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Microinvasive glaucoma surgery represents dawn of new era
New MIGS devices are expected to make glaucoma a more surgically treatable disease.
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Glaucoma is usually a slowly blinding disease that is estimated to affect more than 60 million people worldwide and 2.2 million in the United States. Glaucoma is the leading cause of blindness among African Americans, and it is six to eight times more common in this population segment compared with Caucasians. The World Health Organization ranks glaucoma as the second leading cause of blindness globally. Thus, glaucoma takes center stage as one of the important diseases that erodes human vision, and glaucoma management becomes a priority for all eye care specialists.
Although such management choices include medical and surgical (laser and non-laser) treatment approaches, the major initial thrust of the treatment pattern for glaucoma globally is the use of topical medications that may be supplemented with systemic medications in some selected cases; at some point in the treatment timeline, surgery is introduced when indicated in an attempt to preserve the residual vision. Glaucoma surgery in general takes a seat behind medical treatment.
This has been primarily fueled by the potential surgical complications that may be associated with procedures such as trabeculectomy with mitomycin. This paradigm appears to be shifting favorably toward surgical procedures for IOP control in glaucoma patients by potentially elevating microinvasive glaucoma procedures to a safer interventional strategy that can effectively lower IOP in the postoperative period. When such procedures have smaller entry wound size into the anterior segment, it makes such procedures surgically more attractive because it usually promotes healing in a shorter period of time without introducing iatrogenic astigmatism.
Additionally, when such procedures can be performed as a combined procedure with an elective cataract surgery, it makes these procedures even more appealing to both the surgeon and the patient. Further, such microinvasive glaucoma surgical procedures eliminate conjunctival and scleral dissection that shortens the surgical time and dissipates scleral flap-associated complications. As to whether we should direct the aqueous humor to the subconjunctival or suprachoroidal space as a preferred site is yet to be fully resolved, and further experience will answer some of these questions. Currently, the Trabectome and iStent are approved by the U.S. Food and Drug Administration.
In this column, Drs. Shareef and Ahmed elegantly describe the various microinvasive glaucoma surgery procedures.
Thomas “TJ” John, MD
OSN Surgical Maneuvers Editor
To date, treatment of glaucoma has been limited to medications, laser trabeculoplasty or trabeculectomy, and tube shunt procedures. Several ab interno “angle” procedures, coined microinvasive glaucoma surgery (MIGS), have been introduced as an alternative. Their aim is to either restore physiologic aqueous outflow via Schlemm’s canal by bypassing trabecular meshwork resistance or redirecting aqueous to the suprachoroidal or subconjunctival space. Unlike traditional filtration surgery, MIGS offers a safer procedure profile, minimal tissue trauma and rapid visual recovery with minimal side effects and, more importantly, enables earlier intervention to avert blindness. It lessens dependence upon medications, thus potentially averting more invasive surgeries.
iStent
The iStent trabecular micro-bypass stent (Glaukos), the smallest FDA-approved device, is made of non-ferromagnetic, heparin-coated titanium, 1 mm × 0.33 mm with a snorkel that resides in the anterior chamber having a 120 µm bore diameter.
The iStent is introduced into Schlemm’s canal.
Image: Dump C
Available in two different orientations to facilitate insertion after cataract surgery, under gonioscopic view, the iStent is introduced into the anterior chamber via a temporal incision on a pre-loaded inserter. The anterior third of nasal trabecular meshwork is engaged at 15° to 30° with the device’s self-trephining tip into Schlemm’s canal, advanced along its arc and secured in place with retention arches. Once proper positioning is confirmed, the iStent is disengaged from the inserter. Aqueous bypasses the trabecular meshwork via the snorkel into the half pipe lumen in Schlemm’s canal. From here, aqueous drains via collector channels into episcleral vessels.
Hydrus
The Hydrus microstent (Ivantis), a novel flexible implant made from nitinol, has dimensions and curvature that parallel Schlemm’s canal arc. Under direct gonioscopy, using a preloaded hand-held injector, the device is delivered through the cornea into the nasal iridocorneal angle via the trabecular meshwork into Schlemm’s canal. The distal end of the inlet is placed in the anterior chamber. The implant serves as an intracanalicular scaffold maximally dilating Schlemm’s canal approximately four to five times its cross-sectional area for 3 clock hours (8 mm), enhancing circumferential flow. The non-luminal open design with several “windows” enhances access of aqueous to collector channels.
Under direct gonioscopy, the Hydrus is inserted into Schlemm’s canal.
Image: Kahook M
Trabectome
Using electrocautery, the Trabectome (NeoMedix) surgically ablates points of resistance, the trabecular meshwork and the inner wall of Schlemm’s canal by enabling aqueous to directly access collector channels. Under gonioscopic view, the disposable handpiece is introduced via a 1.8-mm temporal incision in the presence of viscoelastic and advanced toward nasal trabecular meshwork. The footplate is entered into Schlemm’s canal via the trabecular meshwork, and 90° to 180° of tissue is electrocauterized with continuous irrigation. The uniquely designed footplate protects the surrounding and underlying structures, including the outer wall of Schlemm’s canal and collector channels, from heat damage, minimizing thermal injury. Once completed, the handpiece and viscoelastic are removed.
Under gonioscopic view, the Trabectome is introduced toward the trabecular meshwork.
Image: Francis B
CyPass
The CyPass Micro-Stent (Transcend Medical) consists of a 6-mm long fenestrated biocompatible polyimide tube with retention features and a 330-µm lumen. Loaded onto the distal end of a hand-held inserter around a releasable guidewire, under direct gonioscopy, the CyPass is introduced via a 1.5-mm temporal incision and advanced across the anterior chamber toward the scleral spur. It is introduced into the supraciliary space with blunt entry using the guide wire and advanced for almost its entire length, resulting in creation of a controlled cyclodialysis. The distal end is disengaged from the inserter and rests in the iridocorneal angle, resulting in a permanent internal drainage to the suprachoroidal space.
The Cypass device consists of a 6-mm-long fenestrated polyimide tube.
Image: Kahook M
iStent supra
The iStent supra (Glaukos) is a polyethersulfone 4 mm long microstent consisting of 0.16 to 0.17 mm lumen diameter, surface retention ridges to provide stability after implantation, and curved to conform to the suprachoroidal space. In the presence of viscoelastic and under gonioscopic view, the pre-loaded implant on a hand-held inserter is introduced through a 1-mm corneal opening and advanced toward the opposite angle. It is entered posterior to the scleral spur, advanced to an appropriate depth and released from the inserter, leaving behind 0.5 mm of the sleeve opening in the anterior chamber. The hollow lumen allows transportation of aqueous, creating a controlled cyclodialysis cleft.
Xen
The Xen Gel Stent (AqueSys), made from permanent gelatin about the width of a human hair, comes pre-loaded in an IOL-like inserter in a dehydrated state. When it comes in contact with aqueous, it becomes very flexible, making the implant “tissue conforming.” The inserter is placed through a sub-2-mm corneal incision and advanced across the anterior chamber to the desired subconjunctival location for placement. The inserter delivers the implant to the appropriate longitudinal position, with its final resting place connecting the anterior chamber and the subconjunctival space.
The Xen is inserted through a sub-2-mm corneal incision and advanced across the anterior chamber for insertion.
Image: Kahook M
Discussion
The field of MIGS is in its infancy. In the foreseeable future, with introduction of newer MIGS procedures and devices and enhancement of existing prototypes, glaucoma will become more of a surgically treatable disease. By better understanding the drainage pathways distal to Schlemm’s canal as well as the suprachoroidal space, we will be better able to assess the pros and cons of different MIGS and more precisely titrate IOP reduction to a predictable level by either using multiple devices or taking advantage of the synergistic effect from a combination of devices and procedures. This will prevent further visual field loss, enhancing our patients’ quality of life by preserving functional vision.