MORE OPTIONS

iStent inject devices inserted in the trabecular meshwork (Courtesy Barbara Schmidt MD, PhD)

Recent research that challenges 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 (IOP) in glaucoma patients, Barbara A Smit MD, PhD told the 2015 ASCRS•ASOA Symposium & Congress in San Diego, USA.

Traditional filtering surgery is highly effective in lowering IOP, because it shunts fluid to the subconjunctival space, bypassing the natural outflow system and any resistance it entails, noted Dr Smit, Glaucoma Consultant, Spokane Eye Clinic, Spokane, Washington, US, who delivered the Stephen A Obstbaum MD Honoured Lecture. But it also exposes patients to long-term risk of complications including hypotony, scarring and bleb failure, and possibly endophthalmitis.

Filtration surgery is also technically complex and requires a lot of follow-up, making it impractical for many surgeons, particularly given the rising incidence of glaucoma as the population ages, said Dr Smit.“We really need some procedures that require less follow-up, have fewer complications and are technically accessible for the anterior segment surgeon,” she added.

This is where microinvasive glaucoma surgery, or MIGS, comes in. Many of these procedures are what Dr Smit refers to as Schlemm’s canal surgery, including canaloplasty, viscocanalostomy and trabecular meshwork bypass stents. Their goal is to restore access to the eye’s natural distal outflow system, which is presumably blocked by a damaged or clogged trabecular meshwork.

The problem is these techniques don’t always work. And when they do work they usually produce IOPs in the mid-teens, much higher than the 8.0mmHg that might be expected if episcleral venous pressure were the limiting factor, Dr Smit said.

New research offers clues as to why, Dr Smit said. It also suggests that many common assumptions about aqueous outflow may not be correct – and developing a better understanding of outflow mechanisms might result in better surgical outcomes.

New details on outflow

One old idea is that 75 per cent of outflow resistance comes from the trabecular meshwork, and many procedures focus on removing or bypassing it. 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. One way to test whether the distal outflow system is functioning at a particular location is to look for retrograde blood flow into the collector channels and Schlemm’s canal when anterior chamber pressure is lowered. This suggests the outflow mechanism may be intact, and marks a good position for a trabecular meshwork bypass stent.

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. 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.

Evidence also suggests that the trabecular meshwork stiffens as glaucoma progresses, which may inhibit any pumping action, Dr Smit noted. OCT images also suggest that high IOP may push the trabecular meshwork into Schlemm’s canal, effectively closing it off. It’s not known if this effect is temporary or causes permanent damage.

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 Schlemm’s canal will pave the way for more elegant and effective surgeries tomorrow,” she said.

Barbara A Smit: bsmit@spokaneeye.com