Five Things Every OD Should Know About Trabecular Meshwork Bypass and MIGS

More than 4 million Americans will have cataract surgery this year, with more than 20% using at least one topical glaucoma medication.^1,2

This is a timestamp in patients’ lives to perform microinvasive glaucoma surgery (MIGS) during their cataract surgery.

As optometrists, we need to be able to navigate the world of glaucoma interventions so we can provide collaborative care for our patients throughout this process.

MIGS Devices

Today’s MIGS options are exploding. In the United States, there is an enormous potential demand because greater than 75% of patients with glaucoma have mild to moderate disease, where MIGS fits elegantly.³

The goal of MIGS is to intervene earlier in the glaucoma disease process, providing a minimally traumatic procedure to decrease IOP with the potential to reduce, and possibly eliminate, our patients’ medication burden.

To better understand these choices, it is helpful to understand the production and drainage of aqueous in the eye. Aqueous is produced in the posterior segment and travels from the ciliary body epithelium around the lens, through the pupil, into the anterior chamber, and into one of the two outflow pathways.

In the conventional outflow pathway, which accounts for 70 to 95% of outflow, aqueous travels through the trabecular meshwork and into Schlemm’s canal. It is picked up by the collector channels and then the episcleral and aqueous veins.

In the uveoscleral outflow pathway, which accounts for 5 to 30% of outflow, aqueous exits through the root of the iris and between the ciliary muscle bundles, draining the fluid between the suprachoroidal and scleral tissues. Currently, we do not have a commercially available MIGS stenting device for this outflow pathway.

Finally, some glaucoma surgeries create a new outflow path, such as the Xen® Gel Stent (Allergan), trabeculectomies, and tube devices.

About the iStent inject® and iStent inject® W

The FDA approved the iStent® (Glaukos) in 2012, the first MIGS device to come to market. It is a trabecular micro-bypass device made from titanium, designed to restore the pathway for natural outflow from the anterior chamber to Schlemm’s canal. The natural episcleral back pressure of 8 to 11 mm Hg minimizes the risk of hypotony.

Based on the success of the original iStent, the iStent inject and subsequently the iStent inject W (G2W) were introduced. All use the conventional outflow pathway. The iStent inject W injector is preloaded with two stents that are inserted through the trabecular meshwork, 2 to 3 clock hours apart (Figures 1 and 2), designed to create two bypass pathways that provide multidirectional flow through Schlemm’s canal.

Patient Selection for the iStent inject W

Patients with mild to moderate primary open-angle glaucoma are considered on-label candidates for the iStent inject W if stenting is performed during cataract surgery.

Candidates include those who could benefit from better IOP control, patients who want to decrease their medication burden, those who want to avoid more invasive procedures, and patients who are noncompliant with glaucoma medications and spacing doses. More than 90% of patients are noncompliant with topical glaucoma drops, and nearly 50% stop taking their medications in the first 6 months.⁴ Compliance may be hindered by age, number of medications being taken, cost, and/or side effects. Nearly 60% of patients treated with glaucoma medications report ocular surface disease (OSD) symptoms.⁵ Preservatives in the drops can destabilize the tear film, which can adversely impact preoperative calculations and postoperative visual outcomes. Reducing topical medications can help maintain the tear film quality and possibly help improve the refractive outcomes of cataract surgery.⁶

Analyzing the Data

iStent technologies are minimally traumatic for patients, and more than 200 peer-reviewed journal articles have reported on the technology.

In the 2-year pivotal trial of the iStent inject, patients were randomized to receive cataract surgery with the iStent inject or cataract surgery alone. Among those having the combination procedure, 75.8% achieved at least a 20% reduction in unmedicated IOP compared with 61.9% of the group with phacoemulsification alone.⁷ Eighty-four percent of patients who received the iStent inject were medication free versus 67% receiving cataract surgery alone.

In real-world data from our center, 91 patients received the iStent inject during cataract surgery. All patients were using at least one topical glaucoma medication and were not washed out (Figure 3). Seventy-eight of 91 patients had an IOP less than or equal to 18 mm Hg 24 months after surgery.⁸

In a second real-world study, Schweitzer et al at Vance Thompson Vision reported significant improvements in Ocular Surface Disease Index scores 3 months after cataract surgery with iStent or iStent inject. This study also showed decreased IOP and medications in patients using at least one topical glaucoma medication before surgery.⁹

Finally, a report in Clinical Ophthalmology reinforces that the iStent and iStent inject do not impact the outcome of refractive cataract surgery.¹⁰

Postoperative Care

In our practice, we comanage approximately 85 to 90% of MIGS cases with iStent and iStent inject using the same medications and follow-up schedules as for cataract surgery.

Safety is paramount in MIGS, and iStent inject shines here. There are a few considerations to remember. Postoperatively we can still have an IOP spike. For immediate IOP spikes, we add medications and burp the paracentesis if needed. IOP fluctuations occurring 4 to 40 days after the procedure may result from steroids, requiring a change in steroid dosing or addition of glaucoma medications. The second, less common, event is microhyphema, which almost always self-resolves.

At the slit lamp we will see normal inflammation. I recommend gonioscopy to check the stents at least once during the postoperative period. I also encourage optical coherence tomography (OCT) and visual fields testing 3 to 6 months after the procedure to set new baselines.

In the immediate postoperative period (1 to 4 weeks) after trabecular meshwork-Schlemm’s canal-based procedures, the steroid can cause the IOP to be more volatile. After the steroid is removed and fully out of the eye, the new baseline IOP is stable and determined. The visual fields and OCT should guide medication titration. In MIGS, we also need to watch for hypotony, endothelial cell loss, and peripheral anterior synechiae, which are rare events, specifically with iStent inject.

Conclusion

Combined with cataract surgery, the iStent inject W provides short- and long-term IOP reduction to potentially decrease medication burden. Sixteen studies of 4 to 8 years’ duration have been performed for the original iStent and iStent inject.¹¹ All show IOP reduction for the duration of the study. It is an ab interno procedure, lends a high safety profile, and is designed to restore natural physiological outflow through two openings in the trabecular meshwork. It fits elegantly alongside cataract surgery comanagement.