istent

istent inject (Glaukos Corp., Laguna Hills, CA, USA) is inserted through the trabecular meshwork into Schlemm's canal, usually at the time of cataract surgery. It is a titanium stent, 360 microns in length and 230 microns in width.

The Hydrus Microstent ( Ivantis Inc., Irvine, CA, USA) is an implant that holds Schlemm's canal open to allow more fluid out. It is usually inserted at the time of cataract surgery. It is an 8 mm long crescent shaped titanium device.

The istent inject is designed so as to assist in the outflow of aqueous fluid from within the eye by creating a bypass through the trabecular meshwork, the main source of resistance in glaucomatous eyes, resulting in a flow of aqueous fluid into Schlemm's canal and from there it is sent off to be removed into the blood stream. As you are aware, glaucoma is caused by a blockage of the outflow of aqueous fluid from within the eye due to a blocked trabecular meshwork or a blocked Schlemm's canal or a combination of both. So that this raises the question of what happens if Schlemm's canal is either blocked or collapsed?

The istent inject is made by Glaukos Corp., Laguna Hills, CA, USA and is the company's next generation trabecular micro bypass technology and is based on the same fluidic method of action as the company's first generation pioneering istent trabecular micro bypass stent, which has been implanted into more than 450,000 eyes worldwide since its introduction in 2012.

Each istent inject stent is 0.23 mm X 0.36 mm .

Ocular hypotony is a very rare complication of istent implantation. A literature search of the safety profile of istent implantation revealed only 1 case of chronic hypotony. A single case of transient hypotony was reported on the day of surgery, but it resolved without treatment after 1 day. .

Ocular hypotony has been defined as IOP < 6.5 mm Hg.

I refer to Review of Ophthalmology September 2018, istent inject and hydrus: new ways to increase outflow by Christopher Kent, senior editor.

Thomas W. Samuelson MD:

Cataract surgery alone is a meaningful intervention for patients with mild to moderate glaucoma. That's important, because that's the foundation on which MIGS rests, in my opinion.

While the exact mechanism is not clear, we think that removing the cataract is helping physiologic outflow. While speculative, I believe it is mechanical. Removing the lens allows the iris, angle and ciliary zone complex to assume a more posterior position, and that improves outflow.

My philosophy is that if I can augment the pressure lowering caused by the cataract surgery, in a fashion that's synergistic with whatever the cataract has done, that's an added bonus. With canal based procedures you're retaining the outflow pathway that you were born with.

Using the original istent meant placing a single stent, so the surgeon had to choose the most favourable place to put it, a process that has become known as intelligent placement. Many surgeons have found this challenging, which is completely understandable. In contrast, the istent inject employs a 2 stent strategy. If you put both stents in the inferonasal quadrant, where the number of collector channels is greatest, you're very likely to end up in close proximity to a collector channel.

My learning curve for the original istent occurred as an investigator in the FDA trial. Doing my first 20 cases or so, I didn't have nearly as much confidence that I was getting the stent into the canal every time as I did later. In contrast, the new version is easier to implant. It uses a different insertion technique. Instead of circumferential placement requiring a lateral motion and checking the depth of the stent, this is a straight in insertion. I suspect that surgeons will like the new design and find it easier to use than the original istent.

Alan Crandall MD:

Everyone thought that the original istent would be very easy for non glaucoma surgeons to use. However, the number of people using it who are not glaucoma specialists has dropped, because it's not as easy as they thought. It's a little tricky to get into the canal (Schlemm's canal), and you always have the issue of deciding where to put the stent.

The new istent model is different, both because it gives you 2 stents and because it's easier to put in. The advantage of 2 in is that you're more likely to get access to at least 1 outflow channel, and the trial data supports this. In fact, outside the US everybody uses 2 or 3 istents to get a consistently lower IOP. Although the istent inject study design was somewhat different from the original istent trial, the data showed a more consistent and long lasting result than was achieved with the original device - at least for the 2 or 3 years of follow up that's been measured so far. The IOP lowering is at least 1 or 2 points greater than what we get with a single istent.

Although the new device is easier to use, the fact that you're injecting the stent rather than inserting it means that it can easily go into the wrong spot. If the original stent was not in the canal, it was pretty obvious. Then you could make another attempt to get it in. But the new device can be inadvertently injected just above or below the canal, and you won't necessarily realise that it's not in the canal.

My comments:

If you are contemplating this procedure make sure that you have a competent eye surgeon. Do not, under any circumstances, employ someone without the appropriate experience.

One further important improvement needs to be made on the istent inject.

SOURCES

https://sites.google.com/view/dryeye

https://sites.google.com/view/haigis

https://sites.google.com/view/cataractoperation

https://sites.google.com/view/royalperthhospital

https://sites.google.com/view/lionseyeinstituteofwa

https://sites.google.com/view/keratoconus

https://sites.google.com/view/markmcgowan

https://sites.google.com/view/xen45

https://sites.google.com/view/cystoidmacularedema

https://sites.google.com/view/cypass