What You Need to Know About the Hydrus Microstent
The trimodal design maximizes flow through an inlet structure.

Canal-based microinvasive glaucoma surgery (MIGS) devices are now readily available to general ophthalmologists and glaucoma surgeons. Traditional surgical options, though powerful and effective, have typically offered a “one-size-fits-all” approach to treatment, with increased risks and longer recovery times. However, with MIGS continuing to evolve and develop, these devices have established a new niche in the glaucoma treatment algorithm, allowing more individualized treatment options. The safety, efficacy, and reoperability afforded by these devices has led to their recent rise in popularity in the ophthalmology community. This is an exciting time to be involved in glaucoma surgery with a variety of different treatment options. As we continue to learn how to tailor our treatments to each individual patient, a better understanding of the available devices is an important step for anyone involved in MIGS surgeries. One such canal-based MIGS device is the Hydrus Microstent (Ivantis), which as of August 2018, received FDA approval for use in mild-to-moderate open-angle glaucoma patients who are undergoing cataract surgery.

Profile and Mechanism of Action
Roughly the size of an eyelash, the Hydrus Microstent is a device made from nitinol, a super-elastic, biocompatible alloy, which is designed to lower intraocular pressure (IOP) by acting within Schlemm’s canal.1 At 8 mm in length, the Hydrus is curved to match the contour of Schlemm’s canal with alternating “spines” and “windows” to provide structural support while facilitating aqueous humor outflow pathways respectively. It is implanted using an ab-interno, clear-corneal approach with the implant designed to cover 3 clock-hours of Schlemm’s canal. The innovative trimodal mechanism of action works by maximizing flow through an inlet structure, creating a scaffold that dilates Schlemm’s canal to bypass resistance and enhances aqueous flow through conventional outflow systems. The device provides direct flow into the canal through the inlet and stretched inner wall, thereby enhancing the effective filtration area, as well as maintaining the patency of the canal to provide flow in collector channels and aqueous veins.

Patient Selection
The Hydrus microstent has typically been targeted for patients with mild-to-moderate glaucoma on medications who are going to undergo cataract surgery. Due to safety of the device with enhanced IOP lowering vs cataract surgery alone, it promises to reduce or eliminate medication burden in more patients with minimal increased risk. Outside of the United States, centers have also reported encouraging results when the microstent was used as a standalone option in pseudophakic patients, phakic patients, and also in some cases of refractory glaucoma.2

Unique Characteristics and Advantages
As with other MIGS devices, an ab-interno approach allows for insertion of the device through a clear corneal incision. This allows for minimal tissue disruption in the eye, as well as an excellent safety profile. We can thus preserve conjunctival tissue for future glaucoma surgery. There are also several unique advantages to the Hydrus device. First, it does not require the formation of an artificial bleb for fluid egress, which carry potential serious complications such as hypotony or blebitis.3 As Hydrus directly enhances aqueous flow through the conventional outflow system, episcleral venous pressure provides innate protection against hypotony. The device covers 3 clock hours and therefore accesses a large area of episcleral veins. Its shape allows for minimal stressing of the tissues and disruption of the normal anatomy of the eye, as it is designed to conform to the outflow pathway and provide support for the canal over 3 clock hours. Finally, due to the high safety profile of the device, it gives the patient the option of decreasing their drop load at the time of cataract surgery with a quick, low-risk procedure.

Recent Data on Safety
The HYDRUS II study, a randomized, control trial that looked at the efficacy and safety profile of Hydrus, first demonstrated its clinical utility in this population. The study determined significant reduction both in IOP and medication load when the stent procedure was performed in combination with phacoemulsification vs phacoemulsification alone for up to 2 years following surgery (16.9±3.3 mmHg vs 19.2±4.7 mmHg; P=.0093).4 The study also demonstrated comparable safety measures between the Hydrus group and the control group receiving phacoemulsification alone.4

More recently, the HORIZON study, a prospective, multicenter trial, examined the safety and efficacy of the Hydrus Microstent in lowering IOP in patients with mild-to-moderate glaucoma undergoing cataract surgery. It demonstrated similar overall safety measures to the HYDRUS II study, with significant IOP lowering effects and propensity for patients to remain medication free when cataract surgery is combined with Hydrus implantation in comparison with cataract surgery alone, both at 12 and 24 months.5 Based on these results, the device recently received FDA approval in the United States for use in conjunction with cataract surgery to reduce IOP in patients with mild-to-moderate primary open glaucoma.6 Ivantis has provided its preliminary updated 3-year HORIZON study results including endothelial cell density (ECD) data. Patients from the trial exhibited no accelerated loss of ECD from the first postoperative visit through 3 years of follow-up, and the difference between Hydrus and cataract surgery alone was not shown to be statistically significant. A similar outcome was shown for the difference in the rate of ≥30% ECD loss between the Hydrus arm and cataract surgery alone at 3 years. Patient follow-up at 3 years was 100%. The one reported complication associated with Hydrus implantation was an increase in focal peripheral anterior synechiae (PAS) or tissue adhesions (14.9% nonobstructive and 3.8% obstructive).5 The study showed, however, that even with reported PAS and tissue adhesions, the unique trimodal mechanism of action of the device allowed for an IOP lowering effect from the device.5

Conclusion
The Hydrus Microstent is an innovative MIGS device inserted into Schlemm’s canal, acting as a microbypass and a canal scaffold to enhance aqueous flow through conventional outflow systems. Studies have shown promising results when Hydrus is used to target mild-to-moderate glaucoma in combination with cataract surgery. Further long-term studies, including head-to-head comparisons with other novel MIGS procedures, would be useful to fully assess its full potential outcomes in addition to the beneficial outcomes demonstrated. GP

References
1 Ivantis Inc. The Hydrus Microstent. Available at www.ivantisinc.com/hydrus-microstent.php .

2 Fea A, Dallorto L, Lavia C. The SPECTRUM registry: 12 and 24 month results from a global real world study of 2400 glaucoma eyes treated with microinvasive glaucoma surgery using the Hydrus Microstent. Presented at: 9th International Congress on Glaucoma Surgery; September 6-9, 2018; Montreal, Canada.

3 Mastropasqua L, Agnifilli L, Mastropasqua R, Fasanella V. Conjunctival modifications induced by medical and surgical therapies in patients with glaucoma. Curr Opin Pharmacol. 2013;13(1):56-64.

4 Pfeiffer N, Garcia-Feijoo J, Martinez-de-la-Casa JM, et al. A randomized trial of a Schlemm’s canal microstent with phacoemulsification for reducing intraocular pressure in open-angle glaucoma. Ophthalmology. 2015;122(7):1283-1293.

5 Samuelson TW, Chang DF, Marquis R, et al. A Schlemm canal microstent for intraocular pressure reduction in primary open-angle glaucoma and cataract: The HORIZON Study. Ophthalmology. 2018: n. pag.

6 Food and Drug Administration (2018). FDA Summary of Safety and Effectiveness Data (PMA P170034). Silver Spring, MD: US department of Health & Human Services.
https://www.accessdata.fda.gov/cdrh_docs/pdf17/P170034B.pdf