Are Modest Gains From Two-Stent Glaucoma Surgery Worth Pursuing?

Shuchi B. Patel, MD


June 18, 2019

Microinvasive glaucoma surgery (MIGS) has revolutionized the options available to patients with mild to moderate glaucoma. Most MIGS procedures are performed in conjunction with cataract extraction and add only minutes to the intraoperative time, do not lengthen the recovery compared with stand-alone cataract surgery, and have minimal risk for an adverse reaction.

One such MIGS device, iStent (Glaukos Corporation; San Clemente, California), was recently released as a second-generation stent (iStent inject) and approved by the US Food and Drug Administration. iStents are trabecular micro-bypass devices implanted using an ab interno approach. They lower intraocular pressure (IOP) by allowing the aqueous to bypass the trabecular meshwork, which has the area of highest resistance to outflow; thus, the aqueous gets direct access to drain into the Schlemm canal. Although the stent is very small, with only an 80-µm lumen, the four 50-µm side outlets allow for multidirectional outflow so that each stent can carry the total amount of aqueous humor produced by the human body (average 2.5 µL/min). The injector comes preloaded with two stents, with recommendations to implant both 2-3 clock hours apart.

Previous studies have demonstrated that the most effective IOP lowering with iStent implantation occurs with two stents. One stent alone provides minimal change in IOP, whereas more than two stents provides more reduction but with diminishing returns.[1,2]

iStent inject: Two-Year Results

Recently, a multicenter, prospective, randomized, controlled trial[3] was performed to compare outcomes and adverse events over 2 years between a treatment group (n = 387) implanted with two second-generation trabecular micro-bypass iStent inject devices and a control group (n = 118) receiving no stent implantation.

Researchers included eyes with mild to moderate primary open-angle glaucoma with preoperative IOP ≥ 24 mm Hg on one to three medications and unmedicated diurnal IOP (DIOP) in the range of 21-36 mm Hg. The mean treated IOP was 17.5 mm Hg in both groups, and unmedicated DIOP was similar between the treatment and control groups (24.8 and 24.5 mm Hg , respectively). All patients required cataract extraction, which was performed before implantation of the stent in the treatment group.

The study performed several analyses, but overall the results demonstrated a modest reduction in IOP and medication burden.

The mean unmedicated DIOP at 24 months was similar in the treatment and control groups (17.1 and 17.8 mm Hg, respectively), but represented a statistically significant difference. The mean IOP ranged from 15.4 to 17.1 mm Hg in the treatment group and 15.8 to 17.8 mm Hg in the control group among eyes that were able to remain medication-free from 6 to 24 months. This difference was not statistically significant; however, the percentage of patients who were able to remain medication-free was significant, with 84% of treatment eyes medication-free versus 67% of control eyes. Furthermore, the medication burden was reduced in the treatment group. The mean number of ocular hypotensive medications ranged from 0.3 to 0.4 in the treatment group and 0.6 to 0.8 in the control group.

Clinical Takeaways

The results of this study are similar to what I have found clinically beginning with the first-generation iStent. Even with the implantation of two stents, IOP reduction is modest and thus not the procedure of choice in patients with more severe glaucoma or in those who need IOP in a very low range.

The fact that unmedicated DIOP is similar in both groups suggests to me that the reduction in IOP from cataract surgery alone is significant and lasting. More than half of the patients in the control group were able to remain medication-free and have IOP very similar to those who were medication-free with implantation of two stents. Perhaps in patients with such mild glaucoma, there is no great benefit to performing stent implantation in addition to cataract extraction. However, the difficulty comes in predicting which patients would do just as well without a stent, given that about 17% more patients do become medication-free by implanting two stents. Similar to how the Ocular Hypertension Treatment Study[4] performed multiple subgroup analyses to identify the highest-risk patients, it would be interesting to see whether further analyses of the current data could provide more information to identify which patients would benefit from stent implantation to become medication-free, instead of just proceeding with cataract surgery alone.

Without that information or a benefit stratification algorithm at this time, in my opinion it is worthwhile to consider iStent in many patients. As mentioned in the study, even a modest 1 mm Hg reduction of IOP decreases long-term risk for progression by 11%-19%.[5,6] The difference of a 1.6 mm Hg mean reduction of unmedicated DIOP between the treatment and control groups could be very beneficial. If the implant gets approval as a stand-alone procedure in the future, it may become an option to first see the response to cataract surgery and then offer the iStent to pseudophakic patients if the IOP reduction was not optimal with cataract extraction alone.

Another reason I will continue to offer and implant iStents is my belief that it is imperative for both glaucoma surgeons and cataract surgeons to be comfortable with angle-based surgery. The future of MIGS lies in the angle, and without routine gonioscopy and intraoperative gonioscopic-based procedures, I fear this skill could be lost. The second-generation iStent is even simpler than its predecessor. Because it is preloaded, minimal training of surgical staff is required. The implantation is easier with a direct inject approach as opposed to threading it into trabecular meshwork. With some experience, every surgeon could incorporate this procedure into their practice.

In my practice, I am still waiting for a MIGS procedure that can completely replace trabeculectomy. Trabecular micro-bypass shunts are not likely to replace this gold-standard procedure, because the population of patients who benefit from them is not the same as those who are candidates for trabeculectomy and have severe glaucoma or need very low IOP. Regardless, in patients with mild to moderate glaucoma, the small amount of pressure reduction, as well as the increased chance of being medication-free or having a reduction of medication burden, seems a worthwhile reason to offer this option, given the ease of the procedure with almost no adverse sequelae. As an added benefit, patients may also be able to avoid or at least delay the need for other interventions.

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