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Stent helps preserve physiologic outflow in glaucomatous eyes
This approach has short- and long-term benefits and has several advantages over trabeculectomy.
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The majority of aqueous leaves the eye via the conventional outflow pathway through the trabecular meshwork, to Schlemm’s canal and then to collector channels until it enters the systemic venous circulation in the episcleral veins. While this is the most common pathway, up to 30% may also leave through uveoscleral pathways. In glaucoma, IOP can become elevated from an increase in outflow resistance in these drainage pathways, most commonly at the level of the trabecular meshwork.
Traditional approach
The most frequently performed surgery to reduce IOP in open-angle glaucoma is trabeculectomy. For the past 100 years or so, the standard approach to glaucoma has been to create a hole in the eye that allowed aqueous to drain into the subconjunctival space, bypassing the trabecular meshwork and its associated drainage system. While the actual surgery is fairly straightforward, the challenge for the clinician is in managing the various short- and long-term complications. The short-term complications generally relate to suboptimal incisional closure. Leakage or over-filtration allows too much aqueous egress, resulting in hypotony and decreased vision. Too little flow results in scarring and closure of the wound and inadequate IOP lowering. The achievement of optimal flow often requires a significant amount of manipulation of the sclera and the conjunctiva with the use of antifibrotics. Wound healing in patients is a difficult thing to predict: African-Americans tend to have a significant amount of scarring while Caucasians tend to have less scarring.
In the long term, the bleb that is created from the trabeculectomy can create problems on the ocular surface years after the surgery. The bleb can cause irregularity of the cornea, resulting in astigmatism and drying. The bleb can also be a hindrance particularly to people who have to wear contacts or who work in dirty environments, subjecting them to a risk of getting endophthalmitis.
A more optimal solution would be to correct the deficiencies in the outflow system, that is, treat the problem in the trabecular meshwork, Schlemm’s canal or the collector system. As our understanding of the eye’s outflow system has been enhanced over the last 10 years, new therapies have been developed that specifically address the outflow pathways.
Newer treatment
Newer outflow-targeting treatments include the iStent (Glaukos), which is directed at improving aqueous flow through the trabecular meshwork. This 1-mm long, snorkel-shaped stent sits with its long axis in Schlemm’s canal, the “neck” of the stent traversing the trabecular meshwork and the opening of the snorkel protruding slightly into the anterior chamber. Due to the design of the stent, it is stable and does not move along the axis of the canal. While it can be inserted in a standalone procedure, it is most commonly performed in conjunction with cataract surgery and inserted through the same clear corneal incision used to perform phacoemulsification.
The pivotal U.S. Food and Drug Administration trials showed that a single iStent implanted in conjunction with cataract surgery in mild to moderate glaucoma patients resulted in a mean reduction of IOP of 8 mm Hg. In addition, they showed the iStent to be more effective at reducing IOP than phacoemulsification alone and that 72% of phaco-iStent patients were medication free at 1 year after surgery. Additional studies in Canada have shown that multiple iStents may be used for greater reduction of IOP. The mechanism of action with the iStent corresponds to Grant’s studies demonstrating that an incision through the trabecular meshwork into Schlemm’s canal in a series of enucleated glaucomatous eyes eliminated all abnormal outflow resistance and left resistance similar to a normal eye.
The iStent procedure has additional value due to its compatibility with cataract surgery and its medication-lowering impact. Reducing the medication burden for patients has both immediate and long-term benefits. The cost and hassle associated with the use of multiple medications often results in poor compliance, resulting in fluctuating IOP control and disease progression. That makes this low-risk stenting procedure beneficial both to the patient and the health system in general.
When trabeculectomy is combined with cataract surgery, there is a risk of wound leaks and hypotony, which can, in turn, jeopardize the good vision that is the primary goal of cataract surgery. With the ab interno iStent procedure, there is no effect on the cataract wound and no excessive drainage that could induce hypotony or contribute to endophthalmitis. The iStent device does not negatively affect the healing time or risks associated with cataract surgery in any way. As a result, it is an attractive option to be used for the control of glaucoma in patients who are undergoing cataract surgery and wish to decrease their medication burden.
References:
- Brubaker RF. Measurement of aqueous outflow by fluorophotometry. In: Ritch R, Shields MB, eds. The Glaucomas. Vol 1. St. Louis: Mosby-Year Book; 1989:337-344.
- Grant WM. Experimental aqueous perfusion in enucleated human eyes. Arch Ophthalmol. 1963;69:783-801.
- Nilsson SF. The uveoscleral outflow routes. Eye. 1997;11(Pt 2):149-154.
- Samuelson TW, et al. Randomized evaluation of the trabecular micro-bypass stent with phacoemulsification in patients with glaucoma and cataract. Ophthalmology. 2011;118(3):459-467.
- Richard A. Lewis, MD, can be reached at 1515 River Park Drive, Sacramento, CA 95815; 916-649-1515; fax: 916-649-1516; email: rlewiseyemd@yahoo.com.
- Disclosure: Dr. Lewis is a consultant to Alcon, Glaukos and iScience Interventional.
