BLOG: A stint on the iStent

It is estimated that 3.6 million cataract surgeries will be performed in the U.S. this year. Current estimates suggest more than 3 million Americans have glaucoma, and it is estimated this will double to 6 million in the next few decades.

With the incidence of glaucoma and cataracts being fairly high, it is no surprise that these two conditions will frequently co-exist in a clinical setting. Numbers also suggest approximately 20% of eyes that undergo cataract surgery are also on at least one glaucoma drop (Tseng et al.).

In the U.S., the first line of treatment for glaucoma is an antihypertensive drop followed by laser and more invasive surgeries for advanced disease. However, in recent years, there seems to be a shift in the treatment paradigm of managing mild to moderate glaucoma.

One thing we learned from the Ocular Hypertension Treatment Study was that approximately 40% of glaucoma patients need to be on two or more drops to achieve 20% reduction from their baseline IOP (Kass et al.). Most drops have a high safety profile, in general, but they can cause local and systemic side effects.

While administered with minimal effort, it is widely known that glaucoma drop adherence can be very challenging. The Glaucoma Adherence and Persistence Study retrospectively looked at pharmacy refill information to help determine adherence rate of glaucoma patients. It found that 89% of patients claimed to take their glaucoma medication “every day,” yet the research showed that patients had, on average, only enough refilled drop to take their meds 64% of the time. Only 10% of patients reviewed in the study filled prescriptions continuously for 12 months. Some well-recognized barriers to good adherence include: co-existing systemic conditions in the older population (dementia, arthritis, multiple meds), dexterity and cost-prohibitive lifelong drop therapy.

An emerging treatment option, minimally invasive glaucoma surgery (MIGS), is an additional glaucoma management option that is being offered to patients who have mild to moderate glaucoma. MIGS is an umbrella term used for emerging medical devices or procedures that are used to help lower IOP and do not dramatically violate the eye.

The MIGS emphasis is on surgical restoration of the natural outflow of aqueous. These procedures are not intended to take the place of a trabeculectomy or tube shunt, which are often needed for more advanced disease. However, there seems to be a role in utilizing such procedures earlier in the disease process, thereby slowing progression and deferring the need for more complicated surgeries.

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In 2015, a variety of new MIGS procedures hit the scene that theoretically enhance outflow through one of three primary mechanisms of action: removal, bypass and dilation. Regardless of the mechanism of action intended to lower IOP, these MIGS generally share a couple of the same qualities.

First, they are all micro-incisional, which means they are conjunctival sparing and minimally disruptive to the ocular tissue. Second, they are all very low risk with a high safety profile, which allows a quick recovery. In the ophthalmic surgery world, the phrase “ab-interno” refers to a device or procedure that can be approached from the inside of the eye (ie, iStent), as opposed to the term “ab-externo,” which means the procedures are performed from outside the eye (ie, trabeculectomy). Most of the MIGS procedures are ab-interno procedures and utilize a clear corneal incision (CCI) rather than dissection of the conjunctiva.

The iStent (Glaukos) gained FDA approval in 2012 to be used in conjunction with cataract surgery for adult patients with mild to moderate glaucoma who are currently managed with an antihypertensive drop. The iStent is a 1-mm implantable snorkel-shaped shunt that is inserted into a collector channel through the trabecular meshwork at the conclusion of cataract surgery.

We follow a simple checklist in determining patient candidacy for the combined cataract/iStent surgery. Before proceeding, we need to stage the glaucoma, because the iStent is only approved for “mild to moderate” glaucoma. This means either acquiring optic nerve OCT and visual fields from the patient’s OD or performing them preoperatively if none are readily available. It is also imperative that gonioscopy is done prior to surgery to assess the condition of the angle. The iStent is contraindicated in a narrow angle and in patients with secondary angle closure glaucoma, so it needs to predetermined that there is anatomically enough room for the surgeon to place the stent. Not surprisingly, phacomorphic narrowing of the angle does not limit the use of the device, as cataract extraction removes this variable and allows opening of the angle. That said, gonioscopic abnormalities such as peripheral anterior synechiae, iris processes and angle recession may interfere with the placement of the stent.

An iStent in the nasal angle of a patient. You will likely find the stent 180 degrees from the CCI, often inferior-nasal, where a large amount of collector channels are congregated. Source: Aaron Bronner, OD

The risks involved with placement of an iStent are not vastly different from the risk of cataract surgery alone. There is minimal risk of hypotony with the iStent because the trabecular meshwork is preserved in the procedure, and this ensures an episcleral backpressure of 8 mm Hg to 11 mm Hg. Some of the adverse events with the iStent were reported in the U.S. investigational device exemption pivotal study and included early postop corneal edema (8%), posterior capsular opacification (6%), stent obstruction (4%), elevated IOP (4%), blurry vision (3%) and any best corrected visual acuity loss of one line after the 3-month visit (8%).

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In our experience it takes about 6 weeks to have a decent assessment of the postoperative IOP. This allows surgical inflammation to quiet and for the patient to be off their postop steroid drops. We generally keep our iStent patients on their regular glaucoma regimen throughout this postop period. If IOP is lowered at the 6- to 8-week time frame, it is reasonable to consider a glaucoma medication holiday to determine how the iStent is working.

And speaking of working, what are the statistics on the efficacy of the iStent? A prospective, randomized trial conducted by Glaukos and reviewed by the FDA showed that 68% of patients undergoing phaco/iStent met an endpoint of IOP of less than 21 mm Hg with no medications at 12 months, compared to 50% of patients who underwent cataract surgery alone (Samuelson et al.). As with all new medical devices, there is a dearth of information on the duration of efficacy, but it is promising that the 3-year results show a sustained reduction. An independent clinical study published 3-year results that showed an average of 36% IOP reduction and 86% reduction in the number of glaucoma medications (Neuhann).

A second-generation iStent, the iStent inject, consists of two small stents — about a third of the size of the original iStent – which are surgically implanted at the same time and relies on the same fluidic method of action that the iStent currently does. The iStent inject is currently approved in several countries but is not yet approved in the U.S. The company is expected to release efficacy and safety data from the iStent inject trial sometime in the first half of 2018. It is anticipated that with two stents, there will be a larger IOP-lowering effect.

The introduction of MIGS provides a relatively easy, low-risk opportunity for patients to reduce their medication burden by lowering IOP enough to reduce or eliminate the need for topical drops. The safety profile of these procedures paired with rapid recovery, modest efficacy and ab interno approach make MIGS a very appealing option to offer patients with mild to moderate glaucoma. One trade-off for an extremely high safety profile is that most MIGs likely do not have the IOP-lowering efficacy of traditional glaucoma surgery; however, the procedures seem to be gaining momentum in the glaucoma management algorithm.

References:

Kass MA, et al. Arch Ophthalmol. 2002;120(6):701-713; discussion 829-830.

Neuhann TH. J Cataract Refract Surg 2015;doi:10.1016/j.jcrs.2015.06.032.

Samuelson TW, et al. Ophthalmology. 2011;doi:10.1016/j.ophtha.2010.07.007.

Tseng VL, et al. JAMA. 2012;doi:10.1001/jama.2012.9014.