March 2020


Should you be targeting collector channels with MIGS?

by Liz Hillman Editorial Co-Director

Targeting an area with increased pigmentation and blood reflux into Schlemm’s canal (red line) adjacent to a blue mark on the cornea (black line) that identified a collector channel
Source: Paul Harasymowycz, MD

With newer stenting devices bypassing the trabecular meshwork or stenting open Schlemm’s canal in different ways, is it still relevant to target collector channels? The physicians who EyeWorld spoke with said, in short, yes, though perhaps less important than with single stents.
“In every case that we perform—we perform canal stenting procedures for the treatment of glaucoma—the first step of the surgery consists of identifying the episcleral vessels in the nasal quadrant,” said Paul Harasymowycz, MD.
Dr. Harasymowycz said he was encouraged to start targeting areas where segmental outflow would be most likely. While this might have been more relevant with single stents, he said he still identifies collector channels when using an iStent inject (Glaukos), which consists of two multidirectional stents. Targeting iStent inject, however, is more nuanced.
“[T]he size of the wound limits the number of clock hours that one can target as the trocar used for implantation must be very perpendicular to the trabecular meshwork,” he said. “In cases where we use the [iStent] inject, we will modify our main wound location to be between two separate collector channels, or simply make an additional incision to target a ‘ripe’ area.”
Arsham Sheybani, MD, said he was targeting collector channels more when using single stents, and while he still does try to target with larger stents, like the Hydrus Microstent (Ivantis), he said it’s more likely due to the size of the device to hit them. Dr. Harasymowycz also likes to identify the main collector channels with stents like Hydrus, but he sees it as less of a necessity. Stents that bypass the trabecular meshwork and dilate Schlemm’s canal are less sensitive to exact placement and increase larger areas for outflow, he said.
Finally, what about procedures, such as canaloplasty or goniotomy, that open up the angle all together, hitting many collector channels along the way? Dr. Harasymowycz cited research that showed trabecular excision surgeries expose a larger amount of collector channels and have greater areas of outflow as compared to trabecular stenting.1
“One should remember, however, that trabecular excision surgeries may induce postoperative inflammation with angle scarring that may raise IOP but can be reversed with YAG laser goniopuncture,” he said.

How to identify collector channels

Dr. Harasymowycz said collector channels can be identified with gonioscopy in areas where there is increased trabecular meshwork pigmentation or spontaneous blood reflux into the anterior chamber when the IOP is lowered below episcleral venous pressure.
“When the angle has no pigmentation or a homogeneous distribution of pigment, or no blood reflux, it is very difficult to guess how close we are to the collector channels. In these cases, it is even more important to mark the cornea,” he said. Dr. Harasymowycz explained that he marks the cornea at the limbus in order to see the dot during gonioscopy.
Dr. Sheybani said he also looks for areas of hyperpigmentation.
“I’m looking for a place that potentially might have had flow or still has flow because I’d rather get to a collector channel that’s functional. I worry a little bit when it’s been awhile and a collector has not been functional. Say you have an area with a lot of pigmentation compared to one that doesn’t. If there is an area behind a collector that might not have much pigmentation, that collector may have been atrophic,” Dr. Sheybani said.

Verifying you hit a collector channel

When stents are correctly placed, Dr. Harasymowycz said a blood reflux from the collector channel into Schlemm’s canal through the orifice of the stents can be seen.
“In addition, one can see the outflow during irrigation and aspiration of the viscoelastic, and a blanching of the vessels can be seen,” Dr. Harasymowycz said. “Postoperatively at the slit lamp, our group correlated the amount of outflow seen through the collector channels and the amount of IOP lowering and number of medications used.”2
Overall, Dr. Harasymowycz said “we are looking forward to future tools that will help us measure outflow both pre- and postoperatively, and help surgeons decide which specific collector channels to target, similar to coronary angiography when placing cardiac stents,” Dr. Harasymowycz said.

About the doctors

Arsham Sheybani, MD
Assistant professor
Washington University
School of Medicine
St. Louis, Missouri

Paul Harasymowycz, MD
Head of the Glaucoma
Research Unit
University of Montreal
Quebec, Canada


1. Fellman RL, Grover, DS. Episcleral venous fluid wave in the living human eye adjacent to microinvasive glaucoma surgery (MIGS) supports laboratory research: Outflow is limited circumferentially, conserved distally, and favored inferonasally. J Glaucoma. 2019;28:139–45.
2. Bostan C, Harasymowycz P. Episcleral venous outflow: A potential outcome marker for iStent surgery. J Glaucoma. 2017;26:1114–1119.

Relevant disclosures

: Glaukos, Ivantis, New World Medical
Sheybani: Allergan, Katena, Glaukos, Ivantis



Should you be targeting collector channels with MIGS? Should you be targeting collector channels with MIGS?
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