iStent Trabecular Micro-Bypass Device

The trabecular meshwork is likely the primary site of flow restriction in glaucoma.^[1] It makes sense, then, that bypassing the trabecular meshwork would allow more fluid out of the eye resulting in a lower intraocular pressure.

The idea of bypassing the trabecular meshwork has been around for a very long time. Over one hundred years ago a technique to incise the angle from inside the eye was published.^[2] Many methods of bypassing the trabecular meshwork have since been tried with widely varying results. The first trabecular bypass implant to undergo clinical trials in the United States was the Eyepass device.^[3] However, it was never FDA approved for use.

The iStent is the first FDA approved “trabecular microbypass device.” It will likely not be the last as there are a number of other devices in clinical trials awaiting FDA approval.  These devices aim to shunt aqueous fluid directly from the anterior chamber into Schlemm’s canal.

What is the iStent?

The iStent is a 1.0mm long snorkel-like device made out of medical grade titanium. It is, in fact, the smallest medical device approved for implantation in the human body. Weighing only 0.000002 ounces and with a price tag of roughly $1,000 this makes it one of the most valuable items on Earth by weight. An ounce of these little devices would set you back roughly a half billion dollars!

The long end of the snorkel is tapered. Using a specially designed inserter, the surgeon presses the tapered end through the trabecular meshwork and lodges it into the canal of Schlemm. This allows aqueous fluid to flow out of the anterior chamber directly into Schlemm’s canal.

How Well Does the iStent Work?

The answer to this question is not as straightforward as one might desire. Why? Well, the larger studies of the iStent did not look at only placement of the iStent. For every iStent placed, cataract surgery was also done on that eye. Essentially the comparison studied was iStent plus cataract surgery versus cataract surgery alone.

Cataract surgery alone can be expected to lower IOP.^[4] And, the higher the IOP prior to surgery the more cataract surgery can be expected to lower the IOP.^[5] One study noted that patients with elevated IOP prior to cataract surgery experienced IOP lowering by as much as 8.5mmHg after surgery.^[6] That’s pretty impressive. So, how much better did the patients with iStent plus cataract surgery do compared to cataract surgery alone? You be the judge.

One Year Out

In the most frequently quoted study of the iStent, 50% of those with cataract surgery alone achieved an IOP below 22mmHg without use of drops whereas 72% of those with both iStent and cataract surgery achieved this result.^[7] Of course, few glaucoma surgeons would be satisfied with an IOP above 18mmHg so it’s difficult to know what to make of these results.

The same study evaluated the percentage of patients off drops with a reduction of IOP by at least 20% one year out from surgery. In those patients who had cataract surgery alone 48% achieved this IOP reduction whereas when the iStent was added 66% made the cut. Interestingly, the average IOP reduction was similar between those who underwent only cataract surgery (8.5mmHg) and those who also had an iStent placed (8.4mmHg). Fewer patients with an iStent, however, needed medications to control their IOP.

Two Years Out

By two years out from surgery 61% of those with cataract surgery alone achieved an IOP below 22mmHg whereas 71% of those with both iStent and cataract surgery achieved this effect.^[8] Although this difference was “statistically significant”, I don’t know too many surgeons that are not somehow “vested” in this technology who believe this is a real clinical difference. Further supporting the loss of effect over time is that 54% of patients with cataract surgery alone achieved at least a 20% reduction in IOP whereas 61% did so when the iStent was added to cataract surgery. This difference was neither statistically nor meaningfully significant. Finally, there was no significant difference in the number of medications required to control IOP two years out from surgery.

Can More than One iStent be Placed in My Eye?

Placement of one iStent at the time of cataract surgery may not produce much of a “wow” in terms of IOP lowering. Indeed, quite a few eye surgeons have wondered, “Where’s the beef?” If the iStent were a burger, you’d probably want to order a double or triple as more iStents appear to result in greater IOP lowering.^[9] Unfortunately, that’s not on the menu in the USA as the FDA has only approved the use of one iStent at the time of cataract surgery.

“Wait,” you say, “I thought medical doctors could use any FDA approved medication or device ‘off label’?” And, you’d be correct. Sure, a surgeon could implant two or three iStents in your eye but would risk making the surgery center very unhappy. Why? Because each iStent costs the surgery center around $1,000. Surgery centers are paid a set amount for implantation of a trabecular micro-bypass device regardless of how many are placed. This fee is generally enough to cover the cost of one iStent plus the anticipated costs of staffing, supplies, etc. If two or more iStents are placed, however, the surgery center would actually end up paying for the privilege of having you schedule surgery with them.

Risks of iStent Micro-Bypass surgery

Given that a single iStent only lowers the IOP about as much as a single glaucoma eye drop is it worth taking the risk of surgery for such a mild benefit?

Fortunately, there seem to be few vision-threatening risks of having an iStent placed in the eye. In theory the iStent could dislodge, resulting in bleeding or chronic inflammation in the eye. In practice, however, these risks appear to be rare. Compared to cataract surgery alone, there does not appear to be significant added risk to placing an iStent at the time of cataract surgery.^[10]

Who Should Consider Options other than an iStent?

If you have moderate glaucoma that will likely require additional surgical treatment and you would like canaloplasty to be an option then you should not have an iStent placed.  Canaloplasty surgery cannot be performed after placement of an iStent. This is because the iStent blocks the canal. A blocked canal cannot be fully catheterized.

Those who have significant vision loss from glaucoma generally need a lower IOP than can be expected from placing an iStent. As such, the iStent is not an appropriate treatment option for advanced glaucoma.

In practice, if you don’t have a cataract that needs surgery you won’t be eligible for placement of an iStent in the USA. Again, “off label” use without same-setting cataract surgery is allowed but may not be paid for by your insurance. Indeed, even “on label” placement of an iStent may not be covered by insurance.

For those who are paying out of pocket for glaucoma surgery, cost must be considered.  The iStent implant is around $1,000. The surgeon’s fee is often another $1,000.  Considering that at best one implant can be expected to lower the IOP a paltry 1-2mmHg beyond what cataract surgery alone would do, that’s $1,000-2,000 per mmHg lowered! In terms of dollars per unit of IOP lowering there are other glaucoma surgeries that are much better values than the iStent.

In Summary…

If you happen to have ocular hypertension or mild glaucoma that is well-controlled on one or two drops and your cataract is ready to be surgically removed and your insurance covers placement of an iStent or you happen to be obscenely wealthy and just like the idea of having a piece of microscopic titanium eye jewelry with a per ounce value higher than anything other than antimatter^[11] then this might be a reasonable option for you to consider.

If, on the other hand, your glaucoma is moderate or severe, poorly controlled, your insurance doesn’t cover placement of an iStent, or you are likely to need additional glaucoma surgery in the future, then the iStent is likely to disappoint.

Trabecular Bypass Implants for Glaucoma →