Better understanding of aqueous outflow may improve Schlemm’s Canal surgery

Recent research that challenges old assumptions about aqueous outflow could increase the effectiveness of Schlemm’s canal surgery enough to make it a viable – and safer – alternative to trabeculectomy for lowering intraocular pressure in glaucoma patients, said Barbara Smit MD PhD of the University of Washington.

“We need safer, more accessible glaucoma surgeries that are accessible to general ophthalmic surgeons,” said Dr Smit, who delivered the Stephen A Obstbaum MD Honored Lecture at Glaucoma Day at the 2015 American Society of Cataract and Refractive Surgery Symposium in San Diego.

Current procedures that enhance the natural aqueous outflow mechanism, such as canaloplasty, viscocanalostomy and trabecular meshwork bypass stents, eliminate some of the biggest risks of trabeculectomy. These include hypotony, scarring and failure, choroidal haemorrage, and endophthalmitis, Dr Smit noted. But they often fail to lower IOP below the mid-teens, and sometimes don’t work at all.

New research offers clues as to why, Dr Smit said. It also suggests that many common assumptions about aqueous outflow may not be correct. One old idea is that 75 per cent of outflow resistance comes from the trabecular meshwork, and many procedures focus on removing or bypassing the TM. But research using aqueous angiography shows that significant resistance to outflow resides distal to the trabecular meshwork, in collector channels and the deep scleral plexus. “The location of resistance may vary among patients, and resistance also may vary with time and healing,” Dr Smit said.

Similarly, research shows that collector channels are unevenly distributed and outflow only occurs in some, contradicting conventional wisdom that outflow occurs through 360 degrees, Dr Smit noted. This means placement of devices or incisions is critical. Outflow also has been thought static, depending solely on pressure differential. But OCT imaging suggests that outflow may be dynamic and regulated, showing evidence for pumping mechanism, valves and variable resistance, Dr Smit said. Evidence also suggests that the TM stiffens as glaucoma progresses, which may inhibit any pumping action. These observations are buttressed by molecular research by James Tan MD PhD suggesting collector channels and the intrascleral plexus are all lined with contractile tissue.

These insights, and the tools that provide them, are likely to reshape Schlemm’s canal surgery, Dr Smit concluded.

“Clinical tools like OCT and outflow angiography may help us not only to understand the system better, but eventually help us select surgical candidates for the best surgical techniques for them. Current surgical approaches to outflow via Sclemm’s Canal will pave the way for more elegant and effective surgeries tomorrow.”