Pulsatile motion of the trabecular meshwork in healthy human subjects quantified by phase-sensitive optical coherence tomography

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.

Aqueous humor as eye lymph: A crossroad between venous and lymphatic system

Aqueous humor (AQH) is a transparent fluid with characteristics similar to those of the interstitial fluid, which fills the eyeball posterior and anterior chambers and circulates in them from the sites of production to those of drainage. The AQH volume and pressure homeostasis is essential for the trophism of the ocular avascular tissues and their normal structure and function. Different AQH outflow pathways exist, including a main pathway, quite well defined anatomically and referred to as the conventional pathway, and some accessory pathways, more recently described and still not fully morphofunctionally understood, generically referred to as unconventional pathways. The conventional pathway is based on the existence of a series of conduits starting with the trabecular meshwork and Schlemm's Canal and continuing with a system of intrascleral and episcleral venules, which are tributaries to veins of the anterior segment of the eyeball. The unconventional pathways are mainly represented by the uveoscleral pathway, in which AQH flows through clefts, interstitial conduits located in the ciliary body and sclera, and then merges into the aforementioned intrascleral and episcleral venules. A further unconventional pathway, the lymphatic pathway, has been supported by the demonstration of lymphatic microvessels in the limbal sclera and, possibly, in the uvea (ciliary body, choroid) as well as by the ocular glymphatic channels, present in the neural retina and optic nerve. It follows that AQH may be drained from the eyeball through blood vessels (TM-SC pathway, US pathway) or lymphatic vessels (lymphatic pathway), and the different pathways may integrate or compensate for each other, optimizing the AQH drainage. The present review aims to define the state-of-the-art concerning the structural organization and the functional anatomy of all the AQH outflow pathways. Particular attention is paid to examining the regulatory mechanisms active in each of them. The new data on the anatomy and physiology of AQH outflow pathways is the key to understanding the pathophysiology of AQH outflow disorders and could open the way for novel approaches to their treatment.

In-vivo characterization of scleral rigidity in myopic eyes using fundus-pulsation optical coherence elastography

The sclera plays an important role in the structural integrity of the eye. However, as myopia progresses, the elongation of the eyeball exerts stretching forces on the posterior sclera, which typically happens in conjunction with scleral remodeling that causes rigidity loss. These biomechanical alterations can cause localized eyeball deformation and vision impairment. Therefore, monitoring scleral rigidity is clinically important for the management and risk assessment of myopia. In this study, we propose fundus pulsation optical coherence elastography (FP-OCE) to characterize posterior scleral rigidity in living humans. This methodology is based on a choroidal pulsation model, where the scleral rigidity is inversely associated with the choroidal max strain obtained through phase-sensitive optical coherence tomography (PhS-OCT) measurement of choroidal deformation and thickness. Using FP-OCE, we conducted a pilot clinical study to explore the relationship between choroidal strain and myopia severity. The results revealed a significant increase in choroidal max strain in pathologic myopia, indicating a critical threshold beyond which scleral rigidity decreases significantly. Our findings offer a potential new method for monitoring myopia progression and evaluating therapies that alter scleral mechanical properties.

Comparative analysis of traction forces in normal and glaucomatous trabecular meshwork cells within a 3D, active fluid-structure interaction culture environment

This study presents a 3D in vitro cell culture model, meticulously 3D printed to replicate the conventional aqueous outflow pathway anatomical structure, facilitating the study of trabecular meshwork (TM) cellular responses under glaucomatous conditions. Glaucoma affects TM cell functionality, leading to extracellular matrix (ECM) stiffening, enhanced cell-ECM adhesion, and obstructed aqueous humor outflow. Our model, reconstructed from polyacrylamide gel with elastic moduli of 1.5 and 21.7 kPa, is based on serial block-face scanning electron microscopy images of the outflow pathway. It allows for quantifying 3D, depth-dependent, dynamic traction forces exerted by both normal and glaucomatous TM cells within an active fluid-structure interaction (FSI) environment. In our experimental design, we designed two scenarios: a control group with TM cells observed over 20 hours without flow (static setting), focusing on intrinsic cellular contractile forces, and a second scenario incorporating active FSI to evaluate its impact on traction forces (dynamic setting). Our observations revealed that active FSI results in higher traction forces (normal: 1.83-fold and glaucoma: 2.24-fold) and shear strains (normal: 1.81-fold and glaucoma: 2.41-fold), with stiffer substrates amplifying this effect. Glaucomatous cells consistently exhibited larger forces than normal cells. Increasing gel stiffness led to enhanced stress fiber formation in TM cells, particularly in glaucomatous cells. Exposure to active FSI dramatically altered actin organization in both normal and glaucomatous TM cells, particularly affecting cortical actin stress fiber arrangement. This model while preliminary offers a new method in understanding TM cell biomechanics and ECM stiffening in glaucoma, highlighting the importance of FSI in these processes. STATEMENT OF SIGNIFICANCE: This pioneering project presents an advanced 3D in vitro model, meticulously replicating the human trabecular meshwork's anatomy for glaucoma research. It enables precise quantification of cellular forces in a dynamic fluid-structure interaction, a leap forward from existing 2D models. This advancement promises significant insights into trabecular meshwork cell biomechanics and the stiffening of the extracellular matrix in glaucoma, offering potential pathways for innovative treatments. This research is positioned at the forefront of ocular disease study, with implications that extend to broader biomedical applications.

Imaging fine structures of the human trabecular meshwork in vivo using a custom design goniolens and OCT gonioscopy

The trabecular meshwork (TM), located within the iridocorneal angle, is a target for many glaucoma treatments aimed at controlling intraocular pressure. However, structural variations between individuals are poorly understood. We propose a newly designed gonioscopic lens optimized for high-resolution imaging to image fine structures of the human TM in vivo. The body of the new lens is index-matched to the human cornea and includes a choice of two gonioscopic mirrors (59° and 63°) and matching air-spaced doublets placed on the anterior surface of the goniolens. The new design allows a diffraction-limited image plane at the iridocorneal angle structures. The goniolens design was built and then placed on the subjectś eyes coupled to the cornea with goniogel and a 3D adjustable mount. Images were obtained using a commercially available OCT device (Heidelberg Spectralis). The optical resolution was measured in a model eye as 40.32 and 45.25 cy/mm respectively for each mirror angle. In humans, dense OCT scans with minimum spacing oriented tangential to the iris and ICA were performed on 7 healthy subjects (23-73 yrs). The TM was successfully imaged in all subjects. The custom goniolens improved the contrast of the uveoscleral meshwork structures and corneoscleral meshwork revealing limbus parallel striations, not visible with previous goniolens designs. Transverse OCT images were constructed along the segmentation line, providing an enface image of the TM structures including corneoscleral beams, previously only imaged in vivo using custom adaptive optics systems.

Iridocorneal angle imaging of a human donor eye by spectral-domain optical coherence tomography

Iridocorneal angle (ICA) details particularly the trabecular meshwork (TM), Schlemm's canal (SC), and collector channels (CCs) play crucial roles in the regulation of the aqueous outflow in the eyes and are closely associated with glaucoma. Current clinical gonioscopy imaging provides no depth information, and studies of 3D high-resolution optical coherence tomography (OCT) imaging of these structures are limited. We developed a custom-built spectral-domain (SD-) OCT imaging system to fully characterize the angle details. Imaging of a human cadaver eye reveals the visibility of details in the TM/SC/CC region via a 'crossline' scanning and a series of image processing. This shows that ICA imaging can be used for preoperative glaucoma inspections in the clinical setting with the proposed prototype.

Efficacy and Safety of Pairing iStent Inject Trabecular Micro-Bypass and iAccess Precision Blade Goniotomy in Patients with Open-Angle Glaucoma

INTRODUCTION

This study evaluated efficacy and safety of implanting two second-generation trabecular micro-bypass stents (iStent inject/iStent inject W) with phacoemulsification, either with or without iAccess Precision Blade goniotomy, in patients with mild-to-moderate open-angle glaucoma (OAG).

METHODS

This retrospective, non-randomized, unmasked, dual-arm, single-site, multi-surgeon, consecutive case series evaluated all OAG eyes that underwent phacoemulsification and iStent inject implantation either as a dual procedure (group A) or paired with iAccess goniotomy (group B) from July 2020 to May 2022. Effectiveness outcomes analyzed from 1 month onward included intraocular pressure (IOP), proportions of eyes with IOP ≤ 12/≤ 15/≤ 18, proportion medication-free, and medication number. Safety outcomes at all timepoints included adverse events and secondary surgeries.

RESULTS

In group A, mean IOP reduced from 14.9 ± 3.2 mmHg on 1.22 ± 1.31 mean medications preoperatively (n = 63) to 13.5 ± 2.5 mmHg on 0.24 ± 0.61 medications at month 3 (n = 34; p = 0.048 IOP, p < 0.001 medications). In group B, mean IOP reduced from 16.0 ± 4.2 mmHg on 1.12 ± 1.07 medications preoperatively (n = 93) to 12.2 ± 2.3 mmHg on 0.57 ± 1.27 medications at month 3 (n = 23; p < 0.001 IOP, p = 0.003 medications). From preoperative to 3 months, the percent of eyes with IOP ≤ 12 mmHg remained at 32.4% in group A (p = 1.0) and rose from 21.7% to 60.9% in group B (p = 0.0177); eyes with IOP ≤ 15 mmHg rose from 52.9% to 76.5% in group A (p = 0.0963) and from 43.5% to 91.3% in group B (p = 0.0034). Adjusting for baseline between-group differences, group B had significantly greater postoperative IOP reduction than group A (p = 0.043); medication reductions were similar. Safety was favorable in both groups.

CONCLUSIONS

Phacoemulsification and iStent inject with or without iAccess Precision Blade goniotomy produced clinically meaningful and safe IOP and medication reductions. The paired iStent inject + iAccess + phacoemulsification procedure enabled greater IOP reduction and lower IOP thresholds than iStent inject + phacoemulsification. The study provides some of the first data on this paired approach and on the novel iAccess Precision Blade.

Rare protective variants and glaucoma-relevant cell stressors modulate Angiopoietin-like 7 expression

Rare missense and nonsense variants in the Angiopoietin-like 7 (ANGPTL7) gene confer protection from primary open-angle glaucoma (POAG), though the functional mechanism remains uncharacterized. Interestingly, a larger variant effect size strongly correlates with in silico predictions of increased protein instability (r = -0.98), suggesting that protective variants lower ANGPTL7 protein levels. Here, we show that missense and nonsense variants cause aggregation of mutant ANGPTL7 protein in the endoplasmic reticulum (ER) and decreased levels of secreted protein in human trabecular meshwork (TM) cells; a lower secreted:intracellular protein ratio strongly correlates with variant effects on intraocular pressure (r = 0.81). Importantly, accumulation of mutant protein in the ER does not increase expression of ER stress proteins in TM cells (P > 0.05 for all variants tested). Cyclic mechanical stress, a glaucoma-relevant physiologic stressor, also significantly lowers ANGPTL7 expression in primary cultures of human Schlemm's canal (SC) cells (-2.4-fold-change, P = 0.01). Collectively, these data suggest that the protective effects of ANGPTL7 variants in POAG stem from lower levels of secreted protein, which may modulate responses to physiologic and pathologic ocular cell stressors. Downregulation of ANGPTL7 expression may therefore serve as a viable preventative and therapeutic strategy for this common, blinding disease.

Developing an experimental-computational workflow to study the biomechanics of the human conventional aqueous outflow pathway

The aqueous humor actively interacts with the trabecular meshwork (TM), juxtacanalicular tissue (JCT), and Schlemm's canal (SC) through a dynamic fluid-structure interaction (FSI) coupling. Despite the fact that intraocular pressure (IOP) undergoes significant fluctuations, our understanding of the hyperviscoelastic biomechanical properties of the aqueous outflow tissues is limited. In this study, a quadrant of the anterior segment from a normal human donor eye was dynamically pressurized in the SC lumen, and imaged using a customized optical coherence tomography (OCT). The TM/JCT/SC complex finite element (FE) with embedded collagen fibrils was reconstructed based on the segmented boundary nodes in the OCT images. The hyperviscoelastic mechanical properties of the outflow tissues' extracellular matrix with embedded viscoelastic collagen fibrils were calculated using an inverse FE-optimization method. Thereafter, the 3D microstructural FE model of the TM, with adjacent JCT and SC inner wall, from the same donor eye was constructed using optical coherence microscopy and subjected to a flow load-boundary from the SC lumen. The resultant deformation/strain in the outflow tissues was calculated using the FSI method, and compared to the digital volume correlation (DVC) data. TM showed larger shear modulus (0.92 MPa) compared to the JCT (0.47 MPa) and SC inner wall (0.85 MPa). Shear modulus (viscoelastic) was larger in the SC inner wall (97.65 MPa) compared to the TM (84.38 MPa) and JCT (56.30 MPa). The conventional aqueous outflow pathway is subjected to a rate-dependent IOP load-boundary with large fluctuations. This necessitates addressing the biomechanics of the outflow tissues using hyperviscoelastic material-model. STATEMENT OF SIGNIFICANCE: While the human conventional aqueous outflow pathway is subjected to a large-deformation and time-dependent IOP load-boundary, we are not aware of any studies that have calculated the hyperviscoelastic mechanical properties of the outflow tissues with embedded viscoelastic collagen fibrils. A quadrant of the anterior segment of a normal humor donor eye was dynamically pressurized from the SC lumen with relatively large fluctuations. The TM/JCT/SC complex were OCT imaged and the mechanical properties of the tissues with embedded collagen fibrils were calculated using the inverse FE-optimization algorithm. The resultant displacement/strain in the FSI outflow model was validated versus the DVC data. The proposed experimental-computational workflow may significantly contribute to understanding of the effects of different drugs on the biomechanics of the conventional aqueous outflow pathway.

Morphological and biomechanical analyses of the human healthy and glaucomatous aqueous outflow pathway: Imaging-to-modeling

BACKGROUND AND OBJECTIVE

Intraocular pressure (IOP) is maintained via a dynamic balance between the production of aqueous humor and its drainage through the trabecular meshwork (TM), juxtacanalicular connective tissue (JCT), and Schlemm's canal (SC) endothelium of the conventional outflow pathway. Primary open angle glaucoma (POAG) is often associated with IOP elevation that occurs due to an abnormally high outflow resistance across the outflow pathway. Outflow tissues are viscoelastic and actively interact with aqueous humor dynamics through a two-way fluid-structure interaction coupling. While glaucoma affects the morphology and stiffness of the outflow tissues, their biomechanics and hydrodynamics in glaucoma eyes remain largely unknown. This research aims to develop an image-to-model method allowing the biomechanics and hydrodynamics of the conventional aqueous outflow pathway to be studied.

METHODS

We used a combination of X-ray computed tomography and scanning electron microscopy to reconstruct high-fidelity, eye-specific, 3D microstructural finite element models of the healthy and glaucoma outflow tissues in cellularized and decellularized conditions. The viscoelastic TM/JCT/SC complex finite element models with embedded viscoelastic beam elements were subjected to a physiological IOP load boundary; the stresses/strains and the flow state were calculated using fluid-structure interaction and computational fluid dynamics.

RESULTS

Based on the resultant hydrodynamics parameters across the outflow pathway, the primary site of outflow resistance in healthy eyes was in the JCT and immediate vicinity of the SC inner wall, while the majority of the outflow resistance in the glaucoma eyes occurred in the TM. The TM and JCT in the glaucoma eyes showed 1.32-fold and 1.13-fold larger beam thickness and smaller trabecular space size (2.24-fold and 1.50-fold) compared to the healthy eyes.

CONCLUSIONS

Characterizing the accurate morphology of the outflow tissues may significantly contribute to constructing more accurate, robust, and reliable models, that can eventually help to better understand the dynamic IOP regulation, hydrodynamics of the aqueous humor, and outflow resistance dynamic in the human eyes. This model demonstrates proof of concept for determining changes to outflow resistance in healthy and glaucomatous tissues and thus may be utilized in larger cohorts of donor tissues where disease specificity, race, age, and gender of the eye donors may be accounted for.

The Effect of Intraocular Pressure Load Boundary on the Biomechanics of the Human Conventional Aqueous Outflow Pathway

BACKGROUND

Aqueous humor outflow resistance in the trabecular meshwork (TM), juxtacanalicular connective tissue (JCT), and Schlemm's canal (SC) endothelium of the conventional outflow pathway actively contribute to intraocular pressure (IOP) regulation. Outflow resistance is actively affected by the dynamic outflow pressure gradient across the TM, JCT, and SC inner wall tissues. The resistance effect implies the presence of a fluid-structure interaction (FSI) coupling between the outflow tissues and the aqueous humor. However, the biomechanical interactions between viscoelastic outflow tissues and aqueous humor dynamics are largely unknown.

METHODS

A 3D microstructural finite element (FE) model of a healthy human eye TM/JCT/SC complex was constructed with elastic and viscoelastic material properties for the bulk extracellular matrix and embedded elastic cable elements. The FE models were subjected to both idealized and a physiologic IOP load boundary using the FSI method.

RESULTS

The elastic material model for both the idealized and physiologic IOP load boundary at equal IOPs showed similar stresses and strains in the outflow tissues as well as pressure in the aqueous humor. However, outflow tissues with viscoelastic material properties were sensitive to the IOP load rate, resulting in different mechanical and hydrodynamic responses in the tissues and aqueous humor.

CONCLUSIONS

Transient IOP fluctuations may cause a relatively large IOP difference of ~20 mmHg in a very short time frame of ~0.1 s, resulting in a rate stiffening in the outflow tissues. Rate stiffening reduces strains and causes a rate-dependent pressure gradient across the outflow tissues. Thus, the results suggest it is necessary to use a viscoelastic material model in outflow tissues that includes the important role of IOP load rate.

Viscoelastic Biomechanical Properties of the Conventional Aqueous Outflow Pathway Tissues in Healthy and Glaucoma Human Eyes

BACKGROUND

Although the tissues comprising the ocular conventional outflow pathway have shown strong viscoelastic mechanical response to aqueous humor pressure dynamics, the viscoelastic mechanical properties of the trabecular meshwork (TM), juxtacanalicular connective tissue (JCT), and Schlemm's canal (SC) inner wall are largely unknown.

METHODS

A quadrant of the anterior segment from two human donor eyes at low- and high-flow (LF and HF) outflow regions was pressurized and imaged using optical coherence tomography (OCT). A finite element (FE) model of the TM, the adjacent JCT, and the SC inner wall was constructed and viscoelastic beam elements were distributed in the extracellular matrix (ECM) of the TM and JCT to represent anisotropic collagen. An inverse FE-optimization algorithm was used to calculate the viscoelastic properties of the ECM/beam elements such that the TM/JCT/SC model and OCT imaging data best matched over time.

RESULTS

The ECM of the glaucoma tissues showed significantly larger time-dependent shear moduli compared to the heathy tissues. Significantly larger shear moduli were also observed in the LF regions of both the healthy and glaucoma eyes compared to the HF regions.

CONCLUSIONS

The outflow tissues in both glaucoma eyes and HF regions are stiffer and less able to respond to dynamic IOP.

Characterization of the human iridocorneal angle in vivo using a custom design goniolens with OCT gonioscopy

Intraocular pressure (IOP) is the only modifiable risk factor for glaucoma progression, and many treatments target the trabecular meshwork (TM). Imaging this region in vivo is challenging due to optical limitations of imaging through the cornea at high angles. We propose a gonioscopic OCT approach using a custom goniolens and a commercially available OCT device to improve imaging of the TM, Schlemm's canal (SC) and adjacent structures within the iridocorneal angle (ICA). The goniolens is modified with a plano-convex focusing lens and placed on the eye optically mated with goniogel and aided by a 3D adjustable mount. Gonioscopic OCT volume scans are acquired to image SC. Transverse enface images allowed measurements of SC over a 45° section of the ICA for the first time and revealed locations of SC narrowing. The band of extracanalicular limbal lamina and corneoscleral bands were imaged in most subjects and these bands were confirmed using exterior OCT imaging. The polarization dependence of the visibility of these structures is studied by polarization rotation the OCT beam with a half-wave plate, allowing increased contrast of SC. Gonioscopic OCT has successfully been used to image the human ICA in 3D in vivo. This approach provides more detailed characterization of the TM and SC, enhancing their contrast against their birefringent backgrounds.

Novel insight into the role of clusterin on intraocular pressure regulation by modifying actin polymerization and extracellular matrix remodeling in the trabecular meshwork

This study provides comprehensive mechanistic evidence for the role of clusterin, a stress-response secretory chaperone protein, in the modulation of intraocular pressure (IOP) by regulating the trabecular meshwork (TM) actin cytoskeleton and the extracellular matrix (ECM). The pathological stressors on TM known to elevate IOP significantly lowered clusterin protein levels indicating stress-related clusterin function loss. Small interfering RNA-mediated clusterin loss in human TM cells in vitro induced actin polymerization and stabilization via protein kinase D1, serine/threonine-protein kinase N2 (PRK2), and LIM kinase 1 (LIMK1), and the recruitment and activation of adhesome proteins including paxillin, vinculin, and integrin αV and β5. A complete loss of clusterin as seen in clusterin knockout mice (Clu-/- ) led to significant IOP elevation at postnatal Day 70. Contrarily, constitutive clusterin expression using adenovirus (AdCLU) in HTM cells resulted in the loss of actin polymerization via decreased PRK2, and LIMK1 and negative regulation of integrin αV and β5. Furthermore, we found that AdCLU treatment in HTM cells significantly decreased the ECM protein expression and distribution by significantly increasing matrix metalloprotease 2 (MMP2) activity and lowering the levels of pro-fibrotic proteins such as transforming growth factor-β2 (TGFβ2), thrombospondin-1 (TSP-1), and plasminogen activator inhibitor-1 (PAI-1). Finally, we found that HTM cells supplemented with recombinant human clusterin attenuated the pro-fibrotic effects of TGFβ2. For the first time this study demonstrates the importance of clusterin in the regulation of TM actin cytoskeleton - ECM interactions and the maintenance of IOP, thus making clusterin an interesting target to reverse elevated IOP.

Biomechanics of human trabecular meshwork in healthy and glaucoma eyes via dynamic Schlemm's canal pressurization

BACKGROUND AND OBJECTIVE

The trabecular meshwork (TM) consists of extracellular matrix (ECM) with embedded collagen and elastin fibers providing its mechanical support. TM stiffness is considerably higher in glaucoma eyes. Emerging data indicates that the TM moves dynamically with transient intraocular pressure (IOP) fluctuations, implying the viscoelastic mechanical behavior of the TM. However, little is known about TM viscoelastic behavior. We calculated the viscoelastic mechanical properties of the TM in n = 2 healthy and n = 2 glaucoma eyes.

METHODS

A quadrant of the anterior segment was submerged in a saline bath, and a cannula connected to an adjustable saline reservoir was inserted into Schlemm's canal (SC). A spectral domain-OCT (SD-OCT) provided continuous cross-sectional B-scans of the TM/JCT/SC complex during pressure oscillation from 0 to 30 mmHg at two locations. The TM/JCT/SC complex boundaries were delineated to construct a 20-µm-thick volume finite element (FE) mesh. Pre-tensioned collagen and elastin fibrils were embedded in the model using a mesh-free penalty-based cable-in-solid algorithm. SC pressure was represented by a position- and time-dependent pressure boundary; floating boundary conditions were applied to the other cut edges of the model. An FE-optimization algorithm was used to adjust the ECM/fiber mechanical properties such that the TM/JCT/SC model and SD-OCT imaging data best matched over time.

RESULTS

Significantly larger short- and long-time ECM shear moduli (p = 0.0032), and collagen (1.82x) and elastin (2.72x) fibril elastic moduli (p = 0.0001), were found in the TM of glaucoma eyes compared to healthy controls.

CONCLUSIONS

These findings provide additional clarity on the mechanical property differences in healthy and glaucomatous outflow pathway under dynamic loading. Understanding the viscoelastic properties of the TM may serve as a new biomarker in early diagnosis of glaucoma.

Anterior segment imaging in minimally invasive glaucoma surgery - A systematic review

Minimally invasive glaucoma surgery (MIGS) has grown in popularity over the past decade. This systematic review explores the peri-operative and intraoperative application of anterior segment imaging to maximize the efficacy and safety of MIGS. A review of the PubMed, EMBASE and CINAHL databases was conducted, with inclusion criteria restricted to MIGS that had received United States Food and Drug Administration (FDA) premarket approval, FDA 510(K) premarket notification, or were listed as a class 1 device exempt from FDA approval or notification. 21 manuscripts from 21 unique studies were identified pertaining to MIGS devices including the XEN Gel Stent, Trabectome, iStent Inject, 1st-generation iStent and the Kahook Dual Blade (KDB). Anterior segment imaging modalities included anterior segment optical coherence tomography (AS-OCT), ultrasound biomicroscopy (UBM), aqueous angiography, OCT volumetric scans and in vivo confocal microscopy. Identification and evaluation of aqueous outflow pathways before and after MIGS have potential for improving patient preoperative patient selection and postoperative outcomes. Intraoperative imaging potentially provides the resolution needed for good visualization of angle anatomy and accurate evaluation of surgical endpoints in angle-based MIGS. Anterior segment imaging has been used to identify procedural complications, provide objective information on implant location in relation to surrounding anatomy, assess the post-implantation structural impact of MIGS devices and manage bleb failure and scarring. Technical difficulties in incorporating imaging modalities into the surgical microscope, variable quality of images and optical interference from ocular structures or surgical instruments are remaining barriers, which discourage the widespread clinical use of this technology.

Valve-Like Outflow System Behavior With Motion Slowing in Glaucoma Eyes: Findings Using a Minimally Invasive Glaucoma Surgery–MIGS-Like Platform and Optical Coherence Tomography Imaging

Purpose

This study aimed to investigate anatomic relationships and biomechanics of pressure-dependent trabecular meshwork and distal valve-like structure deformation in normal and glaucoma eyes using high-resolution optical coherence tomography (HR-OCT).

Methods

We controlled Schlemm’s canal (SC) pressure during imaging with HR-OCT in segments of three normal (NL) and five glaucomatous (GL) ex vivo eyes. The dissected limbal wedges were studied from 15 locations (5 NL and 10 GL). A minimally invasive glaucoma surgery (MIGS)-like cannula was inserted into the SC lumen, whereas the other end was attached to a switch between two reservoirs, one at 0, the other at 30 mm Hg. A steady-state pressure of 30 mm Hg was maintained to dilate SC and collector channels (CC) during 3D volume imaging. The resulting 3D lumen surface relationships were correlated with internal structural features using an image mask that excluded tissues surrounding SC and CC. While imaging with HR-OCT, real-time motion responses in SC and CC areas were captured by switching pressure from 0 to 30 or 30 to 0 mm Hg. NL vs. GL motion differences were compared.

Results

Lumen surface and internal relationships were successfully imaged. We identified SC inlet and outlet valve-like structures. In NL and GL, the mean SC areas measured at the steady-state of 0 and 30 mm Hg were each significantly different (p &lt; 0.0001). Synchronous changes in SC and CC lumen areas occurred in &lt;200 ms. Measured SC area differences at the steady-state 0 and 30 mmHg, respectively, were larger in NL than GL eyes (p &lt; 0.0001). The SC motion curves rose significantly more slowly in GL than NL (p &lt; 0.001). Pressure waves traveled from the cannula end along the SC lumen to CC and deep intrascleral channels.

Conclusion

HR-OCT provided simultaneous measurements of outflow pathway lumen surfaces, internal structures, and biomechanics of real-time pressure-dependent dimension changes. We identified SC inlet and outlet valve-like structures. GL tissues underwent less motion and responded more slowly than NL, consistent with increased tissue stiffness. A MIGS-like shunt to SC permitted pulse waves to travel distally along SC lumen and into CC.

Endogenous expression of Notch pathway molecules in human trabecular meshwork cells

PURPOSE

Cells in the trabecular meshwork sense and respond to a myriad of physical forces through a process known as mechanotransduction. Whilst the effect of substratum stiffness or stretch on TM cells have been investigated in the context of transforming growth factor (TGF-β), Wnt and YAP/TAZ pathways, the role of Notch signaling, an evolutionarily conserved pathway, recently implicated in mechanotransduction, has not been investigated in trabecular meshwork (TM) cells. Here, we compare the endogenous expression of Notch pathway molecules in TM cells from glaucomatous and non-glaucomatous donors, segmental flow regions, and when subjected to cyclical strain, or grown on hydrogels of varying rigidity.

METHODS

Primary TM from glaucomatous (GTM), non-glaucomatous (NTM) donors, and from segmental flow regions [high flow (HF), low flow (LF)], were utilized between passages 2-6. Cells were (i) plated on tissue culture plastic, (ii) subjected to cyclical strain (6 h and 24 h), or (iii) cultured on 3 kPa and 80 kPa hydrogels. mRNA levels of Notch receptors/ligands/effectors in the TM cells was determined by qRT-PCR. Phagocytosis was determined as a function of substratum stiffness in NTM-HF/LF cells in the presence or absence of 100 nM Dexamethasone treatment.

RESULTS

Innate expression of Notch pathway genes were significantly overexpressed in GTM cells with no discernible differences observed between HF/LF cells in either NTM or GTM cells cultured on plastic substrates. With 6 h of cyclical strain, a subset of Notch pathway genes presented with altered expression. Expression of Notch receptors/ligands/receptors/inhibitors progressively declined with increasing stiffness and this correlated with phagocytic ability of NTM cells. Dexamethasone treatment decreased phagocytosis regardless of stiffness or cells isolated from segmental outflow regions.

CONCLUSIONS

We demonstrate here that the Notch expression in cultured TM cells differ intrinsically between GTM vs NTM, and by substratum cues (cyclical strain and stiffness). Of import, the most apparent differences in gene expression were observed as a function of substratum stiffness which closely followed phagocytic ability of cells. Interestingly, on soft substrates (mimicking normal TM stiffness) Notch expression and phagocytosis was highest, while both expression and phagocytosis was significantly lower on stiffer substrates (mimicking glaucomatous stiffness) regardless of DEX treatment. Such context dependent changes suggest Notch pathway may play differing roles in disease vs homeostasis. Studies focused on understanding the mechanistic role of Notch (if any) in outflow homeostasis are thus warranted.

Pulsatile tissue deformation dynamics of the murine retina and choroid mapped by 4D optical coherence tomography

Irregular ocular pulsatility and altered mechanical tissue properties are associated with some of the most sight-threatening eye diseases. Here we present 4D optical coherence tomography (OCT) for the quantitative assessment and depth-resolved mapping of pulsatile dynamics in the murine retina and choroid. Through a pixel-wise analysis of phase changes of the complex OCT signal, we reveal spatiotemporal displacement characteristics across repeated frame acquisitions. We demonstrate in vivo fundus elastography (FUEL) imaging in wildtype mouse retinas and in a mouse model of retinal neovascularization and uncover subtle structural deformations related to ocular pulsation. Our data in mouse eyes hold promise for a powerful retinal elastography technique that may enable a new paradigm of OCT-based measurements and image contrast.

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.

In-vivo imaging of the conventional aqueous outflow system

PURPOSE OF REVIEW

The aim of this study was to provide a comprehensive summary of in-vivo imaging techniques of the aqueous outflow system and discuss its role in improving our understanding of glaucoma pathogenesis and management.

RECENT FINDINGS

Our understanding of the aqueous outflow system is largely derived from ex-vivo studies. Recent innovations in imaging technology and techniques enable in-vivo evaluation of the conventional outflow system in real-time. Optical coherence tomography allows for noninvasive, high-resolution, volumetric imaging of ocular tissues. Dynamic structural changes have been observed at the trabecular meshwork and Schlemm's canal. In parallel, aqueous angiography using injected tracers show a similar dynamism with variable and pulsatile flow signals.

SUMMARY

In-vivo imaging enable real-time evaluation of the conventional aqueous outflow pathway. This emerging field shows great promise to expand our understanding of the pathogenesis and treatment of glaucoma.

Normal and glaucomatous outflow regulation

Glaucoma remains only partially understood, particularly at the level of intraocular pressure (IOP) regulation. Trabecular meshwork (TM) and Schlemm's canal inner wall endothelium (SCE) are key to IOP regulation and their characteristics and behavior are the focus of much investigation. This is becoming more apparent with time. We and others have studied the TM and SCE's extracellular matrix (ECM) extensively and unraveled much about its functions and role in regulating aqueous outflow. Ongoing ECM turnover is required to maintain IOP regulation and several TM ECM manipulations modulate outflow facility. We have established clearly that the outflow pathway senses sustained pressure deviations and responds by adjusting the outflow resistance correctively to keep IOP within an appropriately narrow range which will not normally damage the optic nerve. The glaucomatous outflow pathway has in many cases lost this IOP homeostatic response, apparently due at least in part, to loss of TM cells. Depletion of TM cells eliminates the IOP homeostatic response, while restoration of TM cells restores it. Aqueous outflow is not homogeneous, but rather segmental with regions of high, intermediate and low flow. In general, glaucomatous eyes have more low flow regions than normal eyes. There are distinctive molecular differences between high and low flow regions, and during the response to an IOP homeostatic pressure challenge, additional changes in segmental molecular composition occur. In conjunction with these changes, the biomechanical properties of the juxtacanalicular (JCT) segmental regions are different, with low flow regions being stiffer than high flow regions. The JCT ECM of glaucomatous eyes is around 20 times stiffer than in normal eyes. The aqueous humor outflow resistance has been studied extensively, but neither the exact molecular components that comprise the resistance nor their exact location have been established. Our hypothetical model, based on considerable available data, posits that the continuous SCE basal lamina, which lies between 125 and 500 nm beneath the SCE basal surface, is the primary source of normal resistance. On the surface of JCT cells, small and highly controlled focal degradation of its components by podosome- or invadopodia-like structures, PILS, occurs in response to pressure-induced mechanical stretching. Sub-micron sized basement membrane discontinuities develop in the SCE basement membrane and these discontinuities allow passage of aqueous humor to and through SCE giant vacuoles and pores. JCT cells then relocate versican with its highly charged glycosaminoglycan side chains into the discontinuities and by manipulation of their orientation and concentration, the JCT and perhaps the SCE cells regulate the amount of fluid passage. Testing this outflow resistance hypothesis is ongoing in our lab and has the potential to advance our understanding of IOP regulation and of glaucoma.

Comparison of Pulse Wave Signal Monitoring Techniques with Different Fiber-Optic Interferometric Sensing Elements

Pulse wave (PW) measurement is a highly prominent technique, used in biomedical diagnostics. Development of novel PW sensors with increased accuracy and reduced susceptibility to motion artifacts will pave the way to more advanced healthcare technologies. This paper reports on a comparison of performance of fiber optic pulse wave sensors, based on Fabry–Perot interferometer, fiber Bragg grating, optical coherence tomography (OCT) and singlemode-multimode-singlemode intermodal interferometer. Their performance was tested in terms of signal to noise ratio, repeatability of demodulated signals and suitability of demodulated signals for extraction of information about direct and reflected waves. It was revealed that the OCT approach of PW monitoring provided the best demodulated signal quality and was most robust against motion artifacts. Advantages and drawbacks of all compared PW measurement approaches in terms of practical questions, such as multiplexing capabilities and abilities to be interrogated by portable hardware are discussed.

The vital role for nitric oxide in intraocular pressure homeostasis

Catalyzed by endothelial nitric oxide (NO) synthase (eNOS) activity, NO is a gaseous signaling molecule maintaining endothelial and cardiovascular homeostasis. Principally, NO regulates the contractility of vascular smooth muscle cells and permeability of endothelial cells in response to either biochemical or biomechanical cues. In the conventional outflow pathway of the eye, the smooth muscle-like trabecular meshwork (TM) cells and Schlemm's canal (SC) endothelium control aqueous humor outflow resistance, and therefore intraocular pressure (IOP). The mechanisms by which outflow resistance is regulated are complicated, but NO appears to be a key player as enhancement or inhibition of NO signaling dramatically affects outflow function; and polymorphisms in NOS3, the gene that encodes eNOS modifies the relation between various environmental exposures and glaucoma. Based upon a comprehensive review of past foundational studies, we present a model whereby NO controls a feedback signaling loop in the conventional outflow pathway that is sensitive to changes in IOP and its oscillations. Thus, upon IOP elevation, the outflow pathway tissues distend, and the SC lumen narrows resulting in increased SC endothelial shear stress and stretch. In response, SC cells upregulate the production of NO, relaxing neighboring TM cells and increasing permeability of SC's inner wall. These IOP-dependent changes in the outflow pathway tissues reduce the resistance to aqueous humor drainage and lower IOP, which, in turn, diminishes the biomechanical signaling on SC. Similar to cardiovascular pathogenesis, dysregulation of the eNOS/NO system leads to dysfunctional outflow regulation and ocular hypertension, eventually resulting in primary open-angle glaucoma.

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.

Detection of localized pulsatile motion in cutaneous microcirculation by speckle decorrelation optical coherence tomography angiography

SIGNIFICANCE

Pulsatility is a vital characteristic of the cardiovascular system. Characterization of the pulsatility pattern locally in the peripheral microvasculature is currently not readily available and would provide an additional source of information, which may prove important in understanding the pathophysiology of arterial stiffening, vascular ageing, and their linkage with cardiovascular disease development.

AIM

We aim to confirm the suitability of speckle decorrelation optical coherence tomography angiography (OCTA) under various noncontact/contact scanning protocols for the visualization of pulsatility patterns in vessel-free tissue and in the microvasculature of peripheral human skin.

RESULTS

Results from five healthy subjects show distinct pulsatile patterns both in vessel-free tissue with either noncontact or contact imaging and in individual microvessels with contact imaging. Respectively, these patterns are likely caused by the pulsatile pressure and pulsatile blood flow. The pulse rates show good agreement with those from pulse oximetry, confirming that the pulsatile signatures reflect pulsatile hemodynamics.

CONCLUSIONS

This study demonstrates the potential of speckle decorrelation OCTA for measuring localized peripheral cutaneous pulsatility and defines scanning protocols necessary to undertake such measurements. Noncontact imaging should be used for the study of pulsatility in vessel-free tissue and contact imaging with strong mechanical coupling in individual microvessels. Further studies of microcirculation based upon this method and protocols are warranted.

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.

A model of the oscillatory mechanical forces in the conventional outflow pathway

Intraocular pressure is regulated by mechanosensitive cells within the conventional outflow pathway, the primary route of aqueous humour drainage from the eye. However, the characteristics of the forces acting on those cells are poorly understood. We develop a model that describes flow through the conventional outflow pathway, including the trabecular meshwork (TM) and Schlemm’s canal (SC). Accounting for the ocular pulse, we estimate the time-varying shear stress on SC endothelium and strain on the TM. We consider a range of outflow resistances spanning normotensive to hypertensive conditions. Over this range, the SC shear stress increases significantly and becomes highly oscillatory. TM strain also increases, but with negligible oscillations. Interestingly, TM strain responds more to changes in outflow resistance around physiological values, while SC shear stress responds more to elevated levels of resistance. A modest increase in TM stiffness, as observed in glaucoma, suppresses TM strain and practically eliminates the influence of outflow resistance on SC shear stress. As SC and TM cells respond to mechanical stimulation by secreting factors that modulate outflow resistance, our model provides insight regarding the potential role of SC shear and TM strain as mechanosensory cues for homeostatic regulation of outflow resistance and hence intraocular pressure.

A review of aqueous outflow resistance and its relevance to microinvasive glaucoma surgery

Primary open-angle glaucoma is the leading cause of irreversible blindness worldwide, and intraocular pressure reduction remains the only proven treatment strategy. Elevated intraocular pressure occurs as the result of impaired aqueous humor outflow. Both a passive model and a dynamic model have been used to explain trabecular outflow resistance. The passive model posits that the trabecular meshwork acts as a static filter that exerts stable and passive resistance to outflow. In contrast, the dynamic model involves a "biomechanical pump." In recent years, the range of surgical management options for glaucoma has dramatically expanded, particularly the class of procedures known as microinvasive glaucoma surgery. These procedures typically target and enhance specific outflow routes. Optimal patient outcomes with microinvasive glaucoma surgery require a clear understanding of aqueous outflow and a surgical approach that is targeted to overcome the site of abnormal resistance in the individual. We review the anatomy and physiology of trabecular and suprachoroidal outflow that is of relevance to microinvasive glaucoma surgery-performing surgeons.

Cyclic Pattern of Intraocular Pressure (IOP) and Transient IOP Fluctuations in Nonhuman Primates Measured with Continuous Wireless Telemetry

Purpose: Most studies on intraocular pressure (IOP) to monitor IOP "fluctuations" in glaucoma patients have been performed with snapshot tonometry techniques that obtain IOP measurements at single time points weeks to months apart. However, IOP telemetry has shown that IOP varies from second-to-second due to blinks, saccades, and systolic vascular filling. The purpose of this study was to characterize the cyclic pattern of baseline IOP and transient IOP fluctuations in 3 nonhuman primates (NHPs).Methods: Bilateral IOP was measured using a proven implantable telemetry system and recorded 500 times per second, 24 hours a day, up to 451 continuous days in 3 male rhesus macaques aged 4 to 5 years old. The IOP transducers were calibrated every two weeks via anterior chamber cannulation manometry and all data were continuously corrected for signal drift via software, filtered for signal noise and dropout, and peaks and troughs were quantified and counted using a finite impulse response filter; waking hours were defined as 6:00-18:00 hours based on room light cycle.Results: Fourier transform analyses of baseline IOP and the hourly mean frequency of transient IOP fluctuations > 0.6 mmHg, 0.6-5 mmHg and > 5 mmHg above baseline during waking hours exhibited an approximate 16- to 91-day cyclic pattern in all NHPs. There were no measured environmental or experimental factors associated with this cyclical pattern.Conclusions: While the importance of the cyclic pattern identified in IOP and its fluctuations is unknown at this time, it is plausible that this pattern is relevant to both homeostasis and pathophysiology of the ONH, corneoscleral shell, and aqueous outflow pathways.

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.

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.

Quantification of Pulse-Dependent Trabecular Meshwork Motion in Normal Humans Using Phase-Sensitive OCT

Purpose

The purpose of this study was to characterize the pulsatile motion of trabecular meshwork (TM) in normal subjects and demonstrate its changes in accommodation with phase-sensitive optical coherence tomography (PhS-OCT).

Methods

A new PhS-OCT laboratory prototype was designed to measure pulsatile TM motion in 13 healthy humans. Two sets of images were captured in 10 subjects, first with best corrective refraction and the other with an additional 3.0 diopters of accommodation. In each image, both maximum velocity (MV) and cumulative displacement (CD) in two selected regions of TM, the internal (IMV and ICD) and external (EMV and ECD) region, were measured.

Results

For all parameters the intraclass correlation coefficient was >0.75. Neither MV nor CD was significantly different between eyes in individual subjects (PIMV = 0.967, PEMV = 0.391, PICD = 0.603, PECD = 0.482). In 26 eyes, with best corrective refraction, the EMV was higher than the IMV (23.9 ± 9.8 vs. 18.9 ± 8.08 μm/s; P = 0.0001), as was the ECD compared with the ICD (0.340 ±0.125 vs. 0.264 ± 0.111 μm; P = 0.000004). With accommodation, MV and CD significantly increased (PIMV = 0.0003, PEMV = 0.0003, PICD = 0.019, and PECD = 0.007), whereas MV and CD in the external region were still larger than those in the internal area (PEMV vs. IMV = 0.009, PECD vs. ICD = 0.023).

Conclusions

This study demonstrates the differences in TM motion between the internal and external regions of TM and displays its change with accommodation. The findings and good reproducibility suggest PhS-OCT helps to understand TM function in regulation of IOP, and, with further refinements, it may be useful in clinical management of glaucoma.

Deep tissue analysis of distal aqueous drainage structures and contractile features

Outflow resistance in the aqueous drainage tract distal to trabecular meshwork is potentially an important determinant of intraocular pressure and success of trabecular bypass glaucoma surgeries. It is unclear how distal resistance is modulated. We sought to establish: (a) multimodal 2-photon deep tissue imaging and 3-dimensional analysis of the distal aqueous drainage tract (DT) in transgenic mice in vivo and ex vivo; (b) criteria for distinguishing the DT from blood and lymphatic vessels; and (c) presence of a DT wall organization capable of contractility. DT lumen appeared as scleral collagen second harmonic generation signal voids that could be traced back to Schlemm's canal. DT endothelium was Prox1-positive, CD31-positive and LYVE-1-negative, bearing a different molecular signature from blood and true lymphatic vessels. DT walls showed prominent filamentous actin (F-actin) labeling reflecting cells in a contracted state. F-actin co-localized with mesenchymal smooth muscle epitopes of alpha-smooth muscle actin, caldesmon and calponin, which localized adjacent and external to the endothelium. Our findings support a DT wall organization resembling that of blood vessels. This reflects a capacity to contract and support dynamic alteration of DT caliber and resistance analogous to the role of blood vessel tone in regulating blood flow.

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.

Ocular fundus pulsations within the posterior rat eye: Chorioscleral motion and response to elevated intraocular pressure

A multi-functional optical coherence tomography (OCT) approach is presented to determine ocular fundus pulsations as an axial displacement between the retina and the chorioscleral complex in the albino rat eye. By combining optical coherence elastography and OCT angiography (OCTA), we measure subtle deformations in the nanometer range within the eye and simultaneously map retinal and choroidal perfusion. The conventional OCT reflectivity contrast serves as a backbone to segment the retina and to define several slabs which are subsequently used for quantitative ocular pulsation measurements as well as for a qualitative exploration of the multi-functional OCT image data. The proposed concept is applied in healthy albino rats as well as in rats under acute elevation of the intraocular pressure (IOP). The evaluation of this experiment revealed an increased pulsatility and deformation between the retinal and chorioscleral complex while increasing the IOP level from 15 mmHg to 65 mmHg. At IOP levels exceeding 65 mmHg, the pulsatility decreased significantly and retinal as well as choroidal perfusion vanished in OCTA. Furthermore, the evaluation of the multi-parametric experiment revealed a spatial correlation between fundus pulsatility and choroidal blood flow. This indicates that the assessed pulsatility may be a valuable parameter describing the choroidal perfusion.

VEGF as a Paracrine Regulator of Conventional Outflow Facility

Purpose

Vascular endothelial growth factor (VEGF) regulates microvascular endothelial permeability, and the permeability of Schlemm's canal (SC) endothelium influences conventional aqueous humor outflow. We hypothesize that VEGF signaling regulates outflow facility.

Methods

We measured outflow facility (C) in enucleated mouse eyes perfused with VEGF-A₁₆₄a, VEGF-A₁₆₅b, VEGF-D, or inhibitors to VEGF receptor 2 (VEGFR-2). We monitored VEGF-A secretion from human trabecular meshwork (TM) cells by ELISA after 24 hours of static culture or cyclic stretch. We used immunofluorescence microscopy to localize VEGF-A protein within the TM of mice.

Results

VEGF-A₁₆₄a increased C in enucleated mouse eyes. Cyclic stretch increased VEGF-A secretion by human TM cells, which corresponded to VEGF-A localization in the TM of mice. Blockade of VEGFR-2 decreased C, using either of the inhibitors SU5416 or Ki8751 or the inactive splice variant VEGF-A₁₆₅b. VEGF-D increased C, which could be blocked by Ki8751.

Conclusions

VEGF is a paracrine regulator of conventional outflow facility that is secreted by TM cells in response to mechanical stress. VEGF affects facility via VEGFR-2 likely at the level of SC endothelium. Disruption of VEGF signaling in the TM may explain why anti-VEGF therapy is associated with decreased outflow facility and sustained ocular hypertension.

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 outflow regulation: Optical coherence tomography implicates pressure-dependent tissue motion

Glaucoma is a leading cause of blindness worldwide and results from damage to the optic nerve. Currently, intraocular pressure is the only treatable risk factor. Changes in aqueous outflow regulate pressure; regulation becomes abnormal in glaucoma. From inside the eye aqueous flows out through the trabecular meshwork into a venous sinus called Schlemm's canal, next into collector channels and finally returns to the episcleral vessels of the venous system. The location of aqueous outflow regulation is unknown. Ex vivo and in vivo studies implicate both pressure-dependent trabecular tissue motion and tissues distal to Schlemm's canal in regulation of aqueous outflow. Technologies have not previously been available to study these issues. New ex vivo imaging in human eyes identifies hinged flaps or leaflets at collector channel entrances using a high-resolution spectral domain optical coherence tomography (SD-OCT) platform. The hinged flaps open and close in synchrony with pressure-dependent trabecular meshwork motion. The SD-OCT platform images from the trabecular meshwork surface while experimentally changing transtrabecular pressure gradients. New in vivo imaging in human eyes uses a motion sensitive technology, phase-sensitive OCT to quantitate real-time pulse-dependent trabecular tissue motion as well as absence of such motion when aqueous access to the outflow system is blocked. The recent studies suggest that aqueous outflow regulation results from synchronous pressure-dependent motion involving a network of interconnected tissues including those distal to Schlemm's canal. The new imaging technologies may shed light on glaucoma mechanisms and provide guidance in the management of medical, laser and surgical decisions in glaucoma.

Highly sensitive, wide dynamic range displacement sensor combining chromatic confocal system and phase-sensitive spectral optical coherence tomography

A displacement sensor with nanometer-sensitivity and a submillimeter dynamic range is proposed. It integrates a chromatic confocal system and phase-sensitive spectral optical coherence tomography (PhS-SOCT) into the fiber-based Michelson interferometer and codes interference and confocal signals with spectral multiplexing. A displacement is evaluated using depth-resolved phase information decoded from the interference signal, which is unwrapped based on the position information decoded from the confocal signal. A sensor system with a 0.102mm dynamic range was built to validate the method. The temperature induced sample surface displacement was measured with a root mean square error of 3.9nm.

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.

OCT Study of Mechanical Properties Associated with Trabecular Meshwork and Collector Channel Motion in Human Eyes

We report the use of a high-resolution optical coherence tomography (OCT) imaging platform to identify and quantify pressure-dependent aqueous outflow system (AOS) tissue relationships and to infer mechanical stiffness through examination of tissue properties in ex vivo human eyes. Five enucleated human eyes are included in this study, with each eye prepared with four equal-sized quadrants, each encompassing 90 degrees of the limbal circumference. In radial limbal segments perfusion pressure within Schlemm’s canal (SC) is controlled by means of a perfusion cannula inserted into the canal lumen, while the other end of the cannula leads to a reservoir at a height that can control the pressure in the cannula. The OCT system images the sample with a spatial resolution of about 5 μm from the trabecular meshwork (TM) surface. Geometric parameters are quantified from the 2D OCT images acquired from the sample subjected to controlled changes in perfusion pressures; parameters include area and height of the lumen of SC, collector channel entrances (CCE) and intrascleral collector channels (ISCC). We show that 3D OCT imaging permits the identification of 3-D relationships of the SC, CCE and ISCC lumen dimensions. Collagen flaps or leaflets are found at CCE that are attached or hinged at only one end, whilst the flaps are connected to the TM by cylindrical structures spanning SC. Increasing static SC pressures resulted in SC lumen enlargement with corresponding enlargement of the CCE and ISCC lumen. Pressure-dependent SC lumen area and height changes are significant at the 0.01 levels for ANOVA, and at the 0.05 for both polynomial curves and Tukey paired comparisons. Dynamic measurements demonstrate a synchronous increase in SC, CCE and ISCC lumen height in response to pressure changes from 0 to 10, 30 or 50 mm Hg, respectively, and the response time is within the 50-millisecond range. From the measured SC volume and corresponding IOP values, we demonstrate that an elastance curve can be developed to infer the mechanical stiffness of the TM by means of quantifying pressure-dependent SC volume changes over a 2 mm radial region of SC. Our study finds pressure-dependent motion of the TM that corresponds to collagen leaflet configuration motion at CCE; the synchronous tissue motion also corresponds with synchrony of SC and CCE lumen dimension changes.

Aqueous Humor Outflow Structure and Function Imaging At the Bench and Bedside: A Review

Anterior segment glaucoma clinical care and research has recently gained new focus because of novel imaging modalities and the advent of angle-based surgical treatments. Traditional investigation drawn to the trabecular meshwork now emphasizes the entire conventional aqueous humor outflow (AHO) pathway from the anterior chamber to the episcleral vein. AHO investigation can be divided into structural and functional assessments using different methods. The historical basis for studying the anterior segment of the eye and AHO in glaucoma is discussed. Structural studies of AHO are reviewed and include traditional pathological approaches to modern tools such as multi-model two-photon microscopy and optical coherence tomography. Functional assessment focuses on visualizing AHO itself through a variety of non-real-time and real-time techniques such as aqueous angiography. Implications of distal outflow resistance and segmental AHO are discussed with an emphasis on melding bench-side research to viable clinical applications. Through the development of an improved structure: function relationship for AHO in the anterior segment of the normal and diseased eye, a better understanding of the eye with improved therapeutics may be developed.

Rapid dynamic changes of the geometry of the anterior segment of the eye: A method of automatic spatial correction of a temporal sequence of OCT images

INTRODUCTION

Ocular dynamics is a very complex phenomenon which has not been well studied and understood yet. The way in which the eye responds to pulsatile changes of the blood pressure or even the electric activity of the heart depends not only on the mechanical properties of each individual structure of the eye globe, but also on its internal conditions, such as the degree of accommodation or intraocular pressure (IOP).

METHOD

This paper presents a method for correcting these undesired movements that could increase the sensitivity of the technique and its reliability in the estimation of pulsatile dynamics. The presented algorithm uses fully automatic detection of the structures that form an angle and fully automatic stabilisation of each individual image being a part of a captured sequence.

RESULTS AND CONCLUSIONS

The procedures described above were applied to the data of nine subjects taking part in the study. For all of them it turned out that the most representative area for fitting the images is the area of the irido-corneal angle apex. The presented algorithm significantly improved the spatial stability of the images in the temporal sequences of the tomographic images. The use of such correction makes it possible to distinguish the subtle pulsatile fluctuations of the ocular structures in the anterior segment, that can be associated with the activity of the retinal blood vessels and/or electric activity of the heart, from the undesired involuntary movements of the eye or the whole head.

Aqueous Angiography: Real-Time and Physiologic Aqueous Humor Outflow Imaging

Purpose

Trabecular meshwork (TM) bypass surgeries attempt to enhance aqueous humor outflow (AHO) to lower intraocular pressure (IOP). While TM bypass results are promising, inconsistent success is seen. One hypothesis for this variability rests upon segmental (non-360 degrees uniform) AHO. We describe aqueous angiography as a real-time and physiologic AHO imaging technique in model eyes as a way to simulate live AHO imaging.

Methods

Pig (n = 46) and human (n = 6) enucleated eyes were obtained, orientated based upon inferior oblique insertion, and pre-perfused with balanced salt solution via a Lewicky AC maintainer through a 1mm side-port. Fluorescein (2.5%) was introduced intracamerally at 10 or 30 mm Hg. With an angiographer, infrared and fluorescent (486 nm) images were acquired. Image processing allowed for collection of pixel information based on intensity or location for statistical analyses. Concurrent OCT was performed, and fixable fluorescent dextrans were introduced into the eye for histological analysis of angiographically active areas.

Results

Aqueous angiography yielded high quality images with segmental patterns (p<0.0001; Kruskal-Wallis test). No single quadrant was consistently identified as the primary quadrant of angiographic signal (p = 0.06–0.86; Kruskal-Wallis test). Regions of high proximal signal did not necessarily correlate with regions of high distal signal. Angiographically positive but not negative areas demonstrated intrascleral lumens on OCT images. Aqueous angiography with fluorescent dextrans led to their trapping in AHO pathways.

Conclusions

Aqueous angiography is a real-time and physiologic AHO imaging technique in model eyes.