Trabecular Bypass: iStent

28 Trabecular Bypass: iStent

John P. Berdahl, Christine L. Larsen, and Thomas W. Samuelson

Case Presentation

A 69-year-old man presented for a second opinion regarding surgical intervention for relatively well-controlled primary open-angle glaucoma (POAG) in the setting of visually significant cataracts. He had borderline elevated intraocular pressure (IOP) despite maximally tolerated medical therapy. His referring physician was recommending a trabeculectomy, but the patient had researched various options and wanted to explore iStent implantation.

The patient was a high myope with a moderate superior arcuate field defect in the right eye and nonspecific visual field changes in the left eye (Fig. 28.1). Nuclear sclerosis reduced his best corrected vision to a level of 20/40 in both eyes. Angles were open on gonioscopy, with a moderate amount of trabecular meshwork pigment. Applanation tonometry assessed the IOP as 23 OD and 22 OS on three topical medications (Lumigan and Cosopt) and in the setting of average pachymetry.

In preoperative discussion with the patient, he expressed his expectations for visual improvement after the cataracts were removed. The surgeon informed him of the typical risks involved with the surgery. In addition, the patient was aware that the goal of stent implantation was to reduce the medication burden required to maintain adequate IOP. He understood that he would likely still require some topical therapy to keep his glaucoma under control.

The patient underwent a successful cataract surgery with iStent placement in the right eye, and, 1 month later, in the left eye. Postoperatively, the patient’s Cosopt medication was discontinued, but he continued to use Lumigan in both eyes. His topical steroid was tapered relatively rapidly over 2 weeks, as inflammation was under good control. Approximately 6 weeks after the second eye surgery, he had uncorrected distance vision of 20/20 in both eyes and an IOP of 19 OD and 18 OS on Lumigan alone.

The Procedure

The iStent (or trabecular micro-bypass stent) has become a key procedure in minimally invasive glaucoma surgery (MIGS). The MIGS procedures have a higher safety profile and a more rapid recovery time in comparison with more invasive filtering surgery. They also have demonstrated the ability to reduce both the IOP as well as the patient’s need for medications, a significant benefit considering concerns regarding compliance rates among glaucoma patients.1 Moreover, unlike most other glaucoma surgical interventions, iStent implantation does not diminish the superb visual and refractive outcomes inherent to modern phacoemulsification. The iStent was developed by Glaukos Corp. (Laguna Hills, CA), with the first implantation in the United States performed in 2005.2 The stent is designed to fit into and remain within Schlemm’s canal. Made from nonferromagnetic titanium, it consists of an inlet (or “snorkel”) connected at a 40-degree angle to the implanted portion. The stent itself is then attached to the tip of a 26-gauge disposable insertion instrument, which has been sterilized by gamma radiation (Fig. 28.2). The pointed end facilitates entry into the canal and the direction of this point corresponds to the designation of a right- or left-handed model. Both right and left iStents have been developed to ease implantation, depending on the preference of the surgeon. The implanted portion includes a half-cylinder opening, which combined with heparin coating, helps to prevent blockage or fibrosis. Three retention arches help to ensure that the device will be held in place within the canal. The iStent is the smallest known medical device to be implanted into the human body. It is 1.0 mm in length, 0.33 mm in height, and with a weight of 60 µg. The snorkel has a length of 0.25 mm and bore diameter of 120 µm3 (Fig. 28.3).

Rationale Behind the Procedure

As with many other MIGS procedures, the iStent works at the level of the trabecular meshwork (TM). Research on the physiology of POAG has demonstrated that the diseased TM is the primary site of reduced outflow facility resulting from increased outflow resistance.4 Approximately 75% of the resistance occurs at the juxtacanalicular meshwork. As the alternative designation for the iStent (the trabecular micro-bypass stent) implies, implantation of the device enables aqueous to bypass the increased TM resistance to outflow and provides a direct channel into Schlemm’s canal and the subsequent collector channels (Fig. 28.4).

The U.S. iStent Study Group performed a large comparative study in POAG patients already undergoing planned cataract surgery to compare the effect of cataract removal alone with cataract removal in combination with iStent placement.5 Prior to this study, several pilot studies had been performed demonstrating the effectiveness of iStent implantation at lowering IOP. Zhou and Smedley6 investigated the effectiveness of a trabecular bypass on outflow facility and IOP. A series of equations explored this relationship and demonstrated that in normal healthy eyes, the facility of outflow increases by 13% and 26% in the presence of a unidirectional and bidirectional bypass, respectively. Via enhanced outflow facility, the IOP could be reduced to physiological levels. Bahler et al7 took this a step further, investigating the effect of a TM bypass on IOP in cultured human anterior segments. A single stent placed into Schlemm’s canal provided the greatest change in pressure (21.4 ± 3.8 mm Hg to 12.4 ± 4.2, p < 0.001) with the addition of more stents providing further lowering of pressure, but to a lesser degree.

Patient Selection

In 2012, the Food and Drug Administration (FDA) approved the iStent for use in combination with cataract extraction for patients with mild to moderate open-angle glaucoma who were using one to three ocular hypotensive medications.

Ideal candidates are those with stable and well-controlled disease. Those who are demonstrating progression on their current medication regimen may require more aggressive surgical intervention, such as filtration surgery. The aforementioned ideal candidate also typically requires pressure lowering, but not to an extreme level (i.e., target IOP around the mid-teens or higher). The general goal is to reduce the dependency on topical medication and not necessarily increase the aggressiveness of treatment.

As can be extrapolated from the characteristics of an ideal candidate, poor candidates would be those with a very low target IOP. Patients with a very shallow anterior chamber should also be avoided, as implantation can be more difficult, with an increased risk of iris or endothelial damage, although the angle in such patients will likely be much deeper once the native lens has been removed. Similarly, iStent placement has been reported in primary angle-closure glaucoma patients in combination with goniosynechialysis. The aforementioned difficulties with implantation may be encountered in these patients, and there may also be a predisposition to recurrent scarring over the angle. As the functionality of the iStent relies on bypassing the diseased TM to allow aqueous to access an otherwise functional outflow system, secondary glaucomas related to elevated episcleral venous pressure (e.g., Sturge-Weber) would not be amenable to stent implantation. Patients with neovascular glaucoma are contraindicated because of both the increased bleeding risk and the reduced function of the outflow system.8

While surgeons are first developing their implantation skills and becoming more comfortable with the procedure, it may be of benefit to select patients who would do well with cataract surgery alone. These patients will still likely do well postoperatively should implantation be unsuccessful. Other favorable traits for initial cases might include highly cooperative individuals with at least moderate pigmentation of the TM and well discernible angle structures. Also, if surgeons favor right or left eyes for phacoemulsification, they are likely to favor those eyes for initial iStent cases as well.

Surgical Technique

Mastering intraoperative gonioscopy is a key to success with iStent implantation. For surgeons who do not perform gonioscopy often, it is useful to examine patients in the clinic to better familiarize oneself with the angle anatomy. In addition, practicing intraoperative gonioscopy during routine cataract cases proves to be beneficial prior to implanting the first stent. Gently touching the anterior meshwork with a viscoelastic cannula can also help one become more comfortable with the hand position.

Once the cataract surgery is completed and the intraocular lens (IOL) has been implanted, injection of a miotic helps to pull the iris away from the angle, and the insertion of viscoelastic material will aid in anterior chamber maintenance (Fig. 28.5a). For initial cases, it is desirable to remove all of the ophthalmic viscosurgical device (OVD) from within the retropupillary space and capsular bag before the pupil is constricted. Later, once more experience is achieved, many surgeons will choose to wait until the iStent has been successfully implanted before the OVD is removed and the miotic instilled. The patient’s head and the operating microscope are rotated 30 to 40 degrees in opposite directions to facilitate a gonioscopic view of the angle. The surgical goniolens is placed on the cornea with a coupling solution (Goniosol, OVD), and the angle is viewed under high magnification. Care is taken not to place pressure on the eye with the goniolens, as resultant corneal striae will impede the view (Fig. 28.5b) Likewise, the surgeon should avoid putting pressure on the wound with the insertion trochar to avoid expressing the OVD from the eye with the subsequent loss of visualization. Once a clear view of the TM is achieved, the applicator is inserted into the anterior chamber through the clear cornea temporal incision and advanced across the anterior chamber toward the nasal angle. As mentioned previously, there are two different designs designating the direction of the pointed end. The intent of the unique iStent design is that after implantation, the body of the stent points toward the inferior angle such that right stents are used in right eyes and left stents are used in left eyes. Evidence that right or left orientation makes any clinical difference, however, is lacking. As such, most surgeons believe that right-and left-hand models are interchangeable (i.e., right and left iStents can be used in both right and left eyes) depending on what feels more comfortable (forehand or backhand) in the dominant hand of the surgeon.

The anterior one third of the TM is approached at a 15- degree angle and is perforated by the tip. The implanted portion is advanced into the canal. By slightly adjusting the angle after perforation (lowering the heel and raising the toe), the stent will slide into the canal more easily (Fig. 28.5c). Once securely positioned with the ridges of stent covered by meshwork tissue, the device is released by pushing the button on the applicator (Fig. 28.5d). Subtle posterior pressure and relaxing of the hand will ensure a stable release.

Oct 29, 2018 | Posted by in OPHTHALMOLOGY | Comments Off on Trabecular Bypass: iStent
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