Suprachoroidal Devices

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 by Jeffrey Bruce Kennedy, MD on February 1, 2016.


Introduction

The uveoscleral outflow pathway was first described in animal experiments by Anders Bill in 1965 (1) (2). Aqueous exits the anterior chamber (AC) across the ciliary body and iris root, driven by the negative pressure gradient from the AC to the suprachoroidal space (3). From there, fluid is resorbed into scleral vessels, the choriocapillaris, and through scleral pores into the episclera (4). Prostaglandin analogues, commonly prescribed as first line medical therapy for glaucoma, increase aqueous outflow through this uveoscleral pathway, thereby lowering intraocular pressure (5). Surgical access to the suprachoroidal space to lower intraocular pressure in patients with glaucoma is not a novel concept. Surgical creation of a cyclodialysis cleft to lower intraocular pressure was described by Heine in 1905 for intractable glaucoma (6); however, this procedure was largely abandoned due to frequent postoperative hypotony and a rapid increase in intraocular pressure after spontaneous closure of the cleft. Various techniques, implants, and space-maintaining substances have been described in an attempt to create a safer and more stable suprachoroidal drainage pathway, with limited success (7) (8) (9) (10) (11). With advances in device design and biocompatibility, the uveoscleral outflow pathway is again a target of active research and development. Several new microsurgical devices seek to lower intraocular pressure by exploiting the outflow facility of the suprachoroidal space and providing a controlled and sustainable fluid egress from the anterior chamber. Both ab externo and ab interno approaches to the suprachoroidal space are possible, each with unique advantages and drawbacks.


Ab Externo Approach

The suprachoroidal space can be accessed ab externo via a trans-scleral dissection. This approach has the disadvantage of requiring both conjunctival and scleral manipulation, which may make additional glaucoma filtering surgery difficult in the future due to scarring. However, an ab externo approach may allow for more extensive tissue dissection and potentially greater IOP lowering compared to an ab interno approach. Three notable ab externo devices under investigation include The Gold Micro-Shunt (GMS, SOLX Corp, Waltham, MA), the STARflo (iStar Medical, Isnes, Belgium), and the Aquashunt (OPKO Health, Miami, FL).

Gold Micro-Shunt

Design

The GMS is made of medical-grade 24 karat gold. The concept for this device was based on the observation that a gold foreign body was well tolerated in the eye (12). The original GMS is 3.2mm wide, 5.2mm long, 44um thick and weighs 6.2mg. The shunt is composed of two rectangular fused leaflets with a rounded proximal end, and a distal end with fins to anchor the device in the suprachoroidal space. The proximal end of the shunt has sixty 100um holes and one 300um hole to allow aqueous to enter. The distal end contains a grid of 117 110um holes on each side of the implant to allow free flow of fluid into the suprachoroidal space. Nineteen channels are present within the shunt of which 9 are open, each with a width of 24um and a height of 50um. The GMS+, a revised model, is 3.2mm wide and 5.5mm long and weighs 9.2mg. The design has been modified such that the fins are located at the proximal end of the device in the anterior chamber. The external gold plates of the GMS+ are separated by posts rather than channels, creating a larger area of aqueous flow (Figure 1). The GMS is currently approved for use in Canada, as well as in some European countries. Investigational studies are underway in the United States.

Surgical Technique

Following creation of a fornix-based conjunctival flap, a full-thickness scleral incision is made 2-3mm posterior to the limbus, exposing the suprachoroidal space. The anterior chamber is deepened using either viscoelastic or an anterior chamber maintainer. A crescent blade is then used to tunnel anteriorly into the AC at approximately 90% scleral thickness. Blunt dissection is also directed posteriorly into the suprachoroidal space. The proximal end of the GMS is fed into the anterior chamber using the inserter which comes with the device, such that 1-1.5mm of the device is visible in the AC. The distal portion of the shunt is then tucked underneath the posterior lip of the scleral flap. The scleral and conjunctival flaps are the then closed tightly to avoid leakage of aqueous and bleb formation.

Clinical Results

Skaat et al (13) compared two different models of the GMS (24 um internal channels or 48 um internal channels) with the Ahmed Glaucoma Valve in a three-armed randomized, prospective, interventional trial for patients with refractory glaucoma. All patients included in the study had either primary open angle glaucoma, pseudoexfoliation glaucoma or pigment dispersion glaucoma and had undergone at least one failed trabeculectomy. Twenty-nine patients were randomized for implantation with either an Ahmed glaucoma valve, 24um GMS or 48um GMS. Success was defined as IOP greater than 5mmHg and less than 22mmHg with reduction of IOP at least 20% below preoperative IOP. IOP was significantly lowered in all 3 groups after 5 years of follow up. In the Ahmed group, IOP was lowered from 33.5±6.7 to 17.3±2.6 mm Hg. In the 24um GMS group, IOP was lowered from 25.7±0.7 to 17.8±2.4 mm Hg. In the 48um GMS group, IOP was lowered from 35.6±2.2 to 19.6±5.2 mm Hg. The number of glaucoma medications required was not significantly different between groups. Cumulative success rates at 5 years were 77.8%, 77.8% and 72.7% in the Ahmed, 24um GMS and 48m GMS, respectively.

Hueber et al (14) reported very different results in a retrospective review of 31 eyes which were implanted with the second-generation GMS+. The patients in this study were diverse; 48% of eyes had undergone previous glaucoma surgery, 55% had primary open angle glaucoma, 16% had pseudoexfoliation glaucoma, 13% had secondary glaucoma, 10% had pseudophakic angle closure glaucoma, and 6% had pigmentary glaucoma. Failure was defined as IOP greater than 21 or less than 5 at any visit at least 6 months after GMS+ implantation, any serious complications (retinal detachment, endophthalmitis, suprachoroidal hemorrhage, low grade inflammation or new onset rubeosis iridis) or need for additional glaucoma surgery. At the end of 4 years, 97% of eyes met criteria for failure, with 71% of eyes failing within the first year. 77% of eyes required a secondary glaucoma surgery for uncontrolled IOP, with mean time to second surgery 7.3 months (range 0.5-37.1). Two GMS+ were explanted because of newly developed rubeosis iridis, and two were explanted for persistent low-grade inflammation. At the conclusion of the study, mean IOP for all patients was 27.9±10.4mmHg, slightly increased from the preoperative mean IOP of 26.58 ± 10.14 mmHg. No definitive rationale for the poor performance of the GMS in this study was provided by the authors.

Figus et al (15) reported on 55 eyes of 55 patients with refractory glaucoma. All patients had uncontrolled IOP on maximum tolerated medical therapy and had undergone at least 1 previous glaucoma surgery (range 1-5). Patients underwent GMS implantation and were followed for two years. Success was defined by IOP less than 21 mmHg and >33% IOP reduction. At 2 years, 67.3% of patients were considered a qualified success, while only 5.5% were considered a complete success. The most common complication was transient hyphema (21.8%). Choroidal detachment was noted in 10.9% of eyes. Explantation was required in three eyes (5.45%): two secondary to corneal edema due to endothelial-shunt contact and one secondary to over-filtration causing exudative retinal detachment. Most failures occurred within the first year. The most common cause of failure was an inflammatory membrane forming on the anterior end of the shunt, obstructing inflow from the AC (66.6%).


STARflo

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