It is mandatory that we glaucoma surgeons evaluate the ocular surface and the health of the conjunctiva. A conjunctiva that is scarred from years of exposure to topical medications can make the surgical procedure long and difficult. Because the tissue is more susceptible to postoperative scarring, it can compromise surgical outcomes. Since the adverse effects of antiglaucoma medications and benzalkonium chloride (the most commonly used preservative) is dose dependent, each additional eye drop decreases the chances of success with conventional glaucoma surgical procedures. Thus, we need to judiciously choose between the potentially deleterious effects of exposing the eye to multiple antiglaucoma medications versus the risks and benefits of surgery.
The current surgical options for glaucoma surgery include trabeculectomy (which is the gold standard), tube shunts, nonpenetrating glaucoma surgery, cyclophotocoagulation, and conjunctiva-independent surgery. Conjunctivaindependent surgery is considered the “new kid on the block” among the current surgical interventions for glaucoma. According to the available evidence based on limited experience, these ab interno approaches can be performed quickly, do not damage the conjunctiva, and appear to be safe. Their main advantage is that they do not rely on the conjunctiva for surgical success, leaving it unaffected and available for later use in case of surgical failure. Three established conjunctiva-independent approaches include conjunctiva-independent Trabectome surgery (NeoMedix, Inc., Tustin, CA), the trabecular microstent (the iStent; Glaukos Corp., Laguna Hills, CA; not available in the United States), and the CyPass implant (Transcend Medical, Inc., Menlo Park, CA; not available in the United States).
TRABECULECTOMY, TUBES
Nonpenetrating glaucoma surgery and trabeculectomy both essentially depend on the limbal conjunctiva for success and thus are contraindicated in a scarred limbal conjunctiva. A potential course of action in these eyes is a tube, which shunts the aqueous to an area posterior to the limbus and thus bypasses the limbal conjunctiva. This procedure is not without significant complications, however. Extrusions are particularly problematic, according to the Tube Versus Trabeculectomy (TVT) study, and they occur in up to 5% of cases.1 Such a risk can be lessened by placing a tube in a tunnel to protect it, creating a deep trench in which to bury the tube, and using a large envelope flap (9 x 5 mm; Figure 1). Conjunctival scarring remains a determinant of surgical success, however.
Therefore, we must consider other options that circumvent the conjunctiva. Conjunctiva-independent surgery is essentially an ab interno approach, which has the potential to drastically influence the results of glaucoma surgery.
AB INTERNO TRABECULECTOMY
One of the more widely accepted conjunctiva-independent surgeries is ab interno trabeculectomy with the Trabectome. The procedure involves an electrocautery ablation of the trabecular meshwork and inner wall of Schlemm's canal through a paracentesis incision under gonioscopic guidance. I was privileged to participate in the early clinical trial of the first 101 cases of ab interno trabeculectomy with George Baerveldt, MD, and Donald S. Minckler, MD, both at the University of California, Irvine.2 Our early results showed that the procedure spared the conjunctiva and was minimally invasive. IOP outcomes to date in our clinical case series have been in the mid-to-low teens, so the procedure may not be appropriate for eyes in which a very low IOP goal is deemed necessary. As with any new glaucoma surgery, patient selection should take into account the expected IOP outcome.
MICROSTENT SURGERY
The rationale for using a trabecular microstent (the iStent; Figure 2) is that it provides a channel for direct transtrabecular aqueous outflow from the anterior chamber to collector channels. The device is self-retaining, constructed of implant-grade titanium (6AL4V), and coated with heparin. The microstent is implanted under gonioscopic guidance using an applicator introduced via an anterior chamber paracentesis.
Most clinical studies of the iStent have been encouraging, showing decreased medication requirements postoperatively and reduced IOP to an average of approximately 17 mm Hg after iStent instillation. Complications have been infrequent, with the most common being malpositioning of the iStent presumed to lead to clinical failure. Reflux bleeding from Schlemm's canal after viscoelastic removal intraoperatively has been common.
THE CYPASS MICROSTENT
The final ab interno approach is the CyPass supraciliary device (Figure 3). Unlike other tubes, it does not target the conventional pathways such as the trabecular meshwork and Schlemm's canal system. Again, the surgeon creates a paracentesis by injecting viscoelastic into the anterior chamber under the guidance of a gonioscopy lens (Figure 4). After inserting the CyPass, the goal is to target the aqueous outflow through the supraciliary space by positioning the outlet of the tube between the sclera and the ciliary body, almost like a surgical prostaglandin. This approach taps into the eye's strategic reserves of uveoscleral outflow. The first time I attempted this procedure, I was skeptical about how easy it would be to properly place the CyPass. I found that the sclera's rigidity guides the tube into the space between the ciliary body and the sclera. The procedure is quite straightforward and is not particularly challenging for the trained surgeon. Long-term follow-up results are necessary.
ADVANTAGES OF CONJUNCTIVA-SPARING PROCEDURES
I am interested in conjunctiva-sparing procedures for glaucoma therapy, because they are fast and relatively safe, based on the data thus far. However, these modalities need more long-term data on their safety and ability to lower IOP. Nevertheless, they appear to offer viable options for driving IOP below critical levels without damaging the conjunctiva, which leaves those tissues available for conventional filtration procedures if and when required. Controlled, randomized, head-to-head comparisons with trabeculectomy are essential before these devices can gain widespread use.
Tarek Shaarawy, MD, is head of glaucoma in the Ophthalmology Service, Department of Clinical Neurosciences, Geneva University Hospitals, Switzerland. He acknowledged no financial interest in the products or companies mentioned herein. Dr. Shaarawy may be reached at tarek.shaarawy@hcuge.ch.
