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MIGS at 4 years: Where are we now and where are we going?
Innovative products that may soon be approved would provide surgeons with multiple options for their patients.
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Welcome to another edition of CEDARS/ASPENS Debates. CEDARS/ASPENS is a joint society of cornea, cataract and refractive surgery specialists, here to discuss some of the latest hot topics in ophthalmology.
While much of the attention regarding new technology in anterior segment surgery has been surrounding advances in IOLs and femtosecond laser technology, minimally invasive glaucoma surgery (MIGS) is now becoming a topic of interest for the anterior segment surgeon. This month, Carlos Buznego, MD, summarizes the current landscape in the world of MIGS. We hope you enjoy the discussion.
Kenneth A. Beckman, MD, FACS OSN CEDARS/ASPENS Debates Editor
“What do you mean my child needs glaucoma surgery?” the concerned father asked the ophthalmologist. “And the risk of complications and reoperation are that high? Why can’t you just fix the natural drainage system?” It was that last question that triggered California glaucoma specialist Dr. Richard Hill to discuss his novel idea of a trabecular bypass with the parent, a venture capitalist. The two came together to bring the original idea of minimally invasive glaucoma surgery to fruition. It took nearly a decade and the work of hundreds of engineers, medical scientists, surgeons and patients, but the iStent (Glaukos) finally reached the hands of eye surgeons in the summer of 2012 when it received FDA approval.
Four years have passed since the introduction of the first MIGS implant — where are we now and where are we going?
Saheb and Ahmed first coined the term “microinvasive glaucoma surgery” in 2012. They stressed that this new class of procedure utilizes an ab interno approach with a corneal microincision that spares the conjunctiva. These minimally traumatic procedures should have at least modest efficacy with an extremely high safety profile and rapid recovery.
The first FDA-approved MIGS implant, the titanium iStent, is the smallest device implanted in humans. It measures 1 mm × 0.5 mm and is implanted through the trabecular meshwork into Schlemm’s canal, facilitating aqueous drainage through the conventional pathway. The FDA trial investigated the use of the iStent in conjunction with cataract surgery. It revealed that the iStent group (plus cataract surgery) was statistically superior to the control group (cataract surgery alone) in both study endpoints of 20% IOP reduction and unmedicated IOP of less than 21 mm Hg. It should be noted that the iStent is the first glaucoma surgical device to stand the rigor of a controlled clinical trial.
Although many felt a trabecular stent would be primarily utilized by glaucoma surgeons, the indications made it immediately attractive to cataract surgeons. Patients with mild to moderate glaucoma are not usually managed by subspecialists but rather comprehensive ophthalmologists. As both conditions are associated with aging, there is significant comorbidity, with about 20% of cataract patients also suffering from glaucoma. The significant interest in MIGS among cataract surgeons may have been unexpected but not surprising considering the gap in therapeutic options between the low-risk topical medications and trabeculoplasty and the higher-intensity surgical management and risk of penetrating glaucoma surgeries such as trabeculectomy and tube shunt implants.
Four years after initial approval of the iStent, it remains the only MIGS implant commercially available in the U.S. However, earlier this year, Alcon received FDA approval for the CyPass suprachoroidal stent. Commercial launch is expected in the fourth quarter. Several other MIGS devices are in the pipeline for FDA consideration. How can we classify these new devices to help us understand their potential utilization?
Aqueous humor outflow is principally achieved (80%) via the “conventional pathway,” passing through the trabecular meshwork, Schlemm’s canal, collector channels, aqueous and episcleral veins. The “secondary pathway” or uveoscleral pathway is responsible for about 20% of outflow as fluid escapes through the iris, ciliary body and suprachoroidal space. These are the two physiologic mechanisms that most of the MIGS devices utilize to reduce IOP by increasing aqueous outflow. In addition, there are a few novel devices that do not neatly fall into the classic definition of MIGS in that, despite being minimally invasive, they do utilize diversion of the aqueous in a non-physiologic manner into a conjunctival bleb. Let’s take a look at how each of the devices works.
Newer devices for trabecular bypass are in the process of FDA approval. The Hydrus implant (Ivantis) is a 4-mm stent made of nitinol, a flexible biocompatible alloy. It not only provides a stent into Schlemm’s canal, it also scaffolds the canal open. This FDA PMA trial has completed enrollment, and initial data look promising. However, 2-year follow-up data is pending. It has an estimated approval date of 2018.
Glaukos is in the midst of an FDA PMA trial for its second-generation trabecular stent, known as the iStent inject. In this procedure, two tiny 0.5-mm rivet-like titanium stents are injected into the trabecular meshwork and Schlemm’s canal. Enrollment for this trial has been completed, and follow-up data are being collected. An estimated date of 2018 or 2019 is expected for commercial approval.
What approaches should be considered if improving the conventional drainage pathway is not sufficient? Two companies are developing suprachoroidal stents to utilize this pathway. The CyPass stent is a 6.35-mm device delivered into the suprachoroidal space via an ab interno approach. The device was developed by Transcend, which was acquired by Alcon earlier this year. It was recently approved by the FDA for use in combination with cataract surgery. Market launch of the new product will take place in the fourth quarter.
Glaukos is currently enrolling for an FDA PMA trial for a suprachoroidal stent. The iStent Supra is implanted in an identical fashion as the CyPass and is also being investigated in conjunction with cataract surgery.
Like the iStent, the other devices discussed so far are approved or seeking approval for implantation in conjunction with cataract surgery and following the arduous path of a randomized controlled clinical trial with an expected indication of “cataract plus mild to moderate glaucoma.”
Two new investigational devices are intended to divert aqueous into the subconjunctival space but in a minimally invasive fashion. The Xen implant (Allergan) is a 6-mm soft gel shunt preloaded in a delivery needle and inserted ab interno through the angle into the subconjunctival space. The current iteration has a 45-µm diameter lumen that limits flow to reduce the risk of hypotony. The clinical trial is completed and is under review by the FDA with approval likely in 2017.
The MicroShunt by InnFocus is another device to divert aqueous into a conjunctival bleb. This 8.5-mm tube is made of SIBS, which is commonly used in vascular stents. It is inserted ab externo; after opening the conjunctiva, a small needle track is made into the anterior chamber to allow insertion of this flexible tube.
Both the Xen and MicroShunt do not meet the exact criteria for MIGS but are seen as significant advances in changing our impression of penetrating glaucoma surgery. Laboratory studies show these blebs are less likely to trigger fibrovascular scarring. They are being investigated for implantation independent of cataract surgery. These devices are following the 510(k) approval process with an expected indication of “refractory glaucoma.” This does not require a randomized controlled trial but allows a more expeditious route by comparing the new surgery vs. trabeculectomy in patients who have failed medical therapy options.
Four years have passed since the initial approval of the iStent, the first MIGS implant. It has been adopted by a significant number of both glaucoma specialists and cataract surgeons. This success has drawn much attention to MIGS from industry, ophthalmologists and patients. The second approved device, the CyPass suprachoroidal stent, is set to launch imminently, with high expectations. Several other innovative products expect approval soon, which will provide surgeons with multiple new surgical options for their patients. It is truly exciting to witness a sea change in the therapeutic options for managing glaucoma, one of the most common ocular conditions.
- For more information:
- Carlos Buznego, MD, can be reached at Center for Excellence in Eye Care, Baptist Medical Arts Building, 8940 North Kendall Drive, Suite 400-E, Miami, FL 33176; email: cbuznego19@gmail.com.
Disclosure: Buznego reports he is a speaker, consultant and investigator for Glaukos and Allergan.