MIGS – how much is too much?
Choices give needed options, but search for ideal continues
Norbert Pfeiffer MD
As minimally invasive glaucoma surgery (MIGS) devices proliferate, including several stents for augmenting Schlemm’s canal or routing aqueous outflow to the supraciliary or subconjunctival space already on or near the market, it may seem like too much to choose from. Yet given the diversity of patient needs and the pluses and minuses of each approach, there’s no such thing as too many MIGS options, Norbert Pfeiffer MD told ESCRS Glaucoma Day 2019 in Paris, France.
Dr Pfeiffer, of Johannes Gutenberg University, Mainz, Germany, characterised MIGS devices as those delivered via paracentesis using an “ab interno” approach that leaves the conjunctiva untouched, or nearly so. He reviewed three categories of MIGS stents currently or soon to be available.
SCHLEMM’S CANAL STENTS
Stenting Schlemm’s canal is intended to increase aqueous outflow by overcoming resistance in the trabecular meshwork. Combined with phacoemulsification cataract surgery, the original iStent (Glaukos) showed moderate lowering of mean intraocular pressure (IOP) compared with phaco alone, 8.5±4.3mmHg vs 8.4±3.6mmHg, with slightly lower use of medications, 0.2 vs 0.4, with significantly more iStent patients meeting target IOP with and without medications, Dr Pfeiffer pointed out (Samuelson et al. Ophthalmology. 2011 Mar;118(3):459-67).
Dr Pfeiffer suggested the reason the stent did not lower IOP more may be its small size. “Schlemm’s canal is not a garden hose; there are outflow channels in some places but not others. If you put a stent in just one place it may not be enough.”
The Hydrus Microstent (Ivantis) is designed to address this issue by holding open three clock hours of Schlemm’s canal, ensuring multiple outflow channels are exposed.
“It is a maximally large minimally invasive stent” which is fed into the canal lengthwise, Dr Pfeiffer said.
In a study he conducted with colleagues, eyes treated with both the Microstent and cataract surgery showed washout IOP more than 2.0mmHg lower than eyes receiving cataract surgery alone at 24 and 36 months (Fea A et al. ESCRS 2018. Pfeiffer N et al. Ophthalmology.2015 Jul;122(7):1283-93).
The second-generation iStent inject allows insertion of multiple stents and appears to reduce IOP more than a single device. All three intracanalicular devices require a bimanual approach with gonioscope in one hand to provide a good view of the angle while inserting the device with the other hand.
A second approach is the supraciliary stent, which drains aqueous into the space between the uvea and sclera, which separate easily, making them less challenging to insert than canal shunts, Dr Pfeiffer said. The CyPass Micro-Shunt (Alcon) was an early example.
When combined with cataract surgery CyPass reduced mean IOP 7.4mmHg vs 5.4mmHg for cataract surgery alone, and reduced the need for glaucoma medication dramatically compared with cataract surgery alone (Vold S et al. Ophthalmology. 2016 Oct;123(10):2103-12). However, the device was removed from the market due to increased loss of corneal endothelial cells five years after surgery.
The MINIject (iSTAR Medical) is a supraciliary stent now in pivotal clinical trials that has reduced IOP by nearly one-third. “We will see what the results will be,” Dr Pfeiffer said. The potential for late side-effects must be closely monitored, he added.
Subconjunctival stents, such as the XEN (Allergan), are inserted from inside the eye, routing aqueous under the conjunctiva to form a filtration bleb much like a trabeculectomy, Dr Pfeiffer said. A clear view of the angle is required to insert the device through the sclera and under Tenon’s capsule, holding them apart to form a bleb. Potential complications include hypotony and subchoroidal bleeding.
The InnFocus MicroShunt (Santen) is not really a minimally invasive device since it is inserted from the outside under the conjunctiva, Dr Pfeiffer noted. It is often inserted with antifibrotics to preserve the bleb. While it appears to significantly reduce IOP, establishing its safety relative to trabeculectomy will require more time and study, he added.
“Variety helps and it is good to choose from. But variety shows the search for the ideal stent is still going on,” Dr Pfeiffer concluded.
Norbert Pfeiffer: firstname.lastname@example.org