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  • Rethinking Glaucoma Surgery: Aqueous within the Eyewall

    Glaucoma
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    For nearly a century, ophthalmologists have been trying with little success to improve glaucoma surgical techniques, and the current standard glaucoma filtering surgery remains trabeculectomy, a procedure in which the surgeon creates a “hole” in the eye to drain aqueous from the anterior chamber into the subconjunctival space. Though the procedure and related devices have developed, patients continue to suffer from complications related to external drainage, including hypotony, flat anterior chamber, choroidal effusions, bleb leakage, late infection, and scarring.

    In a glaucomatous eye, the outflow faces increased resistance, and it is assumed that aqueous flow through the trabecular meshwork and juxtacanalicular tissues is partially obstructed. Based on this assumption, a new concept discussed at the 2005 American Glaucoma Society meeting prompts surgeons to drain the aqueous inside the eye, thereby avoiding external fistulization under the conjunctiva. At least 5 new devices motivated by this concept are under investigation, and each utilizes Schlemm’s canal or the suprachoroidal space as an alternate route of aqueous egress. In bypassing only the diseased portion of the eye's drainage system and using the fluid pathways downstream from the obstruction, the flow more closely approximates the normal functioning of the eye and may help avoid many of the potential complications associated with trabeculectomy.

    EyePass

    Courtesy Mark Sherwood, MD.
    Figures 1 and 2. EyePass implant.

    The EyePass implant (GMP Vision Solutions) is a tiny silicone Y-shaped tube that has a dual-bonded end and is small enough to fit inside conventional silicone tubes (Figures 1 and 2). It requires the creation of a deep scleral flap and the unroofing of a portion of Schlemm's canal, similar to a nonpenetrating sclerostomy procedure. After Schlemm’s canal is defined, the two legs of the tube are inserted in opposite directions down Schlemm’s canal, and the main stem is inserted through a small incision into the anterior chamber. Once in place, there is generally some blood reflux down the implant and into the anterior chamber, which serves to confirm the correct insertion of the device. Finally, the scleral flap is tightly closed. In bypassing the juxtacanalicular portion of the trabecular meshwork, the EyePass ensures drained fluid stays within the eyewall, where it drains from the eye more naturally.

    The implant is undergoing phase III clinical trials in up to 20 investigational sites in the United States. Preliminary data from a non-randomized study described 11 patients after 6 months follow-up, showing a 30% drop in mean intraocular pressure (IOP) from a baseline of 27.6 mm Hg and no sight threatening complications (Brown et al, 2005).

    Glaukos iStent

    Courtesy Mark Sherwood, MD.
    Figures 3 and 4. iStent implant.

    This micro-stent implantation is also a bypass procedure, but the iStent (Glaukos Corp.) is inserted into Schlemms’s canal internally through a clear cornea incision that does not affect the conjunctiva (Figures 3 and 4). It allows the aqueous humor to flow directly into Schlemm’s canal toward the episcleral drainage system, thereby avoiding the trabecular meshwork. The device stents open the trabecular meshwork and Schlemm’s canal to re-establish normal outflow.

    A European multicenter prospective study collected data from 34 patients with mean IOP pre-operative of 26.9 ± 4.8 mm Hg and mean follow-up of 3.8 ± 2.1 months. The study showed that 76.5% of patients reached an IOP of less than or equal to 21 mm Hg and 17.7% reached less than or equal to 15 mm Hg (Feo et al, 2005). Also, it may be possible to place more than one Glaukos iStent in different areas of Schlemm’s canal, and in-vitro studies have suggested that this tactic may lower IOP further (Bahler et al, 2004:988-994).

    Trabectome

    Courtesy 2005 NeoMedix Corp.
    Figures 5 and 6. Trabectome.

    The Trabectome (NeoMedix Corp.) is a 19.5-gauge device that is inserted through a 1.5-mm clear corneal incision and advanced nasally across the anterior chamber (Figures 5 and 6). By using electrocautery, it ablates and removes a small portion of the trabecular meshwork overlying Schlemm’s canal. An incorporated infusion system maintains the anterior chamber depth and a gonioscope is used to visualize the trabecular meshwork. After insertion of the footplate of the Trabectome into Schlemm’s canal, the footswitch activates the aspiration and electrosurgical elements and ablates a strip of trabecular meshwork, opening a 40° to 60° section. The back plate of the device is designed to protect the deep wall of Schlemm’s canal and the exiting vascular system. Transient bleeding into the anterior chamber occurs from the cut meshwork and is washed out through the temporal incision.

    A recent paper reported that 37 Mexican Hispanic and Caucasian patients with primary open-angle glaucoma (POAG) reached a mean IOP of 17.4 mm Hg at 6 months (n=25) and 16.3 mm Hg at 1 year (n=15), on or off medication. Complications included some blood reflux in the majority of patients, which resolved within a week without long-term visual consequences. Corneal injury complications occurred in 12% of patients, as a transient sub-Descemet’s heme, Descemet’s change, and Descemet’s scroll (Minckler et al, 2005:962-967).

    Excimer Laser Trabeculostomy (ELT)

    Courtesy Mark Sherwood, MD.
    Figures 7 and 8. TuiLaser excimer fiberoptic delivery system probe.

    ELT is a minimally invasive surgical treatment for open-angle glaucoma and may work even in patients who have previously had Laser Trabeculoplasty (Figures 7 and 8). The ELT procedure creates a pathway from the anterior chamber into Schlemm's canal. Several shots (about 10 spread over 90o) are delivered from the XeCl 308-nm excimer laser to the trabecular meshwork and the inner wall of Schlemm's canal, removing the tissue by photoablation and limiting damage and scarring. Through a corneal paracentesis, the fiberoptic delivery system crosses the anterior chamber and makes contact with the trabecular meshwork. Laser pulses remove tissue to create a fistula into Schlemm's canal, causing no thermal damage to surrounding tissues. The positioning of the fiberoptic is viewed directly with either a goniolens or an endoscope.

    Data from one recent study suggest that ELT does particularly well if combined with phacoemulsification. The 12-month Kaplan-Meier success rate based on a 20% reduction in IOP and a pressure lower than 21 mm Hg was 46% for the ELT procedure alone (n=37) and 66% for a combined procedure (n=35) (Wilmsmeyer et al, 2005:1-7). In both scenarios, postoperative ocular side effects were minimal and uncommon.

    Gold Micro-Shunt (GMS)

    Courtesy Mark Sherwood, MD.
    Figures 9 and 10. Gold micro-shunt (GMS).

    The GMS (SOLX) is a flat, 24-karat gold plate approximately 5.2 mm long and 3.2 mm wide with a thickness of 44 µm (Figures 9 and 10). The GMS is made of 99.95% pure gold, which benefits aqueous flow by eliminating the formation of scar tissue and minimizing tissue ingrowth and protein adherence. The shunt is inserted into the suprachoroidal space through a single 3.5-mm wide subscleral or clear cornea incision. It contains a number of microtubular channels that connect the anterior chamber with the suprachoroidal space. Studies have shown a difference of about 5 mm Hg in pressure between the anterior chamber and the suprachoridal space. The basis behind the fluid flow mechanics in the shunt is the eye's natural pressure differential, which reduces IOP without requiring a bleb.

    The GMS is also novel because it allows the surgeon to potentially control the amount of fluid flowing from one area to another. Ten of the 20 channels on the GMS are open upon implantation, but with the DeepLight 790-nm Titanium Sapphire Laser, the surgeon may activate additional originally sealed channels, each representing about a 1 mm Hg reduction in IOP when functioning. Even years after surgery, the surgeon may open up these additional channels and achieve incremental reduction in IOP without administering medication.

    The shunt has been implanted in 220 eyes in pilot clinical studies. In one study involving 3 centers, patients achieved an average IOP reduction of 12 mm Hg after implantation (a 34% reduction compared with the pre-operative maximal medicated baseline). Patients were followed for up to 2 years. Complications and adverse events were low and transient in almost all cases (Groves et al, 2005: Special Report).

    Summary & Outlook

    The paradigm in glaucoma surgery may be shifting: by leveraging an alternative pathway for aqueous drainage, new procedures and instruments could help decrease IOP to safe levels and avoid complications related to external drainage.

    While these new methods have great theoretical advantages over external drainage surgery and the initial results are encouraging, the procedures remain under investigation and have been tested only in patients with open-angle glaucoma. Those devices that require internal entry into Schlemm’s canal from the anterior chamber are probably impractical for patients with 360o-angle closure glaucoma, and for some techniques, iridocorneal epithelial (ICE) syndromes or neovascular glaucoma may be a contraindication. Additionally, investigators have not shown whether the devices can reduce IOP levels to the point required by many glaucoma patients who suffer from moderate to advanced disc damage.

    The exact role of these techniques in the surgical armamentarium for glaucoma must be determined. Some of these surgeries may fit into the spectrum of therapies that can be performed prior to trabeculectomy and may not preclude such surgery in the future. More patient studies and longer follow-up are needed to confirm the enduring safety, efficacy, and potential role of these new procedures in our management of glaucoma.

    References

    1. Brown RH, Fellman RL, Ball SF, Lynch MG. EyePass bi-directional glaucoma implant: clinical study [abstract]. Paper presented at: American Glaucoma Society Annual Meeting; March 2004; Sarasota, FL.
    2. Feo F, Traverso C, Lopez FMH, et al. Microtrabecular bypass stent for open-angle glaucoma patients. Poster presented at: Association of International Glaucoma Societies World Glaucoma Congress; July 6–9, 2005; Vienna, Austria.
    3. Bahler CK, Smedley GT, Zhou J, Johnson DH. Trabecular bypass stents decrease intraocular pressure in cultured human anterior segments. Am J Ophthalmol. 2004;138: 988-994.
    4. Minckler DS, Baerveldt G, Alfaro MR, Francis BA. Clinical results with the Trabectome for treatment of open-angle glaucoma. Ophthalmology. 2005;112(6):962-967.
    5. Wilmsmeyer S, Philippin H, Funk J. Excimer laser trabeculotomy: a new, minimally invasive procedure for patients with glaucoma. Graefes Arch Clin Exp Ophthalmol. 2005; 19:1-7.
    6. Groves N. Micro-shunt, laser duo a novel glaucoma therapy. Ophthalmology Times. August 1, 2005.

    Author Disclosure

    The authors state that they have no financial relationship with the manufacturer or provider of any product or service discussed in this article or with the manufacturer or provider of any competing product or service. Dr. Sherwood, however, is principal investigator at one of the clinical sites for a Glaukos iStent study.