Focus on Glaucoma

How MIGS fits into the glaucoma treatment paradigm

Four pathways exist to lower IOP surgically, including Schlemm’s canal and aqueous humor production.

By Brian A. Francis, MD, MS

Trabeculectomy, or guarded filtration surgery, was first described by Cairns in the 1970s.¹ The same decade saw the invention of the first aqueous tube shunt to an extraocular reservoir, by Molteno.² Since that time, many refinements were made, but glaucoma surgery remained largely unchanged, until now. Recently, glaucoma surgical treatment has undergone major changes due to the advent of a new category of procedures called MIGS. Whether one defines this acronym as minimally invasive glaucoma surgery, micro-invasive glaucoma surgery or micro-incisional glaucoma surgery, the procedures have several characteristics in common.

Methods of action

The first is an ab interno surgical approach via a small incision that spares the conjunctiva. The surgery involves minimal trauma and minimal tissue disruption. As such, the safety profile is very high, especially when compared to traditional glaucoma filtration surgery. As a result, the recovery is usually very rapid. One may expect moderate to high levels of intraocular pressure lowering, usually with a return to “physiologic levels” in the mid-teens.³

The Ex-Press mini-shunt is placed under a scleral flap to prevent erosion and allow a diffuse bleb to form.
COURTESY: ALCON

When evaluating new glaucoma surgical techniques, it is helpful to categorize them by the method of action. There are four pathways to lower IOP surgically: Schlemm’s canal, the suprachoroidal space, aqueous humor production and the subconjunctival space.

Schlemm’s canal procedures include two FDA-approved treatments: trabeculotomy by internal approach, or Trabectome (NeoMedix) and trabecular microbypass stenting, or iStent (Glaukos). Both procedures are designed to reduce the outflow resistance caused by the diseased trabecular meshwork. Trabectome accomplishes this decrease by removing a 90º to 180º portion of the trabecular meshwork via a plasma thermocautery handpiece. iStent accomplishes it by the placement of a small titanium stent through the trabecular meshwork that allows aqueous to flow from the anterior chamber into Schlemm’s canal.

Shifting the paradigm

This category of surgery is used to treat mild to moderate glaucoma in patients with a target IOP in the mid-teens. Both procedures are easily combined with cataract surgery and are commonly used in this setting of combined surgery. Both can also be used as a stand-alone procedure (off-label with iStent). Schlemm’s canal surgery has changed the way we approach glaucoma treatment, shifting the surgical treatment paradigm to earlier in the disease process.

Future advances in this area will focus on stent design, with multiple stent placements and larger stents to open more of the canal. Individual mapping of the trabecular outflow system will match outflow collector channels with targeted placement of stents. Pharmacologic treatment will be used to modulate fibrosis and to maintain the patency of the distal outflow pathway.

The suprachoroidal space

The suprachoroidal space is a potential pathway for enhancing aqueous outflow. The pressure gradient between the anterior chamber and the suprachoroidal space allows for the potential flow of fluid in this direction. While there are no FDA-approved devices in this area, two are in trials in the United States: Supra (Glaukos) and CyPass (Transcend Medical). This surgery involves the placement of a small stent on the potential plane between the scleral spur and ciliary body band that establishes communication between the anterior chamber and the suprachoroidal space. Like Schlemm’s canal procedures, it is performed via an ab interno approach that spares the conjunctiva and combines well with cataract extraction.

The patient population is also very similar, aimed at mild to moderate glaucoma patients with target IOP in the mid-teens. Future research is likely to involve stent design and modulation of healing to prevent fibrosis and closure of the distal end of the stent.

An aside: a non-MIGS procedure

Lowering of aqueous humor production is performed by laser treatment of the ciliary processes, or cyclophotocoagulation. The trans-scleral approach is performed by the external application of energy via a handheld probe placed near the corneal limbus. But because of the tissue destruction involved, this approach is not considered a MIGS procedure.

An endoscopic approach

In contrast, endoscopic cyclophotocoagulation (ECP) (Endo Optiks/Beaver Visitec) involves the application of laser energy directly to the ciliary epithelium via an internal, endoscopic approach. Because of the lower energy requirements and decrease in tissue damage, many surgeons consider this procedure to be part of the MIGS category.

When used in combination with cataract extraction, ECP involves the treatment of 270º or more of the ciliary processes via a temporal corneal incision. The energy delivery is usually lower and thus results in moderate IOP lowering with high safety and rapid recovery. ECP can also be used more aggressively to treat glaucoma refractory to filtration surgery. In these cases, 330º to 360º of the ciliary processes are usually treated with a higher amount of energy delivered. The safety profile is still high, but with a longer recovery period and more postoperative inflammation.

Investigating the subconjunctival space

The subconjunctival space is the area accessed by traditional glaucoma filtration surgery, and as such, it is at the periphery of MIGS. Depending on one’s definition of MIGS, the formation of a filtration bleb may exclude this category. There are no FDA-approved devices in this classification, but several are under investigation. The Xen implant (AqueSys Inc.) is a collagen shunt that passes from the anterior chamber to the subconjunctival space. It is implanted using an ab interno approach with or without the help of antifibrotic agents (mitomycin C). The MicroShunt (InnFocus) is a small stent made of SIBS (polystyrene-block- isobutylene-block-styrene) also connecting the anterior chamber to the subconjunctival space, but it is implanted via an external approach with conjunctival dissection.

The goal of these procedures is to duplicate the IOP-lowering capabilities of trabeculectomy while decreasing the risks of hypotony and bleb-related infections. Therefore, they may be appropriate for patients with more advanced glaucoma disease, requiring a lower-target IOP.

Arrival of an era

In summary, the advent of micro-invasive glaucoma surgery, or MIGS, has the potential to change substantially the way we treat our glaucoma patients. With these alternatives in our treatment armamentarium, we are beginning to treat glaucoma as a surgical disease in its earlier stages. Thus, surgery is now being considered in much the same way as medical or laser therapy, rather than as a last resort. Glaucoma treatment can be more tailored to meet a patient’s disease characteristics and therapeutic needs. Using the four categories of glaucoma surgical treatment (Schlemm’s canal, suprachoroidal space, aqueous humor production, and subconjunctival space), we can treat patients with the most appropriate surgical intervention, based on their anatomy, disease status and treatment history. In addition, we can potentially combine surgeries that utilize different pathways for greater IOP reduction. For example, a Schlemm’s canal procedure can be combined with a suprachoroidal shunt or an aqueous humor-decreasing procedure.

Glaucoma surgery is undergoing a paradigm shift with the sudden explosion of surgical treatment alternatives. The MIGS era has arrived. OM

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About the Author
	Brian A. Francis, MD, MS, is a professor and director of Glaucoma Services at the Doheny Eye Institute, Geffen School of Medicine, UCLA Los Angeles. He has no relevant business relationships with any company mentioned in this article.