Effects of Schlemm's Canal Expansion: Biomechanics and MIGS Implications

Two-Year Retrospective Study of Cataract Surgery and Schlemm Canal Hydrus Microstent in Black and Afro-Latino Patients

Background

The Hydrus Microstent has been shown to be efficacious in the treatment of Open-angle Glaucoma. We previously reported 1-year results of the Hydrus Microstent in Black and Afro-Latino patients.

Objective

Here we report two-year results of the Hydrus Microstent in Black and Afro-Latino patients with mild to moderate open-angle glaucoma.

Design

This was a retrospective, nonrandomized study that was carried out at two private practices in Harlem, NY and Queens, NY.

Methods

This study consisted of Black and Afro Latino patients who had a Hydrus Microstent implanted for the treatment of glaucoma. The evaluated parameters were intraocular pressure (IOP), reductions in the number of medications, mean deviation on visual field test and visual acuity. We excluded persons with advanced glaucoma and incomplete data. We included those with mild to moderate glaucoma and had complete two-year follow-up data.

Results

The study consisted of 86 patients and 86 eyes. The mean age was 69. The mean pre-op IOP was 14.1 on 2.17 medications. At two years the mean IOP was 14 (p = 0.651) on 0.42 medications (p = 0.000003); 88% of patients were medication free (p = 0.651), The MD on the visual field changed little from -5.73 preoperatively to -5.21 at two years. There were 3 intraocular pressure spikes and 1 case of hyphema that were treated and had complete resolution. No patients required secondary surgery.

Conclusion

Our results show that at two years the Hydrus Microstent can maintain low intraocular pressures, reduce medication burden and, in aggregate, experienced little change in visual field Mean Deviation in Black and Afro Latino patients with mild to moderate glaucoma.

Historical and Contemporary Debates in Schlemm's Canal-Based MIGS

Glaucoma is one of the primary causes of blindness worldwide. Canal opening surgery, a type of minimally invasive glaucoma surgery (MIGS) applied in cases of mild to moderate glaucoma, has gained increasing popularity in recent years due to its efficacy in reducing the intraocular pressure, its safety profile, the simplicity of its technique, and the reduced likelihood of compromised vision. Nevertheless, the existing body of histopathological studies remains insufficient for a comprehensive understanding of post-surgical wound healing. Consequently, debates persist among researchers regarding the mechanism through which Schlemm's canal opening surgery reduces the intraocular pressure, as well as the surgical techniques that may impact the outcomes and the factors influencing surgical success. As the history of MIGS is relatively short and lacks sufficient systemic reviews or meta-analyses evaluating the influence of individual factors, this review was conducted to illuminate the disparities in researchers' opinions at the current stage of research.

Evaluation of neural innervation in the human conventional outflow pathway distal to Schlemm's canal

The distal outflow pathway of the human eye consists of the outer wall of Schlemm's canal, collector channels, and the deep-scleral, mid-scleral and episcleral vessels. It is the last region of transit for aqueous humor before returning to the venous system. While the trabecular meshwork, scleral spur, and inner wall of Schlemm's canal have been extensively analyzed to define their contributions to aqueous outflow, the role of the distal outflow pathway is not completely understood. Collector channels, emanating from Schlemm's canal were previously thought to be passive conduits for aqueous humor. However, recent studies have shown many collector channels contain flap-like appendages which move with changes in pressure. These findings, along with studies demonstrating innervation of episcleral vessels, have led to questions regarding whether other structures in the distal outflow pathway are under neural regulation and how this may influence aqueous humor outflow. This study evaluates the innervation of the outer wall of Schlemm's canal and collector channels, along with the deep-scleral, mid-scleral and episcleral vasculature with microcomputed tomography and 3-dimensional reconstruction, correlative light microscopy, immunohistochemistry, and transmission electron microscopy. Peripheral, autonomic, and sensory nerve fibers were found to be present adjacent to Schlemm's canal outer wall endothelium, collector channel endothelium, and in the different regions of the distal outflow vasculature. Nerves were more commonly identified in regions that contained collector channels when compared to regions without collector channels. These findings regarding the neural anatomy suggest an active neural regulation of aqueous humor outflow throughout the proximal and distal regions of the conventional outflow pathway.

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.

Gonioscopy-assisted Transluminal Trabeculotomy (GATT) combined phacoemulsification surgery: Outcomes at a 2-year follow-up

BACKGROUND/OBJECTIVES

This study aimed to provide a 24-month follow-up on the surgical success and safety of gonioscopy-assisted transluminal trabeculotomy (GATT) combined with phacoemulsification and intraocular lens (IOL) implantation in the treatment of patients with primary open-angle glaucoma (POAG) combined cataract.

SUBJECTS/METHODS

We included 124 consecutive cases of POAG with microcatheter-assisted GATT or GATT combined with phacoemulsification and IOL implantation at Beijing Tongren Eye Centre between October 2019 and November 2020. Main outcome measures included surgical success rate, changes in IOP, number of antiglaucoma medications, best corrected visual acuity (BCVA), postoperative complications at baseline, and follow-up period of up to 24 months.

RESULTS

In total, 58 eyes received GATT combined with phacoemulsification surgery and 66 eyes received GATT alone. The overall qualified success rate was 86.21% for eyes with GATT combined with phacoemulsification surgery, and 83.48% for eyes with GATT only at 24 months. IOP was reduced from 26.40 ± 6.37 mmHg on 3.12 ± 0.80 medications preoperatively to 14.61 ± 2.28 mmHg on 0.27 ± 0.71 medications at 12 months and 16.08 ± 2.38 mmHg on 0.45 ± 0.96 medications at 24 months after combined surgery. Additionally, mean BCVA improved from 0.75 ± 0.43 logMAR units preoperatively to 0.22 ± 0.18 logMAR units 24 months after combined surgery. No vision-threatening complications occurred during the 24-month follow-up.

CONCLUSIONS

The 24-month follow-up results of our study suggest that GATT combined with cataract surgery is a safe and effective treatment for decreasing IOP and number of medications in patients with POAG combined cataract.

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.

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.

Trabecular Meshwork Motion Profile from Pulsatile Pressure Transients: A New Platform to Simulate Transitory Responses in Humans and Nonhuman Primates

Trabecular meshwork (TM) motion abnormality is the leading cause of glaucoma. With technique limitations, how TM moves is still an enigma. This study describes a new laboratory platform to investigate TM motion responses to ocular transients in ex vivo eyes. The anterior segments of human cadaver and primate eyes were mounted in a perfusion system fitting. Perfusion needles were placed to establish mean baseline pressure. A perfusion pump was connected to the posterior chamber and generated an immediate transient pressure elevation. A phase-sensitive optical coherent tomography system imaged and quantified the TM motion. The peak-to-peak TM displacements (ppTMD) were determined, a tissue relaxation curve derived, and a time constant obtained. This study showed that the ppTMD increased with a rise in the pulse amplitude. The ppTMD was highest for the lowest mean pressure of 16 mmHg and decreased with mean pressure increase. The pulse frequency did not significantly change ppTMD. With a fixed pulse amplitude, an increase in mean pressure significantly reduced the time constant of recoil from maximum distension. Our research platform permitted quantitation of TM motion responses to designed pulse transients. Our findings may improve the interpretation of new TM motion measurements in clinic, aiding in understanding mechanisms and management.