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With the Food and Drug Administration’s approval in June, 2012 of the iStent Trabecular Micro-Bypass Stent (Glaukos Corporation), glaucoma patients now have access to the first of the microinvasive glaucoma surgery or MIGS devices to reach the market. This development makes earlier surgical intervention in patients with mild to moderate glaucoma possible, providing a safe alternative to medications in patients undergoing concurrent cataract surgery.
MECHANISM OF ACTION
Most glaucoma treatments address the buildup of aqueous in the anterior chamber, a process that elevates intraocular pressure (IOP) and damages nerve fibers. Conventional surgeries, such as trabeculectomy or tube shunts, essentially bypass the natural outflow system in the eye and create a new one. The iStent is a minimally invasive option that improves aqueous outflow through the natural physiologic pathway.
Research by Johnson and colleagues confirmed the importance of mining traditional outflow pathways. When comparing three types of eyes: those that had undergone filtration surgery, eyes with primary openangle glaucoma (POAG) that had not undergone surgery, and normal eyes, they found that Schlemm canal was significantly smaller in eyes with POAG after filtration surgery than in either normal eyes or eyes with medically treated POAG.1 As a significant decrease in canal size is otherwise not a finding in POAG, Johnson theorized that bypassing normal outflow structures results in underperfusion of drainage pathways. This discovery corresponds to additional clinical work that links underperfusion of the meshwork and canal to accumulation of extracellular material and increased outflow resistance.2,3
If the blockage is repaired so that aqueous drains through the trabecular meshwork and out Schlemm canal, patients will fare better than with an artificial drainage pathway. Up to 75% of resistance to outflow is located in the juxtacanalicular tissue.4
The iStent is a titanium stent that improves physiologic outflow by creating a patent bypass between the anterior chamber and Schlemm canal, facilitating outflow through the latter.5 Increasing outflow through the infranasal quadrants significantly lowers IOP due to the high presence of collector channels. The physiologic preservation of the trabecular meshwork ensures a natural episcleral back pressure of 8 to 11 mm Hg with a minimal risk of hypotony.6,7
Inserted ab interno through the phaco incision in conjunction with cataract surgery, the device reduces IOP, and the procedure has a similar safety profile to cataract surgery alone.8 The iStent could provide an option for glaucoma patients whose previous treatment consisted solely of topical ophthalmic medications. Results from a terminal washout study of the iStent plus cataract surgery versus cataract surgery alone in patients with POAG showed that patients who received the device had an approximately 3 mm Hg greater drop in IOP.9 An additional 3 mm Hg reduction in IOP is the mean reduction commonly associated with adjunctive glaucoma medications. This procedure has the potential to control POAG without other ocular hypotensive medications, thus representing an excellent treatment option for patients with mild to moderate disease.
Glaucoma is chronic and must be monitored for the remainder of a patient’s life. Drops that require instillation two to three times daily may have a significant impact on a patient’s quality of life. Compliance with glaucoma medications is statistically dismal, with more than half of patients nonadherent to their dosing regimens and nearly 50% not renewing their medications at 6 months.10 I believe the iStent could provide an effective solution for noncompliant patients. In the study, 72% of patients who received the device were able to sustain target IOPs of less than 21 mm Hg without needing medication versus 50% of those who underwent cataract surgery alone.8
The safety and efficacy of the iStent, which I experienced as an early investigator and has been shown in long-term data, is very appealing. I believe efforts at controlling glaucoma have been suboptimal and have lacked the possibility of earlier intervention. This combined procedure provides a sustainable therapy that does not depend on patients’ compliance and boasts the safety profile of cataract surgery alone. I look forward to offering my patients this new and exciting option.
Richard A. Lewis, MD, is in private practice in Sacramento, California. He is a consultant to Alcon Laboratories, Inc.; Glaukos, Inc.; Aquesys; and Ivantis, Inc. Dr. Lewis may be reached at (916) 649-1515; firstname.lastname@example.org.
- 1. Johnson DH, Matsumoto Y. Schlemm’s canal becomes smaller after successful filtration surgery. Arch Ophthalmol. 2000;118:1251-1256.
- 2. Lutjen-Drecoll E, Barany EH. Functional and electron microscope changes in the trabecular meshwork remaining after trabeculectomy in cynomolgus monkeys. Invest Ophthalmol.1974;13:511-524.
- 3. Kuchle M, Naumann GOH. Direct cyclopexy for traumatic cyclodialysis with persisting hypotony: report in 29 consecutive patients. Ophthalmology. 1995; 102:322-333.
- 4. Rosenquist R, Epstein D, Melamed S, et al. Outflow resistance of enucleated human eyes at two different perfusion pressures and different extents of trabeculectomy. Curr Eye Res. 1989;8:1233-1240.
- 5. Bahler CK, Smedly FT, Zhou J, Johnson DH. Trabecular bypass stents decrease intraocular pressure in cultured human anterior segments. Am J Ophthalmol. 2004; 138:988-994.
- 6. Dvorak-Theobald G. Schlemm’s canal: its anastomoses and anatomic relations. Trans Am Ophthalmol Soc. 1934;32:574-595.
- 7. Zhou J, Smedley GT. Trabecular bypass: effect of Schlemm canal and collector channel dilation. J Glaucoma. 2006;15:446-455.
- 8. Samuelson TW, Katz LJ, Wells JM, et al; US iStent Study Group. Randomized evaluation of the trabecular microbypass stent with phacoemulsification in patients with glaucoma and cataract. Ophthalmology. 2011;118(3):459-467.
- 9. Konstas AG, Mikropoulos D, Dimopoulos AT, et al. Secondline therapy with dorzolamide/timolol or latanoprost/ timolol fixed combination versus adding dorzolamide/timolol fixed combination to latanoprost monotherapy. Br J Ophthalmol. 2008;92:1498-1502
- 10. Nordstrom BL, Friedman DS, Mozaffari E, et al. Persistence and adherence with topical glaucoma therapy. Am J Ophthalmol. 2005;140:598-596.