The Aqueous Outflow System as a Mechanical Pump

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.

Imaging of aqueous outflow in health and glaucoma. Justifying the re-direction of aqueous

A wave of less invasive surgical options that target or bypass the conventional aqueous outflow system has been incorporated into routine clinical practice to mitigate surgical risks associated with traditional glaucoma drainage surgery. A blanket surgical approach for open-angle glaucoma is unlikely to achieve the desired IOP reduction in an efficient or economical way. Developing a precise approach to selecting the most appropriate surgical tool for each patient is dependent upon understanding the complexities of the aqueous outflow system and how devices influence aqueous drainage. However, homoeostatic control of aqueous outflow in health and glaucoma remains poorly understood. Emerging imaging techniques have provided an opportunity to study aqueous outflow responses non-invasively in clinic settings. Haemoglobin Video Imaging (HVI) studies have demonstrated different patterns of aqueous outflow within the episcleral venous system in normal and glaucomatous eyes, as well as perioperatively after trabecular bypass surgery. Explanations for aqueous outflow patterns remain speculative until direct correlation with findings from Schlemm's canal and the trabecular meshwork are possible. The redirection of aqueous via targeted stent placement may only be justifiable once the role of the aqueous outflow system in IOP homoeostasis has been defined.

Aqueous humor TGFβ and fibrillin-1 in Tsk mice reveal clues to POAG pathogenesis

Aqueous humor (AH) and blood levels of transforming growth factor β (TGFβ) are elevated in idiopathic primary open angle glaucoma (POAG) representing a disease biomarker of unclear status and function. Tsk mice display a POAG phenotype and harbor a mutation of fibrillin-1, an important regulator of TGFβ bioavailability. AH TGFβ2 was higher in Tsk than wild-type (WT) mice (by 34%; p = 0.002; ELISA); similarly, AH TGFβ2 was higher in human POAG than controls (2.7-fold; p = 0.00005). As in POAG, TGFβ1 was elevated in Tsk serum (p = 0.01). Fibrillin-1 was detected in AH from POAG subjects and Tsk mice where both had similar levels relative to controls (p = 0.45). 350 kDa immunoblot bands representing WT full-length fibrillin-1 were present in human and mouse AH. A 418 kDa band representing mutant full-length fibrillin-1 was present only in Tsk mice. Lower molecular weight fibrillin-1 antibody-reactive bands were present in similar patterns in humans and mice. Certain bands (130 and 32 kDa) were elevated only in human POAG and Tsk mice (p ≤ 0.04 relative to controls) indicating discrete isoforms relevant to disease. In addition to sharing a phenotype, Tsk mice and human POAG subjects had common TGFβ and fibrillin-1 features in AH and also blood that are pertinent to understanding glaucoma pathogenesis.

Segmental biomechanics of the normal and glaucomatous human aqueous outflow pathway

The conventional aqueous outflow pathway, encompassing the trabecular meshwork (TM), juxtacanalicular connective tissue (JCT), and inner wall endothelium of Schlemm's canal (SC), governs intraocular pressure (IOP) regulation. This study targets the biomechanics of low-flow (LF) and high-flow (HF) regions within the aqueous humor outflow pathway in normal and glaucomatous human donor eyes, using a combined experimental and computational approach. LF and HF TM/JCT/SC complex tissues from normal and glaucomatous eyes underwent uniaxial tensile testing. Dynamic motion of the TM/JCT/SC complex was recorded using customized green-light optical coherence tomography during SC pressurization in cannulated anterior segment wedges. A hyperviscoelastic model quantified TM/JCT/SC complex properties. A fluid-structure interaction model simulated tissue-aqueous humor interaction. FluoSpheres were introduced into the pathway via negative pressure in the SC, with their motion tracked using two-photon excitation microscopy. Tensile test results revealed that the elastic moduli of the LF and HF regions in glaucomatous eyes are 3.5- and 1.5-fold stiffer than the normal eyes, respectively. The FE results also showed significantly larger shear moduli in the TM, JCT, and SC of the glaucomatous eyes compared to the normal subjects. The LF regions in normal eyes demonstrated larger elastic moduli compared to the HF regions in glaucomatous eyes. The resultant strain in the outflow tissues and velocity of the aqueous humor in the FSI models were in good agreement with the digital volume correlation and 3D particle image velocimetry data, respectively. This study uncovers stiffer biomechanical responses in glaucomatous eyes, with LF regions stiffer than HF regions in both normal and glaucomatous eyes. STATEMENT OF SIGNIFICANCE: This study delves into the biomechanics of the conventional aqueous outflow pathway, a crucial regulator of intraocular pressure and ocular health. By analyzing mechanical differences in low-flow and high-flow regions of normal and glaucomatous eyes, this research unveils the stiffer response in glaucomatous eyes. The distinction between regions' properties offers insights into aqueous humor outflow regulation, while the integration of experimental and computational methods enhances credibility. These findings have potential implications for disease management and present a vital step toward innovative ophthalmic interventions. This study advances our understanding of glaucoma's biomechanical basis and its broader impact on ocular health.

Implementing new computational methods for the study of JCT and SC inner wall basement membrane biomechanics and hydrodynamics

PURPOSE

The conventional aqueous outflow pathway, which includes the trabecular meshwork (TM), juxtacanalicular tissue (JCT), and the inner wall endothelium of Schlemm's canal (SC), regulates intraocular pressure (IOP) by controlling the aqueous humor outflow resistance. Despite its importance, our understanding of the biomechanics and hydrodynamics within this region remains limited. Fluid-structure interaction (FSI) offers a way to estimate the biomechanical properties of the JCT and SC under various loading and boundary conditions, providing valuable insights that are beyond the reach of current imaging techniques.

METHODS

In this study, a normal human eye was fixed at a pressure of 7 mm Hg, and two radial wedges of the TM tissues, which included the SC inner wall basement membrane and JCT, were dissected, processed, and imaged using 3D serial block-face scanning electron microscopy (SBF-SEM). Four different sets of images were used to create 3D finite element (FE) models of the JCT and inner wall endothelial cells of SC with their basement membrane. The outer JCT portion was carefully removed as the outflow resistance is not in that region, leaving only the SCE inner wall and a few µm of the tissue, which does contain the resistance. An inverse iterative FE algorithm was then utilized to calculate the unloaded geometry of the JCT/SC complex at an aqueous humor pressure of 0 mm Hg. Then in the model, the intertrabecular spaces, pores, and giant vacuole contents were replaced by aqueous humor, and FSI was employed to pressurize the JCT/SC complex from 0 to 15 mm Hg.

RESULTS

In the JCT/SC complex, the shear stress of the aqueous humor is not evenly distributed. Areas proximal to the inner wall of SC experience larger stresses, reaching up to 10 Pa, while those closer to the JCT undergo lower stresses, approximately 4 Pa. Within this complex, giant vacuoles with or without I-pore behave differently. Those without I-pores experience a more significant strain, around 14%, compared to those with I-pores, where the strain is roughly 9%.

CONCLUSIONS

The distribution of aqueous humor wall shear stress is not uniform within the JCT/SC complex, which may contribute to our understanding of the underlying selective mechanisms in the pathway.

Estrogen dysregulation, intraocular pressure, and glaucoma risk

Characterized by optic nerve atrophy due to retinal ganglion cell (RGC) death, glaucoma is the leading cause of irreversible blindness worldwide. Of the major risk factors for glaucoma (age, ocular hypertension, and genetics), only elevated intraocular pressure (IOP) is modifiable, which is largely regulated by aqueous humor outflow through the trabecular meshwork. Glucocorticoids such as dexamethasone have long been known to elevate IOP and lead to glaucoma. However, several recent studies have reported that steroid hormone estrogen levels inversely correlate with glaucoma risk, and that variants in estrogen signaling genes have been associated with glaucoma. As a result, estrogen dysregulation may contribute to glaucoma pathogenesis, and estrogen signaling may protect against glaucoma. The mechanism for estrogen-related protection against glaucoma is not completely understood but likely involves both regulation of IOP homeostasis and neuroprotection of RGCs. Based upon its known activities, estrogen signaling may promote IOP homeostasis by affecting extracellular matrix turnover, focal adhesion assembly, actin stress fiber formation, mechanosensation, and nitric oxide production. In addition, estrogen receptors in the RGCs may mediate neuroprotective functions. As a result, the estrogen signaling pathway may offer a therapeutic target for both IOP control and neuroprotection. This review examines the evidence for a relationship between estrogen and IOP and explores the possible mechanisms by which estrogen maintains IOP homeostasis.

'Intraoperative predictors for clinical outcomes after microinvasive glaucoma surgery"

PURPOSE

To evaluate the clinical applicability of intraoperative predictors for surgical outcomes after gonioscopy-assisted transluminal trabeculotomy (GATT) and microincisional trabeculectomy (MIT).

METHODS

Consecutive patients with primary, or secondary glaucoma (trauma, aphakic, or status post-retinal surgeries) with uncontrolled IOP>21mm Hg, who were scheduled to undergo GATT or MIT with or without significant cataract surgery, at a tertiary eye centre in East India between September 2021 to March 2023, were included. All surgeries were done by a single surgeon. Blanching and Trypan blue (0.4%) staining after intracameral injection using a 25 canula, were analysed in each video. The extent/pattern of blanching and blue staining in each eye was analysed objectively using an overlay of a circle with 12 sectors and a protractor tool to quantify the degrees or quadrants of blanching/staining. Multivariate regression was used to identify predictors for surgical success or the need for medications after surgery.

RESULT

Of 167 eyes that were included (male: female- 134: 33), 49 eyes and 118 eyes underwent GATT and MIT, respectively, with 81 of 167 eyes undergoing concurrent cataract surgery. All eyes had a significant reduction in the number of medications after surgery. Blanching was seen in 154 of 167 eyes in a mean of 2±1.8 quadrants with 41% of eyes showing a blanching effect in >3 quadrants. Of 99 of 167 eyes where Trypan blue staining was assessed, staining in a venular, diffuse haze, or reticular pattern of staining was seen in 73 eyes, 26 eyes showed blue staining in >2 quadrants, with 16% staining in >3 quadrants. Surgical success was not predicted by the quadrants of blanching, blue staining, or other clinical variables (age, visual field, baseline intraocular pressure, type of surgery). The variables significantly predicting the need for medications included blanch (r = -0.1, p = 0.03), and blue staining (r = -0.1, p = 0.04) in <2 quadrants.

CONCLUSIONS

Blanching and Trypan blue staining in >2 quadrants after GATT or MIT can serve as surrogate predictors for the need for medications. However more studies are mandated to find predictors for surgical success after GATT or MIT.

Comparing corneal biomechanic changes among solo cataract surgery, microhook ab interno trabeculotomy and iStent implantation

Minimally invasive glaucoma surgery has expanded the surgical treatment options in glaucoma, particularly when combined with cataract surgery. It is clinically relevant to understand the associated postoperative changes in biomechanical properties because they are influential on the measurement of intraocular pressure (IOP) and play an important role in the pathogenesis of open-angle glaucoma (OAG). This retrospective case-control study included OAG patients who underwent cataract surgery combined with microhook ab interno trabeculotomy (µLOT group: 53 eyes of 36 patients) or iStent implantation (iStent group: 59 eyes of 37 patients) and 62 eyes of 42 solo cataract patients without glaucoma as a control group. Changes in ten biomechanical parameters measured with the Ocular Response Analyzer and Corneal Visualization Scheimpflug Technology (Corvis ST) at 3 and 6 months postoperatively relative to baseline were compared among the 3 groups. In all the groups, IOP significantly decreased postoperatively. In the µLOT and control groups, significant changes in Corvis ST-related parameters, including stiffness parameter A1 and stress‒strain index, indicated that the cornea became softer postoperatively. In contrast, these parameters were unchanged in the iStent group. Apart from IOP reduction, the results show variations in corneal biomechanical changes from minimally invasive glaucoma surgery combined with cataract surgery.

Trabecular Meshwork Movement Controls Distal Valves and Chambers: New Glaucoma Medical and Surgical Targets

Herein, we provide evidence that human regulation of aqueous outflow is by a pump-conduit system similar to that of the lymphatics. Direct observation documents pulsatile aqueous flow into Schlemm's canal and from the canal into collector channels, intrascleral channels, aqueous veins, and episcleral veins. Pulsatile flow in vessels requires a driving force, a chamber with mobile walls and valves. We demonstrate that the trabecular meshwork acts as a deformable, mobile wall of a chamber: Schlemm's canal. A tight linkage between the driving force of intraocular pressure and meshwork deformation causes tissue responses in milliseconds. The link provides a sensory-motor baroreceptor-like function, providing maintenance of a homeostatic setpoint. The ocular pulse causes meshwork motion oscillations around the setpoint. We document valves entering and exiting the canal using real-time direct observation with a microscope and multiple additional modalities. Our laboratory-based high-resolution SD-OCT platform quantifies valve lumen opening and closing within milliseconds synchronously with meshwork motion; meshwork tissue stiffens, and movement slows in glaucoma tissue. Our novel PhS-OCT system measures nanometer-level motion synchronous with the ocular pulse in human subjects. Movement decreases in glaucoma patients. Our model is robust because it anchors laboratory studies to direct observation of physical reality in humans with glaucoma.

Biomechanics of the JCT and SC Inner Wall Endothelial Cells with Their Basement Membrane Using 3D Serial Block-Face Scanning Electron Microscopy

BACKGROUND

More than ~70% of the aqueous humor exits the eye through the conventional aqueous outflow pathway that is comprised of the trabecular meshwork (TM), juxtacanalicular tissue (JCT), the inner wall endothelium of Schlemm's canal (SC). The flow resistance in the JCT and SC inner wall basement membrane is thought to play an important role in the regulation of the intraocular pressure (IOP) in the eye, but current imaging techniques do not provide enough information about the mechanics of these tissues or the aqueous humor in this area.

METHODS

A normal human eye was perfusion-fixed and a radial wedge of the TM tissue from a high-flow region was dissected. The tissues were then sliced and imaged using serial block-face scanning electron microscopy. Slices from these images were selected and segmented to create a 3D finite element model of the JCT and SC cells with an inner wall basement membrane. The aqueous humor was used to replace the intertrabecular spaces, pores, and giant vacuoles, and fluid-structure interaction was employed to couple the motion of the tissues with the aqueous humor.

RESULTS

Higher tensile stresses (0.8-kPa) and strains (25%) were observed in the basement membrane beneath giant vacuoles with open pores. The volumetric average wall shear stress was higher in SC than in JCT/SC. As the aqueous humor approached the inner wall basement membrane of SC, the velocity of the flow decreased, resulting in the formation of small eddies immediately after the flow left the inner wall.

CONCLUSIONS

Improved modeling of SC and JCT can enhance our understanding of outflow resistance and funneling. Serial block-face scanning electron microscopy with fluid-structure interaction can achieve this, and the observed micro-segmental flow patterns in ex vivo perfused human eyes suggest a hypothetical mechanism.

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.

Hydrus microstent for the treatment of primary open-angle glaucoma: overview of its safety and efficacy

INTRODUCTION

Minimally invasive glaucoma surgeries (MIGS) are now a consolidated reality in many surgical units. The Hydrus Microstent is one of several MIGS devices bypassing trabecular outflow and had excellent results over the years. This article aims to review the key features of the Hydrus Microstent in terms of design, efficacy, and safety.

AREAS COVERED

The present review analyses the main characteristics of the device by evaluating the technical and physical details of its functioning. The evidence that supports a clinical decision summarizes the most influential clinical trials and the most accurate systematic reviews.

EXPERT OPINION

The Hydrus device has been extensively studied regarding biocompatibility and outflow potential. The subsequent clinical studies have been well-built and proved that the device effectively reduces intraocular pressure (IOP) and the eyedrop load. The device covers almost a quarter of Schlemm's canal circumference, offering at least two advantages: cannulating the Schlemm's canal provides evidence that the device has been implanted correctly; covering a larger area potentially allows to target multiple collector channels or at least areas of active outflow. This scaffold may prove more effective in naïve patients or subjects who used antiglaucoma eyedrops for a limited period, as the prolonged use of hypotonic medications has been associated with the surgical failure of ab interno microhook trabeculotomy.

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.

Short-Term Effect of Micropulse Transscleral Laser Therapy on Intraocular Pressure in Untreated Fellow Eyes of Glaucoma Patients: Preliminary Results

It has been observed that an intraocular pressure (IOP) altering intervention in one eye is followed by a consensual response in the untreated fellow eye. The underlying mechanisms remain unclear. Involvement of neuronal, cytokine, and hormonal regulation of aqueous humor dynamics, as well as improved treatment adherence or systemic absorption of topically administered medical compounds, have been suggested. Our aim was to investigate the short-term effects of unilateral micropulse transscleral laser therapy on IOP in the fellow eye. All medical records of glaucoma patients who underwent micropulse transscleral laser therapy in a tertiary referral center between May 2019 and February 2023 were collected and analyzed. We found a significant reduction in IOP in the treated eyes, indicating successful treatment. In the fellow eyes, despite not having changed any of the pharmacological IOP-reducing therapies, a significant reduction in IOP from 17.0 ± 5.1 mmHg to 13.5 ± 4.4 mmHg (p < 0.01) was observed. This reduction was, however, short-term and reached statistical significance on the first postoperative day only. Our findings support the concept of consensual inter-eye responses to unilateral IOP changes. Further research is warranted to elucidate the mechanisms underlying this phenomenon.

Acoustic radiation force optical coherence elastography: A preliminary study on biomechanical properties of trabecular meshwork

Evaluating biomechanical properties of trabecular meshwork (TM) is of great significance for understanding the mechanism of aqueous humor circulation and its relationship to some eye diseases such as glaucoma; however, there is almost no relevant study due to the lack of clinical measurement tool. In this paper, an acoustic radiation force optical coherence elastography (ARF-OCE) system is developed with the advantages of noninvasive detection, high resolution, high sensitivity, and high-speed imaging, by which elastic modulus of the porcine and human TMs is accurately quantified. As the first OCE imaging of TM, our study demonstrates that ARF-OCE may be an effective approach to advance the research of diseases related to aqueous humor circulation.

Modeling the Endothelial Glycocalyx Layer in the Human Conventional Aqueous Outflow Pathway

A layer of proteoglycans and glycoproteins known as glycocalyx covers the surface of the trabecular meshwork (TM), juxtacanalicular tissue (JCT), and Schlemm's canal (SC) inner wall of the conventional aqueous outflow pathway in the eye. This has been shown to play a role in the mechanotransduction of fluid shear stress and in the regulation of the outflow resistance. The outflow resistance in the conventional outflow pathway is the main determinant of the intraocular pressure (IOP) through an active, two-way, fluid-structure interaction coupling between the outflow tissues and aqueous humor. A 3D microstructural finite element (FE) model of a healthy human eye TM/JCT/SC complex with interspersed aqueous humor was constructed. A very thin charged double layer that represents the endothelial glycocalyx layer covered the surface of the elastic outflow tissues. The aqueous humor was modeled as electroosmotic flow that is charged when it is in contact with the outflow tissues. The electrical-fluid-structure interaction (EFSI) method was used to couple the charged double layer (glycocalyx), fluid (aqueous humor), and solid (outflow tissues). When the IOP was elevated to 15 mmHg, the maximum aqueous humor velocity in the EFSI model was decreased by 2.35 mm/s (9%) compared to the fluid-structure interaction (FSI) model. The charge or electricity in the living human conventional outflow pathway generated by the charged endothelial glycocalyx layer plays a minor biomechanical role in the resultant stresses and strains as well as the hydrodynamics of the aqueous humor.

Cell senescence alters responses of porcine trabecular meshwork cells to shear stress

Mechanical microenvironment and cellular senescence of trabecular meshwork cells (TMCs) are suspected to play a vital role in primary open-angle glaucoma pathogenesis. However, central questions remain about the effect of shear stress on TMCs and how aging affects this process. We have investigated the effect of shear stress on the biomechanical properties and extracellular matrix regulation of normal and senescent TMCs. We found a more significant promotion of Fctin formation, a more obvious realignment of F-actin fibers, and a more remarkable increase in the stiffness of normal cells in response to the shear stress, in comparison with that of senescent cells. Further, as compared to normal cells, senescent cells show a reduced extracellular matrix turnover after shear stress stimulation, which might be attributed to the different phosphorylation levels of the extracellular signal-regulated kinase. Our results suggest that TMCs are able to sense and respond to the shear stress and cellular senescence undermines the mechanobiological response, which may lead to progressive failure of cellular TM function with age.

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.

Multiomics analysis reveals the mechanical stress-dependent changes in trabecular meshwork cytoskeletal-extracellular matrix interactions

Trabecular meshwork (TM) tissue is subjected to constant mechanical stress due to the ocular pulse created by the cardiac cycle. This brings about alterations in the membrane lipids and associated cell–cell adhesion and cell–extracellular matrix (ECM) interactions, triggering intracellular signaling responses to counter mechanical insults. A loss of such response can lead to elevated intraocular pressure (IOP), a major risk factor for primary open-angle glaucoma. This study is aimed to understand the changes in signaling responses by TM subjected to mechanical stretch. We utilized multiomics to perform an unbiased mRNA sequencing to identify changes in transcripts, mass spectrometry- (MS-) based quantitative proteomics for protein changes, and multiple reaction monitoring (MRM) profiling-based MS and high-performance liquid chromatography (HPLC-) based MS to characterize the lipid changes. We performed pathway analysis to obtain an integrated map of TM response to mechanical stretch. The human TM cells subjected to mechanical stretch demonstrated an upregulation of protein quality control, oxidative damage response, pro-autophagic signal, induction of anti-apoptotic, and survival signaling. We propose that mechanical stretch-induced lipid signaling via increased ceramide and sphingomyelin potentially contributes to increased TM stiffness through actin-cytoskeleton reorganization and profibrotic response. Interestingly, increased phospholipids and diacylglycerol due to mechanical stretch potentially enable cell membrane remodeling and changes in signaling pathways to alter cellular contractility. Overall, we propose the mechanistic interplay of macromolecules to bring about a concerted cellular response in TM cells to achieve mechanotransduction and IOP regulation when TM cells undergo mechanical stretch.

The physiological and pathophysiological roles of the autophagy lysosomal system in the conventional aqueous humor outflow pathway: More than cellular clean up

During the last few years, the autophagy lysosomal system is emerging as a central cellular pathway with roles in survival, acting as a housekeeper and stress response mechanism. Studies by our and other labs suggest that autophagy might play an essential role in maintaining aqueous humor outflow homeostasis, and that malfunction of autophagy in outflow pathway cells might predispose to ocular hypertension and glaucoma pathogenesis. In this review, we will collect the current knowledge and discuss the molecular mechanisms by which autophagy does or might regulate normal outflow pathway tissue function, and its response to different types of stressors (oxidative stress and mechanical stress). We will also discuss novel roles of autophagy and lysosomal enzymes in modulation of TGFβ signaling and ECM remodeling, and the link between dysregulated autophagy and cellular senescence. We will examine what we have learnt, using pre-clinical animal models about how dysregulated autophagy can contribute to disease and apply that to the current status of autophagy in human glaucoma. Finally, we will consider and discuss the challenges and the potential of autophagy as a therapeutic target for the treatment of ocular hypertension and glaucoma.

A Novel Technique Identifies Valve-Like Pathways Entering and Exiting Schlemm’s Canal in Macaca nemestrina Primates With Similarities to Human Pathways

Purpose: The aim of the study was 1) to describe a novel combination of techniques that permit immunohistochemistry imaging of Schlemm’s canal inlet (SIV) and outlet (SOV) valve-like structures, 2) to identify tissue-level SIV adhesive relationships linking the trabecular meshwork (TM) to hinged collagen leaflets at the Schlemm’s canal (SC) external wall, and 3) to determine whether the SIV lumen wall’s adhesive vascular markers are similar to those of the SC inner wall endothelium.

Materials and Methods: Anterior segments of 16 M. nemestrina primates underwent immunohistochemistry (IHC) labeling. We perfused fluorescent microspheres into 12 of the eyes. Limbal tissues were divided into quadrants, viscoelastic introduced into SC, tissues fixed, immunohistochemistry performed, radial segments cut, tissues clarified, and confocal microscopy performed. Finally, we generated ImageJ 3D projections encompassing the TM, SC, and distal pathways.

Results: IHC imaging identified 3D relationships between SIV, collector channel ostia, collector channels (CC), SOV, and intrascleral channels. Imaging depth increased 176.9%, following clarification (p < 0.0001). Imaging demonstrated CD31, collagen type 1 and 4 in the walls of the SIV lumen and more distal pathways. In eight eyes, 384 segments were examined, 447 SIV identified, and 15.4% contained microspheres.

Conclusion: Our technique’s imaging depth permitted the identification of SIV linkage between the TM and SOV. We found comparable cell–cell adhesion molecules (CD31) and basement membrane components in the SC inner wall and SIV lumen walls. Recent OCT studies have suggested that SIV tensional relationships may control CC entrance dimensions that regulate distal resistance. Cellular adhesive properties sustain SIV tensional relationships. These SIV cell–cell and cell-basement membrane properties warrant further study because abnormalities could be a factor in the IOP elevation of glaucoma.

Fibrillin-1 mutant mouse captures defining features of human primary open glaucoma including anomalous aqueous humor TGF beta-2

Primary open angle glaucoma (POAG) features an optic neuropathy, elevated aqueous humor (AH) TGFβ2, and major risk factors of central corneal thickness (CCT), increasing age and intraocular pressure (IOP). We examined Tight skin (Tsk) mice to see if mutation of fibrillin-1, a repository for latent TGFβ, is associated with characteristics of human POAG. We measured: CCT by ocular coherence tomography (OCT); IOP; retinal ganglion cell (RGC) and optic nerve axon counts by microscopic techniques; visual electrophysiologic scotopic threshold responses (STR) and pattern electroretinogram (PERG); and AH TGFβ2 levels and activity by ELISA and MINK epithelial cell-based assays respectively. Tsk mice had open anterior chamber angles and compared with age-matched wild type (WT) mice: 23% thinner CCT (p < 0.003); IOP that was higher (p < 0.0001), more asymmetric (p = 0.047), rose with age (p = 0.04) and had a POAG-like frequency distribution. Tsk mice also had RGCs that were fewer (p < 0.04), declined with age (p = 0.0003) and showed increased apoptosis and glial activity; fewer optic nerve axons (p = 0.02); abnormal axons and glia; reduced STR (p < 0.002) and PERG (p < 0.007) visual responses; and higher AH TGFβ2 levels (p = 0.0002) and activity (p = 1E-11) especially with age. Tsk mice showed defining features of POAG, implicating aberrant fibrillin-1 homeostasis as a pathogenic contributor to emergence of a POAG phenotype.

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.

Schlemm's canal: the outflow 'vessel'

In a healthy eye, the aqueous humour (AH) flows via the ciliary body and trabecular meshwork into the collector channels, which carry it to the episcleral veins. In glaucoma, a heterogeneous group of eye disorders affecting approximately 60 million individuals worldwide, the juxtacanalicular meshwork offers greater resistance to the outflow of the AH, leading to an increase in outflow resistance that gradually results in elevated intraocular pressure (IOP). The present review comprehensively covers the morphology of Schlemm’s canal (SC) and AH pathways. The path of the AH from the anterior chamber through the trabeculum into suprascleral and conjunctival veins via collector channels is described, and the role of SC in the development of glaucoma and outflow resistance is discussed. Finally, channelography is presented as a precise method of assessing the conventional drainage pathway and facilitating localization of an uncollapsed collector and aqueous veins. Attention is also given to the relationship between aqueous and episcleral veins and heartbeat. Possible directions of future research are proposed.

Modeling the biomechanics of the conventional aqueous outflow pathway microstructure in the human eye

BACKGROUND AND OBJECTIVE

Intraocular pressure (IOP) is determined by aqueous humor outflow resistance, which is a function of the combined resistance of Schlemm's canal (SC) endothelium and the trabecular meshwork (TM) and their interactions in the juxtacanalicular connective tissue (JCT) region. Aqueous outflow in the conventional outflow pathway results in pressure gradient across the TM, JCT, and SC inner wall, and induces mechanical stresses and strains that influence the geometry and homeostasis of the outflow system. The outflow resistance is affected by alteration in tissues' geometry, so there is potential for active, two-way, fluid-structure interaction (FSI) coupling between the aqueous humor (fluid) and the TM, JCT, and SC inner wall (structure). However, our understanding of the biomechanical interactions of the aqueous humor with the outflow connective tissues and its contribution to the outflow resistance regulation is incomplete.

METHODS

In this study, a microstructural finite element (FE) model of a human eye TM, JCT, and SC inner wall was constructed from a segmented, high-resolution histologic 3D reconstruction of the human outflow system. Three different elastic moduli (0.004, 0.128, and 51.5 MPa based on prior reports) were assigned to the TM/JCT complex while the elastic modulus of the SC inner wall was kept constant at 0.00748 MPa. The hydraulic conductivity was programmed separately for the TM, JCT, and SC inner wall using a custom subroutine. Cable elements were embedded into the TM and JCT extracellular matrix to represent the directional stiffness imparted by anisotropic collagen fibril orientation. The resultant stresses and strains in the outflow system were calculated using fluid-structure interaction method.

RESULTS

The higher TM/JCT stiffness resulted in larger stresses, but smaller strains in the outflow connective tissues, and resulted in a 4- and 5-fold larger pressure drop across the SC inner wall, respectively, compared to the most compliant model. Funneling through µm-sized SC endothelial pores was evident in the models at lower tissue stiffness, but aqueous flow was more turbulent in models with higher TM/JCT stiffness.

CONCLUSIONS

The mechanical properties of the outflow tissues play a crucial role in the hydrodynamics of the aqueous humor in the conventional outflow system.

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.

Anterior Chamber Angle Changes in Primary Angle-closure Glaucoma Following Phacoemulsification Versus Phacotrabeculectomy: A Prospective Randomized Clinical Trial

PRCIS

We used anterior segment optical coherence tomography to investigate anterior chamber angle in primary angle-closure glaucoma (PACG) eyes following phacoemulsification and phacotrabeculectomy. Angle widening was significantly greater after phacoemulsification up to 6 months after the surgery.

PURPOSE

To compare anterior chamber angle following 2 common surgeries for PACG.

METHODS

One hundred ten glaucoma patients were evaluated in this single center, prospective, randomized clinical trial. Those with concomitant PACG and senile cataract and without a history of ocular surgery, trauma, or chronic miotic use were recruited. Monocular patients were also excluded. Finally, 52 eligible subjects were randomly assigned to phacoemulsification ("Phaco" group, 25 eyes) or phacotrabeculectomy ("Combined" group, 27 eyes) surgeries. A swept-source, anterior segment optical coherence tomography device (CASIA SS-1000 OCT) was used to image the anterior segment. Mask graders used the images to measure the following parameters before and 1 week, 1 month, and 6 months after surgery: angle opening distance at 500 µm, trabecular iris surface area at 500 µm, and trabecular iris angle at 500 µm.

RESULTS

There was no significant difference between study groups regarding best-corrected visual acuity, intraocular pressure (IOP), and the number of glaucoma medications in preoperative or postoperative visits (P>0.076). Also, the measured angle parameters were not statistically significantly different between the 2 groups before surgery (P>0.123). After surgery, all measured parameters were significantly increased in both groups (P<0.0001). At the 6-month follow-up, nasal angle opening distance at 500 µm was 0.383±0.027 vs. 0.349±0.017, trabecular iris surface area at 500 µm was 0.141±0.007 vs. 0.125±0.005, and trabecular iris angle at 500 µm was 40.1±12.9 vs. 34.6±3.1 in Phaco and Combined groups, respectively (P<0.0001 for all).

CONCLUSIONS

Anterior chamber angle widening by anterior segment optical coherence tomography was observed in PACG patients following surgery and was significantly greater after phacoemulsification compared with combined phacotrabeculectomy at all time points. Similar IOP and medication burden were noted for up to 6 months as secondary outcomes. The contribution of angle changes to the IOP-lowering effect of phacoemulsification in PACG eyes needs further study.

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.

Minimally Invasive Glaucoma Surgery-to Remove or Preserve the Trabecular Meshwork: That is the Question?

How to cite this article: Dada T, Mahalingam K, Bhartiya S. Minimally Invasive Glaucoma Surgery—to Remove or Preserve the Trabecular Meshwork: That is the Question? J Curr Glaucoma Pract 2021;15(2):47–51.

Membrane cholesterol regulates TRPV4 function, cytoskeletal expression, and the cellular response to tension

Despite the association of cholesterol with debilitating pressure-related diseases such as glaucoma, heart disease, and diabetes, its role in mechanotransduction is not well understood. We investigated the relationship between mechanical strain, free membrane cholesterol, actin cytoskeleton, and the stretch-activated transient receptor potential vanilloid isoform 4 (TRPV4) channel in human trabecular meshwork (TM) cells. Physiological levels of cyclic stretch resulted in time-dependent decreases in membrane cholesterol/phosphatidylcholine ratio and upregulation of stress fibers. Depleting free membrane cholesterol with m-β-cyclodextrin (MβCD) augmented TRPV4 activation by the agonist GSK1016790A, swelling and strain, with the effects reversed by cholesterol supplementation. MβCD increased membrane expression of TRPV4, caveolin-1, and flotillin. TRPV4 did not colocalize or interact with caveolae or lipid rafts, apart from a truncated ∼75 kDa variant partially precipitated by a caveolin-1 antibody. MβCD induced currents in TRPV4-expressing Xenopus laevis oocytes. Thus, membrane cholesterol regulates trabecular transduction of mechanical information, with TRPV4 channels mainly located outside the cholesterol-enriched membrane domains. Moreover, the biomechanical milieu itself shapes the lipid content of TM membranes. Diet, cholesterol metabolism, and mechanical stress might modulate the conventional outflow pathway and intraocular pressure in glaucoma and diabetes in part by modulating TM mechanosensing.

Hemoglobin Video Imaging Provides Novel In Vivo High-Resolution Imaging and Quantification of Human Aqueous Outflow in Patients with Glaucoma

Purpose

Noninvasive, detailed measurement of the dynamics of human aqueous outflow is difficult to achieve with currently available clinical tools. We used hemoglobin video imaging (HVI) to develop a technique to image and quantify human aqueous outflow noninvasively and in real time.

Design

A prospective observational study to describe characteristics of aqueous veins and a pilot prospective interventional feasibility study to develop quantification parameters.

Participants

Patients were recruited from the Cambridge University Hospitals NHS Foundation Trust Glaucoma clinic. The observational study included 30 eyes, and the pilot interventional feasibility study was performed on 8 eyes undergoing selective laser trabeculoplasty (SLT). Our SLT protocol also included the installation of pilocarpine and apraclonidine eye drops.

Methods

Participants underwent HVI alongside their usual clinic visit.

Main Outcome Measures

The change in cross-sectional area (CSA) of the aqueous column within episcleral veins was correlated with intraocular pressure (IOP) reduction and change in visual field mean deviation (MD) before and after intervention. Fluctuations in contrast and pixel intensity of red blood cells in an aqueous vein were calculated to compare the flow rate before and after intervention using autocorrelation analysis.

Results

Hemoglobin video imaging enables the direct observation of aqueous flow into the vascular system. Aqueous is seen to centralize within a laminar venous column. Flow is pulsatile, and fluctuations of flow through globe pressure or compression of the aqueous vein are observed. There was a significant increase in the aqueous column after the administration of our SLT protocol (n = 13; P < 0.05). This correlated with the degree of IOP reduction (n = 13; Pearson’s correlation coefficient 0.7; P = 0.007) and the improvement in MD observed postintervention (n = 8; Pearson’s correlation coefficient 0.75; P = 0.03). Autocorrelation analysis demonstrated a faster rate of decay in an aqueous vein after intervention, indicating an increase in flow rate.

Conclusions

Hemoglobin video imaging can be incorporated into a routine clinic slit-lamp examination to allow a detailed assessment and quantification of aqueous outflow in real time. It has the potential to be used to help target therapeutic interventions to improve aqueous outflow and further advance our understanding of aqueous outflow dysregulation in the pathogenesis of glaucoma.

Heart rhythm-synchronized fibrin flap in a glaucoma tube shunt: The heartbeat acts as a drainage pump for the aqueous humor: A case report

RATIONALE

The amount of aqueous humor that is constantly produced by the ciliary processes and the equal amount that flows out through the trabecular meshwork via the drainage angle or through the uveoscleral route is very small (2-3 μg/min each), representing approximately 1% of the content of the anterior chamber; therefore, it is challenging to visualize its flow.

PATIENT CONCERNS

A 69-year-old man who had high intraocular pressure (IOP) (>20 mm Hg) with the maximum glaucoma eyedrop dose and presented with severe visual field loss (Humphrey Field Analyzer 30-2: -26.32 dB) had been implanted with a 350-mm2 Baerveldt tube of the aqueous chamber type for refractory open-angle glaucoma. The IOP ultimately decreased (<15 mm Hg) with no need for glaucoma eyedrops.

DIAGNOSES

After the procedures, a fibrin membrane repeatedly formed on the anterior surface of the intraocular lens.

INTERVENTIONS

This issue was resolved by two rounds of neodymium-doped yttrium aluminum granet (Nd:YAG) laser surgery and prescription steroidal eyedrops.

OUTCOMES

During the laser surgery, an unusual and unintended fibrin flap appeared at the opening of the Baerveldt tube; this flap moved synchronously with the heartbeat, as verified by checking the pulse at the radial artery of the wrist. The fibrin flap mimicked the behavior of a cardiac valve, and the aqueous humor and stray fibrin particles mimicked the blood in the chambers of the heart. Although the Baerveldt tube itself is an artificial instrument that is not present in normal human eyes, we hypothesize that our observation shows the fundamental mechanism of aqueous humor drainage.

LESSONS

This novel, vividly descriptive observation highlights the important role of the heartbeat as a drainage pump in aqueous humor flow dynamics and IOP homeostasis, which are treatment targets for glaucoma, the leading cause of blindness.

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.

Aqueous humor metabolite profile of pseudoexfoliation glaucoma is distinctive

Pseudoexfoliation (PEX) is a known cause of secondary open angle glaucoma. PEX glaucoma is associated with structural and metabolic changes in the eye. Despite similarities, PEX and primary open angle glaucoma (POAG) may have differences in the composition of metabolites. We analyzed the metabolites of the aqueous humor (AH) of PEX subjects sequentially first using nuclear magnetic resonance (1H NMR: HSQC and TOCSY), and subsequently with liquid chromatography tandem mass spectrometry (LC-MS/MS) implementing isotopic ratio outlier analysis (IROA) quantification. The findings were compared with previous results for POAG and control subjects analyzed using identical sequential steps. We found significant differences in metabolites between the three conditions. Principle component analysis (PCA) and partial least squares discriminant analysis (PLS-DA) indicated clear grouping based on the metabolomes of the three conditions. We used machine learning algorithms and a percentage set of the data to train, and utilized a different or larger dataset to test whether a trained model can correctly classify the test dataset as PEX, POAG or control. Three different algorithms: linear support vector machines (SVM), deep learning, and a neural network were used for prediction. They all accurately classified the test datasets based on the AH metabolome of the sample. We next compared the AH metabolome with known AH and TM proteomes and genomes in order to understand metabolic pathways that may contribute to alterations in the AH metabolome in PEX. We found potential protein/gene pathways associated with observed significant metabolite changes in PEX.

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.

The ocular pulse decreases aqueous humor outflow resistance by stimulating nitric oxide production

Intraocular pressure (IOP) is not static, but rather oscillates by 2-3 mmHg because of cardiac pulsations in ocular blood volume known as the ocular pulse. The ocular pulse induces pulsatile shear stress in Schlemm's canal (SC). We hypothesize that the ocular pulse modulates outflow facility by stimulating shear-induced nitric oxide (NO) production by SC cells. We confirmed that living mice exhibit an ocular pulse with a peak-to-peak (pk-pk) amplitude of 0.5 mmHg under anesthesia. Using iPerfusion, we measured outflow facility (flow/pressure) during alternating periods of steady or pulsatile IOP in both eyes of 16 cadaveric C57BL/6J mice (13-14 weeks). Eyes were retained in situ, with an applied mean pressure of 8 mmHg and 1.0 mmHg pk-pk pressure amplitude at 10 Hz to mimic the murine heart rate. One eye of each cadaver was perfused with 100 µM L-NAME to inhibit NO synthase, whereas the contralateral eye was perfused with vehicle. During the pulsatile period in the vehicle-treated eye, outflow facility increased by 16 [12, 20] % (P < 0.001) relative to the facility measured during the preceding and subsequent steady periods. This effect was partly inhibited by L-NAME, where pressure pulsations increased outflow facility by 8% [4, 12] (P < 0.001). Thus, the ocular pulse causes an immediate increase in outflow facility in mice, with roughly one-half of the facility increase attributable to NO production. These studies reveal a dynamic component to outflow function that responds instantly to the ocular pulse and may be important for outflow regulation and IOP homeostasis.

TRPV4-Rho signaling drives cytoskeletal and focal adhesion remodeling in trabecular meshwork cells

Intraocular pressure (IOP) is dynamically regulated by the trabecular meshwork (TM), a mechanosensitive tissue that protects the eye from injury through dynamic regulation of aqueous humor flow. TM compensates for mechanical stress impelled by chronic IOP elevations through increased actin polymerization, tissue stiffness, and contractility. This process has been associated with open angle glaucoma; however, the mechanisms that link mechanical stress to pathological cytoskeletal remodeling downstream from the mechanotransducers remain poorly understood. We used fluorescence imaging and biochemical analyses to investigate cytoskeletal and focal adhesion remodeling in human TM cells stimulated with physiological strains. Mechanical stretch promoted F-actin polymerization, increased the number and size of focal adhesions, and stimulated the activation of the Rho-associated protein kinase (ROCK). Stretch-induced activation of the small GTPase Ras homolog family member A (RhoA), and tyrosine phosphorylations of focal adhesion proteins paxillin, focal adhesion kinase (FAK), vinculin, and zyxin were time dependently inhibited by ROCK inhibitor trans-4-[(1R)-1-aminoethyl]-N-4-pyridinylcyclohexanecarboxamide dihydrochloride (Y-27632), and by HC-067047, an antagonist of transient receptor potential vanilloid 4 (TRPV4) channels. Both TRPV4 and ROCK activation were required for zyxin translocation and increase in the number/size of focal adhesions in stretched cells. Y-27632 blocked actin polymerization without affecting calcium influx induced by membrane stretch and the TRPV4 agonist GSK1016790A. These results reveal that mechanical tuning of TM cells requires parallel activation of TRPV4, integrins, and ROCK, with chronic stress leading to sustained remodeling of the cytoskeleton and focal complexes.

Microinvasive Glaucoma Surgery: A Review of Schlemm's Canal-Based Procedures

Microinvasive glaucoma surgery has gained popularity over the past decade. It can be performed using three different mechanisms. In the present review, we focused on Schlemm’s canal (SC)-based surgery, which increases aqueous humor (AH) outflow into the aqueous veins by either removal of the trabecular meshwork (TM) or an increase in the tension in the TM. In primary open-angle glaucoma (POAG), the TM is the most likely region for increased AH outflow resistance. Theoretically, removal of the TM can improve the AH outflow; hence, glaucoma specialists focus on microsurgical dissection of the TM. In this review, we analyzed the available literature to examine SC-related microsurgical modalities based on the histopathological proofs of the localization of resistance of the AH outflow. First, we considered the role, anatomy, and physiology of the TM and SC. We referred to studies that describe the mechanisms and potential pathways, related to increased intraocular pressure in the POAG, that are targeted using the SC-related microsurgical interventions. Next, we took a closer look at the gonioscopic tools necessary for an ab-interno approach and explored incision canal surgery: ab-interno trabeculectomy using different instrumentation (Trabectome^®, Kahook Dual Blade) and variations of the technique. Thereafter, we discussed ab-interno canaloplasty, explaining the technique and reviewing its effectiveness. Finally, we presented the scope for future research in the field. Although the iStent also targets SC by bypassing it, this device has been reviewed extensively elsewhere.

Ab Interno Canaloplasty in Open-angle Glaucoma Patients Combined With In Vivo Trypan Blue Aqueous Venography

PURPOSE

To demonstrate canalogram patterns observed when trypan blue tracer is combined with oculoviscoelastic device during ab interno canaloplasty, and discuss surgical results and potential implications for diagnosis, prognosis, and treatment.

METHODS

This is a retrospective small case series study.

RESULTS

The authors performed this procedure on 5 patients with informed consent with 4 to 18 months of follow-up. All procedures have been complication free. The average preoperative intraocular pressure was 16.4 mm Hg with an average number of topical glaucoma medications being equal to 3.6. Postoperatively, the average intraocular pressure for the group was reduced to 13 mm Hg with a concurrent drop in the average number of topical medications used equaling 1.8.

CONCLUSIONS

Currently, there are limited practical options to evaluate the patency of the aqueous outflow system in our patients. The primary purpose of this paper is to describe canalogram patterns observed during ab interno canaloplasty with trypan blue. This technique may allow us to further our understanding of the aqueous outflow system and its role in the underlying pathophysiology of glaucoma. Further research is required to evaluate the prognostic and diagnostic significance of this technique.

Trabodenoson on trabecular meshwork rejuvenation: a comprehensive review of clinical data

INTRODUCTION

Trabodenoson is an adenosine mimetic acting selectively at the A1 receptor (A1R) subtype, involved in multiple signaling pathways including matrix metalloproteinase (MMP-2) associated with glaucoma pathological processes. It has been developed as a Phase 3 candidate for the treatment of patients with primary open-angle glaucoma (POAG) or ocular hypertension (OH).

AREA COVERED

This review summarizes the molecular traits of Trabodenoson in intraocular pressure (IOP) regulations and provides a scientific interpretation of the Phase 2 clinical study results. This article sheds light on the root causes of the two pivotal Phase 3 clinical trial failures in patients with POAG or OH; it further highlights the discovery of MMP-2 in trabecular meshwork (TM) rejuvenation, which has strategic importance in long-term glaucoma patient care.

EXPERT OPINION

Trabodenoson is a BID glaucoma eye drop with a possible QD dose as maintenance. Its Phase 3 pivotal clinical trials failed at the wrong dose and dosing regimen because of the misinterpretation of the complex IOP results from the Phase 2 monotherapy and combination studies. The future development should focus on the TM benefits whilst unleashing its potential of neural protection through nanoparticle eye drops, medical coating, and sustained release drug delivery.

The role of Piezo1 in conventional aqueous humor outflow dynamics

Controlling intraocular pressure (IOP) remains the mainstay of glaucoma therapy. The trabecular meshwork (TM), the key tissue responsible for aqueous humor (AH) outflow and IOP maintenance, is very sensitive to mechanical forces. However, it is not understood whether Piezo channels, very sensitive mechanosensors, functionally influence AH outflow. Here, we characterize the role of Piezo1 in conventional AH outflow. Immunostaining and western blot analysis showed that Piezo1 is widely expressed by TM. Patch-clamp recordings in TM cells confirmed the activation of Piezo1-derived mechanosensitive currents. Importantly, the antagonist GsMTx4 for mechanosensitive channels significantly decreased steady-state facility, yet activation of Piezo1 by the specific agonist Yoda1 did not lead to a facility change. Furthermore, GsMTx4, but not Yoda1, caused a significant increase in ocular compliance, a measure of the eye's transient response to IOP perturbation. Our findings demonstrate a potential role for Piezo1 in conventional outflow, likely under pathological and rapid transient conditions.

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Piezo1 is functionally expressed in the TM, the most important tissue controlling IOP

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Suppression of mechanosensitive channel leads to a significant decrease in facility

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Our data suggest a role for Piezo in pathological situations and rapid IOP transients

Piezo1 channels mediate trabecular meshwork mechanotransduction and promote aqueous fluid outflow

KEY POINTS

Trabecular meshwork (TM) is a highly mechanosensitive tissue in the eye that regulates intraocular pressure through the control of aqueous humour drainage. Its dysfunction underlies the progression of glaucoma but neither the mechanisms through which TM cells sense pressure nor their role in aqueous humour outflow are understood at the molecular level. We identified the Piezo1 channel as a key TM transducer of tensile stretch, shear flow and pressure. Its activation resulted in intracellular signals that altered organization of the cytoskeleton and cell-extracellular matrix contacts and modulated the trabecular component of aqueous outflow whereas another channel, TRPV4, mediated a delayed mechanoresponse. This study helps elucidate basic mechanotransduction properties that may contribute to intraocular pressure regulation in the vertebrate eye.

ABSTRACT

Chronic elevations in intraocular pressure (IOP) can cause blindness by compromising the function of trabecular meshwork (TM) cells in the anterior eye, but how these cells sense and transduce pressure stimuli is poorly understood. Here, we demonstrate functional expression of two mechanically activated channels in human TM cells. Pressure-induced cell stretch evoked a rapid increase in transmembrane current that was inhibited by antagonists of the mechanogated channel Piezo1, Ruthenium Red and GsMTx4, and attenuated in Piezo1-deficient cells. The majority of TM cells exhibited a delayed stretch-activated current that was mediated independently of Piezo1 by TRPV4 (transient receptor potential cation channel, subfamily V, member 4) channels. Piezo1 functions as the principal TM transducer of physiological levels of shear stress, with both shear and the Piezo1 agonist Yoda1 increasing the number of focal cell-matrix contacts. Analysis of TM-dependent fluid drainage from the anterior eye showed significant inhibition by GsMTx4. Collectively, these results suggest that TM mechanosensitivity utilizes kinetically, regulatory and functionally distinct pressure transducers to inform the cells about force-sensing contexts. Piezo1-dependent control of shear flow sensing, calcium homeostasis, cytoskeletal dynamics and pressure-dependent outflow suggests potential for a novel therapeutic target in treating glaucoma.

Turn Back Elevation of Once Reduced IOP After Trabeculotomy Ab Externo and Kahook Dual Blade Surgeries Combined with Cataract Surgery

Purpose

To evaluate the surgical outcome of internal trabeculectomy by Kahook dual blade (KDB) with that of external trabeculotomy, each combined with phaco-lensectomy and intraocular lens implantation (Phaco).

Patients and Methods

This is a retrospective comparative study. The primary and secondary outcome measures are postsurgical intraocular pressure and postsurgical hyphema. One eye each of 76 primary open angle glaucoma (POAG), pseudoexfoliation glaucoma (PEG) and ocular hypertension (OH) patients underwent external phaco-trabeculotomy, and that of 40 POAG, PEG and OH patients underwent phaco-KDB surgery.

Results

Reduction of the intraocular pressure (IOP) by phaco-KDB at one and two years was 28.4 and 27.8%, respectively, and was not significantly different from that by external phaco-trabeculotomy of 32.7% (P=0.256) and 31.5% (P=0.468), respectively. Three months postsurgical IOP after phaco-KDB and external phaco-trabeculotomy was 16.1 and 15.9 mmHg, respectively. There was a significant turn back elevation of once reduced IOP to 17.1 (P=0.0207) and 17.0 mmHg (P=0.0096) at 24 months, respectively. There were no differences in success probability to achieve IOP below 17 mmHg (P=0.120), 21 mmHg (P=0.719) and >20% IOP reduction (P=0.309) with medication(s) at two years between the phaco-KDB and external phaco-trabeculotomy cohorts. Younger age was a significant (P<0.001) risk factor for failure; however, presurgical IOP (P=0.466), the type of surgery (P=0.219) and presence of postsurgical IOP spike (P=0.737) were not significant risk factors by the Cox proportional hazard model. Hyphema and spike of the IOP in phaco-KDB and external phaco-trabeculotomy cohorts were 40% and 88% (P<0.001), and 53% and 41% (P=0.238), respectively.

Conclusion

The IOP reduction by the phaco-KDB was equivalent to that by external phaco-trabeculotomy up to two years. In both cohorts, once reduced postsurgical IOP tend to increase up to 24 months.

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.

How many aqueous humor outflow pathways are there?

The aqueous humor (AH) outflow pathways definition is still matter of intense debate. To date, the differentiation between conventional (trabecular meshwork) and unconventional (uveoscleral) pathways is widely accepted, distinguishing the different impact of the intraocular pressure on the AH outflow rate. Although the conventional route is recognized to host the main sites for intraocular pressure regulation, the unconventional pathway, with its great potential for AH resorption, seems to act as a sort of relief valve, especially when the trabecular resistance rises. Recent evidence demonstrates the presence of lymphatic channels in the eye and proposes that they may participate in the overall AH drainage and intraocular pressure regulation, in a presumably adaptive fashion. For this reason, the uveolymphatic route is increasingly thought to play an important role in the ocular hydrodynamic system physiology. As a result of the unconventional pathway characteristics, hydrodynamic disorders do not develop until the adaptive routes cannot successfully counterbalance the increased AH outflow resistance. When their adaptive mechanisms fail, glaucoma occurs. Our review deals with the standard and newly discovered AH outflow routes, with particular attention to the importance they may have in opening new therapeutic strategies in the treatment of ocular hypertension and glaucoma.

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.