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

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          Abstract

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

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          Most cited references103

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          Optical coherence tomography.

          A technique called optical coherence tomography (OCT) has been developed for noninvasive cross-sectional imaging in biological systems. OCT uses low-coherence interferometry to produce a two-dimensional image of optical scattering from internal tissue microstructures in a way that is analogous to ultrasonic pulse-echo imaging. OCT has longitudinal and lateral spatial resolutions of a few micrometers and can detect reflected signals as small as approximately 10(-10) of the incident optical power. Tomographic imaging is demonstrated in vitro in the peripapillary area of the retina and in the coronary artery, two clinically relevant examples that are representative of transparent and turbid media, respectively.
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            Micro-invasive glaucoma surgery: current perspectives and future directions.

            There is an increasing interest and availability of micro-invasive glaucoma surgery (MIGS) procedures. It is important that this increase is supported by sound, peer-reviewed evidence. This article will define MIGS, review relevant publications in the period of annual review and discuss future directions. The results of the pivotal trial comparing a trabecular micro-bypass stent (iStent, Glaukos Corporation, Laguna Hills, CA, USA) combined with phacoemulsification to phacoemulsification alone showed a significantly higher percentage of patients with unmedicated intraocular pressure (IOP) ≤ 21 mmHg, and a comparable safety profile. Initial results are published regarding a second-generation micro-bypass stent (iStent inject, Glaukos Corporation, Laguna Hills, CA, USA), a canalicular scaffold (Hydrus, Ivantis Inc., Irvine, CA, USA) and an ab interno suprachoroidal microstent (CyPass, Transcend Medical, Menlo Park, CA, USA), showing a decrease in mean postoperative IOP. Phaco-Trabectome (Ab interno trabeculectomy Trabectome, NeoMedix Inc., Tustin, CA, USA) was compared to phacotrabeculectomy and showed less IOP reduction, less postoperative complications, and a similar success rate. Similar success rates were found with the comparison of excimer laser trabeculostomy (ELT, AIDA, Glautec AG, Nurnberg, Germany) and selective laser trabeculoplasty. A number of publications review the importance of the location of implantable devices, intraoperative gonioscopy, cost-effectiveness and quality-of-life studies, and randomized clinical trials. MIGS procedures offer reduction in IOP, decrease in dependence on glaucoma medications and an excellent safety profile. Their role within our glaucoma treatment algorithm continues to be clarified and differs from the role of more invasive glaucoma surgeries such as trabeculectomy or glaucoma drainage devices.
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              Minimally invasive glaucoma surgery: current status and future prospects

              Minimally invasive glaucoma surgery aims to provide a medication-sparing, conjunctival-sparing, ab interno approach to intraocular pressure reduction for patients with mild-to-moderate glaucoma that is safer than traditional incisional glaucoma surgery. The current approaches include: increasing trabecular outflow (Trabectome, iStent, Hydrus stent, gonioscopy-assisted transluminal trabeculotomy, excimer laser trabeculotomy); suprachoroidal shunts (Cypass micro-stent); reducing aqueous production (endocyclophotocoagulation); and subconjunctival filtration (XEN gel stent). The data on each surgical procedure for each of these approaches are reviewed in this article, patient selection pearls learned to date are discussed, and expectations for the future are examined.
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                Author and article information

                Journal
                0370707
                3647
                Exp Eye Res
                Exp. Eye Res.
                Experimental eye research
                0014-4835
                1096-0007
                20 January 2017
                11 June 2016
                May 2017
                01 May 2018
                : 158
                : 171-186
                Affiliations
                [a ]Department of Bioengineering, University of Washington, USA
                [b ]Department of Ophthalmology, University of Washington, USA
                [c ]Department of Ophthalmology, Beijing Anzhen Hospital, Capital Medical University, China
                [d ]Department of Ophthalmology, Cook County Hospital System, USA
                Author notes
                [* ]Corresponding author. Department of Ophthalmology, University of Washington, Eye Institute, 1259 NE Pacific St., HSB T163KBox 357190, Seattle, WA 98195-7190, USA.
                Article
                NIHMS844141
                10.1016/j.exer.2016.06.007
                5272871
                27302601
                ba51500e-80b3-4eda-a4bb-f86fedf0acaf

                This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/).

                History
                Categories
                Article

                Vision sciences
                glaucoma,aqueous,intraocular pressure,schlemm's canal,trabecular meshwork,collector channels,lymphatics,pulsatile flow,optical coherence tomography

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