Visualization of the conventional outflow pathway in the living human eye

Anterior segment and Schlemm's canal alterations during the menstrual cycle in reproductive-age Turkish women

PURPOSE

To investigate variations in central corneal thickness (CCT), corneal aberrations, and anterior chamber depth througout the follicular and luteal phases of the menstrual cycle in reproductive-age women with regular menstruation using corneal topography and changes in Schlemm's canal (SC) diameter and area using the anterior segment (AS) module of enchanced depth imaging optical coherence tomography (EDI-OCT).

METHODS

Corneal topography and AS-EDI-OCT of 69 volunteers included in this prospective study were performed between days 1-5 of the cycle for the follicular phase and between days 25-35 for the luteal phase. SC diameter and area calculations were calculated using the ImageJ software.

RESULTS

During the luteal phase, there was a significant increase in CCT and SC diameter and trefoil 0° aberrations compared with the follicular phase. Additionally, a significant positive correlation was obtained between intraocular pressure and SC diameter, and a significant negative correlation between CCT and SC diameter and area.

CONCLUSIONS

Changes in corneal and anterior chamber angle were observed due to hormonal changes in the menstrual cycle. This is the first study to investigate the changes in SC diameter and area and corneal aberrations during the menstrual cycle.

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.

Reduced Aqueous Humor Outflow Pathway Arborization in Childhood Glaucoma Eyes

Purpose

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

Methods

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

Results

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

Conclusions

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

Translational Relevance

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

First-Generation iStent Bypass Implantation versus ab Externo Canaloplasty Combined with Phacoemulsification in Patients with Primary Open Angle Glaucoma-12-Month Follow-Up

This prospective, non-randomized, interventional clinical study evaluated the efficacy and safety profile of first-generation iStent bypass implantation versus ab externo canaloplasty, both combined with phacoemulsification, in patients with primary open-angle glaucoma (POAG) after 12-month follow-up. A total of 138 patients with POAG and their 138 eyes (69 phaco-iStent and 69 phacocanaloplasty) were included. Postoperatively at 12 months, the mean intraocular pressure (IOP) decreased from 18.44 ± 3.88 to 15.51 ± 2.50 mmHg and from 17.20 ± 4.04 to 14.97 ± 2.37 mmHg in the phaco-iStent (PiS) and phacocanaloplasty (PC) groups, respectively (p = 0.480). In both groups, 35.7% achieved >20% IOP reduction from baseline. A total of 86% and 71.4% of the eyes were medication-free at 12 months in the PiS and PC groups, respectively. In both groups, all eyes showed improvement in best-corrected visual acuity compared with baseline and demonstrated similar safety profiles throughout the 12-month follow-up period. This study showed equal hypotensive effects of PiS and PC. Both procedures significantly reduced the IOP and the requirement for IOP-lowering medications for at least 12 months postoperatively, with no significant differences between the groups.

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.

Circumferential (360°) trabeculotomy for steroid-induced glaucoma in adults

PURPOSE

To evaluate the safety and efficacy of 360° circumferential trabeculotomy (TO) for steroid induced glaucoma (SIG) of short duration.

METHODS

Retrospective analysis of surgical results of 46 eyes of 35 patients undergoing microcatheter-assisted TO. All eyes had high intraocular pressure for at most about 3 years due to steroid use. Follow-up was between 2.63 and 47.9 months (mean 23.9, median 25.6).

RESULTS

Intraocular pressure (IOP) before surgery was 30.8 ± 8.3 mm Hg, with 3.8 ± 1.0 pressure-lowering medications. After 1 to 2 years, mean IOP was 11.2 ± 2.6 mm Hg (n = 28); mean number of IOP-lowering medications was 0.9 ± 1.3. At their last follow-up, 45 eyes had an IOP < 21 mm Hg, and 39 eyes had an IOP < 18 mm Hg with or without medication. After 2 years, the estimated probability of having an IOP below 18 mm Hg (with or without medication) was 85 ± 6%, and the estimated probability of not using medication was 56 ± 7%. Steroid response was no longer present in all eyes receiving steroids after surgery. Minor complications consisted of hyphema, transient hypotony, or hypertony. One eye proceeded to receiving a glaucoma drainage implant.

CONCLUSION

TO is particularly effective in SIG with relative short duration. This concurs with the pathophysiology of the outflow system. This procedure seems particularly suited for eyes for which target pressures in the mid-teens are acceptable, particularly when chronic use of steroids is necessary .

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.

Canaloplasty in Pseudoexfoliation Glaucoma. Can It Still Be Considered a Good Choice?

Purpose: The aim of this study was to assess the long-term outcomes of canaloplasty surgery in pseudoexfoliation glaucoma (PEXG) patients. Material and Methods: A total of 116 PEXG patients with an intraocular pressure (IOP) > 21 mm/Hg and maximum tolerated local medical therapy who underwent canaloplasty from February 2008 to January 2022 were considered. Every six months, all subjects underwent a complete ophthalmic examination. The period of follow-up ranged from 2 to 167 months. Inclusion criteria included only patients for whom the entire procedure could be completed with a follow-up of at least 2 years. Results: Amongst the 116 PEXG patients, the entire procedure could not be performed in 10 eyes (8.6%), and thus they were not considered in the analysis. Twenty-three patients did not reach the two-year follow-up and another 16 patients during this time period were lost. A total of 67 patients with a mean follow-up of 49 ± 32.3 months were considered in the analysis. The pre-operative mean IOP was 31.2 ± 8.7 mm/Hg (range 20−60). The mean IOP at the two-year follow-up was 17.2 ± 6.7 mmHg, with a mean reduction from baseline of 44.9%. After two years, the qualified success rates according to three different criteria (IOP ≤ 21, ≤18 and ≤16 mmHg) were 80.6%, 73.1% and 61.0%, respectively. The total number of medications used pre- and at the follow-up at 2 years was 3.5 ± 0.8 and 1.2 ± 1.4, respectively. Early complications included: hyphema, in about 30% of cases; Descemet membrane detachment (4.9%); and IOP spikes > 10 mmHg (9.7%). A late failure with an acute IOP rise of up to 50 mmHg was observed in 41 cases (61.2%) after 3 to 72 months. Conclusions: Long-term post-operative outcomes of canaloplasty in PEXG patients appear to be quite good on average; however, an acute rise in IOP can be observed in more than 60% of the cases after a long period of satisfactory IOP control. For this reason, canaloplasty may not be suitable in eyes with PEXG, especially in patients with severe functional damage.

Evaluation of different OCT systems in quantitative imaging of human Schlemm's canal

We examined the performance of human Schlemm's canal (SC) imaging using different OCT devices: CIRRUS 5000 (840 nm, spectral-domain (SD)-OCT), PLEX Elite 9000 (1060 nm, swept-source (SS)-OCT) and CASIA SS-1000 (1310 nm, SS-OCT), and analyzed potential impact factors on visualization and the quantitative assessment of SC morphology in a pilot study. Ten healthy subjects were imaged using three OCT devices by a single experienced operator on the same day. Each eye underwent two cubic scans by each device, one on nasal and the other on temporal quadrant. The B-scan showing the largest SC was manually selected for processing. Four quantitative metrics, including one morphological metric as cross-sectional area (CSA), and three performance metrics as contrast, continuity, and coverage, were derived from the datasets. Repeated-measures ANOVA was used to investigate the difference between these parameters from the three devices (P < 0.05). We found the CSA measured from CIRRUS was significantly larger than PLEX, followed by CASIA. The contrast was highest in CIRRUS, followed by PLEX and CASIA. The coverage was also higher in CIRRUS as compared to PLEX and CASIA. No significant difference was seen in the continuity from the three devices. In summary, we showed the measurements from the three devices were not interchangeable.

Morphological changes in the trabecular meshwork and Schlemm's canal after treatment with topical intraocular pressure-lowering agents

Glaucoma treatment is usually initiated with topical medication that lowers the intraocular pressure (IOP) by reducing the aqueous production, enhancing the aqueous outflow, or both. However, the effect of topical IOP-lowering medications on the microstructures of the aqueous outflow pathway are relatively unknown. In this retrospective, observational study, 56 treatment-naïve patients with primary open-angle glaucoma were enrolled. Images of the nasal and temporal corneoscleral limbus were obtained using anterior segment optical coherence tomography (AS-OCT). The conjunctival vessels and iris anatomy were used as landmarks to select the same limbal area scan, and the trabecular meshwork (TM) width, TM thickness, and Schlemm’s canal (SC) area were measured before and after using the IOP-lowering agents for 3 months. Among the 56 patients enrolled, 33 patients used prostaglandin (PG) analogues, and 23 patients used dorzolamide/timolol fixed combination (DTFC). After 3 months of DTFC usage, the TM width, TM thickness, and SC area did not show significant changes in either the nasal or temporal sectors. Conversely, after prostaglandin analog usage, the TM thickness significantly increased, and the SC area significantly decreased (all P < 0.01). These findings warrant a deeper investigation into their relationship to aqueous outflow through the conventional and unconventional outflow pathways after treatment with PG analogues.

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.

Full circumferential morphological analysis of Schlemm's canal in human eyes using megahertz swept source OCT

We performed full circumferential imaging of the Schlemm's canal (SC) of two human eyes using a Fourier domain mode-lock laser (FDML) based 1.66-MHz SS-OCT prototype at 1060 nm. Eight volumes with overlapping margins were acquired around the limbal area with customized raster scanning patterns designed to fully cover the SC while minimizing motion artifacts. The SC was segmented from the volumes using a semi-automated active contour segmentation algorithm, whose mean dice similarity coefficient was 0.76 compared to the manual segmentation results. We also reconstructed three-dimensional (3D) renderings of the 360° SC by stitching the segmented SCs from the volumetric datasets. Quantitative metrics of the full circumferential SC were provided, including the mean and standard deviation (SD) of the cross-sectional area (CSA), the maximum CSA, the minimum and maximum SC opening width, and the number of collector channels (CC) stemming from the SC.

Development, Validation, and Innovation in Ophthalmic Laser-Based Imaging: Report From a US Food and Drug Administration-Cosponsored Forum

In April 2019, the US Food and Drug Administration, in conjunction with 11 professional ophthalmic, vision science, and optometric societies, convened a forum on laser-based imaging. The forum brought together the Food and Drug Administration, clinicians, researchers, industry members, and other stakeholders to stimulate innovation and ensure that patients in the US are the first in the world to have access to high-quality, safe, and effective medical devices. This conference focused on the technology, clinical applications, regulatory issues, and reimbursement issues surrounding innovative ocular imaging modalities. Furthermore, the emerging role of artificial intelligence in ophthalmic imaging was reviewed. This article summarizes the presentations, discussion, and future directions.

Looking into the future: Gene and cell therapies for glaucoma

Glaucoma is a complex group of optic neuropathies that affects both humans and animals. Intraocular pressure (IOP) elevation is a major risk factor that results in the loss of retinal ganglion cells (RGCs) and their axons. Currently, lowering IOP by medical and surgical methods is the only approved treatment for primary glaucoma, but there is no cure, and vision loss often progresses despite therapy. Recent technologic advances provide us with a better understanding of disease mechanisms and risk factors; this will permit earlier diagnosis of glaucoma and initiation of therapy sooner and more effectively. Gene and cell therapies are well suited to target these mechanisms specifically with the potential to achieve a lasting therapeutic effect. Much progress has been made in laboratory settings to develop these novel therapies for the eye. Gene and cell therapies have already been translated into clinical application for some inherited retinal dystrophies and age-related macular degeneration (AMD). Except for the intravitreal application of ciliary neurotrophic factor (CNTF) by encapsulated cell technology for RGC neuroprotection, there has been no other clinical translation of gene and cell therapies for glaucoma so far. Possible application of gene and cell therapies consists of long-term IOP control via increased aqueous humor drainage, including inhibition of fibrosis following filtration surgery, RGC neuroprotection and neuroregeneration, modification of ocular biomechanics for improved IOP tolerance, and inhibition of inflammation and neovascularization to prevent the development of some forms of secondary glaucoma.

Isolation and characterization of novel primary cells from the human distal outflow pathway

Ocular hypertension occurs due to increased resistance to aqueous humor removal through the conventional outflow pathway. Unlike the proximal region of the conventional outflow pathway, the distal region has not been well studied, mostly due to lack of model systems. Here we describe isolation and characterization of human primary vascular distal outflow pathway (VDOP) cells from the distal region of the conventional outflow pathway. Tissue from the distal region was isolated from human corneo-scleral rims, digested with collagenase type I (100 U/ml) and placed on gelatin coated plates to allow cellular growth in Dulbecco's Modified Eagle's Medium (low glucose) containing fetal bovine serum and antibiotic/antimycotic. VDOP cells showed consistent proliferation for up to 7 passages, retained endothelial-like nature of the parent tissues and showed a unique marker phenotype of Lectin+VEGFR2-CD34-NG2- that was distinct from neighboring trabecular meshwork (Lectin+VEGFR2-CD34-NG2+) and Schlemm's canal (Lectin+VEGFR2+CD34+NG2+) cells. Dexamethasone treated VDOP cells did not express myocilin and did not form cross-linked actin networks, in contrast to trabecular meshwork cells. These data show that VDOP cells are unique to the distal outflow region and can be used as a viable in vitro model system to understand the biology of the distal outflow pathway and intraocular pressure regulation.

Evaluation of Blood-filling Patterns in Schlemm Canal for Trabectome Surgery

PRéCIS:: Regardless of the blood-filling patterns in Schlemm canal (SC) before the trabecular meshwork (TM) ablation, the trabectome surgery, combined with phacoemulsification, is effective for mild to moderate primary open-angle glaucoma patients.

PURPOSE

The purpose of this study was to evaluate the association between trabectome surgery outcomes and the blood filling patterns in SC before TM ablation.

MATERIALS AND METHODS

This retrospective cohort study included 105 eyes of 84 Japanese primary open-angle glaucoma patients who had undergone trabectome surgery in combination with cataract surgery. Provocative gonioscopy was performed before TM ablation to classify the blood filling patterns in SC into 3 groups: no filling (group 1); patchy/irregular filling (group 2); and complete filling (group 3). The subjects were divided into 3 groups according to the blood filling patterns and the trabectome surgery outcomes were compared, including intraocular pressure (IOP), the percentage reduction in IOP, surgical success rate, and the number of glaucoma medications. Success was defined by IOP ≤15 mm Hg and a >20% reduction in IOP with/without glaucoma medication, and without additional glaucoma surgery after trabectome surgery combined with cataract surgery.

RESULTS

Twenty-four eyes were assigned to group 1, 48 to group 2, and 33 to group 3. Between-group analyses showed no significant intergroup differences in age (P=0.213), preoperative mean deviation (P=0.505), preoperative and postoperative IOP (P=0.941 and 0.458, respectively), preoperative and postoperative number of glaucoma medications (P=0.805 and 0.077, respectively), percentage IOP reduction (P=0.256), and success rates (P=0.540).

CONCLUSION

Trabectome surgery is effective for mild to moderate primary open-angle glaucoma patients, independent of the blood-filling patterns in SC before the TM ablation.

Schlemm Canal and Trabecular Meshwork Features in Highly Myopic Eyes With Early Intraocular Pressure Elevation After Cataract Surgery

PURPOSE

To investigate the morphologic features of the Schlemm canal and trabecular meshwork in highly myopic eyes with early intraocular pressure (IOP) elevation after cataract surgery.

DESIGN

Retrospective case-control study.

METHODS

Eighty-eight highly myopic eyes of 88 patients after uneventful cataract surgery were included, 31 of which had early postoperative IOP elevation and 57 of which did not. The morphologic features of the Schlemm canal and trabecular meshwork, collected with swept-source optical coherence tomography before surgery, were reviewed. Backwards stepwise multiple linear regression was used to investigate the anatomic risk factors for early IOP elevation in highly myopic eyes.

RESULTS

Highly myopic eyes with early postoperative IOP elevation had smaller Schlemm canal vertical diameter and area, as well as smaller trabecular meshwork thickness and width, in each quadrant than the non-elevation group. There was no significant difference in Schlemm canal horizontal diameter between the IOP elevation and non-elevation groups. In the highly myopic eyes, average Schlemm canal vertical diameter, Schlemm canal area, trabecular meshwork thickness, and width were all correlated negatively with the IOP elevation. A multivariate analysis showed that average Schlemm canal vertical diameter (β = -0.262, P = .004) and trabecular meshwork thickness (β = -0.173, P < .001) were significantly associated with early transient IOP elevation in highly myopic cataract eyes.

CONCLUSIONS

A smaller vertical diameter of Schlemm canal and a thinner trabecular meshwork are 2 anatomic risk factors for early IOP elevation after cataract surgery in highly myopic eyes.

Episcleral Venous Fluid Wave in the Living Human Eye Adjacent to Microinvasive Glaucoma Surgery (MIGS) Supports Laboratory Research: Outflow is Limited Circumferentially, Conserved Distally, and Favored Inferonasally

PURPOSE

The purpose of this study was to describe downstream patterns of outflow with the episcleral venous fluid wave (EVFW) in the living human eye adjacent to microinvasive glaucoma surgery (MIGS) and determine if the EVFW supports existing ex-vivo laboratory outflow research.

DESIGN

Retrospective, noncomparative case series.

PATIENTS

A total of 10 eyes of 10 patients who underwent phaco-Trabectome and 10 eyes of 10 patients who underwent phaco-iStent consecutively at Glaucoma Associates of Texas for cataract and uncontrolled glaucoma who demonstrated an episcleral wave.

METHODS

The EVFW was visualized and recorded during irrigation and aspiration. To describe the hydrodynamic properties of the fluid wave, its degrees, extent, and characteristics were measured with a protractor in Photoshop.

RESULTS

The incised Trabectome arc produced adjacent episcleral blanching of 134±11 degrees (range, 112 to 150 degrees) with an additional 54 degrees of marginal recruitment (41 degrees inferonasal plus 13 degrees superonasal) adjacent to the ends of the Trabectome incision. The mean episcleral blanch for the iStent was 51±19 degrees (range, 19 to 90 degrees), comprised of 29 degrees inferonasal plus 22 degrees superonasal.

CONCLUSIONS

Downstream episcleral flow in the living human eye adjacent to the iStent is variable and mainly confined to 2 clock hours indicating a lack of significant circumferential flow in glaucomatous eyes. Flow distal to the Trabectome site encompasses the Trabectome incisional arc with an additional 2 clock hours of lateral fluid wave favoring the inferonasal over superonasal quadrant 3 to 1. These in-vivo findings made visible with MIGS, corroborate recent in-vivo and long-standing ex-vivo laboratory research that outflow is largely segmented, favored inferonasally and conserved distally.

Anterior Segment Optical Coherence Tomography Angiography Imaging of Conjunctiva and Intrasclera in Treated Primary Open-Angle Glaucoma

PURPOSE

To investigate conjunctival and intrascleral vasculature in glaucoma eyes using anterior segment (AS)-optical coherence tomography angiography (OCTA) and assess the factors contributing to the vessel density in AS-OCTA images.

DESIGN

Prospective, cross-sectional study.

METHODS

Thirty-four patients with primary open-angle glaucoma and 20 healthy subjects were included. A swept-source OCT system was used to obtain the AS-OCTA images of the corneoscleral limbus at the nasal and temporal quadrants. Vessel densities were measured in the superficial (from the conjunctival epithelium to a depth of 200 μm) and deep (from a depth of 200 to 1000 μm) layers. The vessel density was compared between healthy and glaucoma eyes, and the associations of the vessel density with possible confounding factors were analyzed using univariable and multivariable analyses.

RESULTS

The vessel density was not significantly different between healthy eyes and eyes with glaucoma. There was a significant association of superficial vessel density with the use of a prostaglandin analog (P = .007) and with nasal location (P = .016) in eyes with glaucoma. Deep vessel density was significantly smaller with advancing age (P = .029) in healthy eyes and greater with higher intraocular pressure (P = .021) in eyes with treated glaucoma.

CONCLUSIONS

AS-OCTA images may be useful for the objective assessment of conjunctival hyperemia and helpful for understanding the pathophysiology of post-trabecular aqueous humor outflow.

Ab Externo Imaging of Human Episcleral Vessels Using Fiberoptic Confocal Laser Endomicroscopy

Purpose

There is a growing interest in targeting minimally invasive surgery devices to the aqueous outflow system to optimize treatment outcomes. However, methods to visualize functioning, large-caliber aqueous and episcleral veins in-vivo are lacking. This pilot study establishes an ex-vivo system to evaluate the use of a confocal laser microendoscope to noninvasively image episcleral vessels and quantify regional flow variation along the limbal circumference.

Methods

A fiber-optic confocal laser endomicroscopy (CLE) system with lateral and axial resolution of 3.5 μm and 15 μm, respectively, was used on three porcine and four human eyes. Diluted fluorescein (0.04%) was injected into eyes kept under constant infusion. The microprobe was applied to the sclera 1 mm behind the limbus to acquire real-time video. Image acquisition was performed at 15-degree intervals along the limbal circumference to quantify regional flow variation in human eyes.

Results

Vascular structures were visualized in whole human eyes without processing. Schlemm's canal was visualized only after a scleral flap was created. Fluorescent signal intensity and vessel diameter variation were observed along the limbal circumference, with the inferior quadrant having a statistically higher fluorescein signal compared to the other quadrants in human eyes (P < 0.05).

Conclusion

This study demonstrates for the first time that the fiber-optic CLE platform can visualize the episcleral vasculature with high resolution ex-vivo with minimal tissue manipulation. Intravascular signal intensities and vessel diameters were acquired in real-time; such information can help select target areas for minimally invasive glaucoma surgery (MIGS) to achieve greater intraocular pressure reduction.

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.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

TRANSLATIONAL RELEVANCE

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

Lipid Emulsion-Based OCT Angiography for Ex Vivo Imaging of the Aqueous Outflow Tract

Purpose

Contrast agents applicable for optical coherence tomography (OCT) imaging are rare. The intrascleral aqueous drainage system would be a potential application for a contrast agent, because the aqueous veins are of small diameter and located deep inside the highly scattering sclera. We tested lipid emulsions (LEs) as candidate OCT contrast agents in vitro and ex vivo, including milk and the anesthetic substance Propofol.

Methods

Commercial OCT and OCT angiography (OCTA) devices were used. Maximum reflectivity and signal transmission of LE were determined in tube phantoms. Absorption spectra and light scattering was analyzed. The anterior chamber of enucleated porcine eyes was perfused with LEs, and OCTA imaging of the LEs drained via the aqueous outflow tract was performed.

Results

All LEs showed a significantly higher reflectivity than water (P < 0.001). Higher milk lipid content was positively correlated with maximum reflectivity and negatively with signal transmission. Propofol exhibited the best overall performance. Due to a high degree of signal fluctuation, OCTA could be applied for detection of LE. Compared with blood, the OCTA signal of Propofol was significantly stronger (P = 0.001). As a proof of concept, time-resolved aqueous angiography of porcine eyes was performed. The three-dimensional (3D) structure and dynamics of the aqueous outflow were significantly different from humans.

Conclusions

LEs induced a strong signal in OCT and OCTA. LE-based OCTA allowed the ability to obtain time-resolved 3D datasets of aqueous outflow. Possible interactions of LE with inner eye's structures need to be further investigated before in vivo application.

The future of canine glaucoma therapy

Canine glaucoma is a group of disorders that are generally associated with increased intraocular pressure (IOP) resulting in a characteristic optic neuropathy. Glaucoma is a leading cause of irreversible vision loss in dogs and may be either primary or secondary. Despite the growing spectrum of medical and surgical therapies, there is no cure, and many affected dogs go blind. Often eyes are enucleated because of painfully high, uncontrollable IOP. While progressive vision loss due to primary glaucoma is considered preventable in some humans, this is mostly not true for dogs. There is an urgent need for more effective, affordable treatment options. Because newly developed glaucoma medications are emerging at a very slow rate and may not be effective in dogs, work toward improving surgical options may be the most rewarding approach in the near term. This Viewpoint Article summarizes the discussions and recommended research strategies of both a Think Tank and a Consortium focused on the development of more effective therapies for canine glaucoma; both were organized and funded by the American College of Veterinary Ophthalmologists Vision for Animals Foundation (ACVO-VAF). The recommendations consist of (a) better understanding of disease mechanisms, (b) early glaucoma diagnosis and disease staging, (c) optimization of IOP-lowering medical treatment, (d) new surgical therapies to control IOP, and (e) novel treatment strategies, such as gene and stem cell therapies, neuroprotection, and neuroregeneration. In order to address these needs, increases in research funding specifically focused on canine glaucoma are necessary.

Structure-Function Changes of the Porcine Distal Outflow Tract in Response to Nitric Oxide

Purpose

To correlate outflow function and outflow tract vessel diameter changes induced by nitric oxide (NO).

Methods

In a porcine anterior segment perfusion model, the effects of a nitric oxide donor (100 μM DETA-NO) on outflow facility were compared with controls (n = 8 per group) with trabecular meshwork (TM) and after circumferential ab interno trabeculectomy (AIT). Outflow structures were assessed with spectral-domain optical coherence tomography (SD-OCT) before and after NO, or an NO synthase inhibitor (100 μM L-NAME) and the vasoconstrictor, endothelin-1 (100 pg/mL ET-1). Scans were processed with a custom macroscript and aligned for automated reslicing and quantification of cross-sectional outflow tract areas (CSA).

Results

The facility increased after DETA-NO (Δ of 0.189 ± 0.081 μL/min·mm Hg, P = 0.034) and AIT (Δ of 0.251 ± 0.094 μL/min·mm Hg, P = 0.009), respectively. Even after AIT, DETA-NO increased the facility by 61.5% (Δ of 0.190 ± 0.074 μL/min·mm Hg, P = 0.023) and CSA by 13.9% (P < 0.001). L-NAME + ET-1 decreased CSA by -8.6% (P < 0.001). NO increased the diameter of focal constrictions 5.0 ± 3.8-fold.

Conclusions

NO can dilate vessels of the distal outflow tract and increase outflow facility in a TM-independent fashion. There are short, focally constricting vessel sections that display large diameter changes and may have a substantial impact on outflow.

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.

Effect of age on the morphologies of the human Schlemm's canal and trabecular meshwork measured with swept‑source optical coherence tomography

Purpose

We aimed to measure the sizes of Schlemm’s canal (SC) and the trabecular meshwork (TM) in healthy individuals and to evaluate variations with age from childhood to old age by using swept-source optical coherence tomography (OCT).

Methods

Anterior chamber angle imaging of the superior, inferior, nasal, and temporal regions of the right and left eyes was performed with swept-source OCT. The diameter and area of SC and TM width and thickness were measured manually from OCT images.

Results

A total of 114 healthy individuals were enrolled and included 48 male subjects and 66 female subjects; their ages ranged from 7 to 83 years. Both the SC diameter and area in the four quadrants decreased significantly with aging (P < 0.001) and were wider in the nasal and temporal quadrants compared with the superior and inferior quadrants. Changes in SC size showed significant positive correlations with axial length (AL) and anterior chamber depth (P < 0.001). There was a significant positive association between age and TM thickness in the nasal and temporal quadrants (P < 0.05). The inferior quadrant TM width was the widest among the quadrants. The superior quadrant TM thickness was the thinnest among the quadrants. Changes in TM thickness in the nasal and temporal quadrants showed a significant negative correlation with AL (P < 0.05). There was no statistically significant correlation in SC and TM parameters with central corneal thickness, intraocular pressure, sex, or right or left eye (P > 0.05).

Conclusions

With aging, the SC diameter and area became smaller, TM thickness increased, and TM width seemed to remain constant. Measurements of the sizes of SC and the TM with swept-source OCT could assist in clinical assessments and treatment planning for glaucoma.

Age-Related Changes in Human Schlemm's Canal: An in Vivo Optical Coherence Tomography-Based Study

Purpose: To investigate age-related changes in human Schlemm’s canal (SC) using spectral-domain optical coherence tomography (SD-OCT).

Methods: A total of 125 normal eyes were imaged using SD-OCT nasally and temporally. The age-related variations of SC sagittal diameter and cross-sectional area (CSA) from four age groups [A (16–20 years), B (21–40 years), C (41–60 years), and D (61–80 years)] were analyzed with Spearman correlation.

Results: The positive detection rates of SC showed a significantly downward trend with age. The mean CSA was 13,296 ± 1,897 μm² nasally and 14,552 ± 2,589 μm² temporally. The mean CSA was significantly larger in the temporal than in the nasal region (P < 0.05). Nasal CSA values varied among the four age groups (P = 0.004).

Conclusion: Our study found for the first time that SC in vivo exhibits a morphological variant with age in healthy humans. Clinicians may need to consider this phenomenon when performing examinations targeting SC for glaucoma patients.

Randomized Controlled Comparison of Titanium-Sapphire Versus Standard Q-Switched Nd: YAG Laser Trabeculoplasty

PURPOSE

To evaluate the clinical effects of laser trabeculoplasty using a novel 790 nm wavelength titanium-sapphire laser (TLT) compared with a 532 nm Q-switched Nd:YAG laser used in standard selective laser trabeculoplasty (SLT).

METHODS

Single institution prospective clinical trial of open-angle glaucoma patients randomized to TLT or SLT at the Yale Eye Center from 2011 to 2012. Patients with previous glaucoma surgery or trabeculoplasty were excluded. Trabeculoplasty was performed by a single surgeon unmasked to treatment group. All patients received 360 degrees of treatment. The main outcome measure was intraocular pressure (IOP) at 2 years.

RESULTS

Thirty-seven patients were enrolled. The mean baseline IOPs were 19.7 in TLT (n=18) and 20.4 mm Hg in SLT (n=19, P=0.69).At 2 years, the mean IOP was 12.9 mm Hg (35% decrease, P<0.001) in the TLT group and 15.4 mm Hg (25% decrease, P=0.006) in the SLT group. The 2 groups did not differ in glaucoma medication use. Success, defined as IOP<21 mm Hg with >20% reduction from baseline without the need for secondary glaucoma procedures, occurred in 44% of TLT patients and 61% of SLT patients at 1 year and in 22% of TLT patients and 46% of SLT patients at 2 years (P=0.11). No patients experienced significant pain, anterior chamber reaction, corneal edema, or loss of vision.

CONCLUSIONS

TLT may be a safe and effective alternative to SLT to lower IOP in patients with open-angle glaucoma.

Functional assessment of the aqueous humour distal outflow pathways in bovine eyes using time-of-flight magnetic resonance tomography

The major part of the aqueous humor leaves the eye through the "conventional outflow pathway", consisting of the trabecular meshwork, Schlemm's canal, collector channels, an intrasceral plexus and the episcleral veins. While the trabecular meshwork is well characterized, little is known about anatomical and functional features of the peripheral outflow tract beyond Schlemm's canal. The emergence of minimally-invasive glaucoma surgery directly targeting the outflow resistance in the trabecular meshwork has elicited growing interest in these structures. We used time-of-flight magnetic resonance imaging in ex vivo bovine eyes to map fluid flow under physiological conditions. We were able to identify the peripheral outflow vessels solely by the signal detected from the fluid flow inside their lumina. A question of clinical relevance is, whether localized opening of the trabecular meshwork leads to only localized or to a 360° increase in intrascleral flow. To address this, a goniotomy ab interno was performed in 3 eyes and the flow signal intensity was quantified sectorially. A significant increase in fluid flow was observed in the sector distal to the goniotomy (p = 0.0005) but not in the other sectors (p = 0.1). As a proof of concept we demonstrated that TOF-MRI based detection of flow in the peripheral aqueous outflow tract is feasible. The functional link observed between trabecular meshwork sectors and their distal outflow tract sectors may be relevant for minimally-invasive glaucoma surgery in humans.

Structural and functional imaging of aqueous humour outflow: a review

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

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

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

Segmental outflow of aqueous humor in mouse and human

The main and only modifiable risk factor in glaucoma, the group of usually late onset progressive and irreversible blinding optic neuropathies, is elevated intraocular pressure (IOP). The increase in IOP is due to impeded aqueous humor (AH) outflow through the conventional pathway. The aberrant increased resistance at the trabecular meshwork (TM), the filter-like region in the anterior eye chamber is the major contributory factor in causing the impeded outflow. In normal as well as in glaucoma eyes the regions of the TM are divided into areas of high and low flow. The collector channels and distal outflow regions are now increasingly being recognized as potential players in contributing to impede AH outflow. Structural and molecular make-up contributing to the segmental blockage to outflow is likely to provide greater insight. Establishing segmental blockage to outflow in model systems of glaucoma such as the mouse in parallel to human eyes will expand our repertoire of tools for investigation. Further study into this area of interest has the potential to ultimately lead to the development of new therapeutics focused on lowering IOP by targeting the various components of segmental blockage of outflow in the TM and in the distal outflow region.

Aqueous outflow - A continuum from trabecular meshwork to episcleral veins

In glaucoma, lowered intraocular pressure (IOP) confers neuroprotection. Elevated IOP characterizes glaucoma and arises from impaired aqueous humor (AH) outflow. Increased resistance in the trabecular meshwork (TM), a filter-like structure essential to regulate AH outflow, may result in the impaired outflow. Flow through the 360° circumference of TM structures may be non-uniform, divided into high and low flow regions, termed as segmental. After flowing through the TM, AH enters Schlemm's canal (SC), which expresses both blood and lymphatic markers; AH then passes into collector channel entrances (CCE) along the SC external well. From the CCE, AH enters a deep scleral plexus (DSP) of vessels that typically run parallel to SC. From the DSP, intrascleral collector vessels run radially to the scleral surface to connect with AH containing vessels called aqueous veins to discharge AH to blood-containing episcleral veins. However, the molecular mechanisms that maintain homeostatic properties of endothelial cells along the pathways are not well understood. How these molecular events change during aging and in glaucoma pathology remain unresolved. In this review, we propose mechanistic possibilities to explain the continuum of AH outflow control, which originates at the TM and extends through collector channels to the episcleral veins.

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.

Ultrahigh-resolution OCT imaging of the human cornea

We present imaging of corneal pathologies using optical coherence tomography (OCT) with high resolution. To this end, an ultrahigh-resolution spectral domain OCT (UHR-OCT) system based on a broad bandwidth Ti:sapphire laser is employed. With a central wavelength of 800 nm, the imaging device allows to acquire OCT data at the central, paracentral and peripheral cornea as well as the limbal region with 1.2 µm x 20 µm (axial x lateral) resolution at a rate of 140 000 A-scans/s. Structures of the anterior segment of the eye, not accessible with commercial OCT systems, are visualized. These include corneal nerves, limbal palisades of Vogt as well as several corneal pathologies. Cases such as keratoconus and Fuchs's endothelial dystrophy as well as infectious changes caused by diseases like Acanthamoeba keratitis and scarring after herpetic keratitis are presented. We also demonstrate the applicability of our system to visualize epithelial erosion and intracorneal foreign body after corneal trauma as well as chemical burns. Finally, results after Descemet's membrane endothelial keratoplasty (DMEK) are imaged. These clinical cases show the potential of UHR-OCT to help in clinical decision-making and follow-up. Our results and experience indicate that UHR-OCT of the cornea is a promising technique for the use in clinical practice, but can also help to gain novel insight in the physiology and pathophysiology of the human cornea.

Differential Canalograms Detect Outflow Changes from Trabecular Micro-Bypass Stents and Ab Interno Trabeculectomy

Recently introduced microincisional glaucoma surgeries that enhance conventional outflow offer a favorable risk profile over traditional surgeries, but can be unpredictable. Two paramount challenges are the lack of an adequate training model for angle surgeries and the absence of an intraoperative quantification of surgical success. To address both, we developed an ex vivo training system and a differential, quantitative canalography method that uses slope-adjusted fluorescence intensities of two different chromophores to avoid quenching. We assessed outflow enhancement by trabecular micro-bypass (TMB) implantation or by ab interno trabeculectomy (AIT). In this porcine model, TMB resulted in an insignificant (p > 0.05) outflow increase of 13 ± 5%, 14 ± 8%, 9 ± 3%, and 24 ± 9% in the inferonasal, superonasal, superotemporal, and inferotemporal quadrant, respectively. AIT caused a 100 ± 50% (p = 0.002), 75 ± 28% (p = 0.002), 19 ± 8%, and 40 ± 21% increase in those quadrants. The direct gonioscopy and tactile feedback provided a surgical experience that was very similar to that in human patients. Despite the more narrow and discontinuous circumferential drainage elements in the pig with potential for underperformance or partial stent obstruction, unequivocal patterns of focal outflow enhancement by TMB were seen in this training model. AIT achieved extensive access to outflow pathways beyond the surgical site itself.

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.

Evaluation of Outflow Structures In Vivo after the Phacocanaloplasty

Purpose. To evaluate the in vivo changes in Schlemm's canal (SC) and the trabecular meshwork (TM) in patients with primary open-angle glaucoma (POAG) after phacocanaloplasty using anterior segment optical coherence tomography (AS-OCT). Methods. Ten eyes of nine patients with POAG (6 men and 3 women) who underwent phacocanaloplasty. Preoperative and postoperative visual acuity (VA), intraocular pressure (IOP), and use of glaucoma medications were evaluated. The main outcome measures were the area of SC and TM thickness assessed using AS-OCT before and 12 months after surgery. Results. We found statistically significant reduction in IOP (from 26.4 (8.6) mmHg to 12.9 (2.5) (p < 0.05) mmHg), increase in VA from 0.7 (0.4) to 0.9 (0.2), and decrease in glaucoma medication from 2.6 (1.2) to 1.1 (1.3) at 12 months postoperatively. There was a significant increase in the SC area (3081.7 (842.8) μm² versus 5098.8 (1190.5) μm², p < 0.001) and a decrease in mean TM thickness (91.2 (18.6) μm versus 81.3 (15.1) μm, p = 0.001) after surgery. We found negative correlations between SC area and IOP before surgery (r = −0.67, p = 0.03) and also between SC area before and IOP reduction 12 months after the phacocanaloplasty (r = −0.80, p = 0.005). Conclusions. Our results showed statistically significant dilation of SC area and reduction of TM thickness after phacocanaloplasty in POAG patients. The degree of SC expansion was related to the IOP decrease.

New developments in anterior segment optical coherence tomography for glaucoma

PURPOSE OF REVIEW

The aim of the present review was to summarize the new developments in anterior segment optical coherence tomography (AS-OCT) for glaucoma.

RECENT FINDINGS

Recent years have demonstrated significant advances in the measurement of glaucoma through the use of AS-OCT. Furthermore, a more widespread use of AS-OCT in the clinical study of various glaucomas warrants review, which includes angel assessment, trabecular meshwork and Schlemm's canal assessment, and assessment of the filtering bleb and tube.

SUMMARY

AS-OCT was recently developed and has become a crucial tool in glaucoma clinical practice. AS-OCT is a noncontact imaging device that provides the detailed structure of the anterior part of the eyes. In this review, the author will discuss the various clinical applications of AS-OCT for glaucoma disease, such as angle assessment, trabecular meshwork and Schlemm's canal assessment, or assessment of the filtering bleb and tube.

In Vivo Near-Infrared Fluorescence Imaging of Aqueous Humor Outflow Structures

The aim of this study has been to visualize the aqueous outflow system in patients affected by primary open angle glaucoma. A solution of indocyanine green (ICG) plus high viscosity viscoelastic solution was injected into the Schlemm canal during surgery in 10 glaucomatous patients undergoing canaloplasty. Soon after injection of the dye the borders of the scleral flap were completely stained due to partial reflux caused by the intrachannel resistance; progression of the dye along the Schlemm canal starting from the site of injection was then visualized. The filling of the collector channels was observed only in the patent portions of the Schlemm canal. The only noticeable aqueous veins were located in correspondence of the quadrant in which both the Schlemm canal and the collectors were patent. Lastly, a retrograde filling, of glomerular-shaped structures, deepest to the Schlemm canal was observed in the quadrants where the pathway was functioning. Our findings show that injection of a mixture composed of ICG and viscoelastic solution into the Schlemm canal allows a clear visualization of the functioning portions of the conventional outflow pathway. In addition, a retrograde filling of structures presumably located into the iris was also recorded. Clinical Trial Registration. Our study is registered in ISRCTN registry, number 54005880, DOI 10.1186/ISRCTN54005880.

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.

Biomechanical assessment in models of glaucomatous optic neuropathy

The biomechanical environment within the eye is of interest in both the regulation of intraocular pressure and the loss of retinal ganglion cell axons in glaucomatous optic neuropathy. Unfortunately, this environment is complex and difficult to determine. Here we provide a brief introduction to basic concepts of mechanics (stress, strain, constitutive relationships) as applied to the eye, and then describe a variety of experimental and computational approaches used to study ocular biomechanics. These include finite element modeling, direct experimental measurements of tissue displacements using optical and other techniques, direct experimental measurement of tissue microstructure, and combinations thereof. Thanks to notable technical and conceptual advances in all of these areas, we are slowly gaining a better understanding of how tissue biomechanical properties in both the anterior and posterior segments may influence the development of, and risk for, glaucomatous optic neuropathy. Although many challenging research questions remain unanswered, the potential of this body of work is exciting; projects underway include the coupling of clinical imaging with biomechanical modeling to create new diagnostic tools, development of IOP control strategies based on improved understanding the mechanobiology of the outflow tract, and attempts to develop novel biomechanically-based therapeutic strategies for preservation of vision in glaucoma.

A Closer Look at Schlemm's Canal Cell Physiology: Implications for Biomimetics

Among ocular pathologies, glaucoma is the second leading cause of progressive vision loss, expected to affect 80 million people worldwide by 2020. A primary cause of glaucoma appears to be damage to the conventional outflow tract. Conventional outflow tissues, a composite of the trabecular meshwork and the Schlemm's canal, regulate and maintain homeostatic responses to intraocular pressure. In glaucoma, filtration of aqueous humor into the Schlemm's canal is hindered, leading to an increase in intraocular pressure and subsequent damage to the optic nerve, with progressive vision loss. The Schlemm's canal encompasses a unique endothelium. Recent advances in culturing and manipulating Schlemm's canal cells have elucidated several aspects of their physiology, including ultrastructure, cell-specific marker expression, and biomechanical properties. This review highlights these advances and discusses implications for engineering a 3D, biomimetic, in vitro model of the Schlemm's canal endothelium to further advance glaucoma research, including drug testing and gene therapy screening.

Complete 360° circumferential gonioscopic optical coherence tomography imaging of the iridocorneal angle

Clinically, gonioscopy is used to provide en face views of the ocular angle. The angle has been imaged with optical coherence tomography (OCT) through the corneoscleral limbus but is currently unable to image the angle from within the ocular anterior chamber. We developed a novel gonioscopic OCT system that images the angle circumferentially from inside the eye through a custom, radially symmetric, gonioscopic contact lens. We present, to our knowledge, the first 360° circumferential volumes (two normal subjects, two subjects with pathology) of peripheral iris and iridocorneal angle structures obtained via an internal approach not typically available in the clinic.