16 Minimally Invasive Glaucoma Surgery and Valves
Summary
The chapter describes various types of minimally invasive glaucoma surgery and the different devices used for the minimally invasive glaucoma surgery (MIGS) procedure along with the mechanism of the pathway of function.
16.1 Minimally Invasive Glaucoma Surgery
MIGS is an abbreviation that stands for minimally invasive glaucoma surgery. Traditionally, the treatment of primary open-angle glaucoma 1 , 2 , 3 , 4 , 5 , 6 , 7 comprises a medical line of management followed by trabeculectomy for advanced cases and shunt surgeries for refractory cases of glaucoma. While they are very often effective at lowering eye pressure and preventing the progression of glaucoma, they have a long list of potential complications due to the invasive nature of the procedure.
MIGS has been introduced that potentially helps in mild to moderate cases of glaucoma. These procedures have a higher safety profile with fewer complications and more rapid recovery time than other invasive techniques (▶Video 16.1).
MIGS procedures work by using microscopic-sized equipment and tiny incisions (▶Video 16.2).
There are four main approaches by which MIGS works.
16.1.1 Enhancing Trabecular Meshwork Outflow
Trabecular meshwork forms an important component of the aqueous humor drainage pathway. The trabecular meshwork can either be destroyed (Trabectome) or bypassed using a tiny snorkel-like device (the iStent). These procedures are Food and Drug Administration (FDA) approved but generally do not get the eye pressure very low and therefore are most useful in early to moderate stages of glaucoma. Currently, three devices (iStent, iStent inject [Glaukos Inc., Laguna Hills, CA, USA], and Hydrus [Ivantis Inc., Irvine, CA, USA]) target the juxtacanalicular part of the trabecular meshwork, which is believed to represent the greatest resistance to aqueous humor outflow in patients with open-angle glaucoma. One limitation of all of these procedures is that the postoperative intraocular pressure (IOP) cannot fall below the episcleral venous pressure, which is difficult to evaluate but is reported in different studies in a range of 7.6 to 9.1 mm Hg.
iStent
iStent is the first-generation device, whereas iStent inject is the second-generation device that is FDA approved. iStent measures 0.3 mm in height and 1 mm in length. It is a heparin-coated, nonferromagnetic titanium stent with a snorkel shape to facilitate implantation. The device is placed using a single-use, sterile inserter through a 1.5-mm corneal incision. The applicator is inserted into the anterior chamber (AC) and across the nasal angle. The pointed tip allows penetration of the trabecular meshwork and insertion into Schlemm’s canal and three retention arches ensure that the device will be held in place.
iStent inject is a much smaller device with a length of 360 µm and a diameter of 230 µm. The iStent inject stents are delivered in an injector system, which injects the stents automatically into Schlemm’s canal through a stainless steel insertion tube. The injector is released by the surgeon by pressing a button. Usually, two iStent inject stents are implanted nasally into the trabecular meshwork and Schlemm’s canal with a distance of 30 to 60°.
Trabectome
Trabectome (NeoMedix, CA) allows a trabeculotomy to be performed via an internal approach. In this procedure, a strip of trabecular meshwork is removed along with the inner wall of Schlemm’s canal to create a path for the drainage of aqueous humor. It is a single-use disposable device that allows electrocautery, irrigation, and aspiration. It has a three-stage foot pedal control that initiates irrigation, aspiration, and electrocautery in sequence. Continuous irrigation and aspiration allow for removal of debris and regulation of temperature. Ablation of 60 to 120° is recommended for re-establishment of the drainage pathway.
Hydrus Microstent
The Hydrus Microstent (Ivantis Inc.) is made from superelastic, biocompatible, nickel–titanium alloy (nitinol) and is 8 mm in size. The “intracanalicular scaffold” is inserted into Schlemm’s canal to maintain patency and establish outflow. The procedure can be performed in conjunction with cataract surgery and uses the same corneal incision. The microstent is implanted ab interno with a preloaded injector through a clear corneal incision into Schlemm’s canal. The beveled tip of the injector is used to perforate the trabecular meshwork and to position the microstent in Schlemm’s canal. After implantation, the Hydrus Microstent dilates Schlemm’s canal in the complete nasal quadrant, allowing aqueous humor to bypass the trabecular meshwork through multiple collector channels.
Gonioscopy-Assisted Transluminal Trabeculotomy (GATT)
GATT is a type of ab interno trabeculotomy where in under the guidance of a gonioscopy lens, a goniotomy is made in the nasal trabecular meshwork, which serves as the entry point for the iTrack microcatheter (iScience Interventional Corp, Menlo Park, CA). Alternatively, a 4–0 nylon suture can also be employed instead of the microcatheter. Microsurgical forceps are used to advance the microcatheter into Schlemm’s canal circumferentially 360°, tracking its progress with its illuminated distal tip. Once it has been passed through the entire canal, the catheter is externalized to create a 360° trabeculotomy.
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