BY IKE AHMED, MD, FRCSC
The supraciliary space has been untapped in the last few years since the CyPass Micro-Stent (Alcon) was withdrawn from the market in 2018 due to long-term endothelial cell loss.
This is an aqueous outflow space with vast potential; there is no episcleral venous floor, so the potential to lower pressure is quite great. It is freely accessed and a large, absorptive space that we can take advantage of because the gradient from the anterior chamber to the suprachoroidal space allows for filtration as well.
The question in this space has always been, can we balance safety and efficacy to achieve outcomes? The biggest challenge is controlled outflow and dealing with wound healing that can prevent success and cause failure.
Targeting Suprachoroidal Flow and GradientWe know the CyPass was, of course, a suprachoroidal MIGS procedure combined with phaco and a pressure-lowering stand-alone procedure. We also know the problems we had with CyPass. Most of us believe it’s not necessarily an issue with the supraciliary space; the issue was more the device itself and placement of the device.
When we were developing the CyPass, we debated how stiff or soft the implant should be. We chose to use a stiff, non-conformable implant we felt allowed tenting of the space, and this device had a lumen designed to drain aqueous. We are moving away from these single-lumen stiff devices.
We know the suprachoroidal space has outflow. We know that devices can drain aqueous. But we also know the concerns around implantable hardware. While I am certainly not opposed to implantable hardware, there is always a concern about its effect on the tissue surrounding the implant. The more similar an implant is to surrounding tissue, the less likely it is to have a foreign body reaction. High stiffness and less homologous tissue are likely to elicit more macrophages and fibrosis.Advancing Device Technology
Two new approaches in the supraciliary space are the iStar Medical MINIject and the Iantrek AlloPass.
The MINIject (Figure 1) is a very soft silicon sponge. Stenting opens the supraciliary space to allow flow around and through the device, which acts like a wick. We have already published early results of a two-year study.1 On a cursory look, I would say the pressure lowering seems more effective than what we have seen with trabecular procedures, although it is not as much as bleb-forming procedures.
The AlloPass BioTissue device uses sclera inside the eye (Figure 2).
We have a lot of experience managing sclera. It’s homologous, porous, hydrophilic, and readily available in terms of access. What are some of the device’s features? It’s conformable; there’s no hardware; the device can be placed with adequate depth with the right injector and hydrate and adhere to surrounding tissue. Sclera shows very good permeability, potentially allowing this to act as a wick for the aqueous into the supraciliary space from the anterior chamber.
The AlloPass can be placed with direct viewing of the angle or with a gonio-free approach. Early results from 10 patients over 12 months show fairly good pressure lowering down into the low- to mid-teens, with fortunately no serious adverse events (Figures 3, 4). Combined and stand-alone procedures have shown similar effects. So far, we have seen 10% endothelium loss combined with phaco, which is not too different from other devices in this space.
So, we’ll have to see where these types of devices fit in the zone of pressure lowering and risk. Can we go beyond what we see with MIGS and micro-invasive bleb surgery? Perhaps the suprachoroidal space may provide this middle ground of efficacy balancing with safety.
Dr. Ahmed is a fellowship-trained glaucoma, cataract, and anterior segment surgeon with clinical and research focuses on managing glaucoma, complex cataracts, and intraocular lens complications. He has done pioneering work in innovative glaucoma therapeutics and coined the term MIGS as a new genre of surgical approaches and devices. He is director of Moran’s Alan S. Crandall Center for Glaucoma Innovation.
REFERENCE
1. Denis P, Hirneiss CW, Durr GM, et al. Two-year outcomes of the MINIject drainage system for uncontrolled glaucoma from the STAR-I first-in-human trial. Br J Ophthalmology. 2022;106(1):65-70.
FINANCIAL DISCLOSURE
Dr. Ahmed is a consultant/advisor for iStar, maker of the Medical MINIject, and a consultant/advisor for Iantrek, maker of the AlloPass.