evaluation of flow is performed using various methods, including aqueous angiography. Advances in structural and functional evaluation of AHO are reviewed with discussion of strengths, weaknesses and potential future directions.

Automated circumferential construction of first-order aqueous humor outflow pathways using spectral-domain optical coherence tomography

The purpose was to create a three-dimensional (3-D) model of circumferential aqueous humor outflow (AHO) in a living human eye with an automated detection algorithm for Schlemm’s canal (SC) and first-order collector channels (CC) applied to spectral-domain optical coherence tomography (SD-OCT). Anterior segment SD-OCT scans from a subject were acquired circumferentially around the limbus. A Bayesian Ridge method was used to approximate the location of the SC on infrared confocal laser scanning ophthalmoscopic images with a cross multiplication tool developed to initiate SC/CC detection automated through a fuzzy hidden Markov Chain approach. Automatic segmentation of SC and initial CC’s was manually confirmed by two masked graders. Outflow pathways detected by the segmentation algorithm were reconstructed into a 3-D representation of AHO. Overall, only <1% of images (5114 total B-scans) were ungradable. Automatic segmentation algorithm performed well with SC detection 98.3% of the time and <0.1% false positive detection compared to expert grader consensus. CC was detected 84.2% of the time with 1.4% false positive detection. 3-D representation of AHO pathways demonstrated variably thicker and thinner SC with some clear CC roots. Circumferential (360 deg), automated, and validated AHO detection of angle structures in the living human eye with reconstruction was possible.

Aqueous Angiography in Living Nonhuman Primates Shows Segmental, Pulsatile, and Dynamic Angiographic Aqueous Humor Outflow

PURPOSE

To evaluate the feasibility of safely performing aqueous angiography in intact eyes of living nonhuman primates (NHPs) for evaluating aqueous humor outflow and segmental patterns.

DESIGN

Cross-sectional, observational study.

SUBJECTS

Six nonhuman primates.

METHODS

Aqueous angiography was performed in 6 nonhuman primates. After anesthesia, an anterior chamber (AC) maintainer was placed through a temporal 1-mm side-port wound. Indocyanine green (ICG; 0.4%) or 2.5% fluorescein was introduced (individually or in sequence) into the eye with a gravity-driven constant-pressure system. Aqueous angiography images were obtained with a Spectralis HRA+OCT (Heidelberg Engineering GmbH, Heidelberg, Germany) suspended over the NHP eye using a custom-designed surgical boom arm. Concurrent anterior segment optical coherence tomography (OCT) was performed on distally angiographically positive and negative regions.

MAIN OUTCOME MEASURES

Angiographic patterns described by location, time-course, choice of tracer, and anterior-segment OCT.

RESULTS

Aqueous angiography in the living NHP eye demonstrated mostly stable angiographic patterns. With multimodal imaging, angiographically positive signal co-localized with episcleral veins as identified by infrared imaging and intrascleral lumens, as demonstrated by anterior segment OCT. Sequential aqueous angiography in individual eyes with ICG followed by fluorescein showed similar angiographic patterns. A pulsatile nature of aqueous angiographic outflow was sometimes observed. Aqueous angiographic patterns could also dynamically change. In some instances, positive angiographic flow suddenly arose in regions previously without an angiographic signal. Alternatively, an angiographic signal could suddenly disappear from regions in which an angiographic signal was initially documented.

CONCLUSIONS

Aqueous angiography in living NHPs demonstrated segmental and pulsatile patterns with a newly described ability to dynamically shift. These characteristics further the understanding of live aqueous humor outflow biology and may be useful in improving glaucoma surgeries aimed at trabecular meshwork bypass.