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Glaucoma surgeons are constantly clamoring for better surgical techniques. Traditional, incisional glaucoma surgery shunts fluid to the subconjunctival space and provides a route for aqueous to leave the eye by bypassing the normal outflow system and avoiding sources of resistance to outflow, said Barbara Smit, MD, PhD.
San Diego-Glaucoma surgeons are constantly clamoring for better surgical techniques. These new procedures will have to include less follow-up, fewer complications, and be technically accessible, said Barbara Smit, MD, PhD.
Traditional incisional glaucoma surgery shunts fluid to the subconjunctival space and provides a route for aqueous to leave the eye by bypassing the normal outflow system and avoiding sources of resistance to outflow.
“We all perform it, but it comes with numerous potential complications,” including hypotony, scarring, choroidal hemorrhage, and endophthalmitis, said Dr. Smit during Glaucoma Day at the 2015 meeting of the American Society of Cataract and Refractive Surgery.
One of her patients, “Norm,” had undergone bilateral trabeculectomy in the 1990s, and developed bacterial endophthalmitis in one eye in 1999 (now no light perception in that eye). He then developed fungal endophthalmitis in his other eye in 2004 (now counting fingers vision only).
“Norm would like to inspire us to do better,” she said. With an aging population, glaucoma specialists are going to be inundated with patients and the demand for better procedures cannot go unanswered.
These new procedures will have to include less follow-up, fewer complications, and be technically accessible, she said.
One approach to microinvasive glaucoma surgery (MIGS) is Schlemm’s canal.
“Ideally, we’d access the natural distal outflow system via Schlemm’s canal,” she said.
In theory, this technique would eliminate trabecular outflow resistance, eliminate the risk of subconjunctival filtering, and limit IOP by episcleral venous pressure.
“But does it work?” she asked. “For decades, glaucoma surgeons have tried to figure out how to circumvent the obstruction to normal aqueous outflow bypassing the trabecular meshwork and allowing outflow via Schlemm’s canal and the distal outflow system-the collector channels and aqueous veins.”
There are only three ways to eliminate trabecular meshwork resistance, she said: remove the meshwork, expand Schlemm’s, or bypass the meshwork.
Removing the trabecular meshwork has been investigated with the Trabectome and a combined trabeculectomy/GATT Trab 360, she said. Expanding Schlemm’s can be achieved through viscocanalostomy, ab interno viscodilation, or canaloplasty. And the iStent and Hydrus bypass the meshwork altogether.
Combined glaucoma and cataract surgery results in mean IOPs in the mid-teens over the course of 12 months, “which is higher than the EVP of 8 mm Hg,” Dr. Smit said. “But why can’t we get lower pressures with any of these techniques?”
It’s the outflow dogma-75% of the resistance is in the meshwork, but outflow occurs throughout the 360° of Schlemm’s. And outflow is passive, she said.
“For every complex problem, there is an answer that is clear, simple, and wrong,” she said, citing H.L. Mencken.
For one, the percentage of resistance in the meshwork is “substantially less” than originally thought; the location of resistance may vary among the phenotypes of glaucoma, and there is greater resistance in distal outflow system.
Outflow improves in some eyes after meshwork removal or bypass, but not in all eyes-for reasons unknown, she said.
“The location of resistance may vary among patients,” Dr. Smit said. “Resistance to outflow may vary with time and healing.”
Current understanding of outflow is that the collector channels are unevenly located around Schlemm’s canal, and distal outflow is segmental and variable. Flow only occurs via some collector channels and there are multiple patterns of outflow.
Aqueous angiography is a “much needed” tool to evaluate distal outflow and should give clinicians a better understanding of the outflow system.
Finally, outflow is not really passive-it very well may be dynamic and regulated. There is some evidence for pumping mechanisms, valves, and variable resistance.
“We need safer, more effective glaucoma surgeries that are accessible to general ophthalmic surgeons” to meet the upcoming demand, Dr. Smit said.
Only by gaining a better understanding of the complexities of the outflow system can true surgical innovation occur, she said.
“The current surgical approaches to outflow via Schlemms’ will pave the way for more elegant and effective surgeries tomorrow,” she said.