Residency · Residency · Ophthalmology

Minimally Invasive Glaucoma Surgery (MIGS)

Overview and Classification

Defining MIGS

Minimally invasive glaucoma surgery encompasses a family of procedures that share several defining characteristics: they are performed using an ab interno approach through a corneal incision, they cause minimal tissue disruption with rapid recovery, and they offer a favorable safety profile at the cost of more modest IOP reduction compared with traditional filtering surgery. Most MIGS procedures are combined with cataract surgery, and their target population is patients with mild-to-moderate glaucoma who have an IOP goal in the mid-to-low teens. MIGS is not a substitute for trabeculectomy or tube shunt surgery in advanced glaucoma requiring aggressive IOP lowering.

Classification by Mechanism

MIGS procedures are classified by their mechanism of action and anatomic target. Trabecular bypass and Schlemm canal stents (iStent, iStent inject, Hydrus) create a direct pathway from the anterior chamber into Schlemm canal, bypassing the trabecular meshwork resistance. Trabecular ablation procedures (Kahook Dual Blade goniotomy, Trabectome, and GATT) remove or incise the trabecular meshwork to open Schlemm canal. Suprachoroidal drainage devices (iStent SUPRA, and the withdrawn CyPass) shunt aqueous into the suprachoroidal space. Subconjunctival filtration devices (Xen Gel Stent, PRESERFLO MicroShunt) drain aqueous to the subconjunctival space, functioning on a principle similar to trabeculectomy. Ciliary body-targeted procedures such as endoscopic cyclophotocoagulation are covered in the laser therapy topic.

Trabecular Bypass Devices

iStent and iStent inject W

The first-generation iStent is a single heparin-coated titanium stent placed ab interno into Schlemm canal. The second-generation iStent inject W consists of two preloaded titanium stents, each 360 micrometers long with four lateral outlet lumens. The stents are placed through the nasal trabecular meshwork into Schlemm canal under gonioscopic guidance, and the two-stent design allows access to more collector channels than a single device. The additional IOP reduction beyond cataract surgery alone is approximately 3 to 5 mmHg. The iStent inject is FDA-approved for use in conjunction with cataract surgery in patients with mild-to-moderate open-angle glaucoma. Its safety profile is excellent, adding minimal surgical risk when combined with phacoemulsification. Multiple randomized controlled trials have demonstrated statistically significant IOP and medication reduction compared with phacoemulsification alone.

Hydrus Microstent

The Hydrus Microstent is an 8 mm scaffold made of nitinol (nickel-titanium alloy) that is placed into Schlemm canal, spanning approximately 90 degrees (three clock hours) of the canal. It functions both as a trabecular meshwork bypass and as a scaffold that keeps Schlemm canal open, providing access to multiple collector channel ostia along its length. The HORIZON trial compared Hydrus combined with phacoemulsification to phacoemulsification alone and demonstrated greater IOP reduction and a higher proportion of medication-free patients at 24 months, with approximately 20% IOP reduction from medicated baseline. Based on comparative data, the Hydrus is generally considered more efficacious than the iStent.

<image>Gonioscopic view showing Hydrus microstent spanning three clock hours of Schlemm canal, with iStent inject stents visible in the trabecular meshwork for comparison</image>

Trabecular Ablation Procedures

Kahook Dual Blade (KDB) Goniotomy

The Kahook Dual Blade is a disposable instrument with two parallel blades that excises a strip of trabecular meshwork tissue. By completely removing the meshwork rather than simply incising it, the KDB reduces the risk of trabecular tissue re-closure. It can treat 90 to 180 degrees of the angle and can be combined with or performed independently of cataract surgery. IOP reduction is comparable to other trabecular MIGS procedures. The KDB offers the advantages of leaving no permanent implant in the eye, lower cost, repeatability, and suitability as a standalone procedure.

Goniotomy (Traditional)

Goniotomy involves making an incision through the trabecular meshwork to open Schlemm canal directly. The technique has a long history in pediatric glaucoma, where it is a primary treatment for congenital glaucoma. It has been revived in adult glaucoma practice with the development of improved gonioscopic visualization and specialized surgical instruments. The ab interno approach can be performed with various instruments including the Trabectome, KDB, or manual goniotomy blades.

Trabectome

The Trabectome is an electrosurgical device that performs ab interno trabeculectomy by ablating the trabecular meshwork and the inner wall of Schlemm canal using bipolar electrocautery. It treats 60 to 120 degrees of the angle and achieves 20 to 30% IOP reduction from baseline. Reflux bleeding during the procedure is common and expected -- it is actually a favorable sign indicating that the collector channels downstream of the treated area are patent.

GATT (Gonioscopy-Assisted Transluminal Trabeculotomy)

GATT is the most aggressive of the canal-based MIGS procedures. A suture or illuminated microcatheter is threaded into Schlemm canal and advanced around the full 360-degree circumference. Once both ends of the suture or catheter emerge, they are pulled to unroof the entire canal, performing a complete ab interno trabeculotomy. This 360-degree approach produces more significant IOP reduction than partial-angle procedures, with the ability to achieve IOP in the low-to-mid teens. The trade-off is a higher rate of hyphema, which is self-limited in most cases. GATT is useful across a range of glaucoma types including open-angle, pseudoexfoliation, pigmentary, and juvenile glaucoma. Importantly, it can be performed as a standalone procedure without requiring concurrent cataract surgery.

<image>GATT procedure showing microcatheter cannulation of Schlemm canal 360 degrees with subsequent ab interno trabeculotomy by pulling both suture ends</image>

Subconjunctival Filtration MIGS

Xen Gel Stent (Xen45)

The Xen Gel Stent is a 6 mm hydrophilic gelatin tube with a 45-micrometer inner lumen that is placed ab interno from the anterior chamber through the sclera into the subconjunctival space. It creates controlled aqueous drainage to the subconjunctival space, operating on the same fundamental principle as a trabeculectomy. Mitomycin C must be injected into the subconjunctival space preoperatively to prevent scarring around the drainage site. The Xen achieves 30 to 40% IOP reduction from baseline -- substantially more than trabecular MIGS procedures. Because a bleb forms, bleb-related complications can occur, though at a lower rate than with traditional trabeculectomy. The needling rate is 30 to 40% in the first year. The Xen can be performed as a standalone procedure or combined with cataract surgery and is suitable for moderate-to-advanced glaucoma.

PRESERFLO MicroShunt

The PRESERFLO MicroShunt is an 8.5 mm tube made of SIBS (poly-styrene-block-isobutylene-block-styrene), a highly biocompatible material, with a 70-micrometer lumen. Unlike other MIGS procedures, it is placed ab externo and requires conjunctival dissection, with MMC application similar to trabeculectomy. The device is designed for more significant IOP lowering and is not technically an ab interno procedure, though it is classified as MIGS due to its small size and design philosophy. Studies have shown IOP reduction comparable to trabeculectomy with potentially fewer complications.

<image>Xen Gel Stent placement showing the stent traversing from the anterior chamber through sclera into the subconjunctival space, with filtering bleb formation</image>

MIGS Procedures Comparison

Suprachoroidal MIGS

Historical Context

The CyPass Micro-Stent was an FDA-approved supraciliary space drainage device that was voluntarily withdrawn from the market after post-market analysis of the COMPASS trial five-year data revealed significant endothelial cell loss associated with the device. This withdrawal highlighted the critical importance of long-term safety monitoring for all MIGS devices and served as a cautionary tale for the field. Current suprachoroidal approaches are under development with modified designs intended to address the endothelial safety concern.

iStent SUPRA

The iStent SUPRA is a suprachoroidal micro-bypass stent currently under investigation and not yet widely available. Its theoretical advantage is that by draining aqueous to the suprachoroidal space, IOP reduction is independent of episcleral venous pressure, potentially allowing lower IOP targets than trabecular MIGS can achieve.

Evidence Summary and Patient Selection

Candidacy for MIGS

The ideal MIGS candidate has mild-to-moderate POAG on one to three medications and is undergoing concurrent cataract surgery -- this is the most common clinical scenario. MIGS is also appropriate for patients with medication intolerance, adherence challenges, or a desire to reduce their medication burden. It is not appropriate as the sole intervention for advanced glaucoma or when a very low target IOP is required.

Comparative Efficacy (Approximate)

The various MIGS approaches offer a spectrum of IOP-lowering efficacy. Trabecular MIGS procedures (iStent, Hydrus, KDB) typically achieve 15 to 25% IOP reduction and reduce the medication burden by one to two agents. GATT and 360-degree trabeculotomy provide 25 to 40% IOP reduction. The Xen and PRESERFLO achieve 30 to 45% IOP reduction, approaching the efficacy of trabeculectomy. Trabeculectomy itself remains the gold standard at 40 to 60% IOP reduction.

Clinical Pearls

MIGS procedures are additive to phacoemulsification, so the anterior chamber angle should always be assessed before cataract surgery to identify potential MIGS candidates. A good gonioscopic view is essential for successful ab interno surgery, and proper head positioning and lens selection are critical. Reflux hyphema during trabecular MIGS procedures is a good sign, indicating patent collector channels downstream. GATT provides the most robust IOP lowering among canal-based MIGS procedures and can be performed as a standalone operation. The Xen and PRESERFLO bridge the gap between trabecular MIGS and trabeculectomy, offering more substantial IOP lowering with a more favorable safety profile than traditional filtering surgery. The CyPass withdrawal serves as a cautionary tale -- long-term endothelial cell counts should be monitored with any device positioned near the anterior chamber angle. A fundamental limitation of all trabecular MIGS procedures is that they cannot lower IOP below episcleral venous pressure (approximately 8 mmHg), because they work by improving access to the conventional outflow pathway that ultimately drains into the episcleral veins. Insurance coverage and cost remain barriers to wider MIGS adoption.

References

• Samuelson TW, et al. Prospective, randomized, controlled pivotal trial of an ab interno implanted trabecular micro-bypass in primary open-angle glaucoma and cataract (iStent). Ophthalmology. 2011;118(3):459-467.
• Samuelson TW, et al. Randomized evaluation of the trabecular micro-bypass stent with phacoemulsification in patients with glaucoma and cataract (HORIZON). Ophthalmology. 2019;126(8):1114-1126.
• Grover DS, et al. Gonioscopy-assisted transluminal trabeculotomy: an ab interno circumferential trabeculotomy. Ophthalmology. 2014;121(4):855-861.
• Reitsamer H, et al. The Xen45 Gel Stent for management of refractory glaucoma. Expert Rev Med Devices. 2020;17(2):155-162.
• American Academy of Ophthalmology. Basic and Clinical Science Course, Section 10: Glaucoma.

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