For more than 3 years, surgery with the iStent Trabecular Micro-Bypass Stent (Glaukos Corporation) has been one of the surgical options available to patients with glaucoma in Canada. This practical review is based on more than 135 eyes of 100 patients who underwent phacoemulsification combined with the implantation of two iStents at the University of Montreal and the Montreal Glaucoma Institute.
SELECTION OF PATIENTS
Initially, I suggest choosing patients with no abnormal ocular or head movements (nystagmus, etc.) who have clear corneas, deep anterior chambers, and angles that are completely open on gonioscopy. A gonioscopic evaluation should be performed on all potential patients, especially in the nasal quadrant where implantation usually takes place, and the findings should be noted in the chart. In general, patients with open-angle glaucoma, including primary open angle, pseudoexfoliative, and pigmentary, are good candidates. I also initially recommend selecting patients with cataracts and early or moderate glaucomatous damage whose IOP is well controlled by one or two medications and who do not require very low target IOPs. With experience, ophthalmologists may choose to perform the combined procedure on patients with angles that are slightly narrow but that are expected to open after cataract surgery and on patients with more significant glaucomatous damage who may have to continue using IOP-lowering drops postoperatively.
In my experience, the iStent may also benefit patients with thin or scarred conjunctiva, contact lens wearers, individuals at high risk of complications with glaucoma filtration procedures, and those with glaucoma or ocular hypertension who do not tolerate topical medication. In addition, the device can be used for primary surgery on patients newly diagnosed with glaucoma.
I do not recommend the stent for patients with a history of angle recession or trauma, those with increased episcleral venous pressure, or individuals with narrow angles secondary to mechanisms other than pupillary block (eg, plateau iris, iris cysts, neovascular glaucoma, iridocorneal endothelial syndrome).
SURGICAL PEARLS
Preparation
I have found the positioning of the microscope and the patient to be the most critical steps in ensuring adequate visualization of the angle structures. I recommend tilting the microscope 30º to 45º away from the surgeon and having the patient rotate his or her head and look in the opposite direction. Adequate corneal hydration and the use of lidocaine 2% jelly (Xylocaine; AstraZeneca LP) or methylcellulose (Gonio Gel; Aurolab; not available in the United States) are needed to couple with the modified Swan-Jacob goniolens (Ocular Instruments, Inc.). The surgeon must avoid incising conjunctival or corneal vessels, because even a small amount of blood under the goniolens can prevent crisp visualization. The formation of bubbles upon the injection of viscoelastic material into the anterior chamber may also compromise the visibility of Schlemm canal.
Placement
I place both left- and right-handed iStents in the nasal angle a few clock hours apart. After creating the corneal wounds, I find that slight hypotony can help me to identify Schlemm canal, because I can often see blood reflux. This technique is especially useful in patients with little or no angle pigmentation. It may also be beneficial to place the two stents adjacent to areas of heavy pigmentation, because these locations are often near collector channel openings. For right-handed surgeons, placing the right stent is often more challenging, and using extra torque of the hand or placing it with the left hand may facilitate proper insertion of the device. Visual as well as tactile clues are of help, and a small amount of blood reflux from the snorkel end confirms proper placement.
If I encounter much resistance and the stent does not move forward, I find it advisable to pull out the stent and reinsert it in the same spot or an adjacent area. Occasionally, this extra maneuvering will cause bleeding in the angle that will obstruct my view. At this point, I inject extra viscoelastic material to push away and tamponade the blood. When trabecular tissue is very fragile, a manual goniotomy may be inadvertently performed with the device, after which a new area must be chosen for its insertion. Other times, it may be necessary to place one or both stents in the inferior angle through a new superior paracentesis.
I strongly recommend that ophthalmologists practice regrasping the stent with the inserter before surgery (ie, wet lab), because they may find the technique necessary.
There are certain advantages to placing the iStent either before or after phacoemulsification. In the former situation, there is no corneal edema, and viscoelastic material must be reordered less frequently. In the latter situation, the angle may deepen after the lens is removed, facilitating the stent's placement.
POSTOPERATIVE CONSIDERATIONS
I often ask my patients to discontinue some or all of their IOP-lowering drops after surgery, depending on their amount of glaucomatous damage. In certain cases, I may prescribe pilocarpine to prevent the iris from obstructing the stent. I find that oral acetazolamide at the end of surgery and on the first evening can help avert early pressure spikes.
A hypertensive phase of IOP between the second and fourth postoperative weeks is not uncommon. Patients may require additional IOP-lowering medications until approximately 2 weeks after stopping topical steroid drops. It has been postulated that this hypertensive phase may be the result of patients' sensitivity to steroids, but in some cases, the IOP has risen despite steroids' being absent.
On average, much less surgical time and fewer postoperative visits are needed with this technique compared with a combined phacoemulsification-filtering procedure. The conjunctiva is not violated, and subsequent glaucoma filtration procedures can be performed if needed. My patients' visual and functional rehabilitation is very rapid, and their satisfaction is quite high. On average, 1 year postoperatively, my patients have had an IOP in the low to midteens on two fewer IOP-lowering medications than needed preoperatively.
COMPLICATIONS
The most common complication of the iStent is a transitory hyphema, often absent on the first postoperative day. Inadvertent goniotomy, irido- or cyclodialysis, and a lost or misplaced stent may also occur. High IOP spikes can occur, and patients with advanced glaucomatous damage should be monitored closely. Finally, the iris stroma may block the stent's opening (Figure), necessitating treatment with an Nd:YAG or argon laser to remove the obstructing tissue.
CONCLUSION
In my experience, trabecular bypass surgery with the iStent combined with phacoemulsification significantly lowers IOP and is associated with a very low rate of intra- or postoperative complications. The issues of cost and reimbursement are important; cost-effectiveness and quality-adjusted life year studies must be performed. Also needed are long-term IOP results and research on the risk factors for failure, the timing of surgery (early in disease process?), and the device's use in phakic patients.
The availability of this combined procedure represents a major paradigm shift in the surgical treatment of patients with open-angle glaucoma, and I believe it will directly affect all ophthalmologists caring for patients with this disease.
Paul Harasymowycz, MSc, MD, DABO, FRCSC, is the chief of glaucoma at the University of Montreal and medical director of the Montreal Glaucoma Institute. He is a consultant to Alcon Laboratories, Inc., and Solx, Inc.; is a presenter for Abbott Medical Optics Inc.; and performs research with Ivantis Inc. Dr. Harasymowycz may be reached at pharasymowycz@sympatico.ca.
