Glaucoma Surgical Innovation Nearing the Tipping Point
(Note that this was commentary originally authored in 2011-2012)
We are at a tipping point in the surgical management of glaucoma. Although most agree that trabeculectomy (guarded filtration surgery) has many imperfections, the irreversible nature of glaucoma requires a conservative approach to surgical innovation. Indeed, given the pernicious nature of some forms of glaucoma, trepidation toward surgical innovation is understandable. However, until recently, technological advancement in surgical glaucoma has been virtually nonexistent. This, despite the fact that it has been clear to many in the field that trabeculectomy, an operation for advanced and particularly high-risk glaucoma, is not a procedure for early disease. The heterogeneous nature of glaucoma, ranging from relentlessly progressive to easily controllable, mandates a more nuanced approach than the “one surgical procedure for all patients” (ie, trabeculectomy) approach that has been utilized for the past several decades. The simple fact that a filtration bleb renders a patient at risk for infection for life is unacceptable for patients with early disease or those with a more benign prognosis. It is unacceptable to subject patients at low risk for severe functional vision loss to bleb-related complications, including devastating late infections. Few procedures in medicine subject the patient to such lifelong iatrogenic risk. Further, the “therapeutic index” for trabeculectomy is unacceptably narrow to be widely used in patients with early to moderate disease. For example, a conservative filtration procedure performed with little or no antimetabolite may function well for several years but has a significant risk of failure over time. Although the bleb-related risks with this more conservative approach are diminished, this approach subjects the trabecular meshwork to several years of reduced perfusion. Subsequently, should the bleb fail, the meshwork is likely to function more poorly than prior to the procedure, owing to disuse atrophy of the distal collector system. On the other hand, antimetabolites may ensure long-term bleb stability and function. However, such measures subject the patient to the risks inherent to a weakened conjunctival barrier. Thus, the bleb that functions transiently but is prone to failure is suboptimal, and the bleb that becomes ischemic and well- established is suboptimal due to the risk of late leaks and infection. Aqueous drainage devices are a reason-able alternative to trabeculectomy with less risk of infection or conjunctiva-related complications, yet tube shunts also subject patients to significant risk, including diplopia, hypotony, corneal decompensation, and tube exposure.
Although traditional transscleral procedures, such as trabeculectomy and tube shunts, remain excellent options for advanced glaucoma, there is great need for safer surgical options for use in early glaucoma, prefer-ably procedures that augment physiological outflow rather than bypassing it. Fortunately, the glaucoma surgical space is in the crosshairs of several well-capitalized new companies. Moreover, the purchase of the Ex-Press device by Alcon during the last year has brought new attention and ample resources to glaucoma surgical innovation as well as a safer and more precise procedure in the Ex-Press-assisted trabeculectomy.
Statistics confirm that the number of trabeculectomies performed by United States surgeons has declined considerably in recent years, despite a more aged population. Improvements in medical therapy and increased utilization of laser trabeculoplasty are important factors contributing to this decline. Another explanation for the diminishing role of external filtration surgery is the recent evidence that cataract surgery lowers intraocular pres-sure (IOP) more than previously appreciated. Cataract and glaucoma are com-mon conditions and each is more prevalent with advancing age. The natural history of cataract and glaucoma is often parallel, thus it is a tempting strategy to manage both conditions with a single operation. Traditionally, a significant percentage of trabeculectomy procedures have been performed in conjunction with cataract surgery. However, this trend has decreased in recent years. The collective influence of improved glaucoma medications, the knowledge that cataract surgery often enhances physiologic outflow, and the development of less invasive surgical methods to enhance conventional outflow have made many surgeons reluctant to bypass trabecular outflow via trabeculectomy until later in the disease process or at least until after cataract surgery has been performed.
Although the mechanism of IOP reduction following cataract surgery is unclear, most believe it is due to improved physiological outflow facility. In theory, the IOP-lowering effect of cataract surgery and trabeculectomy are mutually exclusive, one negating the effect of the other. Therefore, for mild to moderate glaucoma occurring in conjunction with cataract, it seems prudent to perform either phacoemulsification alone or phacoemulsification combined with a minimally invasive glaucoma procedure, preferably one that enhances or preserves conventional outflow facility. Although a transscleral bypass procedure, such as trabeculectomy, might be the best option for patients with advanced glaucoma, eg, when fixation is threatened, patients with mild or moderate disease may be better served with a procedure that enhances physiological outflow.
Canaloplasty is an ab externo approach to Schlemm’s canal during which the canal is intubated with a microsurgical catheter allowing 360-degree visco-dilation and subsequent suture tensioning of the trabecular meshwork. The procedure has been shown to provide a reasonable balance between safety and efficacy. Published data show fewer complications than with trabeculectomy with slightly less efficacy. Although the procedure has gained gradual acceptance, its adoption rate has been less than universal due to its technical difficulty, the length of the procedure, and the fact that it expends valuable conjunctiva and scleral tissue, rendering the superior limbus suboptimal for subsequent filtration surgery if needed. My own experience with canaloplasty has been favorable for patients in the intermediate range of glaucoma severity. If a canaloplasty fails in my hands, I generally will follow it with an aqueous drainage device such as a Baerveldt implant. The Tube Versus Trabeculectomy (TVT) Study results provide reasonable validation for the use of tubes without prior trabeculectomy.
Ab interno trabeculotomy (Trabectome) is another approach to Schlemm’s canal. It is appealing due to the fact that it utilizes a clear corneal conjunctival-sparing approach. It is generally performed in conjunction with cataract surgery. Published data suggest that it adds additional IOP reduction as compared to phacoemulsification alone. Yet, a controlled study has not been done to compare the effect of Trabectome alone versus Trabectome with cataract removal. Finally, the iStent, another ab interno approach to Schlemm’s canal, is currently under review by the Food and Drug administration (FDA). A panel convened by the FDA in July 2010 provided a favorable review of the iStent United States Investigational Device Exemption (IDE) trial. However, as of the time of the writing of this chapter, the FDA has not yet approved the iStent. Although the IOP-lowering effect of a single iStent was modest, patients receiving iStent required significantly less postoperative medications, and there was no measurable increased risk of phacoemulsifica-tion combined with iStent as compared with cataract surgery alone. Moreover, data from outside the United States suggest that additional efficacy is achieved when more than one stent is placed.
In summary, I currently reserve trabeculectomy for advanced glaucoma. I prefer safer, minimally invasive surgery, such as cataract extraction alone, Trabectome, canaloplasty, and when available, the iStent for patients with early to moderate glaucoma. We have made tremendous strides in our quest for a better surgery for use in early to moderate glaucoma. Given the momentum currently in place and the considerable attention dedicated to glaucoma surgery by our industry partners, I strongly believe that the best is yet to come for the surgical glaucoma patient.