48 Microinvasive Glaucoma Surgery The development of microinvasive glaucoma surgery (MIGS) over the last decade has brought this field to an exciting new era. MIGS has a highly favorable risk profile compared with traditional surgeries and can easily be performed in conjunction with cataract surgery. The opportunity to improve vision and to reduce both intraocular pressure (IOP) and the need for topical medications with a single surgery is very appealing. This chapter describes MIGS techniques and management strategies, using the micro-stent as the prime example. Cataract and glaucoma are the leading causes of blindness world-wide.1 They are often present simultaneously; hence, it is reasonable to treat these diseases together in one combined surgery. Glaucoma and ocular hypertension affect nearly 20% of patients undergoing cataract surgery in the United States.2 In the past, glaucoma treatment primarily included topical or systemic medication and lasers before advancing to filtration surgery. The treatment algorithm remained standard regardless of the severity of glaucomatous disease. In addition to the huge safety gap between the nonsurgical and surgical options, the latter options are associated with vision-threatening complications. The Tube Versus Trabeculectomy (TVT) study demonstrated that early and late postoperative complications occurred in 73% of trabeculectomies and 55% of aqueous drainage devices.3 To fill this gap, safer options are now available: microinvasive glaucoma surgery. MIGS is defined by five main characteristics: an ab interno conjunctiva-sparing approach, minimal disruption of normal anatomy and physiology, modest to high IOP-lowering efficacy, a positive safety profile, and rapid recovery by the patient.4 Trabecular micro-bypass surgery is a physiological option as it restores the natural drainage pathway and avoids blebs and its related complications. Furthermore, it preserves potential future treatment options including incisional glaucoma surgery. After its first introduction in Europe in 2004, the iStent (Glaukos, Laguna Hills, CA), a trabecular micro-bypass stent targeting the conventional outflow system, was approved by the Food and Drug Administration (FDA) in 2012.5 It is the smallest known medical device to be implanted in the human body. It is made of nonferromagnetic titanium measuring 1 mm by 0.33 mm, and the diameter of the snorkel part is 120 µm (Fig. 48.1).6 The other trabecular micro-bypass stent targeting the conventional outflow system is the Hydrus microstent (Ivantis, Irvine, CA), which is still undergoing clinical trials.5 The latest version of this micro-stent is a crescent-shaped 8-mm device designed to scaffold and dilate three clock hour positions of Schlemm’s canal (SC). Another FDA-approved MIGS device for performing an ab interno trabeculectomy is the Trabectome (NeoMedix, Tustin, CA). It uses electrocautery to ablate a segment of the trabecular meshwork (TM) and the inner wall of SC to achieve direct flow of aqueous into the SC and the collector channels. Studies have shown a significant decrease in IOP and medication load with a favorable safety profile for patients with mild to moderate open-angle glaucoma undergoing MIGS in combination with phacoemulsification.7–11 Recognizing angle anatomy variations and understanding the physiology of the ocular outflow system are essential for MIGS success. The normal conventional outflow pathway of the aqueous humor is from the anterior chamber to the TM, entering the SC to be further drained by collector channels or aqueous veins to the episcleral venous system.12 An average of four to six aqueous veins, most commonly found in the inferonasal quadrant, are responsible for most of the eye’s outflow system.13,14 They are of larger caliber, with an average diameter of 50 µm (Fig. 48.2).13–15 In primary open-angle glaucoma (POAG), the location of highest resistance to outflow is the juxtacanalicular TM.12 A trabecular micro-bypass stent can thus enhance the conventional, pressure-dependent outflow pathway by allowing aqueous to directly bypass the TM.16 This causes an increase in the pressure applied on the downstream collector channels and also increases the aqueous drainage, especially in the area where the bypass is located.16,17 The inferonasal quadrant appears to be the ideal implantation site, as a higher density of collector channels and aqueous veins are typically present.13,16,17 Therefore, trabecular bypass should optimize outflow, thus reducing the IOP. However, the IOP reduction is limited by the distal outflow system and the episcleral venous pressure (EVP), which is normally between 8 and 11 mm Hg.12 Fig. 48.1 iStent trabecular micro-bypass device (Glaukos). (Courtesy of Glaukos.) Fig. 48.2 (a) Drainage routes of aqueous humor. (b) An aqueous vein (arrows). (a from Sampaolesi R, Sampaolesi JR, Zárate J. Structure and function of the tissues that are in contact with the aqueous humor. In: Sampaolesi R, Sampaolesi JR, Zárate J. The Glaucomas. Berlin: Springer-Verlag, 2014. Reprinted by permission.) Performing MIGS is a highly technical procedure with a steep learning curve. Surgical success depends on adequate planning and proper knowledge of anatomic landmarks of the iridocorneal angle. It is imperative to review the gonioscopy findings in the office and essential to learn intraoperative gonioscopy before performing surgery. Practicing in a wet-lab facilitates becoming acquainted with the surgical steps under direct gonioscopic view. The best way to get started with MIGS is to simulate angle surgery with a goniolens in routine cataract surgeries before attempting it on patients. One may manipulate an instrument in the angle to get a feel for the magnification and ergonomics of the maneuvers. One can also practice tilting the microscope and the patient’s head. Mastering the steps without the pressure of implanting the device will help make the surgeon more comfortable in preparing for the first surgical procedure. It may also be useful to watch surgical videos of the implantation technique and review the steps in detail. Appropriate equipment is key to MIGS success. The micro-bypass stent comes preloaded in a single-use, sterile injector.6 It has a secure, rotatable grip and reacquisition capability to facilitate manipulation and placement into the SC. The inserter has a button that is pressed to release or disengage the device at the appropriate time. There are two orientations of the micro-stent: the right and the left.6 The left micro-stent is easier to implant by most right-handed surgeons, as they may feel more comfortable with a forehand insertion technique (Fig. 48.3). For right micro-stent, they must use a backhand technique, which may be more challenging (Fig. 48.4). If implanting one micro-stent, surgeons should opt for the most comfortable approach, regardless of the design. At our center, two micro-stents per eye are typically implanted, one right and one left. Use of a cohesive ophthalmic viscoelastic device (OVD) in the anterior chamber (AC) provides the clearest view. If needed, the surgeon should not hesitate to inject more to clear any blood in the angle, making sure to release some from the wound to prevent over pressurization of the AC and collapsing the SC. Several goniolens prototypes are available, designed to optimize angle visualization including clarity, globe stability, accessibility, and simultaneous surgical manipulation of angle structures.18 The Swan-Jacob goniolens (Ocular Instruments, Bellevue, WA) is commonly used for manipulation during angle surgery. However, it requires repositioning of the patient’s head and the microscope. Thus, access to a surgical microscope with large tilt capabilities is necessary. Micro-tying forceps (MicroSurgical Technologies, Redmond, WA) are very useful tools in various situations.19 They may be used for regrasping, repositioning, and ensuring appropriate positioning of the device. It may be easier to manipulate the device with the micro-tying forceps rather than the inserter once it has released the micro-stent. Indications for MIGS are mild to moderate open-angle glaucoma in the presence of a cataract in patients who are able to tolerate topical medications, if needed postoperatively.7,11 For initial cases, it is preferable to schedule a combined procedure for patients in whom cataract surgery alone would be sufficient, but who may derive additional benefit from MIGS. If the device cannot be implanted for some reason, the patient can be expected to still do well with or without topical IOP-lowering medication. It is also initially recommended to select patients who have prominent angle landmarks, a well-defined pigmented TM, and wide-open angles. Also, avoid performing the surgery during a busy operative schedule, as the time pressure can lead to added stress. The sedation must be titrated so that the patient may participate and cooperate as much as possible. A retrobulbar or peribulbar block may be used during the first few cases to minimize patient movement.5 However, this implies no cooperation from the patient with respect to eye movements during surgery. It is preferable to perform MIGS with the patient under topical anesthesia, as the patient can assist by moving the eye as directed to optimize the surgical view. It therefore is important to select patients who are able to cooperate, who do not have a language barrier, and whose cognitive status is good. The lack of any of these patient factors can compromise the procedure intraoperatively.
Background
Anatomy
Preoperative Considerations
Preparation
Instrumentation
Patient Selection and Preoperative Management
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