The 2 most commonly performed surgeries for medically uncontrolled glaucoma, at present, are trabeculectomy and glaucoma drainage devices. Although proven effective in many cases, both use a nonphysiologic pathway to direct aqueous humor from the anterior chamber to the subconjunctival space and form a filtration bleb to lower intraocular pressure (IOP). A variety of postoperative complications related to the filtration bleb are encountered with both procedures, including bleb fibrosis and failure. Bleb formation after trabeculectomy carries a life-long risk of bleb leak, hypotony, blebitis, endophthalmitis, etc. In addition to these, the glaucoma drainage devices are associated with a unique set of tube-related complications, such as occlusion, migration, erosion, and corneal decompensation.
Because our conventional surgical procedures are far from optimal, there is significant interest in exploring newer surgical approaches that may enhance the existing physiologic outflow pathways, the trabecular out-flow system, or the uveoscleral outflow system. Newer surgical approaches targeting the former pathway include Kahook Dual Blade (New World Medical, Rancho Cucamonga, CA), ABiC Canaloplasty (Ellex Medical Lasers Pty Ltd, Adelaide, Australia), and iStent. This chapter will discuss the uveoscleral outflow pathway system and suprachoroidal devices currently being investigated to enhance aqueous outflow.
Uveoscleral Outflow System
The uveoscleral pathway consists of ciliary body, suprachoroidal space, choroid, and sclera. The aqueous humor flows from the anterior chamber, through the uveal portion (the longitudinal muscle of the ciliary body), to the suprachoroidal space. The fluid then exits the suprachoroidal space through the scleral portion, either the episcleral tissues (or the actual sub-stance of sclera) or the choroidal vascular system. Animal studies have revealed a hydrostatic pressure differential of approximately 3.7 mm Hg between the anterior chamber and the suprachoroidal space that facilitates aqueous outflow through the uveoscleral system. Although initial studies reported a total aqueous outflow of 5% to 15%, subsequent and more recent studies have reported a much greater percentage, 20% to 54%, of outflow through the uveoscleral pathway.
The uveoscleral pathway is medically augmented by the prostaglandin analogues, which are the most potent IOP-lowering medications currently available. Although the exact mechanism by which this class of drugs improves uveoscleral outflow remains poorly understood, the two potential mechanisms are relaxation of the ciliary body and, more importantly, remodelling of extracellular matrix of the ciliary muscle. Historically, this pathway was used in the cyclodialysis procedure, which is an operation for glaucoma first described by Heine in 1905. Cyclodialysis involves separating the military body from the scleral spur, creating a direct conduit between the anterior chamber and suprachoroidal space. This procedure was later abandoned because of unpredictable results and frequent complications of hypotony, hemorrhage, or failure resulting from the closure of the cyclodialysis cleft. Various types of implants and materials have been investigated to keep the cyclodialysis cleft patent, including Teflon tube implant, hydroxyethyl methacrylate capillary strip, scleral strip, air, and sodium hyaluronate, but with limited success. Ozdamar et al implanted a modified Krupin valve into the suprachoroidal space in 4 blind eyes with a reported surgical success of 75%, defined as final IOP lower than 21 mm Hg without the use of adjunctive glaucoma medications at about 8 months’ follow-up. Recently, Jordan et al reported a high failure rate of 75% in 28 eyes with intractable glaucoma after a viscoelastic-assisted cyclodialysis ab interno procedure.
Recently, surgical interest in the uveoscleral pathway has been re-ignited using stents or shunts to allow a more controlled outflow of aqueous humor from the anterior chamber to the suprachoroidal space by using either an ab interno or ab externo approach. Three competing suprachoroidal devices under development include the Gold Shunt (SOLX Corp), Aquashunt (Opko Health Inc., Miami, FL), and CyPass Micro-Stent (Transcend Medical, Menlo Park, California).
SOLX Gold Shunt
The SOLX Gold Shunt is a 24-karat miniature gold implant that connects the anterior chamber to the suprachoroidal space through an ab externo approach. This device is the most researched of various suprachoroidal devices currently available and has evolved over 3 generations. The first-generation models are the GMS and GMS Plus. These implants are 5.2-mm long by 3.2-mm wide but have different thicknesses of 44 and 68 µm in the GMS and GMS Plus models, respectively. The GMS weighs 6.2 mg and the GMS Plus weighs 9.2 mg. All devices are composed of 2 leaflets fused together and consist of 9 channels in the body to divert aqueous humor from the anterior chamber to the suprachoroidal space. The GMS Plus has larger channels designed to increase uveoscleral outflow. The second-generation model, sGMS Plus, is 80 µm thick with one open window on the distal end of the device with flow resistance being 5 times that of the first-generation implants. Additional windows can be opened with a 790-nm titanium:sapphire pulsed laser, as needed, to reach target IOP gonioscopically postoperatively. The third-generation model, mGMS Plus, has all 9 posterior windows open with flow resistance that is 25 times that of the first-generation implants. The biocompatibility and inertness of gold have previously been reported.
The Gold Shunt can be implanted in any quadrant as long as the integrity of the conjunctiva and sclera is well established. Preoperative gonioscopy should be performed to avoid areas of peripheral anterior synechiae. After a fornix-based conjunctival peritomy, a scleral incision approximately 3-mm long is created approximately 2 mm from the limbus at 90% depth. In eyes with high myopia, the scleral incision should be farther away from the limbus. A paracentesis is made, and viscoelastic is inserted into the anterior chamber to keep the eye pressurized or, alternatively, an anterior chamber maintainer may be used. A crescent blade or keratome is used to make a scleral tunnel into the cornea. The scleral incision is then deepened into a full-thickness incision in the suprachoroidal space. A cyclodialysis spatula may be used to correctly locate the suprachoroidal space. Attention is then diverted anteriorly, an entry is made into the anterior chamber at the level of the scleral spur, and the proximal end of the shunt is placed in the anterior chamber. The shape of the proximal end is concave to minimize contact with the iris or corneal endothelium. The distal end is then tucked into the suprachoroidal space. A small amount of balanced salt solution or viscoelastic may be injected into the suprachoroidal space to ease the shunt insertion. A Sinskey hook (Katena Products, Denville, New Jersey) or 27-gauge needle may be used to align the shunt properly at either end. All posterior drainage openings of the implant should be covered by the posterior scleral lip, and the anterior channels of the “head” of the device should be visible anteriorly; the correct position may be confirmed with intraoperative gonioscopy. The scleral wound is closed with nylon sutures, and the conjunctiva is closed with Vicryl sutures. A sterile inserter is supplied with each device to facilitate handling and insertion of the implant.
Melamed et al reported the efficacy and safety of the Gold Shunt in 38 patients with uncontrolled glaucoma. They reported an IOP reduction of 32.6% from baseline and a surgical success of 79% (13% complete and qualified), which was defined as IOP >5 mm Hg and <22 mm Hg. Eight patients had mild to moderate hyphema, which was the most commonly encountered complication of the surgery. In 2010, Mastropasqua and colleagues  described the conjunctival features with in vivo confocal microscopy (IVCM) after Gold Shunt implantation in the suprachoroidal space. No bleb formation was noted clinically. A significantly increased conjunctival microcyst density and area were observed by IVCM at the site of successful Gold Shunt implantation compared with unsuccessful Gold Shunt implantation. The authors concluded that these features suggest that aqueous filtration across the sclera may be one of the mechanisms for IOP reduction with this device.</ref>
The Aquashunt is made of biocompatible polypropylene material that conforms to the shape of the globe. The device is 10-mm long, 4-mm wide, and 0.75-mm thick. The body of the device has 250-µm openings. Unlike other ab externo suprachoroidal devices, this shunt is implanted through a full-thickness incision of the sclera to the level of the suprachoroidal space. The shunt is advanced through the suprachoroidal space toward the anterior chamber with its shearing leading edge separating the attachments between the ciliary body and the scleral spur and creating an opening into the anterior chamber, allowing aqueous outflow through the shunt lumen. The device comes with an insertion tool that serves as an obturator during placement and keeps tissue from blocking the lumen of the device. The shunt and tool essentially replace a cyclodialysis spatula. After the proximal end is properly positioned in the anterior chamber, the insertion tool is removed, and the device is secured to the sclera with a single biodegradable suture. The distal end of the device is tucked beneath the posterior lip of the scleral incision, followed by closure of the conjunctiva.
A small clinical trial of Aquashunt was recently completed in Mexico and the Dominican Republic. Only 15 patients were in the clinical trial, and IOP reduction of 30% to 40% was noted in 13 patients at 12-month follow-up, although 6 required adjunctive medications. The main problem identified in the trial was fibrosis in the suprachoroidal space, suggesting the need for highly biocompatible materials and possibly antifibrotic agents, which are currently under investigation.
Ab Interno Approach
The CyPass shunt (Alcon Labs, Fort Worth TX) and the iStent SUPRA (Glaukos, San Clemente CA) are a biocompatible tubular device designed for implantation in the suprachoroidal space. Unlike the previously mentioned devices, both the CyPass and iStent Supra devices use an ab interno approach to direct fluid into the suprachoroidal space. The procedure is performed through a clear corneal incision alone or in combination with cataract extraction. A goniolens is used to visualize the angle for proper insertion of the shunt. The device and the inserter are advanced across the anterior chamber until the scleral spur and iris root are identified. The iris is gently pushed away with viscoelastic, and a small cyclodialysis is created, placing the distal end of the device into the suprachoroidal space and the proximal collar in the anterior chamber. The CyPass device was pulled from the market by Alcon and subsequently recalled by the FDA due to loss of corneal endothelial cells and concerns over the long term implications on corneal health. It is unclear if the iStent Supra will have the same effect on the cornea as data remain unavailable at this time. A more recent device, the MINIject implant (iSTAR Medical SA, Wavre, Belgium), is also under investigation for shunting fluid into the suprachoroidal space but available data on safety and efficacy are early.
Suprachoroidal devices have been extesnsively studied for the surgical treatment of glaucoma by targeting the uveoscleral outflow system to lower IOP. These devices and procedures may potentially be safer than other options for pressure lowering that require bleb formation such as trabeculectomy and glaucoma drainage device implantation. However, long-term safety of these devices remain unknown and the recent recall of CyPass sheds new light on complications that may at first go by un-appreciated. The efficacy of these devices over long-term follow up also remains to be determined.
- Suprachoroidal devices have been in existence for several decades and in various forms.
- These devices allow for avoidance of a bleb and thus might lead to safer long-term IOP lowering with lower risks of endophthalmitis.
- No long-term data are available showing safety of these devices in various types of glaucoma.
- The potential for fibrosis with early or late failure remains an issue, and future techniques to lessen fibrosis will likely be needed to improve the long-term performance of these devices.
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