Progress in Surgery for Glaucoma
(Note that this was commentary originally authored in 2011-2012)
Progress lies not in enhancing what is, but in advancing toward what will be.
My first thought when I reflect upon the state of glaucoma surgery is that not much has changed. I performed my first incisional glaucoma surgery 25 years ago. I created a partial-thickness defect in the eye wall under a scleral flap. Although it is true that we have benefited from numerous incremental refinements, the fundamental technique of trabeculectomy is quite recognizable when I refer back to the glaucoma textbooks in my library from the 1980s. We have enhanced what is. Glaucoma surgery remains a “trade” practiced by skilled craftsmen. Where are we? How did we get here? Where should we be headed? Are we advancing toward what will be?
Glaucoma was historically regarded as a disease of elevated intraocular pressure (IOP). Experimental evidence indentified that resistance to outflow resides largely in the inner wall of Schlemm’s canal. It seemed logical under that model to “normalize” IOP by removing a portion of the wall to provide aqueous humor direct access to Schlemm’s canal. This was the primary objec-tive of trabeculectomy (not the diversion of aqueous to the subconjunctival space). Eventually, we abandoned this notion of using trabeculectomy to restore aqueous access to Schlemm’s canal and refined the procedure as an aqueous diversion. The study of glaucoma contains a wonderful history of observation, experimentation, and refinement of hypotheses. A lifetime devoted to trying to understand glaucoma is humbling as we continue to provide care in the face of fundamental uncertainties.
We do not understand well the nature of the optic neuropathies that are pressure-sensitive and deserve to be included as such in our current schema of glaucomas. And we have only one proven effective surgical strategy: lower the IOP. I have the chance to repeat in print a lesson I learned from my grandfather: “If all you have is a hammer, everything looks like a nail.” We continue to hammer away at IOP with surgery that provides, at best, a nonphysiologic solution to a poorly understood problem.
Modern cataract surgery might serve as our model for surgery that continues to progress toward what will be. When I was a resident, we still had some attending surgeons who performed elegant and artful intracapsular cataract surgery. Aphakic refraction was a required fundamental skill. The task of cataract surgery was clearly defined—we sought to remove the clouded crystalline lens and replace it with a lens implant without damaging the cornea or inducing astigmatism. In pursuit of this goal, we had the evolution of the intraocular lens (IOL). Extracapsular techniques invited a variety of innovative posterior chamber lens designs. Ultrasonic phacoemulsification allowed incisions smaller than the diameter of the rigid Polymethylmethacrylate (PMMA) lenses. Continuous curvilinear capsulorrhexis improved success. Lens materials advanced to allow smaller incisions. Lasers can perform some steps of the procedure. Each advance opens the door for the next one. As the techniques and technology improve, we add rapid visual rehabilitation and excellent uncorrected visual function to our goals.
Contrast this with trabeculectomy. The first trabeculectomies I per-formed were during my residency, following publication of the Fluorouracil Filtering Surgery Study. My attending physician carefully supervised both the surgery and the postoperative administration of the fluorouracil injections. I spent the next 2 months managing the persistent wound leaks and corneal toxicity I had induced. Things have gotten better since then. I have migrated away from a large fornix incision to a small limbal incision. Instead of retrobulbar anesthesia and a facial block, I use topical anesthesia and sub-Tenon’s irrigation of additional anesthesia. I cannot recall the last time I used a superior rectus bridle suture for exposure. Surgical sponges moistened with mitomycin C (MMC) are applied to sclera posteriorly. My scleral flap is created with a micrometer diamond blade and a beveled micro blade. The dimensions of the scleral flap are much smaller than when I first trained. I use a small punch to create the ostium in peripheral cornea, or I can place a small aperture glaucoma filtration device through the sclera and eliminate the need for an ostium. The flap is closed tightly with planned suture lysis. Postoperative care is still rather intensive with planned suture lysis with elegantly designed laser lenses, titration of medications, and supplemental antifibrosis treatment based on the wound healing response. The theme is unchanged, but the myriad variations and evolution of tech-nique have improved the intra- and postoperative course. Certainly, I believe we have enhanced what is. Yet, a beautifully performed trabeculectomy unleashes the paramount uncertainties of unpredictable wound healing and a lifetime risk of complications.
What Could Be
Why should not the goals of IOP-lowering surgery be the achievement of an effective, predictable, and technically reproducible procedure with rapid visual rehabilitation, excellent long-term control of IOP, and few early or late complications? I hope that describes what will be. How might we advance in that direction?
The subconjunctival space is a convenient target for the diversion of aqueous and is easily accessable from the anterior chamber. The surgical approach is straightforward. However, we still lack predictable techniques to modulate the wound healing response. If we could simply open conjunctiva, create aqueous outflow with modulated resistance, and close the conjunctiva in a watertight fashion with rapid and complete epithelial healing but limited subconjunctival wound healing, we might have a predictable pres-sure-lowering operation. The introduction of 5-fluorouracil and MMC have greatly enhanced our ability to achieve lower IOP but still in a highly unpredictable fashion. We are now plagued with fragile blebs with long-term risk of infection, encapsulated blebs with return of IOP to preoperative levels, or the occasional diffuse healthy bleb with excellent long-term control of IOP. Improved strategies for customized wound-healing modulation remain the greatest opportunity for evolutionary improvement of trabeculectomy. This would also invite the development of devices for active control of outflow to the subconjunctival space in response to continuous IOP monitoring.
The conventional outflow pathway is nature’s solution for modulating IOP. This should remain a primary target for aqueous diversion. Several steps are potentially necessary for surgical success. Aqueous must exit the anterior chamber and enter Schlemm’s canal. A strategy for bypassing the resistance of trabecular meshwork is appealing. We tried that; it is called goniotomy. Although goniotomy can work in pediatric glaucoma, it is not sufficiently successful in adults. A better approach is needed.
A contemporary effort is the development of surgical devices to elimi-nate or bypass TM resistance and allow aqueous direct access to Schlemm’s canal. Accurate placement would seem essential, but, even then, Schlemm’s canal must be a functional system in communication with the downstream outflow pathways. The segmental nature of Schlemm’s canal might necessitate multiple bypasses. Is it possible to identify the functioning segments and recruit those for outflow?
A modern goniotomy can be performed with ablation of the trabecular meshwork. Yet, IOP does not approach the level of episcleral venous pres-sure in most patients who have this procedure. This suggests failure of outflow can reside further downstream. What is the nature of downstream failure? Can we rehabilitate collector channels and aqueous veins? These seem like opportunities for advancement.
The suprachoroidal space is another target for aqueous diversion. We tried that as well. It is called cyclodialysis. This was a highly unpredictable operation that could result in extremely low pressures and poor vision or sudden closure and very high IOPs. This target space has regained interest for an improved surgical approach. Several pilot studies have been initiated with devices to provide controlled access of aqueous to this space. Optimal or even acceptable devices need demonstrated utility. The wound healing in this suprachoroidal space is incompletely characterized. We need to understand this better. Yet, I believe the suprachoroidal space also remains a target worthy of further exploration.
Lowering IOP with trabeculectomy is effective. This is an unqualified statement, and glaucoma surgery is quite nuanced. I have often had conversations with colleagues who said, “I am comfortable with my glaucoma surgery procedure.” I now interpret this statement to mean that the experienced glaucoma surgeon has developed a level of comfort with an operation that has many well-known shortcomings. The unpredictability of IOP control, slow and sometimes incomplete visual rehabilitation, and a lifelong potential for complications characterize our current state of IOP-lowering surgery. I often think the very best we now do is that the patient will be no worse, and we do not always achieve that goal. When faced with the alternatives of losing vision from glaucoma progression or undergoing trabeculectomy, it is not surprising that our patients are looking for another option. If we are honest, as glaucoma surgeons—so are we. I believe these needs will continue to drive our progress toward what will be.