The PreserFlo Ab-Externo MicroShunt: A Conversation With Dr Ike Ahmed

Shuchi B. Patel, MD


May 19, 2022

Iqbal "Ike" K. Ahmed, MD, is a world-renowned ophthalmologist whose clinical work and research focus on glaucoma, complex cataracts, and intraocular lens complications. In 2020, The Ophthalmologist Power List named Dr Ahmed the second most influential ophthalmologist worldwide. He has done pioneering work in innovative glaucoma therapeutics and coined the term micro-invasive glaucoma surgery (MIGS) as a new genre of surgical approaches and devices. These microscopic MIGS devices are implanted into the eye to reduce intraocular pressure.

Here, Dr Patel interviews Dr Ahmed about one of those devices, the PreserFlo MicroShunt.

Shuchi B. Patel, MD: Good morning, Dr Ahmed. Thanks for joining us. Today, we want to get your expert opinion about the Ab-Externo MicroShunt, which was formerly known as the InnFocus but now is marketed as PreserFlo. To get started, can you tell us a little bit about what the shunt is, how it's supposed to work, and what makes it unique from other devices and procedures, such as the Xen, for example?

Iqbal "Ike" K. Ahmed, MD: A pleasure to be here. PreserFlo is technically defined as an ab externo SIBS microshunt. It's a bleb-forming procedure that utilizes mitomycin C to establish a functional bleb to lower pressure. It is basically inserted externally through a scleral dissection. The design of the device, which is 8.5 mm long with a 70-μm lumen, is to prevent hypotony on the basis of Newtonian physics.

The material of the device itself has been shown to be quite biocompatible; in fact, it's used as part of many coronary stent coatings and has been found to be quite stable and less inflammatory. It does have some similarities to other microstents, and people often compare this with the Xen. There are some differences though. As I mentioned, this is placed externally; Xen is placed internally, ab interno, but can be done externally as well. This device is a bit longer, it's a bit larger, and the material is different. There is a small, little fin that is designed in the middle part of the device which is used to hold it in place and prevent any flow around the tube. That's a little bit of a design difference as well, compared with other microshunts.

Dr Patel: Thank you for explaining that because in the US, this device is awaiting FDA approval, so many of us haven't had a chance to see the shunt or have any experience with it. Now, this device has been approved in Canada and Europe, and one of the reasons for the delayed approval in the US, I'm just guessing, is that the study that tried to establish noninferiority of PreserFlo to a trabeculectomy found that the endpoint was not confirmed by the parameters that were set[1]. So, in your experience, do you think that the PreserFlo is comparable to a trabeculectomy in terms of pressure lowering and efficacy or do you find that it is still not as good as our gold standard?

Dr Ahmed: You're speaking about the large pivotal trial, which is 500-plus patients comparing PreserFlo vs trabeculectomy, with mitomycin 0.2 mg/mL used in both groups. This was a high-quality randomized controlled trial, and as you said, the main outcome measures, which were noninferiority for pressure lowering, weren't met. So this device is currently still undergoing regulatory review.

There are, of course, many aspects to an approval, and I'm not a regulatory expert, but this includes efficacy, safety, and other potential patient benefits. In our experience, and we've published our work in peer-reviewed journals, the pressure lowering for PreserFlo can be further optimized with some attention to surgical technique[2,3]. In the pivotal clinical trial, there are very experienced trab surgeons performing the cases, but it was these surgeons' first cases of PreserFlo, and of course you expect the results of the trabs to be good in that group. And the trabs did do very well. In fact, the trabs actually did better than we've seen in pretty much any trial out there.

The PreserFlo did lower pressure, no question, but the noninferiority wasn't met, and I think part of it is surgical technique. As I said, these are learning cases of PreserFlo, the first few cases per surgeon, and so that can contribute toward those results. Some of the study design made it a little bit difficult to achieve success also. One thing we found is that mitomycin — and this is something we published — is particularly important. Although increasing mitomycin may increase risks in some procedures, with PreserFlo, it's important to have enough mitomycin C, considering the way the aqueous is draining from the device. So our results with PreserFlo and higher doses of mitomycin C seem to be more in line with what we get with trabeculectomy. I would still give trabeculectomy an edge, but I think it's within the range. In fact, the average pressures in our studies with typically nonrefractory or refractory glaucoma, or combined procedures, are generally running between 12 mm Hg and 13 mm Hg. I think most surgeons and other published studies would say that is very similar to what we see as our results for trabs.

With attention to technique, our experience has been that we're very close in intraocular pressure (IOP) with PreserFlo and trabs. The other side of it is that there was, even in the pivotal clinical study we are referring to, a reduction in complications, particularly hypotony, in comparing the groups. There are other aspects, like visual recovery, including decreased surgical-induced astigmatism and stability of pressure in the early postoperative period, that are further benefits of PreserFlo. We have found it to be a very patient-friendly bleb, if you want to call it that, in terms of the stability, the recovery, the safety, and so on and so forth. So, it's filled an important void in our armamentarium, and I think we'll see, as more experience comes out around the world, whether in fact PreserFlo is similar to trabs.

Dr Patel: That's great to hear because as I said, I have no experience with this device, but just from reading these publications that you have mentioned, the PreserFlo would be a great asset for a glaucoma surgeon to have. Especially because, as you mentioned, the pressure reduction was significant even if not as good as a trab. Even in the trial that established that it was inferior, the pressure reduction for PreserFlo was significant, from approximately 21 mm Hg to 14 mm Hg, while trabeculectomy went from 21 mm Hg to 11 mm Hg, and they had established inferiority at > 2.5 mm Hg difference in end IOP. So the difference was just half a point higher than needed to be determined equal, and I'm pretty sure most surgeons would be quite happy with a pressure in the 13-14 range if there was a significant reduction of complications and post-op care involved. I would agree from reading the results of this paper and hearing your experience that this seems like a very promising new addition.

So, for surgeons who haven't had a chance and who hopefully will want to add this to their surgical skill set, do you have any suggestions or pitfalls to avoid? You already mentioned that using more mitomycin C is essential, but any other suggestions on how we can make this procedure more successful in our hands?

Dr Ahmed: Yes. I think there are a couple of things. As I mentioned earlier, in our work, mitomycin dosing is important. You may say, "Well, why do we need more mitomycin with PreserFlo and maybe not so much for trabs?" With a single-lumen device, placed posteriorly, under a thick Tenon layer I believe that we just need more mitomycin. That's one big suggestion.

Surgical technique is important, and we often talk about how do we manage Tenon layer? For this procedure, an important aspect is how we open and close the conjunctiva and Tenon layer. For trabeculectomies, surgeons do different things. But in PreserFlo, it's important that we open and close the Tenon layer in a way that doesn't cause any entrapment of the device within Tenon's capsule. This can increase resistance and potentially increase failure.

The other aspect, which I think wasn't so diligent in the study, was checking for flow. The device is placed in the scleral tunnel and depending on that design, there could be compression or there could be issues in regard to flow. The device should be primed and checked, and flow should be ensured to be at an adequate rate so there will be desired pressure reduction. We also typically place the device in one of the superior quadrants. Initially, surgeons were placing this device at 12 o'clock, and for a trab that's fine, but for PreserFlo, since you're draining 6 mm back, that would put the device right at the insertion of the superior rectus muscle. I think we all would agree that the ability to form a bleb would be more limited there than in the supernasal quadrant, for example, between the superior and medial rectus muscles.

Those are a couple of things that would help with increased success. I would also say that placement into the anterior chamber is important. The pivotal study showed no difference in endothelial cell loss between the groups, which may be surprising to some. To achieve this, it's important to place the device properly in the angle of the iris plane and away from the cornea.

Dr Patel: Those are really great tips that may not have been very intuitive to a surgeon using the device for the first time, so that's very useful. You did mention the endothelial cell count. I noticed that there were some further publications after this pivotal study that do mention there was some loss of endothelial cell count[4]. Have you noted in your experience that there have been any long-term complications that we should be concerned about?

Dr Ahmed: A couple of things, absolutely. Any filtering procedure seems to create endothelial cell loss, and this includes trabeculectomy. So yes, I think PreserFlo, trabeculectomy, and other bleb-forming procedures can result in endothelial cell loss. I don't believe that the rate is similar to the bigger tube shunts, like Ahmed and Baerveldt [implants], although we don't have any head-to-head studies. I think this has to do with the size of the device, the material, and the position of it. We have to be careful for monitoring for that. But we have not seen, over the last 8 years and 1000we've done here, anything standing out. There are some patients who have had some issues, often with preexisting corneal conditions, which I believe would have been the case regardless. If the device is not placed properly and directed toward the cornea, that risk is certainly higher.

People often ask about the risk for erosion, of course, because this is a foreign body material, but this is extremely rare in our experience. In our experience, these have been cases, for example, with previously poor or scarred conjunctiva, and those cases we have to be careful using these devices. We don't typically cover with any graft material because of the 3 mm tunnel and the size of the device, so I don't think it's necessary. But in high-risk eyes, that may be something to consider. But those are the things that I think people probably are the most commonly worried about long-term safety–wise. And fortunately, in our hands, at least over 1000 eyes, 8 years, we haven't seen anything of significance.

Dr Patel: That's great to hear, and wow that's a lot of patients. It sounds like this is a device that we should be really looking forward to here in the US, and hopefully will significantly add to what we can offer our patients with glaucoma. So, thank you so much for sharing all your advice, and giving us your opinion and experience about the shunt. We really appreciate it.

Shuchi B. Patel, MD, is director of glaucoma services in the department of ophthalmology at West Palm Beach VA Medical Center in Florida. She explores the ever-changing glaucoma space for Medscape, including advances in diagnostics and treatments.

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